Multilevel Tunnelling Systems and Fractal Clustering in the Low‐Temperature Mixed Alkali‐Silicate Glasses

Bonfanti

Kob

2015

Philosophical Magazine

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The discovery of magnetic and compositional effects in the low temperature properties of multicomponent glasses has prompted the need to extend the standard two-level systems (2LSs) tunneling model. A possible extension [1] assumes that a subset of tunneling quasi-particles is moving in a threewelled potential (TWP) associated with the ubiquitous inhomogeneities of the disordered atomic structure of the glass. We show that within an alternative, cellular description of the intermediate-range atomic structure of glasses the tunneling TWP can be fully justified. We then review how the experimentally discovered magnetic effects can be explained within the approach where only localized atomistic tunneling 2LSs and quasi-particles tunneling in TWPs are allowed. We discuss the origin of the magnetic effects in the heat capacity, dielectric constant (real and imaginaryi parts), polarization echo and SQUID magnetization in several glassy systems. We conclude by commenting on a strategy to reveal the mentioned tunneling states (2LSs and TWPs) by means of atomistic computer simulations and discuss the microscopic nature of the tunneling states in the context of the potential energy landscape of glass-forming systems.arXiv:1508.02424v1 [cond-mat.dis-nn] 10 Aug 2015 1 IntroductionThe physics of glasses, especially at low temperatures, continues to attract considerable interest. At low temperatures glasses are believed to be characterized by low-energy excitations going under the name of tunneling systems, or states (TSs) which are normally described in terms of double-welled potentials (DWPs) and two-level systems (2LSs) with energy asymmetry and tunneling barrier uniformly distributed in the amorphous solid [2,3]. Little is still known about the nature of the TSs, but the general consensus is still that the intermediate-range atomic structure of glasses is well described by Zachariasen's 1932 continuous random network model [4,5] (thus, homogeneously disordered like for a liquid) and the 2LSs arise out of two slightly similar, localized atomic configurations. With this characterization the 2LSs have been employed in the 1970s and 80s to explain with some success the anomalies in the properties of glasses at low temperatures. Recently, the field has been witnessing a renaissance since the discovery that there are unexpected magnetic effects in insulating glasses [6,7,8,9] and that the TSs are responsible for the decoherence of phase qubits made of superconducting Josephson-junctions [10,11,12,13,14]. The 2LSs are thought to be ubiquitous in the junction's tunneling barrier which, though very thin, is considered to be amorphous and described by Zachariasen-Warren's model [4,5]. On the other hand, the low-temperature glasses have also been studied as a paradigm where to conduct research on the physics of aging so common to many disordered systems out of equilibrium [15,16]. In this context, the relevant degrees of freedom have also been described in terms of 2LSs, with some success [17]. Yet, deviations from the behaviour pred...

Section: Bas Glassmentioning

confidence: 71%

Section: Bas Glassmentioning

confidence: 92%

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Realistic tunnelling states for the magnetic effects in non-metallic real glasses

Bonfanti

Kob

2015

Philosophical Magazine

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“…present Author in 2004, has been able to explain so far the magnetic effects in the heat capacity [1], in the real [38] and imaginary [39] parts of the dielectric constant and in the polarization echo amplitude [39] data reported to date for a variety of glasses at low temperatures. The ETM has also shed much new light into the composition-dependent anomalies [7,37]. The new physics provided by the magnetic-field dependent TS DOS, as mentioned, comes from a term due to the near-degenerate TWPs [1] and that gets to be added up to the constant DOS from the STM 2LSs (having constant density n 2LS ):…”

Section: The Cellular Model For the Atomic Structure Of Glasses And Tmentioning

confidence: 98%

The Polycluster Theory for the Structure of Glasses: Evidence from Low Temperature Physics

Springer Proceedings in Physics

2017

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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 of the behaviour near the glass transition.

“…These TSs are dynamical defects that give rise to quasi-universal physical properties that can also be exploited in low-temperature thermometry [5]. A celebrated example is the excess TS contribution to the real part ǫ ′ of the dielectric constant at low frequency, which depends logarithmically on the temperature T [6,7,8]. After 40 years of research, however, the precise microscopic nature of the TSs is still a mystery.…”

Section: Introductionmentioning

confidence: 99%

On the Paramagnetic Impurity Concentration of Silicate Glasses from Low-Temperature Physics

Bonfanti

2015

J Low Temp Phys

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The concentration of paramagnetic trace impurities in glasses can be determined via precise SQUID measurements of the sample's 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 overestimated. This analysis represents a further positive test for the structural inhomogeneity theory of the magnetic effects in the cold glasses.