[Paper Review] Coexisting Fermi Liquid and Strange Metal Phenomena in Sr$_2$RuO$_4$

Ali Husain, Matteo Mitrano+7 more|arXiv (Cornell University)|Jul 13, 2020

Advanced Condensed Matter Physics4 citations

TL;DR

This study reveals that Sr₂RuO₄ hosts both strange metal and Fermi liquid behaviors simultaneously at the same temperature, as evidenced by coexisting non-propagating charge fluctuations and a dispersing collective mode. Using momentum-resolved electron energy-loss spectroscopy, the authors observe spectral weight redistribution and velocity renormalization upon cooling through the coherence temperature, demonstrating that these two exotic states can coexist and interact in a single quantum material.

ABSTRACT

The strange metal is an enigmatic phase whose properties are irreconcilable with the established Fermi liquid theory of conductors. A fundamental question is whether a strange metal and a Fermi liquid are distinct phases of matter, or whether a material can be intermediate between or in a superposition of the two. We studied the collective density response of the correlated metal Sr$_2$RuO$_4$ by momentum-resolved electron energy-loss spectroscopy (M-EELS). We discovered that a broad continuum of non-propagating charge fluctuations (a characteristic of strange metals) and also a dispersing Fermi liquid-like collective mode at low energies and long wavelengths coexist in the same material at the same temperature. These features exhibit a spectral weight redistribution and velocity renormalization when we cool the material through the quasiparticle coherence temperature. Our results show not only that strange metal and Fermi liquid phenomena can coexist but also that Sr$_2$RuO$_4$ serves as an ideal test case for studying the interaction between the two.

Motivation & Objective

• To determine whether strange metal and Fermi liquid behaviors can coexist in a single quantum material.
• To investigate the nature of collective charge responses in the correlated metal Sr₂RuO₄.
• To examine how spectral weight and mode velocity evolve across the quasiparticle coherence temperature.
• To assess whether the interplay between strange metal and Fermi liquid features can be probed in a single system.

Proposed method

• Momentum-resolved electron energy-loss spectroscopy (M-EELS) was used to map the momentum- and energy-dependent collective density response in Sr₂RuO₄.
• Measurements were performed across a range of temperatures, including below and above the quasiparticle coherence temperature.
• The spectral function was analyzed to identify non-propagating charge fluctuations characteristic of strange metals.
• A dispersing collective mode at low energies and long wavelengths was identified as a Fermi liquid-like feature.
• Spectral weight redistribution and velocity renormalization were quantified as the system cooled through the coherence temperature.
• The data were interpreted in the context of competing or coexisting quantum critical and Fermi liquid behaviors.

Experimental results

Research questions

• RQ1 Can strange metal and Fermi liquid phenomena coexist in the same material at the same temperature?
• RQ2 How does the collective charge response evolve across the quasiparticle coherence temperature in Sr₂RuO₄?
• RQ3 What is the nature of the non-propagating charge fluctuations observed in the material?
• RQ4 How do the spectral weight and velocity of the collective mode change upon cooling through the coherence temperature?
• RQ5 Can Sr₂RuO₄ serve as a model system for studying the interplay between strange metal and Fermi liquid physics?

Key findings

• A broad continuum of non-propagating charge fluctuations, characteristic of strange metals, is observed in Sr₂RuO₄ at the same temperature as a dispersing Fermi liquid-like collective mode.
• The Fermi liquid-like mode exhibits a clear dispersion at low energies and long wavelengths, indicating coherent quasiparticle behavior.
• Spectral weight is redistributed between the non-propagating fluctuations and the dispersing mode upon cooling through the quasiparticle coherence temperature.
• The velocity of the Fermi liquid-like mode is renormalized as the system crosses the coherence temperature.
• The coexistence of these two distinct response features in the same material demonstrates that strange metal and Fermi liquid behaviors are not mutually exclusive.
• Sr₂RuO₄ emerges as a unique platform for studying the interplay between strange metal and Fermi liquid physics in a single quantum system.