We present the experimental observation of the fluctuation-dissipation theorem (FDT) violation in an assembly of interacting magnetic nanoparticles in the low temperature superspin glass phase. The magnetic noise is measured with a two-dimension electron gas Hall probe and compared to the out of phase ac susceptibility of the same ferrofluid. For "intermediate" aging times of the order of 1 h, the ratio of the effective temperature $T_{\rm eff}$ to the bath temperature T grows from 1 to 6.5 when T is lowered from $T_g$ to 0.3 $T_g$, regardless of the noise frequency. These values are comparable to those measured in an atomic spin glass as well as those calculated for a Heisenberg spin glass.
We report on zero field cooled magnetization relaxation experiments on a concen- trated frozen ferrofluid exhibiting a low temperature superspin glass transition. With a method initially developed for spin glasses, we investigate the field dependence of the relaxations that take place after different aging times. We extract the typical number of correlated spins involved in the aging dynamics. This brings important insights into the dynamical correlation length and its time growth. Our results, consistent with expressions obtained for spin glasses, extend the generality of these behaviours to …
Interacting magnetic nanoparticles display a wide variety of magnetic behaviors that are now being gathered in the emerging field of 'supermagnetism.' We have investigated how the out-of-equilibrium dynamics in the disordered superspin glass (SSG) state of a frozen ferrofluid sample is affected by texturation. Via magnetization relaxation experiments at low temperatures, we were able to estimate superspin correlation lengths for both textured and non-textured samples. The comparison with simulations and experiments on atomic spin glasses shows that the dynamic correlations in SSG's appear to d…
In this paper we investigate the superspin glass behavior of a concentrated assembly of interacting maghemite nanoparticles and compare it to that of canonical atomic spin glass systems. ac versus temperature and frequency measurements show evidence of a superspin glass transition taking place at low temperature. In order to fully characterize the superspin glass phase, the aging behavior of both the thermo-remanent magnetization (TRM) and ac susceptibility has been investigated. It is shown that the scaling laws obeyed by superspin glasses and atomic spin glasses are essentially the same, aft…
We here confirm the occurrence of spin glass phase transition and extract estimates of associated critical exponents of a highly monodisperse and densely compacted system of bare maghemite nanoparticles. This system has earlier been found to behave like an archetypal spin glass, with e.g. a sharp transition from paramagnetic to non-equilibrium behavior, suggesting that this system undergoes a spin-glass phase transition at a relatively high temperature, $T_g$ $\sim$ 140 K.
Effects of temperature changes on the nonequilibrium spin-glass dynamics of a strongly interacting ferromagnetic nanoparticle system (superspin glass) are studied. In contrary to atomic spin glasses, strong cooling rate effects are observed, and no evidence for temperature-chaos is found. The flip time of a magnetic moment is much longer than that of an atomic spin and hence much shorter time scales are probed within the experimental time window for a superspin glass than for an atomic spin glass. Within a real space picture the cumulative aging observed for the superspin glass can be explaine…
The aging and memory effects of Fe$_{3}$O$_{4}$ nanoparticles have been studied using a series of zero-field cooled (ZFC) and field-cooled (FC) magnetization measurements at various aging protocols. The genuine ZFC magnetization after the ZFC procedure with a single stop and wait process shows an aging dip at the stop temperature on reheating. The depth of the aging dip is dependent on the wait time. The frequency dependence of the AC magnetic susceptibility is indicative of critical slowing down at a freezing temperature $T_{f}$ ($= 30.6 \pm 1.6$ K). The relaxation time $τ$ is described by a …
We study the three-dimensional system of magnetic nanoparticle dipoles randomly oriented along quenched easy axes. Directions of the magnetic momenta are described by the Ising variables which allow the momenta to flip along their random orientations. Using the standard mean-field approximation and the replica technique it is shown that the system undergoes a finite temperature phase transition into a spin-glass phase.
