Photoinduced Chemical Potential shift in Topological Material

Topological crystalline insulators (TCI) are new class of quantum materials with an even number of Dirac surface states protected by crystal symmetry. External excitation via pressure, magnetic field and physical doping can be applied to break the symmetry of such materials and hence tune their electronic properties. Using time and angle resolved photoemission spectroscopy, we report the observation of a shift in the chemical potential of NaCd₄As₃, a mirror symmetry protected TCI. This material exhibits a topological phase transition below 200 K from a TCI to a topological insulator (TI), resulting from the breaking of the mirror symmetry due to a structural phase transition. Within 500 fs of the laser excitation pulse, a change in chemical potential of 170 meV is observed before it gradually relaxes to the ground state after 6 ps. This shift results from an asymmetric distribution of charge carriers induced by the femtosecond laser within sensitive electron bands near the Fermi level. Our results show that ultrafast light pulses can tune the electronic properties of phase-rich topological materials while keeping their intrinsic properties intact.