Fluctuation conductivity in superconductors in strong electric fields

We study the effect of a strong electric field on the fluctuation conductivity within the time-dependent Ginzburg-Landau theory for the case of arbitrary dimension. Our results are based on the analytical derivation of the velocity distribution law for the fluctuation Cooper pairs, from the Boltzmann equation. Special attention is drawn to the case of small nonlinearity of conductivity, which can be investigated experimentally. We obtain a general relation between the nonlinear conductivity and the temperature derivative of the linear Aslamazov-Larkin conductivity, applicable to any superconductor. For the important case of layered superconductors we derive an analogous relation between the small nonlinear correction for the conductivity and the fluctuational magnetoconductivity. On the basis of these relations we provide new experimental methods for determining both the lifetime constant of metastable Cooper pairs above T_c and the coherence length. A systematic investigation of the 3rd harmonic of the electric field generated by a harmonic current can serve as an alternative method for the examination of the metastable Cooper-pair relaxation time.Comment: 18 pages, REVTeX, submitted to Phys. Rev.

Theory of terahertz electric oscillations by supercooled superconductors

We predict that below T_c a regime of negative differential conductivity (NDC) can be reached. The superconductor should be supercooled to T<T_c in the normal phase under DC voltage. In such a nonequilibrium situation the NDC of the superconductor is created by the excess conductivity of the fluctuation Cooper pairs. We propose NDC of supercooled superconductors to be used as an active medium for generation of electric oscillations. Such generators can be used in the superconducting electronics as a new type THz source of radiation. Oscillations can be modulated by the change of the bias voltage, electrostatic doping by a gate electrode when the superconductor is the channel of a field effect transistor, or by light. When small amplitude oscillations are stabilized near the critical temperature T_c the generator can be used as a bolometer. The essential for the applications NDC is predicted by the solution of the Boltzmann kinetic equation for the metastable in the normal phase Cooper pairs. Boltzmann equation for fluctuation Cooper pairs is a result of state-of-the-art application of the microscopic theory of superconductivity. Our theoretical conclusions are based on some approximations like time dependent Ginzburg-Landau theory, but nevertheless can reliably predict appearance of NDC. The maximal frequency at which superconductors can operate as generators is determined by the critical temperature \hbar omega_max ~ k_B T_c. For high-T_c superconductors this maximal frequency falls well inside the terahertz range. Technical conditions to avoid nucleation of the superconducting phase are briefly discussed. We suggest that nanostructured high-T_c superconductors patterned in a single chip can give the best technical performance of the proposed oscillator.Comment: 7 page

Tight-binding modelling of the electronic band structure of layered superconducting perovskites

A detailed tight-binding analysis of the electron band structure of the CuO_2 plane of layered cuprates is performed within a sigma-band Hamiltonian including four orbitals - Cu3d_x^2-y^2, Cu4s, O2p_x, and O2p_y. Both the experimental and theoretical hints in favor of Fermi level located in a Cu or O band, respectively, are considered. For these two alternatives analytical expressions are obtained for the LCAO electron wave functions suitable for the treatment of electron superexchange. Simple formulae for the Fermi surface and electron dispersions are derived by applying the Loewdin down-fold procedure to set up the effective copper and oxygen Hamiltonians. They are used to fit the experimental ARUPS Fermi surface of Pb_0.42Bi_1.73Sr_1.94Ca_1.3Cu_1.92O_8+x and both the ARPES and LDA Fermi surface of Nd_2-xCe_xCuO_4-delta. The value of presenting the hopping amplitudes as surface integrals of ab initio atomic wave functions is demonstrated as well. The same approach is applied to the RuO_2 plane of the ruthenate Sr_2RuO_4. The LCAO Hamiltonians including the three in-plane pi-orbitals Ru4d_xy, O_a 2p_y, O_b 2p_x and the four transversal pi-orbitals Ru4d_zx, Ru4d_yz, O_a 2p_z, O_b 2p_z, are separately considered. It is shown that the equation for the constant energy curves and the Fermi contours has the same canonical form as the one for the layered cuprates.Comment: 21 pages, 10 figures, published in J. Phys.: Condens. Matter (complete and corrected References section

Superconducting Diamagnetic Fluctuations in MgB2

The fluctuating diamagnetic magnetization Mfl at constant field H as a function of temperature and the isothermal magnetization Mfl vs H are measured in MgB2, above the superconducting transition temperature. The expressions for Mfl in randomly oriented powders are derived in the Gaussian approximation of local Ginzburg-Landau theory and used for the analysis of the data. The scaled magnetization Mfl/H^{1/2}*T is found to be field dependent. In the limit of evanescent field the behaviour for Gaussian fluctuations is obeyed while for H>~ 100 Oe the field tends to suppress the fluctuating pairs, with a field dependence of Mfl close to the one expected when short wavelength fluctuations and non-local electrodynamic effects are taken into account. Our data, besides providing the isothermal magnetization curves for T>Tc(0) in a BCS-type superconductor such as MgB2, evidence an enhancement of the fluctuating diamagnetism which is related to the occurrence in this new superconductor of an anisotropic spectrum of the superconducting fluctuations.Comment: Tex file, 4 pages, 3 ps figures, submitted to Phys. Rev. Let