Memory and rejuvenation in spin glasses: aging systems are ruled by more than one length scale

TL;DR

This paper demonstrates that aging dynamics in spin glasses involve multiple length scales, challenging the traditional focus on a single correlation length, through advanced simulations on the Janus II supercomputer.

Contribution

The study reveals the crucial role of three different length scales in aging dynamics, supported by novel numerical experiments and analysis.

Findings

Memory and rejuvenation effects are reproduced reliably in simulations.

Three distinct length scales are essential in aging dynamics.

Simulation results align with experimental observations.

Abstract

Memory and rejuvenation effects in the magnetic response of off-equilibrium spin glasses have been widely regarded as the doorway into the experimental exploration of ultrametricity and temperature chaos (maybe the most exotic features in glassy free-energy landscapes). Unfortunately, despite more than twenty years of theoretical efforts following the experimental discovery of memory and rejuvenation, these effects have thus far been impossible to simulate reliably. Yet, three recent developments convinced us to accept this challenge: first, the custom-built Janus II supercomputer makes it possible to carry out "numerical experiments" in which the very same quantities that can be measured in single crystals of CuMn are computed from the simulation, allowing for parallel analysis of the simulation/experiment data. Second, Janus II simulations have taught us how numerical and experimental length scales should be compared. Third, we have recently understood how temperature chaos materializes in aging dynamics. All three aspects have proved crucial for reliably reproducing rejuvenation and memory effects on the computer. Our analysis shows that (at least) three different length scales play a key role in aging dynamics, while essentially all theoretical analyses of the aging dynamics emphasize the presence and the crucial role of a single glassy correlation length.