Field-Direction Dependence of the Upper Critical Field in Organic Superconductors

In this work, we investigate the anisotropy of the in-plane critical field in conventional and spatially modulated phases of layered superconductors within the quasi-classical approach, taking into account the interlayer Josephson coupling. We show that the anisotropy of the onset of superconductivity may change dramatically in the FFLO state as compared with the conventional superconducting phase.     

A Simple Fabrication of FeSe Superconductors with High Upper Critical Field

A simple and reproducible fabrication process for the synthesis of FeSe superconductors has been developed. The properties were characterized by X-ray diffraction, scanning electron microscope, superconductivity quantum interference device, and physical property measurement system. It showed a zero resistance superconducting critical temperature of ??7.9?K and an onset critical temperature of ??13.9?K. The upper critical magnetic field at zero temperature was estimated to be 31?T. The growth mechanism of FeSe flakes was also discussed.     

Van Hove Non-Fermi Superconductors and Critical Temperature Degradation

We analyzed the non-Fermi behavior of copper oxides superconductors with an energy-dependent density of states of the van Hove (logarithmic) form, using the model proposed by Anderson and Yin and Chakravarty. The critical temperature degradation, due to a small amount of non-magnetic impurities, is also analyzed. It is shown that the decrease of the critical temperature is not of the Abrikosov–Gorkov type.     

Van Hove Non-Fermi Superconductors and Critical Temperature Degradation

We analyzed the non-Fermi behavior of copper oxides superconductors with an energy-dependent density of states of the van Hove (logarithmic) form, using the model proposed by Anderson and Yin and Chakravarty. The critical temperature degradation, due to a small amount of non-magnetic impurities, is also analyzed. It is shown that the decrease of the critical temperature is not of the Abrikosov-Gorkov type.     

Introduction to Ginzburg-Landau and Gross-Pitaevskii Theories for Superconductors and Superfluids

An elementary overview is given of the most frequently used models in the theory of superfluidity and superconductivity. The Gross-Pitaevskii, Ginzburg-Landau and time-dependent Ginzburg-Landau equations are derived, and their conditions of applicability are discussed. Some applications are considered including the critical current and vortices.     

Temperature Dependence of the Upper Critical Field in bcc Solid 3He

The temperature dependence of the upper critical field for the antiferromagnetic high-field phase of bcc solid ³He has been calculated by Iwahashi and Masuda utilizing the Green-function method with Tyablikov decoupling. In the low temperature limit the upper critical field, H _c2(T), is found to decrease from H _c2(0) with increasing temperature as a power law with exponent 3/2. Interestingly, the same power law dependence has been predicted for a system of dilute magnons undergoing Bose–Einstein condensation and has been observed in the spin-gap antiferromagnets TlCuCl₃, BaCuSi₂O₆, and NiCl₂–4SC(NH₂)₂. An experiment has been attempted to determine precisely the temperature dependence of the upper critical field in bcc solid ³He by measuring the pressure of the solid during adiabatic demagnetization through the critical field. The theoretical framework motivating the experimental study is presented along with details of the experimental setup.     

Magnetotransport Studies of Bi-Based 2212 and 2223 High Critical Temperature Superconductors

When a magnetic field is superimposed on a superconducting sample the electric and temperature gradients allow one to observe phenomena called excess electrical resistivity, excess thermoelectric power, Nernst effect, and excess magnetothermal conductivity. Interesting parameter values can be obtained on vortex and quasi-particle scatterings in the mixed state of materials. For illustration we examine the case of several Bi-based 2212 and 2223 superconducting ceramics. A new (linear) relationship is found between the Ginzburg–Landau parameter and the vortex viscosity. A discussion is given on a new way to get some information on the order parameter symmetry from such transport properties through the analysis of the temperature-integrated excess quantity field dependence in the mixed state.     

Calculation of the Critical Field and the Nernst Effect in Cuprate Superconductors

A recent experiment which measured the Nernst effect, the diamagnetic signal and the H _c2 field was interpreted as a support to the scenario which the pseudogap has the pair condensate without long-range phase coherence. We present here calculations which qualitatively reproduces the onset of the Nernst signal temperature T _ν (ρ) and T _c (ρ) as function of the doping level ρ. Together with our previous calculations to the magnetization and H _c2, we conclude that the phase separation scenario supports also the new Nernst effect experiments. This work has been partially supported by CAPES and CNPq.     

