Quantum Phenomena in Structural Glasses: The Intrinsic Origin of Electronic and Cryogenic Anomalies (original) (raw)

The Microscopic Quantum Theory of Low Temperature Amorphous Solids

Advances in Chemical Physics

The quantum excitations in glasses have long presented a set of puzzles for condensed matter physicists. A common view is that they are largely disordered analogs of elementary excitations in crystals, supplemented by two level systems which are chemically local entities coming from disorder. A radical revision of this picture argues that the excitations in low temperature glasses are deeply connected to the energy landscape of the glass when it vitrifies: the excitations are not low excited states built on a single ground state but locally defined resonances, high in the energy spectrum of a solid. According to a semiclassical analysis, the two level systems involve resonant collective tunneling motions of around two hundred molecular units which are relics of the mosaic of cooperative motions at the glass transition temperature Tg. The density of states of the TLS is determined by Tg and the mosaic's length scale, which is a weak function of the cooling rate. The universality of phonon scattering in insulating glasses is explained. The Boson Peak and the plateau in thermal conductivity, observed at higher temperatures, are also quantitatively understood within the picture as arising from the same cooperative motions, but now accompanied by thermal activation of the mosaic's vibrational modes. The dynamics of some of the local structural transitions have significant quantum corrections to the semiclassical picture. These corrections lead to a deviation of the heat capacity and conductivity from the standard tunneling model results and explain the anomalous time dependence of the heat capacity. Interaction between tunneling centers contributes to the large and negative value of the Grüneisen parameter often observed in glasses.

An intrinsic formation mechanism for midgap electronic states in semiconductor glasses

The Journal of Chemical Physics, 2010

We argue that semiconducting quenched liquids and frozen glasses may exhibit a set of peculiar electronic states of topological origin. These states reside at strained regions arising during structural reconfigurations between distinct aperiodic states intrinsic to quenched melts. The strained regions are domain walls separating the distinct aperiodic states; their number is about 10 20 cm −3 in all glassformers owing to the universal dynamics of deeply supercooled melts. Even though located near the middle of the forbidden gap, the topological states are rather extended in one direction while being centered at under-and overcoordinated atoms. The states exhibit the reverse charge-spin relation, the majority of states being diamagnetic and charged. The topological states may be sufficient to account for a number of irradiation-induced phenomena in amorphous semiconductors, including electron spin resonance signal, midgap absorption, photoluminescence, and the fatigue of photoluminescence. We propose experiments to test the present microscopic picture.

Intrinsic Quantum Excitations of Low Temperature Glasses

Physical Review Letters, 2001

Several puzzling regularities concerning the low temperature excitations of glasses are quantitatively explained by quantizing domain wall motions of the random first order glass transition theory. The density of excitations agrees with experiment and scales with the size of a dynamically coherent region at Tg, being about 200 molecules. The phonon coupling depends on the Lindemann ratio for vitrification yielding the observed universal relation l/λ ≃ 150 between phonon wavelength λ and mean free path l. Multilevel behavior is predicted to occur in the temperature range of the thermal conductivity plateau.

A new look at low-temperature anomalies in glasses

Advances in Solid State Physics

We review a model-based rather than phenomenological approach to lowtemperature anomalies in glasses. Specifically, we present a solvable model inspired by spin-glass theory that exhibits both, a glassy low-temperature phase, and a collection of double-and single-well configurations in its potential energy landscape. The distribution of parameters characterizing the local potential energy configurations can be computed , and is found to differ from those assumed in the standard tunneling model and its variants. Still, low temperature anomalies characteristic of amorphous materials are reproduced. More importantly perhaps, we obtain a clue to the universality issue. That is, we are able to distinguish between properties which can be expected to be universal and those which cannot. Our theory also predicts the existence, under suitable circumstances of amorphous phases without low-energy tunneling excitations.

