# Alexei KoshelevArgonne National Laboratory | ANL · Materials Science Division

42.33

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Introduction

**Current Institution**

Materials Science Division

Lemont, Illinois

**Current position**

Scientist

**Skills and Expertise**

248

Research items

4,202

Reads

7,392

Citations

Research Experience

**Nov 1993**

**Scientist**

Argonne National Laboratory · Division of Materials Science

Lemont, United States

**Sep 1991 - Sep 1993**

**Visiting Researcher**

Leiden University · Kamerlingh-Onnes Laboratory

**Sep 1986 - Nov 1991**

**Researcher**

Institute of Solid State Physics RAS · Department of Theory (TD)

Education

**Sep 1983 - May 1986**

**Landau Institute of Theoretical Physics**

Theoretical Physics

**Sep 1977 - May 1983**

**Moscow Institute of Physics and Technology**

Physics

Projects

Research

Research Items (248)

Sr$_x$Bi$_2$Se$_3$ and the related compounds Cu$_x$Bi$_2$Se$_3$ and Nb$_x$Bi$_2$Se$_3$ have attracted considerable interest, as these materials may be realizations of unconventional topological superconductors. Superconductivity with T$_c$ ~ 3 K in Sr$_x$Bi$_2$Se$_3$ arises upon intercalation of Sr into the layered topological insulator Bi$_2$Se$_3$. Here we elucidate the anisotropy of the normal and superconducting state of Sr$_{0.1}$Bi$_2$Se$_3$ with angular dependent magnetotransport and thermodynamic measurements. High resolution x-ray diffraction studies underline the high crystalline quality of the samples. We demonstrate that the normal state electronic and magnetic properties of Sr$_{0.1}$Bi$_2$Se$_3$ are isotropic in the basal plane while we observe a large two-fold in-plane anisotropy of the upper critical field in the superconducting state. Our results support the recently proposed odd-parity nematic state characterized by a nodal gap of $E_u$ symmetry in Sr$_x$Bi$_2$Se$_3$.

Theoretical and experimental studies of intrinsic Josephson junctions that naturally occur in high-Tc superconducting Bi2Sr2CaCu2O8+{\delta} (Bi-2212) have demonstrated their potential for novel types of compact devices for the generation and sensing of electromagnetic radiation in the THz range. Here, we show that the THz-on-a-chip concept may be realized in liquid phase epitaxial-grown (LPE) thick Bi-2212 films. We have grown {\mu}m-thick Bi-2212 LPE films on MgO substrates. These films display excellent c-axis alignment and single crystal grains of about 650x150 {\mu}m2 in size. A branched current-voltage characteristic was clearly observed in c-axis transport, which is a clear signature of underdamped intrinsic Josephson junctions, and a prerequisite for THz-generation. We discuss LPE growth conditions allowing improvement of the structural quality and superconducting properties of Bi-2212 films for THz applications.

The current-carrying capacity of type-II superconductors is decisively determined by how well material defect structures can immobilize vortex lines. In order to gain deeper insights into intrinsic pinning mechanisms, we have explored the case of vortex trapping by randomly distributed spherical inclusions using large-scale simulations of the time-dependent Ginzburg-Landau equations. We find that for a small density of particles having diameters of two coherence lengths, the vortex lattice preserves its structure and the critical current $j_c$ decays with the magnetic field following a power-law $B^{-\alpha}$ with $\alpha \approx 0.66$, which is consistent with predictions of strong pinning theory. For higher density of particles and/or larger inclusions, the lattice becomes progressively more disordered and the exponent smoothly decreases down to $\alpha \approx 0.3$. At high magnetic fields, all inclusions capture a vortex and the critical current decays faster than $B^{-1}$ as would be expected by theory. In the case of larger inclusions with diameter of four coherence length, the magnetic-field dependence of the critical current is strongly affected by the ability of inclusions to capture multiple vortex lines. We found that at small densities, the fraction of inclusions trapping two vortex lines rapidly grows within narrow field range leading to a shallow peak in $j_c(B)$-dependence within this range. With increasing inclusion density, this peak transforms into a plateau, which then smooths out. Using the insights gained from simulations, we determine the limits of applicability of strong pinning theory and provide different routes to describe vortex pinning beyond those bounds.

Micro-electronic devices often undergo significant self-heating when biased to their typical operating conditions. This paper describes a convenient optical micro-imaging technique which can be used to map and quantify such behavior. Europium thenoyltrifluoroacetonate (EuTFC) has a 612 nm luminescence line whose activation efficiency drops strongly with increasing temperature, due to T-dependent interactions between the Eu³⁺ ion and the organic chelating compound. This material may be readily coated on to a sample surface by thermal sublimation in vacuum. When the coating is excited with ultraviolet light (337 nm) an optical micro-image of the 612 nm luminescent response can be converted directly into a map of the sample surface temperature. This technique offers spatial resolution limited only by the microscope optics (about 1 micron) and time resolution limited by the speed of the camera employed. It offers the additional advantages of only requiring comparatively simple and non-specialized equipment, and giving a quantitative probe of sample temperature.

We investigate the onset of superconductivity in magnetic field for a clean two-dimensional multiple-band superconductor in the vicinity of the Lifshitz transition when one of the bands is very shallow. Due to small number of carriers in this band, the quasiclassical Werthamer-Helfand approximation breaks down and Landau quantization has to be taken into account. We found that the transition temperature $T_{C2}(H)$ has giant oscillations and is resonantly enhanced at the magnetic fields corresponding to full occupancy of the Landau levels in the shallow band. This enhancement is especially pronounced for the lowest Landau level. As a consequence, the reentrant superconducting regions in the temperature-field phase diagram emerge at low temperatures near the magnetic fields at which the chemical potential matches the Landau levels. The specific behavior depends on the relative strength of the intraband and interband pairing interactons and the reentrance is most pronounced in the purely interband coupling scenario. The reentrant behavior is suppressed by the Zeeman spin splitting in the shallow band, the separated regions disappear already for very small spin-splitting factors. On the other hand, the reentrance is restored in the resonance cases when the spin-splitting energy exactly matches the separation between the Landau levels. The predicted behavior may realize in the gate-tuned FeSe monolayer.

The behavior of vortex matter in high-temperature superconductors (HTS) controls the entire electromagnetic response of the material, including its current carrying capacity. Here, we review the basic concepts of vortex pinning and its application to a complex mixed pinning landscape to enhance the critical current and to reduce its anisotropy. We focus on recent scientific advances that have resulted in large enhancements of the in-field critical current in state-of-the-art second generation (2G) YBCO coated conductors and on the prospect of an isotropic, high-critical current superconductor in the iron-based superconductors. Lastly, we discuss an emerging new paradigm of critical current by design-a drive to achieve a quantitative correlation between the observed critical current density and mesoscale mixed pinning landscapes by using realistic input parameters in an innovative and powerful large-scale time dependent Ginzburg-Landau approach to simulating vortex dynamics.

Multiple-band electronic structure and proximity to antiferromagnetic (AF) instability are the key properties of iron-based superconductors. We explore the influence of scattering by the AF spin fluctuations on transport of multiple-band metals above the magnetic transition. A salient feature of scattering on the AF fluctuations is that it is strongly enhanced at the Fermi surface locations where the nesting is perfect ("hot spots" or "hot lines"). We review derivation of the collision integral for the Boltzmann equation due to AF-fluctuations scattering. In the paramagnetic state, the enhanced scattering rate near the hot lines leads to anomalous behavior of electronic transport in magnetic field. We explore this behavior by analytically solving Boltzmann transport equation with approximate transition rates. This approach accounts for return scattering events and is more accurate than the relaxation-time approximation. The magnetic-field dependences are characterized by two very different field scales, the lower scale is set by the hot-spot width and the higher scale is set by the total scattering amplitude. A conventional magnetotransport behavior is limited to magnetic fields below the lower scale. In the wide range in between these two scales the longitudinal conductivity has linear dependence on the magnetic field and the Hall conductivity has quadratic dependence. The linear dependence of the diagonal component reflects growth of the Fermi-surface area affected by hot spots proportional to the magnetic field. We discuss applicability of this theoretical framework for describing of anomalous magnetotransport properties in different iron pnictides and selenides in the paramagnetic state.

Electronic nematicity plays important role in iron-based superconductors. These materials have layered structure and theoretical description of their magnetic and nematic transitions has been well established in two-dimensional approximation, i.e., when the layers can be treated independently. However, the interaction between iron layers mediated by electron tunneling may cause non-trivial three-dimensional behavior. Starting from the simplest model for orbital nematic in a single layer, we investigate the influence of interlayer tunneling on bulk nematic order and possible preemptive state where this order is only formed near the surface. We found that the interlayer tunneling suppresses the bulk nematicity which makes favorable formation of a surface nematic above the bulk transition temperature. The purely electronic tunneling Hamiltonian, however, favors alternating from layer-to-layer nematic order parameter in the bulk. The uniform bulk state typically observed experimentally may be stabilized by the coupling with the elastic lattice deformation. Depending on strength of this coupling, we found three regimes: (i) surface nematic and alternating bulk order, (ii) surface nematic and uniform bulk order, and (iii) uniform bulk order without the intermediate surface phase. The intermediate surface-nematic state may resolve the current controversy about the existence of the weak "meta-nematic transition" in the compound BaFe$_2$As$_{2-x}$P$_{x}$.

