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

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 over-estimated. This analysis represents a further positive test for the structural inhomogeneity theory of the magnetic effects in the cold glasses.Comment: 27 pages, 11 figures, 10 tables, 41 references: submitted to Journal of Low Temperature Physic

Low temperature theoretical and numerical study of structural glasses.

Le proprietà termiche e dielettriche dei vetri alle basse temperature (1K) sono generalmente attribuite all’esistenza dei sistemi di tunneling in potenziali a doppia buca asimmetrica del landscape di energia potenziale. Attraverso un’estensione del modello teorico che descrive la fenomenologia dei vetri alle basse temperature e assumendo un nuovo tipo di sistema di tunneling, costituito da un potenziale a tre buche, abbiamo spiegato dei recenti dati sperimentali in presenza di campo magnetico. I fit ottenuti con il modello teorico per la magnetizzazione sono in accordo con i dati sperimentali. Inoltre i calcoli effettuati con un modello esteso sino a quattro buche di potenziale hanno mostrato che il modello più semplice in grado di spiegare le anomalie dei dati sperimentali nei vetri reali è proprio quello a tre buche. Le estensive simulazioni numeriche condotte presso il Laboratorio dell’Università di Montpellier hanno permesso di far luce sulla natura dei sistemi di tunneling. Il nuovo metodo EIS che abbiamo implementato è stato preliminarmente validato at- traverso simulazioni di un cristallo FCC di tipo Lennard-Jones. Nel seguito la stessa procedura è stata applicata ad un modello di vetro, la miscela binaria Lennard-Jones. Le configurazioni analizzate, a bassissime temperature (ottenute con la tecnica del parallel tempering) mostrano l’esistenza di “valli” interne nel bacino dei minimi del landscape di energia potenziale. Le geometrie trovate sembrerebbero essere legate ai sistemi di tunneling, dunque tale risultato può essere considerato in linea di principio come una prima evidenza di tali sistemi nelle simulazioni numeriche

Mapping the routes of perception: Hemispheric asymmetries in signal propagation dynamics

The visual system has long been considered equivalent across hemispheres. However, an increasing amount of data shows that functional differences may exist in this regard. We therefore tried to characterize the emergence of visual perception and the spatiotemporal dynamics resulting from the stimulation of visual cortices in order to detect possible interhemispheric asymmetries. Eighteen participants were tested. Each of them received 360 transcranial magnetic stimulation (TMS) pulses at phosphene threshold intensity over left and right early visual areas while electroencephalography was being recorded. After each single pulse, participants had to report the presence or absence of a phosphene. Local mean field power analysis of TMS-evoked potentials showed an effect of both site (left vs. right TMS) of stimulation and hemisphere (ipsilateral vs. contralateral to the TMS): while right TMS determined early stronger activations, left TMS determined later stronger activity in contralateral electrodes. The interhemispheric signal propagation index revealed differences in how TMS-evoked activity spreads: left TMS-induced activity diffused contralaterally more than right stimulation. With regard to phosphenes perception, distinct electrophysiological patterns were found to reflect similar perceptual experiences: left TMS-evoked phosphenes are associated with early occipito-parietal and frontal activity followed by late central activity; right TMS-evoked phosphenes determine only late, fronto-central, and parietal activations. Our results show that left and right occipital TMS elicits differential electrophysiological patterns in the brain, both per se and as a function of phosphene perception. These distinct activation patterns may suggest a different role of the two hemispheres in processing visual information and giving rise to perception

Graph analysis of TMS–EEG connectivity reveals hemispheric differences following occipital stimulation

(1) Background: Transcranial magnetic stimulation combined with electroencephalography (TMS–EEG) provides a unique opportunity to investigate brain connectivity. However, possible hemispheric asymmetries in signal propagation dynamics following occipital TMS have not been investigated. (2) Methods: Eighteen healthy participants underwent occipital single-pulse TMS at two different EEG sites, corresponding to early visual areas. We used a state-of-the-art Bayesian estimation approach to accurately estimate TMS-evoked potentials (TEPs) from EEG data, which has not been previously used in this context. To capture the rapid dynamics of information flow patterns, we implemented a self-tuning optimized Kalman (STOK) filter in conjunction with the information partial directed coherence (iPDC) measure, enabling us to derive time-varying connectivity matrices. Subsequently, graph analysis was conducted to assess key network properties, providing insight into the overall network organization of the brain network. (3) Results: Our findings revealed distinct lateralized effects on effective brain connectivity and graph networks after TMS stimulation, with left stimulation facilitating enhanced communication between contralateral frontal regions and right stimulation promoting increased intra-hemispheric ipsilateral connectivity, as evidenced by statistical test (p < 0.001). (4) Conclusions: The identified hemispheric differences in terms of connectivity provide novel insights into brain networks involved in visual information processing, revealing the hemispheric specificity of neural responses to occipital stimulation

Universal low-frequency vibrational modes in silica glasses

It was recently shown that different simple models of glass formers with binary interactions define a universality class in terms of the density of states of their quasi-localized low-frequency modes. Explicitly, once the hybridization with standard Debye (extended) modes is avoided, a number of such models exhibit a universal density of state, depending on the mode frequencies as $D(\omega) \sim \omega^4$. It is unknown however how wide is this universality class, and whether it also pertains to more realistic models of glass formers. To address this issue we present analysis of the quasi-localized modes in silica, a network glass which has both binary and ternary interactions. We conclude that in 3-dimensions silica exhibits the very same frequency dependence at low frequencies, suggesting that this universal form is a generic consequence of amorphous glassiness.Comment: 4 pages, 5 figures, Submitted in parallel to "Universality of the nonphononic vibrational spectrum across different classes of computer glasses" to arXiv on March 17th, 202

Oscillatory Instabilities in 3-Dimensional Frictional Granular Matter

The dynamics of amorphous granular matter with frictional interactions cannot be derived in general from a Hamiltonian and therefore displays oscillatory instabilities stemming from the onset of complex eigenvalues in the stability matrix. These instabilities were discovered in the context of one and two dimensional systems, while the three dimensional case was never studied in detail. Here we fill this gap by deriving and demonstrating the presence of oscillatory instabilities in a three dimensional granular packing. We study binary assemblies of spheres of two sizes interacting via classical Hertz and Mindlin force laws for the longitudinal and tangent interactions, respectively. We formulate analytically the stability matrix in 3D and observe that a couple of complex eigenvalues emerges at the onset of the instability as in the case of frictional disks in two-dimensions. The dynamics then shows oscillatory exponential growth in the Mean-Square-Displacement, followed by a catastrophic event. The generality of these results for any choice of forces that break the symplectic Hamiltonian symmetry is discussed.Comment: arXiv admin note: substantial text overlap with arXiv:1903.10887, arXiv:1901.0237

Realistic Tunneling States for the Magnetic Effects in Non-Metallic Real Glasses

The discovery of magnetic and compositional effects in the low temperature properties of multi-component glasses has prompted the need to extend the standard two-level systems (2LSs) tunneling model. A possible extension \cite{Jug2004} assumes that a subset of tunneling quasi-particles is moving in a three-welled 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 imaginary 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.Comment: 48 pages, 27 figures; mini-review for the Proceedings of the XIV International Workshop on Complex Systems (Fai della Paganella, Trento, March 2015) (submitted to Phil.Mag.). arXiv admin note: text overlap with arXiv:cond-mat/0210221 by other author