Theory of the structural glass transition: a pedagogical review

Advances in Physics, 2015

The random first-order transition (RFOT) theory of the structural glass transition is reviewed in a pedagogical fashion. The rigidity that emerges in crystals and glassy liquids is of the same fundamental origin. In both cases, it corresponds with a breaking of the translational symmetry; analogies with freezing transitions in spin systems can also be made. The common aspect of these seemingly distinct phenomena is a spontaneous emergence of the molecular field, a venerable and well-understood concept. In crucial distinction from periodic crystallisation, the free energy landscape of a glassy liquid is vastly degenerate, which gives rise to new length and time scales while rendering the emergence of rigidity gradual. We obviate the standard notion that to be mechanically stable a structure must be essentially unique; instead, we show that bulk degeneracy is perfectly allowed but should not exceed a certain value. The present microscopic description thus explains both crystallisation and the emergence of the landscape regime followed by vitrification in a unified, thermodynamics-rooted fashion. The article contains a self-contained exposition of the basics of the classical density functional theory and liquid theory, which are subsequently used to quantitatively estimate, without using adjustable parameters, the key attributes of glassy liquids, viz., the relaxation barriers, glass transition temperature, and cooperativity size. These results are then used to quantitatively discuss many diverse glassy phenomena, including: the intrinsic connection between the excess liquid entropy and relaxation rates, the non-Arrhenius temperature dependence of α-relaxation, the dynamic heterogeneity, violations of the fluctuation-dissipation theorem, glass ageing and rejuvenation, rheological and mechanical anomalies, super-stable glasses, enhanced crystallisation near the glass transition, the excess heat capacity and phonon scattering at cryogenic temperatures, the Boson peak and plateau in thermal conductivity, and the puzzling midgap electronic states in amorphous chalcogenides.

Role of Disorder in the Thermodynamics and Atomic Dynamics of Glasses

Physical Review Letters, 2014

We measured the density of vibrational states (DOS) and the specific heat of various glassy and crystalline polymorphs of SiO 2 . The typical (ambient) glass shows a well-known excess of specific heat relative to the typical crystal (α-quartz). This, however, holds when comparing a lower-density glass to a higherdensity crystal. For glassy and crystalline polymorphs with matched densities, the DOS of the glass appears as the smoothed counterpart of the DOS of the corresponding crystal; it reveals the same number of the excess states relative to the Debye model, the same number of all states in the low-energy region, and it provides the same specific heat. This shows that glasses have higher specific heat than crystals not due to disorder, but because the typical glass has lower density than the typical crystal.

Structural atomistic mechanism for the glass transition entropic scenario

arXiv: Soft Condensed Matter, 2019

A popular Adam--Gibbs scenario has suggested that the excess entropy of glass and liquid over crystal dominates the dynamical arrest at the glass transition with exclusive contribution from configurational entropy over vibrational entropy. However, an intuitive structural rationale for the emergence of frozen dynamics in relation to entropy is still lacking. Here we study these issues by atomistically simulating the vibrational, configurational, as well as total entropy of a model glass former over their crystalline counterparts for the entire temperature range spanning from glass to liquid. Besides confirming the Adam--Gibbs entropy scenario, the concept of Shannon information entropy is introduced to characterize the diversity of atomic-level structures, which undergoes a striking variation across the glass transition, and explains the change found in the excess configurational entropy. Hence, the hidden structural mechanism underlying the entropic kink at the transition is reveal...

Local structure and glass transition temperature in binary glasses

Journal of Non-Crystalline Solids, 1998

We present in this article a new relationship between the glass transition temperature, g , and the local glass structure, dealing with binary glasses and involving the rate of local coordination. In IV±VI based glasses, the predicted coordination number ratio is 3/4, according to the presence of Q 4 and Q 3 units. Experimental measurements lead to the value of 0.66±0.85. The new relationship is applied to B 2 S 3 and P 2 S 5 based glasses, and the unusual behavior of g is explained by the possibility of modi®er-rich clustering eects. Ó