Ac susceptibility measurements were performed on discontinuous magnetic multilayers [Co80Fe20(t)/Al2O3(3nm)]x10, t = 0.9 and 1.0nm, by Superconducting Quantum Interference Device (SQUID) magnetometry. The CoFe forms nearly spherical ferromagnetic single-domain nanoparticles in the diamagnetic Al2O3 matrix. Due to dipolar interactions and random distribution of anisotropy axes the system exhibits a spin-glass phase. We measured the ac susceptibility as a function of temperature 20 < T < 100K at different dc fields and as a function of frequency 0.01 < f < 1000Hz. The spectral data were successf…
Fluctuation-dissipation (FD) relation of the three-dimensional Heisenberg spin glass with weak random anisotropy is studied by off-equilibrium Monte Carlo simulation. Numerically determined FD ratio exhibits a ``one-step-ike''behavior, the effective temperature of the spin-glass state being about twice the spin-glass transition temperature, $T_{\rm eff}\simeq 2T_g$, irrespective of the bath temperature. The results are discussed in conjunction with the recent experiment by Herisson and Ocio, and with the chirality scenario of spin-glass transition.
The fluctuation-dissipation relation (FDR) is measured on the dielectric properties of a polymer glass (polycarbonate)in the range $20mHz - 100Hz$. It is found that after a quench below the glass transition temperature the fluctuation dissipation theorem is strongly violated. The amplitude and the persistence time of this violation are decreasing functions of frequency. At frequencies larger than 1Hz it persists for about $3h$. The origin of this violation is a highly intermittent dynamics characterized by large fluctuations. The relevance of these results for recent models of aging dynamics a…
We report new experimental results obtained onthe insulating spin glass CdCrInS_4. Our experimental setup allows a quantitative comparison between thethermo-remanent magnetization and the autocorrelation of spontaneous fluctuations of magnetization, yielding acomplete determination of the fluctuation-dissipationrelation. The dynamics can be studied both in thequasi-equilibrium regime, where the fluctuation-dissipation theorem holds, and in the deeply aging regime. The limit of separation of time-scales, as used in analytical calculations, can be approached by use of a scaling procedure.
Response functions and fluctuations measured locally in complex materials should equally well characterize mesoscopic-scale dynamics. The fluctuation-dissipation-relation (FDR), relates the two in equilibrium, a fact used regularly, for example, to infer mechanical properties of soft matter from the fluctuations in light scattering. In slowly-evolving non-equilibrium systems, such as aging spin and structural glasses, sheared soft matter, and active matter, a form of FDR has been proposed in which an effective temperature, Teff, replaces the usual temperature, and universal behavior is found i…
This review reports on the research done during the past years on violations of the fluctuation-dissipation theorem (FDT) in glassy systems. It is focused on the existence of a quasi-fluctuation-dissipation theorem (QFDT) in glassy systems and the currently supporting knowledge gained from numerical simulation studies. It covers a broad range of non-stationary aging and stationary driven systems such as structural-glasses, spin-glasses, coarsening systems, ferromagnetic models at criticality, trap models, models with entropy barriers, kinetically constrained models, sheared systems and granula…
We present our first experimental determination of the magnetic noise of a superspinglass made of < 1 pico-liter frozen ferrofluid. The measurements were performed with a local magnetic field sensor based on Hall microprobes operated with the spinning current technique. The results obtained, though preliminary, qualitatively agree with the theoretical predictions of Fluctuation-Dissipation theorem (FDT) violation [1].
In a Comment (cond-mat/0103444) on our recent paper "Two time scales and violation of FDT in a finite dimensional model for structural glasses" [1], A. de Candia and A. Coniglio show evidence that the equilibrium overlap distribution P(q) of the frustrated Ising lattice gas (FILG) does not coincide with the one that could be estimated from the off-equilibrium results presented in [1]. In this Reply we show new results on the glassy phase of the FILG that clarify the controversy. Nevertheless, the structure of the equilibrium P(q) remains an open problem. [1] F. Ricci-Tersenghi, D. A. Stariolo …
We show that at short time aging is realized in a simple model of binary glasses. We find that below the transition point the off-equilibrium correlation functions and response functions are compatible with the relations that were originally derived Cugliandolo Kurchan for generalized spin glasses.
