Seiichi Seki

The Influence of the Singularity-Enhanced Density of States on the Isotope Effect in a Bond Asymmetric Model for a Cu-O Based High- Tc Superconductor

In a Cu-O based high- Tc superconductor, we investigate possible singularities in the density of states of a bond asymmetric model with intralayer hoppings between nearest and next nearest neighbor O-sites and extremely small interlayer hopping. A combination of such hoppings and the bond asymmetry leads to various types of singularities. By considering the Debye cutoff dependence of Tc, we point out that the singularity-enhanced density of states induces an interesting chemical potential dependence of the isotope effect.

The Shielding Effect of the Meissner Current on the Magnetic Dipole Interactions in the Coexistence State of Superconductivity and Ferromagnetism. II

By considering the finite mass effect of the photons (or the shielding effect of the Meissner current), the expression for the magnetic dipole interactions (MDI) in a superconductor is obtained. The resultant expression shows that when the system is in the coexistence state of superconductivity and fer· romagnetism, the quantity Ad R becomes an important factor in discussing the degeneracy of the spin-wave energies in the long wavelength region. Here AL and R are Londons penetration depth and the size of the system, respectively. Under the strong shielding effect, the contribution from MDI is almost negligible. We also derive MDI in a superconducting film, and point out that in the coexistence state the uniaxial magnetic shape anisotropy is weakened by the surface effect.

The Effect of Magnetic Dipolar Interaction on the Stabilities of the Antiferromagnetic and Ferromagnetic Spin Configurations in a Superconductor

For both of the London and Pippard types of superconductors, the lattice sum of magnetic dipolar interaction is calculated on the assumption that the spin system forms a lattice structure with cubic symmetry. From this result, we prove that the dipolar interaction influenced by supercurrents tends to lead the stability (instability) of the antiferromagnetic (ferromagnetic) configuration of localized moments.

Proximity Effect on the Magnetic Dipolar Interaction near the Interface between a Superconductor and an Insulator

Considering the boundary conditions for the magnetic field at the interface between a superconductor and an insulator (or normal metal), we discuss how the magnetic dipolar interaction (MDI) between localized spins embedded in the insulating (or normal metal) region is modified by the neighboring superconductor. Estimating the MDI energy of a ferromagnetic material deposited on a slab of superconductor, we suggest that such a proximity effect induces the non-negligible modification of the anisotropy energy of the magnet.

The Meissner effect of the supercurrent on a magnetocrystalline energy

Abstract Magnetic dipole interaction in a superconductor is formulated. From numerical calculations for some lattice structures with tetragonal deformations, we point out that the supercurrent yields the significant reduction of the magnetocrystalline energy.

Spin Glass Effect on Superconductivity. A Note on the Lowering of the Effective Attraction.

Considering the magnetic exchange interaction between localized spins and conduction electrons, we discuss the spin glass effect on superconductivity. In particular we investigate the spin glass order parameter-dependence and superconducting critical temperature-dependence of the effective attraction modified by the spin glass phase.

Shielding effect of the quasi-2D supercurrent on the photon-mediated spin-spin interaction

Abstract We investigate the shielding effect of the quasi-2D supercurrent on the photon-mediated spin-spin interaction and estimate the coupling energy renormalization. We also discuss this effect on the spins placed in the normal region in a S-N junction.

In-plane anisotropy-induced reduction of the effective coulomb repulsion in a Cu-O based superconductor

Abstract In a single plane model for a Cu-O based high- T c superconductor, we point out that the in-plane anisotropy leads to the significant reduction of the effective Coulomb repulsion, and eventually makes the effective attraction between charge carriers stronger than that in the absence of the anisotropy.

Magnetic dipolar interaction in a slab of superconductor

Abstract Formulating magnetic dipolar interaction in a slab of superconductor, we show that uniaxial magnetic shape anisotropy of the slab is under the strong influence of the Meissner-Ochsenfeld effect. This effect also leads to appreciable reduction of the spin coupling energy for the antiferromagnetic spin configuration.