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

Critical Currents and Order-Disorder Phase Transition in the Vortex States of YBa2Cu4O8 with Chemically Introduced Disorder

A series of YBa_{2-x}Sr_xCu_4O_8 single crystals was measured to study the influence of site disorder on the transition line Hss(T) between quasi-ordered vortex lattice and highly disordered vortex glass, as well as on the maximum critical current density within the glass phase, jcmax. When 32% of Ba is replaced by Sr, jcmax is an order of magnitude higher than in the unsubstituted compound. In contrast, the transition field Hss first drops by a factor of about five with a substitution of just 10% of Sr for Ba, and then remains approximately constant for higher Sr contents. Our results indicate that in very clean systems the order-disorder transition is affected very strongly by any crystallographic disorder, while above a certain threshold it is relatively robust with respect to additional disorder. In all substituted crystals Hss monotonically decreases with an increase of temperature.Comment: Text revised, some figures replaced, 11 pages, 10 figures, to appear in PR

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

Dielectric properties of charge ordered LuFe2O4 revisited: The apparent influence of contacts

We show results of broadband dielectric measurements on the charge ordered, proposed to be mul- tiferroic material LuFe2O4. The temperature and frequency dependence of the complex permittivity as investigated for temperatures above and below the charge-oder transition near T_CO ~ 320 K and for frequencies up to 1 GHz can be well described by a standard equivalent-circuit model considering Maxwell-Wagner-type contacts and hopping induced AC-conductivity. No pronounced contribution of intrinsic dipolar polarization could be found and thus the ferroelectric character of the charge order in LuFe2O4 has to be questioned.Comment: 4 pages, 3 figure

Hypomania Checklist-32 - cross-validation of shorter versions screening for bipolar disorders in an epidemiological study.

Self-reports such as Hypomania Checklist (HCL-32) can be used to enhance recognition of bipolar disorders, but they are often too long and only validated in clinical samples. The objectives of this study are therefore to test whether (i) the HCL-32 can be used for screening in the community and (ii) whether two previously suggested shorter versions would do as well. Data stemmed from the CoLaus|PsyColaus, a prospective cohort study which included randomly selected residents aged 35-66 years from an urban area. Participants underwent semistructured interviews to assess DSM-IV disorders and 1712 of them completed the HCL-32. Forty individuals (2.3%) were diagnosed as having BD. Compared to others, participants with BD scored significantly higher on the HCL-32. The HCL-32 had a sensitivity of 0.78 and specificity of 0.68. Very similar figures were found for two previously proposed shorter versions with 16 and 20 items. The results of confirmatory factor analysis and item response theory (IRT) models supported the postulated two-factor structure for the three HCL versions. Despite the low base rate of BD in this sample, the screening properties of the HCL-32 remained almost as good. Importantly, two previously proposed shorter versions performed as well, suggesting that those could be used without losing essential information