Generalized thermodynamic closure in ultrafast phonon dynamics

TL;DR

This paper demonstrates that a driven phonon mode can be described by a generalized thermodynamic framework that incorporates both energy and coherence, revealing a coherence-extended thermodynamic regime in nonequilibrium quantum materials.

Contribution

It introduces a novel thermodynamic description for driven phonons that includes coherence, supported by experimental evidence and Lindblad dynamics analysis.

Findings

Observation of delayed ultrafast phonon response beyond a threshold drive

Collapse of density-matrix trajectories onto a common energy-coherence surface

Establishment of a coherence-extended thermodynamic regime for driven phonons

Abstract

Driven-dissipative dynamics underlie a wide range of nonequilibrium phenomena in quantum materials, yet reduced descriptions beyond the quasi-equilibrium picture remain difficult to establish. Here, we experimentally demonstrate that a resonantly driven phonon mode admits a generalized thermodynamic description in which coherence and energy jointly organize the nonequilibrium evolution. Beyond a threshold driving field strength, we observe a delayed ultrafast response of a coherently driven phonon mode. Combined with experimentally constrained Lindblad dynamics, we show that this delay reflects the finite-time spreading of excitations across many phonon levels. At the same time, the full density-matrix trajectories for three driving conditions collapse onto a common surface defined by energy and coherence. Our results establish a coherence-extended thermodynamic regime for driven phonons and provide a framework for broader state engineering in driven-dissipative bosonic excitations.