R. Brusetti

Two-band effects in the angular dependence of H{sub c2} of MgB{sub 2} single crystals.

The angular dependence of the upper critical field H{sub c2} of MgB{sub 2} single crystals is studied at various temperatures by means of specific-heat and transport measurements in magnetic fields up to 17 T. Clear deviations from Ginzburg-Landau behavior are observed at all temperatures and are explained by two-band effects. The angular dependence and temperature dependence of the deviations are in qualitative agreement with theoretical predictions based on band-structure calculations. Quantitative agreement is obtained with an interband coupling slightly stronger than the calculated one, enabling band-structure anisotropies and interband coupling strength to be experimentally estimated. This provides a pathway to the study of disorder and doping effects in MgB{sub 2}.

The role of the lattice at the metal-semimetal transition in nickel sulphide

The characteristics of the lattice on either side of the metal-semimetal transition in Ni1- delta S have been determined using a panoply of experimental methods-specific heat, Mossbauer absorption inelastic neutron scattering, ultrasonic and X-ray diffraction. These different methods provide a coherent picture of the changes in lattice dynamics and thermodynamic properties at the first-order phase transition. Most significant is the softening of the acoustic modes, and in particular the decrease in the shear constant C44 in the high-temperature metallic phase. The transition as a function of temperature is mainly driven by the associated lattice entropy and not directly by electronic or magnetic entropy resulting from the change in electronic structure. Nevertheless, the change in electronic structure is probably indirectly responsible for the change in C44 in the metallic phase through modification of the population regions of the Fermi surface under shear stress.

Thermodynamics of a heavy ion-irradiated superconductor: the zero-field transition

Specific heat measurements show that the introduction of amorphous columnar defects considerably affects the transition from the normal to the superconducting state in {\em zero} magnetic field. Experimental results are compared to numerical simulations of the 3D XY model for both the pure system and the system containing random columnar disorder. The numerics reproduce the salient features of experiment, showing in particular that the specific heat peak changes from cusp-like to smoothly rounded when columnar defects are added. By considering the specific heat critical exponent

Heat capacity of nickel sulfide and its semimetal-metal transition

\alpha

Superconducting and dielectric instabilities in Tl2Mo6Se6: Unusual transport properties and unsaturating critical field.

, we argue that such behavior is consistent with recent numerical work [A. Vestergren et al., Phys. Rev. B {\bf 70}, 054508 (2004)] showing that the introduction of columnar defects changes the universality class of the transition.