Vibrational anomalies and marginal stability of glasses

The experimentally measured vibrational spectrum of glasses strongly deviates from that expected in Debye’s elasticity theory: The density of states deviates from Debye’s ω² law (“boson peak”), the sound velocity shows a negative dispersion in the boson-peak frequency regime, and there is a strong increase in the sound attenuation near the boson-peak frequency. A generalized elasticity theory is presented, based on the model assumption that the shear modulus of the disordered medium fluctuates randomly in space. The fluctuations are assumed to be uncorrelated and have a certain distribution (Gaussian or otherwise). Using field-theoretical techniques one is able to derive mean-field theories for the vibrational spectrum of a disordered system. The theory based on a Gaussian distribution uses a self-consistent Born approximation (SCBA),while the theory for non-Gaussian distributions is based on a coherent-potential approximation (CPA). Both approximate theories appear to be saddle-point approximations of effective replica field theories. The theory gives a satisfactory explanation of the vibrational anomalies in glasses. Excellent agreement of the SCBA theory with simulation data on a soft-sphere glass is reached. Since the SCBA is based on a Gaussian distribution of local shear moduli, including negative values, this theory describes a shear instability as a function of the variance of shear fluctuations. In the vicinity of this instability, a fractal frequency dependence of the density of states and the sound attenuation ∝ ω^1+a is predicted with a ? 1/2. Such a frequency dependence is indeed observed both in simulations and in experimental data. We argue that the observed frequency dependence stems from marginally stable regions in a glass and discuss these findings in terms of rigidity percolation.     

Transport processes in heavy metal fluoride glasses

Na self-diffusion, Li self-diffusion, Na⁺–Li⁺ ion exchange, electrical conductivity, and mechanical relaxation have been studied below T_g on glasses of the system ZrF₄–BaF₂–LaF₃–AF (A=Na, Li), with A=10, 20, 30 mol%. Compared to the transport mechanism in alkali-containing silicate glasses, the mechanisms in these non-oxide glasses are anomalous. Thus the self-diffusion coefficient of Na decreases with increasing NaF content, whereas that of Li increases with increasing LiF content. Both the electrical conductivity and the Na⁺–Li⁺ ion exchange reach a minimum at ≈ 20 mol% LiF, and the mechanical relaxation shows one peak for the 20 and 30 mol% LiF-glasses and two peaks for the glass with 10 mol% LiF, evidencing both a contribution of F⁻ and Li⁺ ions to the transport. Moreover, the presence of the three partially interacting mobile species F⁻, Na⁺, Li⁺ obviously leads to an anionic–cationic mixed ion effect. Applying the Nernst–Einstein equation to the Li⁺ transport in LiF-containing glasses shows that its mechanism is dissimilar to that in oxide glasses. Calculated short jump distances possibly can be interpreted as an Li⁺ movement via energetically suitable sites near F⁻ ions. Likewise the Nernst–Planck model, successfully applied to the ionic transport in mixed alkali silicate glasses, obviously does also not hold for the present heavy metal fluoride glasses.     

Transport and elastic anomalies in ZrTe3

Anomalies were observed in electrical resistivity components, Hall coefficient and Young's modulus of semimetallic ZrTe₃ near 63K. The results suggest that a structural phase transition, e.g. the onset of a charge density wave, occurs. Analysis indicates that the hole band is more strongly affected by the transition, resulting in the strong renormalization of the elastic modulus.     

Transport anomalies and marginal-fermi-liquid effects at a quantum critical point

The conductivity and the tunneling density of states of disordered itinerant electrons in the vicinity of a ferromagnetic transition at low temperature are discussed. Critical fluctuations lead to nonanalytic frequency and temperature dependencies that are distinct from the usual long-time tail effects in a disordered Fermi liquid. The crossover between these two types of behavior is proposed as an experimental check of recent theories of the quantum ferromagnetic critical behavior. In addition, the quasiparticle properties at criticality are shown to be those of a marginal Fermi liquid.     

