Heat capacity of α\alpha-GaN: Isotope Effects

Until recently, the heat capacity of GaN had only been measured for polycrystalline powder samples. Semiempirical as well as \textit{first-principles} calculations have appeared within the past few years. We present in this article measurements of the heat capacity of hexagonal single crystals of GaN in the 20-1400K temperature range. We find that our data deviate significantly from the literature values for polycrystalline materials. The dependence of the heat capacity on the isotopic mass has also been investigated recently for monatomic crystals such as diamond, silicon, and germanium. Multi-atomic crystals are expected to exhibit a different dependence of these heat capacities on the masses of each of the isotopes present. These effects have not been investigated in the past. We also present \textit{first-principles} calculations of the dependence of the heat capacities of GaN, as a canonical binary material, on each of the Ga and N masses. We show that they are indeed different, as expected from the fact that the Ga mass affects mainly the acoustic, that of N the optic phonons. It is hoped that these calculations will encourage experimental measurements of the dependence of the heat capacity on isotopic masses in binary and more complex semiconductors.Comment: 12 pages, 5 Figures, submitted to PR

Thermal conductivity of solid parahydrogen with methane admixtures

The thermal conductivity of solid parahydrogen crystal with methane admixtures has been measured in the temperature range 1.5 to 8 K. Solid samples were grown from the gas mixture at 13 K. Concentration of CH₄ admixture molecules in the gas varied from 5 to 570 ppm. A very broad maximum of thermal conductivity with absolute value of about 110 W/(m×K) is observed at 2.6 K. The data are interpreted by Callaway model considering phonons resonant scattering on quasi-local vibrations of CH₄ molecules, phonon-grain boundary and phonon-phonon scattering processes. The increase of grain boundary scattering leads to the decrease of the maximum broadening. The analysis shows that the solid mixture of p-H₂ and CH₄ is a heterogeneous solution for CH₄ concentration higher than 0.1 ppm

Glassy Anomalies in the Low-Temperature Thermal Properties of a Minimally Disordered Crystalline Solid

The low-temperature thermal and transport properties of an unusual kind of crystal exhibiting minimal molecular positional and tilting disorder have been measured. The material, namely, low-dimensional, highly anisotropic pentachloronitrobenzene has a layered structure of rhombohedral parallel planes in which the molecules execute large-amplitude in-plane as well as concurrent out-of-plane librational motions. Our study reveals that low-temperature glassy anomalies can be found in a system with minimal disorder due to the freezing of (mostly in-plane) reorientational jumps of molecules between equivalent crystallographic positions with partial site occupation. Our findings will pave the way to a deeper understanding of the origin of the above-mentioned universal glassy properties at low temperature.Peer ReviewedPostprint (published version

Effects of internal molecular degrees of freedom on the thermal conductivity of some glasses and disordered crystals

The thermal conductivity κ(T) of the fully ordered stable phase II, the metastable phase III, the orientationally disordered (plastic) phase I, as well as the nonergodic orientational glass (OG) phase, of the glass former cyclohexanol (C 6H 11OH) has been measured under equilibrium vapor pressure within the 2-200 K temperature range. The main emphasis is here focused on the influence of the conformational disorder upon the thermal properties of this material. Comparison of results with those regarding cyanoclyclohexane (C 6H 11CN), a chemically related compound, serves to quantify the role played by the terminal groups -OH and -CN on the phonon scattering processes. The picture that emerges shows that motions of such groups do play a minor role as scattering centers, both within the low-temperature orientationally ordered phases as well as in the OG states. The results are analyzed within the Debye-Peierls relaxation time model for isotropic solids comprising mechanisms for long-wave phonon scattering within the OG and orientational ordered low-temperature phases, as well as others arising from localized short-wavelength vibrational modes as pictured by the Cahill-Pohl model. By means of complementary neutron and Raman scattering we show that in the OG state the energy landscapes for both compounds are very similar. © 2012 American Physical Society.This work was financially supported in part by the Spanish Ministry of Science and Innovation (Grant No. FIS2008-00837) and the Catalan Government (Grant No. 2009SGR-1251)Peer Reviewe

Eliashberg-type equations for correlated superconductors

The derivation of the Eliashberg -- type equations for a superconductor with strong correlations and electron--phonon interaction has been presented. The proper account of short range Coulomb interactions results in a strongly anisotropic equations. Possible symmetries of the order parameter include s, p and d wave. We found the carrier concentration dependence of the coupling constants corresponding to these symmetries. At low hole doping the d-wave component is the largest one.Comment: RevTeX, 18 pages, 5 ps figures added at the end of source file, to be published in Phys.Rev. B, contact: [email protected]

Isotope Effect for the Penetration Depth in Superconductors

We show that various factors can lead to an isotopic dependence of the penetration depth $\delta$. Non-adiabaticity (Jahn-Teller crossing) leads to the isotope effect of the charge carrier concentration $n$ and, consequently, of $\delta$ in doped superconductors such as the cuprates. A general equation relating the isotope coefficients of $T_c$ and of $\delta$ is presented for London superconductors. We further show that the presence of magnetic impurities or a proximity contact also lead to an isotopic dependence of $\delta$; the isotope coefficient turns out to be temperature dependent, $\beta(T)$, in these cases. The existence of the isotope effect for the penetration depth is predicted for conventional as well as for high-temperature superconductors. Various experiments are proposed and/or discussed.Comment: 11 pages, 8 figures, accepted for publication in Phys. Rev.

Water Network Optimization with Wastewater Regeneration Models

The conventional water network synthesis approach greatly simplifies wastewater treatment units by using fixed recoveries, creating a gap for their applicability to industrial processes. This work describes a unifying approach combining various technologies capable of removing all the major types of contaminants through the use of more realistic models. The following improvements are made over the typical superstructure-based water network models. First, unit-specific shortcut models are developed in place of the fixed contaminant removal model to describe contaminant mass transfer in wastewater treatment units. Shortcut wastewater treatment cost functions are also incorporated into the model. In addition, uncertainty in mass load of contaminants is considered to account for the range of operating conditions. Furthermore, the superstructure is modified to accommodate realistic potential structures. We present a modified Lagrangean-based decomposition algorithm in order to solve the resulting nonconvex mixed-integer nonlinear programming (MINLP) problem efficiently. Several examples are presented to illustrate the effectiveness and limitations of the algorithm for obtaining the global optimal solutions.The authors would like to acknowledge financial support from the National Science Foundation for financial support under grant CBET-1437668, the program “Estancias de movilidad en el extranjero “Jose Castillejo” para jóvenes doctores” (JC2011-0051) of the Spanish Ministerio de Educación, and from the University of Alicante (GRE11-19)

Thermal conductivity of high- T c superconductors

This paper reviews existing data on the thermal conductivity of high- T c superconductors. Included are discussions of pristine polycrystalline high- T c ceramics, single crystal specimens, and high- T c materials structurally modified by substitution or by radiation damage. The thermal conductivity of high- T c superconductors is compared with that of conventional superconductors, and dramatic differences are found between the two families. Mechanisms of thermal conductivity applicable to high- T c perovskites are discussed and implications for theories of high- T c superconductivity are noted.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45121/1/10948_2004_Article_BF00617463.pd