Topologically disordered systems at the glass transition

The thermodynamic approach to the viscosity and fragility of amorphous oxides was used to determine the topological characteristics of the disordered network-forming systems. Instead of the disordered system of atoms we considered the congruent disordered system of interconnecting bonds. The Gibbs free energy of network-breaking defects (configurons) was found based on available viscosity data. Amorphous silica and germania were used as reference disordered systems for which we found an excellent agreement of calculated and measured glass transition temperatures. We reveal that the Hausdorff dimension of the system of bonds changes from Euclidian three-dimensional below to fractal 2.55 ± 0.05-dimensional geometry above the glass transition temperature

Particle dynamics in sheared granular matter

The particle dynamics and shear forces of granular matter in a Couette geometry are determined experimentally. The normalized tangential velocity $V(y)$ declines strongly with distance $y$ from the moving wall, independent of the shear rate and of the shear dynamics. Local RMS velocity fluctuations $\delta V(y)$ scale with the local velocity gradient to the power $0.4 \pm 0.05$. These results agree with a locally Newtonian, continuum model, where the granular medium is assumed to behave as a liquid with a local temperature $\delta V(y)^2$ and density dependent viscosity

Double Exchange Alone Does Not Explain the Resistivity of La1−xSrxMnO3La_{1-x} Sr_x MnO_3

The $La_{1-x} Sr_x MnO_3$ system with $0.2 \lesssim x \lesssim 0.4$ has traditionally been modelled with a ``double exchange'' Hamiltonian, in which it is assumed that the only relevant physics is the tendency of carrier hopping to line up neighboring spins. We present a solution of the double exchange model, show it is incompatible with many aspects of the resistivity data, and propose that a strong electron-phonon interaction arising from a Jahn-Teller splitting of the outer Mn d-level plays a crucial role.Comment: Figure available via concentional mail. Contact [email protected]

Connected Network of Minima as a Model Glass: Long Time Dynamics

A simple model to investigate the long time dynamics of glass-formers is presented and applied to study a Lennard-Jones system in supercooled and glassy phases. According to our model, the point representing the system in the configurational phase space performs harmonic vibrations around (and activated jumps between) minima pertaining to a connected network. Exploiting the model, in agreement with the experimental results, we find evidence for: i) stretched relaxational dynamics; ii) a strong T-dependence of the stretching parameter; iii) breakdown of the Stokes-Einstein law.Comment: 4 pages (Latex), 4 eps figure

Ergodic versus nonergodic behavior in oxygen deficient high-T_c superconductors

The oxygen defects induced phase transition from nonergodic to ergodic state in superconductors with intragrain granularity is considered within the superconductive glass model. The model predictions are found to be in a qualitative agreement with some experimental observations in deoxygenated high-T_c single crystals

Collective dynamics of liquid aluminum probed by Inelastic X-ray Scattering

An inelastic X-ray scattering experiment has been performed in liquid aluminum with the purpose of studying the collective excitations at wavevectors below the first sharp diffraction peak. The high instrumental resolution (up to 1.5 meV) allows an accurate investigation of the dynamical processes in this liquid metal on the basis of a generalized hydrodynamics framework. The outcoming results confirm the presence of a viscosity relaxation scenario ruled by a two timescale mechanism, as recently found in liquid lithium.Comment: 8 pages, 7 figure

Nanometer Scale Dielectric Fluctuations at the Glass Transition

Using non-contact scanning probe microscopy (SPM) techniques, dielectric properties were studied on 50 nanometer length scales in poly-vinyl-acetate (PVAc) films in the vicinity of the glass transition. Low frequency (1/f) noise observed in the measurements, was shown to arise from thermal fluctuations of the electric polarization. Anomalous variations observed in the noise spectrum provide direct evidence for cooperative nano-regions with heterogeneous kinetics. The cooperative length scale was determined. Heterogeneity was long-lived only well below the glass transition for faster than average processes.Comment: 4 pages, 4 embedded PS figures, RevTeX - To appear in Phys. Rev. Let

