[Show abstract][Hide abstract] ABSTRACT: We begin with a brief description of the role of the Nernst-Ettingshausen effect in the studies of the high-temperature superconductors and Dirac
materials such as graphene. The theoretical analysis of the NE effect is involved because the standard Kubo formalism has to be modified by the presence of magnetization currents in order to satisfy the third law of thermodynamics. A new generation of the low-buckled Dirac
materials is expected to have a strong spin Nernst effect that represents the spintronics analog of the NE effect. These Dirac
materials can be considered as made of two independent electron subsystems of the two-component gapped Dirac fermions. For each subsystem the gap breaks a time-reversal symmetry and thus plays a role of an effective magnetic field. We explicitly demonstrate how the correct thermoelectric coefficient emerges both by the explicit calculation of the magnetization and by a formal cancelation in the modified Kubo formula. We conclude by showing that the nontrivial dependences of the spin Nersnt signal on the carrier concentration and electric field applied are expected in silicene and other low-buckled Dirac
materials.
Low Temperature Physics 05/2015; 41(5):342-352. DOI:10.1063/1.4919372 · 0.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recent experiments show that the Nernst-Ettingshausen effect is orders of
magnitude stronger than the thermoelectric Seebeck effect in superconductors
above the critical temperature. We explain different magnitudes of the two
effects accounting for the magnetization current of virtual Cooper pairs. The
method allows for detailed understanding of the surprising non-monotonic
dependence of the Nernst-Ettingshausen coefficient on the magnetic field.
Physical Review B 04/2015; 92(2). DOI:10.1103/PhysRevB.92.020514 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Thermoelectric energy conversion is a direct but low-efficiency process, which precludes the devel-opment of long-awaited wide-scale applications. As a breakthrough permitting a drastic performance increase is seemingly out of reach, we fully reconsider the problem of thermoelectric coupling en-hancement. The corner stone of our approach is the observation that heat engines are particularly efficient when their operation involves a phase transition of their working fluid. We derive and compute the thermoelastic coefficients of various systems, including Bose and Fermi gases, and fluc-tuation Cooper pairs. Combination of these coefficients yields the definition of the thermodynamic figure of merit, the divergence of which at finite temperature indicates that conditions are fulfilled for the best possible use of the thermoelectric working fluid. Here, this situation occurs in the fluc-tuation regime only, as a consequence of the increased compressibility of the working fluid near its phase transition. Our results and analysis clearly show that efforts in the field of thermoelectricity can now be productively directed towards systems where electronic phase transitions are possible.
Physical Review B 12/2014; 91(10). DOI:10.1103/PhysRevB.91.100501 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Electron tunneling spectroscopy pioneered by Esaki and Giaever offered a powerful tool for studying electronic spectra and density of states (DOS) in superconductors. This led to important discoveries that revealed, in particular, the pseudogap in the tunneling spectrum of superconductors above their critical temperatures. However, the phenomenological approach of Giaever and Megerle does not resolve the fine structure of low-bias behavior carrying significant information about electron scattering, interactions, and decoherence effects. Here we construct a complete microscopic theory of electron tunneling into a superconductor in the fluctuation regime. We reveal a non-trivial low-energy anomaly in tunneling conductivity due to Andreev-like reflections of injected electrons from superconducting fluctuations. Our findings enable real-time observation of fluctuating Cooper pairs dynamics by time-resolved scanning tunneling microscopy measurements and open new horizons for quantitative analysis of the fluctuation electronic spectra of superconductors.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: A strong spin Nernst effect with nontrivial dependences on the carrier
concentration and electric field applied is expected in silicene and other
low-buckled Dirac materials. These Dirac materials can be considered as made of
two independent electron subsystems of the two-component gapped Dirac fermions.
For each subsystem the gap breaks a time-reversal symmetry and thus plays a
role of an effective magnetic field. Accordingly, the standard Kubo formalism
has to be altered by including the effective magnetization in order to satisfy
the third law of thermodynamics. We explicitly demonstrate this by calculating
the magnetization and showing how the correct thermoelectric coefficient
emerges.