Superionic and ion-conducting chalcogenide glasses: Transport regimes and structural features

Chalcogenide glasses are essentially known as amorphous semiconductors with interesting electronic and optical properties. In contrast to vitreous oxide systems which belong mostly to ionic insulators and/or conductors, the ion transport is not common for chalcogenide glasses and was observed for the first time in the seventies. Nevertheless, a higher polarisability of sulphur, selenium or tellurium compared to oxygen and respectively a higher ionic mobility and diffusivity, makes appropriate chalcogenide glassy systems favourable candidates for both fundamental research and practical applications in the field of solid-state ionics. The observed drastically different ion transport regimes that are closely related to the mobile cation distribution in the structure of silver and copper chalcogenide and chalcohalide glasses will be discussed in the present contribution which represents a compilation of recent results obtained by the author.     

On the nature of low-temperature anomalies of the anelastic properties of metallic glasses

In Memory of A. M. Roshchupkin The low-temperature (30<T<300 K) internal friction and elastic modulus of the metallic glass (MG) Ni₆₀Nb₄₀ subjected to preliminary cold working by rolling, high-temperature uniform straining, or electrolytic hydrogenation is investigated. It is established that cold rolling, which induces localized plastic flow, or hydrogenation radically alters the temperature dependences of the internal friction and elastic modulus: hysteresis appears in the background damping and intense relaxational peaks arise in the internal friction, accompanied by a defect of the elastic modulus. A uniform strain does not affect the low-temperature anelastic behavior of MGs. Microplastic deformation is observed to accompany the hydrogenation of weakly loaded samples. It is asserted that localized microplastic deformation also occurs on hydrogenation with no load. Plastic flow accompanying both rolling and hydrogenation occurs by the formation and motion of dislocationlike defects, which in the presence of an external load of alternating sign give rise to the observed anelastic anomalies. It is concluded that the low-temperature internal-friction peaks, described in the literature, in the “as-quenched,” cold-deformed, or hydrogenated MGs are all of a dislocation nature. Zh. Tekh. Fiz. 67, 35–46 (October 1997) In Memory of A. M. Roshchupkin     

On peculiarities of excitation spectrum and anomalies of properties at moderate low temperatures in glasses

Spectrum of low-energy excitations in anharmonic atomic local potentials characteristic of glasses contains both tunneling states with energies $\text{?<?}{}_{\mathrm{?}}\text{?ω}$ and quasilocal vibrations with ??ω(?10?30K, w being a typical energy. A narrow peak of the spectrum density of states at $\text{??ω}$ is associated with a singularity corresponding to critical points of the spectrum. The vibrations can give rise to anomalies in thermal and other properties of glasses observed at moderate low T (1K < T ? w). Both these anomalies and the well-known anomalies at T <~ 1K can be described coherently in the frame of an unified approach. The theory also is able to predict some other anomalies in glasses, e.g. a peak of neutron inelastic scattering with ernergy transfer～ω.     

Transport and dielectric properties of V2O5-MnO-TeO2 glasses

The frequency (1-10 kHz) and temperature (80-350 K) dependences of the ac conductivity and dielectric constant of the V₂O₅-MnO-TeO₂ system, containing two transition-metal ions, have been measured. The dc conductivity _dc measured in the high-temperature range (200-450 K) decreases with addition of the oxide MnO. This is considered to be due to the formation of bonds such as V--O--Mn and Mn--O--Mn in the glass. The conductivity arises mainly from polaron hopping between V^4+M and V⁵⁺ ions. It is found that a mechanism of adiabatic small-polaron hopping is the most appropriate conduction model for these glasses. This is in sharp contrast with the behaviour of the Mn-free V₂O₅-TeO₂ glass, in which non-adiabatic hopping takes place. High-temperature conductivity data satisfy Mott's small-polaron hopping model and also a model proposed by Schnakenberg in 1968. A power-law behaviour ( _ac = Aω^s , with s < 1) is well exhibited by the ac conductivity σ_ac data of these glasses. Analysis of dielectric data indicates a Debye-type relaxation behaviour with a distribution of relaxation times. The MnO-concentration-dependent σ_ac data follow an overlapping large-polaron tunnelling model over the entire range of temperatures studied. The estimated model parameters are reasonable and consistent with changes in composition.     

