Novel neutron resonance mode in dx2-y2-wave superconductors.

We show that a new resonant magnetic excitation at incommensurate momenta, observed recently by inelastic neutron scattering experiments on YBa2Cu3O6.85 and YBa2Cu3O6.6, is a spin exciton. Its location in the Brillouin zone and its frequency are determined by the momentum dependence of the particle-hole continuum. We identify several features that distinguish this novel mode from the previous resonance mode observed near Q=(pi,pi).

Proximity effects and quantum dissipation in the chains of YBa2Cu3O6+x.

We argue that the results of recent scanning tunneling microscopy, angle-resolved photoemission, and infrared spectroscopy experiments on the CuO chains of YBa2Cu3O6+x are consistently explained within a proximity model by the interplay of a coherent chain-plane and an incoherent and indirect interchain coupling. We show that the CuO2 planes act as an Ohmic heat bath for the chain fermions and induce a substantial quantum dissipation. Below the planar T(c), charge excitations in the chains acquire a universal superconducting gap.

Local defect in metallic quantum critical systems.

We present a theory of a single point, line, or plane defect coupling to the square of the order parameter in a metallic system near a quantum critical point at or above its upper critical dimension. At criticality, a spin droplet is nucleated around the defect with its core size determined by the strength of the defect potential. Outside the core a universal slowly decaying tail of the droplet is found, leading to many dissipative channels coupling to the droplet and to a complete suppression of quantum tunneling. We propose an NMR experiment to measure the impurity-induced changes in the local spin susceptibility.

Resonance peak in Sr(2)RuO(4): signature of spin triplet pairing.

We study the dynamical spin susceptibility, chi(q,omega), in the normal and superconducting states of Sr(2)RuO(4). In the normal state, we find a peak in the vicinity of Q(i) approximately (0.72 pi,0.72 pi) in agreement with recent inelastic neutron scattering experiments. We predict that for spin triplet pairing in the superconducting state a resonance peak appears in the out-of-plane component of chi, but is absent in the in-plane component. In contrast, no resonance peak is expected for spin singlet pairing.