Combining macroscopic and diffraction methods we have studied the electric, magnetic and struc- tural properties of RE_(1-x)Ca_xTiO_3 (RE=Y, Er, Lu) focusing on the concentration range near the metal-insulator transition. The insulating phase, which
is stabilized by a smaller rare-earth ionic ra- dius, exhibits charge order with a predominant occupation of the dxy orbital. The charge and orbital ordering explains the broad stability range of the insulating state in RE_(1-x)Ca_xTiO_3 with smaller rare-earth ions. The strong modulation of the Ti-O bond distances indicates sizeable modulation of the electric charge.
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.
We present experiments on the magnetic field-dependent thermal transport in the spin-1/2 ladder system (C$_5$H$_{12}$N)$_2$CuBr$_4$. The thermal conductivity $kappa(B)$ is only weakly affected by the field-induced transitions between the gapless Lutt
inger-liquid state realized for $B_{c1}< B < B_{c2}$ and the gapped states, suggesting the absence of a direct contribution of the spin excitations to the heat transport. We observe, however, that the thermal conductivity is strongly suppressed by the magnetic field deeply within the Luttinger-liquid state. These surprising observations are discussed in terms of localization of spinons within finite ladder segments and spinon-phonon umklapp scattering of the predominantly phononic heat transport.
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.
