Multiband superconductivity in NbSe_2 from heat transport

The thermal conductivity of the layered s-wave superconductor NbSe_2 was measured down to T_c/100 throughout the vortex state. With increasing field, we identify two regimes: one with localized states at fields very near H_c1 and one with highly delocalized quasiparticle excitations at higher fields. The two associated length scales are most naturally explained as multi-band superconductivity, with distinct small and large superconducting gaps on different sheets of the Fermi surface.Comment: 2 pages, 2 figures, submitted to M2S-Rio 2003 Proceeding

Field-induced quantum critical point in CeCoIn_5

The resistivity of CeCoIn_5 was measured down to 20 mK in magnetic fields of up to 16 T. With increasing field, we observe a suppression of the non-Fermi liquid behavior, rho~T, and the development of a Fermi liquid state, with its characteristic rho=rho_0+AT^2 dependence. The field dependence of the T^2 coefficient shows critical behavior with an exponent of ~4/3. This is evidence for a new field-induced quantum critical point, occuring in this case at a critical field which coincides with the superconducting upper critical field H_c2.Comment: 2 pages, 2 figures, submitted to M2S-Rio 2003 Proceeding

Magnetic phases and reorientation transitions in antiferromagnetically coupled multilayers

In antiferromagnetically coupled superlattices grown on (001) faces of cubic substrates, e.g. based on materials combinations as Co/Cu, Fe/Si, Co/Cr, or Fe/Cr, the magnetic states evolve under competing influence of bilinear and biquadratic exchange interactions, surface-enhanced four-fold in-plane anisotropy, and specific finite-size effects. Using phenomenological (micromagnetic) theory, a comprehensive survey of the magnetic states and reorientation transitions has been carried out for multilayer systems with even number of ferromagnetic sub-layers and magnetizations in the plane. In two-layer systems (N=2) the phase diagrams in dependence on components of the applied field in the plane include ``swallow-tail'' type regions of (metastable) multistate co-existence and a number of continuous and discontinuous reorientation transitions induced by radial and transversal components of the applied field. In multilayers (N \ge 4) noncollinear states are spatially inhomogeneous with magnetization varying across the multilayer stack. For weak four-fold anisotropy the magnetic states under influence of an applied field evolve by a complex continuous reorientation into the saturated state. At higher anisotropy they transform into various inhomogeneous and asymmetric structures. The discontinuous transitions between the magnetic states in these two-layers and multilayers are characterized by broad ranges of multi-phase coexistence of the (metastable) states and give rise to specific transitional domain structures.Comment: Manuscript 34 pages, 14 figures; submitted for publicatio

Extreme Ultra-Violet Spectroscopy of the Lower Solar Atmosphere During Solar Flares

The extreme ultraviolet portion of the solar spectrum contains a wealth of diagnostic tools for probing the lower solar atmosphere in response to an injection of energy, particularly during the impulsive phase of solar flares. These include temperature and density sensitive line ratios, Doppler shifted emission lines and nonthermal broadening, abundance measurements, differential emission measure profiles, and continuum temperatures and energetics, among others. In this paper I shall review some of the advances made in recent years using these techniques, focusing primarily on studies that have utilized data from Hinode/EIS and SDO/EVE, while also providing some historical background and a summary of future spectroscopic instrumentation.Comment: 34 pages, 8 figures. Submitted to Solar Physics as part of the Topical Issue on Solar and Stellar Flare