The ground-state properties of the spin-1 antiferromagnetic Heisenberg model on the cornersharing tetrahedra, pyrochlore lattice, is investigated. By breaking up each spin into a pair of 1/2-spins, the problem is reduced to the equivalent one of the spin-1/2 tetrahedral network in analogy with the valence bond solid state in one dimension. The twofold degeneracy of the spin-singlets of a tetrahedron is lifted by a Jahn-Teller mechanism, leading to a cubic to tetragonal structural transition. It is proposed that the present mechanism is responsible for the phase transition observed in the spin-1 spinel compounds ZnV2O4 and MgV2O4. PACS numbers: 75.10.Jm, 75.40.Cx, 75.80.+q Geometrically frustrated spin systems have been a fascinating subject to study since the Anderson's pioneering work on classical spins with a disordered ground state [1]. The pyrochlore and the spinel compounds, both including the three-dimensional (3D) tetrahedral network sharing the vertexes, are the typical examples of such systems in nature. As recent theoretical progress on pyrochlore spin systems, it may be mentioned that the spin-1/2 antiferromagnetic (AF) Heisenberg model is shown to have a spin liquid ground state [2] and that, even for ferromagnetic Ising model, nontrivial effect of frustration leads to a newly found spin ice ground state [3]. In this letter, we investigate the ground-state properties of the spin-1 AF Heisenberg model on the pyrochlore lattice.We follow a general strategy of constructing effective variational wave functions, which approximately describe the low energy states in the same spirit as the resonating valence bond (RVB) [4] or the one-dimensional (1D) valence bond solid (VBS) approaches [5]. These simple pictures are of great use to understand essential physics of the systems in more intuitive way. Our simple scenario, where the low energy effective Hilbert space is assumed to adiabatically continue to the manifold of the product wave functions of the twofold degenerate spin-singlets at each tetrahedron, leads to a spontaneous breakdown of the lattice symmetry. This new effect of the magnetoelastic interaction in the pyrochlore spin system gives a consistent picture with the recent experimental results on the insulating spin-1 spinel compounds ZnV 2 O 4 andFirst, let us introduce the AF Heisenberg model on the pyrochlore lattice, Fig. 1, in two different but equivalent representations where the interactions are defined for the sets of bonds and tetrahedra, respectively,where S is the spin-1 operator and < i, j > denotes a nearest neighbor pair. The index k, numbered from 1 to N 4 , specifies a tetrahedron and S tot k represents the sum of the four spins on the k-th tetrahedron. For the present spinel compounds, we expect that the relatively small spin value makes the isotropic limit a good starting point. On the other hand, it is well known that there are strong anisotropies for some rare earth titanates [3]. To tackle this problem, we develop an analogous method with that of the VBS state, which captur...

Effect of quantum fluctuations concerned with the orbital degrees of freedom is discussed for the model with SU(4) symmetry in one dimension. An effective Hamiltonian is derived from the orbitally degenerate Hubbard model at quarter filling. This model is equivalent to the Bethe soluble SU(4) exchange model. Quantum numbers of the ground state and the lowest branch of excitations are determined. The spin-spin correlation functions are obtained numerically by the density matrix renormalization group method. It shows a power-law decay with oscillations of the period of four sites. The period originates from the interference between the spin and orbital degrees of freedom. The exponent of the power-law decay estimated from the finite size data is consistent with the prediction by the conformal field theory.Comment: 5 pages, 5 figures, REVTeX, to appear in Phys. Rev.

A NaF/KF postdeposition treatment (PDT) has recently been employed to achieve new record efficiencies of Cu(In,Ga)Se (CIGSe) thin film solar cells. We have used a combination of depth-dependent soft and hard X-ray photoelectron spectroscopy as well as soft X-ray absorption and emission spectroscopy to gain detailed insight into the chemical structure of the CIGSe surface and how it is changed by different PDTs. Alkali-free CIGSe, NaF-PDT CIGSe, and NaF/KF-PDT CIGSe absorbers grown by low-temperature coevaporation have been interrogated. We find that the alkali-free and NaF-PDT CIGSe surfaces both display the well-known Cu-poor CIGSe chemical surface structure. The NaF/KF-PDT, however, leads to the formation of bilayer structure in which a K-In-Se species covers the CIGSe compound that in composition is identical to the chalcopyrite structure of the alkali-free and NaF-PDT absorber.

Traditional ultraviolet/soft X-ray angle-resolved photoemission spectroscopy (ARPES) may in some cases be too strongly influenced by surface effects to be a useful probe of bulk electronic structure. Going to hard X-ray photon energies and thus larger electron inelastic mean-free paths should provide a more accurate picture of bulk electronic structure. We present experimental data for hard X-ray ARPES (HARPES) at energies of 3.2 and 6.0 keV. The systems discussed are W, as a model transition-metal system to illustrate basic principles, and GaAs, as a technologically-relevant material to illustrate the potential broad applicability of this new technique. We have investigated the effects of photon wave vector on wave vector conservation, and assessed methods for the removal of phonon-associated smearing of features and photoelectron diffraction effects. The experimental results are compared to free-electron final-state model calculations and to more precise one-step photoemission theory including matrix element effects.

Nickel acetylacetonate was thermally decomposed in oleylamine under inert atmosphere. Nanocrystals of two cubic phases and a hexagonal phase appeared. The phases were identified using X-ray diffraction, laboratory X-ray photoelectron spectroscopy (XPS), and hard-X-ray photoelectron spectroscopy (HX-PES). The hexagonal phase was a nickel carbide, Ni3C, which had been often identified as a hexagonal close-packed metallic nickel. One cubic phase was a face-centered metallic nickel, and the other cubic phase was proposed as a novel cubic nickel carbide, which was characterized as an intermediate product of the carbidization of metallic face-centered cubic Ni to the hexagonal nickel carbide.

A detailed understanding of the origin of the magnetism in dilute magnetic semiconductors is crucial to their development for applications. Using hard X-ray angle-resolved photoemission (HARPES) at 3.2 keV, we investigate the bulk electronic structure of the prototypical dilute magnetic semiconductor Ga(0.97)Mn(0.03)As, and the reference undoped GaAs. The data are compared to theory based on the coherent potential approximation and fully relativistic one-step-model photoemission calculations including matrix-element effects. Distinct differences are found between angle-resolved, as well as angle-integrated, valence spectra of Ga(0.97)Mn(0.03)As and GaAs, and these are in good agreement with theory. Direct observation of Mn-induced states between the GaAs valence-band maximum and the Fermi level, centred about 400 meV below this level, as well as changes throughout the full valence-level energy range, indicates that ferromagnetism in Ga(1-x)Mn(x)As must be considered to arise from both p-d exchange and double exchange, thus providing a more unifying picture of this controversial material.

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