Scientists Use High-harmonic Spectroscopy To Unlock Electronic Structure Of High-pressure Superconductors

High pressure has created many novel states for condensed matter, revealing new physical and chemical phenomena. Among them, the discovery of near-room-temperature superconductivity (Tc＞ 200 K) in high-pressure hydrides such as H3S and LaH10 has attracted scientists' attention.
The superconducting transition temperature of high-pressure superconductors has been increasing, but the mechanism of superconductivity remains an open question due to the lack of effective probes and the unknown electronic structure and ultrafast dynamical behavior in high-pressure quantum states.
Higher harmonic generation (HHG) is the process of converting an incident laser into strong coherent radiation at several times the laser frequency. As a typical representative of nonlinear optics, HHG in solids originates from the nonlinear driving of intra- and interband electrons by the strong-field laser-matter interaction. As a result, HHG spectra naturally contain a fingerprint of the atomic and electronic properties in the material. Utilizing this nonlinear, nonperturbative dynamical process, scientists are able to peer into the internal nature of materials.
Recently, the team of Sheng Meng, a researcher at the Institute of Physics of the Chinese Academy of Sciences/National Research Center for Condensed Matter Physics in Beijing, explored the ultrafast HHG dynamics in the high-pressure superconductor H3S with the help of first-principles time-containing density-functional theory and the use of a non-adiabatic time-containing density-functional molecular dynamics method and software developed in the group. It is found that HHG in high-pressure superconductors is strongly wavelength-dependent and anisotropic, indicating that the HHG process strongly depends on the electronic structure. The time-frequency analysis of HHG is investigated and the mechanism of in-band scattering dynamics of low-order harmonics is determined. On this basis, using HHG spectra, it is studied to reconstruct the energy band dispersion structure near the Fermi surface. In addition, it is found that there is a strong modulation of the HHG spectrum by coherent phonons, indicating the sensitivity of the HHG process to electroacoustic coupling. Using the HHG spectrum modulated by coherent phonons, the study further reconstructs the electroacoustic coupling matrix elemental strength near the Fermi surface. The study reveals that many-body interactions (electroacoustic coupling) in materials have a significant effect on the behavior of electrons near the Fermi energy level. This supports a phonon-mediated mechanism for high-voltage superconductivity and provides an all-optical approach to probe the electronic structure and electroacoustic coupling in high-voltage quantum states.
The related research results are published as Solid-state high harmonic spectroscopy for all-optical band structure probing of high-pressure quantum states, published in Proceedings of the National Academy of Sciences of the United States of America (PNAS). The research work was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, and the Strategic Pilot Project of the Chinese Academy of Sciences.