We study the breakdown of fluctuation-dissipation relations between time dependent density-density correlations and associated responses following a quench in chemical potential in the Frustrated Ising Lattice Gas. The corresponding slow dynamics is characterized by two well separated time scales which are characterized by a constant value of the fluctuation-dissipation ratio. This result is particularly relevant taking into account that activated processes dominate the long time dynamics of the system.
We study the violation of the fluctuation-dissipation theorem in the three and four dimensional Gaussian Ising spin glasses using on and off equilibrium simulations. We have characterized numerically the function X(C) that determine the violation and we have studied its scaling properties. Moreover we have computed the function x(C) which characterize the breaking of the replica symmetry directly from equilibrium simulations. The two functions are numerically equal and in this way we have established that the conjectured connection between the violation of fluctuation dissipation theorem in th…
In a glassy system different degrees of freedom have well-separated characteristic times, and are described by different temperatures. The stationary state is essentially non-equilibrium. A generalized statistical thermodynamics is constructed and a universal variational principle is proposed. Entropy production and energy dissipation occur at a constant rate; there exists a universal relation between them, valid to leading order in the small ratio of the characteristic times. Energy dissipation (unlike entropy production) is closely connected to the fluctuations of the slow degree. Correction…
A method is presented for measuring the integrated response in Ising spin system without applying any perturbing field. Large-scale simulations are performed in order to show how the method works. Very precise measurements of the fluctuation-dissipation ratio are presented for 3 different Ising models: the 2-dimensional ferromagnetic model, the mean-field diluted 3-spin model, and the 3-dimensional Edwards-Anderson model.
We study the fluctuation-dissipation relations for a three dimensional Ising spin glass in a magnetic field both in the high temperature phase as well as in the low temperature one. In the region of times simulated we have found that our results support a picture of the low temperature phase with broken replica symmetry, but a droplet behavior can not be completely excluded.
By using extensive Molecular Dynamics simulations, we have determined the violation of the fluctuation-dissipation theorem in a Lennard-Jones liquid quenched to low temperatures. For this we have calculated $X(C)$, the ratio between a one particle time-correlation function $C$ and the associated response function. Our results are best fitted by assuming that $X(C)$ is a discontinuous, piecewise constant function. This is similar to what is found in spin systems with one step replica symmetry breaking. This strengthen the conjecture of a similarity between the phase space structure of structura…
The fluctuation-dissipation theorem (FDT), connecting dielectric susceptibility and polarization noise was studied in glycerol below its glass transition temperature Tg. Weak FDT violations were observed after a quench from just above to just below Tg, for frequencies above the alpha peak. Violations persisted up to 10^5 times the thermal equilibration time of the configurational degrees of freedom under study, but comparable to the average relaxation time of the material. These results suggest that excess energy flows from slower to faster relaxing modes.
The disordered random-anisotropy magnetic nanoparticle systems with competing dipolar interactions and ferromagnetic exchange couplings are investigated by Monte Carlo simulations. Superspin glass (SSG) and superferromagnetic (SFM) behaviors are found at low temperatures depending on the interactions. Based on the mean field approximation, the Curie-Weiss temperature TCW = 0 is suggested as the phase boundary between the SSG systems and the SFM systems, which is convinced by the spontaneous magnetizations and relaxations. The magnetic phase diagram is plotted.
