Tsuyoshi Okubo

Z2-Vortex Ordering of the Triangular-Lattice Heisenberg Antiferromagnet

Ordering of the classical Heisenberg antiferromagnet on the triangular lattice is studied by means of a mean-field calculation, a scaling argument and a Monte Carlo simulation, with special attention to its vortex degree of freedom. The model exhibits a thermodynamic transition driven by the Z 2 -vortex binding-unbinding, at which various thermodynamic quantities exhibit an essential singularity. The low-temperature state is a “spin-gel” state with a long but finite spin correlation length where the ergodicity is broken topologically. Implications to recent experiments on triangular-lattice Heisenberg antiferromagnets are discussed.

Signature of a Z2 Vortex in the Dynamical Correlations of the Triangular-Lattice Heisenberg Antiferromagnet

Dynamical properties of the classical Heisenberg antiferromagnet on the triangular lattice are investigated by means of a spin-dynamics simulation and an analytical calculation. While the model was suggested to exhibit a topological transition driven by the Z 2 -vortex binding–unbinding, weakness of the associated thermodynamic singularity has made it difficult to observe the evidence of the Z 2 vortex experimentally so far, only some indirect support. Here, we show that the signature of the Z 2 -vortex excitation and the vortex-induced topological transition can be captured from the dynamical spin correlations. In particular, the dynamical spin structure factor exhibits a characteristic central peak around the Z 2 -vortex transition, and this central peak will be a fingerprint of the Z 2 -vortex excitation.

Various regimes of quantum behavior in anS=12Heisenberg antiferromagnetic chain with fourfold periodicity

We have succeeded in synthesizing single crystals of the verdazyl radical beta-2,6-Cl2-V [= beta-3-(2,6-dichlorophenyl)-1,5-diphenylverdazyl]. The ab initio MO calculation indicated the formation of an S = 1/2 Heisenberg antiferromagnetic chain with four-fold magnetic periodicity consisting of three types of exchange interactions.We have successfully explained the magnetic and thermodynamic properties based on the expected spin model by using the quantum Monte Carlo method. Furthermore, we revealed that the alternating and unique Ising ferromagnetic chains become effective in the specific field regions and observed a cooperative phenomenon caused by the magnetic order and quantum fluctuations. These results demonstrate that verdazyl radical could form unconventional spin model with interesting quantum behavior and provide a new way to study a variety of quantum spin systems.

Random packing of binary hard discs

Random dense packing of binary hard discs generated under the infinitesimal gravity protocol is investigated by a Monte Carlo simulation in the entire region of the size ratio and the concentration. When the size difference of the two species is small, the packing fraction of disc assembly becomes lower than that of the monodisperse system. Defining the adjacent neighbours of a disc through the radical tessellation, we show that the packing fraction can be related to the distance of adjacent discs and that the suppressed packing fraction is caused by an increase of the adjacent neighbours which are not seen in the monodisperse system.

Ground-state properties of Na2IrO3 determined from an ab initio Hamiltonian and its extensions containing Kitaev and extended Heisenberg interactions

We investigate the ground state properties of

Transport properties of metallic LiV2O4 single crystals—heavy mass Fermi liquid behavior

{\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}

Experimental Realization of a Quantum Pentagonal Lattice.

based on numerical calculations of the recently proposed ab initio Hamiltonian represented by Kitaev and extended Heisenberg interactions. To overcome the limitation posed by small tractable system sizes in the exact diagonalization study employed in a previous study [Y. Yamaji et al., Phys. Rev. Lett. 113, 107201 (2014)], we apply a two-dimensional density matrix renormalization group and an infinite-size tensor-network method. By calculating at much larger system sizes, we critically test the validity of the exact diagonalization results. The results consistently indicate that the ground state of

Field-induced incommensurate phase in the strong-rung spin ladder with ferromagnetic legs

{\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}

Snapshot Spectrum and Critical Phenomenon for Two-Dimensional Classical Spin Systems

is a magnetically ordered state with zigzag configuration in agreement with experimental observations and the previous diagonalization study. Applications of the two independent methods in addition to the exact diagonalization study further uncover a consistent and rich phase diagram near the zigzag phase beyond the accessibility of the exact diagonalization. For example, in the parameter space away from the ab initio value of

Cubic and noncubic multiple-qstates in the Heisenberg antiferromagnet on the pyrochlore lattice

{\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}