Mixed State Of high-Tc Superconductors Research Articles

Scaling of the Equilibrium Magnetization in the Mixed State of Type-II Superconductors

We discuss the analysis of mixed-state magnetization data of type-II superconductors using a recently developed scaling procedure. It is based on the fact that, if the Ginzburg-Landau parameter κ does not depend on temperature, the magnetic susceptibility χ(H, T) is a universal function of H/Hc2(T), leading to a simple relation between magnetizations at different temperatures. Although this scaling procedure does not provide absolute values of the upper critical field Hc2(T), its temperature variation can be established rather accurately. This provides an opportunity to validate theoretical models that are usually employed for the evaluation of Hc2(T) from equilibrium magnetization data. In the second part of the paper we apply this scaling procedure for a discussion of the notorious first order phase transition in the mixed state of high-Tc superconductors. Our analysis, based on experimental magnetization data available in the literature, shows that the shift of the magnetization accross the transition may adopt either sign, depending on the particular chosen sample. We argue that this observation is inconsistent with the interpretation that this transition always represents the melting transition of the vortex lattice.

Scaling of the equilibrium magnetization in the mixed state of type-II superconductors

We discuss the analysis of mixed-state magnetization data of type-II superconductors using a recently developed scaling procedure. It is based on the fact that, if the Ginzburg-Landau parameter κ does not depend on temperature, the magnetic susceptibility χ(H,T) is a universal function of H/Hc2(T), leading to a simple relation between magnetizations at different temperatures. Although this scaling procedure does not provide absolute values of the upper critical field Hc2(T), its temperature variation can be established rather accurately. This provides an opportunity to validate theoretical models that are usually employed for the evaluation of Hc2(T) from equilibrium magnetization data. In the second part of the paper we apply this scaling procedure for a discussion of the notorious first order phase transition in the mixed state of high-Tc superconductors. Our analysis, based on experimental magnetization data available in the literature, shows that the shift of the magnetization accross the transition may adopt either sign, depending on the particular chosen sample. We argue that this observation is inconsistent with the interpretation that this transition always represents the melting transition of the vortex lattice.

Equilibrium magnetization in the vicinity of the first-order phase transition in the mixed state of high-Tcsuperconductors

We present the results of a scaling analysis of isothermal magnetization M(H) curves measured in the mixed state of high-Tc superconductors in the vicinity of the established first order phase transition. The most surprising result of our analysis is that the difference between the magnetization above and below the transition may have either sign, depending on the particular chosen sample. We argue that this observation, based on M(H) data available in the literature, is inconsistent with the interpretation that the well known first order phase transition in the mixed state of high-Tc superconductors always represents the melting transition in the vortex system.

Quasiparticle Heat Transport in Mixed State of High-TcSuperconductors

The field dependence of low-temperature thermal conductivity $\kappa(H)$ observed on cuprates is explained by calculating $\kappa(H)$ microscopically. The heat current carried by low-lying quasiparticles around a vortex core decreases with $H$ due to its depletion by the $H$-induced moment centered at a core for an underdoped case. This leads to a common and consistent picture on an ``anomalous core''. $\kappa(H,T)$ is found to be a useful tool for probing quasiparticle structures in the mixed state in general.

The vortex morphology model of ultrasonic attenuation in the mixed state of high-Tc superconductors

This paper presents the vortex morphology model concerning the ultrasonic attenuation in the mixed state of high-Tc superconductor. This model pointed out that under different magnetic field and temperature, different vortex morphology appeared in the high-Tc superconductors. There are three factors that can determine ultrasonic attenuation, which are temperature, magnetic field and anisotropy of superconductors’ structures.

Amplitude dependence of AC response in high Tc superconductors

The response of the vortex system in the mixed state of high-Tc superconductors to ac field is discussed using a new materials equation for type-II superconductors. We compare the amplitude dependence of ac susceptibility derived from this equation with experimental results of high critical current in YBCO materials.

The Pinning Force and the Flux Velocity in the Mixed-State Hall Effect

Using the single flux motion model of Wang, Dong, and Ting (WDT) and taking into account both the pinning and thermal fluctuation, we perform a numerical study to investigate the dependence between the average pinning force F-p and the flux velocity vL in different temperature regions in the mixed-state of high-Tc superconductors. The numerical simulation shows that there does not exist a unified scaling relation between F-p and vL in the whole flux flow temperature region, but F-p and vL keep a linear dependence in the lower temperature region of the negative Hall effect. The variations of the magnetic field or the pinning concentration do not change this linear relation. Employing this linear relation and WDT's theory, we naturally give the scaling law of ρxy ∝ ρβxx with β ≈︂ 2. We also make a power fit for the data of F-p and vL in the high-temperature region of both the negative and the whole positive Hall effect. Finally, we obtain F-p ∼ v—νL with ν ≈︂ 1.5 to 2.0 for different magnetic fields and pinning concentrations.

ANOMALOUS HALL EFFECT IN WEAK PINNING HIGH-Tc SUPERCONDUCTORS

Taking into account both pinning and thermal fluctuation and employing the single flux flow model of Wang, Dong, and Ting (WDT), we have performed numerical study to investigate the flux motion in the weak pinning environment in the mixed-state of high-Tc superconductors. We find that there exist two kinds of weak pinning regime, one possesses low pinning concentration but strong individual pinning potentials, the other consists of high pinning concentration but weak individual pinning potentials for the double sign reversal of Hall resistivity ρxy. Moreover, our simulation and analysis suggest that the weak collective pinning effect may not be a good reason for the abnormal Hall effect. These results are well consistent with recent measurement of Bi2Sr2CaCu2O8 (BSCCO), Tl2Ba2CaCu2O8 (TBCCO) and HgBa2CaCu2O6 (HBCCO) and collective pinning theories for Hall effect.

Generation of ultrasonic waves by ac magnetic fields in the mixed state of high-Tc superconductors.

We discuss the interaction of a vortex array in a superconductor with sound oscillations, taking into account pinning forces between vortices and crystal displacements. The generation of ultrasound by an ac electromagnetic field acting on the vortex array is considered. We find that at low temperatures and in the linear regime the amplitude of the induced sound wave is proportional to the applied dc magnetic field and independent of pinning strength. We show how the subsequent multiple echo amplitudes allow one to obtain information on the Magnus force and viscosity of the vortex array.

Peltier effect in the mixed state of high-Tc superconductors

Abstract The Peltier and Seebeck effects in the mixed state of high-Tc superconductors are proportional to the resistivity due to flux motion. Therefore, both effects also show the broadening of the transition regime characteristic for these superconductors. The origin of the Peltier effect is discussed in detail, and the validity of the Thomson relation is confirmed, as expected.