A. A. Varlamov

Argonne National Laboratory, Lemont, Illinois, United States

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Publications (130)366.76 Total impact

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    A. Glatz, A. A. Varlamov, V. M. Vinokur
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    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.
    EPL (Europhysics Letters) 08/2014; 107(4):47004. · 2.26 Impact Factor
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    A. E. Koshelev, A. A. Varlamov
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    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.
    Superconductor Science and Technology 08/2014; 27(12). · 2.76 Impact Factor
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    V. P. Gusynin, S. G. Sharapov, A. A. Varlamov
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    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.
    06/2014;
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    S V Andreev, A A Varlamov, A V Kavokin
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    ABSTRACT: We address, theoretically, the puzzling similarity observed in the thermodynamic behavior 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.
    Physical Review Letters 01/2014; 112(3):036401. · 7.73 Impact Factor
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    A. Glatz, A. A. Varlamov, V. M. Vinokur
    [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.
    11/2013;
  • Phys.Rev.87, 7. 01/2013; 87(7).
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    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.
  • Physics-Uspekhi 11/2012; 55(11):1146. · 1.87 Impact Factor
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    A. Glatz, A. A. Varlamov, V. M. Vinokur
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    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.
    10/2012;
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    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.
    Physical review. B, Condensed matter 09/2012; 87(7). · 3.77 Impact Factor
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    A. E. Koshelev, A. A. Varlamov
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    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). · 3.66 Impact Factor
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    P. Mosconi, A. Rigamonti, A. A. Varlamov
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    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. · 0.83 Impact Factor
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    S. G. Sharapov, A. A. Varlamov
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    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.
    Physical review. B, Condensed matter 02/2012; 86(3). · 3.77 Impact Factor
  • A. M.cucolo, M.cuoco, C.noce, A. A.varlamov
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    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). · 0.46 Impact Factor
  • G. G. N.angilella, E.piegari, A. A.varlamov
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    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). · 0.46 Impact Factor
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    ABSTRACT: We review a Ginzburg-Landau phenomenological model for the dependence of the critical temperature on microscopic strain in tetragonal high-Tc cuprates. Such a model is in agreement with the experimental results for LSCO under epitaxial strain, as well as with the hydrostatic pressure dependence of Tc in most cuprates. In particular, a nonmonotonic dependence of Tc on hydrostatic pressure, as well as on in-plane or apical microstrain, is derived. From a microscopic point of view, such results can be understood as due to the proximity to an electronic topological transition (ETT). In the case of LSCO, we argue that such an ETT can be driven by a strain-induced modification of the band structure, at constant hole content, at variance with a doping-induced ETT, as is usually assumed.
    International Journal of Modern Physics B 01/2012; 17(04n06). · 0.46 Impact Factor
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    ABSTRACT: SQUID magnetization measurements in oriented powders of Y1-xCaxBa2Cu3Oy from the underdoped to the overdoped regime are discussed in the light of an anomalous precursor fluctuating diamagnetism occurring above the bulk superconducting transition temperature. It is emphasized how both in the underdoped as well as in the overdoped YBCO compounds, the fluctuating magnetisation differs strongly from the predictions based on the Ginzburg-Landau anisotropic functional, at variance of what occurs in optimally doped YBCO. A theoretical description based on the extension of a recent theory for fluctuations of the order parameter in a layered systems of vortices, is briefly outlined. It is shown that such an approach, essentially assuming droplets with a local superconducting temperature above the bulk TC, accounts for most of the experimental findings, including irreversibility effects.
    International Journal of Modern Physics B 01/2012; 17(04n06). · 0.46 Impact Factor
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    Igor A Luk'yanchuk, Andrei A Varlamov, Alexey V Kavokin
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    ABSTRACT: We consider the Nernst-Ettingshausen (NE) effect in the presence of semiclassically strong magnetic fields for a quasi-two-dimensional system with a parabolic or linear dispersion of carriers. We show that the occurring giant oscillations of the NE coefficient are coherent with the recent experimental observation in graphene, graphite, and bismuth. In the 2D case we find the exact shape of these oscillations and show that their magnitude decreases (increases) with enhancement of the Fermi energy for Dirac fermions (normal carriers). With a crossover to the 3D spectrum the phase of the oscillations shifts, their amplitude decreases, and the peaks become asymmetric.
    Physical Review Letters 07/2011; 107(1):016601. · 7.73 Impact Factor
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    ABSTRACT: The observation of a large Nernst signal in cuprates above the superconducting transition temperature has attracted much attention. A potential explanation is that it originates from superconducting fluctuations. Although the Nernst signal is indeed consistent with Gaussian fluctuations for overdoped cuprates, Gaussian theory fails to describe the temperature dependence seen for underdoped cuprates. Here, we consider the vertex correction to Gaussian theory resulting from the pseudogap. This yields a Nernst signal in good agreement with the data.
    Physical review. B, Condensed matter 01/2011; 83. · 3.77 Impact Factor
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    ABSTRACT: A Reply to the Comment by Andrei Sergeev, M. Reizer, and V. Mitin.
    Physical Review Letters 01/2011; 106. · 7.73 Impact Factor

Publication Stats

1k Citations
366.76 Total Impact Points

Institutions

  • 2014
    • Argonne National Laboratory
      • Division of Materials Science
      Lemont, Illinois, United States
    • Northern Illinois University
      • Department of Physics
      DeKalb, Illinois, United States
  • 1993–2014
    • University of Rome Tor Vergata
      • • Dipartimento di Scinze e Tecnologie Chimiche
      • • Dipartimento di Ingegneria Civile e Ingegneria Informatica (DICII)
      Roma, Latium, Italy
  • 1997–2012
    • University of Pavia
      • Department of Physics
      Ticinum, Lombardy, Italy
  • 2008
    • University of Birmingham
      • School of Physics and Astronomy
      Birmingham, ENG, United Kingdom
  • 2005–2007
    • Max Planck Institute for the Physics of Complex Systems
      Dresden, Saxony, Germany
  • 2003
    • Università Degli Studi Roma Tre
      Roma, Latium, Italy
    • Policlinico Tor Vergata
      Roma, Latium, Italy
  • 1999
    • Università degli Studi di Roma "La Sapienza"
      • Department of Physics
      Roma, Latium, Italy
  • 1996–1999
    • University of Florence
      Florens, Tuscany, Italy
  • 1992
    • University of Naples Federico II
      • Department of Physical Sciences
      Napoli, Campania, Italy
  • 1991–1992
    • Abdus Salam International Centre for Theoretical Physics
      Trst, Friuli Venezia Giulia, Italy
  • 1989
    • Georgian National Academy of Sciences
      Tbilsi, T'bilisi, Georgia