Staggered currents in the mixed state

Antiferromagnetism and superconductivity in an electron-doped two-dimensional t-U-V model

The antiferromagnetism (AFM) and superconductivity (SC) in a t-U-V model for an electron-doped two-dimensional square lattice has been investigated with a mean field approximation. Mean field solutions with 2 × 2 unit structures have been sought. The major merit in this study is that all possible superconducting pairings having 2 × 2 unit structures are considered. It has been found that the spin states and orbitals in the superconducting pairing depend on the band filling, and AFM induces a staggered triplet pairing in a system with a d(x(2)-y(2)) orbital singlet SC (dSC). The ratio of the magnitude of the triplet pairing to the magnitude of the singlet pairing has been found to grow monotonically with the antiferromagnetic moment. However, this additional triplet may not have observable effects in many frequently carried out measurements. We have focused on band fillings higher than half-filled. Fundamental quantities including the density of states, local density of states, and single particle spectral weight have been discussed. This study may be related to the electron-doped high-temperature superconducting cuprates, where there have been experimental observations of AFM and SC coexisting, and the nature of the superconducting pairing has been controversial.

Redistributing Chern numbers of Landau subbands: tight-binding electrons under staggered modulated magnetic fields

We investigate the magnetotransport properties of electrons on a square lattice under a magnetic field with the alternate flux strength phi+/-Deltaphi in neighboring plaquettes. A new peculiar behavior of the Hall conductance has been found and is robust against weak disorder: if phi=(p/2N)2pi (p and 2N are coprime integers) is fixed, the Chern numbers of Landau subbands will be redistributed between neighboring pairs and hence the total quantized Hall conductance exhibits a direct transition by +/-Ne2/h at critical fillings when Deltaphi is increased from 0 up to a critical value Deltaphi_{c}. This effect can be an experimental probe of the staggered-flux phase.

Antiferromagnetic vortex core in Tl(2)Ba(2)CuO(6+delta) studied by nuclear magnetic resonance

Spatially resolved NMR is used to probe the magnetism in and around vortex cores of nearly optimally doped Tl(2)Ba(2)CuO(6+delta) (T(c)=85 K). The NMR relaxation rate T(-1)1 at the 205Tl site provides direct evidence that the antiferromagnetic (AF) spin correlation is significantly enhanced in the vortex core region. In the core region Cu spins show a local AF ordering with moments parallel to the layers at T(N)=20 K. Above T(N) the core region is in the paramagnetic state which is a reminiscence of the state above the pseudogap temperature (T(*) approximately 120 K), indicating that the pseudogap disappears within cores.

Mechanism of pseudogap probed by a local impurity

The response to a local strong nonmagnetic impurity in the pseudogap phase is examined in two distinctly different scenarios: phase fluctuation (PF) of pairing field and d-density-wave (DDW) order. In the PF scenario, the resonance state is generally double peaked near the Fermi level, and is abruptly broadened by vortex fluctuations slightly above the transition temperature. In the DDW scenario, the resonance is single peaked and remains sharp up to gradual intrinsic thermal broadening, and the resonance energy is analytically determined to be at minus of the chemical potential.

A four unit cell periodic pattern of quasi-particle states surrounding vortex cores in Bi2Sr2CaCu2O8+delta

Scanning tunneling microscopy is used to image the additional quasi-particle states generated by quantized vortices in the high critical temperature superconductor Bi2Sr2CaCu2O8+delta. They exhibit a copper-oxygen bond-oriented "checkerboard" pattern, with four unit cell (4a0) periodicity and a approximately 30 angstrom decay length. These electronic modulations may be related to the magnetic field-induced, 8a0 periodic, spin density modulations with decay length of approximately 70 angstroms recently discovered in La1.84Sr0.16CuO4. The proposed explanation is a spin density wave localized surrounding each vortex core. General theoretical principles predict that, in the cuprates, a localized spin modulation of wavelength lambda should be associated with a corresponding electronic modulation of wavelength lambda/2, in good agreement with our observations.