Physica B 312–313 (2002) 34–35

Probing the singlet character of the two-hole states in cuprate superconductors G. Ghiringhellia,b,*, N.B. Brookesb, L.H. Tjengc, T. Mizokawad, O. Tjernberge, P.G. Steenekenc, A.A. Menovskyf a

INFM, Politecnico di Milano, piazza Leonardo da Vinci 32, 20133 Milano, Italy b ESRF, BP 220, 38043 Grenoble, France c MSC, University of Groningen, Nijenborgh 4, 9747 AG Groningen, Netherlands d University of Tokyo, Tokyo 113-0033, Japan e Department of Physics, KTH, S-10044 Stockholm, Sweden f University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, Netherlands

Abstract Using spin-resolved resonant photoemission we have probed the singlet vs. triplet character of the two-hole state in the layered cuprates Bi2 Sr2 CaCu2 O8þd ; La2x Srx CuO4 and Sr2 CuO2 Cl2 : The combination of the photon circular polarization with the photoelectron spin detection gives access to the character of the photoemission final states, which correspond to the two-hole configurations localized at a ðCuO4 Þ site. In particular, the lowest energy state is found to have a very high singlet character in all the measured compounds. This can be considered as a strong indication of the existence and stability of the so-called Zhang–Rice singlets in the layered cuprates. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Zhang-Rice singlet; Cuprate; Photoemission

The discovery of superconductivity in the family of layered cuprates has stimulated a tremendous theoretical and experimental effort in order to formulate a new model able to explain this unpredicted effect. If in conventional superconductors the electron pairing (necessary for the superconductivity) is due to the electron– lattice interaction, what can be the pairing mechanism in the cuprate superconductors? Very early it was suggested that in the cuprate hightemperature superconductors the pairing can originate purely from the electron–electron correlation interactions. In particular, Zhang and Rice, within a 1-band Hubbard model [1], and Eskes and Sawatzky, within the cluster model or the Anderson impurity model [2], found that one important property of the coupled particles is their antiparallel mutual spin orientation. The two *Corresponding author. Dipartimento di Fisica, Politecnico di Milano, piazza Leonardo da Vinci 32, 20133 Milano, Italy. E-mail address: giacomo.ghiringhelli@fisi.polimi.it (G. Ghiringhelli).

electron holes form a zero spin particle, usually named Zhang–Rice singlet (ZRS). The existence of the ZRS is nowadays commonly accepted in the superconducting cuprates, but it has never been experimentally proved. Other and more sophisticated models (the t–J and derived models for example) are compatible with the ZRS and the interpretation of numerous spectroscopic experimental data is often based on ZRS. The only experimental evidence of ZRS existence was given up to now for the perfectly undoped compound CuO [3]. The possibility of an experimental investigation of the existence and stability of the ZRS comes from the very particular electronic structure of the cuprates. Nominally the copper is in the 3d9 configuration, and the oxygen is 2p6 in the undoped compounds: the only hole present in each ðCu2þ ÞðO2 Þ4 site is located at the 3d level of the copper ion. The lattice of spin 12 holes is organized in an anti-ferromagnetic two-dimensional arrangement: the undoped compound is a charge transfer insulator. Upon hole doping (the most common case among cuprate superconductors) the additional

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G. Ghiringhelli et al. / Physica B 312–313 (2002) 34–35

hole, according to calculations, resides mainly on the four O atoms, although it strongly couples to the other hole residing on the central Cu. If the two holes have an antiparallel spin orientation then we have a ZRS. The same effect of doping can indeed be obtained by the photoemission process. In addition, the resonant photoemission can lead to the same state. By using circularly polarized photons with the energy corresponding to the 2p3=2 -3d Cu transition (L3 resonance) we can introduce a further selectivity in the formation of the two hole state. The measurement of the spin polarization of the photoelectrons can then directly give the character of the two hole state: pure singlet for a 56 polarization and 5 pure triplet for a 18 polarization, when the photons impinge on a single crystal sample along the c axis. The first demonstration of the applicability of this spectroscopic technique (resonant photoemission with circular polarization of the photons and spin detection, ResSCP) was made on CuO [3]. We present here the first ResSCP measurements on some layered cuprates. In the figure (Fig. 1) we can see the spectra of the first 20 eV binding energy for three different compounds: the optimally hole doped BSCCO ðBi2 Sr2 CaCu2 O8þd ) and LSCO ðLa1:85 Sr0:15 CuO4 Þ and the undoped non-superconductor SCOC ðSr2 CuO2 Cl2 ). In the three cases, the main peak (around 12.2–13:0 eV binding energy) is clearly a pure singlet: the spin polarization is B80%; i.e. not far from the expected 56: This is shown by the decomposition of the spectra into the singlet and the triplet contributions. The most interesting spectral region is that closest to the Fermi level, which is shown in the insets in greater detail. BSCCO shows a very high spin polarization for at least one eV across the Fermi level. LSCO is rather similar to BSCCO, though the singlet character is probably less pure. SCOC shows the presence of a singlet character in the top part of the spectrum: the intensity at the Fermi level is zero because SCOC is an insulator. Those results show that the ZRS are rather pure and stable not only in the undoped compounds (CuO, SCOC), but also in the hole doped superconductors (BSCCO, LSCO). In fact, in both materials the singlet character is very high (> 70%) in a region of more than one eV around the Fermi level. Clearly in BSCCO the purity seems higher than in LSCO, whereas the separation of the singlet states from the first triplet states looks comparable. It is still not clear why in LSCO the spin polarization is lower than in BSCCO: it may depend on the sample quality, or on a weaker resonance in the Fermi level region of the spectrum, or on some more fundamental properties of the sample. A study of LSCO ResSCP as a function of doping could help in better understanding of the first results presented here. The presented results prove that in the p-type superconductors BSCCO and LSCO the first ionization states of the undoped sites have singlet character, which can be

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Fig. 1. The Cu L3 ResSCP spectra of three layered cuprates. From the spin polarization spectra we can calculate the singlet and triplet components of the spin-integrated spectra. In the insets the first ionization state region is put in evidence. In all cases the main peak around 12:5 eV binding energy corresponds to an atomic-like 1 G singlet.

considered as strong experimental evidence for the existence of ZRS in the cuprate superconductors.

References [1] F.C. Zhang, T.M. Rice, Phys. Rev. B 37 (1988) 3759. [2] H. Eskes, G. Sawatzky, Phys. Rev. Lett. 61 (1988) 1415; H. Eskes, G. Sawatzky, Phys. Rev. B 44 (1991) 9656. [3] L.H. Tjeng, et al., Phys. Rev. Lett. 78 (1997) 1126.