Ultrafast studies of nonequilibrium electron-phonon and phonon-phonon interactions in photoexcited lead telluride [electronic resource]
Keywords: Physics.

PbTe is a member of the group IV-VI semiconducting compounds that distinctly crystallize in three closely related structures (cubic, rhombohedral, orthorhombic). The determination of the structure depends on a delicate balance between levels of ionicity and covalency. Classified as an incipient ferroelectric, PbTe lies very close to the cubic/rhombohedral phase boundary, crystallizing in a paraelectric rocksalt cubic configuration, but with a proclivity to distort into a ferroelectric state, although never doing so. This instability leads to anomalous characteristics in the lattice dynamics of PbTe and is thought to contribute to its naturally low thermal conductivity, which makes it so effective as a thermoelectric compound. Specifically, recent inelastic neutron scattering (INS) measurements reveal that the soft transverse optical (TO) mode of the material is far more dispersive than expected (less dispersive trajectory measured in past INS studies) and features a split-peak lineshape at zone center, both indicative of a "giant" anharmonicity. The zone center frequency of this mode is the typical indicator of the nearness to a ferroelectric transition. To more deeply understand the microscopic mechanisms that link the structural instability of PbTe with its unusual phonon behavior, an ultrafast experimental approach is utilized in this thesis to characterize the material. In this tactic, the compound is impulsively photoexcited away from its ground state and a unique perspective is offered on the interactions that govern its relaxation back to equilibrium. Several variations of time-resolved pump-probe methods are employed. One highlighted result proceeds from a series of fluence, temperature, and pressure dependent IR pump-IR probe measurements on PbTe, which show previously unreported, prominent reflectivity oscillations from the rocksalt-structured material. The oscillations increase in amplitude with higher fluence and temperature while blueshifting in frequency with higher pressure. It is determined that these oscillations originate from the photoexcitation of a TO-TA (transverse acoustic) combination via the second-order Raman mechanism. Another result stems from time and momentum resolved x-ray measurements on PbTe, which show evidence that electron-phonon interactions play a significant role in the material's equilibrium ferroelectric instability. Here, we characterize the dispersion of correlated pairs of phonons with equal and opposite momenta produced by the absorption of pump pulses near the energy of the direct band gap. From the dispersion data away from zone center coupled with constrained density functional theory (CDFT) calculations, we find that photoexcitation leads to the TO branch hardening near zone center and softening near zone edge (X point). This is further tied to the weakening of long-range forces along the cubic direction and in the band picture, a reduction of the Peierls-like electronic instability. All of this reinforces the paraelectric state. It is thus determined that electron-phonon coupling drives ferroelectric instability in PbTe. Furthermore, from the dispersion data very near zone center, we find a highly dispersive feature that is identified as a heavily screened longitudinal optical (LO) phonon mode. This feature is attributed to the high photoexcitation density in the measurements and is reminiscent of LO mode softening seen in past INS studies of PbTe. It is concluded that this similarity is due to the effective equivalence between photoexcitation in our measurements and relatively high carrier doping in the samples of the past studies. This is significant since it reconciles the inconsistency between the dispersion results of the aforementioned recent INS measurements (measured on less doped samples) with those seen in the older studies.