We have synthesized for the first time the metastable compound 1T-CrTe2. We have done its complete structural characterization and measured its magnetization, specific heat and electrical resistivity between 4 and 330 K. We have also performed detailed band structure calculations. We have found that it crystallizes in the CdI2 structure type and that its electrical resistance follows a metallic behaviour below room temperature. Its magnetization and specific heat curves show that the compound has a transition to a ferromagnetic state at TC = 310 K, with the magnetic moments ordered parallel to the basal plane. From the specific heat measurements and the ferromagnetic solutions obtained from our DFT calculations, we conclude that the ferromagnetism is of itinerant nature.

We have studied the resistance of 1T-CrSe2, as the Cr atoms are substituted by V or Ti. The V replacement leads to a logarithmic increase in the resistance as the temperature is lowered, proportional to the V concentration. While this behavior is consistent with the Kondo effect, the weak dependence of the resistance with magnetic field and the fact that the system has antiferromagnetic order, rule out a Kondo effect due to spin degeneracy. In contrast to the case of V, Ti substitution does not increase the logarithmic term while application of pressure destroys it. Calculations of the electronic structure within the framework of density functional theory, maximally localized Wannier functions, and many-body calculations in a cluster containing a Cr or V atom and its six nearest-neighbor Se atoms, helped to reveal the existence of an orbital Kondo effect due to orbital degeneracy in the V substitutional impurities.

Depending on the temperature, the charge density wave (CDW) nonlinear conductivity of the blue bronzes A0.30MoO3 (A=K, Rb) shows two different regimes: a strongly damped motion above ∼50K and motion with almost no damping below ∼50K. In a search for an elastic signature of this CDW behaviour, we performed ultrasonic measurements on Rb0.30MoO3 and Rb0.30(Mo1−xVx)O3 single crystals between 4K and 300K. In Rb0.30MoO3, at T∼50K, upon cooling, a large increase of the sound velocity is observed. The ultrasonic attenuation coefficient shows an increase down to 50K followed by a plateau. In Rb0.30(Mo1−xVx)O3 (x=0.4at%) the anomaly broadens and is shifted towards higher temperatures. The results are discussed in terms of CDW glass.

Stochastic resonance (SR) in nonlinear systems is a counterintuitive concept in which a weak periodic signal and noise cooperate and give rise to a maximum in the signal-to-noise ratio at the output of the system when the noise is tuned to a certain value. In the coherence resonance phenomenon (CR), there is no deterministic signal to be enhanced. Intrinsic oscillations are present as transients. Adding an optimum noise turns transients into coherent ones. We discuss the possible application of SR and CR concepts to CDW dynamics in quasi-one dimensional conductors. We show in a preliminary experiment that addition of white noise can modify the behavior of the CDW in the quasi-one dimensional conductor K0.30MoO3.

In the blue bronzes A0.30MoO3 (A=K,Rb), two regimes for the CDW nonlinear conductivity have been identified: strongly damped CDW motion above ∼50K and CDW motion with almost no damping below ∼50K. In a search for an elastic signature of this CDW behavior, we performed ultrasonic measurements on Rb0.30MoO3 single crystals in the temperature range 4–300K. Upon cooling, we have observed at ∼50K a large increase of the sound velocity for the longitudinal mode measured along the [-201], [102] and b directions as well as for the shear mode along the [-201] direction. In addition, a maximum in the ultrasonic attenuation is observed around 50K. The results are discussed in relation with the changes in the CDW rigidity and in the dielectric response.

SessionsThe x=2 composition crystallizes in the C2/c space group (N15) with the doubled unit cell along c-axis.The trimers were found to be of only one type Ni-Cu-Ni.Below TN=20 K a magnetic ordering with the propagation vector k=[1/2,1/2,0] has been found.The magnetic diffraction patterns are well described by the antiferromagnetic structure given by the irreducible representation 2 for both Ni (8f) and Cu (4b) sites.The exchange interactions within the trimers are dominated by Heisenberg-type nearest-neighbor interactions JCu-Cu=-4.92(6)meV, JCu-Ni=-0.85(10)meV and D_Ni=-0.7(1) for x=2.

