Ultrafast Carrier Relaxation and Second Harmonic Generation in a Higher‐Fold Weyl Fermionic System PtAl

In topological materials, shielding of bulk and surface states by crystalline symmetries has provided hitherto unknown access to electronic states in condensed matter physics. Interestingly, photoexcited carriers relax on an ultrafast timescales, demonstrating large transient mobility that can be harnessed for the development of ultrafast optoelectronic devices. In addition, these devices are much more effective than topologically trivial systems because topological states are resilient to the corresponding symmetry‐invariant perturbations. By using optical pump probe measurements, the relaxation dynamics of a topologically nontrivial chiral single crystal, PtAl, is systematically described. Based on the experimental data on transient reflectivity and electronic structures, it is found that the carrier relaxation process involves both acoustic and optical phonons with oscillation frequencies of 0.06 and 2.94 THz, respectively, in picosecond time scale. PtAl with a space group of P213 allows only one non‐zero susceptibility element, i.e., d14, in second harmonic generation (SHG) with a large value of 468(1) pm V–1, which is significantly higher than that observed in standard GaAs(111) and ZnTe(110) crystals. The intensity dependence of the SHG signal in PtAl reveals a non‐perturbative origin. The present study on PtAl provides deeper insight into topological states that will be useful for ultrafast optoelectronic devices. Topological phases with symmetry and chirality offer novel insights into electronic states leading to exotic emergent phenomena. For instance, PtAl gives rise to spin‐3/2 Rarita–Schwinger (RS) fermions at (Γ) and a time‐reversal pair of spin‐1 fermions at (R) of the Brillouin zone (BZ). Through nonlinear optical analysis, the existence of chirality in PtAl is confirmed and its quasiparticle relaxation mechanism is established.