arXiv subjects

T. H. Johansen

Publications and source records attributed to T. H. Johansen.

75 records · Page 5Linked to original sources

arXiv1998

Central peak position in magnetization loops of high-$T_c$ superconductors

Exact analytical results are obtained for the magnetization of a superconducting thin strip with a general behavior J_c(B) of the critical current density. We show that within the critical-state model the magnetization as function of applied field, B_a, has an extremum located exactly at B_a=0. This result is in excellent agreement with presented experimental data for a YBCO thin film. After introducing granularity by patterning the film, the central peak becomes shifted to positive fields on the descending field branch of the loop. Our results show that a positive peak position is a definite signature of granularity in superconductors.

cond-mat.supr-con↗

arXiv1998

Spatially-resolved studies of chemical composition, critical temperature, and critical current density of YBaCuO thin film

Spatially-resolved studies of a YBaCuO thin film bridge using electron probe microanalysis (EPMA), low-temperature scanning electron microscopy (LTSEM), and magneto-optical flux visualization (MO) have been carried out. Variations in chemical composition along the bridge were measured by EPMA with 3 microns resolution. Using LTSEM the spatial distributions of the critical temperature, Tc, and of the local transition width, dTc, were determined with 5 microns resolution. Distributions of magnetic flux over the bridge in an applied magnetic field have been measured at 15 and 50 K by magneto-optical technique. The critical current density Jc as a function of coordinate along the bridge was extracted from the measured distributions by a new specially developed method. Significant correlations between Jc, Tc, dTc and cation composition have been revealed. It is shown that in low magnetic fields deviation from the stoichiometric composition leads to a decrease in both Tc and Jc. The profile of Jc follows the Tc-profile on large length scales and has an additional fine structure on short scales. The profile of Jc along the bridge normalized to its value at any point is almost independent of temperature.

cond-mat.supr-con↗

arXiv1997

Shape Distortion by Irreversible Flux-Pinning-Induced Magnetostriction

Exact analytical results are obtained for the flux-pinning-induced magnetostriction in cylindrical type-II superconductors placed in parallel magnetic field. New modes of irreversible deformation are found: In contrast to the circular cylinder where shape is conserved, it is shown that a square cross-section deforms with considerable distortion. During a field cycle both concave, convex, and even more complicated distortions are predicted. Strong implications for dilatometric measurements on crystals are emphasized. The main results are valid for any critical-state model, j_c = j_c(B).

cond-mat.supr-con↗