Yasufumi Yamashita

Spin-driven jahn-teller distortion in a pyrochlore system

The ground-state properties of the spin-1 antiferromagnetic Heisenberg model on the corner-sharing tetrahedra, the pyrochlore lattice, are 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.

SU(4) SPIN-ORBIT CRITICAL STATE IN ONE DIMENSION

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.

Numerical Study of the One-Dimensional Spin-Orbit Coupled System with SU(2)⊗SU(2) Symmetry

We numerically study the SU(2)⊗SU(2) symmetric spin-orbit coupled model as a lower symmetric generalization of the SU(4) exchange model. On the symmetric line with respect to the spin and orbit, ou...We numerically study the SU(2)

Crossover phenomena in the one-dimensional SU(4) spin-orbit model under magnetic fields

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Dimensional Reduction and Odd-Frequency Pairing of the Checkerboard-Lattice Hubbard Model at 1/4-Filling

SU(2) symmetric spin-orbit coupled model as a lower symmetric generalization of the SU(4) exchange model. On the symmetric line with respect to the spin and orbit, our result shows the essentially singular gap formation in consistent with the analytic approach, which is different from the previous numerical calculation. Furthermore, we find new critical phases around the SU(4) point, surrounding the previously known gapless symmetric line. In these novel phases either spin or orbital excitations around momentum

SU(3) Dirac electrons in the15-depleted square-lattice Hubbard model at14filling

q=\pi

Gap formation for the one-dimensional SU(4) spin–orbital model

form massless continua which split from the excitations belonging to the

Lower symmetric generalization of the SU(4) spin–orbit model

[2^2]

Quantum Ising model coupled with conducting electrons

irreducible representation at the SU(4) point. On the critical symmetric line, the additional coupled spin-orbit excitations around

Valence-bond type approach for the pyrochlore spin system

q=\pi/2