Increase of O 2punoccupied electronic states within theabplane ofYBa2Cu3

Abstract

Temperature-dependent 0 K-edge absorption spectra of single-crystalline YBa2Cu306. 8 films were obtained across T"(77 K), using linearly polarized x rays. When the incident x rays were polarized parallel to the a(b) axis, absorption within -1.4 eV just above the Fermi level increased as the temperature was lowered from above T, (120 K) to below T, (68 K). The increase disappeared upon breaking the superconducting state by applying a dc current. When polarization was parallel to the c axis, lowering the temperature below T, had no effect on the absorption. The increase implies that, in the superconducting state, the 2pcr orbitals on the O(2) and O(3) sites in the Cu02 plane have more holes than in the normal conducting state.Doped holes play a key role in the mechanism of high-T, . superconductivity, because hole doping into the Cu02 plane induces superconductivity of cupreous oxides. ' Electron-energy-loss spectroscopy (EELS),x-rayabsorption spectroscopy (XAS), ' ' and photoelectron spectroscopy' ' indicate that hole doping creates 0 2p unoccupied electronic states above the Fermi level within the -2-eV (Refs. 14 and 15) charge-transfer gap.Wagener et al. ' have reported the increase of 0 2p holes at 60 K compared to 300 K for Y-Ba-Cu-0 and Bi-Sr-Ca-Cu-0 superconductors, using inverse photoelectron spectroscopy (IPS). They interpreted the increase as due to enhanced charge transfer from 0 to Cu at lower temperatures. Their IPS method, 's however, was unable to clarify whether the increase of 0 2p holes was correlated with the superconducting transition, and whether the increase was in the Cu02 plane. In this Rapid Comrnunication, we present experimental evidence that 0 2p unoccupied states of a single-crystalline YBa2Cu306.8 film increase anisotropically in the superconducting state compared with those in the normal conducting state. This suggests that 0 2pcr orbitals in the Cu02 plane have more holes as a direct consequence of the superconducting transition.We obtained the density of 0 2p unoccupied states by measuring 0 K-edge absorption spectra, using Auorescence yield (FY) detection, ' ' while monitoring the superconducting transition in situ. FY detection, ' "' ' which is bulk sensitive and probes to a depth of -0.2 pm, ' was indispensable, because the surface of superconducting YBa2Cu307y is unstable due to the oxygen deficiency and contamination.Thus, surface sensitive methods, such as total-secondary-electron-yield (TEY) detection and IPS, ' would leave ambiguity to their interpretation.Though EELS (Refs. 3-5) is also bulk sensitive, it is subject to electron radiation damage.Linearly polarized x rays from a synchrotron-radiation source was also essential for the present results, because the change of 0 K-edge FY spectrum due to the superconducting transition was only in the Cu02 plane.The experiments were performed using the soft-x-ray analysis system' at Branchline SA, Photon Factory, National Laboratory for High Energy Physics (KEK). We measured 0 K-edge FY spectra with a thin-window Si(Li) detector. ...

Electronic structure studies of the Bi2212 system by polarised X-ray absorption measurement

“…The change was reproducibly observed in our measurements on all the crystals. Such a change was also observed in the O K XAS spectra of the YBCO system by Hirai et al [19]. Its observation in the YBCO [19] and in the Bi2212 system [16] strongly indicates that the superconducting state is accompanied by an increase of the itinerant holes.…”

“…Such a change was also observed in the O K XAS spectra of the YBCO system by Hirai et al [19]. Its observation in the YBCO [19] and in the Bi2212 system [16] strongly indicates that the superconducting state is accompanied by an increase of the itinerant holes. In fact, as reported in our earlier paper [16], the increase in the itinerant holes is at the cost of a decrease in the covalent holes.…”

“…It was also motivated by the fact that Khomskii et al [17] had presented a picture of possible charge redistribution in the Cu-O planes below T c which was supported by inverse photoemission (IPES) measurements of Wagener et al [18] where an increase in the O 2p holes from the normal to the superconducting state was observed with a decrease in the Cu 3d holes in the bulk samples of YBaaCu30 7 (YBCO) and Bi2212 systems. The IPES study of Wagener et al [18] was further followed by polarised XAS measurements of Hirai et al [19] on YBCO thin films where they also concluded to an increase in the O 2px,y holes from the normal to the superconducting state.…”

Bernoulli potential in type-I and weak type-II superconductors: II. Surface dipole

“…For these layered materials it was predicted 14,15 that the superconducting transition induces a charge transfer from CuO 2 planes to charge reservoirs. This transfer caused merely by the pairing mechanism has been confirmed by bulkoriented experiments like the positron annihilation, 16 the x-ray absorption spectroscopy, 17 and the nuclear magnetic resonance. 18 Apparently, there are two groups of contradictory experimental results.…”

Polarized x-ray absorption spectroscopy study of the symmetry of unoccupied electronic states near the Fermi level in the system

“…Another report [35] in the case of the 1-2-3 system indicates an increase in the weight of the O 2p x,y orbital but no change in the weight of the O 2p z orbital when these go from the normal to the superconducting state. This would then be in contradiction to the theoretical predictions such as those in [24,34].…”