Ultrafast dynamics of pressure-probed topological material

Transient reflectivity spectra of Sb2Te3 at different pressures. Credit: Su Fuhai

Prof. Su Fuhai of the Hefei Institutes of Physical Science (HFIPS) of the Chinese Academy of Sciences (CAS), along with researchers from the Aerospace Information Research Institute and the Center for High Pressure Science and Technology Advanced Research, investigated the nonequilibrium electron and phonon dynamics of the topological insulator Sb2Te3 under pressure and explored the ultrafast photophysics across the electronic topological and latticial interfaces.

Physical Review B contains publications of pertinent findings.

Ultrafast spectroscopy can record the evolution of excited states with femtosecond time resolution, allowing direct access to ultrafast dynamics involving the cooling of hot electrons, coherent phonons, electron-phonon couplings, and so on. Pressure modulation utilising a diamond anvil cell (DAC) offers a straightforward and hygienic method for continuously modifying the lattice and electronic structures of materials, resulting in distinct phase transitions. Pressure-induced electron topological transitions (ETTs) without lattice abruption are frequently crucial for thermal electronic properties and superconductivity in high pressure phase materials. However, it remains difficult to investigate the electron-phonon interactions on ETT.

Using femtosecond optical pump-probe spectroscopy (OPPS) and DAC, the researchers investigated the ultrafast photocarrier dynamics of Sb2Te3, one of the prototypical topological insulators, in this study.

OPPS was utilised to monitor the nonequilibrium relaxations of the hot electron and coherent acoustic phonon in the time range of 100 picoseconds under hydrostatic pressures as high as 30 GPa. From the pressure dependence of phonon vibrations, relaxation time constants, and coherent phonons, the researchers were able to identify the ETT and semiconductor-semimetal transition around 3 GPa and 5 GPa.

Intriguingly, OPPS uncovered a hot phonon bottleneck effect at low pressure, which was discovered to be effectively suppressed by the onset of ETT. According to the calculated electronic and lattice structures, this phenomenon was explained in terms of a sudden increase in the density of state and the number of Fermi pockets.

In addition, they discovered that the pressure dependence of photocarrier dynamics accurately reflected the lattice structure transitions, including – and – phase changes, as well as the mixed phase.

This work not only provides a new understanding of the interactions between electron and lattice in Sb2Te3, but it may also provide the impetus to evaluate the pressure-induced topological phase transitions using ultrafast spectroscopies.

Further information: Kai Zhang et al, Nonequilibrium electron and lattice dynamics of Sb₂Te₃ under pressure, Physical Review B (2022). DOI: 10.1103/PhysRevB.105.195109

Journal information: Physical Review B

Source: Chinese Academy of Sciences