J.C. Levet

Ultrasound study on YBa2Cu3O7−δ and GdBa2Cu3O7−δ single crystals

Abstract Sound velocity results on YBa 2 Cu 3 O 7-δ and GdBa 2 Cu 3 O 7-δ single crystals are reported. Discontinuities in the temperature derivatives of the longitudinal and shear sound velocities are observed at the superconducting transition T c . Hardening in both longitudinal (C 33 ) and shear (C 44 ) modes is observed below T c . Estimations of the second stress derivatives of T c are evaluated using a mean field thermodynamics analysis. A second phase transition is observed around 108 K.

Ultrasound study of YBa2Cu3O7−δ single crystals

Abstract Sound velocity measurements on YBa 2 Cu 3 O 7− δ single crystals are reported. No anomalous behaviour within the experimental resolution (≊5 10 −5 ) was observed in the superconducting state. Present results confirm that the elastic properties of this system can be related to the thermodynamic model which predicts a very small velocity change (≊ 10 −5 ) at the superconducting transition temperature.

Magnetization jumps in YBa2Cu3O7 single crystal, up to 18T

Abstract The magnetization of a large single crystal (27 mg) of YBa 2 Cu 3 O 7 exhibits several new features associated to the anisotropy (and sample size) : for H // c axis large magnetization jumps are observed below about 15 K. The magnetization jumps are smaller, less spaced and observed only below 7 K for H ⊥ c. At high temperature the hysteresis loops, M(H), are convex lens shaped which gives a larger density of currents in high fields. The critical currents (6.8 10 5 A/cm 2 or 2.8 10 5 A/cm 2 for H parallel or perpendicular to the c axis, at 4.2 K) can be compared with values obtained for much smaller crystals.

Magnetization jumps and critical current of YBa2Cu3O7 single crystal

Abstract The magnetization of a large YBa 2 Cu 3 O 7 single crystal (27 mg) exhibits magnetic jumps at low temperature, which affect the critical currents. The temperature dependence of J c is roughly exponential up to 50 K with an anisotropy ratio of ∼ 3.4 for the two field directions.

Caractérisation d'un tritellurure d'uranium: β-UTe3 de structure type NdTe3

Abstract A new uranium tritelluride, β-UTe3, has been characterized. The crystal structure was determined from single-crystal data; β-UTe3 crystallizes with the NdTe3-type structure: orthorhombic system, space group Cmcm, with the pseudotetragonal lattice parameters a = 4.338(2) A, b = 24.743(10) A, c = 4.338(2) A. Tellurium atoms with planar Te4Te bonds from (010) tellurium layers separated by Van der Waals gaps, giving a bidimensional character to the structure. Unit cell volume considerations indicate that uranium is in the tetravalent state whereas rare earths and transuranium elements are trivalent in isostructural compounds. Magnetization measurements on powder and single crystal show Curie-Weiss behavior in the paramagnetic range and ferromagnetic ordering below 12 K, with the measured magnetic moments: M = 0.63 μ B U for H//b (the pseudotetragonal axis) and M = 0.22 μB for H//(a, c).

Anisotropy of the superconducting properties of Bi2Sr2CaCu2O8

Abstract We report magnetoresistance measurements up to 20 teslas on the superconducting high-temperature compound Bi2Sr2CaCu2O8. The upper critical field is very anisotropic; H c 2∥ H c 2⊥ ≅ 20 . (H∥ for H∥ c-axis and H⊥ for H∥ (a,b) plane). This value is twice larger than in YBa2Cu3O7, it demonstrates the stronger 2-D character of Bi2Sr2CaCu2O8.

Crystal structure and magnetic properties of the uranium germanide U5Ge4

Abstract The crystal structure of the uranium germanide U5Ge4 was determined and refined from single crystal X-ray diffraction data to R = 0.043, Rw = 0.054. U5Ge4 crystallizes in the hexagonal Ti5Ga4 (P63/mcm) structure type (filled up Mn5Si3 type) with the following lattice parameters: a = 8.744(4) A and c = 5.863(2) A . Investigation of neighbouring compositions, using X-ray powder diffraction refinements and microprobe analysis, revealed that the binary compound U5Ge4 is in equilibrium with the other binary UGe (ThIn type) and U metal dissolving 3% of germanium, whatever the annealing temperature in the range of 1000–1300°C. These results rule out the existence of the binary compositions U5Ge3 and U7Ge, which were previously reported. Magnetic measurements on an annealed polycrystalline powder sample of U5Ge4 indicate nearly temperature independent paramagnetic behaviour down to 2 K.

Magnetic phase diagram of YBa2Cu3O6+x

Abstract Neutron scattering experiments on single crystals have allowed an accurate determination of the magnetic phase diagram of YBa2Cu3O6+ x. For increasing oxygen content the antiferromagnetic ordering of YBa2Cu3O6 is unchanged up to x = 0.2. For x > 0.2 both TN and the ordered moment of Cu2+ ions in the Cu2 site decrease and vanish abruptly around x ⋍ 0.40. Whatever the oxygen content the Cu+ ions in the Cu1 site do not order. For x = 0.30 a reentrant behaviour is observed. Preliminary inelastic neutron scattering experiments on a rather large single crystal (0.2 cm3) with x ⋍ 0.3 are reported.

Low temperature specific heat of YBa2Cu3O7 single crystals

Abstract Specific heat measurements of YBa 2 Cu 3 O 7 single crystals are reported in the temperature range 0.09 to 5 K. Below 3 K, C p is analysed as the sum of a Schottky anomaly, a nuclear hyperfine term, both of which have also been detected in polycrystalline sintered samples, and a large T 0.5 contribution which dominates the other contributions down to 0.1 K.

Critical current densities (up to 20 T) of high Tc REBa2Cu3O7, Bi2Sr2CaCu2O8, Tl2Ba2CaCu2O8 crystals

Abstract High field magnetization measurements have been used to determine the critical current densities Jc(H.T) of single crystals from the Bean model and the anisotropy of the magnetic moment in the normal state.