839 research outputs found
Evaluation of Born and local effective charges in unoriented materials from vibrational spectra
We present an application of the Lorentz model in which fits to vibrational
spectra or a Kramers Kronig analysis are employed along with several useful
formalisms to quantify microscopic charge in unoriented (powdered) materials.
The conditions under which these techniques can be employed are discussed, and
we analyze the vibrational response of a layered transition metal
dichalcogenide and its nanoscale analog to illustrate the utility of this
approach.Comment: 9 pages, 1 figur
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Cathodoluminescence of uranium oxides
The cathodoluminescence of uranium oxide surfaces prepared in-situ from clean uranium exposed to dry oxygen was studied. The broad asymmetric peak observed at 470 nm is attributed to F-center excitation
Pressure-tuning of the c-f hybridization in Yb metal detected by infrared spectroscopy up to 18 GPa
It has been known that the elemental Yb, a divalent metal at mbient pressure,
becomes a mixed-valent metal under external pressure, with its valence reaching
~2.6 at 30 GPa. In this work, infrared spectroscopy has been used to probe the
evolution of microscopic electronic states associated with the valence
crossover in Yb at external pressures up to 18 GPa. The measured infrared
reflectivity spectrum R(w) of Yb has shown large variations with pressure. In
particular, R(w) develops a deep minimum in the mid-infrared, which shifts to
lower energy with increasing pressure. The dip is attributed to optical
absorption due to a conduction c-f electron hybridization state, similarly to
those previously observed for heavy fermion compounds. The red shift of the dip
indicates that the - hybridization decreases with pressure, which is
consistent with the increase of valence.Comment: 2 pages, to appear in J. Phys. Soc. Jpn. Supp
Optical Study of LaO_0.9F_0.1FeAs: Evidence for a Weakly Coupled Superconducting State
We have studied the reflectance of the recently discovered superconductor
LaO_0.9F0.FeAs in a wide energy range from the far infrared to the visible
regime. We report on the observation of infrared active phonons, the plasma
edge (PE) and possible interband transitions. On the basis of this data and the
reported in-plane penetration depth lambda_L(0) about 254 nm [H. Luetkens et
al., Phys. Rev. Lett. v. 101, 0970009 (2008)] a disorder sensitive relatively
small value of the total electron electron-boson coupling constant
lambda_tot=lambda_e-ph+lambda_e-sp ~ 0.6 +- 0.35 can be estimated adopting an
effective single-band picture.Comment: Changed title, updated references, final published versio
Towards device-size atomistic models of amorphous silicon
The atomic structure of amorphous materials is believed to be well described
by the continuous random network model. We present an algorithm for the
generation of large, high-quality continuous random networks. The algorithm is
a variation of the "sillium" approach introduced by Wooten, Winer, and Weaire.
By employing local relaxation techniques, local atomic rearrangements can be
tried that scale almost independently of system size. This scaling property of
the algorithm paves the way for the generation of realistic device-size atomic
networks.Comment: 7 pages, 3 figure
Structure and energetics of the Si-SiO_2 interface
Silicon has long been synonymous with semiconductor technology. This unique
role is due largely to the remarkable properties of the Si-SiO_2 interface,
especially the (001)-oriented interface used in most devices. Although Si is
crystalline and the oxide is amorphous, the interface is essentially perfect,
with an extremely low density of dangling bonds or other electrically active
defects. With the continual decrease of device size, the nanoscale structure of
the silicon/oxide interface becomes more and more important. Yet despite its
essential role, the atomic structure of this interface is still unclear. Using
a novel Monte Carlo approach, we identify low-energy structures for the
interface. The optimal structure found consists of Si-O-Si "bridges" ordered in
a stripe pattern, with very low energy. This structure explains several
puzzling experimental observations.Comment: LaTex file with 4 figures in GIF forma
Large well-relaxed models of vitreous silica, coordination numbers and entropy
A Monte Carlo method is presented for the simulation of vitreous silica.
Well-relaxed networks of vitreous silica are generated containing up to 300,000
atoms. The resulting networks, quenched under the BKS potential, display
smaller bond-angle variations and lower defect concentrations, as compared to
networks generated with molecular dynamics. The total correlation functions
T(r) of our networks are in excellent agreement with neutron scattering data,
provided that thermal effects and the maximum inverse wavelength used in the
experiment are included in the comparison. A procedure commonly used in
experiments to obtain coordination numbers from scattering data is to fit peaks
in rT(r) with a gaussian. We show that this procedure can easily produce
incorrect results. Finally, we estimate the configurational entropy of vitreous
silica.Comment: 7 pages, 4 figures (two column version to save paper
Traveling through potential energy landscapes of disordered materials: the activation-relaxation technique
A detailed description of the activation-relaxation technique (ART) is
presented. This method defines events in the configurational energy landscape
of disordered materials, such as a-Si, glasses and polymers, in a two-step
process: first, a configuration is activated from a local minimum to a nearby
saddle-point; next, the configuration is relaxed to a new minimum; this allows
for jumps over energy barriers much higher than what can be reached with
standard techniques. Such events can serve as basic steps in equilibrium and
kinetic Monte Carlo schemes.Comment: 7 pages, 2 postscript figure
Optical investigation on the electronic structures of Y_{2}Ru_{2}O_{7}, CaRuO_{3}, SrRuO_{3}, and Bi_{2}Ru_{2}O_{7}
We investigated the electronic structures of the bandwidth-controlled
ruthenates, YRuO, CaRuO, SrRuO, and BiRuO, by optical conductivity analysis in a wide energy region of 5 meV
12 eV. We could assign optical transitions from the systematic changes
of the spectra and by comparison with the O 1 x-ray absorption data. We
estimated some physical parameters, such as the on-site Coulomb repulsion
energy and the crystal-field splitting energy. These parameters show that the
4 orbitals should be more extended than 3 ones. These results are also
discussed in terms of the Mott-Hubbard model.Comment: 12 pages (1 table), 3 figure
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