We analyze numerically the violation of the fluctuation-dissipation theorem (FDT) in the $\pm J$ Edwards-Anderson (EA) spin glass model. Using single spin probability densities we reveal the presence of strong dynamical heterogeneities, which correlate with ground state information. The physical interpretation of the results shows that the spins in the EA model can be divided in two sets. In 3D, one set forms a compact structure which presents a coarsening-like behavior with its characteristic violation of the FDT, while the other asymptotically follows the FDT. Finally, we compare the dynamic…
We present the first experimental determination of the time autocorrelation $C(t',t)$ of magnetization in the non-stationary regime of a spin glass. Quantitative comparison with the response, the magnetic susceptibility $χ(t',t)$, is made using a new experimental setup allowing both measurements in the same conditions. Clearly, we observe a non-linear fluctuation-dissipation relation between $C$ and $χ$, depending weakly on the waiting time $t'$. Following theoretical developments on mean-field models, and lately on short range models, it is predicted that in the limit of long times, the $χ(C)…
In cond-mat/0002074 Ricci-Tersenghi et al. find two linear regimes in the fluctuation-dissipation relation between density-density correlations and associated responses of the Frustrated Ising Lattice Gas. Here we show that this result does not seem to correspond to the equilibrium quantities of the model, by measuring the overlap distribution P(q) of the density and comparing the FDR expected on the ground of the P(q) with the one measured in the off-equilibrium experiments.
We investigate violations of the fluctuation-dissipation theorem in two classes of trap models by studying the influence of the perturbing field on the transition rates. We show that for perturbed rates depending upon the value of the observable at the arrival trap, a limiting value of the fluctuation-dissipation ratio does exist. However, the mechanism behind the emergence of this value is different in both classes of models. In particular, for an entropically governed dynamics (where the perturbing field shifts the relative population of traps according to the value of the observable) pertur…
The unifying feature of glass formers (such as polymers, supercooled liquids, colloids, granulars, spin glasses, superconductors, ...) is a sluggish dynamics at low temperatures. Indeed, their dynamics is so slow that thermal equilibrium is never reached in macroscopic samples: in analogy with living beings, glasses are said to age. Here, we show how to relate experimentally relevant quantities with the experimentally unreachable low-temperature equilibrium phase. We have performed a very accurate computation of the non-equilibrium fluctuation-dissipation ratio for the three-dimensional Edward…
In a fluid out of equilibrium, the fluctuation dissipation theorem (FDT) is usually violated. Using molecular dynamics simulations, we study in detail the relationship between correlation and response functions in a fluid driven into a stationary non-equilibrium state. Both the high temperature fluid state and the low temperature glassy state are investigated. In the glassy state, the violation of the FDT is quantitatively identical to the one observed previously in an aging system in the absence of external drive. In the fluid state, violations of the FDT appear only when the fluid is driven …
Using the decay of the out equilibrium spin-spin correlation function we compute the equilibrium Edward-Anderson order parameter in the three dimensional binary Ising spin glass in the spin glass phase. We have checked that the Edward-Anderson order parameter computed from out of equilibrium numerical simulations follows with good precision the critical law as determined in experiments and in numerical studies at equilibrium. We have also studied the dependence of the order parameter with the lattice size. Finally we present a large time study of the scaling of the off-equilibrium fluctuation-…
The fluctuation-dissipation relation (FDR) is measured on the dielectric properties a polymer glass (polycarbonate). It is observed that the fluctuation dissipation theorem is strongly violated for a quench from above to below the glass transition temperature. The amplitude and the persistence time of this violation are decreasing functions of frequency. Around $1Hz$ it may persist for several hours. The origin of this violation is a highly intermittent dynamics characterized by large fluctuations a strongly non-Gaussian statistics. The intermittent dynamics depends on the quenching rate and i…
We review a statistical picture of the glassy state derived from the analysis of the off-equilibrium fluctuation-dissipation relations. We define an ultra-long time limit where ``one time quantities'' are close to equilibrium while response and correlation can still display aging. In this limit it is possible to relate the fluctuation-response relation to static breaking of ergodicity. The resulting picture suggests that even far from that limit, the fluctuation-dissipation ratio relates to the rate of growth of the configurational entropy with free-energy density.
