As part of an international team, University of Warwick researchers have helped redefine long-held theories in a landmark experiment where superheated gold remained solid despite being heated to over 42 times its nominal melting point, that is 14 times above the temperature predicted as the absolute limit of superheating.
Working with, and accurately measuring, extremely hot materials has proven challenging, near impossible, but these measurements are essential to help understand complex hot systems such as planetary cores, solar processes and fusion reactors.
Now, for the first time, researchers report in Nature that they have directly measured the temperature of ions in warm dense matter through X-ray scattering with extremely high resolution and applied this technique to superheated gold. This experiment went beyond a well-established theoretical limit and has broken a temperature record for solid gold, overturning a decades-old theory known as the "entropy catastrophe," which claimed solids couldn't remain stable at such extreme temperatures.
Professor Dirk Gericke, Department of Physics, University of Warwick, played a key role in experimental planning and resolving the theoretical consequences of this discovery. He said: "States far from equilibrium keep surprising us. The old estimates for the entropy catastrophe were pretty robust, and nobody could overcome, or even reach, them for a long time. However, things change fundamentally if one drives matter really hard and doesn't allow enough time to find its equilibrium again."
This experimental achievement was possible thanks to the team using the Linac Coherent Light Source (LCLS), an X-ray laser at the SLAC National Accelerator Laboratory. For nearly a decade, the team has worked to develop a method that circumvents the usual challenges of directly measuring extreme temperatures.
"We have good techniques for measuring density and pressure of these systems, but not temperature," said Bob Nagler, staff scientist at the Department of Energy's SLAC National Accelerator Laboratory. "In these studies, the temperatures are always estimated with huge error bars, which really holds up our theoretical models. It's been a decades-long problem."
The experimental team overcame the challenge by rapidly heating a nanometer-thin gold foil with a laser pulse lasting just 50 quadrillionths (one millionth of a billionth) of a second. Then, using LCLS as a high-precision thermometer, they measured the vibrations of gold ions in the gold foil to determine temperature - 19,000 degrees Kelvin (33,740 degrees Fahrenheit)!
This feat of measurement provides the first direct temperature reading in warm dense matter, a state found in stars, planetary cores, and fusion experiments, paving the way for temperature diagnostics across a broad range of high-energy-density environments.
Dirk added: "In my opinion, overcoming the superheating limit is a very fundamental result. Its biggest impact might be in material sciences, but it might also affect the interpretation of observations in plasma and astro-physics."
Against their expectations, the researchers found the gold foil was still in a solid crystal lattice at these high temperatures. "This is possibly the hottest crystalline material ever recorded," Thomas White, lead author and Clemons-Magee Endowed Professor in Physics at the University of Nevada, Reno said. "I was expecting the gold to heat quite significantly before melting, but I wasn't expecting a fourteen-fold temperature increase".
Superheating materials is not unusual, but the further a material gets beyond its freezing, melting or boiling point, the more likely an entropy catastrophe will be triggered, causing sudden onset of the phase transition. The ultimate theoretical limit for catastrophe was theorised in 1988 at around three times the melting point.
How the gold foil remained solid despite surpassing the theoretical entropy limit appears to be down to the speed with which the gold was heated. The finding suggests that the limit of superheating solids may be far higher - or non-existent - if heating occurs quickly enough to allow these materials to escape entropy catastrophe.
"It's important to clarify that we did not violate the Second Law of Thermodynamics," White added. "What we demonstrated is that these catastrophes can be avoided if materials are heated extremely quickly - in our case, within trillionths of a second."
