280 research outputs found
Interplay of the electronic and lattice degrees of freedom in A_{1-x}Fe_{2-y}Se_{2} superconductors under pressure
The local structure and electronic properties of RbFeSe
are investigated by means of site selective polarized x-ray absorption
spectroscopy at the iron and selenium K-edges as a function of pressure. A
combination of dispersive geometry and novel nanodiamond anvil pressure-cell
has permitted to reveal a step-like decrease in the Fe-Se bond distance at
GPa. The position of the Fe K-edge pre-peak, which is directly
related to the position of the chemical potential, remains nearly constant
until GPa, followed by an increase until GPa. Here, as in
the local structure, a step-like decrease of the chemical potential is seen.
Thus, the present results provide compelling evidence that the origin of the
reemerging superconductivity in FeSe in vicinity of a
quantum critical transition is caused mainly by the changes in the electronic
structure
A hierarchical research by large-scale and ab initio electronic structure theories -- Si and Ge cleavage and stepped (111)-2x1 surfaces --
The ab initio calculation with the density functional theory and plane-wave
bases is carried out for stepped Si(111)-2x1 surfaces that were predicted in a
cleavage simulation by the large-scale (order-N) electronic structure theory
(T. Hoshi, Y. Iguchi and T. Fujiwara, Phys. Rev. B72 (2005) 075323). The
present ab initio calculation confirms the predicted stepped structure and its
bias-dependent STM image. Moreover, two (meta)stable step-edge structures are
found and compared. The investigation is carried out also for Ge(111)-2x1
surfaces, so as to construct a common understanding among elements. The present
study demonstrates the general importance of the hierarchical research between
large-scale and ab initio electronic structure theories.Comment: 5 pages, 4 figures, to appear in Physica
Interplay between local structure and electronic properties on CuO under pressure
The electronic and local structural properties of CuO under pressure have
been investigated by means of X-ray absorption spectroscopy (XAS) at Cu K edge
and ab-initio calculations, up to 17 GPa. The crystal structure of CuO consists
of Cu motifs within CuO square planar units and two elongated apical Cu-O
bonds. The CuO square planar units are stable in the studied pressure
range, with Cu-O distances that are approximately constant up to 5 GPa, and
then decrease slightly up to 17 GPa. In contrast, the elongated Cu-O apical
distances decrease continuously with pressure in the studied range. An
anomalous increase of the mean square relative displacement (EXAFS Debye
Waller, \sigma) of the elongated Cu-O path is observed from 5 GPa up to 13
GPa, when a drastic reduction takes place in \sigma. This is interpreted in
terms of local dynamic disorder along the apical Cu-O path. At higher pressures
(P>13 GPa), the local structure of Cu changes from a 4-fold square
planar to a 4+2 Jahn-Teller distorted octahedral ion. We interpret these
results in terms of the tendency of the Cu ion to form favorable
interactions with the apical O atoms. Also, the decrease in Cu-O apical
distance caused by compression softens the normal mode associated with the
out-of-plane Cu movement. CuO is predicted to have an anomalous rise in
permittivity with pressure as well as modest piezoelectricity in the 5-13 GPa
pressure range. In addition, the near edge features in our XAS experiment show
a discontinuity and a change of tendency at 5 GPa. For P < 5 GPa the evolution
of the edge shoulder is ascribed to purely electronic effects which also affect
the charge transfer integral. This is linked to a charge migration from the Cu
to O, but also to an increase of the energy band gap, which show a change of
tendency occurring also at 5 GPa
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A pressure-induced collapse of magnetic ordering in at GPa has previously been interpreted as evidence for possible emergence of spin liquid states in this hyperhoneycomb iridate, raising prospects for experimental realizations of the Kitaev model. Based on structural data obtained at room temperature, this magnetic transition is believed to originate in small lattice perturbations that preserve crystal symmetry, and related changes in bond-directional anisotropic exchange interactions. Here we report on the evolution of the crystal structure of under pressure at low temperatures and show that the suppression of magnetism coincides with a change in lattice symmetry involving Ir-Ir dimerization. The critical pressure for dimerization shifts from 4.4(2) GPa at room temperature to GPa below 50 K. While a direct transition is observed at room temperature, the low temperature transitions involve new as well as coexisting dimerized phases. Further investigation of the Ir () isotropic branching ratio in x-ray absorption spectra indicates that the previously reported departure of the electronic ground state from a state is closely related to the onset of dimerized phases. In essence, our results suggest that the predominant mechanism driving the collapse of magnetism in is the pressure-induced formation of dimers in the hyperhoneycomb network. The results further confirm the instability of the moments and related noncollinear spiral magnetic ordering against formation of dimers in the low-temperature phase of compressed
Phase transitions in MgSiO3 post-perovskite in super-Earth mantles
The highest pressure form of the major Earth-forming mantle silicate is
MgSiO3 post-perovskite (PPv). Understanding the fate of PPv at TPa pressures is
the first step for understanding the mineralogy of super-Earths-type
exoplanets, arguably the most interesting for their similarities with Earth.
