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Aging in nonequilibrium systems means that the measured response or correlation depends on how long the system has been evolving since preparation, not only on the interval between measurements. A two-time correlation $C(t_w+t,t_w)$ depends separately on waiting time $t_w$ when time-translation invariance has not been established.

One probing protocol quenches a disordered magnet, waits different times and then applies a small field. Comparing the resulting responses distinguishes preparation history from a single equilibrium relaxation time. Analytical spin-glass models exhibit such nonequilibrium correlation and response behavior; simple scaling with $t/t_w$ is a model-dependent form, not the definition of aging.^[ref]

J. Kurchan coauthored a 1993 analytical study of relaxation in a long-range spin glass with Leticia Fernanda Cugliandolo. They solved the long-time behaviour of correlation and response functions in a spherical model with interactions involving several spins.^[ref]

The solution exhibits aging in nonequilibrium systems: the response after a disturbance depends on how long the system has already been evolving, rather than only on elapsed time since the disturbance. It supplies an explicit model in which slow history-dependent dynamics can be studied analytically. The initials are those printed in the primary-paper byline.

Leticia Cugliandolo studies the slow nonequilibrium dynamics of spin glass systems and other complex systems. Her theoretical treatment distinguishes equilibrium relaxation from aging in nonequilibrium systems, where a response depends on how long the system has already been evolving. Two-time correlation functions and responses can violate an equilibrium fluctuation-dissipation theorem, leading to carefully defined effective-temperature descriptions in suitable regimes. Solvable mean-field models connect this dynamics with a complicated free-energy landscape, supplying tests of which features extend beyond a particular material.^[ref]

A physicist researching spin glasses and statistical physics at Sorbonne University.^[ref]