How Stress Can Reduce Dissipation in Glasses

We propose that stress can decrease the internal friction of amorphous solids, either by increasing the potential barriers of defects, thus reducing their tunneling and thermal activation that produce loss, or by decreasing the coupling between defects and phonons. This stress can be from impurities, atomic bonding constraints, or externally applied stress. Externally applied stress also reduces mechanical loss through dissipation dilution. Our results are consistent with the experiments, and predict that stress could substantially reduce dielectric loss and increase the thermal conductivity.Comment: 9 pages, 7 figure

Mechanically probing coherent tunnelling in a double quantum dot

We study theoretically the interaction between the charge dynamics of a few-electron double quantum dot and a capacitively-coupled AFM cantilever, a setup realized in several recent experiments. We demonstrate that the dot-induced frequency shift and damping of the cantilever can be used as a sensitive probe of coherent inter-dot tunnelling, and that these effects can be used to quantitatively extract both the magnitude of the coherent interdot tunneling and (in some cases) the value of the double-dot T_1 time. We also show how the adiabatic modulation of the double-dot eigenstates by the cantilever motion leads to new effects compared to the single-dot case.Comment: 6 pages, 2 figure

Dynamics of a Pair of Interacting Spins Coupled to an Environmental Sea

We solve for the dynamics of a pair of spins, coupled to each other and also to an environmental sea of oscillators. The environment mediates an indirect interaction between the spins, causing both mutual coherence effects and dissipation. This model describes a wide variety of physical systems, ranging from 2 coupled microscopic systems (eg., magnetic impurities, bromophores, etc), to 2 coupled macroscopic quantum systems. We obtain analytic results for 3 regimes, viz., (i) The locked regime, where the 2 spins lock together; (ii) The correlated relaxation regime (mutually correlated incoherent relaxation); and (iii) The mutual coherence regime, with correlated damped oscillations. These results cover most of the parameter space of the system.Comment: 49 pages, To appear in Int J. Mod. Phys.

Dynamic sound attenuation at hypersonic frequencies in silica glass

In order to clarify the origin of the dominant processes responsible for the acoustic attenuation of phonons, which is a much debatted topic, we present Bril louin scattering experiments in various silica glasses of different OH impurities content. A large temperature range, from 5 to 1500 K is investigated, up to the glass transition temperature. Comparison of the hypersonic wave attenuation in various samples allows to identify two different processes. The first one induce s a low temperature peak related to relaxational processes; it is strongly sensitive to the extrinsic defects. The second, dominant in the hig h temperature range, is weakly dependent on the impurities and can be ascribed to anharmonic interactions

A Solvable Model of a Glass

An analytically tractable model is introduced which exhibits both, a glass--like freezing transition, and a collection of double--well configurations in its zero--temperature potential energy landscape. The latter are generally believed to be responsible for the anomalous low--temperature properties of glass-like and amorphous systems via a tunneling mechanism that allows particles to move back and forth between adjacent potential energy minima. Using mean--field and replica methods, we are able to compute the distribution of asymmetries and barrier--heights of the double--well configurations {\em analytically}, and thereby check various assumptions of the standard tunneling model. We find, in particular, strong correlations between asymmetries and barrier--heights as well as a collection of single--well configurations in the potential energy landscape of the glass--forming system --- in contrast to the assumptions of the standard model. Nevertheless, the specific heat scales linearly with temperature over a wide range of low temperatures.Comment: 11 pages, latex, including 5 figures, talk presented at the XIV Sitges Conferenc

Nonlinear acoustic and microwave absorption in glasses

A theory of weakly-nonlinear low-temperature relaxational absorption of acoustic and electromagnetic waves in dielectric and metallic glasses is developed. Basing upon the model of two-level tunneling systems we show that the nonlinear contribution to the absorption can be anomalously large. This is the case at low enough frequencies, where freqeuency times the minimal relaxation time for the two-level system are much less than one. In dielectric glasses, the lowest-order nonlinear contribution is proportional to the wave's intensity. It is negative and exhibits anomalous frequency and temperature dependencies. In metallic glasses, the nonlinear contribution is also negative, and it is proportional to the square root of the wave's intensity and to the frequency. Numerical estimates show that the predicted nonlinear contribution can be measured experimentally

Effect of nuclear quadrupole interactions on the dynamics of two-level systems in glasses

The standard tunneling model describes quite satisfactorily the thermal properties of amorphous solids at temperatures $T<1K$ in terms of an ensemble of two-level systems possessing logarithmically uniform distribution over their tunneling amplitudes and uniform distribution over their asymmetry energies. In particular, this distribution explains the observable logarithmic temperature dependence of the dielectric constant. Yet, experiments have shown that at ultralow temperatures $T<5mK$ such a temperature behavior breaks down and the dielectric constant becomes temperature independent (plateau effect). In this letter we suggest an explanation of this behavior exploiting the effect of the nuclear quadrupole interaction on tunneling. We show that below a temperature corresponding to the characteristic energy of the nuclear quadrupole interaction the effective tunneling amplitude is reduced by a small overlap factor of the nuclear quadrupole ground states in the left and right potential wells of the tunneling system. It is just this reduction that explains the plateau effect . We predict that the application of a sufficiently large magnetic field $B>10T$ should restore the logarithmic dependence because of the suppression of the nuclear quadrupole interaction.Comment: To appear in the Physical Review Letter

The role of localization in glasses and supercooled liquids

This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/104/13/10.1063/1.471147.Localized excitations (tunneling modes, soft harmonic vibrations) are believed to play a dominant role in the thermodynamics and transport properties of glasses at low temperature. Using instantaneous normal‐mode (INM) analysis, we explore the role that such localization plays in determining the behavior of such systems in the vicinity of the glass transition. Building on our previous study [Phys. Rev. Lett. 74, 936 (1995)] we present evidence that the glass transition in two simple model systems is associated with a transition temperature below which all un‐ stable INM’s become localized. This localization transition is a possible mechanism for the change in diffusion mechanism from continuous flow to localized hopping that is believed to occur in fragile glass formers at a temperature just above T g

On the theory of resonant susceptibility of dielectric glasses in magnetic field

The anomalous magnetic field dependence of dielectric properties of insulating glasses in the temperature interval $10mK<T<50mK$ is considered. In this temperature range, the dielectric permittivity is defined by the resonant contribution of tunneling systems. The external magnetic field regulates nuclear spins of tunneling atoms. This regulation suppresses a nuclear quadrupole interaction of these spins with lattice and, thus, affects the dielectric response of tunneling systems. It is demonstrated that in the absence of an external magnetic field the nuclear quadrupole interaction $b$ results in the correction to the permittivity $\delta\chi\sim| b| /T$ in the temperature range of interest. An application of a magnetic field results in a sharp increase of this correction approximately by a factor of two when the Zeeman splitting $m$ approaches the order of $| b|$. Further increase of the magnetic field results in a relatively smooth decrease in the correction until the Zeeman splitting approaches the temperature. This smooth dependence results from tunneling accompanied by a change of the nuclear spin projection. As the magnetic field surpasses the temperature, the correction vanishes. The results obtained in this paper are compared with experiment. A new mechanism of the low temperature nuclear spin-lattice relaxation in glasses is considered.Comment: 9 Pages, 5 Figures, To be submitted to the Physical Review B, please send comment