N. E. Israeloff

Observation of Fluctuation-Dissipation-Theorem Violations in a Structural Glass

Department of Physics and Center for Interdisciplinary Research on Complex Systems, Northeastern University,Boston, MA 02115(November 13, 1999)The fluctuation-dissipation theorem (FDT), connecting dielectric susceptibility and polarizationnoise was studied in glycerol below its glass transition temperature T

Local dielectric spectroscopy of polymer films

A noncontact scanning probe microscopy method of probing local frequency-dependent dielectric spectroscopy is described. Dielectric spectra can be measured with this technique from 0.01to100Hz, in nanometer-scale near-surface regions of materials. The technique is applied to polymer films (polyvinyl acetate), in order to determine if polymer glassy dynamics are altered near a free surface. A small reduction in glass transition temperature and a moderate narrowing of the distribution of relaxation times are found within 20nm of a free surface.

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.

ATOMIC FORCE MEASUREMENT OF LOW-FREQUENCY DIELECTRIC NOISE

Using noncontact scanning probe microscopy techniques, dielectric properties were studied on 50-nm-length scales in poly-vinyl-acetate (PVAc) and poly-methyl-methacrylate films. Low-frequency (1/f ) fluctuations observed in the measurements, peaked in intensity near the glass transition temperature in PVAc. The noise is shown to arise from thermal dielectric polarization fluctuations. Analysis of this noise provides a noninvasive method of probing equilibrium nanometer-scale dynamical processes in dielectric materials and devices.

Local Polarization Fluctuations in an Aging Glass

Polarization fluctuations were measured in nanoscale volumes of a polymer glass during aging following a temperature quench through the glass transition. Statistical properties of the noise were studied in equilibrium and during aging. The noise spectral density had a larger temporal variance during aging; i.e., the noise was more non-Gaussian, suggesting stronger correlations during aging.

Numerical study of ageing in coupled two-level systems

Abstract We study a lattice model of coupled two-level systems by Monte Carlo simulation. We find that it has glassy behaviour (non-exponential relaxation, Vogel-Fulcher relaxation times and ageing) at low temperatures. We study, in particular, ageing of the susceptibility in the frequency domain and find that it differs from spin-glass behaviour in that it does not obey ωt w scaling. Its qualitative features indicate that it may be a good model of ageing in structural glasses.

Imaging spatio-temporal fluctuations and local susceptibility in disordered polymers

Spatial and temporal fluctuations of the electric polarization were imaged in polymer thin films near the glass transition using electric force microscopy. Below the glass transition the fluctuations are quasi-static and spatial fluctuations were found to quantitatively agree with predictions for thermal fluctuations. Temporal fluctuations appear near the glass transition. Images of the space-time nanoscale dynamics near the glass transition are produced and analyzed. Local, complex dielectric susceptibility was also studied, and shows that dynamics on the free-surface are faster relative to the bulk.