We demonstrate a twofold increase in the in-field critical current of AMSC's standard 2G coil wire by irradiation with 18-MeV Au ions. The optimum pinning enhancement is achieved with a dose of 6 × 1011 Au ions/cm2. Although the 77 K, self-field critical current is reduced by about 35%, the in-field critical current (H//c) shows a significant enhancement between 4 and 50 K in fields > 1 T. The process was used for the roll-to-roll irradiation of AMSC's standard 46-mm-wide production coated conductor strips, which were further processed into standard copper laminated coil wire. The long-length wires show the same enhancement as attained with short static irradiated samples. The roll-to-roll irradiation process can be incorporated in the standard 2G wire manufacturing, with no modifications to the current process. The enhanced performance of the wire will benefit rotating machine and magnet applications.

A new critical-current-by-design paradigm is presented. It aims at predicting the optimal defect landscape in superconductors for targeted applications by elucidating the vortex dynamics responsible for the bulk critical current. To this end, critical current measurements on commercial high-temperature superconductors were combined with large-scale time-dependent Ginzburg-Landau simulations of vortex dynamics.

The interaction of vortex matter with defects in applied superconductors directly determines their current carrying capacity. Defects range from chemically grown nanostructures and crystalline imperfections to the layered structure of the material itself. The vortex-defect interactions are non-additive in general, leading to complex dynamic behavior that has proven difficult to capture in analytical models. With recent rapid progress in computational powers, a new paradigm has emerged that aims at simulation-assisted design of defect structures with predictable critical-current-by-design: analogous to the materials genome concept of predicting stable materials structures of interest. Here we demonstrate the feasibility of this paradigm by combining large-scale time-dependent Ginzburg-Landau numerical simulations with experiments on commercial high-temperature superconductor containing well-controlled correlated defects.

We present a critical current analysis of a real high-temperature
superconducting (HTS) sample in a magnetic field by combining state-of-the-art
large-scale Ginzburg-Landau simulations with reconstructive three-dimensional
scanning transmission electron microscopy tomography of the pinning landscape
in Dy-doped YBa$_2$Cu$_3$O$_{7-\delta}$. This methodology provides a unique
look at the vortex dynamics in the presence of a complex pinning landscape,
responsible for the high current-carrying capacity characteristic of commercial
HTS wires. Our method demonstrates very good functional and quantitative
agreement of the critical current between simulation and experiment, providing
a new predictive tool for HTS wires design.

Introducing nanoparticles into superconducting materials has emerged as one
of the most efficient routes to enhance their current-carrying capability. We
address the optimization problem for vortex pinning by randomly distributed
metallic spherical inclusions using large-scale numerical simulations of
time-dependent Ginzburg-Landau equations. We found the optimal size and density
of particles for which the highest critical current is realized in fixed
magnetic field. For each particle size and magnetic field the critical current
reaches a maximum value at a certain particle density, which typically
corresponds to 15--23\% of the total volume being replaced by
nonsuperconducting material. For fixed diameter this optimal particle density
increases with the magnetic field. We found that the optimal particle diameter
slowly decreases with the magnetic field from $\sim 4.5$ to $\sim 2.5$
coherence lengths at a given temperature. Our results show that pinning
landscapes have to be optimized for specific applications.

Coexistence of antiferromagnetic order with superconductivity in many
families of newly discovered iron-based superconductors has renewed interest to
this old problem. Due to competition between the two types of order, one can
expect appearance of the antiferromagnetism inside the cores of the vortices
generated by the external magnetic field. The structure of a vortex in type II
superconductors holds significant importance from the theoretical and the
application points of view. Here we consider the internal vortex structure in a
two-band s$_\pm$ superconductor near a spin-density-wave instability. We treat
the problem in a completely self-consistent manner within the quasiclassical
Eilenberger formalism. We study the structure of the s$_\pm$ superconducting
order and magnetic field-induced spin-density-wave order near an isolated
vortex. We examine the effect of this spin-density-wave state inside the vortex
cores on the local density of states.

We present images of magnetic flux structures in a single crystal of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\text{\ensuremath{-}}\mathrm{d}}$ during remagnetization by fields tilted from the basal plane of the crystal. Depending on the magnitude and angle of the applied field, we observe anisotropic flux penetration along and across the in-plane field component and emergence of vortex instabilities resulting in modulated flux distributions. We associate the observed patterns with flux cutting effects and with tilted vortex structures intrinsic for layered superconductors. Time dependent Ginzburg-Landau simulations show preferential vortex motion across the $c$ axis and reveal the flux structure evolution in anisotropic superconductors under tilted magnetic fields.

We determine the upper and lower critical fields, the penetration depth and the vortex pinning characteristics of single crystals of overdoped Ba0.2K0.8Fe2As2 with Tc∼10K. We find that bulk vortex pinning is weak and vortex dynamics to be dominated by the geometrical surface barrier. The temperature dependence of the lower critical field, Hc1, displays a distinctive upturn at low temperatures, which is suggestive of two distinct superconducting gaps. Furthermore, the penetration depth, λ, varies linearly with temperature below 4 K indicative of line nodes in the superconducting gap. These observations can be well described in a model based on a multiband nodal superconducting gap.

We study the self-heating of a large stack of Bi2Sr2CaCu2O8+δ intrinsic Josephson junctions, of a configuration designed for terahertz generation. We find good qualitative agreement between direct thermoluminescent measurements of the device surface temperature and low-temperature scanning laser microscopy images. In particular, the two techniques both reveal a mode of thermal instability through the asymmetric nucleation of a small hot spot near a corner or edge of the sample. This behavior conforms with a theoretical stability analysis, and the radius of the hot spot is in excellent agreement with theoretical predictions, as is its growth with increasing bias current and bath temperature. Narrow hot spots may offer a possible means of enhancing the terahertz emission power from this type of device.

We performed a magneto-optical study of flux distributions in yttrium barium copper oxide (YBCO) crystal under various applied crossed field orientations to elucidate the complex nature of magnetic flux cutting in superconductors. Our work reveals unusual vortex patterns induced by the interplay between flux cutting and vortex pinning. We observe strong flux penetration anisotropy of the normal flux B-perpendicular to in the presence of an in-plane field H-parallel to and associate the modified flux dynamics with the staircase structure of tilted vortices in YBCO and the flux-cutting process. We demonstrate that flux cutting can effectively delay vortex entry in the direction transverse to H-parallel to. Finally, we elucidate details of the vortex cutting and reconnection process using time-dependent Ginzburg-Landau simulations.

We consider a multiband metal with deep primary bands and a shallow secondary
one. In the normal state the system undergoes Lifshitz transition when the
bottom of the shallow band crosses the Fermi level. In the superconducting
state Cooper pairing in the shallow band is induced by the deep ones. As a
result, the density of electrons in the shallow band remains finite even when
the bottom of the band is above the Fermi level. We study the density of states
in the system and find qualitatively different behaviors on the two sides of
the Lifshitz transition. On one side of the transition the density of states
diverges at the energy equal to the induced gap, whereas on the other side it
vanishes. We argue that this physical picture describes the recently measured
gap structure in shallow bands of iron pnictides and selenides.

Understanding the interaction of vortices with inclusions in type-II
superconductors is a major outstanding challenge both for fundamental science
and energy applications. At application-relevant scales, the long-range
interactions between a dense configuration of vortices and the dependence of
their behavior on external parameters, such as temperature and an applied
magnetic field, are all important to the net response of the superconductor.
Capturing these features, in general, precludes analytical description of
vortex dynamics and has also made numerical simulation prohibitively expensive.
Here we report on a highly optimized iterative implicit solver for the
time-dependent Ginzburg-Landau equations suitable for investigations of type-II
superconductors on massively parallel architectures. Its main purpose is to
study vortex dynamics in disordered or geometrically confined mesoscopic
systems. In this work, we present the discretization and time integration
scheme in detail for two types of boundary conditions. We describe the
necessary conditions for a stable and physically accurate integration of the
equations of motion. Using an inclusion pattern generator, we can simulate
complex pinning landscapes and the effect of geometric confinement. We show
that our algorithm, implemented on a GPU, can provide static and dynamic
solutions of the Ginzburg-Landau equations for mesoscopically large systems
over thousands of time steps in a matter of hours. Using our formulation,
studying scientifically-relevant problems is a computationally reasonable task.

We consider a multiband metal with deep primary bands and a shallow secondary
one. In the normal state the system undergoes Lifshitz transition when the
bottom of the shallow band crosses the Fermi level. In the superconducting
state Cooper pairing in the shallow band is induced by the deep ones. As a
result, the density of electrons in the shallow band remains finite even when
the bottom of the band is above the Fermi level. We study the density of states
in the system and find qualitatively different behaviors on the two sides of
the Lifshitz transition. On one side of the transition the density of states
diverges at the energy equal to the induced gap, whereas on the other side it
vanishes. We argue that this physical picture describes the recently measured
gap structure in shallow bands of iron pnictides and selenides.

