Scientists have simultaneously broken a temperature record, overturned a long-held theory and utilized a new laser spectroscopy method for dense plasmas in a groundbreaking article published today in the journal Nature.

In their research article, “Superheating gold beyond the predicted entropy catastrophe threshold,” physicists revealed they were able to heat gold to over 19,000 Kelvin (33,740 degrees Fahrenheit), over 14 times its melting point, without it losing its solid, crystalline structure.

“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.

This result overturns the long-held theoretical limit known as the entropy catastrophe. The entropy catastrophe theory states that solids cannot remain stable above approximately three times their melting temperature without spontaneously melting. The melting point of gold, 1,337 Kelvin (1,947 degrees Fahrenheit), was far more than tripled in this experiment utilizing an extremely powerful laser at Stanford University’s SLAC National Accelerator Laboratory.

“I was expecting the gold to heat quite significantly before melting, but I wasn’t expecting a fourteen-fold temperature increase,” White said.

To heat the gold, researchers at the University of Nevada, Reno, SLAC National Accelerator Laboratory, the University of Oxford, Queen’s University Belfast, the European XFEL and the University of Warwick designed an experiment to heat a thin gold foil using a laser fired for 50 quadrillionths of a second (one millionth of a billionth). The speed with which the gold was heated seems to be the reason the gold remained solid. The findings suggest that the limit of superheating solids may be far higher – or nonexistent – if heating occurs quickly enough. The new methods used in this study open the field of high energy density physics to more exploration, including in areas of planetary physics and fusion energy research.

White and his team expected that the gold would melt at its melting point, but to measure the temperature inside the gold foil, they would need a very special thermometer.

“We used the Linac Coherent Light Source, a 3-kilometer-long X-ray laser at SLAC, as essentially the world’s largest thermometer,” White said. “This allowed us to measure the temperature inside the dense plasma for the first time, something that hasn’t been possible before.”

"I’m incredibly grateful for the opportunity to contribute to such cutting-edge science using billion-dollar experimental platforms alongside world-class collaborators." - Doctoral student Travis Griffin

“This development paves the way for temperature diagnostics across a broad range of high-energy-density environments,” Bob Nagler, staff scientist at SLAC and coauthor on the paper, said. “In particular, it offers the only direct method currently available for probing the temperature of warm dense states encountered during the implosion phase of inertial fusion energy experiments. As such, it is poised to make a transformative contribution to our understanding and control of fusion-relevant plasma conditions."

Along with the experimental designers, the research article is the result of a decade of work and collaboration between Columbia University, Princeton University, the University of Padova and the University of California, Merced.

“It’s extremely exciting to have these results out in the world, and I’m really looking forward to seeing what strides we can make in the field with these new methods,” White said.

The research, funded by the National Nuclear Security Administration, will open new doors in studies of superheated materials.

“The National Nuclear Security Administrations' Academics Program is a proud supporter of the groundbreaking innovation and continued learning that Dr. White and his team are leading for furthering future critical research areas beneficial to the Nuclear Security Enterprise,” Jahleel Hudson, director at the Techology and Partnerships Office of the NNSA said.

White and his colleagues returned to the Linac Coherent Light Source in July to measure the temperature inside hot compressed iron and are using those results to gain insights into the interiors of planets.

Several of White’s graduate students and one undergraduate student were coauthors on the study, including doctoral student Travis Griffin, undergraduate student Hunter Stramel, Daniel Haden, a former postdoctoral scholar in White’s lab, Jacob Molina, a former undergraduate student currently pursuing his doctoral degree at Princeton University and Landon Morrison, a former undergraduate student pursuing his master’s degree at the University of Oxford. Jeremy Iratcabal, research assistant professor in the Department of Physics, was also a coauthor on the paper.

“I’m incredibly grateful for the opportunity to contribute to such cutting-edge science using billion-dollar experimental platforms alongside world-class collaborators,” Griffin said. “This discovery highlights the power of this technique, and I’m excited by the possibilities it opens for the future of high-energy-density physics and fusion research. After graduation, I’ll be continuing this work as a staff scientist at the European XFEL.”