V. Yushankhai

Bose-Einstein Condensation of Magnons in Cs~2CuCl~4

We report on results of specific heat measurements on single crystals of the frustrated quasi-2D spin-1/2 antiferromagnet Cs2CuCl4 (T(N)=0.595 K) in external magnetic fields B<12 T and for temperatures T>30 mK. Decreasing B from high fields leads to the closure of the field-induced gap in the magnon spectrum at a critical field Bc approximately = 8.51 T and a magnetic phase transition is clearly seen below Bc. In the vicinity of Bc, the phase transition boundary is well described by the power law Tc(B) proportional, variant (Bc-B)(1/phi), with the measured critical exponent phi approximately =1.5. These findings are interpreted as a Bose-Einstein condensation of magnons.

Metal-insulator transition in the quarter-filled frustrated checkerboard lattice

Abstract.We study the electronic structure and correlations in the geometricallynfrustrated two dimensional checkerboard lattice. In the large U limitnconsidered here we start from an extended Hubbard model of spinless fermions atnhalf-filling. We investigate the model within two distinct Green’s functionnapproaches: In the first approach a single-site representation decouplingnscheme is used that includes the effect of nearest neighbor charge fluctuations.nIn the second approach a cluster representation leading to an‘multiorbital’ model is investigated which includes intra-clusterncorrelations more rigorously and those between clusters on a mean field basis. It isndemonstrated that with increasing nearest-neighbor Coulomb interactionxa0Vnboth approaches lead to a metal-insulator transition with an associatedn‘Mott-Hubbard’ like gap caused byxa0V. Within the single site approach we alsonexplore the possibility of charge order. Furthermore we investigate thenevolution of the quasiparticle bands as funtion ofxa0V. n

Magnetic susceptibility in quasi one-dimensional Ba2V3O9: chain segmentation versus the staggered field effect

Abstract:A pronounced Curie-like upturn of the magnetic susceptibility xn(T) of the quasi one-dimensional spin chain compound Ba2V3O9 has been found recently [1]. Frequently this is taken as a signature for a staggered field mechanism due to the presence of g-factor anisotropy and Dzyaloshinskii-Moriya interaction. We calculate this contribution within a realistic structure of vanadium 3 d- and oxygen 2p-orbitals and conclude that this mechanism is far too small to explain experimental results. We propose that the Curie term is rather due to a segmentation of spin chains caused by broken magnetic bonds which leads to uncompensated S = ½ spins of segments with odd numbers of spins. Using the finite-temperature Lanczos method we calculate their effective moment and show that ˜ 1% of broken magnetic bonds is sufficient to reproduce the anomalous low-T behavior of x(T) in Ba2V3O9.

Bose-Einstein condensation of magnons in Cs2CuCl4: A dilute gas limit near the saturation magnetic field

Based on the realistic spin Hamiltonian for the frustrated quasi-two-dimensional spin-1/2 antiferromagnet Cs{sub 2}CuCl{sub 4}, a three-dimensional spin ordering in the applied magnetic field B near the saturation value B{sub c} is studied within the magnon Bose-Einstein condensation scenario. Using a hard-core boson formulation of the spin model, a strongly anisotropic magnon dispersion in Cs{sub 2}CuCl{sub 4} is calculated. In the dilute magnon limit near B{sub c}, the hard-core boson constraint results in an effective magnon interaction which is treated in the Hartree-Fock approximation. The critical temperature T{sub c} is calculated as a function of the magnetic field B and compared with the phase boundary T{sub c}(B) experimentally determined in Cs{sub 2}CuCl{sub 4} [Phys. Rev. Lett. 95, 127202 (2005)].

Ring exchange in transition metal oxides: a new manifestation in Li2CuO2?

Abstract Magnetic edge-sharing chain compound Li 2 CuO 2 is considered. Previously, noted contradiction between the standard model calculations and the experimental estimates of the intrachain spin coupling is resolved, at least qualitatively. The proposed scenario is based on the ring exchange mechanism explaining a ferromagnetic superexchange coupling between distant Cu spins.

Bose-Einstein Condensation of Magnons inCs2CuCl4

We report on results of specific heat measurements on single crystals of the frustrated quasi-2D spin-1/2 antiferromagnet Cs2CuCl4 (T(N)=0.595 K) in external magnetic fields B<12 T and for temperatures T>30 mK. Decreasing B from high fields leads to the closure of the field-induced gap in the magnon spectrum at a critical field Bc approximately = 8.51 T and a magnetic phase transition is clearly seen below Bc. In the vicinity of Bc, the phase transition boundary is well described by the power law Tc(B) proportional, variant (Bc-B)(1/phi), with the measured critical exponent phi approximately =1.5. These findings are interpreted as a Bose-Einstein condensation of magnons.

Cluster approach study of intersite electron correlations in pyrochlore and checkerboard lattices

To treat effects of electron correlations in geometrically frustrated pyrochlore and checkerboard lattices, an extended single-orbital Hubbard model with nearest-neighbor hopping

Quantum chemical study of Co3+ spin states in LaCoO3

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and Coulomb repulsion

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