Direct measure of atomic heat disproves decades-old theory

Amazing stuff!

"It’s notoriously difficult to take the temperature of really hot things. ...

Now, for the first time, a team of researchers report ... that they have directly measured the temperature of atoms in warm dense matter. While other methods rely on complex and hard-to-validate models, this new method directly measures the speed of atoms, and therefore the temperature of the system. Already, their innovative method is changing our understanding of the world: In an experimental debut, the team superheated solid gold far beyond the theoretical limit, unexpectedly overturning four decades of established theory. ...

At SLAC’s MEC instrument, the team used a laser to superheat a sample of gold. As heat flashed through the nanometer-thin sample, its atoms began to vibrate at a speed directly related to their rising temperature. The team then sent a pulse of ultrabright X-rays from the Linac Coherent Light Source (LCLS) through the superheated sample. As they scattered off the vibrating atoms, the X-rays’ frequency shifted slightly, revealing the atoms’ speed and thus their temperature. ...

In their recent study, the team discovered that the gold had been superheated to an astonishing 19,000 kelvins (33,740 degrees Fahrenheit) – more than 14 times its melting point and well beyond the proposed entropy catastrophe limit – all while maintaining its solid crystalline structure. ..."

From the abstract:

"In their landmark study, Fecht and Johnson unveiled a phenomenon that they termed the ‘entropy catastrophe’, a critical point where the entropy of superheated crystals equates to that of their liquid counterparts. This point marks the uppermost stability boundary for solids at temperatures typically around three times their melting point.

Despite the theoretical prediction of this ultimate stability threshold, its practical exploration has been prevented by numerous intermediate destabilizing events, colloquially known as a hierarchy of catastrophes, which occur at far lower temperatures.

Here we experimentally test this limit under ultrafast heating conditions, directly tracking the lattice temperature by using high-resolution inelastic X-ray scattering.

Our gold samples are heated to temperatures over 14 times their melting point while retaining their crystalline structure, far surpassing the predicted threshold and suggesting a substantially higher or potentially no limit for superheating. We point to the inability of our samples to expand on these very short timescales as an important difference from previous estimates. These observations provide insights into the dynamics of melting under extreme conditions."

Direct measure of atomic heat disproves decades-old theory | Stanford Report "An innovative method to measure atom temperature in extremely hot materials is already changing our understanding of the world, overturning long-held beliefs about superheating."

The limit does not exist: Superheated gold survives the entropy catastrophe "Researchers taking the first-ever direct measurement of atom temperature in extremely hot materials inadvertently disproved a decades-old theory and upended our understanding of superheating."

Superheating gold beyond the predicted entropy catastrophe threshold (open access)

Fig. 1: Diagram of the experimental set-up.

Fig. 3: Ion temperature evolution.