More than a decade has passed since the discovery of high temperature (HTc) superconductors. However, despite an intense research effort the mechanism responsible for these phenomena is still not fully understood. In order to test the basic assumptions of HTc superconductivity theories, it is important to have experimental evidence for the electronic structure of these materials. In many mainstream theories, like the single band Hubbard model and the t ­ J model, the relevant states in the (CuO2)2­ planes are of local singlet spin character (two holes with antiparallel spins, S = 0). In the single-band t ­ J model these states are often referred to as Zhang-Rice singlets [1]. Singlet states are contrary to what would normally be expected from Hund's first rule (i.e. triplet states, two holes with parallel spins, S = 1). Calculations have also shown that the photoemission spectrum can be directly related to the ground state electronic structure of the doped material. Consequently, one would like to measure the spin-resolved single-particle excitation spectrum to derive the energies of the different spin-dependent electronic states close to the Fermi level. The observation of the singlet states would provide strong support for the existence of the Zhang-Rice singlets in HTc cuprates [1].

Recently the feasibility of this type of study was demonstrated for CuO [2]. The experiment is based on spin-polarised resonant photoemission measurements made using circularly polarised X-rays. In these new experiments we have studied optimally doped (Tc = 91 K) Bi2Sr2CaCu2O8+ samples. The experiments were carried out on the helical undulator based beamline ID12B and the spectra were recorded using an electron analyser coupled to a spin polarimeter. The resonant photoemission measurements were achieved by tuning the photon energy to the peak of the Cu 2p3/2 (L3) photoabsorption white line (h = 931.5 eV).

In Figure 53a we show the spin integrated resonant photoemission spectra (full line) and in Figure 53b we show the spin polarisation given by the spin difference (using both helicities) normalised to the spin integrated spectrum. The spectrum results principally from Cu 3d8 final states and the peak at ~12 eV binding energy can be assigned to an atomic like 1G state. This is completely analogous to the previous work on CuO [2]. The spin polarisation of this peak is ~80%. This value is consistent with an analysis of the selection rules which give for a 3d9 ion, neglecting the small 3d spin-orbit interaction, 5/6 (83.3%) for pure singlet states and ­1/3* 5/6 (-27.8%) for triplet states. The strong dip in the polarisation at ~9 eV binding energy indicates a significant triplet contribution to the spectra at this energy. Assuming these model values for the polarisation of singlet and triplet states, we can separate the spectra into the two contributions. These are shown by the symbols in Figure 53a.

We are principally interested in the electronic states close to the Fermi level. Although the intensity is extremely low, it is clear that the spin polarisation increases dramatically around the Fermi level (see Figure 53b). This is the first evidence that the states close to the Fermi level are mostly of singlet character. In Figure 53c we show the region close to the Fermi level taken with good statistical quality. The spectrum is separated into its singlet and triplet components. It is quite clear that the intensity closest to the Fermi level and over more than 1 eV is dominated by singlet states. Consequently, we can conclude that for the Bi2Sr2CaCu2O8+ superconductor that the first ionisation state is of nearly pure singlet character. This provides compelling evidence for the existence and stability of the Zhang-Rice singlets in HTc cuprates [1].

References
[1] F.C. Zhang, T.M. Rice, Phys. Rev., B 37, 3759 (1988).
[2] L.H. Tjeng, B. Sinkovic, N.B. Brookes, J.B. Goedkoop, R. Hesper, E. Pellegrin, F.M.F. de Groot, S. Altieri, S.L. Hulbert, E. Shekel, G.A. Sawatzky, Phys. Rev. Lett., 78, 1126 (1997).

Authors
N.B. Brookes (a), G. Ghiringhelli (a), O. Tjernberg (a), L.H. Tjeng (b), T. Mizokawa (b) , T.W. Li (c), A.A. Menovsky (c).

(a) ESRF
(b) Solid State Physics Laboratory, Materials Science Centre, University of Groningen, (The Netherlands)
(c) Van der Waals-Zeeman Laboratory, University of Amsterdam (The Netherlands)