Ultrafast nonthermal restoration of higher crystalline symmetry in topological candidate elemental tellurium
One route to the control of topological properties in materials is through the manipulation of crystalline structure. We demonstrate an ultrafast structural phase transition in elemental tellurium, a prototypical Peierls-distorted topological material candidate. Our static and time-dependent density functional theory simulations predict dynamical concomitant topological and structural transitions upon optical excitation. This transition is probed experimentally through correlated time-resolved second harmonic generation polarimetry and coherent phonon spectroscopy measurements. Upon irradiation with light, we observe a dramatic drop of the second harmonic intensity followed by a coherent A1 phonon mode. Further, as a function of fluence, an inflection point is observed in both the drop of the second harmonic intensity and the phonon dynamics, which is quantitatively reproduced by our theory. Our work not only reveals a metastable structural transition, but also points towards a possible topological switch that has no equilibrium analog.
Structural characterization of magnetic Weyl semimetal candidates CeAlGe and PrAlSi
The crystallographic structure of the magnetic Weyl semimetal candidate PrAlSi is presently unclear. Specifically, it is difficult to distinguish between a non-centrosymmetric tetragonal point group 4mm versus a centrosymmetric group 4/mmm with current diffraction-based techniques. However, the presence or absence of inversion symmetry has a profound effect on optical second harmonic generation (SHG). We employed optical second harmonic generation (SHG) polarimetry and established that even well above the magnetic transition the SHG response from PrAlSi is of predominantly bulk electrical dipolar origin from 4mm group, same as its analog CeAlGe. This result confirms the lattice structure where nontrivial topological electronic band structure can emerge.