高温超导体的热力学性质的研究

以一个修正的依赖时间的金兹堡朗道理论和层状模型去研究高温超导体的热力学性质.计算各向异性超导体的热容量等一些热力学量,并且得到了超导体的熵、能、热容量的具体表达式,并与最近的实验数据进行比较,结果一致符合 关键词： 热容量 热力学量 高温超导体     

Frequency dispersion of effective conductivity of high-temperature superconductors in the normal phase

The paper is devoted to the presently widely discussed question of the electrodynamic properties of high-temperture superconducting ceramics in the normal phase. The effective-dielectric-constant method is used to take into account the influence of the soft phonon modes connected with the structural phase transition accompanying the superconducting transition. The presence of soft phonons gives grounds for assuming that there exists a frequency region in which the contributions of the electron and phonon subsystems can be comparable. The frequency dependences of the effective conductivity and of the dielectric constant are investigated. Estimates show that an anomalous decrease of the effective conductivity should be observed, as was recently done in a number of experiments, in the microwave region. The influence of the dynamic character of the soft modes is also discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 39–42, December, 1989.     

Anomalous thermal expansion of high-temperature superconductors at low temperatures

The origins of the anomalous (negative) thermal expansion of high-temperature superconductors (HTSCs) and of the anomalously strong effect of a magnetic field on the thermal expansion coefficient at low temperatures are discussed. A physical model is proposed based on the stabilizing influence of a charge density wave (CDW) arising in the oxygen sublattice in addition to the antiferromagnetic and spin-Peierls ordering in the copper ion sublattice of an HTSC compound. The effects of temperature, magnetic field, and doping on the interaction of such a CDW with the ionic subsystem of an HTSC compound are studied. The conclusions derived from the model are in agreement with experimental data.     

Low-temperature thermal conductivity of superconductors with gap nodes

We report a detailed analytic and numerical study of electronic thermal conductivity in d-wave superconductors. We compare theory of the crossover at low temperatures from T dependence to T(3) dependence for increasing temperature with recent experiments on YBa(2)Cu(3)O(7) in zero magnetic field for T approximately [0.04 K,0.4 K] by Hill et al. [Phys. Rev. Lett. 92, 027001 (2004)]. Transport theory, including impurity scattering and inelastic scattering within strong-coupling superconductivity, can consistently fit the temperature dependence of the data in the lower half of the temperature regime. We discuss the conditions under which we expect power-law dependences over wide temperature intervals.     

Effective electrical and thermal conductivity of multifilament twisted superconductors

The effective electrical and thermal conductivity of composite wire with twisted superconducting filaments embedded into normal metal matrix is calculated using the extension of Bruggeman method. The resistive conductivity of superconducting filaments is described in terms of symmetric tensor, whereas the conductivity of a matrix is assumed to be isotropic and homogeneous. The dependence of the resistive electrical conductivity of superconducting filaments on temperature, magnetic field, and current density is implied to be parametric. The resulting effective conductivity tensor proved to be non-diagonal and symmetric. The non-diagonal transverse–longitudinal components of effective electrical conductivity tensor are responsible for the redistribution of current between filaments. In the limits of high and low electrical conductivity of filaments the transverse effective conductivity tends to that of obtained previously by Carr. The effective thermal conductivity of composite wires is non-diagonal and radius-dependent even for the isotropic and homogeneous thermal conductivities of matrix and filaments.     

The electronic thermal conductivity of superconductors with paramagnetic impurities

We have measured the thermal conductivitiesK _s in the superconducting state andK _n in the normal state of (La_1–x RE_x) Al₂ single crystals (RE: Gd, Ce;x0.005) between 0.4 and 8 K. The lattice conductivity is small in the superconducting state above 0.5T _c and in the normal state and is independent of annealing. Annealing does, however, strongly depress the ratioK _s ^e /K _n ^e of the electronic thermal conductivities. We also report on a theory of the electronic thermal conductivity which describes the experimental results quantitatively. Interaction effects between impurities show up for the samples with the highest RE-concentration.Work performed within the research program of the Sonderforschungsbereich 125 Aachen/Jülich/Köln     

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.     

