Publications (137)355.54 Total impact
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ABSTRACT: We demonstrate that the partial entropy of a twodimensional electron gas (2DEG) exhibits quantized peaks at resonances between the chemical potential and electron levels of size quantization. In the limit of no scattering, the peaks depend only on the subband quantization number and are independent on material parameters, shape of the confining potential, electron effective mass and temperature. The quantization of partial entropy is a signature of a topological phase transition in a 2DEG. In the presence of stationary disorder, the magnitude of peaks decreases. Its deviation from the quantized values is a direct measure of the disorder induced smearing of the electronic density of states.  [Show abstract] [Hide abstract]
ABSTRACT: The effect of fluctuations on the nuclear magnetic resonance (NMR) relaxation rate, $W$, is studied in a complete phase diagram of a 2D superconductor above the upper critical field line $H_{c2}(T)$ . In the region of relatively high temperatures and low magnetic fields, the relaxation rate $W$ is determined by two competing effects. The first one is its decrease in result of suppression of quasiparticle density of states (DOS) due to formation of fluctuation Cooper pairs (FCP). The second one is a specific, purely quantum, relaxation process of the MakiThompson (MT) type, which for low field leads to an increase of the relaxation rate. The latter describes particular fluctuation processes involving selfpairing of a single electron on selfintersecting trajectories of a size up to phasebreaking length $l_{\phi }$ which becomes possible due to an electron spinflip scattering event at a nucleus. As a result, different scenarios with either growth or decrease of the NMR relaxation rate are possible upon approaching the normal metal  typeII superconductor transition. The character of fluctuations changes along the line $H_{c2}$ from the thermal longwavelength type in weak magnetic fields to the clusters of rotating FCP in fields comparable to $H_{c2}$. We find that below the welldefined temperature $T^*_0\approx 0.6T_{c0}$, the MT process becomes ineffective even in absence of intrinsic pairbreaking. The small scale of FCP rotations ($\xi_{xy}$) in so high fields impedes formation of long (<$l_{\phi }$) selfintersecting trajectories, causing the corresponding relaxation mechanism to lose its efficiency. This reduces the effect of superconducting fluctuations in the domain of high fields and low temperatures to just the suppression of quasiparticle DOS, analogously to the Abrikosov vortex phase below the $H_{c2}$ line.  [Show abstract] [Hide abstract]
ABSTRACT: We begin with a brief description of the role of the NernstEttingshausen effect in the studies of the hightemperature 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 lowbuckled 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 twocomponent gapped Dirac fermions. For each subsystem the gap breaks a timereversal 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 lowbuckled Dirac materials.  [Show abstract] [Hide abstract]
ABSTRACT: Recent experiments show that the NernstEttingshausen 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 nonmonotonic dependence of the NernstEttingshausen coefficient on the magnetic field.  [Show abstract] [Hide abstract]
ABSTRACT: Thermoelectric energy conversion is a direct but lowefficiency process, which precludes the development of longawaited widescale applications. As a breakthrough permitting a drastic performance increase is seemingly out of reach, we fully reconsider the problem of thermoelectric coupling enhancement. 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 fluctuation 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 fluctuation 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.  [Show abstract] [Hide abstract]
ABSTRACT: The theory of current transport in a narrow superconducting channel accounting for thermal fluctuations is revisited. The value of voltage appearing in the sample is found as the function of temperature (close to transition temperature $TT_{\mathrm{c}}$ $\ll T_{\mathrm{c}}$) and bias current $J<J_{\mathrm{c}}$ ( $J_{\mathrm{c}}$ is a value of critical current calculated in the framework of the BCS approximation, neglecting thermal fluctuations). It is shown that the careful analysis of vortex crossing of the stripe results in considerable increase of the activation energy. 
Article: Highresolution tunnel fluctuoscopy
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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 lowbias 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 nontrivial lowenergy anomaly in tunneling conductivity due to Andreevlike reflections of injected electrons from superconducting fluctuations. Our findings enable realtime observation of fluctuating Cooper pairs dynamics by timeresolved 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 twoband superconductor. Multipleband 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 GinzburgLandau picture. As expected, the multipleband 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 inplane 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 zaxis 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 singleband 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 lowbuckled Dirac materials. These Dirac materials can be considered as made of two independent electron subsystems of the twocomponent gapped Dirac fermions. For each subsystem the gap breaks a timereversal 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.  [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 quasi2D 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.  [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 selftrapped 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 lowbias 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 nontrivial lowenergy anomaly in tunneling conductivity due to Andreevlike reflection of injected electrons from superconducting fluctuations. Our findings enable realtime observation of fluctuating Cooper pairs dynamics by timeresolved 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, NernstEttingshausen, Peltier, and Thompson and ReghiLeduc via the temperature derivative of the chemical potential of a system. These general expressions are applied to three, two and onedimensional systems of charge carriers having a parabolic or Dirac spectrum. The method allows for predicting thermoelectric and thermomagnetic properties of novel materials and structures. 
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ABSTRACT: The leading contributions to the caxis 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 AslamazovLarkin and MakiThompson contributions, leading to a peak in the overall caxis 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 YBa2Cu3O7δ 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 HighPressure 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 currentvoltage characteristics of disordered superconducting films placed in a perpendicular magnetic field, $H$, in the whole $H$$T$ phase diagram outside the superconducting region. This tunnelcurrent 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 zerovoltage tunneling conductance, but also discover an important nonlinear contribution, which appears due to dynamic fluctuation modes and results in the formation of a strong zerobias anomaly (ZBA) on the scale $eV\sim k_{\mathrm{B}}(TT_{c0})$. 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 gaplike 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: Angleresolved photoemission spectroscopy (ARPES) is a powerful probe of electron correlations in twodimensional layered materials. In this Letter we demonstrate that ARPES can be used to probe the onset of exciton condensation in spatiallyseparated systems of electrons and holes created by gating techniques in either doublelayer graphene or topologicalinsulator 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 (swave and dwave) 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 phasebreaking length $l_{\phi}$ and its longrange 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 swave superconductor with elastic scatterers the inhomogeneous LDoS is directly connected to the corresponding characteristics of normal metal. 
Article: Magnetic field dependence of the superconducting fluctuation contribution to NMRNQR relaxation
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ABSTRACT: The dependence of the MakiThompson 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 threedimensional (3D) highTc 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 NMRNQR 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 2D spectrum of fluctuations.
Publication Stats
2k  Citations  
355.54  Total Impact Points  
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Institutions

19942015

Argonne National Laboratory
 Division of Materials Science
Лимонт, Illinois, United States


2014

Russian Quantum Center
Skolkovo, MO, Russia


20002014

University of Rome Tor Vergata
 Dipartimento di Scienze e Tecnologie Chimiche
Roma, Latium, Italy 
University of Tours
Tours, Centre, France


19972012

University of Pavia
 Department of Physics
Ticinum, Lombardy, Italy


2009

INO  Istituto Nazionale di Ottica
Florens, Tuscany, Italy


19912008

National Research Council
Roma, Latium, Italy


2004

UNIT
Miami, Florida, United States


2003

Policlinico Tor Vergata
Roma, Latium, Italy


20012003

Università Degli Studi Roma Tre
Roma, Latium, Italy


1999

Università degli Studi di Roma "La Sapienza"
 Department of Physics
Roma, Latium, Italy


19941999

University of Florence
Florens, Tuscany, Italy


1992

University of Naples Federico II
 Department of Physical Sciences
Napoli, Campania, Italy


19911992

Abdus Salam International Centre for Theoretical Physics
Trst, Friuli Venezia Giulia, Italy
