Ultrafast dynamics in square-net nodal-line semimetals

Abstract

Even as an increasing number of topological materials are predicted and experimentallyrealized in an increasingly diverse array of structure types, there remains an underlying de- mand for guiding principles and unifying behavior. This search extends to the optical realm. While generalizations of the low-energy steady-state optical response have been moderately successful, similar descriptions in the time domain have been elusive. This thesis explores the ultrafast optical response in a subset of topological materials known as nodal-line semimet- als through systematic, time-resolved spectroscopic studies, in search of unifying behaviors particular to this class of materials. Chapter 1 introduces topological semimetals — “square- net” nodal-line semimetals in particular — and details what has been gained from prior spec- troscopic studies on them, encompassing steady-state, equilibrium measurements and time- resolved, out-of-equilibrium measurements, as well as more specialized approaches that com- bine optical excitation with other probes. Chapter 2 establishes how two archetypal nodal- line semimetals, ZrSiS and ZrSiSe, behave on the femtosecond – picosecond timescales fol- lowing photoexcitation with near-infrared light. By combining steady-state reflectance and transient reflectivity spectroscopies, a physical description of the excited electronic state is presented. Chapter 3 delves into the response of the next material in this series, ZrSiTe. In contrast to ZrSiS and ZrSiSe, a particular coherent phonon mode is easily and strongly ex- cited; the effects this vibration might have on the material’s electronic structure are discussed, and connected with experimental indications that the material undergoes a photo-induced phase transition. Chapter 4 continues with ZrSiTe, using time- and angle-resolved photoe- mission spectroscopy to further explore the photo-induced effects speculated on in Chapter 3. The findings in this chapter serve to corroborate — in a visually striking manner — the results presented in Chapter 2 and those from similar layered topological semimetals, demon- strating a common response to photoexcitation in this class of material. In Chapter 5, these measurements are extended to a new series of materials, GdSbxTe2−x−δ, which are isostruc- tural to ZrSiS et al. for x > 0.80 but host tunable charge density waves for lower values of x. The evolution of the structural modulations is shown to strongly impact the ultrafast dynamics in the near-infrared and visible regions.