Search results for "Lattice dynamics"

2015

The recently proposed Wigner function for a particle in an infinite lattice [NJP 14, 103009 (2012)] is extended here to include an internal degree of freedom, as spin. The formalism is developed to account for dynamical processes, with or without decoherence. We show explicit solutions for the case of Hamiltonian evolution under a position-dependent potential, and for evolution governed by a master equation under some simple models of decoherence. Discrete processes are also discussed. Finally we discuss the possibility of introducing a negativity concept for the Wigner function in the case in which the spin degree of freedom is included.

Lattice Dynamics Study of HgGa2Se4 at High Pressures

2013

We report on Raman scattering measurements in mercury digallium selenide (HgGa2Se4) up to 25 GPa. We also performed, for the low-pressure defect-chalcopyrite structure, lattice-dynamics ab initio calculations at high pressures which agree with experiments. Measurements evidence that the semiconductor HgGa2Se4 exhibits a pressure-induced phase transition above 19 GPa to a previously undetected structure. This transition is followed by a transformation to a Raman-inactive phase above 23.4 GPa. On downstroke from 25 GPa until 2.5 GPa, a broad Raman spectrum was observed, which has been attributed to a fourth phase, and whose pressure dependence was followed during a second upstroke. Candidate …

Raman scattering in hcp rare gas solids under pressure

2008

We present Raman measurements of hcp rare gas solids (RGSs) at megabar pressures together with lattice dynamics calculations. The ${E}_{2g}$ phonon was measured in Xe up to metallization near 135 GPa and in Ar up to 58 GPa. A comparative analysis of first-principles and semiempirical calculations shows that three-body forces contribute to the energetics at low pressures and that at volume compressions greater than $\ensuremath{\sim}2.6$ higher-order many-body forces become important. The distinct behavior of He under pressure relative to that of the rest of the RGS family is discussed.

Computational and theoretical studies on lattice thermal conductivity and thermal properties of silicon clathrates

2016

The lattice thermal conductivity is usually an intrinsic property in the study of thermoelectricity. In particular, relatively low lattice thermal conductivity is usually a desired feature when higher thermoelectric efficiency is pursued. The mechanisms which lower the lattice thermal conductivity are not known in sufficient detail and deeper understanding about the phenomena is needed and if such understanding is achieved it can be used to design more efficient thermoelectric materials. In this thesis, the lattice thermal conductivity and other thermal properties of several silicon clathrates, which are known to be promising candidates for the thermoelectric applications, are studied by theoreti…