High-Pressure Synthesis and Thermal Conductivity of Semimetallic θ-Tantalum Nitride

The lattice thermal conductivity (κ_ph) of metals and semimetals is limited by phonon-phonon scattering at high temperatures and by electron-phonon scattering at low temperatures or in some systems with weak phonon-phonon scattering. Following the demonstration of a phonon band engineering approach to achieve an unusually high κ_ph in semiconducting cubic-boron arsenide (c-BAs), recent theories have predicted ultrahigh κ_ph of the semimetal tantalum nitride in the θ-phase (θ-TaN) with hexagonal tungsten carbide (WC) structure due to the combination of a small electron density of states near the Fermi level and a large phonon band gap, which suppress electron-phonon and three-phonon scattering, respectively. Here, measurements on the thermal and electrical transport properties of polycrystalline θ-TaN converted from the ε phase via high-pressure synthesis are reported. The measured thermal conductivity of the θ-TaN samples shows weak temperature dependence above 200 K and reaches up to 90 Wm⁻¹K⁻¹, one order of magnitude higher than values reported for polycrystalline ε-TaN and δ-TaN thin films. These results agree with theoretical calculations that account for phonon scattering by 100 nm-level grains and suggest κ_ph increase above the 249 Wm⁻¹ K⁻¹ value predicted for single-crystal WC when the grain size of θ-TaN is increased above 400 nm.     

Study of vibrational modes in superionic lithium oxide

Lithium oxide, like other superionics, finds several technological applications. These applications range from miniature light weight high power density lithium ion batteries for heart pacemakers, mobile phones, laptops computers, etc. to high capacity energy storage devices for next generation. It is also a leading contender for future fusion reactors to convert energetic neutrons to usable heat and to breed tritium necessary to sustain D–T (deuterium–tritium) reactions. In this work, the lattice dynamics of Li₂O have been carried out to study the fast ion phase and the diffusion behavior of lithium and oxygen ions. The investigation of this simple material is a reference point for understanding more complex metal-oxides. A modified rigid ion model has been used to study the phonons in lithium oxide by considering the interatomic interactions up to third nearest neighbor. The calculated results are compared and analyzed with available experimental results.     

Lattice Effects on Charge Localization in Cuprates

Recent experimental results on cuprates and manganites, including those of elastic and inelastic neutron scattering measurements, suggest that charges are not homogeneously distributed even in the metallic state in these compounds. Charge inhomogeneity results from spin/lattice charge constriction. In cuprates the LO phonons strongly reflect the temperature- and composition-dependent charge inhomogeneity and may possibly be involved in causing it. Unlike the static stripes that compete against superconductivity, the charge inhomogeneity seen by the LO phonons is markedly increased in the superconducting phase. A new mechanism of high-temperature superconductivity involving lattice/spin charge constriction is proposed.     

Theory of Thermal Conduction in Thin Ceramic Films

The theory of heat conduction in ceramics by phonons, and at high temperatures also by infrared radiation, is reviewed. The phonon mean free path is limited by three-phonon interactions and by scattering of various imperfections. Point defects scatter high-frequency phonons; extended imperfections, such as inclusions, pores, and grain boundaries, affect mainly low-frequency phonons. Thermal radiation is also scattered by imperfections, but of a larger size, such as splat boundaries and large pores. Porosity also reduces the effective index of refraction. For films there are also external boundaries, cracks, and splat boundaries, depending on the method of deposition. Examples discussed are cubic zirconia, titanium oxide, and uranium oxide. Graphite and graphene sheets, with two-dimensional phonon gas, are discussed briefly.     

Local Structural Distortions and Phase Separation in Cuprates

We report evidence from Raman measurements of local lattice distortions in the cuprates, which are induced at room temperature by varying the hydrostatic pressure and correlate with changes in the superconducting transition temperature. Anomalous nonlinear pressure dependence is observed for almost all A_g phonons of the YBa₂Cu₃O_6.5, YBa₂Cu₃O_over, and YBa₂Cu₄O₈ superconducting single crystals, which occurs at pressures where the transition temperature shows also characteristic changes. The results are compared with a similar correlation found between transition temperature and spectral modifications in Bi₂Sr₂CaCu₂O₈ by pressure and in YBa₂Cu₃O_x and La_2–xSr_xCuO₄ by chemical doping. There are strong indications that the saturation of T_c with pressure or compositional doping in these cuprates is mainly related with lattice instabilities and probably also with phase separation phenomena.     

Plasmons and phonons in anisotropic systems and their effect on high-Tc superconductivity

The structure of the plasmon and phonon bands in conducting, highly anisotropic layered materials is studied. The observed high superconducting transition temperatures are caused, in our view, by the coexistence of strong electronphonon coupling and the plasmon mechanism. A generalized Eliashberg equation describing the effects of phonons and plasmons on the pairing is presented. Our approach is based on the method of thermodynamic Green's functions. We point out connections to experiments and briefly comment on other models.     

Ir studies of disordered zincblende semiconductors III-V-II-VI alloys and GaAs-Ga2/3Se

This paper presents the ir reflectance spectra of several III-V — II-VI alloys, GaAs-ZnSe, GaP-ZnSe and GaAs-ZnS, and the III-V — III₂ -VI₃ system, GaAs-Ga_2/3 Se. GaAs-ZnSe and GaP-ZnSe alloys each display two reflectivity peaks, where-as the GaAs-ZnS system displays a single feature. These behaviors are explained using a linear chain model which considers both mass and force constant differences and which assumes pairing of the III-V and II-VI constituents. The spectrum of Ga₂ Se₃ is dominated by disorder induced features associated with the vacancies. Alloying with GaAs reduces the number of vacancies and is accompanied by im-portant changes in both the intensity and shape of the spectral features.     

Dependence of the Crystal Structure on Particle Size in Barium Titanate

The effect of the sample particle size on the crystal structure and the Curie temperature of BaTiO₃ powder has been investigated in the particle size range 0.1 to 1.0 μm. The transformation from tetragonal to cubic symmetry occurs at a critical particle size of 0.12 μm at room temperature, and the Curie temperature drops below room temperature at the critical particle size.