A. A. Varlamov

Space-time dispersion of graphene conductivity

Abstract.We present an analytic calculation of the conductivity of pure graphene as a function of frequency ω, wave-vector k, and temperature for the range where the energies related to all these parameters are small in comparison with the band parameter γ≃3 eV, but much larger than the collision rate τ-1. The simple asymptotic expressions are given in various limiting cases. For instance, the conductivity for kv0≪ T≪ω is equal to σ(ω,k)=e2/4ħ and independent of the band structure parameters γ and v0. Our results are also used to explain the known dependence of the graphite conductivity on temperature and pressure.

The role of density of states fluctuations in the normal state properties of high T c superconductors

During the last decade a lot of efforts have been undertaken to explain the unusual normal state properties of high temperature superconductors (HTS) in the framework of unconventional theories based on strongly interacting electrons, pre-formed Cooper pairs, polaron mechanism of super conductivity etc. A different approach to this problem would be to develop the perturbation theory for interacting electrons in the normal phase of strongly anisotropic superconductors without specifying the origin of this interaction. The Cooper channel of interelectron interaction is equivalent to the superconducting fluctuations which are unusually strong in HTS. We show that the peculiarities of such systems not only lead to the increase of the magnitude but are also frequently responsible for the change of the hierarchy of different fluctuation effects and even of the sign of the total corrections. As a result the fluctuation contributions can manifest themselves in very unusual forms. The first and well known result i...

Two-gap model for underdoped cuprate superconductors

Various properties of underdoped superconducting cuprates, including the momentum-dependent pseudogap opening, indicate a behavior which is neither BCS- nor Bose-Einstein condensation (BEC)\char21{} like. To explain this issue we introduce a two-gap model. This model assumes an anisotropic pairing interaction among two kinds of fermions with small and large Fermi velocities representing the quasiparticles near the M and the nodal points of the Fermi surface, respectively. We find that a gap forms near the M points resulting in incoherent pairing due to strong fluctuations. Instead, the pairing near the nodal points sets in with phase coherence at lower temperature. By tuning the momentum-dependent interaction, the model allows for a continuous evolution from a pure BCS pairing (in the overdoped and optimally doped regime) to a mixed boson-fermion picture (in the strongly underdoped regime).

Anomalous growth of thermoelectric power in gapped graphene

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.

Fluctuation Phenomena in Superconductors

Reading a major part of the textbooks one can have impression that the BCS theory of superconductivity is the exact one. Proposed review dispels this delusion, clearly indicating the limits of the mean field theory applicability and demonstrating existence of the wide circle of interesting phenomena being beyond its confines. The chapter presents itself both the review and the textbook of fluctuation phenomena in superconductors. Its first half is devoted to presentation of the phenomenological and microscopical methods in the description of fluctuations. The Ginzburg-Landau functional and corresponding general description of fluctuation thermodynamics in framework of the functional integration over fluctuation fields approach is introduced. Time-dependent Ginzburg-Landau equation is is applied for the description of the fluctuation transport in the vicinity of superconducting transition. The basic aspects of the microscopic description of fluctuation phenomena in superconductors in the frameworks of the quantum field theory are presented. In the second half the authors portray a panorama of the superconductive fluctuations manifestation in different observables. The microscopic analysis of the different fluctuation contributions to the conductivity, magnetoconductivity, Hall effect, tunneling conductivity, thermopower, NMR relaxation rate is presented. The physical origin of Aslamazov-Larkin, Maki-Thompson and density of states renormalization corrections are discussed.

Crossover between Aslamazov-Larkin and short-wavelength fluctuation regimes in high-temperature-superconductor conductivity experiments

We present paraconductivity measurements in three different high-temperature superconductors: a melt-textured

Fluctuation conductivity in intercalated superconductors

{\mathrm{YBa}}_{2}

Superconducting properties of [ BaCuO x ] 2 / [ CaCuO 2 ] n artificial structures with ultrathick CaCuO 2 blocks

{\mathrm{Cu}}_{3}