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Optical Probing of Collective Excitonic Modes within Excitonic Insulators

Davari, Elahe | 2025

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 58027 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Kargarian, Mehdi
  7. Abstract:
  8. The excitonic insulator phase is a quantum state of matter resulting from the macroscopic coherent condensation of electron-hole bound states. As such, it shares similarities with superconductivity, exhibiting superfluid-like behavior and collective Higgs and Goldstone modes. The discovery of primary excitonic candidates at cryogenic temperatures has spurred research into materials with higher critical temperatures and stable conditions. Recently, several transition metal chalcogenides have emerged as promising high-temperature excitonic insulators. However, the interplay between excitonic condensation and structural phase transitions in these materials presents a significant challenge to unravel the true nature of the phase. Discrepancies in reported experiments necessitate further studies, motivating us to propose a theoretical approach to understand and control exciton condensation as a distinct feature of this phase. Exciton condensation involves the spontaneous breaking of U(1) symmetry, leading to low-energy collective phase and amplitude modes of the exciton order parameter. To probe the collective modes, we employed an effective two-dimensional spinless model and investigated the excitonic condensate response to laser light at high frequencies (in far-infrared spectrum) with varying polarizations and intensities ranging from (0.4-2.7) mJ/cm2 using Floquet theory. The polarization of the light changes the electronic symmetry of the system. The optical response shows that the amplitude mode oscillates around the single-particle energy gap, while the gapless phase mode evolves linearly over time, influenced by the light's intensity and polarization. To further investigate the dynamics of primarily lattice-driven and primarily electronic exciton collective modes, we examined their response to light by adjusting the electron-phonon interaction strength. The results underscore the distinct dynamics of exciton condensation. In the lattice-driven state, the phase mode oscillates at (fphase=74.88 THz), close to the single-particle gap, whereas in the electronic state, the oscillation frequency is much lower at (fphase=38.65 THz), providing an effective method for differentiating between these two states. To further probe the collective modes, in the next step, we investigated entangled light-matter states using the quantum nature of the light in a cavity. Using a one-dimensional model, we examined the effect of the system's phase mode on the cavity photon dispersion. The results show that the collective modes of the exciton driven by the coulomb interaction significantly affect the cavity modes, creating an avoiding band crossing in the photon dispersion. This phenomenon does not occur for excitonic insulators driven by the electron-phonon interaction. Therefore, the approaches studied in this thesis can be used as a probe to detect the nature of the excitonic insulator phase and its collective modes
  9. Keywords:
  10. Excitonic Insulator ; Floquet Theorem ; Higgs Mode ; Excitonic Higgs and Goldstone Mode

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