Muons and magnets

Abstract:

We have studied the interlayer resistivity of the prototypical quasi-two-dimensional organic superconductor κ- (BEDT-TTF)2 Cu (NCS)2 as a function of temperature, current, and magnetic field, within the superconducting state. We find a region of nonzero resistivity whose properties are strongly dependent on magnetic field and current density. There is a crossover to non-Ohmic conduction below a temperature that coincides with the two-dimensional vortex solid-vortex liquid transition. We interpret the behavior in terms of a model of current- and thermally driven phase slips caused by the diffusive motion of the pancake vortices which are weakly coupled in adjacent layers, giving rise to a finite interlayer resistance. © 2007 The American Physical Society.

Abstract:

The interlayer magnetoresistance rho(zz) of the organic metal kappa-(BEDT-TTF)(2)Cu(NCS)(2) is studied in fields of up to 45 T and at temperatures T from 0.5 to 30 K. The peak in rho(zz) seen in in-plane fields, a definitive signature of interlayer coherence, remains to Ts exceeding the Anderson criterion for incoherent transport by a factor approximately 30. Angle-dependent magnetoresistance oscillations are modeled using an approach based on field-induced quasiparticle paths on a 3D Fermi surface, to yield the T dependence of the scattering rate tau(-1). The results suggest that tau(-1) does not vary strongly over the Fermi surface, and that it has a T(2) dependence due to electron-electron scattering.

Abstract:

Critical exponents closely matching those of the N=2 chiral universality class have been obtained for the layered magnetic system cobalt glycerolate using muon spin relaxation. This class was originally introduced to represent geometrically frustrated triangular stacked-layer XY magnets with chiral noncollinear spin structures. Since the present magnetic system is a canted XY system without geometrical frustration or chiral degeneracy, the results indicate that the order parameter for canting in this system plays a similar role to the chiral order parameter in the geometrically frustrated systems, strongly suggesting that both types of noncollinear system share the same universality class.