Phonons in random alloys: the itinerant coherent-potential approximation

We present the itinerant coherent-potential approximation(ICPA), an analytic, translationally invariant and tractable form of augmented-space-based, multiple-scattering theory in a single-site approximation for harmonic phonons in realistic random binary alloys with mass and force-constant disorder. We provide expressions for quantities needed for comparison with experimental structure factors such as partial and average spectral functions and derive the sum rules associated with them. Numerical results are presented for Ni_{55} Pd_{45} and Ni_{50} Pt_{50} alloys which serve as test cases, the former for weak force-constant disorder and the latter for strong. We present results on dispersion curves and disorder-induced widths. Direct comparisons with the single-site coherent potential approximation(CPA) and experiment are made which provide insight into the physics of force-constant changes in random alloys. The CPA accounts well for the weak force-constant disorder case but fails for strong force-constant disorder where the ICPA succeeds.Comment: 19 pages, 12 eps figures, uses RevTex

Towards a first principles description of phonons in Ni50_{50}Pt50_{50} disordered alloys: the role of relaxation

Using a combination of density-functional perturbation theory and the itinerant coherent potential approximation, we study the effects of atomic relaxation on the inelastic incoherent neutron scattering cross sections of disordered Ni$_{50}$Pt$_{50}$ alloys. We build on previous work, where empirical force constants were adjusted {\it ad hoc} to agree with experiment. After first relaxing all structural parameters within the local-density approximation for ordered NiPt compounds, density-functional perturbation theory is then used to compute phonon spectra, densities of states, and the force constants. The resulting nearest-neighbor force constants are first compared to those of other ordered structures of different stoichiometry, and then used to generate the inelastic scattering cross sections within the itinerant coherent potential approximation. We find that structural relaxation substantially affects the computed force constants and resulting inelastic cross sections, and that the effect is much more pronounced in random alloys than in ordered alloys.Comment: 8 pages, 3 eps figures, uses revtex

The shear modulus of metastable amorphous solids with strong central and bond-bending interactions

We derive expressions for the shear modulus of deeply-quenched, glassy solids, in terms of a Cauchy-Born free energy expansion around a rigid (quenched) reference state, following the approach due to Alexander [Alexander, Phys. Rep. 296, 1998]. Continuum-limit explicit expressions of the shear modulus are derived starting from the microscopic Hamiltonians of central and bond-bending interactions. The applicability of the expressions to dense covalent glasses as well as colloidal glasses with strongly attractive and adhesive bonds is discussed

TEM analysis of apatite surface layers observed on zinc based glass polyalkenoate cements

peer-reviewedGlass polyalkenoate cements (GPCs) are acid base cements formed by the reaction of an aqueous solution of polyalkenoic acid, usually polyacrylic acid (PAA) with an acid degradable aluminosilicate glass. The result of the reaction is cement consisting of reacted and unreacted glass particles embedded in a polysalt matrix. In addition to these conventional GPCs, aluminium free glass polyalkenoate cements based on zinc silicate glasses (Zn-GPCs) exhibit significant potential as bone cements for several reasons. Primarily, they are formulated without the inclusion of aluminium (Al) [1] in the glass phase and thus eliminate clinical complications arising from the release of the Al3+ ion from the cement in vivo. Such complications have, in the past, included aluminium induced encephalopathy [2-5] and defective mineralisation of cancellous bone [6]. Secondly, Zn-GPCs set without a significant evolution of heat, when compared with commercial bone cements such as Spineplex ® (Stryker, Limerick, Ireland). Finally, these materials can be tailored to release clinically beneficial ions into surrounding tissues [7]. In addition to Zn, these cements have been synthesized to contain strontium (Sr) [8, 9]. Both Sr and Zn inhibit osteoclastic turnover and promote osteoblastic turnover, resulting in increased bone strength and decreased fracture risk [10-14].Acceptedpeer-reviewe

Structure and physical properties of paracrystalline atomistic models of amorphous silicon

We have examined the structure and physical properties of paracrystalline molecular dynamics models of amorphous silicon. Simulations from these models show qualitative agreement with the results of recent mesoscale fluctuation electron microscopy experiments on amorphous silicon and germanium. Such agreement is not found in simulations from continuous random network models. The paracrystalline models consist of topologically crystalline grains which are strongly strained and a disordered matrix between them. We present extensive structural and topological characterization of the medium range order present in the paracrystalline models and examine their physical properties, such as the vibrational density of states, Raman spectra, and electron density of states. We show by direct simulation that the ratio of the transverse acoustic mode to transverse optical mode intensities ITA /ITO in the vibrational density of states and the Raman spectrum can provide a measure of medium range order. In general, we conclude that the current paracrystalline models are a good qualitative representation of the paracrystalline structures observed in the experiment and thus provide guidelines toward understanding structure and properties of medium-range-ordered structures of amorphous semiconductors as well as other amorphous materials

Neutron-Scattering Study of Librations and Intramolecular Phonons in Rb_{2.6}K_{0.4}C_{60}

We report the results of inelastic neutron-scattering measurements on Rb2.6K0.4C60. Librational modes were observed as broad peaks with maxima between 4.1 and 4.7 meV, as the temperature is lowered from 300 to 12 K. As in K3C60, no change in the width or position of the librational peak was observed when the sample was cooled through the superconducting transition. Thus any coupling of the librations to electronic states is small. The magnitude of the orientational potential barrier was estimated from the librational peak frequency. A flat background observed in the low-energy inelastic-scattering spectra is ascribed to two-phonon scattering. The density of states of intramolecular modes is similar to that of previously studied M3C60 compounds; modes at 53 and 66 meV in pure C60 are not observed in Rb2.6K0.4C60