The upper critical field and dimensional crossover in superconductors with anisotropic electron spectrum

The theory of superconductivity in metallic superstructures with anisotropic electron spectrum is presented. It is shown that these structures, three-dimensional in the normal state, become layered or fibrous in the superconducting state. The upper critical magnetic field changes the temperature dependence in the vicinity of the critical temperature from one (two)-dimensional to three-dimensional.     

Vortices in anisotropic type-II superconductors

We present a generalized variational model of an isolated vortex, valid in the anisotropic case when the external magnetic field is along one of the symmetry directions. We discuss the effects of the field anisotropy on the core and magnetic pinning of individual vortices.     

Pinning in superconductors with anisotropic defects

By calculating the volume pinning force in superconductors with anisotropic pinning centers we have shown that the critical current densitiesj _c in NbTi by both current and magnetization measurements can be explained. The critical current density in fields parallel toj _c can also be included. We conclude therefore that in superconductors with strong pinning centers the critical state is always determined by the equilibrium condition between the Lorentz force and the volume pinning force and not by the instability of the force-free configuration. By detailed investigation of thej _c dependence on field direction, useful information can be obtained with respect to the volume and surface contributions to the pinning force.     

Vortex structure in high-T c ceramic superconductors

We consider penetration of magnetic flux in the rare earth ceramic superconductors within the framework of a generalized Ginzburg-Landau theory, based on the coexistence of antiferromagnetism and superconductivity. We study vortex structure produced for a magnetic field close to . We make a new prediction that may serve to test the theory experimentally.     

Viscous flux motion in anisotropic magnetic superconductors

Magnetic field penetrates in the form of flux lines or vortex thread into type-II magnetic superconductors (MSC) and induces magnetization of magnetic subsystem over a distance of an order of the London penetration depth surrounding the normal cores. When a flux line moves by, surrounding magnetization moves as a whole through the sample and a free motion of vortices is subjected to magnetic viscous drag, giving rise to dissipation. The flux flow resistance in the mixed state of anisotropic MSC has been studied on the basis of the London theory. Expressions for the dissipation and viscosity coefficient associated with the change of the magnetic subsystem as a vortex moves about are derived.     

Vortex dynamics and decay of supercurrent in 2-d superconductors

The decay of supercurrent in a 2-d superconductor through the homogenous quantum nucleation of vortex-antivortex pairs is formulated as a (2+1)-d scalar quantum electrodynamic problem. The calculation of the quantum decay rate can then be simplified to the usual single particle WKB calculation. We show explicitly that the homogeneous quantum nucleation of vortex-antivortex pairs is incompatible to the existence of the magnus force in the absence of dissipation, and point out more complicated nucleation processes needed to be studied by the vortex dynamics method in such case.     

Vortex fluctuations in two-dimensional superconductors above and below the Berezinskii-Kosterlitz-Thouless temperature

The vortex-unbinding transition in two-dimensional superconductors is revisited. The approach presented here is based on a Thomas-Fermi-type ansatz and allows to calculate the vortex density and the screened interaction below and above the transition temperature within a unified formalism.     

On the critical temperature of anisotropic superconductors

An approximate analytic formula for the increase in the superconducting critical temperature caused by anisotropy is derived and compared with both the results of exact numerical solutions of the Eliashberg equations and with a previous approximate formula due to Allen and Butler. In all the cases considered our new formula is found to be more accurate than the previous one and to agree reasonably well with the exact results for the intermediate and weak coupling superconductors considered.Research supported by the Natural Sciences and Engineering Research Council of Canada.     

Vortex motion in type II superconductors with controllable type defects

A model that connects the nonlinearity of the voltage-current characteristicsV(I) in the mixed state with the statistical distribution of the pinning forces acting on moving vortices is considered. The role of the elastic properties of the vortex lattice in theV(I) behavior is discussed. A method for obtaining the statistical distribution parameters of the pinning force from experimentalV(I) data is considered, based on experimental data onV(I) dependences in Al-Ag. In alloys with controllable defects, the distribution parameters of the pinning force can be found and connected with typical parameters of the defect structure of the specimen.     

Low-temperature properties of anisotropic superconductors with kondo impurities

We present a self-consistent theory of superconductors in the presence of Kondo impurities, using large-N slave-boson methods to treat the impurity dynamics. The technique is tested on the s-wave case and shown to give good results compared to other methods forT _K >T _c . We calculate low temperature thermodynamic and transport properties for various superconducting states, including isotropic s-wave and representative anisotropic model states with line and point nodes on the Fermi surface.     

Vortex glass state in superconductors with fractal clusters of normal phase

The influence of fractal clusters of a normal phase on the dynamics of vortices in a percolation superconductor is considered. It is established that a vortex glass state is formed in superconductors with a fractal cluster structure. The vortex glass phase can be identified by the initial part of its current-voltage characteristic. The dependence of the glassy exponent on the fractal dimension of normal phase cluster boundaries is found.     

