Spectroscopic investigation of the Zero-Point Renormalization, excitonic Mott transition and ultrafast quasiparticle relaxations

Roy, Basabendra (2023) Spectroscopic investigation of the Zero-Point Renormalization, excitonic Mott transition and ultrafast quasiparticle relaxations. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

In this thesis, we spectroscopically explore aspects of electron-phonon and electron-electron interactions in band and correlated-electron insulators. We have studied three problems—the zero-point bandgap renomalization in GaAs, the excitonic Mott transition in silicon, and aspects of ultrafast quasiparticle relaxation in NdNiO₃. NdNiO₃ is a charge-ordered insulator that undergoes a thermally-induced insulator-to-metal transition. The main experimental tools for this work are low-temperature photoluminescence spectroscopy and sub-picosecond time-resolved pump-probe reflectivity. In many semiconductors, the low temperature exponential absorption tails when extrapolated give a convergent bundle, with the point of convergence termed “Urbach focus,” which represents the zerotemperature band gap of the pure crystal with the electron-phonon interactions switched off. The Urbach focus therefore yields the zero-point bandgap renormalization (ZPR), a direct consequence of the quantum zero-point motion. Our temperature dependent study of the absorption edge for GaAs gives a bare crystal gap of 1.581 eV and a ZPR of about 66 meV. In the next chapter, we revisit the well-studied and extensively debated exciton-to-plasma Mott crossover in indirect band gap material silicon. As the density of the exciton gas is gradually enhanced, beyond a point it becomes energetically favourable for the excitations to exist as electron-hole plasma, rather than to be in hydrogenic bound states. This crossover from a bosonic exciton gas to a fermionic plasma is a type of metal-insulator transition that has been of interest for many decades. We have found that the low temperature photoluminescence spectrum around a certain crossover density shows a decrease of the emission intensity with increase in the excitation power. The photoluminescence efficiency (emission per incident photon) decreases by more than an order of magnitude building up to the Mott crossover, after which it becomes almost constant. This drastic loss in the oscillator strength is accompanied onset of a strong broadening of the excitonic peak. Trivial effects arising out of the laser-induced heating of the sample are ruled out. We next switch to ultrafast time-resolved transient reflectivity measurements on NdNiO3, which is a strongly correlated charge-transfer insulator with a first-order insulator-to-metal transition. We perform low temperature pump-probe measurements on thin films of NdNiO3 (on NdGaO3 substrate) which show remarkable reversibility across the insulator-to-metal transition. Using the lack of hysteresis, we estimate the true lattice temperature in our measurements. We report a non-monotonic V-shaped relax ation dynamics with the rise in the transient reflectivity attributed to the emergence of metallic domains as the sample enters the coexistence region. This metallic growth is seen to get stronger with temperature and eventually engulfs the ubiquitous exponential dynamics. We show that the competing ultrafast relaxation mechanisms and the emergent metallicity can be well-modeled by invoking a combination of fast exponential decay along with a slower logistic growth curve. In the next chapter, we report the phase-coexistence across a first-order insulator-to-metal transition on NdNiO3 films grown on the more usual LAO substrate. In the coexistence region, where both the metallic and the insulating domains are present, the transient ultrafast relaxation response is actually an incoherent superposition of the signals arising from the two phases respectively. A systematic study then reveals the gradual evolution of the emergent domains leading to the change in relaxation nature of the sample from insulating to metallic. We resolve our ultrafast relaxation into separate contributions from both the phases and study its gradual evolution from a predominately insulating relaxation to a predominately metallic one, which is captured by the inversion in the effective signal amplitude.

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