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The effects of Andreev reflection on the current-voltage characteristic and differential conductance of a junction between a normal metal and a ${\mathit{d}}_{\mathit{xy}}$-wave superconductor, or equivalently, a ${\mathit{d}}_{{\mathit{x}}^{2}\mathrm{\ensuremath{-}}{\mathit{y}}^{2}}$-wave superconductor with a {110}-oriented surface are investigated using the Bogoliubov--de Gennes equations. Our study elucidates several important consequences of the sign change of a d-wave order parameter. In particular, a zero-bias conductance peak is obtained when an insulating barrier exists at the interface between the normal metal and the d-wave superconductor, consistent with numerous experiments performed on cuprate superconductors. If the insulating barrier is assumed to reside in the normal metal, several coherence lengths away from the superconductor surface, bound states within the energy gap, and consequent subgap resonances in the differential conductance, are predicted. The positions of these resonances are out of phase with respect to those predicted for an isotropic or anisotropic s-wave superconductor, thus providing unique signatures of pairing state symmetry. \textcopyright{} 1996 The American Physical Society.

We derive microscopically the Ginzburg-Landau equations for weak-coupling superconductors with coexisting {ital s} and {ital d} symmetries. The equations are derived from Gor{close_quote}kov equations by using the finite temperature Green{close_quote}s-function method. We explore the physical consequences of such Ginzburg-Landau equations. {copyright} {ital 1996 The American Physical Society.}

The structures of a single vortex and vortex lattice in a superconductor with ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ symmetry are studied self-consistently employing a recently developed Ginzburg-Landau theory. Near a single vortex, we found that an $s$-wave component of the order parameter is always induced, and it causes the local magnetic-field distribution and the $d$-wave order parameter to have a fourfold anisotropy. It is shown that there is a strong correlation between the structure of a single vortex and the shape of the vortex lattice. Our numerical calculation indicates that the structure of the vortex lattice is always oblique except for temperatures very close to ${T}_{c}$ where it becomes triangular. The possible connection of the result with experiment is also discussed.

We study the static configurations of a one-dimensional 0-{pi} Josephson junction, consisting of an {ital s}-{ital d} junction or selected boundaries between three suitably oriented {ital d}-wave superconductors. We find that a {ital d}-wave-type interference pattern can only be observed in the field-modulated critical current if the junction is short compared to the Josephson penetration depth. However, the behavior of a 0-{pi} junction is found to be indistinguishable from that of a 0 junction between {ital s}-wave superconductors in the long junction limit, due to the formation of a {pi} vortex.

The one-dimensional spin-1/2 Ising-Heisenberg ferromagnets are studied by the Green's function method at finite temperatures. The Green's function is decoupled in terms of the correlation functions of 〈${\mathit{S}}_{0}^{\mathit{z}}$${\mathit{S}}_{\mathit{n}}^{\mathit{z}}$〉 and 〈${\mathit{S}}_{0}^{+}$${\mathit{S}}_{\mathit{n}}^{\mathrm{\ensuremath{-}}}$〉 under the constraint of 〈${\mathit{S}}^{\mathit{z}}$〉=0, from which a set of self-consistent equations are derived and solved numerically. The thermodynamic properties such as internal energy, specific heat, and zero-field parallel susceptibility are calculated. Our results predict the correct behavior for the correlation functions and thermodynamic quantities in the entire temperature region, in agreement both with the exact numerical results on the finite chain and the experimental data.

A general spin-wave theory for the low-dimensional quantum spiral magnets with a constraint introduced by Takahashi is presented. We show that this theory can reproduce the main results of the conventional spin-wave theory for Heisenberg magnetic systems without frustrations. For the triangular lattice spin-1/2 Heisenberg antiferromagnet and the frustrated square lattice Heisenberg models, the present theory yields results different from those of the conventional spin-wave theory but in agreement with those of recent numerical simulations for small lattices.

La theorie classique de l'onde de spin ne peut fournir un diagramme de phases correct pour l'antiferromagnetique frustre a reseau carre. Presentation d'une theorie de l'onde de spin modifiee analogue a la theorie recente de Takahashi du modele de Heisenberg bidimensionnel sans frustre. Obtention d'un nouveau diagramme de phases dans lequel il n'existe aucun etat de liquide de spin pour S=1/2. Les resultats sont en bon accord avec ceux des simulations numeriques recentes pour de petits reseaux