What occurs when you subject a metal renowned for its stability, blast it with a laser pulse only 45 femtoseconds in duration, and heat it to 19,000 kelvins but it won’t melt? Physicists had long thought the so-called “entropy catastrophe” placed an unbreakable roof on superheating solids. But one recent experiment not only shattered that boundary, it has overturned the very assumptions forming the basis for thermodynamics of matter at extreme conditions.
The test was conducted, under Thomas White’s direction at the University of Nevada, Reno, using nanometer-thick films of gold half the width of a virus during the experiment. Gold’s melting point, at 1,300 kelvins, is already beyond that of volcanic lava. But as White’s team found, “This was extremely surprising. We were totally shocked when we saw how hot it actually got.” The gold sample, which was exposed to a 45-femtosecond laser pulse, reached temperatures more than 14 times its melting point and did so without crumbling into a liquid. This finding, described in Nature, goes directly against the 1980s forecast that a pushed solid beyond the entropy catastrophe would inevitably melt, since its entropy would be higher than that of the liquid phase, which would be a violation of the second law of thermodynamics.
The key to this paradoxical strength is the ultrafast heating. The laser deposited energy so quickly that the gold atoms started vibrating erratically before the lattice had time to expand or rearrange. “The scale of superheating observed suggests a genuinely new regime,” White explained, adding that neither pressure effects nor ionization could completely explain the measurements. The fleeting nature of the experiment only quadrillionths of a second long kept the system from settling into equilibrium, keeping entropy in check and preventing the disastrous phase transition that theorists had long predicted. Sheng-Nian Luo, a physicist at Southwest Jiaotong University in China, commented, “Melting under such ultrafast, ultrasmall, ultracomplex conditions could be overinterpreted,” but the authors are convinced that their findings demonstrate a new state of matter.
To measure such brief, record temperatures, equally sophisticated diagnostics were needed. The team went to the Linac Coherent Light Source at SLAC National Accelerator Laboratory, the world’s most intense x-ray free-electron laser. This three-kilometer-long instrument accelerates electrons to over 99 percent the speed of light, channeling them through undulating magnetic fields to generate a beam of a trillion x-ray photons. When this ultrabright x-ray pulse hit the superheated gold, the scattered photons carried information about atomic velocities, allowing the researchers to deduce the temperature with unprecedented accuracy. As SLAC staff scientist Bob Nagler described, “The biggest lasting contribution is going to be that we now have a method to really accurately measure these temperatures.”
The high-tech nature of the LCLS and its Matter in Extreme Conditions (MEC) instrument lies behind this achievement. The chamber of the MEC, a two-meter-diameter vacuum vessel, is filled with silicon carbide mirrors, beryllium compound refractive lenses, and an ensemble of spectrometers and detectors that can resolve atomic-scale events on femtosecond timescales. The beam can be narrowed to sub-micrometer spots, allowing imaging and scattering diagnostics with spatial resolutions below one micron. This configuration enables isochoric heating energy input prior to expansion capability essential for studying the warm dense matter regime, filling the gap between condensed matter and plasma physics.
The applications of these results reach far beyond gold. The new temperature-measuring method, which can identify atomic temperatures of 1,000 to 500,000 kelvins, provides access to direct investigations of warm dense matter a condition that exists in planetary interiors and in inertial confinement fusion targets. According to Nagler, “When a fusion fuel target implodes in a fusion reactor, the targets are in a warm dense state. To design useful targets, we need to know at what temperatures they will undergo important state changes. Now, we finally have a way to make those measurements”. In order to design useful targets, we must be told at what temperatures they will experience significant state transitions. Now, at last, we have a means to make those measurements. Previous efforts to define WDM made use of indirect techniques, generally making inferences from the surface emission or matching simulations to sparse experimental data. The LCLS method, with the use of ultrafast x-ray scattering, offers direct, bulk-sensitive temperature diagnostics that are important for verification of theoretical models and for advancing our knowledge of matter in extreme conditions.
The experiment also demonstrates the importance of non-equilibrium physics. In ultrashort laser-matter interactions, electrons are excited prior to the rapid heating to thousands of kelvins as the lattice is relatively cold a situation that can be characterized by the two-temperature model. Only picoseconds after that, electrons and the lattice equilibrate, something that may be interrogated by the same x-ray scattering methods. Computer simulations verify that, at low fluences, gold’s electron temperature can reach more than 5,000 K while the lattice remains below the melting point, in agreement with the experimentally observed non-melt crystallization and structural transitions on sub-picosecond timescales.
This research not only tests the theoretical limits of phase transitions but also gives a solid experimental system for investigating the properties of materials at the edges of temperature and pressure. In White’s own words, “We were surprised to find a much higher temperature in these superheated solids than we initially expected, which disproves a long-standing theory from the 1980s”. This wasn’t our original goal, but that’s what science is about discovering new things you didn’t know existed. This wasn’t our initial intention, but that is what science is all about finding things you didn’t know you knew existed.
