Publications (131)348.07 Total impact
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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.Low Temperature Physics 05/2015; 41(5):342352. DOI:10.1063/1.4919372 · 0.79 Impact Factor  [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.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 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.Physical Review B 12/2014; 91(10). DOI:10.1103/PhysRevB.91.100501 · 3.74 Impact Factor 
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.EPL (Europhysics Letters) 08/2014; 107(4):47004. DOI:10.1209/02955075/107/47004 · 2.10 Impact Factor  [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.Superconductor Science and Technology 08/2014; 27(12). DOI:10.1088/09532048/27/12/124001 · 2.33 Impact Factor  [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.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 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.Low Temperature Physics 04/2014; 40(4):270279. 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 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.Physical Review Letters 01/2014; 112(3):036401. DOI:10.1103/PhysRevLett.112.036401 · 7.51 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 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.EPL (Europhysics Letters) 06/2013; 103(4). DOI:10.1209/02955075/103/47005 · 2.10 Impact Factor 
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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.PhysicsUspekhi 11/2012; 55(11):1146. DOI:10.3367/UFNe.0182.201211j.1229 · 2.61 Impact Factor  [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.Physical review. B, Condensed matter 09/2012; 87(7). DOI:10.1103/PhysRevB.87.075418 · 3.66 Impact Factor  [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.Physical Review B 04/2012; 85(21). DOI:10.1103/PhysRevB.85.214507 · 3.74 Impact Factor 
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.Applied Magnetic Resonance 04/2012; 19(3):345354. 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 selfconsistent 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). DOI:10.1103/PhysRevB.86.035430 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The influence of superconducting fluctuations on the conductance spectra of superconductorinsulatornormal metal (SIN) 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  [Show abstract] [Hide abstract]
ABSTRACT: We review a GinzburgLandau phenomenological model for the dependence of the critical temperature on microscopic strain in tetragonal highTc 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 inplane 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 straininduced modification of the band structure, at constant hole content, at variance with a dopinginduced ETT, as is usually assumed.International Journal of Modern Physics B 01/2012; 17(04n06). DOI:10.1142/S0217979203016200 · 0.94 Impact Factor
Publication Stats
2k  Citations  
348.07  Total Impact Points  
Top Journals
Institutions

2014

Russian Quantum Center
Skolkovo, MO, Russia


20072014

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


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
