925 research outputs found
Role of electron-electron and electron-phonon interaction effect in the optical conductivity of VO2
We have investigated the charge dynamics of VO2 by optical reflectivity
measurements. Optical conductivity clearly shows a metal-insulator transition.
In the metallic phase, a broad Drude-like structure is observed. On the other
hand, in the insulating phase, a broad peak structure around 1.3 eV is
observed. It is found that this broad structure observed in the insulating
phase shows a temperature dependence. We attribute this to the electron-phonon
interaction as in the photoemission spectra.Comment: 6 pages, 8 figures, accepted for publication in Phys. Rev.
Big-bang nucleosynthesis with a long-lived charged massive particle including He spallation processes
We propose helium-4 spallation processes induced by long-lived stau in
supersymmetric standard models, and investigate an impact of the processes on
light elements abundances. We show that, as long as the phase space of helium-4
spallation processes is open, they are more important than stau-catalyzed
fusion and hence constrain the stau property.Comment: 12 pages, 4 figures, 1 table, references added, all figures correcte
Quantum Melting of the Charge Density Wave State in 1T-TiSe2
We report a Raman scattering study of low-temperature, pressure-induced
melting of the CDW phase of 1T-TiSe2. Our Raman scattering measurements reveal
that the collapse of the CDW state occurs in three stages: (i) For P<5 kbar,
the pressure dependence of the CDW amplitude mode energies and intensities are
indicative of a ``crystalline'' CDW regime; (ii) for 5 < P < 25 kbar, there is
a decrease in the CDW amplitude mode energies and intensities with increasing
pressure that suggests a regime in which the CDW softens, and may decouple from
the lattice; and (iii) for P>25 kbar, the absence of amplitude modes reveals a
melted CDW regime.Comment: 5 pages, 4 figure
Two-magnon Raman scattering in insulating cuprates: Modifications of the effective Raman operator
Calculations of Raman scattering intensities in spin 1/2 square-lattice
Heisenberg model, using the Fleury-Loudon-Elliott theory, have so far been
unable to describe the broad line shape and asymmetry of the two magnon peak
found experimentally in the cuprate materials. Even more notably, the
polarization selection rules are violated with respect to the
Fleury-Loudon-Elliott theory. There is comparable scattering in and
geometries, whereas the theory would predict scattering in only
geometry. We review various suggestions for this discrepency and
suggest that at least part of the problem can be addressed by modifying the
effective Raman Hamiltonian, allowing for two-magnon states with arbitrary
total momentum. Such an approach based on the Sawatzsky-Lorenzana theory of
optical absorption assumes an important role of phonons as momentum sinks. It
leaves the low energy physics of the Heisenberg model unchanged but
substantially alters the Raman line-shape and selection rules, bringing the
results closer to experiments.Comment: 7 pages, 6 figures, revtex. Contains some minor revisions from
previous versio
Raman Response of Magnetic Excitations in Cuprate Ladders and Planes
An unified picture for the Raman response of magnetic excitations in cuprate
spin-ladder compounds is obtained by comparing calculated two-triplon Raman
line-shapes with those of the prototypical compounds SrCu2O3 (Sr123),
Sr14Cu24O41 (Sr14), and La6Ca8Cu24O41 (La6Ca8). The theoretical model for the
two-leg ladder contains Heisenberg exchange couplings J_parallel and J_perp
plus an additional four-spin interaction J_cyc. Within this model Sr123 and
Sr14 can be described by x:=J_parallel/J_perp=1.5, x_cyc:=J_cyc/J_perp=0.2,
J_perp^Sr123=1130 cm^-1 and J_perp^Sr14=1080 cm^-1. The couplings found for
La6Ca8 are x=1.2, x_cyc=0.2, and J_perp^La6Ca8=1130 cm^-1. The unexpected sharp
two-triplon peak in the ladder materials compared to the undoped
two-dimensional cuprates can be traced back to the anisotropy of the magnetic
exchange in rung and leg direction. With the results obtained for the isotropic
ladder we calculate the Raman line-shape of a two-dimensional square lattice
using a toy model consisting of a vertical and a horizontal ladder. A direct
comparison of these results with Raman experiments for the two-dimensional
cuprates R2CuO4 (R=La,Nd), Sr2CuO2Cl2, and YBa2Cu3O(6+delta) yields a good
agreement for the dominating two-triplon peak. We conclude that short range
quantum fluctuations are dominating the magnetic Raman response in both,
ladders and planes. We discuss possible scenarios responsible for the
high-energy spectral weight of the Raman line-shape, i.e. phonons, the
triple-resonance and multi-particle contributions.Comment: 10 pages, 6 figure
Two-magnon Raman scattering in spin-ladder geometries and the ratio of rung and leg exchange constants
We discuss ways in which the ratio of exchange constants along the rungs and
legs of a spin-ladder material influences the two-magnon Raman scattering
spectra and hence can be determined from it. We show that within the
Fleury-Loudon-Elliott approach, the Raman line-shape does not change with
polarization geometries. This lineshape is well known to be difficult to
calculate accurately from theory. However, the Raman scattering intensities do
vary with polarization geometries, which are easy to calculate. With some
assumptions about the Raman scattering Hamiltonian, the latter can be used to
estimate the ratio of exchange constants. We apply these results to Sugai's
recent measurements of Raman scattering from spin-ladder materials such as
LaCaCuO and SrCuO.Comment: 5 pages, revtex. Latest version focuses on ladder materials, with a
detailed examination of the role of Heisenberg-like coupling constants which
appear in the Fleury-Loudon-Elliott scattering operator but are rarely
discussed in the literatur
Vibrational states and disorder in continuously compressed model glasses
We present in this paper a numerical study of the vibrational eigenvectors of
a two-dimensional amorphous material, previously deeply studied from the point
of view of mechanical properties and vibrational eigen-frequencies [7-10].
Attention is paid here to the connection between the mechanical properties of
this material in term of elastic heterogeneities (EH), and how these inherent
heterogeneous structures affect the vibrational eigenvectors and their plane
waves decomposition. The systems are analysed for different hydrostatic
pressures, and using results from previous studies, a deeper understanding of
the boson peak scenario is obtained. The vibrational spectrum of a continuously
densified silica glass is also studied, from which it appears that the
pulsation associated with the boson peak follows the same pressure dependence
trend than that of transverse waves with pulsation associated with the EH
characteristic size.Comment: 9 pages, 12 figure
- …