Research

Warm dense matter (WDM) lies between condensed matter and plasma: temperatures of 10⁴–10⁶ K at near-solid density, where electrons are partially degenerate and ions are strongly coupled. Neither solid-state theory nor classical plasma theory applies cleanly. The state is found in the interiors of giant planets and brown dwarfs, in the compression stage of inertial confinement fusion, and transiently in any solid heated by an intense laser or X-ray pulse.

We create WDM by heating thin metal foils with femtosecond optical lasers or XFEL pulses, and probe it with ultrafast X-ray pulses at free-electron laser facilities. In X-ray absorption spectroscopy, for example, the absorption edge records the electron temperature and the shape of the density of states, while its time evolution tracks how energy moves from electrons to the lattice. From these measurements we extract electron–phonon (electron–ion) coupling strengths, thermal conductivities, and the timescales of nonthermal electron relaxation in WDM.

Recent work includes the electron–phonon coupling of warm dense copper, the thermal conductivity of warm dense iron, and the dependence of electron–ion coupling on ion temperature in liquid copper. We are extending the method to transition-metal oxides, where the insulator-to-metal transition under heating is an open question, and to new X-ray imaging techniques at PAL-XFEL. Experiments are interpreted with DFT-MD and two-temperature models, with machine learning where it helps.

Find more information from the following articles.

- G. Kang et al, App. Surf. Sci. (2025)
-  J. W. Lee, M. J. Kim, et al, PRL (2021)
- B. I. Cho, et al, Sci. Rep. 6, 18843 (2016)
- B. I. Cho, et al, PRL 106, 167601 (2011)

An X-ray free-electron laser delivers femtosecond pulses with 10⁹ times the peak brightness of a synchrotron. For HED science this does two things at once: a focused XFEL pulse can heat a solid isochorically to millions of kelvin before it has time to expand, and the same kind of pulse can probe the result with diffraction, scattering, spectroscopy, or imaging on the timescale of atomic motion.

Our group has worked at various XFELs since the early days of these facilities, both as users and as partners in developing the X-ray spectroscopy capabilities we rely on. Our emphasis is on single-shot methods: an HED experiment destroys its sample every shot, and shot-to-shot fluctuations of a SASE source mean that a spectrum must be measured whole, not scanned.

Our core technique is soft X-ray time-resolved absorption spectroscopy, in which the full absorption spectrum is recorded on each pulse and compared before and after laser heating. We have extended it in two directions at PAL-XFEL: a pink-beam XAS setup that uses the entire SASE bandwidth to capture an absorption spectrum from a single pulse, and high-energy-resolution off-resonant spectroscopy (HEROS), which reconstructs the unoccupied density of states from X-ray emission excited below the edge and thus provides XAS-equivalent information without scanning the incident energy.

We also use the XFEL itself as the heater. A focused X-ray pulse deposits energy through inner-shell photoionization throughout the depth of a solid, so the sample reaches warm or hot dense conditions uniformly and at a density that is known, rather than through the steep gradients left by optical-laser heating. The electrons are driven far from equilibrium in the process, and the subsequent Auger cascades, collisional ionization, and thermalization can be followed with femtosecond resolution. This is also the regime of nonlinear X-ray–matter interaction, where absorption itself changes during the pulse.

Find more information from the following articles.

-  J. W. Lee, M. J. Kim, et al, PRL (2021)
- B. I. Cho, et al, PRL 119, 075002 (2017)
- B. I. Cho, et al, PRL 109, 245003 (2012)
- S. M. Vinko, et al. Nature 482, 59-62 (2012)