We consider the behaviour of the fluctuating specific heat and conductivity
in the vicinity of the upper critical field line for a two-band superconductor.
Multiple-band effects are pronounced when the bands have very different
coherence lengths. The transition to superconductive state is mainly determined
by the properties of the rigid condensate of the "strong" band, while the
"weak" band with a large coherence length of the Cooper pairs causes the
nonlocality in fluctuation behaviour and break down of the simple
Ginzburg-Landau picture. As expected, the multiple-band electronic structure
does not change the functional forms of dominating divergencies of the
fluctuating corrections when the magnetic field approaches the upper critical
field. The temperature dependence of the coefficients, however, is modified.
The large in-plane coherence length sets the field scale at which the upper
critical field has upward curvature. The amplitude of fluctuations and
fluctuation width enhances at this field scale due to reduction of the
effective z-axis coherence length. We also observe that the apparent transport
transition displaces to lower temperatures with respect to the thermodynamic
transition. Even though this effect exists already in a single-band case at
sufficiently high fields, it may be strongly enhanced in multiband materials.

We report the direct imaging of a novel modulated flux striped domain phase
in a nearly twin-free YBCO crystal. These domains arise from instabilities in
the vortex structure within a narrow region of tilted magnetic fields at small
angles from the in-plane direction. By comparing the experimental and
theoretically derived vortex phase diagrams we infer that the stripe domains
emerge from a first order phase transition of the vortex structure. The size of
domains containing vortices of certain orientations is controlled by the
balance between the vortex stray field energy and the positive energy of the
domain boundaries. Our results confirm the existence of the kinked vortex chain
phase in an anisotropic high temperature superconductor and reveal a sharp
transition in the state of this phase resulting in regular vortex domains.

Mixed pinning landscapes in superconductors are emerging as an effective strategy to achieve high critical currents in high, applied magnetic fields. Here, we use heavy-ion and proton irradiation to create correlated and point defects to explore the vortex pinning behavior of each and combined constituent defects in the iron-based superconductor Ba0.6K0.4Fe2As2 and find that the pinning mechanisms are non-additive. The major effect of p-irradiation in mixed pinning landscapes is the generation of field-independent critical currents in very high fields. At 7 T ∥ c and 5 K, the critical current density exceeds 5 MA/cm2.

Iron-based superconductors could be useful for electricity distribution and superconducting magnet applications because of their relatively high critical current densities and upper critical fields. SmFeAsO0.8F0.15 is of particular interest as it has the highest transition temperature among these materials. Here we show that by introducing a low density of correlated nano-scale defects into this material by heavy-ion irradiation, we can increase its critical current density to up to 2 × 10(7) A cm(-2) at 5 K-the highest ever reported for an iron-based superconductor-without reducing its critical temperature of 50 K. We also observe a notable reduction in the thermodynamic superconducting anisotropy, from 8 to 4 upon irradiation. We develop a model based on anisotropic electron scattering that predicts that the superconducting anisotropy can be tailored via correlated defects in semimetallic, fully gapped type II superconductors.

In several parent iron-pnictide compounds the resistivity has an extended
range of linear magnetic field dependence. We argue that there is a simple and
natural explanation of this behavior. Spin density wave transition leads to
Fermi-surface reconstruction corresponding to strong modification of the
electronic spectrum near the nesting points. It is difficult for quasiparticles
to pass through these points during their orbital motion in magnetic field,
because they must turn sharply. As the area of the Fermi surface affected by
the nesting points increases proportionally to magnetic field, this mechanism
leads to the linear magnetoresistance. The crossover between the quadratic and
linear regimes takes place at the field scale set by the SDW gap and scattering
rate.

We have studied the terahertz emission from a 720 × 60 × 1.2 μm3 mesa patterned from under-doped Bi2Sr2CaCu2O8+δ. This device has an S-shaped current–voltage characteristic due to self-heating, allowing us to compare its THz emission behaviours at up to three different bias currents for the same voltage. The THz frequency generated along the lowest current branch follows the expected Josephson relation for a stack of intrinsic Josephson junctions connected in series. However, in the high current regimes, THz emission occurs at a significantly lower frequency than expected. We show that this behaviour is consistent with strongly non-uniform self-heating of the mesa at high bias currents.

In this paper we study the role of impurities in a two-band superconductor.
We elucidate the nature of the recently predicted transition from s_{+-} state
to s_{++} state induced by interband impurity scattering. Using a
Ginzburg-Landau theory, derived from microscopic equations, we demonstrate that
close to T_c this transition is necessarily a direct one, but deeper in the
superconducting state an intermediate complex state appears. This state has a
distinct order parameter, which breaks the time-reversal symmetry, and is
separated from the s_{+-} and s_{++} states by phase transitions. Based on our
results, we suggest a phase diagram for systems with weak repulsive interband
pairing, and discuss its relevance to iron-based superconductors.

Stacks of intrinsic Josephson junctions (IJJs) in high-temperature
superconductors enable the fabrication of compact sources of coherent terahertz
radiation. Here we demonstrate that multiple stacks patterned on the same
Bi2Sr2CaCu2O8+delta crystal can - under optimized conditions - be synchronized
to emit high-power THz-radiation. For three synchronized stacks we achieved 610
microwatts of continuous-wave coherent radiation power at 0.51 THz. We suggest
that synchronization is promoted by THz-waves in the base crystal. We note
though that synchronization cannot be achieved in all samples. However, even in
these cases powers on the 100-microwatt scale can be generated.

The powerful terahertz emission from intrinsic Josephson junctions in high-Tc
cuprate superconductors has been detected recently. The synchronization of
different junctions is enhanced by excitation of the geometrical cavity
resonance. A key characteristic of the radiation is its linewidth. In this
work, we study the intrinsic linewidth of the radiation near the internal
cavity resonance. Surprisingly, this problem was never considered before,
neither for a single Josephson junction nor for a stack of the intrinsic
Josephson junctions realized in cuprate superconductors. The linewidth appears
due to the slow phase diffusion, which is determined by the dissipation and
amplitude of the noise. We found that both these parameters are resonantly
enhanced when the cavity mode is excited but enhancement of the dissipation
dominates leading to the net suppression of diffusion and dramatic narrowing of
the linewidth. The line shape changes from Lorentzian to Gaussian when either
the Josephson frequency is shifted away from the resonance or the temperature
is increased.

Many superconducting materials are composed of weakly coupled conducting
layers. Such a layered structure has a very strong influence on the properties
of vortex matter in a magnetic field. This review focuses on the properties of
the Josephson vortex lattice generated by the magnetic field applied in the
layers direction. The theoretical description is based on the Lawrence-Doniach
model in the London limit which takes into account only the phase degree of
freedom of the superconducting order parameter. In spite of its simplicity,
this model leads to an amazingly rich set of phenomena. We review in details
the structure of an isolated vortex line as well as various properties of the
vortex lattice, both in dilute and dense limits. In particular, we present an
extensive consideration on the influence of the layered structure and thermal
fluctuations on the selection of lattice configurations at different magnetic
fields.

Stacks of intrinsic Josephson junctions (IJJs) made from high-temperature superconductors such as Bi2Sr2CaCu2O8+δ (Bi-2212) (BSCCO) are a promising source of coherent continuous-wave terahertz radiation. It is thought that at electrical bias conditions under which THz-emission occurs, hot spots may form due to resistive self-heating, and that these spots may be highly beneficial for the generation of high levels of THz power. Here, we perform an imaging study of the temperature distribution at the surface of BSCCO stacks utilizing the temperature-dependent 612 nm fluorescence line of Eu3+ in a europium chelate. The images directly reveal a highly non-uniform temperature distribution in which the temperature in the middle of the stack can exceed the superconducting transition temperature by tens of Kelvin under biasing conditions typical for THz-emission.

Stacks of intrinsic Josephson-junctions are realized in mesas fabricated
out of layered superconducting single crystals, such as
Bi2Sr2CaCu2O8 (BSCCO).
Synchronization of phase oscillations in different junctions can be
facilitated by the coupling to the internal cavity mode leading to
powerful and coherent electromagnetic radiation in the terahertz
frequency range. An important characteristic of this radiation is the
shape of the emission line. A finite line width appears due to different
noise sources leading to phase diffusion. We investigated the intrinsic
line shape caused by the thermal noise for a mesa fabricated on the top
of a BSCCO single crystal. In the ideal case of fully synchronized stack
the finite line width is coming from two main contributions, the
quasiparticle-current noise inside the mesa and the fluctuating
radiation in the base crystal. We compute both contributions and
conclude that for realistic mesa's parameters the second mechanism
typically dominates. The role of the cavity quality factor in the
emission line spectrum is clarified. Analytical results were verified by
numerical simulations. In real mesa structures part of the stack may not
be synchronized and chaotic dynamics of unsynchronized junctions may
determine the real line width.

Since its discovery, iron based superconductivity has garnered much
interest from the research community for its potential in both
application and fundamental science. One of the questions awaiting an
answer is the pairing symmetry of this new class of superconductors.
Recently, Koshelev and Stanev proposed a fingerprint of s+- symmetry in
the NIS tunneling spectrum where the iron based superconductor is
proximity-coupled to a thin s-wave superconductor[1]. We have prepared
oxygen doped iron telluride (FeTe:Ox) thin films, along with an in-situ
grown tunnel barrier and top electrode by Molecular Beam Epitaxy (MBE).
We have fabricated them into planar tunnel junction and will report the
temperature dependence of both tunneling and point contact spectra. [1].
A. E. Koshelev and V. Stanev, EPL (Europhysics Letters) 96 (2), 27014
(2011).

