摘要:

The studies of electronic transport in the cuprates have played a key role in the attempt to elucidate the mechanism of high temperature superconductivity. In this paper we will provide an overview of the experiments probing the anisotropic nature of the transport in high‐Tcsuperconductors and discuss the implications for our understanding of the normal‐state of these materials.

ISSN:0094-243X    

DOI:10.1063/1.44738

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The normal state carriers give information about the charge carriers and how they are scattered. Measurements on a number of highTcsuperconducting oxides indicate unusual transport properties; as well as these observations, the change to more conventional behavior as doping increases needs to be explained. The in‐plane resistivity &rgr;abis linear in temperature for the materials with the highestTcover a surprisingly broad range; overdoping, however, leads to aT2temperature dependence. The in‐plane thermopowerSabfor Bi‐ and Tl‐based highTcsuperconductors is linear in temperature but with a positive extrapolated (T=0) intercept. In the overdoped limit this intercept approaches zero, and the thermopower shows the Mott temperature dependence,Sab∝T, characteristic of diffusion thermopower. The Hall coefficient in these materials is also remarkable. In a large number of highTcsuperconducting families, a ‘‘universal’’ behavior of the Hall angle, &thgr;H, has been reported: cot &thgr;H=aT2+C. In YBa2Cu3O7−&dgr;, the presence of chains leads to anisotropy of the resistivity and thermopower even in theabplane directions.

ISSN:0094-243X    

DOI:10.1063/1.44739

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

I discuss recent developments in the application of fractional statistics to condensed matter physics. I emphasize Haldane’s recent and novel definition of fractional statistics, partly in order to discuss some very recent work involving this concept, and partly in order to highlight a promising direction for future work. The applicability of both the conventional definition (in terms of exchange phase) and Haldane’s ‘‘fractional exclusion principle’’ are examined for the charged excitations (quasiparticles) of the fractional quantum Hall effect (FQHE). Subsequently, the viability of ‘‘anyon superconductivity’’ (which is defined solely in terms of the former definition) is critically examined in the light of recent experiments. The possible utility of Haldane’s definition for the HTSC is briefly discussed.

ISSN:0094-243X    

DOI:10.1063/1.44740

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Ultrasonic attenuation and velocity measurements have been performed on melt‐textured YBa2Cu3O7. In low magnetic field sweeps at constant temperatures, one sees sharp changes in both the velocity and the attenuation at the lower critical field. The attenuation jump per vortex can be fit to a Debye relaxation formula and the temperature dependence of the relaxation time in the superconducting state can be determined. The final results appear to be consistent with BCS energy gap &Dgr;(0)=1.76 kBTC.

ISSN:0094-243X    

DOI:10.1063/1.44742

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We provide a review for the recently observed even‐denominator fractional quantum Hall effect in GaAs quantum well structures. We argue that the &ngr;=1/2 fractional quantum Hall state observed in two recent experiments is the theoretically predicted 331 state, which is a two‐component generalization of the well‐known Laughlin incompressible state for the single two‐dimensional layer. Our calculations show that the 331 state exhibiting &ngr;=1/2 fractional quantum Hall effect is stabilized in a double quantum well structure due to a competition between intrawell and interwell Coulomb correlations and this state is robust in the presence of some interwell electron tunneling.

ISSN:0094-243X    

DOI:10.1063/1.44743

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The metallic phase in the Quantum Hall Regime is of fundamental importance not only in the theory of the Quantum Hall Effect but also in the field of disorder induced metal‐insulator (Anderson) transitions in general. We give a brief overview of some of the more advanced experimental studies of the critical aspects of this phase. We also propose a new generation of experiments, based on the idea of strong mesoscopic conductance fluctuations near criticality, which could possible resolve many of the outstanding experimental and theoretical questions which mostly relate to the nature of delocalization in strongly disordered systems.

ISSN:0094-243X    

DOI:10.1063/1.44744

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The Wigner‐Dyson distribution of level spacings plays a central role in the classification of quantum chaotic systems. However, this description provides no information about the dispersion of the energy levels,Ei(X) in response to an external perturbation. We demonstrate that the response depends on the detailed properties of the system only through mean‐level spacing, and a conductance which we define. We derive a non‐perturbative expression for the autocorrelator of density of states fluctuations for systems taken from orthogonal and unitary ensembles, showing that the dependence on these two parameters can be removed by a rescaling. We argue that this description applies quite generally to arbitrary perturbations, and that after rescaling the statistical properties of the random functions,Ei(X) become universal.

ISSN:0094-243X    

DOI:10.1063/1.44745

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The transport properties of 0.75 &mgr;m long insulating indium oxide wires and rings have been measured. These devices have no apparent tunnel barriers, yet they exhibit two properties which are characteristic of series arrays of small capacitance tunnel junctions: highly non‐linear IV characteristics and a zero‐bias conductance which is periodic in a voltage applied by means of a lateral gate. Two types of samples can be distinguished based on the behavior of the conductance oscillations at low temperatures. For the first type, the structure of the oscillations remains periodic down to our lowest temperatures, similar to the data from the tunnel junction arrays. For the second type, lowering the temperature results in a transition from periodic to quasi‐periodic conductance peaks. The temperature and magnetic field dependence of these effects suggest that they are due to the influence of single electron charging on transport through localized electron states in the indium oxide.

ISSN:0094-243X    

DOI:10.1063/1.44746

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The existence of mesoscopic conductance fluctuations invalidates the traditional concept of impurity average conductances even for metallic systems which are much larger than the inelastic scattering length. We use the methodology of classical conductance networks in order to incorporate the effect of mesoscopic conductance fluctuations in a self consistent, extended scheme for computing macroscopic conductances. Scaling ideas are reconsidered with a main emphasis on the 2D disordered electron gas and, in particular, the integral quantum Hall regime. We find that the singularity structure of the theories of ‘‘macroscopic’’ and ‘‘impurity averaged’’ conductances are quite generally the same. Significant numerical differences may occur, however, in the regime of strong quantum interference effects.

ISSN:0094-243X    

DOI:10.1063/1.44721

出版商:AIP    

年代:1992

数据来源: AIP

ISSN:0094-243X    

DOI:10.1063/1.44723

出版商:AIP    

年代:1992

数据来源: AIP