Thermal fluctuations in d-wave layered superconductors

We study the thermal fluctuations of anisotropic order parameters (OP) in layered superconductors. In particular, for copper oxides and a d-wave OP, we present some experimental consequences of fluctuations in the direction normal to the layers. It is shown that the c-axis penetration depth λ_c can have a “disorder-like” quadratic temperature dependence at low temperature. The fluctuations are analyzed in the framework of a Lawrence-Doniach model with an isotropic Fermi surface. Anisotropies pin the orientation of the OP to the crystallographic axes of the lattice. Then we study an extended t-J model that fits Fermi surface data of bilayers YBCO and BSCCO. This leads to a d-wave OP with two possible orientations and, including the thermal fluctuations, yields the announced temperature dependence of λ_c. Furthermore a reservoir layer is introduced. It implies a finite density of states at the Fermi energy which is successfully compared to conductance and specific heat measurements.     

Thermal conductivity and competing orders in d-wave superconductors

We derive the expression for the thermal conductivity in the low-temperature limit in d-wave superconductors, taking into account the presence of competing orders such as spin-density wave, is-pairing, etc. The expression is used for analyzing recent experimental data in . Our analysis strongly suggests that competing orders can be responsible for anomalies in behavior of thermal conductivity observed in those experiments.Received: 9 December 2003, Published online: 2 April 2004PACS: 74.25.Fy Transport properties (electric and thermal conductivity, thermoelectric effects, etc.) - 74.72.Dn La-based cuprates - 74.72.-h Cuprate superconductors (high-Tc and insulating parent compounds)V.P. Gusynin: On leave from Bogolyubov Institute for Theoretical Physics, 03143 Kiev, UkraineV.A. Miransky: On leave from Bogolyubov Institute for Theoretical Physics, 03143 Kiev, Ukraine     

Quantum critical fluctuations in disordered d-wave superconductors

To explain the strong quasiparticle damping in the cuprates, Sachdev and collaborators proposed to couple the system to a critically fluctuating id(xy)- or is-order parameter mode. Here we generalize the approach to the presence of static disorder. In the id case, the order parameter dynamics becomes diffusive, but otherwise much of the phenomenology of the clean case remains intact. In contrast, the interplay of disorder and is-order parameter fluctuations leads to a secondary superconductor transition, with a critical temperature exponentially sensitive to the impurity concentration.     

Fluctuation conductivity in cuprate superconductors

We have measured the in-plane resistivity of Bi₂Sr₂CaCu₂O_8+δ and Tl₂Ba₂ CaCu₂O_8+δ single crystals in the temperature range 70–300 K. The thermodynamic fluctuations in the conductivity of both the samples start around ∼ 125 K. We find the Lawrence and Doniach [1] model to be inadequate to describe the fluctuation conductivity in these materials. The modification suggested by Ramallo et al [4] where by the conductivity is enhanced due to the presence of two superconducting layers in each unit cell is also not adequate. We suggest the fluctuation conductivity to be reduced due to the reduction in the density of states (DOS) of the quasiparticles which results due to the formation of Cooper pairs at the onset of the fluctuations. The data agrees with the theory proposed by Dorin et al [5] which takes into account this reduction in DOS.     

Tunneling spectra of layered strongly correlated d-wave superconductors

Tunneling conductance experiments on cuprate superconductors exhibit a large diversity of spectra that appear in different nanosized regions of inhomogeneous samples. In this Letter, we use a mean-field approach to the tt't'J model in order to address the features in these spectra that deviate from the BCS paradigm, namely, the bias sign asymmetry at high bias, the generic lack of evidence for the van Hove singularity, and the absence of coherence peaks at low dopings. We conclude that these features can be reproduced in homogeneous layered d-wave superconductors solely due to a proximate Mott insulating transition. We also establish the connection between the above tunneling spectral features and the strong renormalization of the electron dispersion around (0, pi) and (pi, 0) and the momentum space anisotropy of electronic states observed in angle-resolved photoemission spectroscopy experiments.     