Upper Critical Field and Specific Heat of Cuprates

A representative set of magnetotransport measurements in novel superconductors is analyzed. The resistive upper critical field, H _c2 (T) of many cuprates, of superconducting spin-ladders, and organic (TMTSF)2X systems has a universal nonlinear temperature dependence H _c2 (T_c – T)^3/2 in a wide temperature interval near T _c, while its low-temperature behavior depends on the chemical formula and sample quality. The unusual H _c2(T) is described as the Bose–Einstein condensation field of preformed pairs. Its universal temperature dependence follows from the scaling arguments. Controversy in the determination of H _c2 (T) from the resistivity and specific heat measurements is resolved in the framework of the charged Bose-gas model with the impurity scattering. It is shown that specific heat shows two anomalies. The high-temperature anomaly is strong and shows only weak shift with applied field. The low-temperature anomaly corresponds to resistive transition and is very weak in agreement with the experiments. Both anomalies coincide at H = 0.     

Time-Dependent Ginzburg-Landau Equation for d_{x^2 - y^2 } -wave Superconductors: Hall Effect in the Low Field Regime

The Hall effect is studied by using a phenomenological time-dependent Ginzburg-Landau equation with mixing of s-and d-wave components in -wave superconductors within a low field regime. An equation of motion for a single vortex is derived and the Hall angle is obtained under an external driving current along the crystal axis. We find that not only the imaginary parts of s- and d-wave relaxation time but also the mixed gradient terms may change the sign of the Hall effect.     

Low Temperature Value of the Upper Critical Field H c2(0) of Isotropic Single Band Metals

The strong coupling Schossmann–Schachinger theory for the upper critical field is considered methodically from the user point of view. We check the accuracy of approximate formulas for the upper critical field. In particular, we explain in detail how a convenient expression recently proposed by Shulga et al.(11) can be justified. The connection of H _c2(0) with the critical temperature T _c is considered and the similarity with the Allen–Dynes T _c-formula is shown explicitly.     

Anisotropic Upper Critical Field in Single Crystal MgB2

Recent experiments on MgB₂ single crystals appear to indicate that superconductivity in this material is described not only by two superconducting order parameters attached to the band and the band, respectively, but also these two order parameters have different anisotropy. More explicitly the anisotropy of H _c2(t) requires an oblate order parameter in momentum space attached to the a band while H _c1(t) is described by a prolate order parameter, attached to the band. Therefore the vortex state in MgB₂ should be described by an interplay of these two superconducting order parameters.     

Self-Organization of the Critical State in Granular Superconductors

We study the critical state of a one-dimensional multijunction SQUID with a random arrangement of junctions in an increasing magnetic field. Using two mathematical models (system of differential equations for gauge-invariant phase differences and a simplified algorithm), we show that the system demonstrates a self-organized behavior. An intrinsic spatial randomness introduced into the model allows to obtain self-organization in one-dimensional case under fully deterministic perturbation. We also show that our simplified algorithm represents a new model of a self-organized criticality.     

A Possible Explanation of Critical Temperature and Isotope Effect Coefficient of High T c Cuprate Superconductors

The expressions for superconducting transition temperature (T _c) and isotope effect coefficient (α) have been derived from a generalized integral gap equation for a strongly coupled superconductor, when electrons–phonons and electrons–biexcitons are simultaneously present in high-T _c cuprate superconductors.     

Critical Current Anisotropy of Tl-1223 Superconductors

A comparative study of the critical current anisotropy of Tl-1223 conductors manufactured by different techniques, prepared from different compositions and originating from different research laboratories is presented and discussed. For highly oriented layers, with the magnetic field (B) parallel to the ab plane, the normalised critical current density (j) versus magnetic field characteristic shows very little drop of j value due to the intrinsic properties of the Tl-1223 phase, while all powder-in-tube conductors show a rapid drop of j value in fields as low as 10 mT. For all powder-in-tube conductors the anisotropy coefficient k shows a very pronounced minima at fields up to 30 mT followed by a monotonic decrease of anisotropy with increase of the magnetic field. This is due to poor grain alignment and a demagnetising effect in the powder-in-tube Tl-1223 tapes.     

Upper Critical Magnetic Field for a Modified Maltifiamental Nb3A1Wire

We measured the upper critical field Μ₀H_c2 for a modified Nb_3A.l sample above 10 K and evaluated the values using a different criterion. The values from theI ^c measurement, converting from the ac susceptibility and direct judging from the transition point on the magnetization curve, were close and larger than that extrapolated from the magnetization basedon the theory of Kramer and the critical state model.     

Scaling of the Resistance and Magnetization for High Temperature Superconductors under Pressure

The scaling relation of single parameter scaling hypothesis is applied to the study of the scaling behavior of high temperature superconductors under pressure. The data of resistance and magnetization under various pressures are scaled onto a universal curve according to this scaling relation. The scaling parameters are pressure dependent while temperature independent. It is found that the controlling parameter B _i equals the relative critical temperature t _cP , which indicates that the superconducting energy gap at the zero temperature 2Δ _s0 is the controlling parameter in this scaling.