Critical magnetic field in layered superconductors

The nature of the dependence H _c2(T) is greatly affected by the presence of magnetic impurities and their ordering at low temperatures. The scattering leads to a change in the sign of the curvature, and consequently to an increase in the value H _c2. The theory allows one to explain recent experimental data with the overdoped cuprates which exhibits behavior that is drastically different from the conventional picture. The calculations are in excellent agreement with the data.The authors are grateful to A. Mackenzie, J. Cooper, and S. Wolf for fruitful discussions. One of us (YNO) wishes to acknowledge the support of the Humboldt Foundation. The research of VZK is supported by the U.S. Office of Naval Research under contract no. N00014-94-F0006.     

On the BCS theory of paramagnetic impurities in anisotropic superconductors

We develop a theory based on BCS theory which treats the effect of paramagnetic impurities in an anisotropic superconductor. It is found that in a separable model for the order parameter its anisotropy is expected to increase with increasing concentration of paramagnetic impurities. The critical temperature, the order parameter, and the quasiparticle density of states are computed numerically from our theoretical results. We find that there exists a regime which is superconducting by virtue of the fact that the paramagnetic impurities make the material highly anisotropic. In this regime the critical temperature can be reduced rapidly to zero by adding dilute concentrations of normal scattering impurities.Research supported by the Natural Sciences and Engineering Research Council of Canada.This paper is dedicated on the occasion of the 70th birthday of Prof. Dr. E. Ledinegg, head of the Institut für Theoretische Physik, Technische Universität, Graz, Austria.     

Magnetic field effects in thick proximity-induced superconductors

Low-field magnetic screening and breakdown fields have been measured in thick proximity-induced superconducting copper by means of dc magnetization and ac susceptibility in the temperature range 10 to 0.005 K. From the thickness and temperature dependence of the breakdown field the Cooper pair penetration depth in copper is obtained. This value isK _N ^–1 =0.6/T m, withT in kelvins. No saturation effects were observed in the screening length , the breakdown fieldH _b, or the supercooled fieldH _sc down to the lowest temperatures.     

各向异性单晶硅中声波时间反转自适应聚焦

1引言 在各向异性介质中,声波的传播速度与方向有关,这种传播特性会使聚焦声束产生散焦,研究如何克服这种因为传播速度各向异性而造成的相位畸变,具有重要的理论和实际意义.时间反转法(TR,time reversal)是一种新颖的聚焦方法,不需要了解介质的性质和换能器阵的结构就可以实现自适应聚焦.在时间反转处理过程中,用时间反转阵接收声波,并进行存储、时间反转后再发射,这时换能器阵各个阵元激发的声波将同时同相到达原声源处,实现自适应聚焦.近年来,这种聚焦方法引起越来越多的关注,Fink等在TR方法方面开展了大量而深入的研究,从理论上对时间反转法进行了论证,并开展了相应的实验研究.我们实验室也开展了水下波导介质和各向异性介质中时间反转法的研究,研制了一套数字式多通道超声成像系统.各向异性介质中声波时间反转法研究相当少见,在本研究中,选用本征单晶硅作为各向异性介质,研究时间反转法的自适应聚焦特性.     

Vortex motion in high-T c superconductors near the upper critical fieldH c2

The vortex dynamics in layered superconductors such as YBaCuO and BiSrCaCuO near the upper critical field H_c2 is considered. The magnetic field is parallel to the layers and a transport current flows along the layers perpendicular toH. There exists a finite pinning force for the vortex motion across the layers in a macroscopically homogeneous superconductor. The critical current vanishes exponentially in the limit of weakly layered structure, _c (T)s. The current-voltage characteristic is calculated in case _c (T)s. The initial part of the characteristic may have a negative slope depending on the magnetic fieldH.     

Electromagnetic behavior of superconductors in a microwave field

Our research on the surface impedance of superconductors in microwave, millimeter, and submillimeter wave frequency ranges has revealed the nature of the interaction between microwave and superconductors. The complete solution to the Mattis-Bardeen (MB) quadruple [Phys. Rev. 111, 412 (1958)] and the analytical simplification of the MB formula to a double integral have been given. The theoretical calculations for metallic superconductors (Al and Sn) are in excellent agreement with the typical experimental data [Biondi and Garfunkel,Phys. Rev. 116, 853 (1959); S. Sridhar,J. Appl. Phys. 65, 159 (1988)]. Whereas for oxide superconductors a new microscopic model has been suggested because of its distinctive electromagnetic behavior compared to that of conventional superconductors, we think that the nonlinear effect caused by the strong anisotropy in superconductivity is one of the reasons why oxide superconductors give an unusual response.     

Upper critical field and isotope effect in the oxide superconductors

In a model for which the pairing is due mainly to an electronic mechanism supplemented by a small phonon contribution, we study the isotope effect exhibited by the upper critical magnetic field H _c2 and compare with the critical temperature. When paramagnetic impurities are added, we find very different behaviours from that predicted in a pure phonon model. The alignment of the paramagnetic spin by the external magnetic field as well as Pauli limiting is neglected in our calculations.