Details of disorder matter in 2D d-wave superconductors

Within numerically exact solutions of the Bogoliubov-de Gennes equations, we demonstrate that discrepancies between predicted low-energy quasiparticle properties in disordered 2D d-wave superconductors occur because of the unanticipated importance of disorder model details and normal state particle-hole symmetry. For the realistic case, which is best described by a binary alloy model without particle-hole symmetry, we predict density-of-state suppression below an energy scale which appears to be correlated with the corresponding single impurity resonance.     

Weak-field thermal hall conductivity in the mixed state of d-wave superconductors

Thermal transport in the mixed state of a d-wave superconductor is considered within the weak-field regime. We express the thermal conductivity, kappa(xx), and the thermal Hall conductivity, kappa(xy), in terms of the cross section for quasiparticle scattering from a single vortex. Solving for the cross section (neglecting the Berry phase contribution and the anisotropy of the gap nodes), we obtain kappa(xx)(H,T) and kappa(xy)(H,T) in surprisingly good agreement with the qualitative features of the experimental results for YBa2Cu3O6.99. In particular, we show that the simple, yet previously unexpected, weak-field behavior, kappa(xy)(H,T) approximately T squareroot [H], is that of thermally excited nodal quasiparticles, scattering primarily from impurities, with a small skew component provided by vortex scattering.     

Superflow in d-wave superconductors

Superflow in a phenomenological tight-binding model for the superconducting state of some high-temperature superconductors is discussed thoroughly. The formalism used is explicitly gauge invariant and currents are computed exactly within the BCS theory, going therefore beyond linear response theory. The dependence of gap functions, current density, critical currents and free energy as a function of the superfluid velocity for different angles and doping concentrations is investigated. Different sources of anisotropy, like the dispersion relation of the model, the internal symmetry of the order parameter, and orthorhombic distortions of the lattice are also studied.     

Strong vertex corrections from weak disorder in 2D d-wave superconductors

We show that weak static random potentials have pronounced effects on the quasiparticle states of a 2Dd-wave superconductor close to a node. We prove that the vertex correction coming from the simplest crossed diagram is important even for a nonmagnetic potential. The leading frequency and momentum dependent logarithmic singularities in the self-energy are calculated exactly to second order in perturbation theory. The self-energy corrections lead to a modified low energy density of states which depends strongly on the type of random potential and which can be measured in experiments. There is an exceptional case for a potential with extremely local scatterers and opposite nodes separated by (, ) where an exact cancelation takes place eliminating the leading frequency dependent singularity in the simplest crossed diagram. A comparison of the perturbative results with a self-consistent CPA (coherent potential approximation) for the nonmagnetic disorder reveals qualitative differences in the self-energy at the smallest energies which are due to the neglectance of vertex corrections in CPA.     

The upper critical field in two-band layered superconductors

The upper critical field of clean MgB₂ is investigated using the two-band layered Ginzburg--Landau (GL) theory. The calculated results are fitted to the experimental data of clean MgB₂ crystal very well in a broad temperature range. Based on the GL theory for clean superconductors, a phenomenological theory for dirty superconductor is proposed. Selecting appropriate parameters, two-band layered GL theory is successfully applied to the crystal of Mg(B_1-xC_x)₂ and the neutron irradiation samples of MgB₂.     

Zinc impurities in d-wave superconductors

We study the two-dimensional Hubbard model with nonmagnetic Zn impurities modeled by binary diagonal disorder using quantum Monte Carlo within the dynamical cluster approximation. With increasing Zn content we find a strong suppression of d-wave superconductivity and an enhancement of antiferromagnetic spin correlations. T(c) vanishes linearly with Zn impurity concentration. The spin susceptibility changes from pseudogap to Curie-Weiss-like behavior indicating the existence of free magnetic moments in the Zn doped system. We interpret these results within the resonating-valence-bond picture.