99 research outputs found
Optical mode crossings and the low temperature anomalies of SrTiO3
Optical mode crossing is not a plausible explanation for the new broad
Brillouin doublet nor for the strong acoustic anomalies observed at low
temperatures in SrTiO3. Data presented to support that explanation are also
inconclusive.Comment: This is a comment to a paper from J.F. Scott (same ZFP volume
Hyper-Raman scattering from vitreous boron oxide: coherent enhancement of the boson peak
Hyper-Raman scattering spectra of vitreous BO are reported and
compared to Raman scattering results. The main features are indexed in terms of
vibrations of structural units. Particular attention is given to the low
frequency boson peak which is shown to relate to out-of-plane librations of
BO boroxol rings and BO triangles. Its hyper-Raman strength is
comparable to that of cooperative polar modes. It points to a sizeable coherent
enhancement of the hyper-Raman signal compared to the Raman one. This is
explained by the symmetry of the structural units.Comment: 4 pages, 3 figure
Kinetic glass behavior in a diffusive model
Three properties of the Edwards-Anderson model with mobile bonds are
investigated which are characteristic of kinetic glasses. First is two-time
relaxation in aged systems, where a significant difference is observed between
spin and bond autocorrelation functions. The spin subsystem does not show
two-time behavior, and the relaxation is stretched exponential. The bond
subsystem shows two-time behavior, with the first relaxation nearly exponential
and the second similar to the spin one. Second is the two-temperature behavior,
which can be tuned by bond dilution through the full range reported in the
literature. Third is the rigid-to-floppy transition, identified as a function
of bond dilution. Simple Glauber Monte Carlo evolution without extraneous
constraints reproduces the behavior of classical kinetic simulations, with the
bond (spin) degree of freedom corresponding to configurational (orientational)
disorder.Comment: 4 pages, 3 figures, minimal corrections, to appear in Phys. Rev. B
(RC
Hyper-Raman scattering analysis of the vibrations in vitreous boron oxide
Hyper-Raman scattering has been measured on vitreous boron oxide,
BO. This spectroscopy, complemented with Raman scattering and
infrared absorption, reveals the full set of vibrations that can be observed
with light. A mode analysis is performed based on the local D symmetry
of BO triangles and BO boroxol rings. The results show that in
BO the main spectral components can be succesfully assigned using
this relatively simple model. In particular, it can be shown that the
hyper-Raman boson peak arises from external modes that correspond mainly to
librational motions of rigid boroxol rings.Comment: 13 pages, 11 figures, 2 table
XUV Frequency Combs via Femtosecond Enhancement Cavities
We review the current state of tabletop extreme ultraviolet (XUV) sources
based on high harmonic generation (HHG) in femtosecond enhancement cavities
(fsEC). Recent developments have enabled generation of high photon flux (1014
photons/sec) in the XUV, at high repetition rates (>50 MHz) and spanning the
spectral region from 40 nm - 120 nm. This level of performance has enabled
precision spectroscopy with XUV frequency combs and promises further
applications in XUV spectroscopic and photoemission studies. We discuss the
theory of operation and experimental details of the fsEC and XUV generation
based on HHG, including current technical challenges to increasing the photon
flux and maximum photon energy produced by this type of system. Current and
future applications for these sources are also discussed.Comment: invited review article, 38 page
High resolution infrared absorption spectra, crystal field, and relaxation processes in CsCdBr_3:Pr^3+
High resolution low-temperature absorption spectra of 0.2% Pr^3+ doped
CsCdBr_3 were measured in the spectral region 2000--7000 cm-1. Positions and
widths of the crystal field levels within the 3H5, 3H4, 3F2, and 3F3 multiplets
of the Pr^3+ main center have been determined. Hyperfine structure of several
spectral lines has been found. Crystal field calculations were carried out in
the framework of the semiphenomenological exchange charge model (ECM).
Parameters of the ECM were determined by fitting to the measured total
splittings of the 3H4 and 3H6 multiplets and to the observed in this work
hyperfine splittings of the crystal field levels. One- and two-phonon
relaxation rates were calculated using the phonon Green's functions of the
perfect (CsCdBr_3) and locally perturbed (impurity dimer centers in
CsCdBr_3:Pr^3+) crystal lattice. Comparison with the measured linewidths
confirmed an essential redistribution of the phonon density of states in
CsCdBr_3 crystals doped with rare-earth ions.Comment: 16 pages, 5 tables, 3 figure
Third order dielectric susceptibility in a model quantum paraelectric
In the context of perovskite quantum paraelectrics, we study the effects of a
quadrupolar interaction , in addition to the standard dipolar one .
We concentrate here on the nonlinear dielectric response , as
the main response function sensitive to quadrupolar (in our case
antiquadrupolar) interactions. We employ a 3D quantum four-state lattice model
and mean-field theory. The results show that inclusion of quadrupolar coupling
of moderate strength () is clearly accompanied by a
double change of sign of from negative to positive, near the
quantum temperature where the quantum paraelectric behaviour sets in. We
fit our to recent experimental data for SrTiO, where the
sign change is identified close to .Comment: 22 page
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