Recent spectroscopic measurements in a number of strongly correlated metals that exhibit non-Fermi liquid like properties have observed evidence of anomalous frequency and momentum-dependent charge-density fluctuations. Specifically, in the strange metallic regime of the cuprate superconductors, there is a featureless particle-hole continuum exhibiting unusual power-laws, and experiments suggest that the plasmon mode decays into this continuum in a manner that is distinct from the expectations of conventional Fermi liquid theory. Inspired by these new experimental developments, we address the nature of low-energy collective modes and the particle-hole continua for different "solvable" lattice models of non-Fermi liquids that host a critical Fermi surface -- a sharp electronic Fermi surface without any low-energy electronic quasiparticles. We scrutinize theoretically the possible existence of a long-lived zero-sound mode, which is renormalized to the plasma frequency in the presence of long-ranged coulomb interactions, and its decay into the continuum over a wide range of frequencies and momenta. Quite remarkably, some of the models analyzed here can account for certain aspects of the universal experimental observations, that clearly lie beyond the purview of standard Fermi liquid theory.

中文翻译：

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具有临界费米面的奇异金属的集体密度波动

最近对许多表现出非费米液体性质的强相关金属的光谱测量已经观察到异常频率和动量相关的电荷密度波动的证据。具体来说，在铜酸盐超导体的奇异金属状态中，存在一个无特征的粒子-空穴连续体，表现出不寻常的幂律，实验表明等离子体模式衰减到这个连续体中的方式与传统费米液体的预期不同理论。受这些新的实验发展的启发，我们针对具有临界费米面的非费米液体的不同“可解”晶格模型解决了低能集体模式和粒子孔连续体的性质 - 一个尖锐的电子费米表面，没有任何低能电子准粒子。我们从理论上仔细研究了长寿命零声模式的可能存在，该模式在存在长程库仑相互作用的情况下被重新归一化为等离子体频率，并在很宽的频率和动量范围内衰减为连续体。非常值得注意的是，这里分析的一些模型可以解释普遍实验观察的某些方面，这显然超出了标准费米液体理论的范围。