We study magnetic flux distributions in YBCO single crystals
remagnetized by magnetic fields of different orientations using the
magneto-optic indicator technique. Application of the perpendicular
field to the crystals cooled in the in-plane magnetic field, application
of the in-plane field to the crystals cooled in the normal magnetic
field, and remagnetization by magnetic field tilted to the sample
surface result in unusual quasiperiodic vortex structures. These
strongly inhomogeneous vortex patterns can be associated with the flux
cutting and strong anisotropy of the vortex kink motion depending on the
trapped flux and external field orientations. We discuss the effect of
resulting inhomogeneous current distributions on the current carrying
ability of the YBCO coated conductors. Work supported by the US DoE-BES
funded Energy Frontier Research Center and by Department of Energy,
Office of Science, Office of Basic Energy Sciences under Contract No.
DE-AC02-06CH11357.

We investigate the enhancement of vortex pinning by compound defects
that are composed of correlated and point defects in
Ba0.6K0.4Fe2As2 crystals
with Tc 37.5. Initial irradiation by high-energy heavy ions
to a dose matching field of B=21T increases vortex pinning via columnar
defects with no degradation of the superconducting transition
temperature. Subsequent proton irradiations further enhance the critical
current Jc(H) by suppressing the motion of vortex kinks between the
columnar defects. At a temperature of 5K, we find a critical current
density of 5.8 MA/cm^2 that is essentially magnetic field independent in
fields up to 7 T. This work supported by the Center for Emergent
Superconductivity, an Energy Frontier Research Center funded by the U.S.
D.O.E., Office of Science, Office of Basic Energy Sciences and by the
D.O.E, Office of Basic Energy Sciences, under Contract No.
DE-AC02-06CH11357. The operation of the ATLAS facility was supported by
the U.S. D.O.E., Office of Nuclear Physics, under Contract No.
DE-AC02-06CH11357. The work in China was supported by the NSF of China,
the MOST of China (2011CBA00102 and 2012CB821403) and PAPD.

Stacks of intrinsic Josephson junctions (IJJs) made from high-temperature
superconductors such as Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} (Bi-2212) are a
promising source of coherent continuous-wave terahertz radiation. It is thought
that at electrical bias conditions under which THz-emission occurs hot spots
may form due to resistive self-heating, and that these spots may be highly
beneficial for the generation of high levels of THz power. Here we perform an
imaging study of the temperature distribution at the surface of BSCCO stacks
utilizing the temperature-dependent 612nm fluorescence line of Eu^{3+} in a
europium chelate. The images directly reveal a highly non-uniform temperature
distribution in which the temperature in the middle of the stack can exceed the
superconducting transition temperature by tens of Kelvin under biasing
conditions typical for THz-emission.

The s± state in which the order parameter has different signs in different bands is a leading candidate for the superconducting state in the iron-based superconductors. We investigate a Josephson junction between s and s± superconductors within microscopic theory. Frustration, caused by interaction of the s-wave gap parameter with the opposite-sign gaps of the s± superconductor, leads to nontrivial phase diagram. When the partial Josephson coupling energy between the s-wave superconductor and one of the s± bands dominates, s-wave gap parameter aligns with the order parameter in this band. In this case, the partial Josephson energies have different signs corresponding to signs of the gap parameters. In the case of strong frustration, corresponding to almost complete compensation of the total Josephson energy, a nontrivial time-reversal-symmetry breaking (TRSB) state realizes. In this state, all gap parameters become essentially complex. As a consequence, this state provides realization for so-called ϕ-junction with finite phase difference in the ground state. The width of the TRSB state region is determined by the second harmonic in Josephson current, ∝sin(2ϕ), which appears in the second order with respect to the boundary transparency. Using the microscopic theory, we establish a range of parameters where different states are realized. Our analysis shows insufficiency of the simple phenomenological approach for treatment of this problem.

Using mesa array fabricated at the top of Bi2Sr2CaCu2O8 single crystal was demonstrated recently as a promising route to enhance the radiation power generated by the Josephson oscillations in mesas. We study the synchronization in such an array via the plasma waves in the base crystal. First, we analyze plasma oscillations inside the base crystal generated by the synchronized mesa array and the associated dissipation. We then solve the dynamic equation for the superconducting phase numerically to find conditions for synchronization and to check the stability of the synchronized state. We find that the mesas are synchronized when the cavity resonance of mesas matches with that of the base crystal. An optimal configuration of the mesa arrays is also obtained.

We study proximity effects close to a boundary between s and s+-
superconductors. Frustration, caused by interaction of the s-wave gap parameter
with the opposite-sign gaps of s+- superconductor, leads to several anomalous
features. In the case of strong frustration a nontrivial time-reversal-symmetry
breaking (TRSB) state, with nonzero phase angles between all gap parameters, is
possible. In a more typical state, the s-wave order parameter is aligned with
one of the s+- gaps. The other (anti-aligned) gap induces negative feature in
the s-wave density of states, which can serve as a fingerprint of s+- state.
Another consequence of the frustration is an extended region in the parameter
space in which s-wave superconductivity is suppressed, despite being in contact
with nominally stronger superconductor. This negative proximity effect is
always present for the TRSB state, but extends even into the aligned states. We
study these effects within a simple microscopic model assuming dirty limit in
all bands, which allows us to model the system in terms of minimum number of
the most relevant parameters. The described anomalous features provide a route
to establishing the possible s+- state in the iron-based superconductors

Each discovery of a new high temperature superconductor drives the expectation that advanced engineering of materials defect structures will enable effective vortex pinning and high values of the electrical current density. Here, we demonstrate that single crystals of the iron-based superconductor Ba0.6K0.4Fe2As2 with Tc = 37.5 K can accommodate an unprecedented large concentration of strong-pinning defects in the form of discontinuous nm-sized nanorods with no degradation of the superconducting transition temperature. At a temperature of 5 K, we find a critical current density of 5 MA/cm2 that is magnetic field independent in fields up to 7 T.

We explore correlations of inhomogeneous local density of states (LDoS) for
impure superconductors with different symmetries of the order parameter (s-wave
and d-wave) and different types of scatterers (elastic and magnetic
impurities). It turns out that the LDoS correlation function of superconductor
always slowly decreases with distance up to the phase-breaking length
$l_{\phi}$ and its long-range spatial behavior is determined only by the
dimensionality, as in normal metals. On the other hand, the energy dependence
of this correlation function is sensitive to symmetry of the order parameter
and nature of scatterers. Only in the simplest case of s-wave superconductor
with elastic scatterers the inhomogeneous LDoS is directly connected to the
corresponding characteristics of normal metal.

The proximity effect has been proposed as a mechanism to unambiguously
identify the possible s±-state in iron-based
superconductors.ootnotetextA. E. Koshelev, V. Stanev, Europhysics
Letters, Vol. 96, 27014 (2011) With a thin s-wave superconductor atop a
s±-superconductor it is suggested that the s-wave
order parameter will couple to the s±-gaps
differently, inducing a correction to the s-wave density of states that
can be probed using electron tunneling spectroscopy. In this talk, we
will present recent results of the superconducting proximity effect in
s-wave MoGe thin films sputtered on top of bulk superconducting
Ba0.6K0.4Fe2As2
(Tc=35K) pnictide. Electron tunneling spectroscopy
measurements were performed for several MoGe film thicknesses using a
homemade point contact setup. Finally, results will also be presented
for similar measurements using two conventional s-wave superconductors.

Stacks of intrinsic Josephson junctions (IJJs) in high-temperature
superconductors enable the fabrication of compact sources of coherent
THz-radiation. Here we demonstrate 150 microwatts of radiation power at
0.51 THz, using three synchronized stacks patterned on a single
Bi2Sr2CaCu2O8+δ
crystal. The emitted power scales roughly as the square of the number of
energized stacks, while the total power spectrum is monochromatic to
within observational limits. These results imply that the stacks radiate
coherently.

We present studies of the galvanomagnetic effects of compensated
BaFe2(As1-xPx)2
(x=0.32˜0.6) superconductors. The magnetoresistance follows the
relaxed Kohler's scaling for all doping levels. Using a two-band model,
we quantitatively extracted the scattering parameter m*/τ and the
carrier density of the electron and hole bands. The temperature
dependence of the carrier concentration reveals the semimetal properties
of BaFe2(As1-xPx)2. The
Fermi-liquid behavior, m*/τ˜T^2, is observed from optimal
doped x=0.32 to over-doped x=0.6 crystals, suggesting that the proximity
of the SDW state does not play an important role in transport. Our
analysis suggests that the normal state transport properties of
BaFe2(As1-xPx)2 can be well
understood in the framework of a compensated two-band Fermi-liquid
semimetal.

We report specific heat and magnetization measurements on
SmFeAsO0.8F0.15 and
BaFe2(As1-xPx)2 single
crystals irradiated with high energy heavy ions of 1.4GeV Pb to dose
matching fields up to 4 Tesla. We find a nearly one half reduction in
the superconducting anisotropy and doubling of the irreversibility field
in SmFeAsO0.8F0.15 after irradiation and virtually
no change in the zero-field superconducting transition temperature. In
both SmFeAsO0.8F0.15 and
BaFe2(As1-xPx)2 crystals, we
find a substantial increase in the critical current determined from
SQUID and micro-Hall probe magnetization measurements. Pinning force
analysis on proton and heavy ion irradiated pristine overdoped
BaFe2(As1-xPx)2 crystals
indicates presence of induced δTc-type pinning defects
in these samples.

