EuC$_2$ is a ferromagnet with a Curie-temperature of $T_C simeq 15,$K. It is semiconducting with the particularity that the resistivity drops by about 5 orders of magnitude on cooling through $T_C$, which is therefore called a metal-insulator transit
ion. In this paper we study the magnetization, specific heat, thermal expansion, and the resistivity around this ferromagnetic transition on high-quality EuC$_2$ samples. At $T_C$ we observe well defined anomalies in the specific heat $c_p(T)$ and thermal expansion $alpha(T)$ data. The magnetic contributions of $c_p(T)$ and $alpha(T)$ can satisfactorily be described within a mean-field theory, taking into account the magnetization data. In zero magnetic field the magnetic contributions of the specific heat and thermal expansion fulfill a Gruneisen-scaling, which is not preserved in finite fields. From an estimation of the pressure dependence of $T_C$ via Ehrenfests relation, we expect a considerable increase of $T_C$ under applied pressure due to a strong spin-lattice coupling. Furthermore the influence of weak off stoichiometries $delta$ in EuC$_{2 pm delta}$ was studied. It is found that $delta$ strongly affects the resistivity, but hardly changes the transition temperature. In all these aspects, the behavior of EuC$_2$ strongly resembles that of EuO.
LiFeAs is unique among the broad family of FeAs-based superconductors, because it is superconducting with a rather large $T_csimeq 18$ K under ambient conditions although it is a stoichiometric compound. We studied the electrical transport on a high-
quality single crystal. The resistivity shows quadratic temperature dependence at low temperature giving evidence for strong electron-electron scattering and a tendency towards saturation around room temperature. The Hall constant is negative and changes with temperature, what most probably arises from a van Hove singularity close to the Fermi energy in one of the hole-like bands. Using band structure calculations based on angular resolved photoemission spectra we are able to reproduce all the basic features of both the resistivity as well as the Hall effect data.
The crystal structure and the magnetism of BaMn$_2$O$_3$ have been studied by thermodynamic and by diffraction techniques using large single crystals and powders. BaMn$_2$O$_3$ is a realization of a $S = 5/2$ spin ladder as the magnetic interaction i
s dominant along 180$^circ$ Mn-O-Mn bonds forming the legs and the rungs of a ladder. The temperature dependence of the magnetic susceptibility exhibits well-defined maxima for all directions proving the low-dimensional magnetic character in BaMn$_2$O$_3$. The susceptibility and powder neutron diffraction data, however, show that BaMn$_2$O$_3$ exhibits a transition to antiferromagnetic order at 184 K, in spite of a full frustration of the nearest-neighbor inter-ladder coupling in the orthorhombic high-temperature phase. This frustration is lifted by a remarkably strong monoclinic distortion which accompanies the magnetic transition.
We found that under pressure SnO with alpha-PbO structure, the same structure as in many Fe-based superconductors, e.g. beta-FeSe, undergoes a transition to a superconducting state for p > 6 GPa with a maximum Tc of 1.4 K at p = 9.3 GPa. The pressure
dependence of Tc reveals a dome-like shape and superconductivity disappears for p > 16 GPa. It is further shown from band structure calculations that SnO under pressure exhibits a Fermi surface topology similar to that reported for some Fe-based superconductors and that the nesting between the hole and electron pockets correlates with the change of Tc as a function of pressure.
We present high-resolution measurements of the $c^star$-axis thermal expansion and magnetostriction of piperidinium copper bromide hp. The experimental data at low temperatures is well accounted for by a two-leg spin-ladder Hamiltonian. The thermal e
xpansion shows a complex behaviour with various sign changes and approaches a $1/sqrt{T}$ divergence at the critical fields. All low-temperature features are semi-quantitatively explained within a free fermion model; full quantitative agreement is obtained with Quantum Monte Carlo simulations.
We report measurements of the thermal expansion for two Eu$^{+2}$- and two Gd$^{+3}$-based intermetallics which exhibit ferro- or antiferromagnetic phase transitions. These materials show sharp positive (EuAgMg and GdAuMg) and negative (EuAuMg and Gd
AgMg) peaks in the temperature dependence of the thermal expansion coefficient $alpha$ which become smeared and/or displaced in an external magnetic field. Together with specific heat data we determine the initial pressure dependences of the transition temperatures at ambient pressure using the Ehrenfest or Clausius-Clapeyron relation. We find large pressure dependences indicating strong spin-phonon coupling, in particular for GdAgMg and EuAuMg where a quantum phase transition might be reached at moderate pressures of a few GPa.
