arXiv subjects

Giancarlo Jug

Publications and source records attributed to Giancarlo Jug.

At least 19 recordsLinked to original sources

arXiv2021

A new Phenomenon: Glass Paramagnetism. Further Experimental and Theoretical Details

In a recent manuscript, the discovery of a new phenomenon in glasses has been reported: {\it glass paramagnetism}, that is the intrinsic magnetisation developed by a substance that has failed to crystallise in a temperature quench when placed in an external magnetic field. The field- ($H$) and especially the temperature- ($T$) dependence of the intrinsic magnetisation $M_{intr}=M_{intr}(H,T)$ is very unexpected, with broad peaks developing for high $H/T$ and marked $T$- oscillations at fixed $H$ for intermediate-to-high temperatures. In this work we present details on the samples, new data for other glassy systems and especially the theoretical background that is capable of explaining most of the experimental data. New phenomena are however emerging from these data, for example a deviation from the Curie-like behaviour of the magnetic susceptibility ($M_{intr}/H$ at low fields) due to the glassy structure alone as well as evidence for new and intriguing quantum coherence effects at the lowest temperatures.

cond-mat.dis-nn↗

arXiv2021

Revealing the Intrinsic Magnetism of Non-Magnetic Glasses

Ordinary multi-component silicate glasses belong to a class of amorphous insulators normally displaying no special form of magnetism, save for the Larmor dominant diamagnetism from the constituent atoms' core electrons and the extrinsic Langevin paramagnetism due to the ubiquitous Fe-group dilute paramagnetic impurities. Here we show that the macroscopic magnetisation of three case-study glass types measured in a SQUID-magnetometer cannot be explained solely by means of the Larmor-Langevin contributions. In particular, we reveal a novel {\em intrinsic} contribution to the bulk magnetisation due to the amorphous structure itself, a contribution that is peculiar both in its temperature and magnetic-field dependence and represents the first true magnetic effect in nominally non-magnetic glasses. The only theoretical interpretation we know of for such an effect and which can consistently explain the experimental data demands the re-thinking of the atomic organisation of glasses at the nanometric scale.

cond-mat.mes-hall↗

arXiv2019

The Making Of A Theory Of The Vitreous Solid State: From 1 milli-K To 1 kilo-K

This is a short account of the basic principles of a comprehensive theory of the vitreous state, looking at glasses and their eventual < > into a liquid state (the inverse glass < >) from the perspective of their low-temperature properties, then working all the way up to the melting temperature (and back again). The theory is still in the making, so only essential guidelines will be provided. There appears to be no ideal-glass transition, no ideal-glass state, but a first theoretical justification of the Vogel-Fulcher-Tamman law can be provided, making glasses truly fascinating quantum as well as topological substances.

cond-mat.soft↗

arXiv2017

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

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.

cond-mat.mes-hall↗

arXiv2015

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.

cond-mat.dis-nn↗

arXiv2015

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.

cond-mat.dis-nn↗

arXiv2013

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

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.

cond-mat.dis-nn↗

arXiv2009

Direct Evidence for a Two-component Tunnelling Mechanism in the Multicomponent Glasses at Low Temperatures

The dielectric anomalies of window-type glasses at low temperatures ($T<$ 1 K) are rather successfully explained by the two-level systems (2LS) tunneling model (TM). However, the magnetic effects discovered in the multisilicate glasses in recent times \cite{ref1}-\cite{ref3}, 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 \cite{ref4}, indicate the need for a suitable generalization of the 2LS TM. We show that, not only for the magnetic effects \cite{ref3,ref5} but also for the mixed glasses in the absence of a field, the right extension of the 2LS TM is provided by the (anomalous) multilevel tunneling systems approach proposed by one of us. It appears that new 2LS develop via dilution near the hull of the SiO$_4$-percolating clusters in the mixed glasses.

cond-mat.dis-nn↗

arXiv2009

Spin-size disorder model for granular superconductors with charging effects

A quantum pseudo-spin model with random spin sizes is introduced to study the effects of charging-energy disorder on the superconducting transition in granular superconducting materials. Charging-energy effects result from the small electrical capacitance of the grains when the Coulomb charging energy is comparable to the Josephson coupling energy. In the pseudo-spin model, randomness in the spin size is argued to arise from the inhomogeneous grain-size distribution. For a particular bimodal spin-size distribution, the model describes percolating granular superconductors. A mean-field theory is developed to obtain the phase diagram as a function of temperature, average charging energy and disorder.

