Microscopic origins of electrical conductivity in superheated solids revealed at FLASH

Scientists used terahertz radiation for measurements of strongly excited material

Origins of electrical conductivity in superheated solids measured with THZ radiation at FLASH

Artist's impression: origins of the electrical conductivity in superheated solids measured with THZ radiation at FLASH at DESY (Credit: Z. Chen, SLAC).

multi-cycleTHz pulses

The FLASH XUV pulse heats the gold nanofoil samples to electron temperature above 16,000 °C while the multi-cycle THz pulse enables measuring the electrical conductivity of the nanofoil samples as thermal energy transfers from electrons to ions (Credit: Z. Chen, SLAC).

The temporal evolution of the electrical conductivity is determined from the THz transmission through the sample

The temporal evolution of the electrical conductivity is determined from the THz transmission through the sample. Each THz cycle provides an individual measurement. The transmission ratio increases as the conductivity decreases (credit: Z. Chen, SLAC)

In-depth understanding of the electrical conductivity of matter is the key to many cutting-edge research and applications, ranging from phase-change memory in microelectronics to magnetospheres rooted in planetary interiors due to the motion of the conductive fluid. Unique states of material created by ultrafast table-top lasers or free-electron lasers (FEL) allow us to gain insight into atomic levels. However, it also requires sub-picosecond resolution to capture the details on the timescale of atomic motion. Therefore, in conductivity measurements it prevents the use of contact diagnostics such as multimeter and four-point-probe. Although ultrafast optical or X-ray measurements can provide information on high frequency electrical conductivity, they require complex models to extrapolate the intrinsic direct current (DC) conductivity of material.

The terahertz radiation (1 THz= 1012 Hz (cycles per second)) offers a unique solution to tackle this dilemma. The THz electromagnetic wave behaves like DC electric-field to the sample because the oscillation of its electric field is slow compared to the electron momentum relaxation frequencies in solid and liquid materials (typically 1013Hz or larger), and the width of each THz cycle is short enough to resolve sub-picosecond dynamics. Nevertheless, to measure the conductivity of strongly excited materials in the irreversible regime still requires high brightness THz radiation in order to penetrate the dense electron cloud as well as high sensitivity to detect the THz temporal profile in a single shot.

An international research team, led by scientists from the SLAC National Accelerator Laboratory and DESY, have recently measured the electrical conductivity of strongly heated material using the THz FEL radiation at FLASH. In this study, gold nano-foil samples were heated by the FLASH extreme ultraviolet (XUV) FEL pulses to electron temperatures up to 16,000 °C. As the thermal energy transfers from the electrons to the ions, the sample transits from cold to superheated solid and eventually melts into warm dense liquid. The researchers have determined the DC electrical conductivity by measuring the transmitted THz electric field through the heated samples. The multi-cycle THz pulses from FLASH provide continuous measurements with temporal resolutions better than 500 femtoseconds.

The ultrafast time-resolved measurements offer unique insight into the electrical conductivity. It allows the research team to differentiate between the individual contributions from electron-electron and electron-ion scatterings which limit the conductivity at high temperatures. They found that the electron-electron scattering frequency increased drastically after ultrafast heating of samples, following the so-called Fermi-Liquid theory that was only observed in cryogenic cooled samples in previous studies. They also demonstrated the close relation between electron-ion scattering frequency and the structure of the ions, based entirely on experimental data. The high-quality data acquired from this study will help researchers to test and improve their models on strongly heated materials. In this regime, sophisticated theories such as quantum molecular dynamics are needed to describe the phenomena.

In addition, this study has also demonstrated the capabilities of a novel experimental platform that utilise the unique features of both the THz and XUV pulses at FLASH. Similar experimental configuration is now being planned for the FLASH 2020+ upgrade at DESY. It is expected to inspire new studies for future FEL users.


Z. Chen, C.B. Curry, R. Zhang, F. Treffert, N. Stojanovic, S. Toleikis, R. Pan, M. Gauthier, E. Zapolnova, L. E. Seipp, A. Weinmann, M. Z. Mo, J. B. Kim, B. B. L. Witte, S. Bajt, S. Usenko, R. Soufli, T. Pardini, S. Hau-Riege, C. Burcklen, J. Schein, R. Redmer, Y. Y. Tsui, B. K. Ofori-Okai and S. H. Glenzer, Ultrafast Multi-cycle Terahertz Measurements of the Electrical Conductivity in Strongly Excited Solids, Nat. Comm. 12, 1638 (2021), DOI: /10.1038/s41467-021-21756-6