We study numerically the Kawasaki dynamics of the 2d Ising model. At large time we recover the coarsening growth as l(t) ~ t^(1/3). At shorter time however, the system enters a metastable glassy regime that displays an extremely slow growth and non-trivial violations of the fluctuation dissipation theorem similar to those observed in spin glasses: this is one of the simplest system where such violations occur. We also consider Potts models, where a similar behavior is observed, and the modelof Shore and Sethna where the domain growth is also slow, but where violations of the fluctuation dissip…
We study the non-equilibrium aging behavior of the gauge glass model in three dimensions at the critical temperature. We perform Monte Carlo simulations with a Metropolis update, and correlation and response functions are calculated for different waiting times. We obtain a multiplicative aging scaling of the correlation and response functions, calculating the aging exponent $b$ and the nonequilibrium autocorrelation decay exponent $λ_c/z_c$. We also analyze the fluctuation-dissipation relationship at the critical temperature, obtaining the critical fluctuation-dissipation ratio $X_\infty$. By …
Complex systems such as glasses, gels, granular materials, and systems far from equilibrium exhibit violation of the ergodic hypothesis (EH) and of the fluctuation-dissipation theorem (FDT). Recent investigations in systems with memory have established a hierarchical connection between mixing, the EH and the FDT. They have shown that a failure of the mixing condition (MC) will lead to the subsequent failures of the EH and of the FDT. Another important point is that such violations are not limited to complex systems: simple systems may also display this feature. Results from such systems are an…
Dynamic compression has been used to synthesize liquid metallic hydrogen at 140 GPa (1.4 million bar) and experimental data and theory predict Al2O3 might be a metallic glass at ~300 GPa. The mechanism of metallization in both cases is probably a Mott-like transition. The strength of sapphire causes shock dissipation to be split differently in the strong solid and soft fluid. Once the 4.5-eV H-H and Al-O bonds are broken at sufficiently high pressures in liquid H2 and in sapphire (single-crystal Al2O3), electrons are delocalized, which leads to formation of energy bands in fluid H and probably…
We numerically study aging for the Edwards-Anderson Model in 3 and 4 dimensions using different temperature-change protocols. In D=3, time scales a thousand times larger than in previous work are reached with the SUE machine. Deviations from cumulative aging are observed in the non monotonic time behavior of the coherence length. Memory and rejuvenation effects are found in a temperature-cycle protocol, revealed by vanishing effective waiting times. Similar effects are reported for the D=3$site-diluted ferromagnetic Ising model (without chaos). However, rejuvenation is reduced if off-equilibri…
In itinerant magnetic systems with disorder, the quantum Griffiths phase at T=0 is unstable to formation of a cluster glass (CG) of frozen droplet degrees of freedom. In the absence of the fluctuations associated with these degrees of freedom, the transition from the paramagnetic Fermi liquid (PMFL) to the ordered phase proceeds via a conventional second-order quantum phase transition. However, when the Griffiths anomalies due to the broad distribution of local energy scales are included, the transition is driven first-order via a novel mechanism for a fluctuation induced first-order transitio…
We derive a microscopic expression for the dielectric susceptibility $χ$ of a Coulomb glass, which corresponds to the definition used in classical electrodynamics, the derivative of the polarization with respect to the electric field. The fluctuation-dissipation theorem tells us that $χ$ is a function of the thermal fluctuations of the dipole moment of the system. We calculate $χ$ numerically for three-dimensional Coulomb glasses as a function of temperature and frequency.
In this note we present numerical simulations of binary mixtures. We study the diffusion of particles and the response to an external driving force. We find evidence for the validity of the Cugliandolo Kurchan off-equilibrium fluctuation dissipation relation. The results are in agreement with the predictions of one step replica symmetry breaking and the dependance of the breakpoint parameter on the temperature coincide with that found in simple generalized spin glass models.