cond-mat.supr-con↗

arXiv2009

Multi-Welled Tunneling Model for the Magnetic-Field Effect in Ultracold Glasses

Puzzling observations of both thermal and dielectric responses in multi-silicate glasses at low temperatures $T$ to static magnetic fields $B$ have been reported in the last decade and call for an extension of the standard two-level systems tunneling model. An explanation is proposed, capable of capturing at the same time the $T$- and $B$-dependence of the specific heat $C_p$ and of the dielectric constant $ε$ in these glasses. This theory points to the existence of anomalous multi-welled tunneling systems in the glasses -- alongside the standard two-level systems -- and indications are given for glasses which should achieve larger electric magnetocapacitive enhancements.

cond-mat.dis-nn↗

arXiv2005

Quantized Transport in Two-Dimensional Spin-Ordered Structures

We study in detail the transport properties of a model of conducting electrons in the presence of double-exchange between localized spins arranged on a 2D Kagome lattice, as introduced by Ohgushi, Murakami, and Nagaosa (2000). The relationship between the canting angle of the spin texture $θ$ and the Berry phase field flux per triangular plaquette $ϕ$ is derived explicitly and we emphasize the similarities between this model and Haldane's honeycomb lattice version of the quantum Hall effect (Haldane, 1988). The quantization of the transverse (Hall) conductivity $σ_{xy}$ is derived explicitly from the Kubo formula and a direct calculation of the longitudinal conductivity $σ_{xx}$ shows the existence of a metal-insulator transition as a function of the canting angle $θ$ (or flux density $ϕ$). This transition might be linked to that observable in the manganite compounds or in the pyrochlore ones, as the spin ordering changes from ferromagnetic to canted.

cond-mat.str-el↗

arXiv1999

Tails of Localized Density of States of Two-dimensional Dirac Fermions

The density of states of Dirac fermions with a random mass on a two-dimensional lattice is considered. We give the explicit asymptotic form of the single-electron density of states as a function of both energy and (average) Dirac mass, in the regime where all states are localized. We make use of a weak-disorder expansion in the parameter g/m^2, where g is the strength of disorder and m the average Dirac mass for the case in which the evaluation of the (supersymmetric) integrals corresponds to non-uniform solutions of the saddle point equation. The resulting density of states has tails which deviate from the typical pure Gaussian form by an analytic prefactor.

cond-mat.mes-hall↗

arXiv1999

Duality symmetry, strong coupling expansion and universal critical amplitudes in two-dimensional Φ^{4} field models

We show that the exact beta-function β(g) in the continuous 2D gΦ^{4} model possesses the Kramers-Wannier duality symmetry. The duality symmetry transformation \tilde{g}=d(g) such that β(d(g))=d'(g)β(g) is constructed and the approximate values of g^{*} computed from the duality equation d(g^{*})=g^{*} are shown to agree with the available numerical results. The calculation of the beta-function β(g) for the 2D scalar gΦ^{4} field theory based on the strong coupling expansion is developed and the expansion of β(g) in powers of g^{-1} is obtained up to order g^{-8}. The numerical values calculated for the renormalized coupling constant g_{+}^{*} are in reasonable good agreement with the best modern estimates recently obtained from the high-temperature series expansion and with those known from the perturbative four-loop renormalization-group calculations. The application of Cardy's theorem for calculating the renormalized isothermal coupling constant g_{c} of the 2D Ising model and the related universal critical amplitudes is also discussed.

cond-mat.stat-mech↗

arXiv1998

Low-temperature renormalization group study of uniformly frustrated models for type-II superconductors

We study phase transitions in uniformly frustrated SU(N)-symmetric $(2+ε)$-dimensional lattice models describing type-II superconductors near the upper critical magnetic field $H_{c2}(T)$. The low-temperature renormalization-group approach is employed for calculating the beta-function $β(T,f)$ with $f$ an arbitrary rational magnetic frustration. The phase-boundary line $H_{c2}(T)$ is the ultraviolet-stable fixed point found from the equation $β(T,f)=0$, the corresponding critical exponents being identical to those of the non-frustrated continuum system. The critical properties of the SU(N)-symmetric complex Ginzburg-Landau (GL) model are then examined in $(4+ε)$ dimensions. The possibility of a continuous phase transition into the mixed state in such a model is suggested.

