Anisotropic properties of MgB2 by torque magnetometry

Anisotropic properties of superconducting MgB2 obtained by torque magnetometry are compared to theoretical predictions, concentrating on two issues. Firstly, the angular dependence of Hc2 is shown to deviate close to Tc from the dependence assumed by anisotropic Ginzburg-Landau theory. Secondly, from the evaluation of torque vs angle curves it is concluded that the anisotropy of the penetration depth gamma_lambda has to be substantially higher at low temperature than theoretical estimates, at least in fields higher than 0.2 T.Comment: 2 p.,2 Fig., submitted to Physica C (M2S-Rio proceedings); v2: 1 ref adde

Explosive Ising

We study a two-dimensional kinetic Ising model with Swendsen-Wang dynamics, replacing the usual percolation on top of Ising clusters by explosive percolation. The model exhibits a reversible first-order phase transition with hysteresis. Surprisingly, at the transition flanks the global bond density seems to be equal to the percolation thresholds.Comment: 7 pages, 5 figure

Differential thermal analysis and solution growth of intermetallic compounds

To obtain single crystals by solution growth, an exposed primary solidification surface in the appropriate, but often unknown, equilibrium alloy phase diagram is required. Furthermore, an appropriate crucible material is needed, necessary to hold the molten alloy during growth, without being attacked by it. Recently, we have used the comparison of realistic simulations with experimental differential thermal analysis (DTA) curves to address both these problems. We have found: 1) complex DTA curves can be interpreted to determine an appropriate heat treatment and starting composition for solution growth, without having to determine the underlying phase diagrams in detail. 2) DTA can facilitate identification of appropriate crucible materials. DTA can thus be used to make the procedure to obtain single crystals of a desired phase by solution growth more efficient. We will use some of the systems for which we have recently obtained single-crystalline samples using the combination of DTA and solution growth as examples. These systems are TbAl, Pr$_7$Ni$_2$Si$_5$, and YMn$_4$Al$_8$.Comment: 17 pages, 8 figure

Anisotropy of the superconducting state properties and phase diagram of MgB2 by torque magnetometry on single crystals

The angular and temperature dependence of the upper critical field Hc2 in MgB2 was determined from torque magnetometry measurements on single crystals. The Hc2 anisotropy gamma_H was found to decrease with increasing temperature, in disagreement with the anisotropic Ginzburg-Landau theory, which predicts that the gamma_H is temperature independent. This behaviour can be explained by the two band nature of superconductivity in MgB2. An analysis of measurements of the reversible torque in the mixed state yields a field dependent effective anisotropy gamma_eff, which can be at least partially explained by different anisotropies of the penetration depth and the upper critical field. It is shown that a peak effect in fields of about 0.85 Hc2 is a manifestation of an order-disorder phase transition of vortex matter. The H-T phase diagram of MgB2 for H//c correlates with the intermediate strength of thermal fluctuations in MgB2, as compared to those in high and low Tc superconductors.Comment: 12 pages, 8 figures, Physica C in print (invited paper for a special issue on MgB2