We study intersubband spin density collective modes in double-layer quantum Hall systems at ν = 2 within the time-dependent Hartree-Fock approximation. We find that these intersubband spin density excitations may soften under experimentally accessible conditions, signaling a phase transition to a new quantum Hall state with interlayer inplane antiferromagnetic spin correlations. We show that this novel canted antiferromagnetic phase is energetically stable and that the phase transition is continuous. 73.40.Hm 73.20.Mf 73.20.Dx Typeset using REVT E X 1 Electron systems in confined geometries exhibit a richer variety of physical properties than their higher-dimensional counterparts due to enhanced interaction effects in reduced dimensions. Interaction in a low-dimensional system does not merely result in stronger renormalization of physical quantities, but can in many cases drive the system into completely new phases with peculiar properties. For a two-dimensional electron gas in a perpendicular magnetic field, the interaction effects are especially important because of Landau level quantization. When electrons are entirely restricted to the lowest Landau level by a large magnetic field, electron-electron interaction completely dominates the properties of the system as the electron kinetic energy is quenched to an unimportant constant. One of the most interesting phenomena in this strongly-correlated system is the quantum Hall effect

We address the question of whether anisotropic superconductivity is compatible with the evidently weak sensitivity of the critical temperature T c to sample quality in the high-T c copper oxides. We examine this issue quantitatively by solving the strongcoupling Eliashberg equations numerically as well as analytically for s-wave impurity scattering within the second Born approximation. For pairing interactions with a characteristically low energy scale, we find an approximately universal dependence of the d-wave superconducting transition temperature on the planar residual resistivity which is independent of the details of the microscopic pairing. These results, in conjunction with future systematic experiments, should help elucidate the symmetry of the order parameter in the cuprates.

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Recent measurements of the London penetration depth tensor in the cuprates find a weak temperature dependence along the c-direction which is seemingly inconsistent with evidence for d-wave pairing deduced from in-plane measurements. We demonstrate in this paper that these disparate results are not in contradiction, but can be explained within a theory based on incoherent quasiparticle hopping between the CuO2 layers. By relating the calculated temperature dependence of the penetration depth λc(T ) to the c-axis resistivity, we show how the measured ratio λ 2 c (0)/λ 2 c (T ) can provide insight into the behavior of c-axis transport below Tc and the related issue of "confinement."

Experiments on the cuprate superconductors demonstrate that these materials may be viewed as a stack of Josephson junctions along the direction normal to the CuO 2 planes (the c-axis). In this paper, we present a model which de-