Optimal doped crystals of
(Ba0.6K0.4)Fe2As2 were
irradiated with 1.4 GeV Pb ions to dose-matching fields ranging from 4
Tesla to 21 Tesla. Plan-view transmission electron microscopy shows
creation of defects with diameters of 2 ˜ 5 nm. Post-irradiation
characterization shows that the superconducting anisotropy is reduced to
near unity, probably due to the increase in intra-band scattering. In
addition, the critical current density JC determined from
magnetization measurements shows systematic enhancement up to ˜5
MA/cm^2 at T=5K. We show that the decay of the critical current with
magnetic field can be greatly mitigated with dense defects with
approximately 20nm spacing produced by a dose matching field irradiation
of 21T. Remarkably, the superconducting transition temperature remain
unchanged for all matching field irradiation, suggesting that inter-band
scattering due to non-magnetic impurity does not play a dominant role in
pair-breaking.

We report on the systematic evolution of vortex pinning behavior in isovalent doped single crystals of BaFe2(As1−xPx)2. Proceeding from optimal doped to overdoped samples, we find a clear transformation of the magnetization hysteresis from a fishtail behavior to a distinct peak effect, followed by a reversible magnetization and Bean-Livingston surface barriers. Strong point pinning dominates the vortex behavior at low fields whereas weak collective pinning determines the behavior at higher fields. In addition to doping effects, we show that particle irradiation by energetic protons can tune vortex pinning in these materials.

We suggest a straightforward and unambiguous test to identify possible opposite signs of the superconducting order parameter in different bands proposed for iron-based superconductors (s±-state). We consider the proximity effect in a weakly coupled sandwich composed of a
s±-superconductor and a thin layer of the s-wave superconductor. In such system the s-wave order parameter is coupled differently with different s±-gaps and it typically aligns with one of these gaps. This forces the other s±-gap to be anti-aligned with the s-wave gap. In such situation the aligned band induces a peak in the s-wave density of states (DoS), while the anti-aligned band induces a dip. Observation of such contact-induced negative feature in the s-wave DoS would provide a definite proof for s±-superconductivity.

We have measured coherent terahertz emission spectra from Bi${}_{2}$Sr${}_{2}$CaCu${}_{2}$O${}_{8+$\delta${}}$ mesa devices as a function of temperature and mesa bias voltage. The emission frequency is found to be tunable by up to 12% by varying the temperature and bias voltage. We attribute the appearance of tunability to asymmetric boundaries at the top and bottom and the nonrectangular cross section of the mesas. This interpretation is consistent with numerical simulations of the dynamics of intrinsic Josephson junctions in the mesa. Easily tunable emission frequency may have important implications for the design of terahertz devices based on stacked intrinsic Josephson junctions.

The pinning of vortex lines by an array of nanoparticles embedded inside
superconductors has become the most efficient practical way to achieve high
critical currents. In this situation pinning occurs via trapping of the
vortex-line segments and the critical current is determined by the typical
length of the trapped segments. To verify analytical estimates and develop a
quantitative description of strong pinning, we numerically simulated isolated
vortex lines driven through an array of nanoparticles. We found that the
critical force grows roughly as the square root of the pin density and it is
strongly suppressed by thermal noise. The configurations of pinned lines are
strongly anisotropic, displacements in the drive directions are much larger
than in the transverse direction. Moreover, we found that the roughening index
for the longitudinal displacements exceeds one. This indicates that the local
stresses in the critical region increase with the total line length and the
elastic description breaks down in the thermodynamic limit. Thermal noise
reduces the anisotropy of displacements in the critical regions and straightens
the lines.

We report on the systematic evolution of vortex pinning behavior in isovalent
doped single crystals of BaFe2(As1-xPx)2. Proceeding from optimal doped to
ovedoped samples, we find a clear transfor- mation of the magnetization
hysteresis from a fishtail behavior to a distinct peak effect followed by a
reversible magnetization and Bean Livingston surface barriers. Strong point
pinning dominates the vortex behavior at low fields whereas weak collective
pinning determines the behavior at higher fields. In addition to doping
effects, we show that particle irradiation by energetic protons can tune vortex
pinning in these materials.

In strongly anisotropic layered superconductors in tilted magnetic fields the
Josephson vortex lattice coexists with the lattice of pancake vortices. Due to
the interaction between them, the dissipation of the Josephson-vortex lattice
occurs to be very sensitive to the presence of the pancake vortices. If the
c-axis magnetic field is smaller then the corresponding lower critical field,
the pancake stacks are not formed but the individual pancakes may exist in the
fluctuational regime either near surface in large-size samples or in the
central region for small-size mesas. We calculate the contribution of such
fluctuating pancake vortices to the c-axis conductivity of the Josephson vortex
lattice and compare the theoretical results with measurements on small mesas
fabricated out of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ crystals. A
fingerprint of fluctuating pancakes is characteristic exponential dependence of
the c-axis conductivity observed experimentally. Our results provide strong
evidence of the existence of the fluctuating pancakes and their influence on
the Josephson-vortex-lattice dissipation.

We report on the specific-heat determination of the anisotropic phase diagram of single crystals of optimally doped SmFeAsO1-xFx. In zero field, we find a clear cusplike anomaly in C/T with ΔC/Tc=24 mJ/mol K2 at Tc=49.5 K. In magnetic fields along the c axis, pronounced superconducting fluctuations induce broadening and suppression of the specific-heat anomaly which can be described using three-dimensional lowest-Landau-level scaling with an upper critical field slope of –3.5 T/K and an anisotropy of Γ =8. The small value of ΔC/Tc yields a Sommerfeld coefficient γ ∼ 8 mJ/mol K2, indicating that SmFeAsO1-xFx is characterized by a modest density of states and strong coupling.

We report specific heat measurements on a series of BaFe2(As1-xPx)2 single crystals with phosphorous contents ranging from optimal doping (x˜0.3, Tc = 29.5 K) to highly overdoped (x˜0.6, Tc = 11K). We find a sharp superconducting transition at Tc for all doping levels, a suppression of the deltaC-step at Tc with increasing doping and enhanced magnetic field dependence at higher doping. The phase diagrams determined from specific heat data show a decrease of dHc2/dT with increasing doping and a nearly constant superconducting anisotropy of gamma˜2.5. Our results will be compared with the proposed "universal" scaling of deltaCp/Tc and dHc2/dT due to quantum criticality and non Fermi liquid behavior [1] and due to strong pair-breaking and non-magnetic interband scattering [2], respectively. [4pt] [1] J. Zaanen, Phys. Rev. B 80, 212502 (2009) [0pt] [2] V. G. Kogan, Phys. Rev. B 80, 214532 (2009)

Pinning of vortex lines by array of nanoparticles embedded inside superconductors became the most efficient practical way to achieve high critical currents. In this situation pinning occurs via trapping of the vortex-line segments and the critical current is determined by the typical length of trapped segment. To verify analytical estimates and develop a quantitative description of strong pinning, we use large-scale numerical simulations. We study the dependence of the critical force on the density of pins in the regime of independently pinned lines, statistical properties of trapped lines, and suppression of the apparent critical force by thermal fluctuations.

The nature of the superconducting order parameter (OP) in iron pnictides and chalcogenides is a hotly debated issue. It was theoretically proposed that the OP has opposite signs on the hole and the electron bands, i.e., it belongs to the unconventional class of s± (or extended s)-wave. There are, however, very few experiments that can directly distinguish this state from the ordinary s-wave OP. One way to address this problem is to study the proximity effects in a sandwich composed of conventional and iron pnictide superconductors (SC). If the pnictides indeed have the s± OP this system is intrinsically frustrated. In the case of strong frustration, a time-reversal symmetry-breaking (TRSB) SC state emerges, in which the OP phases in different bands are tilted at an angle, different from pi, and controlled by the coupling strength. Observation of such state in the iron-based SC materials would give definite evidence for the s± OP. We present a microscopic, fully self-consistent approach to this problem, based on Usadel equations. We have studied the conditions for existence of the TRSB state and its experimental signatures.

We report on magnetization measurements on doped single crystals of superconducting BaFe2(As1-xPx)2 . For optimum doped crystals, we observe a second magnetization peak effect (fish tail). With further doping of phosphur for arsenic, the fish tail effect evolves into a peak effect close to Hc2, similar to that found in conventional type II superconductors. In heavily overdoped crystals, the magnetization loop is mostly reversible and no peak effect is observed. The evolution of the peak effect with doping is attributed to the reduction in defects as the crystal's purity is increased, going from optimum doping to over-doping. Possible pinning mechanism for the peak effect will be discussed within the framework of recent heat capacity and resistivity measurements.

The so-called "terahertz gap," covering frequencies from approximately
0.3 to 1.5 THz, is of particular interest for a number of scientific and
security applications, although no bright sources of coherent radiation
presently exist in this range. However, stacks of high-temperature
superconducting intrinsic Josephson junctions are a promising candidate.
[1] Here we discuss recent progress in improving the performance of
these devices. In particular, we demonstrate that via control of bias
voltage and operating temperature, the emission from an 80-μm wide
Bi2Sr2CaCu2O8 mesa can be
tuned continuously over a frequency range in excess of 10% in the
vicinity of 0.5 THz. We find that as the emission frequency increases
from 0.420 to 0.492 THz, the linewidth increases from <2.25 GHz
(limited by instrument resolution) to ˜9 GHz. [4pt] [1] L. Ozyuzer
et al., Science 318 (2007) 1291-1293.

