M. Castonguay

Mott transition, compressibility divergence, and the P-T phase diagram of layered organic superconductors: an ultrasonic investigation.

The phase diagram of the organic superconductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl has been investigated by ultrasonic velocity measurements under helium gas pressure. Different phase transitions were identified through several elastic anomalies characterized from isobaric and isothermal sweeps. Our data reveal two crossover lines that end on the critical point terminating the first-order Mott transition line. When the critical point is approached along these lines, we observe a dramatic anomaly of the sound velocity which is consistent with a diverging compressibility of the electronic degrees of freedom.

Elastic study of antiferromagnetic fluctuations in the layered organic superconductorsκ−(BEDT−TTF)2X

In an ultrasonic experiment, we have investigated the temperature profile of the velocity of longitudinal elastic waves propagating along a direction perpendicular to the layers in the organic superconductors kappa-(BEDT-TTF)_2X, X = Cu(SCN)_2 and Cu[N(CN)_2]Br. Although a small decrease of the velocity is observed at the superconducting transition, the most anomalous behavior is obtained in the normal metallic state where an important softening is identified around 40-50 K. In order to characterize the origin of this anomaly, we have studied its behavior under the application of hydrostatic pressure. The observed behavior is found to mimic those of the transport and magnetic properties of these materials which have been attributed to the magnetic fluctuations. Following the example of one-dimensional insulating systems where coupling between longitudinal acoustic waves and magnetic fluctuations is known to occur, our results suggest that the pseudo-gap regime of these two-dimensional organic superconductors is dominated by a similar mechanism.

Optical Zeeman effect measurements on isolated and ferromagnetically coupled Nd3+ ions in weakly doped YVO4 crystals: evidence for lack of crystal field perturbations in ion pair interactions

Optical Zeeman effect measurements are performed at liquid helium temperature on Nd3+ ions in weakly doped YVO4 crystals in the range 11 350-11 390 cm-1 of the 4F3/2→ 4I9/2 neodymium transitions. This study provides confirmation that the concentration dependent satellite lines accompanying the optical transitions are due to ferromagnetically coupled pairs of Nd3+ ions in undistorted Y3+ sites of D2d point site symmetry. Nonlinear Zeeman splittings are observed in the range of experimental magnetic fields. These splittings are compared with those simulated from crystal field calculations by introducing the Zeeman Hamiltonian into the secular determinant before diagonalization. A reasonably good agreement between the calculated and experimental g-values is obtained, confirming that the exchange coupled neodymium pairs experience no crystal field perturbations.