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Register a new userOn the Paramagnetic Impurity Concentration of Silicate Glasses from Low-Temperature Physics
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The concentration of paramagnetic trace impurities in glasses can be determined via precise SQUID measurements of the samples magnetization in a magnetic field. However the existence of quasi-ordered structural inhomogeneities in the disordered solid causes correlated tunneling currents that can contribute to the magnetization, surprisingly, also at the higher temperatures. We show that taking into account such tunneling systems gives rise to a good agreement between the concentrations extracted from SQUID magnetization and those extracted from low-temperature heat capacity measurements. Without suitable inclusion of such magnetization contribution from the tunneling currents we find that the concentration of paramagnetic impurities gets considerably over-estimated. This analysis represents a further positive test for the structural inhomogeneity theory of the magnetic effects in the cold glasses.
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A relation between the freezing temperature ($T^{}_{rm g}$) and the exchange couplings ($J^{}_{ij}$) in metallic spin-glasses is derived, taking the spin-correlations ($G^{}_{ij}$) into account. This approach does not involve a disorder-average. The expansion of the correlations to first order in $J^{}_{ij}/T^{}_{rm g}$ leads to the molecular-field result from Thouless-Anderson-Palmer. Employing the current theory of the spin-interaction in disordered metals, an equation for $T^{}_{rm g}$ as a function of the concentration of impurities is obtained, which reproduces the available data from {sl Au}Fe, {sl Ag}Mn, and {sl Cu}Mn alloys well.
The thermal and dielectric anomalies of window-type glasses at low temperatures ($T<$ 1 K) are rather successfully explained by the two-level systems (2LS) standard tunneling model (STM). However, the magnetic effects discovered in the multisilicate glasses in recent times, magnetic effects in the organic glasses and also some older data from mixed (SiO$_2$)$_{1-x}$(K$_2$O)$_x$ and (SiO$_2$)$_{1-x}$(Na$_2$O)$_x$ glasses indicate the need for a suitable extension of the 2LS-STM. We show that -- not only for the magnetic effects, but already for the mixed glasses in the absence of a field -- the right extension of the 2LS STM is provided by the (anomalous) multilevel tunnelling systems (A-TS) proposed by one of us for multicomponent amorphous solids. Though a secondary type of TS, different from the standard 2LS, was invoked long ago already, we clarify their physical origin and mathematical description and show that their contribution considerably improves the agreement with the experimental data.
We present a novel mechanism for the anomalous behaviour of the specific heat in low-temperature amorphous solids. The analytic solution of a mean-field model belonging to the same universality class as high-dimensional glasses, the spherical perceptron, suggests that there exists a crossover temperature above which the specific heat scales linearly with temperature while below it a cubic scaling is displayed. This relies on two crucial features of the phase diagram: (i) The marginal stability of the free-energy landscape, which induces a gapless phase responsible for the emergence of a power-law scaling (ii) The vicinity of the classical jamming critical point, as the crossover temperature gets lowered when approaching it. This scenario arises from a direct study of the thermodynamics of the system in the quantum regime, where we show that, contrary to crystals, the Debye approximation does not hold.
Comparisons and analogies are drawn between materials ferroic glasses and conventional spin glasses, in terms of both experiment and theoretical modelling, with inter-system conceptual transfers leading to suggestions of further issues to investigate.
The problems of the intermediate-range atomic structure of glasses and of the mechanism for the glass transition are approached from the low-temperature end in terms of a scenario for the atomic organization that justifies the use of an extended tunneling model. The latter is crucial for the explanation of the magnetic and compositional effects discovered in non-metallic glasses in the Kelvin and milli-Kelvin temperature range. The model relies on the existence of multi-welled local potentials for the effective tunneling particles that are a manifestation of a non-homogeneous atomic structure deriving from the established dynamical heterogeneities that characterize the supercooled liquid state. It is shown that the extended tunneling model can successfully explain a range of experiments at low temperatures, but the proposed non-homogeneous atomic structure scenario is then tested in the light of available high resolution electron microscopy imaging of the structure of some glasses and on the behaviour near the transition.
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