PRL Shanghai Jiaotong University's Zhang Wentao's team achieves quantum materials-related breakthrough

Recently, the ultrafast laser manipulation of the electronic structure of the quantum material 1T-TiSe₂ has been investigated by Wentao Zhang's research group at the School of Physics and Astronomy, Shanghai Jiao Tong University using a self-developed high-energy and high-time-resolution angular-resolution photoelectron spectroscopy system. The laser-induced ultrafast transition from the charge density wave phase to the substable metallic state was observed in 1T-TiSe₂ using ultrafast light excitation of coherent phonons associated with the charge density wave phase, which induces collective displacement of atoms within the lattice, and the photostable substable state was found to have a tunable lifetime.

The related research work was published in Physical Review Letters on June 1 under the title "Ultrafast Switching from the Charge Density Wave Phase to a Metastable Metallic State in 1T-TiSe₂".

There are numerous novel physical phenomena in quantum materials, and precise control of the physical properties in quantum materials is the key to move these exotic phenomena to practical applications. The use of ultrafast laser-matter interaction to modulate the macroscopic properties of quantum materials is an important direction being developed internationally, which can realize novel physical phenomena that do not exist or are difficult to achieve in equilibrium, and provides a new method for the study of the mechanism of matter in quantum materials and the exploration of new matter states.

The interaction of ultrafast lasers with quantum materials to generate coherent phonon modes is a common phenomenon in condensed matter physics, but whether such coherent phonon modes can be used to induce changes in crystal structure and thus induce new states in materials has not been reported.

Left: Schematic diagram of time-resolved angle-resolved photoelectron spectra; (a) transient electronic structures before (left) and after (right) laser pumping; (b) ultrafast evolution of the electronic structure in the center of the Brillouin zone with time at high pumping excitation densities, with the point Se₂ energy band moving toward the Fermi energy level after laser pumping and its energy almost constant with time within 1 ps and the appearance of plateau features.

The transition metal-sulfide compound 1T-TiSe₂ is a typical charge density wave material whose charge density wave phase transition mechanism has been controversial and may be the result of the combined effect of exciton and electron-phonon coupling. There are also the emergence of novel physical phenomena such as superconductivity in 1T-TiSe₂ under pressurized and Cu-doped conditions. The intricate microscopic interactions and rich physical phase diagrams in 1T-TiSe₂ provide a unique platform for the study of macroscopic properties of laser manipulated quantum materials.

In this work, the electronic states of the charge density wave material 1T-TiSe₂ are excited using infrared light pumping, followed by irradiation of the material with extreme ultraviolet light to excite electrons outside the sample through photoelectric effect, and these escaped photoelectrons are captured by a hemispherical electron analyzer; by varying the time delay between pumping and probing light, the ultrafast evolution of the electronic structure with time can be studied in the femtosecond time scale.

It is found that the charge density wave in 1T-TiSe₂ is instantaneously melted within 200 femtoseconds after the pumping of the infrared light, and the Se1 energy band in the center of its Brillouin zone crosses the Fermi energy level with the appearance of an instantaneous metallic state; at the same time, the Se₂ energy band in the center of the Brillouin zone moves about 120 meV toward the Fermi energy level, and its energy is almost constant with time within 1 picosecond, indicating that the system enters a sub-stable state. Further study reveals that the lifetime of this photostable state depends on the excitation density of the pump light, which can last more than 1 picosecond at the highest pump excitation density.

(a) Ultrafast evolution of the potential energy surface (left) and transient potential energy surface with order parameter and time before and after laser pumping (right); (b) Comparison of electronic structure and order parameter kinetic processes at high pump excitation densities.

The novel phenomena observed above are based on the principle of electroacoustic interaction, caused by laser manipulation of the atomic positions in the lattice, i.e., an increase in the amplitude of the laser-induced coherent phonon modes at high excitation densities, leading to a collective displacement of the corresponding atoms in the lattice into sub-stable positions and thus into new states of matter. Further, the present study reproduces the experimental results using Ginzburg-Landau model numerical simulations. At high pumping excitation densities, the potential energy surface of the system is instantaneously changed after laser pumping and induces the emergence of coherent phonon modes based on electro-acoustic interactions related to the charge density wave phases, and the vibrations of coherent phonons cause large amplitude collective displacement of atoms in the lattice and stay far from their equilibrium positions, leading to sub-stable states with novel physical properties. This leads to the formation of sub-stable states with novel physical properties.

The present work further supports the team's previous findings that ultrafast laser light can induce the inversion of the order parameter at the surface of the sample at a specific pump excitation density, which can induce the inversion of the order parameter hierarchy inside the quantum material to construct two-dimensional electronic states with novel physical properties (Nature 595,239 (2021)). This light-induced collective displacement of atoms in quantum materials provides a new mechanism and a new way for laser modulation of macroscopic properties of quantum materials.

The continuous advancement of experimental techniques is the key to make new discoveries, and Wentao Zhang's research group has been working on the development of high-performance and versatile angle-resolved photoelectron spectroscopy systems. In the past few years, Wentao Zhang's research group has developed an international leading level time-resolved angular-resolved photoelectron spectrometer (Rev. Sci. Instrum. 90, 063905 (2019)), and recently implemented an automatic sample position positioning and correction system with submicron accuracy in the angular-resolved photoelectron spectrometry system based on the image recognition binocular vision system, which provides the opportunity to realize the above research in This provides an important guarantee for the realization of time-resolved experiments and fine-grained variable temperature experiments in the above study (Rev. Sci. Instrum. 93, 103905 (2022)).

This work was mainly funded by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the Natural Science Foundation of Shanghai, the Innovation Program of Shanghai Municipal Education Commission, and the Major Project of Shanghai Science and Technology, and the Postdoctoral Program, in collaboration with Prof. Yanfeng Guo's group at Shanghai University of Science and Technology, Prof. Dong Qian and Prof. Wei at Shanghai Jiao Tong University. The first author of the paper is Duan Shaofeng, a postdoctoral fellow in the School of Physics and Astronomy of Shanghai Jiao Tong University, and the corresponding author is Professor Wentao Zhang.