Frustrated percolation,spin glasses and glasses

Summary The static and dynamic properties of the frustrated percolation model are investigated. This model, which contains frustration as an essential ingredient, exhibits two transitions: a percolation transition at a temperatureT _p with critical exponents of the ferromagnetic (s=1/2)-state Potts model, and a second transition at a lower temperatureT _g in the same universality class of the Ising spin glass model. AboveT _p the time-dependent autocorrelation function is characterized by a single exponential, while forT _p>T>T _g preliminary numerical results show a broad shoulder or plateau typical of a structural glass transition. BelowT _g the system is in glassy state with an infinitely long relaxation time. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Microexplosions in tellurite glasses

Femtosecond laser pulses were used to produce localized damage in the bulk and near the surface of baseline, Al₂O₃-doped and La₂O₃-doped sodium tellurite glasses. Single or multiple laser pulses were non-linearly absorbed in the focal volume by the glass, leading to permanent changes in the material in the focal volume. These changes were caused by an explosive expansion of the ionized material in the focal volume into the surrounding material, i.e. a microexplosion. The writing of simple structures (periodic array of voxels, as well as lines) was demonstrated. The regions of microexplosion and writing were subsequently characterized using scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS) and atomic force microscopy (AFM). Fingerprints of microexplosions (concentric lines within the region and a concentric ring outside the region), due to the shock wave generated during microexplosions, were evident. In the case of the baseline glass, no chemistry change was observed within the region of the microexplosion. However, Al₂O₃-doped and La₂O₃-doped glasses showed depletion of the dopant from the edge to the center of the region of the microexplosions, indicating a chemistry gradient within the regions. Interrogation of the bulk- and laser-treated regions using micro-Raman spectroscopy revealed no structural change due to the microexplosions and writing within these glasses. Received: 27 December 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. +1-509/376-3108, E-mail: sk.sundaram@pnl.gov     

Confined phonons in glasses

The European Physical Journal E - We have applied nuclear inelastic absorption (NIA) to the molecular glass former dibutyl phthalate/ferrocene, both in bulk and in nanoporous matrices having pore...     

Planar rings in glasses

An energy minimization argument is used to predict that small order rings of bonds are likely to be planar in many glasses. The 606 and 495 cm^-1 Raman “defect” lines in fused silica are then shown to be consistent with small concentrations (?1%) of planar 3-fold and 4-fold rings, respectively. The argument predicts this form of intermediate range order at larger concentrations in several glasses, including 3-fold planar rings in B₂O₃, while it predicts no planar rings in other glasses, including ZnCl₂.     

Fluorescence linewidths in glasses

The homogeneous optical linewidth of an impurity ion in a glass is described in terms of a process which arises from thermal transitions between the states of the two level systems of a glass; these give shifts in the electronic energy levels via the strain field coupling. With available values of the parameters, this contribution gives reasonably good agreement for the overall magnitude as well as for the temperature and time dependences observed recently for the “anomalous” fluorescence linewidth of Eu³⁺ in silicate glass for temperatures up to $\text{? 100}\text{K}$.     

Superconductivity in zirconium-nickel glasses

The superconducting transition temperatures (T_c) and magnetic susceptibilities of amorphous Zr_100?xNi_x alloys have been measured. T_c decreases linearly with increasing x. The results are compared to those for amorphous ZrPd and ZrCu alloys and discussed in terms of changes in the electron to atom ratio on alloying.     

Lightguiding in ion-exchanged glasses

A comparison is made of different Gaussian refractive index distributions in waveguides formed by thermal diffusion of silver ions into glass. Guides prepared for various diffusion times are investigated. It is shown that only one set of mode indices is insufficient to determine whether some index profile asymmetry exists.