220 research outputs found
The thermal conductivity reduction in HgTe/CdTe superlattices
The techniques used previously to calculate the three-fold thermal
conductivity reduction due to phonon dispersion in GaAs/AlAs superlattices
(SLs) are applied to HgTe/CdTe SLs. The reduction factor is approximately the
same, indicating that this SL may be applicable both as a photodetector and a
thermoelectric cooler.Comment: 5 pages, 2 figures; to be published in Journal of Applied Physic
Anthropogenic Noise Affects Behavior across Sensory Modalities
Many species are currently experiencing anthropogenically driven environmental changes. Among these changes, increasing noise levels are specifically a problem for species using acoustic signals (i.e., species relying on signals that use the same sensory modality as anthropogenic noise). Yet many species use other sensory modalities, such as visual and olfactory signals, to communicate. However, we have only little understanding of whether changes in the acoustic environment affect species that use sensory modalities other than acoustic signals. We studied the impact of anthropogenic noise on the common cuttlefish Sepia officinalis, which uses highly complex visual signals. We showed that cuttlefish adjusted their visual displays by changing their color more frequently during a playback of anthropogenic noise, compared with before and after the playback. Our results provide experimental evidence that anthropogenic noise has a marked effect on the behavior of species that are not reliant on acoustic communication. Thus, interference in one sensory channel, in this case the acoustic one, affects signaling in other sensory channels. By considering sensory channels in isolation, we risk overlooking the broader implications of environmental changes for the behavior of animals
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
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
Small Fermi energy and phonon anharmonicity in MgB_2 and related compounds
The remarkable anharmonicity of the E_{2g} phonon in MgB_2 has been suggested
in literature to play a primary role in its superconducting pairing. We
investigate, by means of LDA calculations, the microscopic origin of such an
anharmonicity in MgB_2, AlB_2, and in hole-doped graphite. We find that the
anharmonic character of the E_{2g} phonon is essentially driven by the small
Fermi energy of the sigma holes. We present a simple analytic model which
allows us to understand in microscopic terms the role of the small Fermi energy
and of the electronic structure. The relation between anharmonicity and
nonadiabaticity is pointed out and discussed in relation to various materials.Comment: 5 pages, 2 figures replaced with final version, accepted on Physical
Review
Purpose and benefits of hybrid simulation: Contributing to the convergence of its definition
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Hybrid simulation challenges and opportunities: a life-cycle approach
The last 10 years have witnessed a marked upsurge of attention on Hybrid Simulation (HS). The majority of authors define HS as a joint modelling approach which includes two or more simulation approaches (mainly Discrete Event Simulation, System Dynamics and Agent Based Simulation). Whilst some may argue that HS has been in existence for more than 5 decades, the recent rise tended to be more problem driven rather than technical experimentation. Winter Simulation Conference (WSC) 2015, 2016, 2017 have witnessed 3 panels on the purpose, history and definition of HS, respectively. This paper reports on a comprehensive review conducted by the panelists on HS and its applications. The aim of the paper is to move the debate forward by exploring potential platforms for developing concrete avenues for research on HS in conjunction with the modelling life cycle
Lattice Dynamics of II-VI materials using adiabatic bond charge model
We extend the adiabatic bond charge model, originally developed for group IV
semiconductors and III-V compounds, to study phonons in more ionic II-VI
compounds with a zincblende structure. Phonon spectra, density of states and
specific heats are calculated for six II-VI compounds and compared with both
experimental data and the results of other models. We show that the 6-parameter
bond charge model gives a good description of the lattice dynamics of these
materials. We also discuss trends in the parameters with respect to the
ionicity and metallicity of these compounds.Comment: 16 pages of RevTex with 3 figures submitted as a uuencode compressed
tar fil
Thermal Stabilization of the HCP Phase in Titanium
We have used a tight-binding model that is fit to first-principles
electronic-structure calculations for titanium to calculate quasi-harmonic
phonons and the Gibbs free energy of the hexagonal close-packed (hcp) and omega
crystal structures. We show that the true zero-temperature ground-state is the
omega structure, although this has never been observed experimentally at normal
pressure, and that it is the entropy from the thermal population of phonon
states which stabilizes the hcp structure at room temperature. We present the
first completely theoretical prediction of the temperature- and
pressure-dependence of the hcp-omega phase transformation and show that it is
in good agreement with experiment. The quasi-harmonic approximation fails to
adequately treat the bcc phase because the zero-temperature phonons of this
structure are not all stable
Disorder induced collapse of the electron phonon coupling in MgB observed by Raman Spectroscopy
The Raman spectrum of the superconductor MgB has been measured as a
function of the Tc of the film. A striking correlation is observed between the
onset and the frequency of the mode. Analysis of the data with
the McMillan formula provides clear experimental evidence for the collapse of
the electron phonon coupling at the temperature predicted for the convergence
of two superconducting gaps into one observable gap. This gives indirect
evidence of the convergence of the two gaps and direct evidence of a transition
to an isotropic state at 19 K. The value of the electron phonon coupling
constant is found to be 1.22 for films with T 39K and 0.80 for films with
T19K.Comment: 5 pages, 4 figure
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