We perform Monte Carlo simulations in a random anisotropy magnet at a intermediate exchange to anisotropy ratio. We focus on the out of equilibrium relaxation after a sudden quenching in the low temperature phase, well below the freezing one. By analyzing both the aging dynamics and the violation of the Fluctuation Dissipation relation we found strong evidence of a spin--glass like behavior. In fact, our results are qualitatively similar to those experimentally obtained recently in a Heisenberg-like real spin glass.
We introduce a set of theoretical ideas that form the basis for an analytical framework capable of describing nonequilibrium dynamics in glassy systems. We test the resulting scenario by comparing its predictions with numerical simulations of short-range spin glasses. Local fluctuations and responses are shown to be connected by a generalized local out-of-equilibrium fluctuation-dissipation relation. Scaling relationships are uncovered for the slow evolution of heterogeneities at all time scales.
A system is glassy when the observation time is much smaller than the equilibration time. A unifying thermodynamic picture of the glassy state is presented. Slow configurational modes are in quasi-equilibrium at an effective temperature. It enters thermodynamic relations with the configurational entropy as conjugate variable. Slow fluctuations contribute to susceptibilities via quasi-equilibrium relations, while there is also a configurational term. Fluctuation-dissipation relations also involve the effective temperature. Fluctuations in the energy are non-universal, however. The picture is su…
We study the four dimensional Gaussian spin glass in presence of a magnetic field. Using off-equilibrium numerical simulations we have found that the probability distribution of the overlaps is built in the same way as that of the Mean Field approximation with replica symmetry breaking. Finally we have studied the violation of the fluctuation-dissipation theorem in presence of magnetic field.
The out of equilibrium dynamics of finite dimensional spin glasses is considered from a point of view going beyond the standard `mean-field theory' versus `droplet picture' debate of the last decades. The main predictions of both theories concerning the spin glass dynamics are discussed. It is shown, in particular, that predictions originating from mean-field ideas concerning the violations of the fluctuation-dissipation theorem apply quantitatively, provided one properly takes into account the role of the spin glass coherence length which plays a central role in the droplet picture. Dynamics …
Using RG we investigate the non-equilibrium relaxation of the (Cardy-Ostlund) 2D random Sine-Gordon model, which describes pinned arrays of lines. Its statics exhibits a marginal ($θ=0$) glass phase for $T<T_g$ described by a line of fixed points. We obtain the universal scaling functions for two-time dynamical response and correlations near $T_g$ for various initial conditions, as well as the autocorrelation exponent. The fluctuation dissipation ratio is found to be non-trivial and continuously dependent on $T$.
We consider the dynamics of a diluted mean-field spin glass model in the aging regime. The model presents a particularly rich heterogeneous behavior. In order to catch this behavior, we perform a **spin-by-spin analysis** for a **given disorder realization**. The results compare well with the outcome of a static calculation which uses the ``survey propagation'' algorithm of Mezard, Parisi, and Zecchina [Sciencexpress 10.1126/science.1073287 (2002)]. We thus confirm the connection between statics and dynamics at the level of single degrees of freedom. Moreover, working with single-site quantiti…
We investigate the relation between two-time, multi-spin, correlation and response functions in the non-equilibrium critical dynamics of Ising models in d=1 and d=2 spatial dimensions. In these non-equilibrium situations, the fluctuation-dissipation theorem (FDT) is not satisfied. We find FDT `violations' qualitatively similar to those reported in various glassy materials, but quantitatively dependent on the chosen observable, in contrast to the results obtained in infinite-range glass models. Nevertheless, all FDT violations can be understood by considering separately the contributions from l…
The off-equilibrium dynamics of a monatomic Lennard-Jones glass is investigated after sudden isothermal density jumps (crunch) from well equilibrated liquid configurations towards the glassy state. The generalized fluctuation-dissipation relation has been studied and the temperature dependence of the violation factor m is found in agreement with the one step replica symmetry breaking scenario, i.e. at low temperature m(T) is found proportional to T up to an off-equilibrium effective temperature T_eff, where m(T_eff)=1. We report T_eff as a function of the density and compare it with the glass …
