Electrodynamics of the vortex lattice in untwinned YBaCuO by complex impedance measurements

We report complex impedance measurements in an untwinned YBaCuO crystal. Our broad frequency range covers both the quasi static response and the resistive response of the vortex lattice. It allow us to characterize the irreversibility line without the need of any frequency dependent pinning parameters. We confirm the validity of the two modes model of vortex dynamic, and extract both the surface critical current and the flux flow resistivity around the first order transition T_m. This latter is identified by the abrupt loss of pinning and by an unexpected step of (T) at T_m. Received 22 November 2002 / Received in final form 17 February 2003 Published online 20 June 2003 RID="a" ID="a"e-mail: alain.pautrat@ismra.fr RID="b" ID="b"UMR 6508 associée au CNRS     

Transformation of indices of planes and directions in twinned crystals

The transformation formulae for indices of planes and directions in twinned crystals are computed for the general case of an arbitrary crystal system. Using the notation of tensor calculus all the formulae are obtained in a relatively simple form and can be used for the interpretation of X-ray diffraction patterns and electron microscopy diffraction patterns as well as in other problems concerning twinning.     

The procedure in solving the structure of some twinned crystals

Difficulties in solving the crystal structure of substances which crystallize as twins are discussed. The procedure used in certain special cases of twinning is demonstrated on two examples.     

Absorbance spectra of polycrystalline samples and twinned crystals of oligothiophenes

We propose optical absorption technique at oblique incidence as one of the spectroscopic tools that allow experimentally recognizing the macroscopic order and structural features of molecular solids of conjugated molecules, from single crystals to polycrystalline or twinned samples. We apply this spectroscopy to quaterthiophene as representative of a wide class of materials that usually possess optical transitions of Frenkel exciton origin with strong directional dispersion. The comparison between experimental and simulated data gives evidence of the high sensitivity of this technique for determining quantitatively the polycrystallinity of the measured samples, whose domains may show mirror-like orientation of the unit cell with respect to one of its faces.Frenkel exciton; Oligothiophene, Optical properties     

Change in the Bauschinger effect during the relaxation of twinned crystals

Changes taking place in the quantitative characteristics of the Bauschinger effect in wedge-shaped twins are studied in relation to the period for which these are held before untwinning. All the measurements are made in the cleavage plane of single-crystal zinc and Zn-Cu alloys. The Bauschinger displacement of the twin boundaries first increases slightly and then starts diminishing during the delay period. In the course of untwinning, the region of easy untwinning on the (b) relationship diminishes with time in the softened zone; the extent of the softened zone remains constant during the relaxation of the crystals. The suppression of the Bauschinger effect takes place more intensively if the crystal is held under load before untwinning. The change in the quantitative characteristics of the Bauschinger effect may be explained by the diffusion of impurities to the twinning dislocations and dislocation sources and the consequent blocking of these, and also by the relaxation of stresses in the twin boundaries as a result of plastic shears and dislocation reactions between complete and twinning dislocations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 113–117, April, 1971.     

Vortex dynamics in YBaCuO single crystals with point- and line-like defects-flux creep studies

Studies of vortex dynamics and pinning were conducted in irradiated Y₁Ba₂Cu₃O₇ single crystals by observing the decay of supercurrents with time. The materials contained either point-like defects from proton irradiation or line-like defects from irradiation with energetic heavy ions. Analysis of the first case gives clear support for collective creep theory. With linear defects, the experiments give evidence for thermal smearing of the vortex pinning potential at higher temperatures, as predicted theoretically.     

Why an ac magnetic field shifts the irreversibility line in type-II superconductors

We show that for a thin superconducting strip placed in a transverse dc magnetic field--the typical geometry of experiments with high-T(c) superconductors--the application of a weak ac magnetic field perpendicular to the dc field generates a dc voltage in the strip. This voltage leads to the decay of the critical currents circulating in the strip, and eventually the equilibrium state of the superconductor is established. This relaxation is not due to thermally activated flux creep but to the "walking" motion of vortices in the two-dimensional critical state of the strip with in-plane ac field. Our theory explains the shaking effect that was used for detecting phase transitions of the vortex lattice in superconductors.     

Gravitation and irreversibility

The internal time operatorM is an alternative to the usual dynamic time, an independent parameter of motion. Even when the dynamical entity is the three-geometry⁽³⁾ G and we are concerned with its evolution in superspace (the problem of cosmological evolution), dynamical time remains an independent parameter associated with a choice of lapse and shift functions. The quantityM is, on the contrary, an ensemble-dependent parameter related to the age of a process: the entirety of the ensemble's evolution. With this different view of time as age, we seek a geometrical counterpart toM for the⁽³⁾ G as an ensemble. For a closed negatively curved universe, a Lyapounov function can be established which allows anM to be defined for the Robertson-Walker universe. The time component of superspace momentum _uv is introduced, and we identify its conjugate energyS/ _uv with dissipation due to the evolving universe. A geometrical counterpart ofM is introduced by a conformal invariant . This quantity simultaneously expresses (i) the topological feature of orientation-preserving transformations, and (ii) the Hamiltonian treatment of dissipative systems. This dual character of , which links topological change to dissipative systems, suggests a geometrical basis forM. In this sense irreversibility is incorporated into the geometric structure of space-time, along with gravitation.