[en] Spin ladder compounds are valuable in understanding the physics of crossover from one dimensional to the two dimensional nature of spin systems. They have been known to show interesting magnetic ground states and even superconductivity upon doping. Gapped and ungapped spin excitation spectra have been predicted and experimentally verified for spin ladders with even and odd number of ladder-legs respectively. Therefore, this class of materials has been of particular interest in the field of low dimensional quantum magnets. It is known that in the thermodynamic phase diagram of La₂O₃-CuO there exists a compound whose spin structure resembles that of a five leg ladder, though there has not been much progress towards understanding its magnetism as it is challenging to grow it as a single crystal. Here, we have used the travelling floating zone method to grow single crystals of La₈Cu₇O₁₉ and measured some of its physical properties. This compound magnetically orders below 103 K and shows anisotropic magnetic behavior, the nature of which is not yet clear.

[en] La_2_-_xBa_2CuO_4 displays an anomalous doping dependence associated with a deep suppression of superconductivity at the hole concentration x=1/8. The so-called 1/8-anomaly is accompanied by a structural transition in the average rotational symmetry of the CuO_2 planes coinciding with the onset of a charge stripe order. It has been claimed that static stripe order destroys the superconducting phase coherence, while dynamic stripe correlations may promote superconductivity. In order to achieve more information about the relationship between superconductivity, stripe order and crystal structure we performed magnetization measurements under pressure up to 3 GPa on the single crystalline La_2_-_xBa_2CuO_4 with 0.095 ≤ x ≤ 0.125. Moreover, we relate the magnetization data to pressure dependent X-Ray diffraction studies. This way, we show that the specific superconducting properties as function of pressure are clearly correlated to structural changes.

[en] The interplay between electronic correlations and spin-orbit coupling in heavy transition metal compounds has been intensively studied in the last years due to their interesting properties and unusual ground states like quantum spin liquids. Particularly α-RuCl_3 seems to be a suitable candidate for the experimental realisation of the Kitaev-Model due to its J_e_f_f = 1/2 state and its layered honeycomb lattice of Ru"3"+ ions in the 4d"5 configuration. This leads to highly anisotropic magnetic properties in this compound. We report on specific heat and magnetisation measurements for α-RuCl_3 single crystals grown by means of chemical transport reactions. Furthermore magnetisation experiments under high pressure were conducted on this compound.

[en] We report on the effect of nearly-hydrostatic pressure P ≤ 23 kbar in an optimally-doped LaFeAsO_1_-_xF_x sample. Our results of muon spin rotation show no dependence of T_c on P and, at the same time, a remarkable enhancement of the superfluid density (n_s). This is a dramatic effect, the saturation value of n_s being increased by ∝ 30 % at the maximum P value. We provide evidence from density-functional theory (DFT) calculations that this increase should not be associated to an induced change in the fermiology of LaFeAsO_1_-_xF_x. Accordingly, we suggest that the experimental data can be explained by assuming a modification of the ratio between intra- and inter-band impurity scattering, only possible in multi-band SC.