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Dispersive charge density wave excitations in Bi 2 Sr 2 CaCu 2 O 8 + δ
| Content Provider | Semantic Scholar |
|---|---|
| Author | Chaix, Laura Ghiringhelli, Gianluca Peng, Y. Y. Hashimoto, M. Moritz, B. Kummer, Kurt Brookes, Nick B. He, Yitao Chen, Shoulong Ishida, Shigeyuki Yoshida, Yukihiro Eisaki, Hiroshi Salluzzo, M. Braicovich, L. Shen, Z.-X. Devereaux, Thomas P. |
| Copyright Year | 2017 |
| Abstract | Experimental evidence on high-Tc cuprates reveals ubiquitous charge density wave (CDW) modulations, which coexist with superconductivity. Although the CDWhad been predicted by theory, important questions remain about the extent to which the CDW influences lattice and charge degrees of freedom and its characteristics as functions of doping and temperature. These questions are intimately connected to the origin of the CDW and its relation to the mysterious cuprate pseudogap. Here, we use ultrahigh-resolution resonant inelastic X-ray scattering to reveal new CDW character in underdopedBi2.2Sr1.8Ca0.8Dy0.2Cu2O8+δ.At lowtemperature,we observe dispersive excitations from an incommensurate CDW that induces anomalously enhanced phonon intensity, unseen using other techniques. Near the pseudogap temperature T, the CDW persists, but the associated excitations significantly weaken with an indication of CDW wavevector shift. The dispersive CDW excitations, phonon anomaly, and analysis of the CDW wavevector provide a comprehensive momentumspace picture of complex CDW behaviour and point to a closer relationship with the pseudogap state. With sufficient energy resolution, resonant inelastic X-ray scattering (RIXS) can be an ideal probe for revealing the CDW excitations in cuprates. By tuning the incident photon energy to the Cu L3-edge (Fig. 1a), the resonant absorption and emission processes can leave the system in excited final states, which couple to a variety of excitations arising from orbital, spin, charge, and lattice degrees of freedom. Thus, information of these elementary excitations in energy and momentum space can be deduced from analysing the RIXS spectra as functions of the energy loss and the momentum transfer of the photons (Fig. 1a). This is highlighted by the pivotal role that RIXS has recently played in revealing orbital and magnetic excitations in cuprates. In addition, RIXS provided the first X-ray scattering evidence for an incommensurate CDW in (Y,Nd)Ba2Cu3O6+δ (ref. 4), owing to energy resolution that separated the quasi-elastic CDW signal (bright spot in Fig. 1b, limited by the instrumental resolution ∼130meV) from other intense higher-energy excitations. Notably this quasi-elastic signal is asymmetric with respect to zero energy loss (Fig. 1c), which indicates the possible existence of additional low-energy excitations near the CDW wavevector (QCDW). In this work, we exploit the newly commissioned ultrahighresolution RIXS instrument at the European Synchrotron Radiation Facility to reveal these low-energy excitations near the CDW. We choose the double-layer cuprate Bi2.2Sr1.8Ca0.8Dy0.2Cu2O8+δ (Bi2212), whose electronic structure has been extensively studied by surface-sensitive spectroscopy, such as scanning tunnelling microscopy and angle-resolved photoemission, and in which a short-range CDW order was recently reported. With improved energy resolution up to 40meV, we see additional features in the previous quasi-elastic region (Fig. 1d). Figure 2a presents an energy–momentum RIXS intensity map of our high-resolution data. Two excitation branches are clearly observed with a momentum-dependent intensity distribution, also evident in the energy-loss spectra at representativemomenta (Fig. 2b). The first branch centred at zero energy should contain the scattering signal due to an underlying CDW. Indeed, as plotted in Fig. 2c, the momentum distribution of the RIXS intensity averaged over a small energy window exhibits a symmetric peak at a finite momentum, with a peak-to-background ratio of∼2, unambiguously confirming the existence of the CDW in Bi2212. The peak position is located at QCDW∼ 0.3 reciprocal lattice units (r.l.u.), consistent with previous STM studies and with a full-width at half-maximum (FWHM) of approximately 0.085 r.l.u., corresponding to a short correlation length of approximately 15Å. Interestingly, despite the short correlation length, the CDW strength, estimated by the integrated intensity of the CDW peak in the quasi-elastic region, is not weaker than that in YBa2Cu3O6+x (see Supplementary Information). The second branch of excitations possesses an energy scale of approximately 60meV, whose energy–momentum-dispersion can be reliably extracted (Fig. 2d and Supplementary Fig. 2). The extracted dispersion agrees well with that of the Cu–O bond-stretching phonon measured by non-resonant inelastic X-ray scattering (Supplementary Information). Owing to the high momentum-resolution of our data, Fig. 2e shows mode softening by approximately 25% atQCDW, with a corresponding broadening of the fitted peakwidth, indicating that the short-ranged orderedCDW unambiguously affects the lattice. This observation is reminiscent of the bond-stretching phonon anomaly reported in the stripedordered cuprates and the acoustic phonon anomaly in YBCO, where CDW correlation lengths are notably longer. Curiously, the |
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| Alternate Webpage(s) | https://www.dora.lib4ri.ch/psi/islandora/object/psi:3448/datastream/PDF/Chaix-2017-Dispersive_charge_density_wave_excitations-(published_version).pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
