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No abstract available

[en] We report magnetostriction measurements on Pr_0.65Ca_0.35MnO₃ polycrystalline samples in magnetic fields up to 14 T. The charge ordering occurs below T_CO=230 K. The application of a magnetic field causes the transition from the ordered insulator state to metallic state. It is found that this insulator-metal transition is accompanied by large negative jumps in the longitudinal and transverse magnetostrictions with a temperature-dependent hysteresis. The H-T phase diagram is determined from the magnetostriction

[en] We present ultrasonic measurements on magnetoresistance manganites Pr_1-xCa_xMnO₃ (x=0.35,0.5) in magnetic fields up to 27 T. Charge ordering occurs in Pr_1-xCa_xMnO₃ in the 0.35< x<0.5 composition regime. The associated lattice distortions also exhibit long-range ordering. A large decrease (1%) of the sound velocity was observed at the insulator-to-metal transition induced by magnetic field destroying the charge ordering. The insulator-metal transition for Pr_0.5Ca_0.5MnO₃ is accompanied with a large hysteresis which extends up to 20 T at the lowest temperatures (2 K). The field dependence of the longitudinal sound velocity measured on polycrystalline samples are reported and the magnetic diagram is determined

[en] In the present work, we report elastic measurements performed on the manganese perovskite La_2/3Sr_1/3MnO₃, for which the ferromagnetic ordering temperature was determined to be 377 K, coincident with a metal-insulator transition. Measurements in the temperature range 250-480 K at various frequencies were performed, showing a weak dip in the modulus (1.9%) and no Q^-1 peak at the magnetic transition, unlike La_2/3Ca_1/3MnO₃. Two Q^-1 peaks were observed, one above T_c (414.5±0.2 K at 1 Hz) and another below T_c (303±6 K at 1 Hz). With increasing measurement frequency between 0.3 and 20 Hz, a shift to higher temperatures of the centers of both peaks was observed. Such shifts yielded an activation energy of 3.77±0.05 eV for the high-temperature peak and 0.56±0.07 eV for the low-temperature peak. Evidence was found that the high-temperature peak is not a simple relaxation process, but shows a relaxation time distribution. Relaxation peaks are attributed to jumps of oxygen ions within the crystal under applied stress

[en] Specific heat measurements of neutron-irradiated crystalline quartz are reported between 0.2 and 4 K. A large specific heat, similar to that found in Suprasil W vitreous silica, is observed in crystalline quartz after an irradiation dose of 5 x 10¹⁸ neutrons/cm². (author)

[en] Audiofrequency dielectric measurements down to 4 mK on gamma-irradiated natural quartz crystals revealed the existence of anomalous behaviour. The results show a dielectric relaxation time which decreases with decreasing temperature and suggest a new relaxation mechanism in smoky quartz. (author)

[en] Audiofrequency dielectric measurements down to 4 mK on γ-irradiated quartz are reported. At the lowest temperatures, the mean relaxation rate increases with dipole concentration and shows a field dependence in small ac measuring fields. Similar properties are observed in other dielectric systems containing randomly distributed electric dipoles. (author)

[en] The charge density wave (CDW) nonlinear conductivity of the blue bronzes A_0.30MoO₃ (A = K, Rb) shows two different regimes depending on the temperature: a strongly damped CDW motion above ∼50 K and a CDW motion with almost no damping below ∼50 K. In a search for an elastic signature of this CDW behaviour, we performed ultrasonic measurements on A_0.30MoO₃ single crystals in the temperature range 4-300 K. In Rb_0.30MoO₃, at T∼50 K, upon cooling, a large increase of the sound velocity for the longitudinal mode measured along the 2-bar01, [102] and b directions is observed. The ultrasonic attenuation coefficient shows an increase down to 50 K followed by a plateau. Similar results are found in K_0.30MoO₃. In V-doped samples, Rb_0.30(Mo_1-xV_x)O₃ (x = 0.4%) the anomaly broadens and is shifted towards higher temperatures. The results are discussed in relation to the changes in the CDW rigidity, disorder and dielectric response. A scenario based on a glass transition for the CDW superstructure is proposed.

[en] We report thermal expansion measurements in La_2/3(Ca_1-x Sr_x) _1/3MnO₃ perovskites. For x=0 and 0.05 a very narrow and symmetric cusp is observed at T_C, indicating the transition is first order, changing to second order above x=0.15. The data of an x=1 single crystal was fitted in the critical region yielding α=-0.14±0.10 for the critical exponent and A₊/A_-=1.4±0.4 for the amplitude ratio, both characteristic of 3d-Heisenberg systems. The magnetic field dependence is observed to decrease with increasing x