Thermal conduction in the vortex state of unconventional superconductors

When a magnetic field is applied to a type II superconductor, the field penetrates in the form of vortices wherein the superconducting order parameter vanishes over a length scale of 1 0 0 A or so. Each vortex supports localized electronic states. Above the upper critical field Hc2, the normal state is recovered and the electronic states are fully delocalized. A basic question is to ask how the electronic states evolve from being localized to delocalized as the intervortex distance is reduced (i.e. as the magnetic field is increased). For that purpose, we have performed systematic studies of the electronic thermal conductivity ne/ T through the vortex state at temperatures as low as 50 mK and fields as high as 13 Tesla. This was done in a variety of systems: VsSi, LuNi2 B2 C, NbSe2 and Tl2 Ba2 Cu0 6 +5 . The work performed on the phonon-mediated V3 SL LuNi2 B2C and NbSe2 led to many surprises. V3Si is an archetypal type-II superconductor and displays the expected activated behavior. It provides a solid basis for comparison. However, we found that the borocarbide superconductor LuNi2 B2C has a highly anisotropic gap of unprecedented magnitude (a factor of 1 0 ), while the layered compound NbSe2 is found to display multiband superconductivity. We also studied the overdoped cuprate superconductor Tl2 Ba2 Cu0 6 + < 5 where the upper critical field was low enough for us to reach the normal state. The Wiedemann-Franz law was tested and found to be satisfied with a 1 % experimental accuracy. This represents the first unambiguous evidence that the overdoped side of the phase diagram is a Fermi liquid and displays no sign of spin-charge separation and thus provides a solid basis for a comprehensive theory of the phase diagram of cuprate superconductors. In the superconducting state, we find that Tl2 Ba2CuO 6 + < 5 is the textbook example of a d-wave superconductor as it shows quantitative agreement with theory. In summary we obtained a range of behaviors in several different types of superconductors, from the test case of the conventional V3Si to the highly unconventional cuprates and through highly anisotropic and multi-band superconductors.Ph.D