Glen R. Kowach

Phonon density of states and negative thermal expansion in ZrW2O8

Thermal expansion of solids arises from anharmonic lattice dynamics. The contrasting phenomenon of negative thermal expansion (NTE)—where expansion occurs on cooling rather than heating—was discovered in ZrW2O8 in 1968. Recently, this material has attracted interest in the context of NTE for several reasons: the magnitude of the effect is relatively large (−9 p.p.m. K−1); the temperature range over which NTE occurs is also large (from close to absolute zero up to the decomposition temperature of about 1,050 K); and the NTE effect is isotropic, evidenced by the fact that ZrW2O8 remains cubic at all temperatures. These characteristics make ZrW2O8 an important system in which to study unusual lattice dynamics of this type, and potentially well suited for application in composite materials with an engineered thermal expansion coefficient. Here we report neutron-scattering measurements of ZrW2O8 that allow us to investigate its phonon spectrum, and hence determine the energy scale for the lattice motions governing NTE. We find that NTE can be modelled by several low-energy phonon modes, suggesting that the effect arises from the unusual crystal structure of ZrW2O8, which supports highly anharmonic vibrational modes.

High mobility AlGaN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy on semi-insulating GaN templates prepared by hydride vapor phase epitaxy

We report on an extensive study of the growth and transport properties of the two-dimensional electron gas (2DEG) confined at the interface of AlGaN/GaN heterostructures grown by molecular beam epitaxy (MBE) on thick, semi-insulating GaN templates prepared by hydride vapor phase epitaxy (HVPE). Thick (∼20 μm) GaN templates are characterized by low threading dislocation densities (∼5×108 cm−2) and by room temperature resistivities of ∼108 Ω cm. We describe sources of parasitic conduction in our structures and how they have been minimized. The growth of low Al containing (x⩽0.05) AlxGa1-xN/GaN heterostructures is investigated. The use of low Al containing heterostructures facilitates the study of the 2DEG transport properties in the previously unexplored regime of carrier density ns⩽2×1012 cm−2. We detail the impact of MBE growth conditions on low temperature mobility. Using an undoped HVPE template that was residually n type at room temperature and characterized an unusually low dislocation density of ∼2×1...

Unusual low-energy phonon dynamics in the negative thermal expansion compound ZrW2O8.

An infrared study of the phonon spectra of ZrW2O8 as a function of temperature which includes the low-energy (2-10 meV) region relevant to negative thermal expansion is reported and discussed in the context of specific heat and neutron density of state results. The prevalence of infrared active phonons at low energy and their observed temperature dependence are highly unusual and indicative of exotic low-energy lattice dynamics. Eigenvector calculations indicate a mixing of librational and translational motion within each low-frequency IR mode. The role of the underconstrained structure in establishing the nature of these modes and the relationship between the IR spectra and the large negative thermal expansion in ZrW2O8 are discussed.

Dual on-fiber thin-film heaters for fiber gratings with independently adjustable chirp and wavelength

Dual, independently addressable thin-film resistive heaters fabricated in a multilayer geometry on the surface of an optical fiber provide a new, flexible means for thermally tuning the properties of intracore gratings. In particular, control of the current that is applied to each of these heaters permits the chirp and the central wavelength of the grating to be adjusted independently. The designs and simple fabrication procedures for these types of device, the important physics of heat flow in them, and a tunable add-drop filter that demonstrates essential aspects of their operation are described.

Heat capacities, third-law entropies and thermodynamic functions of the negative thermal expansion materials, cubic α-ZrW2O8 and cubic ZrMo2O8, from K

Abstract The molar heat capacities of crystalline cubic α-ZrW2O8 and cubic ZrMo2O8 have been measured at temperatures from (0.6 to 400) K. At T=298.15 K , the standard molar heat capacities are (207.01±0.21) J · K −1 · mol −1 for the tungstate and (210.06±0.42) J · K −1 · mol −1 for the molybdate. Thermodynamic functions have been generated from smoothed fits of the experimental results. The standard molar entropies for the tungstate and molybdate are (257.96±0.50) J · K −1 · mol −1 and (254.3±1) J · K −1 · mol −1 , respectively. The uncertainty of the entropy of the cubic ZrMo2O8 is larger due to the presence of small chemical and phase impurities whose effects cannot be corrected for at this time. The heat capacities of the negative thermal expansion materials have been compared to the weighted sums of their constituent binary oxides. Both negative thermal expansion materials have heat capacities which are significantly greater than the sum of the binary oxides over the entire temperature region.

Geometrical frustration, spin ice and negative thermal expansion – the physics of underconstraint

Abstract The idea that some systems could have a thermodynamically large number of accessible ground states was presaged in the work of Pauling on ice (Pauling, Cornell University Press, Ithaca, NY, 1945) [1] . With the advent of spin glasses, the methodology for describing ground states changed dramatically, and in particular it was realized that the observed slow dynamics were due to relaxation among a large number of nearly degenerate ground states. Now the accepted wisdom is that both “frustration”, as well as structural disorder, is responsible for spin glass behavior. However, well before spin-glasses were identified as a distinct class of systems, it had been appreciated that even for structurally periodic systems, bond frustration could lead to a thermodynamically large number of states. There is now a well-defined class of magnets which display effects of macroscopic ground state degeneracy. This class of geometrically frustrated magnets presents some new paradigms with which to view condensed matter systems – marginal underconstraint and downward shift of spectral weight. We discuss possible realizations of these phenomena in both in spin ice and also outside the context of local-moment magnetism.

Fabrication and optical measurements of germanium-doped silica ridge waveguides using a colloidal suspension approach

Germanium-doped silica ridge waveguides for planar lightwave circuits were fabricated using colloidal suspensions. After consolidation at 1300 °C, fully dense micron-scale thick films were obtained. The refractive index of these films was tuned with germanium dopant up to 11.1 mol %. The propagation loss of a 3.1 mol % germanium-doped silica sample without an overcladding layer was 3.3±0.5dB∕cm. Postannealing of the patterned waveguides at 1200 °C for 30 min was conducted in order to reflow waveguides, but it did not significantly smooth the sidewall roughness due to the high viscosity of the 3.1 mol % germanium-doped silica at that temperature.

Coulomb glass origin of defect-induced dielectric loss in thin-film oxides

The dielectric loss in amorphous, thin-film oxide insulators produces a real part of the ac conductivity σ′(ω) that scales as ωs with s∼1. Conventional models explain this frequency dependence by hopping or tunneling of charge between neighboring defect sites. These models fail at low temperatures since they predict that σ′ should vanish at T=0. We observe that the ac conductivity of Ta2O5, ZnO, and SiO2 has a nonzero extrapolated value at T=0. We propose that this behavior is consistent with the predictions of a Coulomb glass, an insulator with a random distribution of charged defects.The dielectric loss in amorphous, thin-film oxide insulators produces a real part of the ac conductivity σ′(ω) that scales as ωs with s∼1. Conventional models explain this frequency dependence by hopping or tunneling of charge between neighboring defect sites. These models fail at low temperatures since they predict that σ′ should vanish at T=0. We observe that the ac conductivity of Ta2O5, ZnO, and SiO2 has a nonzero extrapolated value at T=0. We propose that this behavior is consistent with the predictions of a Coulomb glass, an insulator with a random distribution of charged defects.