We demonstrate that it is possible to obtain spin-resolved valence band spectra with a very high degree of spin polarization from antiferromagnetic transition metal materials if the excitation light is circularly polarized and has an energy close to the cation 2p 3͞2 (L 3 ) white line. We are able to unravel the different spin states in the single-particle excitation spectrum of CuO and show that the top of the valence band is of pure singlet character, which provides strong support for the existence and stability of Zhang-Rice singlets in high-T c superconductors. [S0031-9007(97)02343-0] PACS numbers: 74.25.Jb, 75.25. + z, To determine the nature and behavior of quasiparticles in strongly correlated transition metal oxides, including high-T c superconductors, it is highly desirable to have experimental information about the energies and band widths of the different spin and multiplet states in the single-particle excitation spectrum. Identification of these states, which have their meaning within the Anderson impurity model, could facilitate the modeling of the lowenergy excitations of the lattice in terms of those of the impurity. Knowledge of the character of the first ionization states is important for a better understanding of the behavior of the charge carriers in the doped materials, which could be quite intricate, especially when bound states occur with a compensated local spin contrary to that expected from Hund's first rule. There is a tendency for such to occur in charge transfer insulators, with perhaps the high-T c cuprates as the most famous of them. In fact, the basic assumption in main stream theories concerning high-T c superconductivity, like the single band Hubbard model [1] and the t-J model [2], is that the relevant states in the CuO 2 planes are of local singlet character, based on theoretical estimates [3][4][5][6][7]. Up to now, however, no direct experimental observation of such spin compensated states have been reported, mainly because spin-resolved photoemission [8][9][10][11], which is the obvious spectroscopic tool to use, cannot be applied due to the fact that most of the oxide materials, including the high-T c cuprates, are macroscopically not magnetic, so that all the spinresolved signals from the magnetically opposite cation sites cancel each other. For the same reason, magnetic circular dichroism experiments [12 -14] at the cation 2p and O 1s photoabsorption edges of hole doped oxides [15] and cuprates [16,17] would provide no information about the magnetic coupling between the cation and oxygen holes.In this paper we report the combined use of circularly polarized light and electron spin detection in our resonant photoemission study on CuO. Of all strongly correlated transition materials, CuO has the simplest atomic multiplet structure, and may therefore serve as a first test for this new type of spin-resolved photoemission technique applied to antiferromagnets. CuO may also serve as a model compound for high-T c cuprates, since in comparing it with the insulating parent compou...

We report the measurement of the local Ni 3d spin polarization, not only below but also above the Curie temperature ͑T C ͒, using the newly developed spin-resolved circularly polarized 2p ͑L 3 ͒ resonant photoemission technique. The experiment identifies the presence of 3d 8 singlets at high energies and 3d 8 triplets at low energies extending all the way to the Fermi energy, both below and above T C , showing that it is the orbital degeneracy of the 3d band and the Hund's rule splitting which is of utmost importance to understand Ni and other 3d ferromagnets. [S0031-9007(97) PACS numbers: 75.20.En, 75.25. + z, 79.60.Bm Itinerant ferromagnetism in late transition metals at finite temperatures has been for a long time a subject of scientific debate [1]. Neutron scattering experiments [2] and also photoemission measurements on Ni [3,4] at high temperatures have in particular stimulated much discussion on the validity of the simple Stoner-Wohlfarth meanfield theory [5] which predicts the collapse of exchange splitting above the Curie temperature ͑T C ͒ and the disappearance of local moments. Observations of different k dependent exchange splitting behavior with temperature gave rise not only to fluctuating band theory [6] which assumes the persistence of a certain degree of short-range magnetic order above T C , but also to the two-pole ansatz plus effective medium approach for the Hubbard model [7] which predicts the retention of local moments above T C together with a collapse of the exchange splitting. However, despite a large body of experimental data, a complete description of the finite temperature magnetism remains controversial, with some recent results on Ni supporting a Stoner-like behavior [8], fluctuating band theory [9], or suggesting even more complex behavior [10]. And as far as the electronic structure is concerned, most of this discussion can be reduced to the question as to whether or not the atomic Hund's rule correlations have survived the strong band formation. Such local exchange interactions, together with the suppression of charge fluctuations due to Coulomb interactions, may not account only for the failure of mean-field theories to calculate T C properly, but, more important, may also give a plausible explanation for the retention of local moments and short-range magnetic order above T C in late transition metals [11].To provide a better insight in these phenomena, we have investigated the local electronic structure of Ni and its temperature dependence, with special emphasis on the spin polarization of the atomiclike 3d orbitals. For this we have used the spin-resolved circularly polarized 2p ͑L 3 ͒ resonant photoemission technique, a newly developed spectroscopic tool with the unique property that it is capable of measuring the local 3d spin polarization independent of the orientation of the local moment, which is a necessary condition to study local moments above T C . We have been able to observe a strong spin polarization in the valence band of Ni, not only below but also above T ...

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Spin polarized Fe L 3 M 2,3 M 2,3 and L 3 M 2,3 V Auger-electron spectra were measured and calculated using on-resonance excitation energies of linearly polarized photons ͑at the 2p 3/2 threshold͒ as well as in the offresonance region. Both the intensity distribution and the spin polarization in the experimentally obtained Auger spectra are explained by means of the theoretical model taking into account spin-orbit splitting of core levels and exchange interaction of the core states with the polarized valence band. Different contributions to the Auger spin polarization of both CCC ͑Fe L 3 M 2,3 M 2,3 ) and CCV ͑Fe L 3 M 2,3 V) Auger spectra are discussed. Auger electron spin polarization is positive by transitions to the singlet final two-hole states and is increased with increasing core state-valence state exchange interaction, whereas for the triplet final states the opposite behavior is found.