cond-mat.supr-con↗

arXiv1997

Vacuum Instability and Pair Nucleation in a Dissipative Medium

We present a systematic and unifying treatment of the problem of spontaneous nucleation of particle-antiparticle pairs in a (2+1)-dimensional system due to a static and uniform electromagnetic-like field, in the presence of quantum dissipation. We first describe a direct derivation of the Caldeira-Leggett type of mechanism for quantum dissipation within the context of string theory and of the ensuing Born-Infeld action, pointing out the difference with the physical context in which vacuum decay can occur. We then evaluate the particle-antiparticle pair production rate, working out all the details of the calculation and including also the effects of a possible periodic background potential and of the Coulomb-like particle-antiparticle attraction. The former induces a dissipation-driven localization which interferes with the effect of the driving electric-like field. We also hint at a possible application to the problem of the decay of a supercurrent in a superconducting thin film due to vortex-antivortex nucleation in the presence of a pinning lattice.

hep-th↗

arXiv1996

Vortex Quantum Nucleation and Tunneling in Superconducting Thin Films: Role of Dissipation and Periodic Pinning

We investigate the phenomenon of decay of a supercurrent in a superconducting thin film in the absence of an applied magnetic field. The resulting zero-temperature resistance derives from two equally possible mechanisms: 1) quantum tunneling of vortices from the edges of the sample; and 2) homogeneous quantum nucleation of vortex-antivortex pairs in the bulk of the sample, arising from the instability of the Magnus field's ``vacuum''. We study both situations in the case where quantum dissipation dominates over the inertia of the vortices. We find that the vortex tunneling and nucleation rates have a very rapid dependence on the current density driven through the sample. Accordingly, whilst normally the superconductor is essentially resistance-free, for the high current densities that can be reached in high-$T_c$ films a measurable resistance might develop. We show that edge-tunneling appears favoured, but the presence of pinning centres and of thermal fluctuations leads to an enhancement of the nucleation rates. In the case where a periodic pinning potential is artificially introduced in the sample, we show that current-oscillations will develop indicating an effect specific to the nucleation mechanism where the vortex pair-production rate, thus the resistance, becomes sensitive to the corrugation of the pinning substrate. In all situations, we give estimates for the observability of the studied phenomena.

cond-mat.supr-con↗

arXiv1996

Supercurrent Decay by Vortex Nucleation in the Presence of an Array of Pinning Centres: A Renormalization Group Approach

We study the phenomenon of decay of a supercurrent in a superconducting thin film due to the spontaneous homogeneous nucleation of quantized vortex-antivortex pairs in the absence of an external magnetic field and in the presence of a periodic pinning potential. We describe the vortex nucleation by means of a Schwinger-type path-integral calculation including the effects of quantum dissipation. The overdamped case is treated exactly, whilst the case in which a periodic array of pinning centres is present is dealt with by means of a renormalization group (RG) approach in frequency space. This yields an approximate but very appealing analytic result for the dependence of the vortex nucleation rate on the externally-driven supercurrent. In this formula the rate dependence displays oscillations which are connected to the pinning periodicity and correspond to the vortex nucleation length probing over the pinning sites as the current density is varied. Our RG treatment describes the vortex nucleation process in both the confined and the mobile phases of the dissipation-driven localization transition characterising the motion of dissipating quantum particles in a periodic potential.

cond-mat↗

arXiv1996

Edge Tunneling of Vortices in Superconducting Thin Films

We investigate the phenomenon of the decay of a supercurrent due to the zero-temperature quantum tunneling of vortices from the edge in a thin superconducting film in the absence of an external magnetic field. An explicit formula is derived for the tunneling rate of vortices, which are subject to the Magnus force induced by the supercurrent, through the Coulomb-like potential barrier binding them to the film's edge. Our approach ensues from the non-relativistic version of a Schwinger-type calculation for the decay of the 2D vacuum previously employed for describing vortex-antivortex pair-nucleation in the bulk of the sample. In the dissipation-dominated limit, our explicit edge-tunneling formula yields numerical estimates which are compared with those obtained for bulk-nucleation to show that both mechanisms are possible for the decay of a supercurrent.

cond-mat↗