A new type of charged defect in amorphous chalcogenides

We report on density-functional-based tight-binding (DFTB) simulations of a series of amorphous arsenic sulfide models. In addition to the charged coordination defects previously proposed to exist in chalcogenide glasses, a novel defect pair, [As4]--[S3]+, consisting of a four-fold coordinated arsenic site in a seesaw configuration and a three-fold coordinated sulfur site in a planar trigonal configuration, was found in several models. The valence-alternation pairs S3+-S1- are converted into [As4]--[S3]+ pairs under HOMO-to-LUMO electronic excitation. This structural transformation is accompanied by a decrease in the size of the HOMO-LUMO band gap, which suggests that such transformations could contribute to photo-darkening in these materials.Comment: 5 pages, 2 figure

Simulation of physical properties of the chalcogenide glass As2S3 using a density-functional-based tight-binding method

We have used a density-functional-based tight-binding method in order to create structural models of the canonical chalcogenide glass, amorphous As2S3. The models range from one containing defects that are both chemical (homopolar bonds) and topological (valence-alternation pairs) in nature to one that is defect-free (stoichiometric). The structural, vibrational, and electronic properties of the simulated models are in good agreement with experimental data where available. The electronic densities of states obtained for all models show clean optical band gaps. A certain degree of electron-state localization at the band edges is observed for all models, which suggests that photoinduced phenomena in chalcogenide glasses may not necessarily be attributed to the excitation of defects of only one particular kind

Influence of copper on the electronic properties of amorphous chalcogenides

We have studied the influence of alloying copper with amorphous arsenic sulfide on the electronic properties of this material. In our computer-generated models, copper is found in two-fold near-linear and four-fold square-planar configurations, which apparently correspond to Cu(I) and Cu(II) oxidation states. The number of overcoordinated atoms, both arsenic and sulfur, grows with increasing concentration of copper. Overcoordinated sulfur is found in trigonal planar configuration, and overcoordinated (four-fold) arsenic is in tetrahedral configuration. Addition of copper suppresses the localization of lone-pair electrons on chalcogen atoms, and localized states at the top of the valence band are due to Cu 3d orbitals. Evidently, these additional Cu states, which are positioned at the same energies as the states due to ([As4]-)-([S_3]+) pairs, are responsible for masking photodarkening in Cu chalcogenides

On the decay of Burgers turbulence

This work is devoted to the decay ofrandom solutions of the unforced Burgers equation in one dimension in the limit of vanishing viscosity. The initial velocity is homogeneous and Gaussian with a spectrum proportional to $k^n$ at small wavenumbers $k$ and falling off quickly at large wavenumbers. In physical space, at sufficiently large distances, there is an ``outer region'', where the velocity correlation function preserves exactly its initial form (a power law) when $n$ is not an even integer. When $1<n<2$ the spectrum, at long times, has three scaling regions : first, a $|k|^n$ region at very small $k$\ms1 with a time-independent constant, stemming from this outer region, in which the initial conditions are essentially frozen; second, a $k^2$ region at intermediate wavenumbers, related to a self-similarly evolving ``inner region'' in physical space and, finally, the usual $k^{-2}$ region, associated to the shocks. The switching from the $|k|^n$ to the $k^2$ region occurs around a wave number $k_s(t) \propto t^{-1/[2(2-n)]}$, while the switching from $k^2$ to $k^{-2}$ occurs around $k_L(t)\propto t^{-1/2}$ (ignoring logarithmic corrections in both instances). The key element in the derivation of the results is an extension of the Kida (1979) log-corrected $1/t$ law for the energy decay when $n=2$ to the case of arbitrary integer or non-integer $n>1$. A systematic derivation is given in which both the leading term and estimates of higher order corrections can be obtained. High-resolution numerical simulations are presented which support our findings.Comment: In LaTeX with 11 PostScript figures. 56 pages. One figure contributed by Alain Noullez (Observatoire de Nice, France

Universal Features of Terahertz Absorption in Disordered Materials

Using an analytical theory, experimental terahertz time-domain spectroscopy data and numerical evidence, we demonstrate that the frequency dependence of the absorption coupling coefficient between far-infrared photons and atomic vibrations in disordered materials has the universal functional form, C(omega) = A + B*omega^2, where the material-specific constants A and B are related to the distributions of fluctuating charges obeying global and local charge neutrality, respectively.Comment: 5 pages, 3 fig

Nature of vibrational eigenmodes in topologically disordered solids

We use a local projectional analysis method to investigate the effect of topological disorder on the vibrational dynamics in a model glass simulated by molecular dynamics. Evidence is presented that the vibrational eigenmodes in the glass are generically related to the corresponding eigenmodes of its crystalline counterpart via disorder-induced level-repelling and hybridization effects. It is argued that the effect of topological disorder in the glass on the dynamical matrix can be simulated by introducing positional disorder in a crystalline counterpart.Comment: 7 pages, 6 figures, PRB, to be publishe

Extended point defects in crystalline materials: Ge and Si

B diffusion measurements are used to probe the basic nature of self-interstitial 'point' defects in Ge. We find two distinct self-interstitial forms - a simple one with low entropy and a complex one with entropy ~30 k at the migration saddle point. The latter dominates diffusion at high temperature. We propose that its structure is similar to that of an amorphous pocket - we name it a 'morph'. Computational modelling suggests that morphs exist in both self-interstitial and vacancy-like forms, and are crucial for diffusion and defect dynamics in Ge, Si and probably many other crystalline solids