Crossover of Charge Fluctuations across the Strange Metal Phase Diagram

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 (M-EELS) we recently showed that, rather than a plasmon, optimally-doped Bi2.1Sr1.9Ca1.0Cu2.0O8+x (Bi-2212) exhibits a featureless, temperature-independent continuum with a power-law form over most energy and momentum scales [M. Mitrano, PNAS 115, 5392-5396 (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 towards 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. arXiv:1903.04038v2 [cond-mat.str-el]

Holographic plasmon relaxation with and without broken translations

“…An interesting future direction would be to understand the role of our results in relation to the recent experiments performed in strange metals and quantum critical materials [14,15]. Finally, similar dispersion relations have been discussed in the hydrodynamic description of collective modes in Weyl semimetals [84].…”

“…The results are quite different from the weakly coupled paradigm, in the sense that the plasmon modes display an anomalously strong damping. Not only that, but there are recent observations [14,15] which indicate that when increasing the momentum, plasmons stop existing as welldefined quasi-particles and get smoothed out in the collective and incoherent "quantum soup" losing a characteristic momentum scale. The strong damping of plasmons, even at zero momentum, is expected for strongly coupled quantum critical systems and was first observed in a holographic model in [16].…”

Charge density response and fake plasmons in holographic models with strong translation symmetry breaking

“…This is also seen in Eq. (3.7): increasing the plasma frequency by increasing e 2 makes the two purely imaginary modes approach each other and, at a certain critical value of ω p , they collide and form a pair of sound-like modes 3. A similar behaviour was found in[13], however we would not draw a direct analogy since we do not consider spontaneous breaking of translations here.…”

“…This competition between Γ k and e 2 , which was anticipated in [12] in a metallic setup with irrelevant translation symmetry-breaking, will be important for us in the present situation. 3 In fact, it complicates our search for a featureless density-density response, because we should avoid the regime where the modes become sound-like. In order to prevent this from happening at finite dressing e 2 and momentum k, we focus on the strong momentum-dissipation regime which goes beyond the applicability of the hydrodynamical approximation.…”

“…It is well-established by now that strongly correlated electron systems, in particular high temperature superconductors and strange metals, display anomalous phenomenology [1]; the charge density response is not an exception. For instance, recent momentum resolved electron energy-loss spectroscopy (M-EELS) experiments found that the spectrum of cuprate strange metals cannot be understood in terms of the conventional physics of a coherent plasmon [2,3]. These unconventional condensed matter systems require unconventional theoretical approaches to describe their phenomenology and the holographic duality provides a suitable framework for this task [4].…”