Synthesis and crystal growth

Abstract:

An ac photopyroelectric calorimeter has been used to measure the thermal diffusivity of the perovskite manganite family La1-xSr xMnO3 on a set of single crystals with doping range 0≤x≤0.35. Taking into account that the inverse of the thermal diffusivity has the same critical behavior as the specific heat, the critical exponent a of the magnetic transitions has been obtained. The results point to short-range interaction models for pure magnetic transitions. In the pure and lightly doped samples (x<0.10), where the transition is antiferromagnetic-paramagnetic, the critical exponent is consistent with the Heisenberg model (α=-0.11). For the highly doped samples (x>0.28), where there is a pure ferromagnetic- paramagnetic transition, the critical exponent is exactly that of an Ising behavior (α=+0.11). For 0.10≤x<0.28 no universality class was found and this behavior has been discussed taking into account the complexity of the phase diagram in this concentration range.

Abstract:

Muonium centres in Cu2O, Ag2O and CdO show hyperfine parameters spanning four orders of magnitude. They exemplify the three different categories of hydrogen defect centre in semiconducting and dielectric solids, with very different electronic structure and electrical activity, namely quasi-atomic (possibly deep acceptor), deep donor and shallow donor. © Springer 2005.

Abstract:

We have investigated the optimum conditions for the growth of Na xCoO2 single crystals by the optical floating-zone technique. A significant reduction in Na loss, was achieved by using a high-pressure growth atmosphere. A high-pressure oxygen-rich atmosphere during crystal growth was found to reduce the presence of CO3O4 impurity phase in the grown crystal. The magnetic properties of powder and single crystal samples were measured by superconducting quantum interference device (SQUID) magnetometry and found to vary according to the preparation conditions. © 2004 Elsevier B.V. All rights reserved.