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Journal Articles

Spin nanotubes with the ring exchange interactions

Sakai, Toru; Kasahara, Toshihiro*; Hijii, Keigo*; Ota, Hitoshi*; Nakano, Hiroki*

Synthetic Metals, 208, p.26 - 28, 2015/10

 Times Cited Count:1 Percentile:4.84(Materials Science, Multidisciplinary)

The S=1/2 three-leg spin nanotube with the ring exchange interaction at each plaquette is investigated using the numerical diagonalization of finite-size systems. The previous work suggested that the system without the ring exchange interaction has a spin excitation gap between the singlet ground state and the triplet excitation, because a spontaneous dimerization occurs in the ground state. The present study indicates that as the ring exchange increases, a quantum phase transition occurs at some critical value to another spin gap phase where the pattern of the dimerization is different from the initial one.

Journal Articles

Possible singlet-triplet transition of ESR in the kagome-lattice antiferromagnet

Sakai, Toru; Hijii, Keigo*; Okubo, Susumu*; Ota, Hitoshi*; Nakano, Hiroki*; Miyashita, Seiji*

Applied Magnetic Resonance, 46(9), p.997 - 1002, 2015/09

 Times Cited Count:1 Percentile:6.54(Physics, Atomic, Molecular & Chemical)

The S=1/2 kagome-lattice antiferromagnet is investigated by the numerical diagonalization of 18-spin finite-size cluster. The matrix elements proportional to the intensity of the singlet-triplet ESR transition are calculated in the presence of the Dzyaloshinsky-Moriya interaction. Some angle-dependent selection rules is also proposed.

Journal Articles

Ground state phase diagram of an $$S=frac{1}{2}$$ two-leg Heisenberg spin ladder system with negative four-spin interaction

Hijii, Keigo; Sakai, Toru

Physical Review B, 88(10), p.104403_1 - 104403_8, 2013/09

 Times Cited Count:5 Percentile:25.3(Materials Science, Multidisciplinary)

We investigate the ground-state phase diagram and critical properties of the $$S=frac{1}{2}$$ two-leg Heisenberg spin ladder system with a negative four-spin interaction. We numerically determine a phase boundary between the rung singlet phase and the columnar dimer (CD) phase by the phenomenological renormalization group method, and one between the CD phase and the Haldane phase by the twisted boundary condition method. We confirm that the phase transition between the CD phase and the Haldane phase is of second order and this universality class is described by the $$k$$ = 2 $$SU$$(2) Wess-Zumino-Novikov-Witten nonlinear $$sigma$$ model, calculating the central charge and scaling dimensions.

Oral presentation

An SU(2) symmetry of the spin-1 BQ model with single ion anisotropy

Hijii, Keigo

no journal, , 

We show that the spin-1 BQ model with single ion anisotropy has a nontrivial SU(2) symmetry. We can explain some degeneracies of excitation states on a ground state phase boundary between the XY2 phase and the Neel phase which were reported in previous numerical results.

Oral presentation

A Theoretical study of the ground state phase transition of S=1/2 4-leg spin ladder system

Hijii, Keigo; Sakai, Toru

no journal, , 

We investigate ground states of the S=1/2 4-leg frustrated spin ladder system. We find a sign of a quantum phase transition using twisted boundary condition method and the conformal field theory. We will report a finding about the quantum phase in the ground state.

Oral presentation

The Ground state phase diagram of S=1/2 3-leg spin ladder system

Hijii, Keigo; Sakai, Toru

no journal, , 

We investigate the ground state phase diagram and properties of the frustrated three-leg S=1/2 quantum spin ladder systems. We make the ground state phase diagram clear using the exact diagonalization. And we find out magnon bound states in the nontrivial intermediate phase.

Oral presentation

Ground state phase diagram of an S=1/2 frustrated 3-leg spin ladder system and properties of an intermediate phase

Hijii, Keigo; Sakai, Toru

no journal, , 

We investigate an S=1/2 frustrated 3-leg spin ladder system. Using a perturbation theory from a strong coupling limit and numerical diagonalization, we clarify the ground state phase diagram and find out a nontrivial intermediate phase. Furthermore we find out a possibility that magnon bound states appear.

Oral presentation

Ground state phase transitions of the S=1/2 spin ladder system with the four spin exchange

Hijii, Keigo; Sakai, Toru

no journal, , 

We investigate ground state phase transitions of the S=1/2 quantum spin ladder system with the four spin exchange. We find out a second order quantum phase transition using numerical calculations based on the exact diagonalization method. And we discussed the universality class of its quantum phase transition.

Oral presentation

Ground state phase diagram of S=1/2 spin ladder system with four spin exchange

Hijii, Keigo; Sakai, Toru

no journal, , 

We study a ground state phase diagram of the S=1/2 two leg Heisenberg spin ladder system with a four spin exchange. We find a quantum phase transition investigating energy level structures by a numerical exact diagonalization method and the phenomenological renormalization group method.

Oral presentation

Numerical Simulation Study on the Spin Nanotube with the ring exchange interaction

Kasahara, Toshihiro*; Hijii, Keigo*; Sakai, Toru

no journal, , 

The spin nanotube with the ring exchange interaction is investigated using the numerical diagonalization. As a result, it is theoretically found that another spin gap phase due to a different singlet dimer configuration from the initial ground state is realised for sufficiently large ring exchange interaction.

Oral presentation

Singlet-triplet transitions of ESR in gapped spin systems

Sakai, Toru; Hijii, Keigo*; Okubo, Susumu*; Ota, Hitoshi*

no journal, , 

The spin gap of the low-dimensional quantum spin systems and the strongly correlated electron systems is one of the interesting macroscopic quantum phenomena. Since the spin gap is the energy gap between the singlet ground state and the triplet excitation, the ESR direct transition is prohibited because of the spin conservation law. Thus the properties of the spin gap systems has been observed by some ESR transitions among the triplet excitations. Recently, however, the ESR direct transition corresponding to the spin gap was observed in several gapped spin systems. The mechanism of appearance of these prohibited ESR transition are supposed to be the Dzyaloshinsky-Moriya interaction and/or the staggered g-tensor. In order to identify the origin of the ESR direct transition, we gave some selection rules depending on the relative angle among the crystal axes of each material, the polarization of the microwave and the direction of external magnetic field.

Oral presentation

Quantum phase transition of the S=1/2 three-leg spin tube

Sakai, Toru; Kasahara, Toshihiro*; Nakano, Hiroki*; Hijii, Keigo*; Okunishi, Koichi*; Okamoto, Kiyomi*

no journal, , 

We report the result from the theoretical study based on the numerical diagonalization and the finite-size scaling analysis, on the quantum phase transition which occurs when the ratio of the rung coupling, the leg coupling and the ring exchange interactions is varied, in the S=1/2 three-leg spin tube.

Oral presentation

Numerical study on the dynamical susceptibility of the S=1/2 trimerized quantum spin chain

Hijii, Keigo*; Sakai, Toru; Miyashita, Seiji*; Ota, Hitoshi*

no journal, , 

We numerically investigated the dynamical susceptibility of the S=1/2 trimerized quantum spin chain. In order to solve the mechanism of the special feature, which is characteristic of the composite spins different from the single S=3/2, observed in the previous high frequency ESR measurement by Prof. Ajiro, we will report the numerical result of the external field dependence and the temperature dependence of the dynamical susceptibility.

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