PHYSICAL REVIEW B 96, 115148 (2017) Observation of momentum-dependent charge excitations in hole-doped cuprates using resonant inelastic x-ray scattering at the oxygen K edge Kenji Ishii, 1 Takami Tohyama, 2 Shun Asano, 3 Kentaro Sato, 3 Masaki Fujita, 3 Shuichi Wakimoto, 4 Kenji Tustsui, 1 Shigetoshi Sota, 5 Jun Miyawaki, 6 Hideharu Niwa, 6 Yoshihisa Harada, 6 Jonathan Pelliciari, 7, * Yaobo Huang, 7 Thorsten Schmitt, 7 Yoshiya Yamamoto, 8 and Jun’ichiro Mizuki 8 1 Synchrotron Radiation Research Center, National Institutes for Quantum and Radiological Science and Technology, Hyogo 679-5148, Japan 2 Department of Applied Physics, Tokyo University of Science, Tokyo 125-8585, Japan 3 Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan 4 Materials Sciences Research Center, Japan Atomic Energy Agency, Ibaraki 319-1195, Japan 5 Computational Materials Science Research Team, RIKEN Advanced Institute for Computational Science (AICS), Kobe, Hyogo 650-0047, Japan 6 Institute for Solid State Physics, University of Tokyo, Chiba 277-8581, Japan 7 Research Department Synchrotron Radiation and Nanotechnology, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 8 Graduate School of Science and Technology, Kwansei Gakuin University, Hyogo 669-1337, Japan (Received 2 March 2017; revised manuscript received 5 September 2017; published 22 September 2017) We investigate electronic excitations in La 2−x (Br,Sr) x CuO 4 using resonant inelastic x-ray scattering (RIXS) at the oxygen K edge. RIXS spectra of the hole-doped cuprates show clear momentum dependence below 1 eV. The broad spectral weight exhibits positive dispersion and shifts to higher energy with increasing hole concentration. Theoretical calculation of the dynamical charge structure factor on oxygen orbitals in a three-band Hubbard model is consistent with the experimental observation of the momentum and doping dependence, and therefore the momentum-dependent spectral weight is ascribed to intraband charge excitations which have been observed in electron-doped cuprates. Our results confirm that the momentum-dependent charge excitations exist on the order of the transfer energy (t ), and the broad spectral line shape indicates damped and incoherent character of the charge excitations at the energy range in the doped Mott insulators. DOI: 10.1103/PhysRevB.96.115148 I. INTRODUCTION Strongly correlated transition-metal oxides display vari- ous interesting physical properties, including metal-insulator transition, high-temperature superconductivity, and colossal magnetoresistance, and some of the oxides are classified into doped Mott insulators where electron correlation significantly modifies their band structure, which diverges from that of the noninteracting simple metal [1]. Among the doped Mott insulators, superconducting cuprates are most intensively studied [2]. This is mainly due to the superconductivity at the high transition temperature and related phenomena such as pseudogap and a competing phase with charge order [3]. In addition to the interest of superconductivity, doped cuprates are important and suitable for the study of the electronic structure of the doped Mott insulator because relatively simple theoretical models with a few orbitals are applicable to describe the electronic structure near the Fermi energy. They represent the benchmark of the doped Mott insulators, and the clarification of the fundamental electronic structure is essential for understanding the mechanism of the physical phenomena in the doped cuprates. In the undoped cuprates, only the spin degree of freedom contributes to the low-energy electron dynamics. When carri- ers are doped, the charge degree of freedom becomes active and the electron dynamics is characterized by the motion of spin and charge. Therefore, we consider that both spin and charge * Present address: Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. excitations must be investigated on equal footing in order to understand the electron dynamics characterizing the physics of the cuprates. Inelastic neutron scattering (INS) has been widely used for studying the spin dynamics in the reciprocal lattice space, and high-resolution resonant inelastic x-ray scattering (RIXS) at the Cu L 3 edge has recently become an alternative to measure momentum-resolved spin excitations up to several hundred meV [4,5]. Charge excitations in the doped cuprates extend to higher energy than the spin excitations, and the electron correlation affects the charge excitations of the order of a few electronvolts. Optical studies [6,7] demonstrated that the spectral weight of the intraband charge excitations emerges below the charge-transfer gap in the doped cuprates, and the weight characterizes the charge excitations in the doped Mott insulators. The high-energy part of momentum-resolved charge excitations below the gap was studied by Cu K -edge RIXS [8–10], but the huge tail of the elastic scattering hampers the observation below several hundred meV. Alternatively, the capability of Cu L 3 -edge RIXS for the detection of the charge excitations is argued theoretically [11,12], and particle- hole charge excitations have been reported in a Ti L 3 -edge RIXS study on the weakly correlated broadband material 1T -TiSe 2 [13]. Nevertheless, momentum-dependent charge excitations in the hole-doped cuprates, to the best of our knowledge, have not been identified experimentally using Cu L 3 -edge RIXS. On the other hand, in electron-doped cuprates, a dispersive mode which is located at higher energy than the spin excitations was found in the Cu L 3 -edge RIXS spectra [14,15] and it is ascribed to the particle-hole charge excitations [14]. However, the origin of this mode is still controversial. 2469-9950/2017/96(11)/115148(8) 115148-1 ©2017 American Physical Society