Spin triplet pairing in high temperature superconductors

Various superconductivity mechanisms show that the electron pair is in spin singlet (S=0) belowTc. In contrast with these, our EPR measurements show that the electron pair is in spin triplet (S=1) rather than the spin singlet. Since an intense zero field absorption was observed in the high temperature superconductors of YBa₂Cu₃O_7–y , and YBa₂Cu₃O_7–y F₁, which violated various spin-pairing singlet superconductivity mechanisms.     

Local density of states around two nonmagnetic impurities in cuprate superconductors

The local density of states (LDOS) around two nonmagnetic impurities which are located at different sites is studied within the two-dimensional t-J-U model. The order parameters are determined in a self-consistent way with the Gutzwiller projected mean-field approximation and the Bogoliubov-de Gennes theory. When the two impurities are located one or two sites away, we find the superconductivity coherence peaks are more strongly suppressed and the zero-energy peak (ZEP) has split into two peaks. Whereas when the two impurities are located next to each other, the ZEP vanished, and LDOS does not change a lot compared with the case away from the impurities.     

States of local moment induced by nonmagnetic impurities in cuprate superconductors

By using a model Hamiltonian with d-wave superconductivity and competing antiferromagnetic (AF) orders, the local staggered magnetization distribution due to nonmagnetic impurities in cuprate superconductors is investigated. We show that the net moment induced by a single impurity corresponds to a local spin with S(z)=0 or 1/2 depending on the strength of the AF interaction U and the impurity scattering strength epsilon. Phase diagram of epsilon versus U for the moment formation is presented. We discuss the connection of this result with the Kondo problem. When two impurities are placed at the nearest neighboring sites, the net moment is always zero, unusually robust to parameter changes. For two neighboring strong impurities, separated by a Cu-ion site, the induced net moment has S(z)=0, 1/2, or 1.     

Influence of nonmagnetic impurities on the kondo effect

The influence of scattering by nonmagnetic impurities is studied in perturbation theory. While the finite lifetime of the electrons in intermediate states due to scattering by nonmagnetic impurities does not lead to a change in the logT-behaviour of the third-order self-energy, certain vertex-corrections give rise to an additional term which varies like 1/√T at low temperatures. Similar correction terms are found to occur in the higher order self-energy contributions. Although these terms diverge more strongly atT=0 than the logarithmic contributions they are quite small at finite temperatures since they depend on the lifetime τ of the electrons through a factor of (? _F τ)^?5/2 (? _F Fermi energy). The possibility of observing these interference effects experimentally is discussed.     

Nature of pairing in high Tc oxide superconductors

In connection with the unique structural features of the recently discovered high T_c YBa₂Cu₃O₇ and (La,Ba)₂CuO₄ we show that in such a multi-narrow-band system the opposite-spin interband pairs can function concurrently as both antiferromagnetic real pairs in x-space and Cooper pairs in k-space. So, contrary to the usually depairing Coulomb pseudotential μ¹ considered in traditional superconductors, the intra-atomic Coulomb repulsive interactions in a multiband system result in a positive contribution to Cooper pairing and enhance the conventional boson-mediated (electron-phonon and/or electron-exciton) superconductivity.     

Transition temperature of anisotropic superconductors with kondo impurities

The Matsuura-Ichinose-Nagoako (MIN) model for Kondo impurities in a superconductor has been generalized to include the effect of gap anisotropy. Analytic results are obtained for the change in the transition temperature. It is shown that, in appropriate limits, one recovers the recent results of Schachinger and Carbotte and Okabe and Nagi. The resulting formulae yield corrections to those of MIN in the BCS isotropic model.     

Effect of nonmagnetic impurities on the critical temperature of the itinerant-electron antiferromagnet

The effect of nonmagnetic impurities on the critical temperature of the itinerant-electron antiferromagnet is investigated using a Fedders-Martin model with an energy dependent density of states.It is found that nonmagnetic impurities can increase the critical temperature T_N if the density of states is changed by the impurities. This result is in agreement with the experimental results for CrRu alloys.     

Magnetic response of nonmagnetic impurities in cuprates

A theory of the local magnetic response of a nonmagnetic impurity in a doped antiferromagnet, as relevant to the normal-state in cuprates, is presented. It is based on the assumption of the overdamped collective mode in the bulk system and on the evidence that equal-time spin correlations are only weakly renormalized in the vicinity of the impurity. The theory relates the Kondo-like behavior of the local susceptibility to the anomalous temperature dependence of the bulk magnetic susceptibility.     