Physical Review B 06/2014; 90(15). DOI:10.1103/PhysRevB.90.155107 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present an overview of our recent results on quantum magnetic oscillations in new functional materials. We begin with the Lifshitz and Kosevich approach for quasi-2D layered materials and obtain general formulas for the oscillatory parts of the grand thermodynamic potential and magnetization. Then we consider the oscillations of the Nernst–Ettingshausen coefficient which consists of thermal and magnetization parts. The difference between normal and Dirac carriers is also discussed. To conclude we consider a model for multilayer grapheme which allows to calculate exactly the Berry phase which remains undetermined in the Lifshitz–Kosevich approach. The magnetic oscillations of the density of states and capacitance for different number of the carbon layers are described.
Low Temperature Physics 04/2014; 40(4):270-279. DOI:10.1063/1.4869583 · 0.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We address theoretically the puzzling similarity observed in the
thermodynamic behaviour of independent clouds of cold dipolar excitons in
coupled semiconductor quantum wells. We argue that the condensation of
self-trapped exciton gas starts at the same critical temperature in all traps
due to the specific scaling rule. As a consequence of the reduced
dimensionality of the system, the scaling parameters appear to be insensitive
to disorder.
[Show abstract][Hide abstract] ABSTRACT: Electron tunneling spectroscopy pioneered by Esaki and Giaever offered a
powerful tool for studying electronic spectra and density of states (DOS) in
superconductors. This led to important discoveries that revealed, in
particular, the pseudogap in the tunneling spectrum of superconductors above
their critical temperatures. However, the phenomenological approach is
insufficient for describing the does not resolve the fine structure of low-bias
behavior carrying significant information about electron scattering,
interactions, and decoherence effects. Here we construct a complete microscopic
theory of electron tunneling into a superconductor in the fluctuation regime.
We reveal a non-trivial low-energy anomaly in tunneling conductivity due to
Andreev-like reflection of injected electrons from superconducting
fluctuations. Our findings enable real-time observation of fluctuating Cooper
pairs dynamics by time-resolved scanning tunneling microscopy measurements and
open new horizons for quantitative analysis of the fluctuation electronic
spectra of superconductors.
[Show abstract][Hide abstract] ABSTRACT: We express the link between conductivity and coefficients of Seebeck,
Nernst-Ettingshausen, Peltier, and Thompson and Reghi-Leduc via the temperature
derivative of the chemical potential of a system. These general expressions are
applied to three-, two- and one-dimensional systems of charge carriers having a
parabolic or Dirac spectrum. The method allows for predicting thermoelectric
and thermomagnetic properties of novel materials and structures.
[Show abstract][Hide abstract] ABSTRACT: The leading contributions to the c-axis conductivity of layered superconductors arising from superconducting fluctuations of the order parameter are discussed for arbitrary intralayer scattering. The contributions from fluctuations of the normal quasiparticle density of states are shown to be opposite in sign to the Aslamazov-Larkin and Maki-Thompson contributions, leading to a peak in the overall c-axis resistivity ρc(T) above Tc. This peak is enhanced by a magnetic field H∥c^. With increasing H, the relative peak maximum in ρc(T,H) increases in magnitude and is shifted to lower temperatures by an amount proportional to H2 for weak fields and to H for strong fields. For comparison, the fluctuation conductivity parallel to the layers has been calculated including the fluctuations of the normal density of states. Our results are discussed in regard to recent experiments with YBa2Cu3O7-δ and Bi2Sr2CaCu2O8+δ.
[Show abstract][Hide abstract] ABSTRACT: scientific session of the Physical Sciences Division of the Russian Academy of Sciences (RAS) devoted to the "Physical properties of graphene" was held on 28 March 2012 in the conference hall of the Lebedev Physical Institute. The agenda of the session announced on the RAS Physical Sciences Division website http://www.gpad.ac.ru included the following reports: (1) Falkovsky L A (Landau Institute of Theoretical Physics, RAS, Moscow; Vereshchagin Institute of High-Pressure Physics, RAS, Moscow) "Magnetooptics of graphene"; (2) Varlamov A A (The University of Rome Tor Vergata, Italy) "Thermoelectric properties of graphene." The papers written on the basis of these reports are given below.