We revisited the problem of the stability of the superconducting state in RbxWO3 and identified the main causes of the contradictory data previously published. We have shown that the ordering of the Rb vacancies in the nonstoichiometric compounds have a major detrimental effect on the superconducting temperature T-c. The order-disorder transition is of first order only near x=0.25, where it cannot be quenched effectively and T-c is reduced below 1 K. We found that the high T-c's that were sometimes deduced from resistivity measurements and attributed to compounds with 0.25less than or similar toxless than or similar to0.30, are to be ascribed to interfacial superconductivity that generates spectacular nonlinear effects. We also clarified the effect of acid etching and set more precisely the low-rubidium-content boundary of the hexagonal phase. This work makes clear that T-c would increase continuously (from approximate to2 K up to approximate to5.5 K) as we approach this boundary (xapproximate to0.20), if no ordering would take place-as it is approximately the case in CsxWO3. This behavior is reminiscent of the tetragonal tungsten bronze NaxWO3 and asks the same question: what mechanism is responsible for this large increase in T-c despite the considerable associated reduction of the electron density of state D-FE? By reviewing the other available data on these bronzes we conclude that the theoretical models that are able to answer this question are probably those where the instability of the lattice plays a major role and, particularly, the model that calls upon local structural excitations, associated with the missing alkali atoms.

Small Angle Neutron Scattering has been used to observe diffraction from the flux line lattice in the fully isotropic (K, Ba)BiO3high Tcsuperconductor. We show that the flux lines are ordered into a hexagonal lattice at low field (H < He∼ 0.6T ≪ Hg (0) where Hgis the second order vortex glass transition field). The diffracted intensity continuously drops to zero as the field is increased towards He. Those observations are in good agreement with recent theoritical works which suggested that the formation of dislocations becomes favorable above Heleading to a glassy structure. The field Helies close to the onset of the second peak in magnetization measurements indicating that this peak is related to the change in the vortex structure.

We report an extensive study on the temperature ([ital T]) dependence of the upper critical field [ital H][sub [ital c]2] on Ba[sub 1[minus][ital x]]K[sub [ital x]]BiO[sub 3] single crystals. The [ital H][sub [ital c]2]([ital T]) curve shows a reproducible upward curvature at [similar to]22 K. This can be due to the presence of two superconducting phases, one with [ital T][sub [ital c]][similar to]30 K and low [ital H][sub [ital c]2] ([[ital dH][sub [ital c]2]/[ital dT]][sub [ital T][ital c]][similar to]0.5 T/K) and another one with [ital T][sub [ital c]][similar to] 25 K and higher [ital H][sub [ital c]2] ([[ital dH][sub [ital c]2]/[ital dT]][sub [ital T][ital c]][similar to]1.1 T/K). However, we find that [ital H][sub [ital c]2] may rise up to 32 T at 1.8 K. It turns out that the reduced critical field [ital h][sub [ital c]2]=[ital H][sub [ital c]2]/[[ital T][sub [ital c]]([ital dH][sub [ital c]2]/[ital dT])[sub [ital T][ital c]]] is [ge]1 at low temperatures for both the 30- and 25-K phases, i.e., much higher than what is expected for a conventional superconductor. We compare the low-temperature [ital h][sub [ital c]2] data with calculations recently performed by Marsiglio and Carbotte in the framework of an extended Werthamer-Helfand-Hohenberg theory.

In order to study the modification of the transport properties of NaxWO3 by substitution in the WO3 sublattice, single crystals of tantalum-substituted tungsten bronzes of formula NaxTayW1−yO3 have been grown by electrolytic reduction of fused Na2WO4WO3Ta2O5 mixtures. The lattice constant of their cubic perovskite like structure increases with Ta content. Electric conductivity measurements on NaxTayW1−yO3 single crystals characterize a metal-non metal transition when x-y, i.e. the number of d electrons, decreases. This transition seems to be of Anderson type.