A team of researchers led by Bob Nagler and Thomas White recently demonstrated a new method to measure the temperature of atoms within warm dense matter—by directly measuring the speed of atoms.
Materials all possess specific melting and boiling points, but can be superheated above them until they reach an entropy “catastrophe” level of sudden melting and boiling.
When the team superheated solid gold far beyond its theoretical limit to 19,000 kelvins, it survived the entropy catastrophe—which suggests there may not be an upper limit for superheated materials if they’re heated quickly enough.
Laser Focus World: Whose idea was it to superheat gold with LCLS? What inspired it?
Bob Nagler: When we set out to do the experiment, our goal was to develop a new method to measure the temperature of warm dense matter. This matter is as dense as a solid, but heated to tens or hundreds of thousands of degrees Kelvin. You find it in giant planet cores and stellar interiors, but when we recreate it in the lab, actually measuring its temperature is infamously difficult.
We launched this project to address this challenge, using the world’s brightest x-ray source, SLAC National Accelerator’s Linac Coherent Light Source (LCLS), to help.
Thomas White: I’d love to say it was a lone-wolf flash of brilliance but, in truth, the idea came out of long-standing frustrations across the field. We knew we needed a better diagnostic, and gold made the idea test material: it scatters x-rays well and can be easily made into the thin foils required for this technique. Our team at the University of Nevada, Reno, SLAC, and other partners expected the gold to heat up under irradiation, but what stood out was how hot the solid remained while maintaining its crystalline structure. Even at these extreme temperatures, the gold lattice persisted beyond the expected limit for structural order. This observation shifted the focus of our project. What began as a practical effort to build a better thermometer evolved into a deeper investigation of superheating and the fundamental limits of solid-state matter under extreme conditions.
LFW: Why LCLS?
White: The method we developed relies on detecting tiny changes in how x-rays scatter off atoms in a material. Specifically, small energy shifts reveal the temperature of the ions. It requires not only an extraordinarily bright source of x-rays, but also extremely narrow bandwidth. Free-electron lasers like LCLS, and a few others such as the European XFEL, are uniquely capable of delivering this combination. They’re up to a billion times brighter than any synchrotron, which is essential because the inelastic scattering is incredibly weak—on the order of just a few photons per shot.
Nagler: LCLS is essentially a kilometer-long x-ray laser that, for this experiment, also acts as a kilometer-long thermometer. Without this combination of brightness, coherence, and spectral precision, this measurement simply wouldn’t be possible.
LFW: What did your experiment involve?
Nagler: We heated an ultrathin gold foil—just 50-nm thick—using a frequency-doubled Ti:sapphire laser, giving us 400-nm wavelength light with pulse durations around 45 fs. Despite the extreme temperatures we reached, the laser itself wasn’t especially powerful by high-energy-density standards. We used only about ~0.3 mJ per pulse. It means the heating part of the experiment, the creation of superheated gold, could, in principle, be reproduced by many laser labs around the world.
White: But measuring the temperature of what you create? It’s the hard part. For this, you need the ultrabright, narrow-bandwidth, femtosecond x-rays that only facilities like LCLS and a few other XFELs can provide. It’s what made this experiment possible.
LFW: What are the key takeaways of this experiment? Any surprises?
Nagler: For us and our field, the major takeaway is that we now have a direct, model-free method for measuring ion temperatures in extreme states of matter—which has been a long-standing challenge in high-energy-density physics. The technique opens the door to benchmarking equations of state, validating hydrodynamic simulations, and exploring matter within regimes that were previously out of reach experimentally.
White: The real surprise came when we saw just how far we could push a solid before it gave in to disorder. We expected the gold to melt once it crossed a certain threshold—but it didn’t. The crystal lattice held together at temperatures more than 14x the melting point—well beyond what standard thermodynamics would predict. This was the ‘aha!’ moment: Not only could we take the temperature, but the system itself defied expectations. In doing so, we found ourselves not only solving a diagnostic challenge, but also uncovering new physics, pushing the limits of superheating, and revisiting assumptions about when and how solids melt under extreme conditions.
LFW: What did it feel like to disprove a decades-old theory?
White: It was a fun and fascinating deep dive into the physics of superheating, exploring how far a solid can be pushed before it breaks down, and realizing that even well-established concepts need careful rethinking when applied to ultrafast, nonequilibrium regimes.
Nagler: It wasn’t so much about disproving a decades-old theory as it was showing that the theory doesn’t necessarily apply to far-from-equilibrium superheated states. The original framework assumes a system in thermal equilibrium, slowly approaching the melting point, not one blasted by a femtosecond laser pulse. Instead of overturning existing theory, this was more like stepping outside its domain.
LFW: What does this discovery mean for superheating?
Nagler: It shows that superheated matter in these nonequilibrium states can behave quite differently than you’d expect for more run-of-the-mill near-equilibrium systems and it would be interesting to explore these differences in more detail.
White: Ultimately, it reopens the question of whether there’s a true limit to superheating in intensely driven, far-from-equilibrium systems, or whether solids can persist well beyond what traditional thermodynamics predicts.