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Nematic order refers to the spontaneous breaking of rotational symmetry while preserving translational symmetry. First identified in classical liquid crystals, nematic order arises from the collective alignment of anisotropic molecules. Its quantum counterpart, electronic nematicity, has been observed in a variety of quantum materials, ranging from unconventional superconductors to kagome metals. Despite its prevalence, there is no universal understanding of the conditions under which nematic order occurs. Electronic nematicity is most firmly established in iron-based superconductors, where it is understood to be a consequence of vestigial spin density wave (SDW) order. However, direct evidence for nematicity arising from other types of order are lacking. Here, we report direct evidence for charge-order-driven electronic nematicity in Ba Sr Ni As , a nickel-based analog of the iron pnictides known to exhibit charge density wave (CDW) order. Using x-ray diffraction under applied uniaxial strain, we observe a pronounced symmetry-breaking response-up to -in the intensity of incommensurate CDW Bragg peaks, even at small strain levels ( ). This effect occurs within the same region of the phase diagram where a giant nematic susceptibility is observed in transport measurements. These results provide direct evidence that long-range CDW order can drive nematic behavior in quantum materials. 

A charge density wave (CDW) is a phase of matter characterized by a periodic modulation of valence electron density coupled with lattice distortion. Its formation is closely tied to the dynamical charge susceptibility, $\chi (q,\omega )$, which reflects the collective electron dynamics of the material. Despite decades of study, $\chi (q,\omega )$near a CDW transition has never been measured at nonzero momentum, $q$, with meV energy resolution. Here, we investigate the canonical CDW transition in ErTe ${}_3$using momentum-resolved electron energy loss spectroscopy, a technique uniquely sensitive to valence band charge excitations. Unlike phonons, which soften via the Kohn anomaly, we find the electronic excitations exhibit purely relaxational dynamics well described by a diffusive model, with the diffusivity peaking just below the critical temperature, $T_{C1}$. Additionally, we report for the first time a divergence in the real part of $\chi (q,\omega )$in the static limit ( $\omega \to 0$), a long-predicted hallmark of CDWs. Unexpectedly, this divergence occurs as $T\to 0$, with only a weak thermodynamic signature at $T=T_{C1}$. Our study necessitates a reexamination of the traditional description of CDW formation in quantum materials. 

A charge density wave (CDW) is a phase of matter characterized by a periodic modulation of the valence electron density accompanied by a distortion of the lattice structure. The microscopic details of CDW formation are closely tied to the dynamic charge susceptibility, χ(q, ω), which describes the behavior of electronic collective modes. Despite decades of extensive study, the behavior of χ(q, ω) in the vicinity of a CDWtransition has never been measured with high energy resolution (∼meV). Here, we investigate the canonical CDW transition in ErTe3 using momentum-resolved electron energy loss spectroscopy (M-EELS), a technique uniquely sensitive to valence band charge excitations. Unlike phonons in these materials, which undergo conventional softening due to the Kohn anomaly at the CDW wavevector, the electronic excitations display purely relaxational dynamics that are well described by a diffusive model. The diffusivity peaks around 250 K, just below the critical temperature. Additionally, we report, for the first time, a divergence in the real part of χ(q, ω) in the static limit (ω → 0), a phenomenon predicted to characterize CDWs since the 1970s. These results highlight the importance of energy- and momentum-resolved measurements of electronic susceptibility and demonstrate the power of M-EELS as a versatile probe of charge dynamics in materials. 

The density fluctuation spectrum captures many fundamental properties of strange metals. Using momentum-resolved electron energy loss spectroscopy (M-EELS), we recently showed that the density response of the strange metal Bi2⁢Sr2⁢CaCu2⁢O8+𝑥 (Bi-2212) at large momentum, 𝑞, exhibits a constant-in-frequency continuum [M. Mitrano et al., Proc. Natl. Acad. Sci. USA 115, 5392 (2018); A. A. Husain et al., Phys. Rev. X 9, 041062 (2019)] reminiscent of the marginal Fermi liquid (MFL) hypothesis of the late 1980s [C. M. Varma et al., Phys. Rev. Lett. 63, 1996 (1989)]. However, reconciling this observation with infrared (IR) optics experiments, which show a well-defined plasmon excitation at 𝑞∼0, has been challenging. Here we report M-EELS measurements of Bi-2212 using 4× improved momentum resolution, allowing us to reach the optical limit. For momenta 𝑞<0.04 reciprocal lattice unites (r.l.u.), the M-EELS data show a plasmon feature that is quantitatively consistent with IR optics. For 𝑞>0.04 r.l.u., the spectra become incoherent with an MFL-like, constant-in-frequency form. We speculate that, at finite frequency, 𝜔, and nonzero 𝑞, some attribute of this Planckian metal randomizes the probe electron, causing it to lose information about its own momentum. 

Abstract The characteristic excitation of a metal is its plasmon, which is a quantized collective oscillation of its electron density. In 1956, David Pines predicted that a distinct type of plasmon, dubbed a ‘demon’, could exist in three-dimensional (3D) metals containing more than one species of charge carrier¹. Consisting of out-of-phase movement of electrons in different bands, demons are acoustic, electrically neutral and do not couple to light, so have never been detected in an equilibrium, 3D metal. Nevertheless, demons are believed to be critical for diverse phenomena including phase transitions in mixed-valence semimetals², optical properties of metal nanoparticles³, soundarons in Weyl semimetals⁴and high-temperature superconductivity in, for example, metal hydrides^3,5–7. Here, we present evidence for a demon in Sr₂RuO₄from momentum-resolved electron energy-loss spectroscopy. Formed of electrons in theβandγbands, the demon is gapless with critical momentumq_c = 0.08 reciprocal lattice units and room-temperature velocityv = (1.065 ± 0.12) × 10⁵m s⁻¹that undergoes a 31% renormalization upon cooling to 30 K because of coupling to the particle–hole continuum. The momentum dependence of the intensity of the demon confirms its neutral character. Our study confirms a 67-year old prediction and indicates that demons may be a pervasive feature of multiband metals.