Diffusion and viscosity in a supercooled polydisperse system

We have carried out extensive molecular dynamics simulations of a supercooled polydisperse Lennard-Jones liquid with large variations in temperature at a fixed pressure. The particles in the system are considered to be polydisperse both in size and mass. The temperature dependence of the dynamical properties such as the viscosity ($\eta$) and the self-diffusion coefficients ($D_i$) of different size particles is studied. Both viscosity and diffusion coefficients show super-Arrhenius temperature dependence and fit well to the well-known Vogel-Fulcher-Tammann (VFT) equation. Within the temperature range investigated, the value of the Angell's fragility parameter (D $\approx 1.4$) classifies the present system into a strongly fragile liquid. The critical temperature for diffusion ($T_o^{D_i}$) increases with the size of the particles. The critical temperature for viscosity ($T_o^{\eta}$) is larger than that for the diffusion and a sizeable deviations appear for the smaller size particles implying a decoupling of translational diffusion from viscosity in deeply supercooled liquid. Indeed, the diffusion shows markedly non-Stokesian behavior at low temperatures where a highly nonlinear dependence on size is observed. An inspection of the trajectories of the particles shows that at low temperatures the motions of both the smallest and largest size particles are discontinuous (jump-type). However, the crossover from continuous Brownian to large length hopping motion takes place at shorter time scales for the smaller size particles.Comment: Revtex4, 7 pages, 8 figure

Electron Dynamics in Nd1.85_{1.85}Ce.15_{.15}CuO4+δ_{4+\delta}: Evidence for the Pseudogap State and Unconventional c-axis Response

Infrared reflectance measurements were made with light polarized along the a- and c-axis of both superconducting and antiferromagnetic phases of electron doped Nd$_{1.85}$Ce$_{.15}$CuO$_{4+\delta}$. The results are compared to characteristic features of the electromagnetic response in hole doped cuprates. Within the CuO$_2$ planes the frequency dependent scattering rate, 1/$\tau(\omega)$, is depressed below $\sim$ 650 cm$^{-1}$; this behavior is a hallmark of the pseudogap state. While in several hole doped compounds the energy scales associated with the pseudogap and superconducting states are quite close, we are able to show that in Nd$_{1.85}$Ce$_{.15}$CuO$_{4+\delta}$ the two scales differ by more than one order of magnitude. Another feature of the in-plane charge response is a peak in the real part of the conductivity, $\sigma_1(\omega)$, at 50-110 cm$^{-1}$ which is in sharp contrast with the Drude-like response where $\sigma_1(\omega)$ is centered at $\omega=0$. This latter effect is similar to what is found in disordered hole doped cuprates and is discussed in the context of carrier localization. Examination of the c-axis conductivity gives evidence for an anomalously broad frequency range from which the interlayer superfluid is accumulated. Compelling evidence for the pseudogap state as well as other characteristics of the charge dynamics in Nd$_{1.85}$Ce$_{.15}$CuO$_{4+\delta}$ signal global similarities of the cuprate phase diagram with respect to electron and hole doping.Comment: Submitted to PR

Violation of the fluctuation-dissipation theorem in glassy systems: basic notions and the numerical evidence

This review reports on the research done during the past years on violations of the fluctuation-dissipation theorem (FDT) in glassy systems. It is focused on the existence of a quasi-fluctuation-dissipation theorem (QFDT) in glassy systems and the currently supporting knowledge gained from numerical simulation studies. It covers a broad range of non-stationary aging and stationary driven systems such as structural-glasses, spin-glasses, coarsening systems, ferromagnetic models at criticality, trap models, models with entropy barriers, kinetically constrained models, sheared systems and granular media. The review is divided into four main parts: 1) An introductory section explaining basic notions related to the existence of the FDT in equilibrium and its possible extension to the glassy regime (QFDT), 2) A description of the basic analytical tools and results derived in the framework of some exactly solvable models, 3) A detailed report of the current evidence in favour of the QFDT and 4) A brief digression on the experimental evidence in its favour. This review is intended for inexpert readers who want to learn about the basic notions and concepts related to the existence of the QFDT as well as for the more expert readers who may be interested in more specific results.Comment: 120 pages, 37 figures. Topical review paper . Several typos and misprints corrected, new references included and others updated. to be published in J. Phys. A (Math. Gen.