Multiorbital charge-density wave excitations and concomitant phonon anomalies in Bi ₂ Sr ₂ LaCuO _6+δ

Abstract

Charge-density waves (CDWs) are ubiquitous in underdoped cuprate superconductors. As a modulation of the valence electron density, CDWs in hole-doped cuprates possess both Cu-3dand O-2porbital character owing to the strong hybridization of these orbitals near the Fermi level. Here, we investigate underdoped Bi2Sr1.4La0.6CuO6+δusing resonant inelastic X-ray scattering (RIXS) and find that a short-range CDW exists at both Cu and O sublattices in the copper-oxide (CuO2) planes with a comparable periodicity and correlation length. Furthermore, we uncover bond-stretching and bond-buckling phonon anomalies concomitant to the CDWs. Comparing to slightly overdoped Bi2Sr1.8La0.2CuO6+δ, where neither CDWs nor phonon anomalies appear, we highlight that a sharp intensity anomaly is induced in the proximity of the CDW wavevector (QCDW) for the bond-buckling phonon, in concert with the diffused intensity enhancement of the bond-stretching phonon at wavevectors much greater than QCDW. Our results provide a comprehensive picture of the quasistatic CDWs, their dispersive excitations, and associated electron-phonon anomalies, which are key for understanding the competing electronic instabilities in cuprates.

Charge order lock-in by electron-phonon coupling in La 1.675 Eu 0.2 Sr 0.125 CuO 4

“…For the copperoxygen bond-stretching mode, a sin 2 (h) dependence of intensity for Q = (h, 0) is expected in the absence of charge order (16,(25)(26)(27). Recent RIXS studies, on different cuprate systems, have observed bond-stretching phonon intensities in good agreement with this theoretical modeling (20,21,25,26). It is also in accord with the intensities measured above T LTT in LESCO (Fig.…”

“…Phase excitations of stripes couple to the lattice and create a splitting of the in-phase and out-of-phase oscillation modes (35). However, the phonon anomaly is expected (35) to exist within a narrow (17,19,20). This is supported by a nonmonotonically increasing phonon intensity versus momentum and an excitation energy softening that exceeds expectations for EPC.…”

“…Recent advances of the resonant IXS (RIXS) technique have enabled studies of optical phonons, charge excitations, and their Fano interference (16)(17)(18)(19)(20)(21)(22). The RIXS phonon scattering cross section is directly proportional to the EPC strength (see Fig.…”

Charge Density Waves in YBa2Cu3O6.67 Probed by Resonant X-Ray Scattering under Uniaxial Com

“…Nevertheless, as they do affect the shape of the Fermi surface via hybridization between energy bands derived from the CuO 2 planes and the CuO chains, they may thus indirectly influence the propensity of the electron system in the CuO 2 planes to CDW formation. The importance of electron-phonon interactions (EPI) for the formation of the CDW in the cuprates has been highlighted [9,26,41,[46][47][48][49], and research on classical CDW compounds has shown that its momentum dependence is also a critical parameter [50][51][52]. We argue here that the pronounced a=b anisotropies of EPI for high-energy phonons reported in YBCO [53,54] reflect the influence of the CuO chains on the EPI and play an important role in destabilizing the 3D a-CDW relative to the b-CDW.…”

Emergent charge order from correlated electron-phonon physics in cuprates

“…However, trends with doping do not show any clear correlation with spin-charge stripes 39 , static charge density waves 42 , or kinks in the electronic dispersion 40 , which seems to rule out conventional el-ph coupling mechanisms of the type contempleted here. Rather, it is proposed that the softening reflects coupling to a collective charge mode of the electron liquid 40,43 . In Forgan et al 32 , oxygen displacements do indeed have in-plane components, indicating that the bondstretching modes contribute alongside the buckling mode, but the out-of-plane components are dominant, and we make the simplifying approximation that the in-plane component can be neglected.…”