18 research outputs found
Synchrotron Mössbauer spectroscopic study of ferropericlase at high pressures and temperatures
The electronic spin state of Fe^(2+) in ferropericlase, (Mg_(0.75)Fe_(0.25))O, transitions from a high-spin (spin unpaired) to low-spin (spin paired) state within the Earth’s mid-lower mantle region. To better understand the local electronic environment of high-spin Fe^(2+) ions in ferropericlase near the transition, we obtained synchrotron Mössbauer spectra (SMS) of (Mg_(0.75),Fe_(0.25))O in externally heated and laser-heated diamond anvil cells at relevant high pressures and temperatures. Results show that the quadrupole splitting (QS) of the dominant high-spin Fe^(2+) site decreases with increasing temperature at static high pressure. The QS values at constant pressure are fitted to a temperature-dependent Boltzmann distribution model, which permits estimation of the crystal-field splitting energy (Δ_3) between the d_(xy_ and d_(xz) or d_(zy) orbitals of the t_(2g) states in a distorted octahedral Fe^(2+) site. The derived Δ_3 increases from approximately 36 meV at 1 GPa to 95 meV at 40 GPa, revealing that both high pressure and high temperature have significant effects on the 3d electronic shells of Fe^(2+) in ferropericlase. The SMS spectra collected from the laser-heated diamond cells within the time window of 146 ns also indicate that QS significantly decreases at very high temperatures. A larger splitting of the energy levels at high temperatures and pressures should broaden the spin crossover in ferropericlase because the degeneracy of energy levels is partially lifted. Our results provide information on the hyperfine parameters and crystal-field splitting energy of high-spin Fe^(2+) in ferropericlase at high pressures and temperatures, relevant to the electronic structure of iron in oxides in the deep lower mantle
P-T phase diagram of iron arsenide superconductor NdFeAsO0.88F0.12
NdFeAsO0.88F0.12 belongs to the recently discovered family of high-TC
iron-based superconductors. The influence of high pressure on transport
properties of this material has been studied. Contrary to La-based compounds,
we did not observe a maximum in TC under pressure. Under compression, TC drops
rapidly as a linear function of pressure with the slope k = -2.8 \pm 0.1 K /
GPa. The extrapolated value of TC at zero pressure is about TC (0) = 51.7 \pm
0.4 K. At pressures higher than ~18.4 GPa, the superconducting state disappears
at all measured temperatures. The resistance changes slope and shows a turn-up
behavior, which may be related to the Kondo effect or a weak localization of
two-dimensional carriers below ~45 K that is above TC and thus competing with
the superconducting phase. The behavior of the sample is completely reversible
at the decompression. On the bases of our experimental data, we propose a
tentative P-T phase diagram of NdFeAsO0.88F0.12
Anomalous high-temperature superconductivity in YH
Pressure-stabilized hydrides are a new rapidly growing class of
high-temperature superconductors which is believed to be described within the
conventional phonon-mediated mechanism of coupling. Here we report the
synthesis of yttrium hexahydride Im3m-YH that demonstrates the
superconducting transition with T = 224 K at 166 GPa, much lower than the
theoretically predicted (>270 K). The measured upper critical magnetic field
B(0) of YH was found to be 116-158 T, which is 2-2.5 times larger
than the calculated value. A pronounced shift of T in yttrium deuteride
YD with the isotope coefficient 0.4 supports the phonon-assisted
superconductivity. Current-voltage measurements showed that the critical
current I and its density J may exceed 1.75 A and 3500 A/mm at 0 K,
respectively, which is comparable with the parameters of commercial
superconductors, such as NbTi and YBCO. The superconducting density functional
theory (SCDFT) and anharmonic calculations suggest unusually large impact of
the Coulomb repulsion in this compound. The results indicate notable departures
of the superconducting properties of the discovered YH from the
conventional Migdal-Eliashberg and Bardeen-Cooper-Schrieffer theories.Comment: arXiv admin note: text overlap with arXiv:1902.1020
PPMS-based set-up for Raman and luminescence spectroscopy at high magnetic field, high pressure and low temperature
We present an experimental set-up permitting Raman and luminescence spectroscopy studies in a commercial Physical Properties Measurement System (PPMS) from Quantum Design. Using this experimental set-up, gaseous, liquid and solid materials, in bulk or thin film form, may be investigated. The set-up is particularly suitable for the study of the spin-lattice coupling in strongly correlated oxide materials utilizing several different stimuli, e.g. magnetic and electric fields, high pressure and low temperatures. Details for the Raman extension, sample holder assembly and optical design, as well as data acquisition and measurement routine are described. Finally, we present exemplary results collected using the set-up, measured on reference materials, as well as on a correlated transition metal oxide
Pressure-induced spin transition and evolution of the electronic excitations of FeBO
A high-pressure resonant inelastic x-ray scattering (RIXS) of at the Fe K pre-edge has been carried out to study the evolution of electronic excitations through the pressure-induced spin transition. Systematic peak shifts with insignificant peak width change are observed with increasing pressure in the high-spin state. An electronic transition occurs in tandem with the high-spin to low-spin transition, observed as the emergence of multiple new low-energy peaks in the spectra. The energy gap is reduced due to these low-energy peaks, not a peak width broadening. The observed electronic excitations are associated with dd excitations, which are calculated using a full-multiplet theory. We consider changes in crystal-field splitting and covalency to explain the observed peak shifts in the high-spin state. The new peaks that emerge upon the high-to low-spin transition are compared with dd excitations for the low-spin configuration
The first-order structural transition in NiO at high pressure
The physics of NiO under applied pressure has long been debated and the material has been a key contributor to our understanding of Mott insulators and strongly correlated materials more generally. Here, the authors perform high-pressure X-ray diffraction measurements reporting a pressure-induced structural phase transition for NiO, which they suggest is linked with the metal-insulator transition of this system