The phase diagram of Sr${}_{1-x}$Eu${}_x$TiO${}_3$ is determined experimentally by electron paramagnetic resonance and resistivity measurements and analyzed theoretically within the self-consistent phonon approximation as a function of $x$ (0.03 ⩽ $x$ ⩽ 1.0). The transition temperature of the structural instability of the system increases nonlinearly to higher temperatures with increasing $x$. This is interpreted theoretically by a substantial alteration in the dynamics caused by a change in the double-well potential from broad and shallow to narrow and deep.

An extended study of the superconducting and normal-state properties of various as-grown and post-annealed Rb${}_x$Fe${}_{2-y}$Se${}_2$ single crystals is presented. Magnetization experiments evidence that annealing of Rb${}_x$Fe${}_{2-y}$Se${}_2$ at 413 K, well below the onset of phase separation $T_{\mathrm{p}}\simeq 489$ K, neither changes the magnetic nor the superconducting properties of the crystals. In addition, annealing at 563 K, well above $T_{\mathrm{p}}$, suppresses the superconducting transition temperature $T_{\mathrm{c}}$ and leads to an increase of the antiferromagnetic susceptibility accompanied by the creation of ferromagnetic impurity phases, which are developing with annealing time. However, annealing at $T=488\mathrm{K}\simeq T_{\mathrm{p}}$ increases $T_{\mathrm{c}}$ up to 33.3 K, sharpens the superconducting transition, increases the lower critical field, and strengthens the screening efficiency of the applied magnetic field. Resistivity measurements of the as-grown and optimally annealed samples reveal an increase of the upper critical field along both crystallographic directions as well as its anisotropy. Muon spin rotation and scanning transmission electron microscopy experiments suggest the coexistence of two phases below $T_{\mathrm{p}}$: a magnetic majority phase of Rb${}_2$Fe${}_4$Se${}_5$ and a nonmagnetic minority phase of Rb${}_{0.5}$Fe${}_2$Se${}_2$. Both microscopic techniques indicate that annealing the specimens just at $T_{\mathrm{p}}$ does not affect the volume fraction of the two phases, although the magnetic field distribution in the samples changes substantially. This suggests that the microstructure of the sample, caused by mesoscopic phase separation, is modified by annealing just at $T_{\mathrm{p}}$, leading to an improvement of the superconducting properties of Rb${}_x$Fe${}_{2-y}$Se${}_2$ and an enhancement of $T_{\mathrm{c}}$.

The basic features of multi-band superconductivity and its implications are derived. In particular, it is shown that enhancements of the superconducting transition temperature take place due to interband interactions. In addition, isotope effects differ substantially from the typical BCS scheme as soon as polaronic coupling effects are present. Special cases of the model are polaronic coupling in one band as realized e.g., in cuprates, coexistence of a flat band and a steep band like in MgB₂, crossovers between extreme cases. The advantages of the multiband approach as compared to the single band BCS model are elucidated and its rather frequent realization in actual systems discussed.

We investigate the low temperature X-band electron paramagnetic resonance (EPR) of YBa(2)Cu(3)O(x) compounds with x congruent with 6.0 doped with Dy(3+), Tb(3+), and Nd(3). The EPR spectra of Dy(3+) and Tb(3+) have been identified. The EPR of Tb(3+) is used also to study the effect of suppression of high T(c) superconductivity by doping with Tb(3+). The EPR of Nd(3+) is probably masked by the intense resonance of Cu(2+). All experimental EPR results compare well with theoretical estimations.

Oxygen isotope effect on the charge-ordering transition was studied in ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3.$ Upon replacing ${}^{16}\mathrm{O}$ with ${}^{18}\mathrm{O},$ the charge-ordering temperature increases by 9 K under zero magnetic field. With an increase of the magnetic field, the oxygen isotope shift increases rapidly and reaches a magnitude of about 40 K under a magnetic field of 5.4 T. The present results cannot be explained by the existing theories of the charge-ordering transition in manganites.

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