[Show abstract][Hide abstract] ABSTRACT: We study the effect of superconducting fluctuations on the tunnel
current-voltage characteristics of disordered superconducting films placed in a
perpendicular magnetic field, $H$, in the whole $H$-$T$ phase diagram outside
the superconducting region. This tunnel-current is experimentally accessible by
STM measurements. In the domain of temperatures $T\geq T_{c0}$ and relatively
weak fields $H\ll H_{c2}(0)$ we reproduce existing results for the zero-voltage
tunneling conductance, but also discover an important nonlinear contribution,
which appears due to dynamic fluctuation modes and results in the formation of
a strong zero-bias anomaly (ZBA) on the scale $eV\sim
k_{\mathrm{B}}(T-T_{c0})$. At large voltages ($eV\sim k_{\mathrm{B}}T_{c0}$)
these modes, together with the contribution from static fluctuations, form a
pseudogap maximum. At low temperatures, with magnetic field values near
$H_{c2}(0)$, fluctuations acquire quantum character and the general picture of
the voltage dependent tunneling conductance resembles that one close to
$T_{c0}$, where the role of temperature and magnetic field are exchanged. In
particular, a gap-like structure appears with maximum at $eV_{\max}\sim
\Delta_{\mathrm{BCS}}$ and a sharp ZBA on the scale $eV\sim
\Delta_{\mathrm{BCS}}(H/H_{c2}(0)-1)$. The complete expression for the
tunneling current at arbitrary fields and temperatures can be evaluated only
numerically, which is presented in detail.
[Show abstract][Hide abstract] ABSTRACT: Angle-resolved photoemission spectroscopy (ARPES) is a powerful probe of
electron correlations in two-dimensional layered materials. In this Letter we
demonstrate that ARPES can be used to probe the onset of exciton condensation
in spatially-separated systems of electrons and holes created by gating
techniques in either double-layer graphene or topological-insulator thin films.
[Show abstract][Hide abstract] ABSTRACT: 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.
Physical Review B 04/2012; 85(21). DOI:10.1103/PhysRevB.85.214507 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The dependence of the Maki-Thompson and of the density of states (DOS) depletion contributions from superconducting fluctuations
to nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) relaxation is derived in the framework of the diagrammatic
theory, applied to layered three-dimensional (3-D) high-Tc superconductors. The regularization procedure devised for the conductivity (A. I. Buzdin, A. A. Varlamov: Phys. Rev. B58, 14195, 1998) is used in order to avoid the divergence of the DOS term. The theoretical results are discussed in the light
of NMR-NQR measurements in YBCO and compared with the recent theory (M. Eschrig et al.: Phys. Rev. B59, 12095, 1999), on the basis of the assumption of a purely 2-D spectrum of fluctuations.
Applied Magnetic Resonance 04/2012; 19(3):345-354. DOI:10.1007/BF03162376 · 1.17 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: There exist experiments indicating that at certain conditions, such as an
appropriate substrate, a gap of the order of 10 meV can be opened at the Dirac
points of a quasiparticle spectrum of graphene. We demonstrate that the opening
of such a gap can result in the appearance of a fingerprint bump of the Seebeck
signal when the chemical potential approaches the gap edge. The magnitude of
the bump can be up to one order higher than the already large value of the
thermopower occurring in graphene. Such a giant effect, accompanied by the
nonmonotonous dependence on the chemical potential, is related to the emergence
of a new channel of quasiparticle scattering from impurities with the
relaxation time strongly dependent on the energy. We analyze the behavior of
conductivity and thermopower in such a system, accounting for quasiparticle
scattering from impurities with the model potential in a self-consistent
scheme. Reproducing the existing results for the case of gapless graphene, we
demonstrate a failure of the simple Mott formula in the case under
consideration.
[Show abstract][Hide abstract] ABSTRACT: We review some results concerning the superconducting properties of a 2D superconductor in the proximity to an electronic topological transition (ETT). In contrast to the 3D case, we find that the superconducting gap at T = 0 is characterized by a nonmonotonic behavior, with maxima occurring close to the ETT, both for s- and for d-wave pairing. Such a result is in good qualitative agreement with the phenomenological trend recently observed for as a function of the hopping ratio t′/t for several cuprate compounds. We also derive a nonmonotonic dependence of the quasiparticle inverse lifetime due to impurity scattering. We further analyze the effect of an ETT on the Ginzburg-Landau stiffness η, for which we recover an expression analogous to the case of an isotropic dispersion relation.
International Journal of Modern Physics B 01/2012; 17(04n06). DOI:10.1142/S0217979203016066 · 0.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The influence of superconducting fluctuations on the conductance spectra of superconductor-insulator-normal metal (S-I-N) tunnel junctions is considered. We discuss how the tunneling conductance is affected as the strength of fluctuations is changed close to the superconducting critical transition.
International Journal of Modern Physics B 01/2012; 13(09n10). DOI:10.1142/S0217979299001326 · 0.94 Impact Factor