Spin-glass in diverse geometrie (da sottili film a sistemi cubici): simulazioni incontrano esperimenti

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física Teórica I, leída el 19-07-2021During my Ph.D., we made small progress in the secular question on the nature of the spin-glass system. However, we demonstrated the unique and powerful combination of experiments, theory, and simulations addressing complex dynamics. On one hand, the experimental progress on the sample preparation and the increased precision on the measurements of key physical observables have opened new prospects in the spinglass investigation. On the other hand, the Janus II special-purpose supercomputer, in combination with theory, is suficient to extend simulation time and length scales tovalues explored experimentally. The text is organized into four parts. In the following paragraphs, we introduce briefly each of them...Esta tesis presenta nuestra modesta contribución a la comprensión y modelización de los vidrios de espín. En particular, hemos mostrado que la combinación resultados teóricos, numéricos y experimentales permite revolucionar la comprensión de la dinámica de estos sistemas complejos. Desde el punto de vista experimental, los progresos en la preparación de muestras se alían con la alta precisión de las medidas para abrir nuevas perspectivas previamente insospechadas. Por otro lado, el uso combinado del superordenador Janus II y el análisis teórico ha permitido obtener (e interpretar)simulaciones en una escala temporal comparable a la de los experimentos. Para explicar de manera coherente estos avances, hemos dividido el texto en cuatro partes cuyo contenido describimos a continuación...Fac. de Ciencias FísicasTRUEunpu

Multifractality in spin glasses

15 pages, 15 figuresInternational audienceWe unveil the multifractal behavior of Ising spin glasses in their low-temperature phase. Using the Janus II custom-built supercomputer, the spin-glass correlation function is studied locally. Dramatic fluctuations are found when pairs of sites at the same distance are compared. The scaling of these fluctuations, as the spin-glass coherence length grows with time, is characterized through the computation of the singularity spectrum and its corresponding Legendre transform. A comparatively small number of site pairs controls the average correlation that governs the response to a magnetic field. We explain how this scenario of dramatic fluctuations (at length scales smaller than the coherence length) can be reconciled with the smooth, self-averaging behavior that has long been considered to describe spin-glass dynamics

Quantifying memory in spin glasses

10 pages, 8 figuresRejuvenation and memory, long considered the distinguishing features of spin glasses, have recently been proven to result from the growth of multiple length scales. This insight, enabled by simulations on the Janus~II supercomputer, has opened the door to a quantitative analysis. We combine numerical simulations with comparable experiments to introduce two coefficients that quantify memory. A third coefficient has been recently presented by Freedberg et al. We show that these coefficients are physically equivalent by studying their temperature and waiting-time dependence

Quantifying memory in spin glasses

10 pages, 8 figuresRejuvenation and memory, long considered the distinguishing features of spin glasses, have recently been proven to result from the growth of multiple length scales. This insight, enabled by simulations on the Janus~II supercomputer, has opened the door to a quantitative analysis. We combine numerical simulations with comparable experiments to introduce two coefficients that quantify memory. A third coefficient has been recently presented by Freedberg et al. We show that these coefficients are physically equivalent by studying their temperature and waiting-time dependence

Superposition principle and nonlinear response in spin glasses

International audienceThe extended principle of superposition has been a touchstone of spin-glass dynamics for almost 30 years. The Uppsala group has demonstrated its validity for the metallic spin glass, CuMn, for magnetic fields H up to 10 Oe at the reduced temperature Tr=T/Tg=0.95, where Tg is the spin-glass condensation temperature. For H>10 Oe, they observe a departure from linear response which they ascribe to the development of nonlinear dynamics. The thrust of this paper is to develop a microscopic origin for this behavior by focusing on the time development of the spin-glass correlation length, ξ(t,tw;H). Here, t is the time after H changes, and tw is the time from the quench for T>Tg to the working temperature T until H changes. We connect the growth of ξ(t,tw;H) to the barrier heights Δ(tw) that set the dynamics. The effect of H on the magnitude of Δ(tw) is responsible for affecting differently the two dynamical protocols associated with turning H off (TRM, or thermoremanent magnetization) or on (ZFC, or zero-field-cooled magnetization). This difference is a consequence of nonlinearity based on the effect of H on Δ(tw). Superposition is preserved if Δ(tw) is linear in the Hamming distance Hd (proportional to the difference between the self-overlap qEA and the overlap q[Δ(tw)]). However, superposition is violated if Δ(tw) increases faster than linear in Hd. We have previously shown, through experiment and simulation, that the barriers Δ(tw) do increase more rapidly than linearly with Hd through the observation that the growth of ξ(t,tw;H) slows down as ξ(t,tw;H) increases. In this paper, we display the difference between the zero-field-cooled ξZFC(t,tw;H) and the thermoremanent magnetization ξTRM(t,tw;H) correlation lengths as H increases, both experimentally and through numerical simulations, corresponding to the violation of the extended principle of superposition in line with the finding of the Uppsala Group