198 research outputs found
Pressure-induced ferromagnetism due to an anisotropic electronic topological transition in Fe1.08Te
A rapid and anisotropic modification of the Fermi-surface shape can be
associated with abrupt changes in crystalline lattice geometry or in the
magnetic state of a material. In this study we show that such an electronic
topological transition is at the basis of the formation of an unusual
pressure-induced tetragonal ferromagnetic phase in FeTe. Around 2 GPa,
the orthorhombic and incommensurate antiferromagnetic ground-state of
FeTe is transformed upon increasing pressure into a tetragonal
ferromagnetic state via a conventional first-order transition. On the other
hand, an isostructural transition takes place from the paramagnetic
high-temperature state into the ferromagnetic phase as a rare case of a `type
0' transformation with anisotropic properties. Electronic-structure
calculations in combination with electrical resistivity, magnetization, and
x-ray diffraction experiments show that the electronic system of FeTe
is instable with respect to profound topological transitions that can drive
fundamental changes of the lattice anisotropy and the associated magnetic
order.Comment: 7 pages, 4 figur
Nesting Induced Peierls-type Instability for Compressed Li-CI16
Alkalies are considered to be simple metals at ambient conditions. However,
recently reported theoretical and experimental results have shown an unexpected
and intriguing correlation between complex structures and an enhanced
superconducting transition temperature in lithium under pressure. In this
article we analyze the pressure induced Fermi surface deformation in bcc
lithium, and its relation to the observed cI16 structure. According to our
calculations, the Fermi surface becomes increasingly anisotropic with pressure
and develops an extended nesting along the bcc [121] direction. This nesting
induces a phonon instability of both transverse modes at N, so that a
Peierls-type mechanism is proposed to explain the stability of Li-cI16.Comment: Proceedings of Fukuoka 2006 Conference on Novel Pressure-induced
Phenomena in Condensed Matter Systems. To be published in J. Phys. Soc. Jpn.
2 pages and 3 figure
Crystal structure of LaTiO_3.41 under pressure
The crystal structure of the layered, perovskite-related LaTiO_3.41
(La_5Ti_5O_{17+\delta}) has been studied by synchrotron powder x-ray
diffraction under hydrostatic pressure up to 27 GPa (T = 295 K). The
ambient-pressure phase was found to remain stable up to 18 GPa. A sluggish, but
reversible phase transition occurs in the range 18--24 GPa. The structural
changes of the low-pressure phase are characterized by a pronounced anisotropy
in the axis compressibilities, which are at a ratio of approximately 1:2:3 for
the a, b, and c axes. Possible effects of pressure on the electronic properties
of LaTiO_3.41 are discussed.Comment: 5 pages, 6 figure
High-pressure study of X-ray diffuse scattering in ferroelectric perovskites
We present a high-pressure x-ray diffuse scattering study of the ABO
ferroelectric perovskites BaTiO_3 and KNbO_3. The well-known diffuse lines are
observed in all the phases studied. In KNbO_3, we show that the lines are
present up to 21.8 GPa, with constant width and a slightly decreasing
intensity. At variance, the intensity of the diffuse lines observed in the
cubic phase of BaTiO_3 linearly decreases to zero at GPa. These
results are discussed with respect to x-ray absorption measurements, which
leads to the conclusion that the diffuse lines are only observed when the B
atom is off the center of the oxygen tetrahedron. The role of such disorder on
the ferroelectric instability of perovskites is discussed.Comment: 4 pages, Accepted in PR
Quantum and Classical Orientational Ordering in Solid Hydrogen
We present a unified view of orientational ordering in phases I, II, and III
of solid hydrogen. Phases II and III are orientationally ordered, while the
ordering objects in phase II are angular momenta of rotating molecules, and in
phase III the molecules themselves. This concept provides quantitative
explanation of the vibron softening, libron and roton spectra, and increase of
the IR vibron oscillator strength in phase III. The temperature dependence of
the effective charge parallels the frequency shifts of the IR and Raman
vibrons. All three quantities are linear in the order parameter.Comment: Replaced with the final text, accepted for publication in PRL. 1 Fig.
