In this work it is studied the Hopfield fermionic spin glass model which allows interpolating from trivial randomness to a highly frustrated regime. Therefore, it is possible to investigate whether or not frustration is an essential ingredient which would allow this magnetic disordered model to present naturally inverse freezing by comparing the two limits, trivial randomness and highly frustrated regime and how different levels of frustration could affect such unconventional phase transition. The problem is expressed in the path integral formalism where the spin operators are represented by bilinear combinations of Grassmann variables. The Grand Canonical Potential is obtained within the static approximation and one-step replica symmetry breaking scheme. As a result, phase diagrams temperature {\it versus} the chemical potential are obtained for several levels of frustration. Particularly, when the level of frustration is diminished, the reentrance related to the inverse freezing is gradually suppressed.
Spin glasses, generally defined as disordered systems with randomized competing interactions, are a widely investigated complex system. Theoretical models describing spin glasses are broadly used in other complex systems, such as those describing brain function, error-correcting codes, or stock-market dynamics. This wide interest in spin glasses provides strong motivation to generate an artificial spin glass within the framework of artificial spin ice systems. Here, we present the experimental realization of an artificial spin glass consisting of dipolar coupled single-domain Ising-type nanoma…
Università del Salento(萨兰托大学);Universität Zürich(苏黎世大学);King’s College London(伦敦国王学院);Scuola Normale Superiore(高等师范学校)
Restricted Boltzmann Machines are described by the Gibbs measure of a bipartite spin glass, which in turn corresponds to the one of a generalised Hopfield network. This equivalence allows us to characterise the state of these systems in terms of retrieval capabilities, both at low and high load. We study the paramagnetic-spin glass and the spin glass-retrieval phase transitions, as the pattern (i.e. weight) distribution and spin (i.e. unit) priors vary smoothly from Gaussian real variables to Boolean discrete variables. Our analysis shows that the presence of a retrieval phase is robust and no…
It is well-known that spontaneous symmetry breaking in one spatial dimension is thermodynamically forbidden at finite energy density. Here we show that mirror-symmetric disorder in an interacting quantum system can invert this paradigm, yielding spontaneous breaking of mirror symmetry only at finite energy density and giving rise to "mirror-glass" order. The mirror-glass transition, which is driven by a finite density of interacting excitations, is enabled by many-body localization, and appears to occur simultaneously with the localization transition. This counterintuitive manifestation of loc…
The infinite volume limit of the Hopfield neural networks in the replica-symmetric regime
Sapienza Università di Roma(罗马第一大学);Istituto Nazionale di Fisica Nucleare, Sezione di Roma(意大利国家核物理研究所罗马分部);CNR Nanotec, Salento Unit(意大利国家研究委员会纳米技术研究所莱切单元);Università del Salento(萨伦托大学);Istituto Nazionale di Fisica Nucleare, Sezione di Lecce(意大利国家核物理研究所莱切分部)
It is presented a theory that describes a spin glass phase at finite temperatures in Kondo lattice systems with an additional RKKY interaction represented by long range, random couplings among localized spins like in the Sherrington- Kirkpatrick (SK) spin glass model. The problem is studied within the functional integral formalism where the spin operators are represented by bilinear combinations of fermionic (anticommuting) Grassmann variables. The Kondo and spin glass transitions are both described with the mean field like static ansatz that reproduces good results in the two well known limit…
我们分析了具有无序磁相互作用的Blume-Emery-Griffiths模型,该模型表现出逆冻结现象。该自旋1模型在晶体场的行为进行了研究,在整个相图和过渡和spinodal线的模型计算使用全复本对称性破缺Ancroplant,总是产生一个稳定的相位。我们将结果与格子气上具有Ising自旋的淬火无序模型(其中没有再进入发生)以及Schupper和Shnerb最近引入的具有广义自旋变量的模型[Phys. Rev. Lett. 93,037202(2004)]。所有这些模型中最简单的一个版本,称为Ghatak-Sherrington模型,它包含了所有表征向非晶相逆转变的一般特征,包括正确的热力学行为。
In the present work, we employ broadband dielectric spectroscopy to study the molecular dynamics of the prototypical glass former glycerol confined in two microporous zeolitic imidazolate frameworks (ZIF-8 and ZIF-11) with well-defined pore diameters of 1.16 and 1.46 nm, respectively. The spectra reveal information on the modified alpha relaxation of the confined supercooled liquid, whose temperature dependence exhibits clear deviations from the typical super-Arrhenius temperature dependence of the bulk material, depending on temperature and pore size. This allows assigning well-defined cooper…
Learning About Learning: A Path from Spin Glasses to Artificial Intelligence
NeuroPhysics Laboratory, Departamento de Física, Universidade Federal de Santa Catarina, Florianópolis SC, Brazil. 88040-900(神经物理实验室,物理系,圣卡塔琳娜联邦大学,弗洛里亚诺波利斯SC,巴西。88040-900)
