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The phase diagram of the heavy fermion compound YbFe 2 Ge 2 under high pressures P ഛ 18.2 GPa was obtained by electrical resistivity measurements. Pressure drives the system from a paramagnetic Fermi liquid state to a magnetically ordered state, with a quantum critical point at P C Ϸ 9.4 GPa. In the vicinity of P C a non-Fermi-liquid behavior ascribed to two-dimensional antiferromagnetic fluctuations is observed. In the magnetic side, the resistivity shows the existence of spin-wave excitations characteristic of an antiferromagnet.

The phonon density of states ͑DOS͒ of ␤-Sn has been directly determined using nuclear inelastic scattering of synchrotron radiation by 119 Sn nuclei. The obtained phonon DOS is in good agreement with theoretical calculations and provides an accurate estimation of the Lamb-Mössbauer factor, the mean-square displacement of the atoms and the lattice contribution to the specific heat in ␤-Sn. RAPID COMMUNICATIONS R14 884PRB 61 A. BARLA et al.

The effect of pressure on the electronic and magnetic properties of the antiferromagnetic ͑T N ϳ 43 K͒ narrow gap semiconductor UNiSn has been investigated by 119 Sn Mössbauer spectroscopy and nuclear forward scattering of synchrotron radiation, electrical resistance, and x-ray diffraction. We show that the decrease of the semiconducting gap which leads to a metallic state at p ϳ 9 GPa is associated with an enhancement of T N. At higher pressures, both T N and the transferred magnetic hyperfine field decrease, with a collapse of magnetism at ϳ18.5 GPa. The results are explained by a volume-dependent competition between indirect Ruderman-Kittel-Kasuya-Yosida interaction and the 5f-ligand hybridization.