Abstract
A normal metal exhibits a valence plasmon, which is a sound wave in its conduction electron density. The mysterious strange metal is characterized by non-Boltzmann transport and violates most fundamental Fermi-liquid scaling laws. A fundamental question is, do strange metals have plasmons? Using momentum-resolved inelastic electron scattering, we recently showed that, rather than a plasmon, optimally doped (Bi-2212) exhibits a featureless, temperature-independent continuum with a power-law form over most energy and momentum scales [M. Mitrano et al., Proc. Natl. Acad. Sci. U.S.A. 115, 5392 (2018)]. Here, we show that this continuum is present throughout the fan-shaped, strange metal region of the phase diagram. Outside this region, dramatic changes in spectral weight are observed: In underdoped samples, spectral weight up to 0.5 eV is enhanced at low temperature, biasing the system toward a charge order instability. The situation is reversed in the overdoped case, where spectral weight is strongly suppressed at low temperature, increasing quasiparticle coherence in this regime. Optimal doping corresponds to the boundary between these two opposite behaviors at which the response is temperature independent. Our study suggests that plasmons do not exist as well-defined excitations in Bi-2212 and that a featureless continuum is a defining property of the strange metal, which is connected to a peculiar crossover where the spectral weight change undergoes a sign reversal.
- Received 11 March 2019
- Revised 20 October 2019
DOI:https://doi.org/10.1103/PhysRevX.9.041062
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
A crowning achievement of condensed-matter physics is the Fermi-liquid theory of metals. It reduces the incredibly complicated problem of interacting electrons to a simple model of weakly interacting electron quasiparticles plus some collective oscillations, called plasmons, which are sound waves in the electron fluid. While this paradigm is widely successful for simple metals, it fails to describe the enigmatic “strange metal” phase found in many strongly correlated quantum materials. Because of its ubiquity in correlated systems ranging from hard metallic oxides to soft organics, understanding the strange metal has become one of the great unsolved problems in condensed-matter physics. To help address this problem, we investigate how plasmons behave in strange metals by directly measuring charge fluctuations in a prototypical sample.
We rely on the newly developed technique of momentum-resolved inelastic electron scattering to measure charge fluctuations in a strange metal at a variety of temperatures and doping strengths. We find that in the strange metal phase, this material lacks a well-defined plasmon excitation but instead exhibits a highly damped continuum of excitations that is featureless in both energy and momentum. This observation strikingly contrasts with conventional metals, in which plasmon excitations are well defined. Outside of the strange metal regime, the continuum is highly structured and temperature dependent because of many-body effects that remain to be understood.
Our study suggests that a momentum-independent continuum of charge fluctuations is the defining fingerprint of the strange metal state, providing a crucial observable to test microscopic theories aiming to describe this enigmatic phase of matter.