added. Misc. text revision
How chemistry controls electron localization in 3d1 perovskites: A Wannier-function study
In the series of 3d1 t2g perovskites, SrVO3--CaVO3--LaTiO3--YTiO3 the
transition-metal d electron becomes increasingly localized and undergoes a Mott
transition between CaVO3 and LaTiO3. By defining a low-energy Hubbard
Hamiltonian in the basis of Wannier functions for the t2g LDA band and solving
it in the single-site DMFT approximation, it was recently shown[1] that
simultaneously with the Mott transition there occurs a strong suppression of
orbital fluctuations due to splitting of the t2g levels. The present paper
reviews and expands this work, in particular in the direction of exposing the
underlying chemical mechanisms by means of ab initio LDA Wannier functions
generated with the NMTO method. The Wannier functions for the t2g band exhibit
covalency between the transition-metal t2g, the large cation-d, and the
oxygen-p states; this covalency, which increases along the series, turns out to
be responsible not only for the splittings of the t2g levels, but also for
non-cubic perturbations of the hopping integrals, both of which are decisive
for the Mott transition. We find good agreement with the optical and
photoemission spectra, with the crystal-field splittings and orbital
polarizations recently measured for the titanates, and with the metallization
volume for LaTiO3. The metallization volume for YTiO3 is predicted. Using
super-exchange theory, we reproduce the observed magnetic orders in LaTiO3 and
YTiO3, but the results are sensitive to detail, in particular for YTiO3 which,
without the Jahn-Teller distortion, would be AFM C- or A-type, rather than FM.
Finally, we show that it possible to unfold the orthorhombic t2g LDA
bandstructure to a pseudocubic zone. In this zone, the lowest band is separated
from the two others by a direct gap and has a width, W_I, which is
significantly smaller than that, W, of the entire t2g band. The progressive
GdFeO3-type distortion favours electron localization by decreasing W, by
increasing the splitting of the t2g levels and by decreasing W_I. Our
conclusions concerning the roles of GdFeO3-type and JT distortions agree with
those of Mochizuki and Imada [2].Comment: Published version, final. For high resolution figures see
http://www.fkf.mpg.de/andersen/docs/pub/abstract2004+/pavarini_02.pd
Importance of correlation effects in hcp iron revealed by a pressure-induced electronic topological transition
We discover that hcp phases of Fe and Fe0.9Ni0.1 undergo an electronic
topological transition at pressures of about 40 GPa. This topological change of
the Fermi surface manifests itself through anomalous behavior of the Debye
sound velocity, c/a lattice parameter ratio and M\"ossbauer center shift
observed in our experiments. First-principles simulations within the dynamic
mean field approach demonstrate that the transition is induced by many-electron
effects. It is absent in one-electron calculations and represents a clear
signature of correlation effects in hcp Fe
Structure stability in the simple element sodium under pressure
The simple alkali metal Na, that crystallizes in a body-centred cubic
structure at ambient pressure, exhibits a wealth of complex phases at extreme
conditions as found by experimental studies. The analysis of the mechanism of
stabilization of some of these phases, namely, the low-temperature Sm-type
phase and the high-pressure cI16 and oP8 phases, shows that they satisfy the
criteria for the Hume-Rothery mechanism. These phases appear to be stabilized
due to a formation of numerous planes in a Brillouin-Jones zone in the vicinity
of the Fermi sphere of Na, which leads to the reduction of the overall
electronic energy. For the oP8 phase, this mechanism seems to be working if one
assumes that Na becomes divalent metal at this density. The oP8 phase of Na is
analysed in comparison with the MnP-type oP8 phases known in binary compounds,
as well as in relation to the hP4 structure of the NiAs-type
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