We discuss the role of orbital ordering in a two-dimensional triangular lattice. Usually, this lattice is considered as "frustrated." We show that a peculiar type of orbital ordering can remove this frustration if we allow for orbital degeneracy. We pay special attention to the d 2 case, for which we present mean field calculations for possible orbital orderings and an exact diagonalization study of a three-site cluster within a degenerate Hubbard model. The results support the possibility of an orbitally ordered singlet ground state. In particular, it provides new insight to the magnetic phase transition and the low temperature phase of LiVO 2 . [S0031-9007(97)02453-8] PACS numbers: 71.70.Gm, 71.30.+ h, 75.30.Et Transition metal (TM) compounds are well known for their large diversity and richness in physical phenomena. The flurry of activity in the field of high-T c superconductors has as yet not culminated in an understanding of their origin, but has opened up a Pandora's box of new effects in sometimes old "well understood" compounds: the colossal magnetoresistance in La 12x Ca x MnO 3 [1], the spiral spin fluctuations in the paramagnetic phase of V 2 O 3 [2], the temperature dependent high energy scale spectral weight transfer in V 2 O 3 [3], and the peculiar charge ordering in La 2 Sr 22x NiO 4 [4] and related materials. This richness of properties is due to the strongly correlated nature of the 3d states in these systems, often rendering them magnetic, and to the strong hybridization with the extended ligand valence states.Another very important aspect is the orbital degeneracy of open 3d shells. In a localized system such orbital degeneracy will be lifted at low temperatures in one way or another: this is the well-known Jahn-Teller effect [5]. In concentrated systems it often leads to structural phase transitions accompanied by a certain ordering of occupied orbitals. Such orbital ordering may be driven by other factors than lattice distortions, e.g., by exchange interactions [6]; of course, the lattice always follows to some extent. The strongest effects of this kind are observed in TM compounds with a twofold e g orbital degeneracy [e.g., those containing Mn 31 ͑d 4 ͒, Co 21 ͑d 7 ͒, or Cu 21 ͑d 9 ͒ in octahedral coordination]. Some wellknown examples are the Mn 31 spinels, LaMnO 3 , and KCuF 3 [6].In this Letter, we discuss the very interesting situation that may exist in systems with partially filled t 2g shells, in which orbital ordering can drive the system into a spin-singlet state without any long-range magnetic order. Specifically, we describe in this Letter a new class of orbitally ordered systems involving threefold degenerate orbitals in a triangular two-dimensional lattice. This model is relevant for a class of d 1 and d 2 TM compounds, including LiVO 2 , NaTiO 2 , and titanium halides, which are often regarded as model systems for the triangular lattice Heisenberg antiferromagnet. These materials contain quasi-two-dimensional hexagonal TM cation layers, the cation 3d states being split into...
We discuss the possibility of a Kondo like effect associated with H in metals resulting from the strong dependence of the H 1s orbital radius on the occupation number. We demonstrate that such a strong breathing property of the orbital radius, which translates directly into a strong occupation dependent hopping, results in the formation of local singlet-like bound states involving one electron on H and one on the surrounding metal orbitals. We also show that already at a mean field level an occupation dependent hopping integral leads to a substantial potential energy correction on hydrogen, and that the failure of band structure methods to incorporate this correction is responsible for the incorrect prediction of a metallic ground state for the YH3 switchable mirror compounds.
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We present temperature-dependent V-2p and O-1s x-ray-absorption spectra of LiVO 2. The aim of this study is to monitor changes in electronic structure on going through the phase transition. The spectral changes turn out to be very small: the V-3d-O-2p hybridization does not change considerably, and the symmetry of the V-ion ground state ͑high-spin 3 T 1) is retained. To explain our results, together with the anomalously low magnetic susceptibility below the transition temperature, we propose a model in which a three-orbital sublattice is formed in the V ͑111͒ planes, which results in a total singlet state. ͓S0163-1829͑97͒00623-1͔
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