EuTiO3 is shown to exhibit novel strong spin-charge-lattice coupling deep in the paramagnetic phase. Its existence is evidenced by an, until now, unknown response of the paramagnetic susceptibility at temperatures exceeding the structural phase transition temperature T(S) = 282 K. The 'extra' features in the susceptibility follow the rotational soft zone boundary mode temperature dependence above and below TS. The theoretical modeling consistently reproduces this behavior and provides reasoning for the stabilization of the soft optic mode other than quantum fluctuations.

The magnetic and lattice properties of the S=1/2 quantum-spin-chain ferromagnet, CuAs2O4, mineral name trippkeite, were investigated. The crystal structure of CuAs2O4 is characterized by the presence of corrugated CuO2 ribbon chains. Measurements of the magnetic susceptibility, heat capacity, electron paramagnetic resonance and Raman spectroscopy were performed. Our experiments conclusively show that a ferromagnetic transition occurs at ∼7.4 K. Ab initio DFT calculations reveal dominant ferromagnetic nearest-neighbor and weaker antiferromagnetic nextnearest-neighbor spin exchange interactions along the ribbon chains. The ratio of Jnn/Jnnn is near -4, placing CuAs2O4 in close proximity to a quantum critical point in the Jnn -Jnnn phase diagram. TMRG simulations used to analyze the magnetic susceptibility confirm this ratio. Single-crystal magnetization measurements indicate that a magnetic anisotropy forces the Cu 2+ spins to lie in an easy plane perpendicular to the c-axis. An analysis of the field and temperature dependent magnetization by modified Arrott plots reveals a 3d-XY critical behavior. Lattice perturbations induced by quasi-hydrostatic pressure and temperature were mapped via magnetization and Raman spectroscopy.

The antiferromagnetic (AFM) phase transition temperature T N of EuTiO 3 has been studied as a function of pressure p. The data reveal a nonlinear dependence of T N on p with T N increasing with increasing pressure. The exchange interactions exhibit an analogous dependence on p as T N (if the absolute value of the nearest neighbor interaction is considered) and there is evidence that the AFM transition is robust with increasing pressure. The corresponding Weiss temperature Θ W remains anomalous since it always exhibits positive values. The data are analyzed within the Bloch power law model and provide excellent agreement with experiment.Pacs-Index: 75.50. Ee, 75.30.Kz, 61.50.Ks