Dispersive charge density wave excitations in Bi2Sr2CaCu2O8+δ

Abstract

Experimental evidence on high-T c cuprates reveals ubiquitous charge density wave (CDW) modulations 1-10 , which coexist with superconductivity. Although the CDW had been predicted by theory 11-13 , 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 10,14 . Here, we use ultrahigh-resolution resonant inelastic X-ray scattering to reveal new CDW character in underdoped Bi 2.2 Sr 1.8 Ca 0.8 Dy 0.2 Cu 2 O 8+δ . At low temperature, 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 L 3 -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 15 . 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 [16][17][18][19][20] . In addition, RIXS provided the first X-ray scattering evidence for an incommensurate CDW in (Y,Nd)Ba 2 Cu 3 O 6+δ (ref. 4), owing to energy resolution that separated the quasi-elastic CDW signal (bright spot in Fig. 1b, limited by the instrumental resolution ∼130 meV) 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 (Q CDW ).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 Bi 2.2 Sr 1.8 Ca 0.8 Dy 0.2 Cu 2 O 8+δ (Bi2212), whose electronic structure has been extensively studied by surface-sensitive spectroscopy, such as scanning tunnelling microscopy 21 and angle-resolved photoemission 22 , and in which a short-range CDW order was recently reported 7,8 . With improved energy resolution up to 40 meV, we see additional features in the pre...

Formation of Incommensurate Charge Density Waves in Cuprates

“…This strongly points towards models in which CDW order is driven by local real-space correlations. Similar short-ranged CDW correlations that persist even to room temperature were recently observed in YBa 2 Cu 3 O 6+δ (YBCO) [18,37], Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) [22], La 2−x Sr x CuO 4 (LSCO) [14,38] and electron doped Nd 2−x Ce x CuO 4 [39], strongly indicating an ubiquitous pCDW phase in underdoped cuprates. Our results are also compatible with previous STM studies of various cuprate families with- out magnetic stripe order at low temperature, such as Bi2212 and Ca 2−x Na x CuO 2 Cl 2 , where CDWs are found to be locally commensurate with large phase slips [40][41][42].…”

Polarization-resolved Cu L3 -edge resonant inelastic x-ray scattering of orbital and spin excitations in NdBa2Cu3O7−<

“…RIXS spectra contain a variety of excitations spanning over a wide range of energies, from phonons below 100 meV [13][14][15] , to magnetic excitations up to 500 meV in cuprates 5,6 , to orbital (dd, crystal field) 9,11,[16][17][18][19][20][21][22] and charge transfer [23][24][25][26] excitations in the eV range. Moreover, the elastic and quasi-elastic intensities collected in RIXS spectra carry information on charge density waves or charge orders and associated excitations 10,[27][28][29] .…”

Single-shot time-resolved magnetic x-ray absorption at a free-electron laser

“…Research activities range from the investigation of fundamental processes in gas phase experiments [1][2][3] to the observation of structural rearrangement in biological macromolecules using novel time resolved crystallography techniques 4,5 . In condensed matter physics, the availability of ultrashort X-ray pulses has allowed to probe ultrafast charge, spin and lattice dynamics with chemical selectivity and nanometer spatial resolution [6][7][8][9][10] . A significant number of these time resolved experiments investigate the relaxation dynamics following an externally induced excitation, which is typically realized by an ultrashort infrared (IR) pump pulse.…”