We have measured coherent terahertz emission spectra from Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} mesa devices as a function of temperature and mesa bias voltage. The emission frequency is found to be tunable by up to 12% by varying the temperature and bias voltage. We attribute the appearance of tunability to asymmetric boundaries at the top and bottom and the nonrectangular cross section of the mesas. This interpretation is consistent with numerical simulations of the dynamics of intrinsic Josephson junctions in the mesa. Easily tunable emission frequency may have important implications for the design of terahertz devices based on stacked intrinsic Josephson junctions.

The phase diagram of vortex matter in the layered superconductor Bi2Sr2CaCu2O8−δ exposed to a magnetic field oblique to the crystalline c-axis contains two first order transition (FOT) lines [1]. The first, HFOTm, separates the vortex solid from the vortex liquid, the second, HFOTct, separates the combined lattice state in the vortex solid from a tilted lattice state. The angular dependence of HFOTm in the tilted lattice region follows the anisotropic Ginzburg–Landau model [2], allowing for the determination of the anisotropy factor γeff and the contribution of magnetic coupling to the mutual interaction of “pancake” vortices in the crossed lattice limit. The later parameter is directly related to the in-plane penetration depth λab. We investigate the evolution of the phase diagram of Bi2Sr2CaCu2O8−δ in oblique fields with point-like disorder, introduced by irradiation with 2.3MeV electrons. Apart from the depression of Tc, point-like disorder induces an increase of γeff and a depression of the superfluid density.

Stacks of intrinsic Josephson junctions in the resistive state can by
efficiently synchronized by the internal cavity mode resonantly excited by the
Josephson oscillations. We study the stability of dynamic coherent states near
the resonance with respect to small perturbations. Three states are considered:
the homogeneous and alternating-kink states in zero magnetic field and the
homogeneous state in the magnetic field near the value corresponding to half
flux quantum per junction. We found two possible instabilities related to the
short-scale and long-scale perturbations. The homogeneous state in modulated
junction is typically unstable with respect to the short-scale alternating
phase deformations unless the Josephson current is completely suppressed in one
half of the stack. The kink state is stable with respect to such deformations
and homogeneous state in the magnetic field is only stable within a certain
range of frequencies and fields. Stability with respect to the long-range
deformations is controlled by resonance excitations of fast modes at finite
wave vectors and typically leads to unstable range of the wave-vectors. This
range shrinks with approaching the resonance and increasing the in-plane
dissipation. As a consequence, in finite-height stacks the stability frequency
range near the resonance increases with decreasing the height.

We evaluate accurate low-field/low-temperature asymptotics of the thermal
conductivity perpendicular to magnetic field for one-band and two-band s-wave
superconductors using Keldysh-Usadel formalism. We show that heat transport in
this regime is limited by tunneling of quasiparticles between adjacent vortices
across a number of local points and therefore widely-used approximation of
averaging over circular unit cell is not valid. In the single-band case, we
obtain parameter-free analytical solution which provides theoretical lower
limit for heat transport in the mixed state. In the two-band case, we show that
heat transport is controlled by the ratio of gaps and diffusion constants in
different bands. Presence of a weaker second band strongly enhances the thermal
conductivity at low fields

We show that shunt capacitor stabilizes synchronized oscillations in
intrinsic Josephson junction stacks biased by DC current. This synchronization
mechanism has an effect similar to the previously discussed radiative coupling
between junctions, however, it is not defined by the geometry of the stack. It
is particularly important in crystals with smaller number of junctions, where
radiation coupling is week, and is comparable with the effect of strong
super-radiation in crystal with many junctions. The shunt also helps to enter
the phase-locked regime in the beginning of oscillations, after switching on
the bias current. Shunt may be used to tune radiation power, which drops as
shunt capacitance increases.

We present a study of the changes of Tc and of the upper critical field of Ba1-xKxFe2As2 that are induced by irradiation with 9 MeV protons and 1.4 GeV Pb-ions. Irradiation to a fluence of 2x10^15 protons/cm^2 creates sparse individual point defects and their clusters. These are sufficient to increase vortex pinning, but do not alter the phase diagram in a noticeable way. In contrast, heavy-ion irradiation to a dose matching field of 2 T induces a suppression of Tc by ˜1K and a reduction of the jump of the specific heat, deltaC, at Tc by ˜50 %. Furthermore, the upper critical field slopes increase to enormously high values of dHc2^c/dT=-12.8 T/K and dHc2^ab/dT=-21 T/K, corresponding to a low anisotropy of gamma ˜ 1.6. Such behavior is not expected for a d-wave superconductor, but is consistent with pair-breaking by non-magnetic scattering centers in a superconductor with s± gap symmetry.

We report on magnetization measurements on Ba0.6K0.4 Fe2As2 single crystals irradiated with 6 MeV protons followed by 1.4 GeV Pb ions to a dose matching field of 2.0 Tesla. We see a systematic increase of the critical current at all temperatures and fields with Jc increasing nearly a factor of ten at 20 K. In addition, we report on magnetization measurements on Ba0.6K0.4 Fe2As2 single crystals irradiated with 1.4GeV Pb ions to dose matching fields of 0.1T and 1.0T. Here, we see a systematic increase of both the irreversibility line and the critical current with increasing irradiation dose. Our results show that both proton and heavy ions are good candidates for increasing vortex pinning in these materials.

There is a strong theoretical reasoning in favor of the s± superconducting state in new iron-based superconductors. The order parameter in such a state has different signs in the electron and hole Fermi pockets. In this situation scattering between different pockets by impurities has pair-breaking effect and introduces states inside the gap which strongly influence low-temperature behavior of superconducting parameters. We solve numerically the two-band Bogolyubov equations for the s± superconductor and explore behavior of density of states and localization properties at different scattering parameters and concentration of impurities. We found that the commonly used self-consistent T-matrix approximation is incomplete and not very accurate in describing subgap states.

There is strong support in favor of an unusual $s_{\pm}$ superconducting
state in the new iron-based superconductors, in which the gap parameter has
opposite signs in different bands. In this case scattering between different
bands by impurities has a pair-breaking effect and introduces states inside the
gap. We studied the statistics of disorder-induced subgap states in $s_{\pm}$
superconductors due to collective effects of impurities. Numerically solving
the two-band Bogolyubov equations, we explored the behavior of the density of
states and localization length. We located the mobility edge separating the
localized and delocalized states for the 3D case and the crossover between the
weak and strong localization regimes for the 2D case. We found that the widely
used self-consistent T-matrix approximation is not very accurate in describing
subgap states.

A compact, solid-state THz source based on the driven Josephson vortex lattice in a highly anisotropic superconductor such as Bi.sub.2Sr.sub.2CaCu.sub.2O.sub.8 that allows cw emission at tunable frequency. A second order metallic Bragg grating is used to achieve impedance matching and to induce surface emission of THz-radiation from a Bi.sub.2Sr.sub.2CaCu.sub.2O.sub.8 sample. Steering of the emitted THz beam is accomplished by tuning the Josephson vortex spacing around the grating period using a superimposed magnetic control field.

Recently, we experimentally demonstrated that rectangular mesa structures of intrinsic Josephson junctions (IJJ) in Bi2Sr2CaCu2O8+d (Bi2212) can be used as a compact solid-state generator of continuous, coherent and polarized terahertz (THz) radiation. In the present work, we will exhibit tall mesas (over 600 junctions) which were fabricated using UV lithography, e-beam lithography with photoresist and e-beam lithography with a Ti selective etching technique. We will present measurements of the c-axis resistance as a function of temperature and of current–voltage characteristics of THz emitting mesas with lateral sizes ranging from 30 × 300 to 100 × 300 µm2. Furthermore, we will discuss the dependence of the characteristics of the mesa structures on the oxygen doping level of the Bi2212 crystals. We will also experimentally show that the voltage–frequency relation of the ac Josephson effect has to match the cavity resonance for successful emission.

Josephson vortices in naturally stacked Bi-2212 tunneling junctions display rich dynamic behavior that derives from the coexistence of three basic states: static Josephson vortex lattice, coherently moving lattice, and incoherent quasiparticle tunneling state. Rich structure of hysteretic branches observed in the current-voltage characteristics can be understood as combinatorial combinations of these three states which are realized in different junctions and evolve separately with magnetic field and bias current. In particular, the multiple Josephson-vortex-flow branches at low bias currents arise from the individual depinning of Josephson vortex rows in each junction. Comment: Submitted to Europhysics Letters

Josephson vortices in naturally stacked Bi-2212 tunneling junctions display
rich dynamic behavior that derives from the coexistence of three basic states:
static Josephson vortex lattice, coherently moving lattice, and incoherent
quasiparticle tunneling state. Rich structure of hysteretic branches observed
in the current-voltage characteristics can be understood as combinatorial
combinations of these three states which are realized in different junctions
and evolve separately with magnetic field and bias current. In particular, the
multiple Josephson-vortex-flow branches at low bias currents arise from the
individual depinning of Josephson vortex rows in each junction.

