Quenching of charge and spin degrees of freedom in condensed matter

TL;DR

Rapid cooling in condensed matter can kinetically stabilize non-equilibrium charge and spin states, enabling control over metastable phases and potential applications in phase-change memory devices.

Contribution

This paper reveals that cooling rate acts as a control parameter for non-equilibrium states, expanding the understanding of phase transitions beyond equilibrium thermodynamics.

Findings

Rapid cooling can bypass first-order phase transitions.

Quenched states differ from equilibrium ground states.

Potential use in non-volatile phase-change memory.

Abstract

Electrons in condensed matter have internal degrees of freedom, such as charge, spin and orbital, leading to various forms of ordered states through phase transitions. However, in individual materials, a charge/spin/orbital ordered state of the lowest temperature is normally uniquely determined in terms of the lowest-energy state, i.e., the ground state. Here, we summarize recent results showing that under rapid cooling, this principle does not necessarily hold, and thus, the cooling rate is a control parameter of the lowest-temperature state beyond the framework of the thermo-equilibrium phase diagram. Although the cooling rate utilized in low-temperature experiments is typically 2*10^-3 - 4*10^-1 K/s, the use of optical/electronic pulses facilitate rapid cooling, such as 10^2-10^3 K/s. Such an unconventionally high cooling rate allows some systems to kinetically avoid a first-order phase transition, resulting in a quenched charge/spin state that differs from the ground state. We also demonstrate that quenched states can be exploited as a non-volatile state variable when designing phase-change memory functions. The present findings suggest that rapid cooling is useful for exploring and controlling the metastable electronic/magnetic state that is potentially hidden behind the ground state.