Observation of insulator-metal transition in EuNiO3_{3} under high pressure

The charge transfer antiferromagnetic (T$_{N}$ =220 K) insulator EuNiO$_{3}$ undergoes, at ambient pressure, a temperature-induced metal insulator MI transition at T$_{MI}$=463 K. We have investigated the effect of pressure (up to p~20 GPa) on the electronic, magnetic and structural properties of EuNiO$_{3}$ using electrical resistance measurements, {151}^Eu nuclear resonance scattering of synchrotron radiation and x-ray diffraction, respectively. With increasing pressure we find at p$_{c}$ =5.8 GPa a transition from the insulating state to a metallic state, while the orthorhombic structure remains unchanged up to 20 GPa. The results are explained in terms of a gradual increase of the electronic bandwidth with increasing pressure, which results in a closing of the charge transfer gap. It is further shown that the pressure-induced metallic state exhibits magnetic order with a lowervalue of T$_{N}$ (T$_{N}$ ~120 K at 9.4 GPa) which disappears between 9.4 and 14.4 GPa.Comment: 10 pages, 3 figure

Valence and magnetic instabilities in Sm compounds at high pressures

We report on the study of the response to high pressures of the electronic and magnetic properties of several Sm-based compounds, which span at ambient pressure the whole range of stable charge states between the divalent and the trivalent. Our nuclear forward scattering of synchrotron radiation and specific heat investigations show that in both golden SmS and SmB6 the pressure-induced insulator to metal transitions (at 2 and about 4-7 GPa, respectively) are associated with the onset of long-range magnetic order, stable up to at least 19 and 26 GPa, respectively. This long-range magnetic order, which is characteristic of Sm(3+), appears already for a Sm valence near 2.7. Contrary to these compounds, metallic Sm, which is trivalent at ambient pressure, undergoes a series of pressure-induced structural phase transitions which are associated with a progressive decrease of the ordered 4f moment.Comment: 15 pages (including 7 figures) submitted to J. Phys.: Condens. Matte

Epitaxial Stabilization of Ultrathin Films of Rare-Earth Nickelates

We report on the synthesis of ultrathin films of highly distorted EuNiO3 (ENO) grown by interrupted pulse laser epitaxy on YAlO3 (YAO) substrates. Through mapping the phase space of nickelate thin film epitaxy, the optimal growth temperatures were found to scale linearly with the Goldschmidt tolerance factor. Considering the gibbs energy of the expanding film, this empirical trend is discussed in terms of epitaxial stabilization and the escalation of the lattice energy due to lattice distortions and decreasing symmetry. These findings are fundamental to other complex oxide perovskites, and provide a route to the synthesis of other perovskite structures in ultrathin-film form.Comment: 7 pages, 3 figure

Short and canonical GRBs

Within the "fireshell" model for the Gamma-Ray Bursts (GRBs) we define a "canonical GRB" light curve with two sharply different components: the Proper-GRB (P-GRB), emitted when the optically thick fireshell of electron-positron plasma originating the phenomenon reaches transparency, and the afterglow, emitted due to the collision between the remaining optically thin fireshell and the CircumBurst Medium (CBM). We outline our "canonical GRB" scenario, with a special emphasis on the discrimination between "genuine" and "fake" short GRBs.Comment: 4 pages, 3 figures, in the Proceedings of the "Gamma Ray Bursts 2007" meeting, November 5-9, 2007, Santa Fe, New Mexico, US

Pressure driven collapse of the magnetism in the Kondo insulator UNiSn

The effect of pressure on the electronic and magnetic properties of the antiferromagnetic (TN~43 K) narrow gap semiconductor UNiSn has been investigated by 119Sn 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~9 GPa is associated with an enhancement of TN. At higher pressures, both TN and the transferred magnetic hyperfine field decrease, with a collapse of magnetism at ~18.5 GPa. The results are explained by a volume-dependent competition between indirect Ruderman-Kittel-Kasuya-Yosida interaction and the 5f-ligand hybridization

Simulation of Intermetallic Solidification using Phase-Field Techniques

We present current ideas towards developing a phase-field model appropriate to the solidification of intermetallic phases. Such simulation presents two main challenges (i) dealing with faceted interfaces and (ii) the complex sub-lattice models used to describe the thermodynamics of such phases. Although models are already existent for the simulation of faceted crystals, some of these can be shown to produce highly unrealistic Wulff shapes. The model present here uses a parameterization of the Wulff shape as a direct input to the model, allowing the simulation of arbitrary crystal shapes. In addition, an anti-trapping current that can be used with arbitrary (including sub-lattice) thermodynamics is presented. Such antitrapping currents are vital in the simulation of intermetallic phases where the steep liquidus slope means small deviations in solute partitioning behaviour can translate to a significant change in tip undercooling