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Register a new userValence and magnetic instabilities in Sm compounds at high pressures
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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.
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The pressure-dependent relation between Eu valence and lattice structure in model compound EuO is studied with synchrotron-based x-ray spectroscopic and diffraction techniques. Contrary to expectation, a 7% volume collapse at $approx$ 45 GPa is accompanied by a reentrant Eu valence transition into a $emph{lower}$ valence state. In addition to highlighting the need for probing both structure and electronic states directly when valence information is sought in mixed-valent systems, the results also show that widely used bond-valence methods fail to quantitatively describe the complex electronic valence behavior of EuO under pressure.
The intermediate valent systems TmSe and SmB6 have been investigated up to 16 and 18 GPa by ac microcalorimetry with a pressure (p) tuning realized in situ at low temperature. For TmSe, the transition from an antiferromagnetic insulator for p<3 GPa to an antiferromagnetic metal at higher pressure has been confirmed. A drastic change in the p variation of the Neel temperature (Tn) is observed at 3 GPa. In the metallic phase (p>3 GPa), Tn is found to increase linearly with p. A similar linear p increase of Tn is observed for the quasitrivalent compound TmS which is at ambiant pressure equivalent to TmSe at p=7 GPa. In the case of SmB6 long range magnetism has been detected above p=8 GPa, i.e. at a pressure slightly higher than the pressure of the insulator to metal transition. However a homogeneous magnetic phase occurs only above 10 GPa. The magnetic and electronic properties are related to the renormalization of the 4f wavefunction either to the divalent or the trivalent configurations. As observed in SmS, long range magnetism in SmB6 occurs already far below the pressure where a trivalent Sm3+ state will be reached. It seems possible, to describe roughly the physical properties of the intermediate valence equilibrium by assuming formulas for the Kondo lattice temperature depending on the valence configuration. Comparison is also made with the appearance of long range magnetism in cerium and ytterbium heavy fermion compounds.
We present thermoelectric power (TEP) studies under pressure and high magnetic field in the antiferromagnet CeRh2Si2 at low temperature. Under magnetic field, large quantum oscillations are observed in the TEP, S(H), in the antiferromagnetic phase. They suddenly disappear when entering in the polarized paramagnetic (PPM) state at Hc pointing out an important reconstruction of the Fermi surface (FS). Under pressure, S/T increases strongly of at low temperature near the critical pressure Pc, where the AF order is suppressed, implying the interplay of a FS change and low energy excitations driven by spin and valence fluctuations. The difference between the TEP signal in the PPM state above Hc and in the paramagnetic state (PM) above Pc can be explained by different FS. Band structure calculations at P = 0 stress that in the AF phase the 4f contribution at the Fermi level (EF) is weak while it is the main contribution in the PM domain. By analogy to previous work on CeRu2Si2, in the PPM phase of CeRh2Si2 the 4f contribution at EF will drop.
In order to probe the changes in the valence state and magnetic properties of Eu metal under extreme pressure, x-ray absorption near-edge spectroscopy, x-ray magnetic circular dichroism and synchrotron Mossbauer spectroscopy experiments have been carried out. The Mossbauer isomer shift exhibits an anomalous pressure dependence, passing through a maximum near 20 GPa. Density functional theory has been applied to give insight into the pressure-induced changes in both Eus electronic structure and Mossbauer isomer shift. Contrary to previous reports, Eu is found to remain nearly divalent to the highest pressures reached (87 GPa) with magnetic order persisting to at least 50 GPa. These results should lead to a better understanding of the nature of the superconducting state found above 75 GPa and of the sequence of structural phase transitions observed to 92 GPa.
We investigate the binary phase diagram of helium and iron using first-principles calculations. We find that helium, which is a noble gas and inert at ambient conditions, forms stable crystalline compounds with iron at terapascal pressures. A FeHe compound becomes stable above 4 TPa, and a FeHe$_2$ compound above 12 TPa. Melting is investigated using molecular dynamics simulations, and a superionic phase with sublattice melting of the helium atoms is predicted. We discuss the implications of our predicted helium-iron phase diagram for interiors of giant (exo)planets and white dwarf stars.
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