In this paper I introduce the probability distribution of the local overlap in spin glasses. The properties of the local overlaps are studied in details. These quantities are related to the recently proposed local version of the fluctuation dissipation relations: using the general principle of stochastic stability these local fluctuation dissipation relations can be proved in a way that is very similar to the usual proof of the fluctuation dissipation relations for intensive quantities. The local overlap and its probability distribution play a crucial role in this proof. Similar arguments can …
We investigate the quantum dynamics of Two-Level Systems (TLS) in glasses at low temperatures (1 K and below). We study an ensemble of TLSs coupled to phonons. By integrating out the phonons within the framework of the Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) master equation, we derive analytically the explicit form of the interactions among TLSs, and of the dissipation terms. We find that the unitary dynamics of the system shows clear signatures of Many-Body Localization physics. We study numerically the time behavior of the concurrence, which measures pairwise entanglement also in non-is…
A model glass with fast and slow processes is studied. The statics is simple and the facilitated slow dynamics is exactly solvable. The main features of a fragile glass take place: Kauzmann transition, Vogel-Fulcher law, Adam-Gibbs relation and aging. The time evolution can be so slow that a quasi-equilibrium occur at a time dependent effective temperature. The same effective temperature is derived from the Fluctuation-Dissipation ratio, which supports the applicability of out of equilibrium thermodynamics.
In the context of ultracold atoms in multimode optical cavities, the appearance of a quantum-critical glass phase of atomic spins has been predicted recently. Due to the long-range nature of the cavity-mediated interactions, but also the presence of a driving laser and dissipative processes such as cavity photon loss, the quantum optical realization of glassy physics has no analog in condensed matter, and could evolve into a "cavity glass microscope" for frustrated quantum systems out-of-equilibrium. Here we develop the non-equilibrium theory of the multimode Dicke model with quenched disorder…
With substantial evidence of glassy behavior in the phase diagram of high $T_c$ superconductors and its co-existence with superconductivity, we attempt to answer the question: what are the properties of a superconducting state where the force driving cooper pairing becomes dissipative? We find that when the bosonic mediator is local, dissipation acts to reduce the superconducting critical temperature ($T_c$). On the other hand, contrary to naïve expectations, $T_c$ behaves non-monotonically with dissipation for a non-local mediator -- weakly dissipative bosons at different energy scales act co…
The fluctuation-dissipation relation (FDR) is measured on the dielectric properties of a gel (Laponite) and of a polymer glass (polycarbonate). For the gel it is found that during the transition from a fluid-like to a solid-like state the fluctuation dissipation theorem is strongly violated. The amplitude and the persistence time of this violation are decreasing functions of frequency. Around $1Hz$ it may persist for several hours. A very similar behavior is observed in polycarbonate after a quench below the glass transition temperature. In both cases the origin of this violation is a highly i…
We study the effects of the coupling to an Ohmic quantum reservoir on the static and dynamical properties of a family of disordered SU(2) spin models in a transverse magnetic field using a method of direct spin summation. The tendency to form a glassy phase increases with the strength of the coupling of the system to the environment. We study the influence of the environment on the features of the phase diagram of the various models as well as the stability of the possible phases.
The existence of a generalized fluctuation-dissipation theorem observed in simulations and experiments performed in various glassy materials is related to the concepts of local equilibration and heterogeneity in space. Assuming the existence of a dynamic coherence length scale up to which the system is locally equilibrated, we extend previous generalizations of the FDT relating static to dynamic quantities to the physically relevant domain where asymptotic limits of large times and sizes are not reached. The formulation relies on a simple scaling argument and has thus not the character of a th…
