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High pressure 149Sm nuclear forward scattering experiments have been performed on the nonmagnetic semiconductor SmS. We present the first clear evidence that the closure of the insulating gap at p(Delta) approximately 2 GPa coincides with the appearance of magnetic order. The pressure-induced magnetic phase transition has some first order character and suggests that the Sm ions are nearly trivalent at p(Delta). A Gamma(8) quartet crystal field ground state with a value of approximately 0.5 micro(B) for the samarium magnetic moment is inferred from our results. Considerable magnetic short range order is observed above the ordering temperature inferred from macroscopic measurements.

We report high-pressure x-ray diffraction and magnetization measurements combined with ab-initio calculations to demonstrate that the high-pressure optical and transport transitions recently reported in TiOCl, correspond in fact to an enhanced Ti Interaction among different degrees of freedom provides a way for opening up a gap for the collective excitations, and different types of long-range order become then possible.Among the latter, a magnetoelastic coupling in S=1/2 chains results in a spin-Peierls phase transition to a low temperature nonmagnetic dimerized structure, 3,4 which is a localized counterpart of a conventional Peierls transition for itinerant electrons. 5 This effect was found in CuGeO 3 , 6 for a long time the only inorganic system with a pure spin-Peierls distortion. Recently, the possibility that TiOCl shows a low temperature spin-Peierls dimerization has been suggested, although the behavior of this material is

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We find that the unusual pressure dependence of the Kondo temperature in Yb-based heavy-fermion (HF) compounds can be explained by the competition between two mechanisms of coupling between f electrons and the lattice. An increase of the hybridization between f and conduction electron states under pressure results in the increase of the Kondo temperature while the suppression of valency fluctuations of rare-earth ions under pressure leads to the decrease of the Kondo temperature. In contrast to Yb compounds, in Ce compounds pressure enhances both the hybridization and valency fluctuations. As a result the Kondo temperature increases. We compare the theory with available experimental data and find a qualitative and quantitative agreement.