Momentum-resolved electron dynamics in a strange metal

Guo, Xuefei

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https://hdl.handle.net/2142/129851 Copy

Description

Title

Momentum-resolved electron dynamics in a strange metal

Author(s)

Guo, Xuefei

Issue Date

2025-07-11

Director of Research (if dissertation) or Advisor (if thesis)

Abbamonte, Peter

Doctoral Committee Chair(s)

Madhavan, Vidya

Committee Member(s)

• Phillips, Philip W.
• Faulkner, Thomas

Department of Study

Physics

Discipline

Physics

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Strange metals
• Strongly correlated electron systems
• Cuprate superconductors
• Pseudogap
• Quantum criticality
• M-EELS
• RSXS

Abstract

The strange metal phase, characterized by its simple linear-in-temperature resistivity, challenges the conventional quasiparticle paradigm. This anomalous behavior, associated with Planckian dissipation, suggests a minimal relaxation timescale and a maximally entangled many-body quantum state. Despite decades of investigation, this strongly interacting electronic phase remains one of the most enduring puzzles in condensed matter physics. This thesis uses advanced scattering and spectroscopy techniques to investigate the strange metal phase in Bi2Sr2CaCu2O8+x (Bi-2212), aiming to shed new light on its underlying dynamics. In the pseudogap regime, putative broken symmetries are considered essential to understanding the emergence of the strange metal state within the framework of quantum criticality. Using resonant soft x-ray scattering at the Cu L3 edge, signatures of a Q ~ 0 order are observed in optimally doped Bi-2212, indicated by the appearance of the nominally forbidden (0, 0, 3) Bragg peak under circularly polarized light within the pseudogap regime. In contrast, this peak remains absent under linear polarization or at temperatures outside the pseudogap regime. These results suggest the presence of a spatially uniform valence band order involving higher-order multipole moments of charge or spin distribution around the copper sites. In the strange metal regime, momentum-resolved electron energy-loss spectroscopy (M-EELS) reveals that the low-energy charge fluctuations in Bi-2212 exhibit w/T scaling invariance. The dynamic charge response is well captured by the Glauber relaxational model, indicating predominantly relaxational dynamics with negligible diffusive contribution. The extracted relaxation rate is consistent with Planckian dissipation. Moreover, the momentum-independent nature of the response suggests that the charge dynamics are local. Strikingly, the data are well described by a conformal field theory with a conformal dimension of 0.05, significantly deviating from the marginal Fermi liquid hypothesis, which predicts a conformal dimension of 0.5. These findings indicate that the charge fluctuations in the strange metal phase of Bi-2212 exhibit conformal invariance, suggesting a connection to holographic descriptions of strange metals. Recent advances in electron detector technology have prompted the development of a new time-of-flight approach for M-EELS measurements, expected to significantly improve the efficiency of the current scheme. As an initial step, a thermally enhanced photocathode was developed using a conventional LaB6 filament illuminated by a 392 nm ultrafast UV laser. Increasing the filament temperature leads to a higher photoemission current, but also introduces a higher continuous thermal background. An optimal operating temperature is identified to maximize the signal-to-noise ratio. Additionally, higher filament temperatures improve the stability of the cathode, which is advantageous for M-EELS measurements with low count rates.

Graduation Semester

2025-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/129851

Copyright and License Information

2025 by Xuefei Guo. All rights reserved.

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