Spectroscopic evidence for strongly correlated electronic states in La-Sr-Cu and Y-Ba-Cu oxides

Abstract

Photoemission studies of (La~-"Sr")2Cu04and YBa2Cu3067 have revealed that their electronic structure is essentially that of a Mott insulator, i.e. , Cu 3d electrons are localized, and the density of states at the Fermi level is very low. This result combined with the magnetic properties suggest a fluctuation of the local moment due to intersite exchange, which may be consistent with a bipolaron state or a resonating valence-bond state.Recently discovered high-T, superconductivity in the La-Ba-Cu-0 system' and related Cu oxides has stimulated extensive research in these materials. So far, most theoretical studies have been based on the twodimensional character of the electronic energy bands giving a high density of states (DOS) near the Fermi level (EF).Density-functional band-structure calculations have yielded a strongly hybridized Cu3d-02p antibonding band at EF, which leads to a strong electron-phonon coupling for a breathing-type displacement of 0 atoms.On the other hand, Anderson has proposed the resonating-valence-bond or quantum-spin-liquid state, which may occur in frustrated Heisenberg spin systems, and also discussed their unusual magnetic and transport properties in the normal state. ' In the light of the above two extreme points of view, the knowledge of the electronic structure is a key step toward understanding not only the superconducting but also the normal-state properties. This Rapid Communication reports a photoemission study on (La~-"Sr") 2Cu04-s and YBa2CusOs 7. The results demonstrate that the Cu 3d electrons are essentially localized due to strong electron correlation at the Cu sites. We have studied sintered samples of (La~"Sr")2Cu04-~and YBa2Cu30s7 which were prepared as described in Refs. 2 and 4 and whose properties are listed in Table I. Room-temperature x-ray and ultraviolet photoemission (XPS and UPS) experiments were performed using a spectrometer fitted with a Mg Ka x-ray source (hv=1253. 6 eV) and a helium resonance lamp (h v =21.2 and 40.8 eV). The energy resolution was -0. 1 eV for UPS and -0.8 eV for XPS. The base pressure in the spectrometer was -1 x 10 ' Torr. The spectra have been corrected for the Mg Ea3 4 satellite and the analyzer transmission efficiency. Clean surfaces were obtained by in situ scraping with a diamond file. Scraping was repeated frequently so that no change in the spectra was observed during the measurements. Figure 1 shows XPS spectra in the Cu 2p core-level region, where one can see that each of the j = 2 and components exhibits a two-peak structure as in previous reports. " The higher and lower binding-energy (satellite and main) peaks correspond, respectively, to 2p3d9 and 2p3d' final-state configurations (2p denotes a Cu 2p core hole), so that the separation between the two peaks is approximately the intra-atomic Coulomb energy between the core hole and the 3d electron U,d. The satellite structure arises not only from the coexistence of the d (Cu +) and d ' (Cu '+ ) configurations in the ground state but also from a ligand-to-d charge transfer in the...

Theoretical Model for Optical Spectrum and Structural Dynamics of La2-xSrxCuO4

“…Part of the misunderstanding regarding LM charge transfer spectra arises from a paper which shows the same satellite structure of La 2-x Sr x CuO 4 and YBa 2 Cu 3 O 6.7 as for other Cu 2+ systems (Fujimori, Takayama-Muromachi, Uchida, & Okai, 1987). The result is correctly interpreted as due to a more attractive Cu site after 2p-ionization, according to the Sawatzky-Larsson model (Larsson, 1976).…”

“…The Cu2p-hole is mainly screened by increased 3d-character in the occupied Cu3d-O2p molecular orbitals. The satellite/main line energy gap is equal to the LM energy gap, and it remains at about 4 eV after Cu2p ionization (Fujimori, Takayama-Muromachi, Uchida, & Okai, 1987). In any case Cu2p ionization is not involved in SC.…”

Mechanism of unconventional superconductivity induced by skyrmion excitations in two-dimensional strongly correlated electron systems

“…In the absence of carriers, the system is a charge-transfer insulator. [7] Due to the large charge-transfer gap, a localized magnetic moment is produced at each Cu site. These localized magnetic moments form antiferromagnetic long-range order below Néel temperature.…”

HOW STRONGLY ARE ELECTRONS CORRELATED IN THE HIGH-Tc SUPERCONDUCTING MATERIALS?

“…Since the discovery of high-T c superconducting oxides (HTSO) [1], there has been increasing evidence that electron correlations are sizeable in these materials [2][3][4][5][6][7][8]. Antiferromagnetic longrange order has been observed in La2CuO 4 [2,3].…”