Aging dynamics across a dynamic Almeida-Thouless line in three dimensional short-range Issuing spin glass Cu$_{0.5}$Co$_{0.5}$Cl$_{2}$-FeCl$_{3}$ graphite bi-intercalation compound
Cu $_{0.5}$ Co $_{0.5}$ Cl $_{2}$ -FeCl $_{3}$石墨层间化合物是具有自旋冻结温度$T_{c}$($= 3.92 \pm 0.11$ K)的三维短程伊辛自旋玻璃。在磁场$H$存在下的自旋玻璃相的稳定性由(i)自旋冻结温度$T_{f}(ω,H)$检验,在该温度下,微分场冷却(FC)磁化率$\partial M_{FC}(T,H)/\partial H$与具有角频率$ω$的色散$χ^{\prime}(ω,T,H=0)$一致,以及(ii)在具有等待时间$t_{W}$($= 1.0 \times 10^{4}$)的ZFC老化协议之后,零场冷却(ZFC)磁化率$χ_{ZFC}$的时间依赖性。弛豫速率$S(t)$(= d $χ_{ZFC}$ /d $\ln t$)在特征时间$t_{cr}$处表现出局部最大值,反映了非平衡老化动力学。在固定温度$T$(2.9 K $\leq T<T_{c}$)下,峰值时间$t_{cr}(T,H)$随$H$的增加而减小。自旋冻结温度$T_{f}(ω,H)$提供了在有限磁场中热平衡时自旋玻璃相不稳定的证据。$t_{cr}(T,H)$和$T_{f}(ω,H)$都表现出一定的缩放行为预测的液滴图片,表明在$H$的存在下,从SG动态顺磁行为的动态交叉。
The interplay between geometric frustration (GF) and bond disorder is studied in the Ising kagome lattice within a cluster approach. The model considers antiferromagnetic (AF) short-range couplings and long-range intercluster disordered interactions. The replica formalism is used to obtain an effective single cluster model from where the thermodynamics is analyzed by exact diagonalization. We found that the presence of GF can introduce cluster freezing at very low levels of disorder. The system exhibits an entropy plateau followed by a large entropy drop close to the freezing temperature. In t…
We investigate the spin-glass transition in the strongly frustrated well-known compound $Fe_2TiO_5$. A remarkable feature of this transition, widely discussed in the literature, is its anisotropic properties: the transition manifests itself in the magnetic susceptibly only along one axis, despite $Fe^{3+}$ $d^5$ spins having no orbital component. We demonstrate, using neutron scattering, that below the transition temperature $T_g = 55 K$, $Fe_2TiO_5$ develops nanoscale surfboard shaped antiferromagnetic regions in which the $Fe^{3+}$ spins are aligned perpendicular to the axis which exhibits f…
Understanding the glassy nature of neural networks is pivotal both for theoretical and computational advances in Machine Learning and Theoretical Artificial Intelligence. Keeping the focus on dense associative Hebbian neural networks, the purpose of this paper is two-fold: at first we develop rigorous mathematical approaches to address properly a statistical mechanical picture of the phenomenon of {\em replica symmetry breaking} (RSB) in these networks, then -- deepening results stemmed via these routes -- we aim to inspect the {\em glassiness} that they hide. In particular, regarding the meth…
The anisotropic spin glass transition, in which spin freezing is observed only along the c-axis in pseudobrookite Fe$_2$TiO$_5$, has long been perplexing because the Fe$^{3+}$ moments (d$^5$) are expected to be isotropic. Recently, neutron diffraction demonstrated that surfboard-shaped antiferromagnetic nanoregions coalesce above the glass transition temperature, T$_g$ $\approx$ 55 K, and a model was proposed in which the freezing of the fluctuations of the surfboards' magnetization leads to the anisotropic spin glass state. Given this new model, we have carried out high resolution inelastic n…
We report on the crossover from the thermal to athermal regime of an artificial spin ice formed from a square array of magnetic islands whose lateral size, 30~nm~$\times$~70~nm, is small enough that they are superparamagnetic at room temperature. We used resonant magnetic soft x-ray photon correlation spectroscopy (XPCS) as a method to observe the time-time correlations of the fluctuating magnetic configurations of spin ice during cooling, which are found to slow abruptly as a freezing temperature $T_0 = 178 \pm 5$~K is approached. This slowing is well-described by a Vogel-Fulcher-Tammann law,…
We perform Metropolis Monte Carlo simulations of the behaviour of a film of insulating material containing a distribution of magnetic nanoparticles. We assume that these particles only interact through dipolar forces and we find that their behaviour at low T shows characteristics of a spin-glass with a freezing T_f at which the linear susceptibility and the specific heat show a maximum. We obtain the spin glass order parameter as a function of temperature and we also calculate the time auto-correlation of the spin at the center of the system. We find that these results are consistent with the …
Observation of associative-memory retrieval and spin-glass phases on a photonic quantum simulator
Sapienza Università di Roma(罗马大学);Università del Salento(萨伦托大学);Institute of Nanotechnology of the National Research Council of Italy, CNR-NANOTEC(意大利国家研究委员会纳米技术研究所);Università degli Studi di Salerno(萨莱诺大学);INFN, Sezione di Napoli, Gruppo Collegato di Salerno(意大利国家核物理研究所那不勒斯分部萨莱诺协作组);Department of Computing Sciences, Bocconi University(博科尼大学计算科学系);Istituto di Fotonica e Nanotecnologie, Consiglio Nazionale delle Ricerche (IFN-CNR)(意大利国家研究委员会光子和纳米技术研究所)