Tandem pairing in heavy-fermion superconductors

We consider the internal structure of a d-wave heavy-fermion superconducting condensate, showing that it necessarily contains two components condensed in tandem: pairs of quasiparticles on neighboring sites and composite pairs consisting of two electrons bound to a single local moment. These two components draw upon the antiferromagnetic and Kondo interactions to cooperatively enhance the superconducting transition temperature. This tandem condensate is electrostatically active, with an electric quadrupole moment predicted to lead to a superconducting shift in the nuclear quadrupole resonance frequency.     

铜氧化物高温超导体中的电子有序态

铜氧化物高温超导现象自30年前被发现以来,对现代凝聚态物理的发展产生了极其重要的影响,然而其微观机制至今依然是一个谜。近年来,多种实验手段的研究结果发现,在铜氧化物高温超导体中电子除了形成库珀对,还可能形成多种其他新奇的有序态,例如自旋有序态、电荷有序态以及库珀对密度波等。这些有序态的起源及其与赝能隙态和超导态的关联对于理解高温超导机理可能具有重要的意义。文章将主要从实验的角度对铜氧化物超导体中的电子有序态做一个概述。     

Andreev bound states in high temperature superconductors

Andreev bound states at the surface of superconductors are expected for any pair potential showing a sign change in different k-directions with their spectral weight depending on the relative orientation of the surface and the pair potential. We report on the observation of Andreev bound states in high temperature superconductors (HTS) employing tunneling spectroscopy on bicrystal grain boundary Josephson junctions (GBJs). The tunneling spectra were studied as a function of temperature and applied magnetic field. The tunneling spectra of GBJ formed by YBa₂Cu₃O (YBCO), Bi₂Sr₂CaCu₂O(BSCCO), and La_1.85Sr_0.15CuO₄ (LSCO) show a pronounced zero bias conductance peak that can be interpreted in terms of Andreev bound states at zero energy that are expected at the surface of HTS having a d-wave symmetry of the order parameter. In contrast, for the most likely s-wave HTS Nd_1.85Ce_0.15CuO_4-y (NCCO) no zero bias conductance peak was observed. Applying a magnetic field results in a shift of spectral weight from zero to finite energy. This shift is found to depend nonlinearly on the applied magnetic field. Further consequences of the Andreev bound states are discussed and experimental evidence for anomalous Meissner currents is presented. Received: 17 February 1998 / Revised: 27 April 1998 / Accepted: 23 June 1998     

Coherence transition in granular high temperature superconductors

We have studied experimentally the electrical conductivity and specific heat near the superconducting transition of granular samples of YBa₂Cu₃O_7−δ, YBa₂(Cu_2.98Zn_0.02)O_7−δ and GdBa₂Cu₃O_7−δ. The results show that the transition proceeds in two stages. Careful analysis of the conductivity in the regime of approach to the zero resistance state reveals the occurrence of a coherence transition, which is related to the connective nature of the granular samples. This transition occurs when the fluctuating phases of the order parameter in individual grains become long-range ordered. We obtain the exponent for fluctuation conductivity and the relevant critical temperature, T_co, which is close to the point where resistivity vanishes. The specific heat results, when analyzed as dC/dT, show a weak but reproducible cusp-like anomaly at T_co. This finding gives strong support to the interpretation of the coherence transition as a genuine critical phenomenon.     

Cu,Pu and Fe high Tc superconductors: Spin holes in anti-ferromagnetic clusters form nonmagnetic bipolarons

The new iron based high T_c superconductors with T_c up to 55 K have stirred new interest in this field. It is consensus that the BCS mechanism is not able to explain the high T_c’s. In the following we propose that spin holes in anti-ferromagnetic clusters combine to make nonmagnetic bipolarons, which can condense and lead to superconductivity.     

Zinc impurities in d-wave superconductors

We study the two-dimensional Hubbard model with nonmagnetic Zn impurities modeled by binary diagonal disorder using quantum Monte Carlo within the dynamical cluster approximation. With increasing Zn content we find a strong suppression of d-wave superconductivity and an enhancement of antiferromagnetic spin correlations. T(c) vanishes linearly with Zn impurity concentration. The spin susceptibility changes from pseudogap to Curie-Weiss-like behavior indicating the existence of free magnetic moments in the Zn doped system. We interpret these results within the resonating-valence-bond picture.