By patterning mesoscopic crystals of Bi2Sr2CaCu2O8 (BSCCO) into
electromagnetic resonators the oscillations of a large number of intrinsic
Josephson junctions can be synchronized into a macroscopic coherent state
accompanied by the emission of strong continuous wave THz-radiation. The
temperature dependence of the emission is governed by the interplay of
self-heating in the resonator and by re-trapping of intrinsic Josephson
junctions which can yield a strongly nonmonotonic temperature dependence of the
emission power. Furthermore, proper shaping of the resonators yields THzsources
with voltage-tunable emission frequencies.

We present a thermal analysis of a patterned mesa on a Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8</sub> (Bi2212) single crystal that is based on tunneling characteristics of the c-axis stack of ~800 intrinsic Josephson junctions in the mesa. Despite the large mesa volume (e.g., 40 times 300 times 1.2 mum<sup>3</sup>) and power dissipation that result in self-heating and backbending of the current-voltage curve (I-V), there are accessible bias conditions for which significant polarized THz-wave emission can be observed. We estimate the mesa temperature by equating the quasiparticle resistance, R<sub>qp</sub>(T), to the ratio V/I over the entire I-V including the backbending region. These temperatures are used to predict the unpolarized black-body radiation reaching our bolometer and there is substantial agreement over the entire I-V. As such, backbending results from the particular R<sub>qp</sub>(T) for Bi2212, as first discussed by Fenton, rather than a significant suppression of the energy gap. This model also correctly predicts the observed disappearance of backbending above ~60 K.

We present specific heat measurements on single crystals of the pnictide superconductors NdFeAsO1−xFx and Ba1−xKxFe2As2. Low-temperature measurements on Ba1−xKxFe2As2 reveal that the Sommerfeld coefficient of the quasiparticle specific heat increases linearly with applied magnetic field. This is the signature of a fully gapped superconducting state as is expected for instance in the extended s-wave scenario. The large value of the normal state Sommerfeld coefficient indicates a large electronic density of states and significant mass enhancements. We determine the phase diagram using an entropy conserving construction, which does not require the choice of a resistivity criterion to define the transition temperature. Both materials display clear mean-field steps in the specific heat at Tc of 47K and 34.6K, respectively, proving bulk superconductivity. They are characterized by a low superconducting anisotropy near Tc of Γ∼4 for NdFeAsO1−xFx and 2.6 for Ba1−xKxFe2As2, which is promising for potential applications. We observe extraordinarily high upper critical field slopes of μ0∂Hc2c/∂T=-6.5T/K and μ0∂Hc2ab/∂T=-17.4T/K for Ba1−xKxFe2As2 which points to the emergence of paramagnetic limiting effects at low temperatures. A thermodynamic analysis reveals that this material is extreme type-II with κc∼100 and κab∼260.

Intrinsic Josephson-junction stacks realized in high-temperature superconductors may generate powerful electromagnetic radiation in terahertz frequency range. A major challenge is to synchronize phase oscillations in many junctions. A promising way of efficient synchronization is to excite an internal cavity mode, with the frequency set by the stack lateral size. We discuss several issues relevant for this mechanism: (i) damping of the resonance mode due to radiation into free space and into the base crystal, (ii) mechanisms of coupling to the internal mode, (iii) structures and stability of coherent states.

We investigate the effect of point-like disorder, introduced by irradiation with 2.3 MeV electrons, on the mixed state phase diagram of Bi2Sr2CaCu2O8 single crystals. We focus on the higher irradiation doses that produce a significant depression of the critical temperature Tc, to as low as 2/3 of the initial value. Surprisingly, the first order phase transition (FOT) of the vortex ensemble, from a crystal to the pancake vortex liquid, persists in those highly disordered samples. The second peak in the irreversible magnetization, observed at low temperatures, is equally observed after high irradiation doses, but at much lower magnetic fields. A simple scaling of the phase diagram for samples with various degrees of disorder is not possible, indicating that several fundamental parameters of the superconductor are affected. From the analysis of the angular dependence of the FOT, we deduce that the effective anisotropy factor increases after irradiation.

Cobalt and Manganese intercalated NbSe2 single crystals have been synthesized and characterized by DC magnetization and scanning tunnelling microscopy (STM) at low temperatures. We observed a pronounced peak effect in magnetization for both Co and Mn intercalated samples that we further investigated by low temperature STM. A structural phase transition of the vortex lattice (VL) has been observed for applied magnetic fields corresponding to the peak in magnetization.

We present a thermodynamic study of the phase diagram of single-crystal Ba{sub 1-x}K{sub x}Fe{sub 2}As{sub 2} using specific-heat measurements. In zero-magnetic field a clear step in the heat capacity of {Delta}C/T{sub c} = 0.1 J/mol K{sup 2} is observed at T{sub c} {approx} 34.6 K for a sample with x = 0.4. This material is characterized by extraordinarily high slopes of the upper critical field of {mu}{sub 0}{partial_derivative}H{sub c2}{sup e}/{partial_derivative}T = -6.5 T/K and {mu}{sub 0}{partial_derivative}H{sub c2}{sup ab}/{partial_derivative}T = -17.4 T/K and a surprisingly low anisotropy of {Lambda} {approx} 2.6 near T{sub c}. A consequence of the large field scale is the effective suppression of superconducting fluctuations. Using thermodynamic relations we determine Ginzburg-Landau parameters of {kappa}{sub c} {approx} 100 and {kappa}{sub ab} {approx} 260 identifying Ba{sub 1-x}K{sub x}Fe{sub 2}As{sub 2} as extreme type II. The large value of the normalized discontinuity of the slopes of the specific heat at T{sub c}, (T{sub c}/{Delta}C){Delta}(dC/dT){sub T{sub c}}, {approx}6, indicates strong-coupling effects in Ba{sub 1-x}K{sub x}Fe{sub 2}As{sub 2}.

We have recently succeeded in extracting coherent cw THz-radiation from intrinsic Josephson junctions in BSCCO [Science 318, 1291, (2007)]. An electromagnetic cavity resonance inside the sample generates a coherent state in which a large number of junctions are synchronized to oscillate in phase resulting in emission powers of up to 5 muW at frequencies up to 0.85 THz. The emission displays a non-monotonic temperature dependence with a sample dependent sharp maximum in the range of 25 to 45 K which we attribute to the interplay of self-heating effects and re-trapping of intrinsic junctions. Application of magnetic fields of less than 100 Oe parallel to the CuO2 planes as well as perpendicular leads to the rapid suppression of the emission.

We present anisotropic heat capacity measurements of the upper critical field of Ba0.6K0.4Fe2As2 single crystals in fields up to 8 Tesla. In zero-magnetic field a clear step in the heat capacity is observed at Tc˜36K. Using an entropy conserving construction we determined the transition temperatures in applied fields and the upper critical field slopes dHc2 ||c/dT = -6.5 T/K and dHc2 ||ab/dT -17.4 T/K, the latter showing record high critical field slope near Tc. The temperature dependence of the specific heat of Ba0.6K0.4Fe2As2 indicates strong coupling effects. Based on the experimental values of the upper critical field slopes, we determined the Ginzburg parameter, coherence and penetration lengths, anisotropy and thermodynamic critical fields. We also present magnetization measurements and discuss their implications on the nature of critical currents in this material.

Intrinsic Josephson-junction stacks are realized in mesas fabricated out of high-temperature superconductors. Phase oscillations in different junctions can be synchronized via coupling to the intrinsic cavity mode leading to powerful electromagnetic radiation in terahertz frequency range [1,2]. As homogeneous oscillations do not couple directly to the cavity modes, the mechanism of mode excitations is a nontrivial issue. New inhomogeneous dynamic state providing such coupling has been demonstrated recently [3]. In this state, the stack spontaneously splits into two subsystems with different phase-oscillation patterns. The phase shift between the oscillations in the two subsystems is static and varies from 0 to 2pi in a narrow region near the stack center (phase kink). The oscillating electric and magnetic fields are almost homogeneous in all the junctions. The formation of this state promotes efficient pumping of the energy into the cavity resonance. We will also discuss (i) stability of coherent states (ii) synchronization in inhomogeneous mesas, and (iii) mechanisms of damping of the resonance mode.[1]L. Ozyuzer et al. , Science 318, 1291 (2007). [2]A. E. Koshelev and L. N. Bulaevskii, Phys. Rev. B 77, 014530 (2008). [3]Sh. Lin and X. Hu Phys.Rev. Lett., 100, 247006 (2008); A. E. Koshelev, Phys. Rev., B 78, 174509 (2008).*In collaboration with L. Bulaevskii (LANL), U. Welp, C. Kurter, K. Gray (MSD, ANL), L. Ozyuzer (Izmir Institute of Technology, Turkey), K. Kadowaki (Tsukuba University, Japan)

We investigated confinement effects on the resistive anisotropy of a superconducting niobium strip with a rectangular cross-section. When the strip's transverse dimensions are comparable to the superconducting coherence length, we find the angle dependent magentoresistances at a fixed temperature can be scaled as R(theta, H) = R(H /Hctheta) where Hctheta = Hc0 (cos^2theta+gamma-2sin^2theta)-1/2 is the angular dependent critical field, gamma = w/d is the width to thickness ratio of the strip, and Hc0 is the out-plane critical field at theta = 0 . Our results can be understood in terms of the anisotropic diamagnetic energy of a one-dimensional superconductor in a magnetic field.