Modeling their internal structure requires knowledge of stable mineral phases,
their properties under compression, and major element abundances. Several
studies of PPv under extreme pressures support the notion that a sequence of
pressure induced dissociation transitions produce the elementary oxides SiO2
and MgO as the ultimate aggregation form at ~3 TPa. However, none of these
studies have addressed the problem of mantle composition, particularly major
element abundances usually expressed in terms of three main variables, the
Mg/Si and Fe/Si ratios and the Mg#, as in the Earth. Here we show that the
critical compositional parameter, the Mg/Si ratio, whose value in the Earth's
mantle is still debated, is a vital ingredient for modeling phase transitions
and internal structure of super-Earth mantles. Specifically, we have identified
new sequences of phase transformations, including new recombination reactions
that depend decisively on this ratio. This is a new level of complexity that
has not been previously addressed, but proves essential for modeling the nature
and number of internal layers in these rocky mantles.Comment: Submitted to Earth Planet. Sci. Lett., 28 pages, 6 figure
Local electronic structure rearrangements and strong anharmonicity in YH3 under pressures up to 180 GPa
The authors acknowledge the ESRF program committee (Grenoble, France) for the opportunity to perform XAFS and XRD measurements. We are grateful to Prof. Dr Marek Tkacz from the Institute of Physical Chemistry, PAS Kasprzaka 44/52, 01-224 Warsaw, Poland, for high quality YH3 samples and to Dr. José A. Flores-Livas for a fruitful discussion. A.P.M. and A.A.I. acknowledge the Russian Foundation for the Basic Research (grant No 18-02-40001_mega) for financial support. J.P., A.K., and I.P. would like to thank the support of the Latvian Council of Science project No. lzp-2018/2-0353. ISSP UL acknowledge the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-20l 6-2017-TeamingPhase2, grant agreement No. 739508, project CAMART2.The discovery of superconductivity above 250 K at high pressure in LaH10 and the prediction of overcoming the room temperature threshold for superconductivity in YH10 urge for a better understanding of hydrogen interaction mechanisms with the heavy atom sublattice in metal hydrides under high pressure at the atomic scale. Here we use locally sensitive X-ray absorption fine structure spectroscopy (XAFS) to get insight into the nature of phase transitions and the rearrangements of local electronic and crystal structure in archetypal metal hydride YH3 under pressure up to 180 GPa. The combination of the experimental methods allowed us to implement a multiscale length study of YH3: XAFS (short-range), Raman scattering (medium-range) and XRD (long-range). XANES data evidence a strong effect of hydrogen on the density of 4d yttrium states that increases with pressure and EXAFS data evidence a strong anharmonicity, manifested as yttrium atom vibrations in a double-well potential.--//--This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.Russian Foundation for the Basic Research (grant No 18-02-40001_mega); Latvian Council of Science project No. lzp-2018/2-0353; European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-20l 6-2017-TeamingPhase2, grant agreement No. 739508, project CAMART2
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