131 research outputs found
Effect of pressure on the Raman modes of antimony
The effect of pressure on the zone-center optical phonon modes of antimony in
the A7 structure has been investigated by Raman spectroscopy. The A_g and E_g
frequencies exhibit a pronounced softening with increasing pressure, the effect
being related to a gradual suppression of the Peierls-like distortion of the A7
phase relative to a cubic primitive lattice. Also, both Raman modes broaden
significantly under pressure. Spectra taken at low temperature indicate that
the broadening is at least partly caused by phonon-phonon interactions. We also
report results of ab initio frozen-phonon calculations of the A_g and E_g mode
frequencies. Presence of strong anharmonicity is clearly apparent in calculated
total energy versus atom displacement relations. Pronounced nonlinearities in
the force versus displacement relations are observed. Structural instabilities
of the Sb-A7 phase are briefly addressed in the Appendix.Comment: 10 pages, 8 figure
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
Raman spectra of MgB2 at high pressure and topological electronic transition
Raman spectra of the MgB2 ceramic samples were measured as a function of
pressure up to 32 GPa at room temperature. The spectrum at normal conditions
contains a very broad peak at ~590 cm-1 related to the E2g phonon mode. The
frequency of this mode exhibits a strong linear dependence in the pressure
region from 5 to 18 GPa, whereas beyond this region the slope of the
pressure-induced frequency shift is reduced by about a factor of two. The
pressure dependence of the phonon mode up to ~ 5GPa exhibits a change in the
slope as well as a "hysteresis" effect in the frequency vs. pressure behavior.
These singularities in the E2g mode behavior under pressure support the
suggestion that MgB2 may undergo a pressure-induced topological electronic
transition.Comment: 2 figure
Structural and superconducting properties of MgBBe
We prepared MgBBe (, 0.2, 0.3, 0.4, and 0.6) samples where
B is substituted with Be. MgB structure is maintained up to .
In-plane and inter-plane lattice constants were found to decrease and increase,
respectively. Superconducting transition temperature decreases with
. We found that the decrease is correlated with in-plane contraction
but is insensitive to carrier doping, which is consistent with other
substitution studies such as MgAlB and MgBC.
Implication of this work is discussed in terms of the 2D nature of -band.Comment: 3 pages,4 figures, to be published in Phys. Rev.
Mixing of magnetic and phononic excitations in incommensurate Spin-Peierls systems
We analyze the excitation spectra of a spin-phonon coupled chain in the
presence of a soliton. This is taken as a microscopic model of a Spin-Peierls
material placed in a high magnetic field. We show, by using a semiclassical
approximation in the bosonized representation of the spins that a trapped
magnetic state obtained in the adiabatic approximation is destroyed by
dynamical phonons. Low energy states are phonons trapped by the soliton. When
the magnetic gap is smaller than the phonon frequencies the only low energy
state is a mixed magneto-phonon state with the energy of the gap. We emphasize
that our results are relevant for the Raman spectra of the inorganic
Spin-Peierls material CuGeO.Comment: 5 pages, latex, 2 figures embedded in the tex
Far-Infrared Spectroscopy in Spin-Peierls Compound CuGeO_3 under High Magnetic Fields
Polarized far-infrared (FIR) spectroscopic measurements and FIR
magneto-optical studies were performed on the inorganic spin-Peierls compound
CuGeO_3. An absorption line, which was found at 98 cm in the dimerized
phase (D phase), was assigned to a folded phonon mode of B symmetry. The
splitting of the folded mode into two components in the incommensurate phase
(IC phase) has been observed for the first time. A new broad absorption
centered at 63 cm was observed only in the axis
polarization, which was assigned to a magnetic excitation from singlet ground
state to a continuum state.Comment: 9 pages multicolREVTeX, 10 figure
Spin-phonon coupled modes in the incommensurate phases of doped CuGeO
The doping effect of the folded phonon mode at 98 cm was investigated
on the Si-doped CuGeO by magneto-optical measurements in far-infrared (FIR)
region under high magnetic field. The folded phonon mode at 98 cm
appears not only in the dimerized (D) phase but also in the
dimerized-anitiferromagnetic (DAF) phase on the doped CuGeO. The splitting
was observed in the incommensurate (IC) phase and the antiferromagnetically
ordered incommensurate (IAF) phase above . The split-off branches exhibit
different field dependence from that of the pure CuGeO in the vicinity of
, and the discrepancy in the IAF phase is larger than that in the IC
phase. It is caused by the interaction between the solitons and the impurities.Comment: 7 pages, 4 figures, resubmitted to Phys. Rev.
Simple Metals at High Pressure
In this lecture we review high-pressure phase transition sequences exhibited
by simple elements, looking at the examples of the main group I, II, IV, V, and
VI elements. General trends are established by analyzing the changes in
coordination number on compression. Experimentally found phase transitions and
crystal structures are discussed with a brief description of the present
theoretical picture.Comment: 22 pages, 4 figures, lecture notes for the lecture given at the Erice
course on High-Pressure Crystallography in June 2009, Sicily, Ital
Raman spectrum and lattice parameters of MgB2 as a function of pressure
We report Raman spectra and synchrotron x-ray diffraction measurements of
lattice parameters of polycrystalline MgB2 under hydrostatic pressure
conditions up to 15 GPa. An anomalously broadened Raman band at 620 cm-1 is
observed that exhibits a large linear pressure shift of its frequency. The
large mode damping and Gruneisen parameter indicate a highly anharmonic nature
of the mode, broadly consistent with theoretical predictions for the E2g
in-plane boron stretching mode. The results obtained may provide additional
constraints on the electron-phonon coupling in the system.Comment: 3 pages, 3 figure
Excitations of the field-induced soliton lattice in CuGeO3
Here we report the first inelastic neutron scattering study of the magnetic
excitations in the incommensurate phase of a spin-Peierls material. The results
on CuGeO3 provide direct evidence of a finite excitation gap, two sharp
magnetic excitation branches and a very low-lying excitation which is
identified as a phason mode, the Goldstone mode of the incommensurate soliton
lattice.Comment: 5 pages, revtex, 4 figures (*.eps), win-zippe
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