Ultrafast spectroscopy of the Dirac semimetal Cd3As2 under pressure

Topological properties of a three-dimensional Dirac semimetal Cd3As2, protected by crystal rotation and time-reversal symmetry, can be tuned with the application of pressure. Ultrafast spectroscopy is a unique tool to investigate the character and time evolution of electronic states, emphasizing the signatures of transition. We designed an experimental setup for in situ pressure-dependent ultrafast optical pump-optical probe spectroscopy of Cd3As2 using a symmetric diamond anvil cell. The fast relaxation processes show significant changes across pressure-induced phase transitions at PC1 3 GPa and PC2 9 GPa. The relaxation time decreases beyond PC1, suggesting a stronger electron-phonon coupling, characteristic of a semiconducting state. A new sub-picosecond timescale relaxation dynamics emerges beyond PC2. Theoretical calculations of differential reflectivity for both interband and intraband processes indicate that the negative (positive) differential reflectivity (AR/R) results from the interband (intraband) processes. The pressure-dependent behavior of relaxation dynamics amplitudes beyond PC1 emphasized the necessity of incorporating quadratic band opening in the calculations, explaining the transition of Cd3As2 from a Dirac semimetal to a semiconducting phase. The time evolution of differential reflectivity is calculated using the electronic temperature as a function of time, as provided by the two-temperature model, which fits the experimental data.