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 ${\mathrm{Bi}}_{2.1}{\mathrm{Sr}}_{1.9}{\mathrm{Ca}}_{1.0}{\mathrm{Cu}}_{2.0}{\mathrm{O}}_{8+x}$ (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.

中文翻译：

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跨奇怪金属相图的电荷波动交叉

普通金属表现出价等离激元，它是在其传导电子密度中的声波。这种神秘的奇怪金属的特征是非玻耳兹曼输运，并且违反了最基本的费米-液结垢定律。一个基本的问题是，奇怪的金属有等离子体吗？最近，使用动量分辨的非弹性电子散射，我们发现了最佳掺杂而不是等离激元${双}_{2.1}{r}_{1.9}{钙}_{1.0}{铜}_{2.0}{\mathrm{Ø}}_{8+X}$（Bi-2212）在大多数能量和动量尺度上均表现出无定律，与温度无关的连续体，并且具有幂律形式[M. Mitrano等。，过程。Natl。学院 科学 美国 115，5392（2018）]。在这里，我们表明该连续体存在于相图的扇形，奇怪的金属区域中。在该区域之外，观察到光谱权重发生了巨大变化：在掺杂不足的样品中，低温下，光谱权重高达0.5 eV增强，使系统趋向于电荷级不稳定性。在过量掺杂的情况下，情况则相反，在低温情况下，光谱权重被强烈抑制，在这种情况下，准粒子相干性增加。最佳掺杂对应于这两个相反行为之间的边界，在该边界处，响应与温度无关。我们的研究表明，等离子激元在Bi-2212中不存在定义明确的激发，并且无特征连续体是奇异金属的定义性质，