Single crystals of layered high-temperature superconductors intrinsically behave as stacks of Josephson junctions. We analyze response of current-biased stack of intrinsic junctions to irradiation by the external electromagnetic (em) wave. In addition to well-known Shapiro steps in the current-voltage characteristics, irradiation promotes stimulated radiation which adds with spontaneous Josephson radiation from the crystal. Such enhancement of radiation from current-biased crystal may be used for amplification of em waves. Irradiation also facilitates synchronization of Josephson oscillations in all intrinsic Josephson junctions of a single crystal as well as oscillations in intrinsic junctions of different crystals.

We present a thermodynamic study of the phase diagram of single-crystal Ba1-xKxFe2As2 using specific heat measurements. In zero-magnetic field a clear step in the heat capacity of deltaC/Tc = 0.1 J/f.u.K2 is observed at Tc = 34.6K for a sample with x = 0.4. This material is characterized by extraordinarily high slopes of the upper critical field of dHc2,c/dT= -6.5 T/K and dHc2,ab/dT= -17.4 T/K and a surprisingly low anisotropy of gamma ~ 2.6 near Tc. A consequence of the large field scale is the effective suppression of superconducting fluctuations. Using thermodynamic relations we determine Ginzburg-Landau parameters of kappac ~ 100 and kappaab ~ 260 identifying Ba1-xKxFe2As2 as extreme type-II. The large value of the normalized discontinuity of the slopes of the specific heat at Tc, (Tc/deltaC)times delta(dC/dT)~ 6 indicates strong coupling effects in Ba1-xKxFe2As2.

We report a pronounced peak effect in the magnetization of CoxNbSe2 single crystals with critical temperatures Tc ranging between 7.1 and 5.0 K, and MnxNbSe2 single crystals with critical temperatures down to 3.4 K. We correlate the peak effect in magnetization with the structure of the vortex lattice across the peak-effect region using scanning-tunneling microscopy. Magnetization measurements show that the amplitude of the peak effect in the case of CoxNbSe2 exhibits a nonmonotonic behavior as a function of the Co content, reaching a maximum for concentration of Co of about 0.4at.% (corresponding to a Tc of 5.7 K) and after that gradually decreasing in amplitude with the increase in the Co content. The normalized value of the peak position Hp/Hc2 has weak dependence on Co concentration. In the case of MnxNbSe2 the features of the peak effect as a function of the Mn content are different and they can be understood in terms of strong pinning.

We present heat-capacity measurements of the upper critical fields of single-crystal NdFeAsO{sub 1?x}F. In zero-magnetic field a clear step in the heat capacity is observed at T{sub c} 47 K. In fields applied perpendicular to the FeAs layers the step broadens significantly whereas for the in-plane orientation the field effects are small. This behavior is reminiscent of the CuO-high-T{sub c} superconductors and is a manifestation of pronounced fluctuation effects. Using an entropy conserving construction we determine the transition temperatures in applied fields and the upper critical-field slopes of H{sup c}c2/T = -0.72 T/K and H{sup ab}c2/T = -3.1 T/K. Zero-temperature coherence lengths of {sub ab} 3.7 nm and {sub c} 0.9 nm and a modest superconducting anisotropy of 4 can be deduced in a single-band model.

We investigated confinement effects on the resistive anisotropy of a superconducting niobium strip with a rectangular cross section. When its transverse dimensions are comparable to the superconducting coherence length, the angle dependent magnetoresistances at a fixed temperature can be scaled as R(theta,H) = R(H/Hctheta) where Hctheta =Hc0(cos2theta + gamma(-2)sin2theta)(-1/2) is the angular dependent critical field, gamma is the width to thickness ratio, and Hc0 is the critical field in the thickness direction at theta=0 degrees . The results can be understood in terms of the anisotropic diamagnetic energy for a given field in a one-dimensional superconductor.

Intrinsic Josephson-junction stacks realized in high-temperature
superconductors provide a very attractive base for developing coherent sources
of electromagnetic radiation in the terahertz frequency range. A promising way
to synchronize phase oscillations in all the junctions is to excite an internal
cavity resonance. We demonstrate that this resonance promotes the formation of
an alternating coherent state, in which the system spontaneously splits into
two subsystems with different phase-oscillation patterns. There is a static
phase shift between the oscillations in the two subsystems which changes from 0
to $2\pi$ in a narrow region near the stack center. The oscillating electric
and magnetic fields are almost homogeneous in all the junctions. The formation
of this state promotes efficient pumping of the energy into the cavity
resonance leading to strong resonance features in the current-voltage
dependence.

We have observed intense, coherent, continuous and monochromatic electromagnetic (EM) emission at terahertz frequencies generated from a single crystalline mesa structure of the high-Tc superconductor Bi2Sr2CaCu2O8+δ intrinsic Josephson junction system. The mesa is fabricated by the Argon-ion-milling and photolithography techniques on the cleaved surface of Bi2Sr2CaCu2O8+δ single crystal. The frequency, ν, of the EM radiation observed from the sample obeys simple relations: ν = c/nλ = c/2nw and ν = 2eV/hN, where c is the light velocity in vacuum, n the refractive index of a superconductor, λ the wave length of the EM emission in vacuum, w the shorter width of the mesa, V the voltage applied to the mesa, N the number of layers of intrinsic Josephson junctions, e and h are the elementary charge and the Planck constant, respectively. These two relations strongly imply that the mechanism of the emission is, firstly, due to the geometrical resonance of EM waves to the mesa like a cavity resonance occuring in the mesa structure, and forming standing waves as cavity resonance modes, and secondly, due to the ac-Josephson effect, which works coherently in all intrinsic Josephson junctions. The peculiar temperature dependence of the power intensity emitted form samples shows a broad maximum in a temperature region between 20 and 40 K, suggesting that the nonequilibrium effect plays an essential role for the emission of EM waves in this system. The estimated total power is significantly improved in comparison with the previous report [L. Ozyuzer et al., Science 318 (2007) 1291, K. Kadowaki, et al., Physica C 437–438 (2006) 111, I.E. Batov, et al., Appl. Phys. Lett. 88 (2006) 262504], and reached as high as 5 μW from single mesa with w = 60 μm at 648 GHz, which enables us to use it for some of applications. So far, we succeeded in fabricating the mesa emitting EM waves up to 960 GHz in the fundamental mode in the w = 40 μm mesa, whereas the higher harmonics up to the 4-th order were observed, resulting in a frequency exceeding 2.5 THz. In sharp contrast to the previous reports [K. Kadowaki, et al., Physica C 437–438 (2006) 111 , M.-H. Bae, et al., Phys. Rev. Lett. 98, (2007) 027002], all the present measurements were done in zero magnetic field. Lastly, a plausible theoretical model for the mechanism of emission is discussed.

Intrinsic Josephson-junction stacks realized in mesas fabricated out of high-Tc superconductors may be used as sources of coherent electromagnetic radiation. The major challenge is to synchronize Josephson oscillations in all junctions to get significant radiation. A simple way to solve this problem is to excite the in-phase Fiske mode when the Josephson frequency matches the Fiske-resonance frequency set by the stack lateral size[1]. A finite direct coupling to such mode exists in mesas with lateral modulation of the Josephson critical current identical in all junctions [2]. The powerful almost standing electromagnetic wave is excited inside the crystal in the resonance promoting full synchronization. We evaluate behavior of the I-V characteristics and radiated power near the resonance. We will discuss several relevant issues including (i) stability of the coherent state, (ii) mechanism of damping including external radiation and leaking of radiation into the bulk crystal, and (iii) angular dependence of external radiation. [1]L. Ozyuzer, et al. Science 318, 1291 (2007) [2]A. E. Koshelev and L. N. Bulaevskii, cond-mat 0708.3269

Compact solid-state sources of terahertz (THz) radiation are being sought for sensing, imaging, and spectroscopy applications
across the physical and biological sciences. We demonstrate that coherent continuous-wave THz radiation of sizable power can
be extracted from intrinsic Josephson junctions in the layered high-temperature superconductor Bi2Sr2CaCu2O8. In analogy to a laser cavity, the excitation of an electromagnetic cavity resonance inside the sample generates a macroscopic
coherent state in which a large number of junctions are synchronized to oscillate in phase. The emission power is found to
increase as the square of the number of junctions reaching values of 0.5 microwatt at frequencies up to 0.85 THz, and persists
up to ∼50 kelvin. These results should stimulate the development of superconducting compact sources of THz radiation.

Paraconductivity of MgB2 has been measured in homogeneous thin films grown by Hybrid physical chemical vapor deposition. In order to reduce the possible effects of spatial inhomogeneities, stripes of different widths were cut on the films. We measured several samples with different resistivity values; after subtracting the normal state resistivity, paraconductivity appears to be of the same order of magnitude in all the samples. The dependence on the reduced temperature ε = ln(T/Tc) is discussed and compared with the existing models.

We derive the power of direct radiation into free space induced by Josephson oscillations in intrinsic Josephson junctions of layered superconductors. We consider the superradiation regime for a crystal cut in the form of a thin slice parallel to the c axis. We find that the radiation correction to the current-voltage characteristic in this regime depends only on crystal shape. We show that at a large number of junctions oscillations are synchronized providing high radiation power and efficiency in the terahertz frequency range. We discuss the crystal parameters and bias current optimal for radiation power and crystal cooling.