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application/pdf en text Submission original under an indefinite embargo labeled 'Open Access'. The submission was exported from vireo on 2025-02-04 without embargo terms The student, Jin Chen, accepted the attached license on 2024-05-22 at 20:36. The student, Jin Chen, submitted this Dissertation for approval on 2024-05-22 at 21:19. This Dissertation was approved for publication on 2024-05-23 at 16:35. DSpace SAF Submission Ingestion Package generated from Vireo submission #20809 on 2025-02-04 at 21:03:24 Long-wavelength density fluctuations and momentum scrambling In the strange metal Bi2Sr2CaCu2O8+x Chen, Jin 2024-05-23 Abbamonte, Peter Chiang, Tai-Chang Phillips, Philip W. Uchoa, Bruno Electron Energy Loss Spectrum Strange Metal Plasmon Unconventional Superconductor eng There is currently a major controversy about plasmons in strange metals, which can be resolved by studying the momentum-dependent density fluctuation spectrum. Recently published papers, based on momentum-resolved energy-loss spectroscopy (M-EELS) at large q, show a frequency-independent continuum in the strange metal Bi2Sr2CaCu2O8+x [Mitrano, PNAS 115, 5392 (2018); Husain, PRX 9, 041062 (2019)], reminiscent of the marginal Fermi liquid (MFL) hypothesis of the late 1980s [Varma, PRL 63, 1996 (1989)]. However, our observations, which were done at large q, have been difficult to reconcile with infrared (IR) optics, done at q = 0, which see a well-defined plasmon excitation. In this thesis, we present M-EELS data taken with ∼4× better momentum resolution, allowing us to reach the optical limit, q ∼ 0, where direct comparison with IR optics can be made. We see a clear plasmon feature for q < 0.04 r.l.u. that, when analyzed with an old theoretical framework of Jain & Allen [Jain, PRB 32, 997 (1985)], shows quantitative consistency between M-EELS and IR experiments. For q > 0.04 r.l.u., the spectra become incoherent. Instead of a conventional dispersive plasmon, the spectra show a constant-in-frequency continuum. Our results resolve the controversy and show how the density response of strange metals evolves from the Brillouin zone boundary down to the optical limit, q ∼ 0. We speculate that electrons in such a planckian metal undergo momentum scrambling at finite frequency, ω, and nonzero q. Black phosphorus, as a layered semiconductor, is well-known for its tunable narrow energy gap and large in-plane optical anisotropy, holding great potential for diverse applications such as optical sensors. However, few studies of the dynamic charge response at finite momentum transfer have been reported. Using M-EELS, we measured the interband transitions of black phosphorus as a function of temperature and momentum transfer. We observe a temperature dependent, non-dispersive, isotropic gap in M-EELS spectrum in bulk black phosphorus. Beyond M-EELS, we propose a new technique called two-electron M-EELS, where there are two electrons coming in and two electrons going out, undergoing momentum and energy transfer. We calculated the cross section in the case of reflection geometry, similar to M-EELS. With such a technique, we are able to measure electron interaction inside the materials, such as the coulomb interaction or phonon-mediated interaction, among others. In this thesis, we take a coulomb interaction as an example and calculate the corresponding cross section. 2024-08 Thesis https://hdl.handle.net/2142/125511 Copyright 2024 Jin Chen Physics Physics University of Illinois at Urbana-Champaign Ph.D. Dissertation