Relation between vortex excitation and thermal conductivity in superconductors

Thermal conductivity κ _xx(T) under a field is investigated in d _{x2 - y2}-wave superconductors and isotropic s-wave superconductors by the linear response theory, using a microscopic wave function of the vortex lattice states. To study the origin of the different field dependence of κ_xx(T) between higher and lower temperature regions, we analyze the spatially-resolved thermal conductivity around a vortex at each temperature, which is related to the spectrum of the local density of states. We also discuss the electric conductivity in the same formulation for a comparison. Received 8 December 2001 and Received in final form 20 March 2002 Published online 6 June 2002     

Universal intrinsic doping behavior of in-plane dc conductivity for hole-doped high-temperature cuprate superconductors

Understanding the normal state transport properties in hole-doped high-temperature cuprate superconductors (HTCSs) is a challenging task which has been widely believed to be one of the key steps toward revealing the pairing mechanism of high-temperature superconductivity. Here, we present a true intrinsic and universal doping dependence of in-plane dc conductivity for all underdoped HTCSs. The doping dependence of in-plane dc conductivity normalized to that at optimal doping can be represented by a simple exponential formula. The doping behavior of the square of the nodal Fermi velocity derived by the high-resolution laser-based angle-resolved photoemission spectroscopy in the superconducting state follows reasonably well the universal intrinsic doping behavior. Our findings suggest a commonality of the low-energy quasiparticles both in the normal and superconducting states that place a true universal and stringent constraint on the mechanism of high-temperature superconductivity for HTCSs.     

Electronic inhomogeneity and breakdown of the universal thermal conductivity of cuprate superconductors

We report systematic, high-precision measurements of the low-T (down to 70 mK) thermal conductivity kappa of YBa2Cu3O(y), La(2-x)Sr(x)CuO4, and Bi2Sr2CaCu2O(8+delta). Careful examinations of the Zn- and hole-doping dependences of the residual thermal conductivity kappa0/T, as well as the in-plane anisotropy of kappa0/T in Bi2Sr2CaCu2O(8+delta), indicate a breakdown of the universal thermal conductivity, a notable theoretical prediction for d-wave superconductors. Our results point to an important role of electronic inhomogeneities, which are not considered in the standard perturbation theory for thermal conductivity, in the underdoped to optimally doped regime.     

Low-temperature behavior of the thermal conductivity in pure superconductors with anisotropic pairing

The effect of the degeneracies of the nodes (which can be different for fixed positions of the zeros for pairing of the same symmetry type) of the superconducting order parameter on the low-temperature behavior of the thermal conductivity in pure superconductors is examined. It is shown that the recent experimental data on the anisotropy of the thermal conductivity of UPt₃ at low temperatures [B. Lussier, B. Ellman, and L. Taillefer, Phys. Rev. Lett. 73, 3294 (1994) and Preprint (cond-mat/9504072) (1995)] can be explained qualitatively on the basis of both two-dimensional representations E _1g and E _2u of the hexagonal group D _6h . Pis’ma Zh. éksp. Teor. Fiz. 63, No. 4, 276–280 (25 February 1996)     

Group-theoretical analysis of possible structural phase transitions in the high-temperature superconductors

A group-theoretical analysis is performed for the complete condensation of order parameters at structural phase transitions (SPT's) in the high-temperature superconductors belonging to the D ¹⁷ _4h -14/mmm space group in the high-symmetry phase. As a rule, such transformations are due to a successive softening of phonons with wave vectors k ₁ = 1/2 b ₃ and k ₂ = 1/2 (b ₁ - b ₃) belonging to the K13(X) star of the Brillouin zone of a tetragonal body-centered Bravais cell. SPT's in system La_2-x Ba _x CuO₄ are considered in detail.