Observation of inhibited electron-ion coupling in strongly heated graphite

TL;DR

This study demonstrates a significantly inhibited electron-ion energy transfer in strongly heated graphite, revealing an energy transfer bottleneck in non-equilibrium warm dense matter through time-resolved x-ray diffraction.

Contribution

It introduces a novel proton-based energy deposition method to heat graphite and uncovers a much slower electron-ion relaxation process than previously observed.

Findings

Electron-ion energy transfer is significantly suppressed.

Relaxation times are approximately three times longer than prior reports.

Evidence of an energy transfer bottleneck in warm dense matter.

Abstract

Creating non-equilibrium states of matter with highly unequal electron and lattice temperatures allows unsurpassed insight into the dynamic coupling between electrons and ions through time-resolved energy relaxation measurements. Recent studies on low-temperature laser-heated graphite suggest a complex energy exchange when compared to other materials. To avoid problems related to surface preparation, crystal quality and poor understanding of the energy deposition and transport mechanisms, we apply a different energy deposition mechanism, via laser-accelerated protons, to isochorically and non-radiatively heat macroscopic graphite samples up to temperatures close to the melting threshold. Using time-resolved x ray diffraction, we show clear evidence of a very small electron-ion energy transfer, yielding approximately three times longer relaxation times than previously reported. This is indicative of the existence of an energy transfer bottleneck in non-equilibrium warm dense matter.