5 research outputs found
Thermodynamics of the vortex liquid in heavy ion-irradiated superconductors
It is shown that the large effect of heavy ion-irradiation on the
thermodynamical properties of the anisotropic superconductor
YBaCuO extends well into the superconducting
fluctuation regime. The presence of the induced amorphous columnar defects
shifts the specific heat maximum at the normal-to-superconducting transition.
This effect is similar to that recently put into evidence in cubic
KBaBiO (). In both compounds, vortex pinning
manifests itself as a sharp angular dependence of the \em equilibrium \rm
torque. In YBaCuO, pinning by the defects appears at
the temperature of the specific heat maximum, well above the
magnetic irreversibility line . In isotropic
KBaBiO, the onset of the pinning-related torque anomaly
tracks the onset of the specific heat anomaly and the irreversibility line. In
YBaCuO, fluctuations of the amplitude of the order
parameter (and not vortex line wandering) are ultimately responsible for the
vanishing of pinning. In KBaBiO, vortex pinning disappears
only at the superconducting-to-normal transition. The results indicate that in
both compounds, the pinning energy at the ``Bose glass'' transition is large
with respect to the total free energy gain in the superconducting state. By
implication, the mechanism of this latter transition should be reconsidered.Comment: 9 pages, 9 figures, resubmitted to PRB 23-09-200
Effect of pressure on the superconductivity of Rb0.19WO3
International audienceWe have performed electrical resistivity measurements under pressures up to 20GPa between 1 and 300K on monocrystalline hexagonal Rb0.19WO3. For pressures lower than ~5GPa, we observe a decrease of the metallic-like resistivity at room temperature as well as a small decrease of Tc. At this pressure, the resistivity starts to increase slowly up to 10GPa accompanied by a sharper decrease of Tc .The resistivity curves above 10GPa denote an activated behaviour and a Tc lower than 3K indicating that there is a phase transition that takes place gradually between 5 and 10GPa. We interpret our measurements as the result of structural transformations under high pressure
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 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 alpha, we argue that such
behavior is consistent with recent numerical work [Vestergren et al., PRB 70,
054508 (2004)] showing that the introduction of columnar defects changes the
universality class of the transition.Comment: 4 pages, 2 figure
La transition de phase électronique dans le monosulfure de nickel
No abstractPas de résum
Superconductivity in the tungsten bronze RbxWO3 in connection with its structure, electronic density of states and phonon density of states
International audienceWe have measured the magnetic susceptibility of the title compound and examined its structural properties and lattice dynamics, using elastic and inelastic neutron scattering (INS) experiments, in order to gain further insight into the unusual features of its superconducting (SC) state, namely: i) the stabilizing effect resulting from the reduction of rubidium content, i.e. of the conduction electron density (what we shall name the "Tc (x) paradox"), ii) the destabilizing effect of the ordering of the Rb ions. We also performed density-functional calculations of the phonon dispersion in the "stoichiometric" Rb0.33WO3 and Cs0.33WO3 to identify the main features of the phonon spectra. These calculations give a very satisfactory description of the INS data and confirm the assignment to these bronzes of a lower (orthorhombic) symmetry than previously proposed. Our results contradict the previous interpretations of the "Tc (x) paradox" and of the ordering effect: i) no general softening of the lattice accompanies the increase of the Rb-vacancy population, ii) no general decrease of the electron density of states DEF distinguishes the ordered non-superconducting Rb0.25WO3 from its neighboring disordered parents. It appears therefore that the electron-electron coupling in this system probably proceeds through well-defined electronic states and phonons. This is a feature these "hexagonal" tungsten bronzes (HTB) apparently share with several high-Tc materials. We discuss what could be the mechanisms responsible for the very selective electron-phonon (e-p) coupling in the HTB