Temperature Of high-Tc Superconductors Research Articles

Comparative analysis of the temperature dependences of the resistivity, pseudogap, and thermoelectric power in polycrystals YBa2Cu3O7–δ with a decrease in the density of charge carriers

Comparative analysis of the temperature dependences of resistivity ρ(T), excess conductivity σ′(T), pseudogap (PG) Δ*(T), and thermoelectric power S(T), measured on textured YBa2Cu3O7–δ (YBCO) polycrystals with different charge carrier density nf, depending on the level of doping with oxygen, modified by annealing, has been carried out. It is shown that for an optimally doped (OD) sample with Tc = 90 K (sample S1), σ′(T) near Tc is well described by the Aslamazov–Larkin (AL−3D) and Maki–Thompson (MT-2D) fluctuation theories, demonstrating 3D–2D crossover with increasing temperature. The crossover temperature T0 was used to determine the coherence length along the с axis, ξс(0). With a decrease in nf (samples S2 with Tc = 84 K and S3 with Tc = 80 K), the MT contribution is suppressed, and the σ′(T) dependence obeys the Lawrence–Doniach model, which is typical for samples with defects. The dependence Δ*(T) obtained for S1 has a form typical for OD single crystals of YBCO with a maximum at Tpair ∼114 K and a linear section descending to T01 ∼94 K, which limits the region of superconducting fluctuations above Tc. As nf decreases, the shape of Δ*(T) noticeably changes and becomes typical for YBCO films with a symmetric maximum at Tpair, which is the BEC–BCS transition temperature in high-Tc superconductors. As nf decreases, the slope S(T) changes from positive to negative, demonstrating a feature at the PG opening temperature T*. Accordingly, the dependence of S(T)/T on log T changes from linear to nonlinear, which indicates a change in the nature of interactions in the YBCO electronic subsystem with decreasing nf, since S/T ∼1/nf.

CRITICAL TEMPERATURE OF HIGH-Tc SUPERCONDUCTORS AND BOUNDARY CONDITIONS IN GINZBURG-LANDAU THEORY

New boundary condition for the order parameter in the Ginzburg-Landau theory is applied to the case of CuO 2 planes which are the main structural elements responsible for superconductivity in high-Tc superconductors. It was found that the order parameter in these superconductors is significantly depressed in the CuO 2 planes. As a result, this boundary condition to the GL equations is found to limit the critical temperature of high-Tc superconductors. Thus, in order to increase Tc of cuprate superconductors, the number of CuO 2 planes that are within a short distance of each other in unit cell or insulating properties of the layers located in the vicinity to the CuO 2 planes should be increased.

Internal structure of fluctuating Cooper pairs

ABSTRACT In order to obtain information about the internal structure of fluctuating Cooper pairs in the pseudogap state and below the transition temperature of high Tc superconductors, we solve the Bethe-Salpeter equation for the two-electron propagator in order to calculate a “pair structure function” gP that depends on the internal distance between the partners and on the center of mass momentum P of the pair. We use an attractive Hubbard model with a local potential for s-wave and a separable potential for d-wave symmetry. The amplitude of gP for small ρ depends on temperature, chemical potential and interaction symmetry, but the ρ dependence itself is rather insensitive to the interaction strength. Asymptotically gP decreases as an inverse power of ρ for weak coupling, but exponentially when a pseudogap develops for stronger interaction. Some possibilities of observing the pair structure experimentally are mentioned.

The variation of the transition temperature of high Tc superconductors by electric field effect

High T c superconductor is considered as an alternately stacked metal and insulating layers. The electric field effect in this layer superconductor is investigated by the Thomas-Fermi(TF) approximation for the screening of an applied electric field and in the frame of Ginzburg-Landau(GL) theory with the coupling between metallic layers for the transition temperature. The change of the transition temperature related to the coupling constant and applied voltage is derived for two and three CuO 2 layers. The transition temperature for more than three layers is also briefly discussed.

The ac loss near the glass-liquid transition temperature in high-Tc superconductors - an extended Bean model

The ac loss in high-Tc superconductors near the glass-liquid transition temperature Tg is studied numerically and theoretically. It is found that the ac loss in a slab sample under a dc bias magnetic field depends remarkably on the angular frequency, , which cannot be described by the usual critical state model using the critical current density Jc defined with the electric field criterion. However, it is shown that the ac loss including the flux flow loss is described quantitatively by the well known expression based on the Bean critical state model, where Jc is replaced by the effective critical current density Jce(). The applicability of the present expression is also discussed.

Fluctuation-induced diamagnetism below the critical temperature in high-Tcsuperconductors

We have calculated the fluctuation-induced diamagnetism (\ensuremath{\Delta}\ensuremath{\chi}) of the high-${T}_{c}$ superconductors below the critical temperature ${T}_{c}$ and in the weak-field region. The theoretical formulation estimates the relation between \ensuremath{\Delta}\ensuremath{\chi} and the reduced temperature.

Effect of gap anisotropy and plasmons on the critical temperature of high-Tc superconductors

Abstract We study the nature of the transition temperature of a high temperature superconductor with an anisotropic gap using the three square well model of electron-phonon, electron-electron, and electron-plasmon interactions. The critical temperature is calculated as a function of anisotropic parameter and interaction strengths. To show a variety of electron-plasmon coupling constant dependence, λpl, we present curves of transition temperature versus λpl with different values of anisotropic average. We see from these results that both plasmon and anisotropy play key roles in raising the critical temperature of the cuprates and when λpl increases the transition temperature is insensitive to anisotropy.

Simple balance technique for determining the transition temperature of high T c superconducting powders

A simple technique is described that permits the measurement of the transition temperature of high Tc superconductors in powder form. The method employs a modification of a single-pan balance and has sensitivity for measurements on powder amounts as low as a few mg.