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Journal Articles

Magnetic structure and dispersion relation of the $$S$$=1/2 quasi-one-dimensional Ising-like antiferromagnet BaCo$$_{2}$$V$$_{2}$$O$$_{8}$$ in a transverse magnetic field

Matsuda, Masaaki*; Onishi, Hiroaki; Okutani, Akira*; Ma, J.*; Agrawal, H.*; Hong, T.*; Pajerowski, D. M.*; Copley, J. R. D.*; Okunishi, Koichi*; Mori, Michiyasu; et al.

Physical Review B, 96(2), p.024439_1 - 024439_8, 2017/07

 Times Cited Count:8 Percentile:50.2(Materials Science, Multidisciplinary)

Journal Articles

Exotic quantum phase transition of the spin nanotube

Sakai, Toru; Nakano, Hiroki*; Okunishi, Koichi*

Journal of Physics; Conference Series, 568(4), p.042024_1 - 042024_5, 2014/00

 Times Cited Count:3 Percentile:80.74

The spin nanotube is investigated by the numerical diagonalization and the level spectroscopy analysis. As a result, the quantum phase transition between the spin gap and gapless phases was revealed to occur with respect to the ratio of the rung and leg exchange interactions.

Journal Articles

Magnetization plateau of the quantum spin nanotube

Okamoto, Kiyomi*; Sato, Masahiro*; Okunishi, Koichi*; Sakai, Toru; Itoi, Chigaku*

Physica E, 43(3), p.769 - 772, 2011/01

 Times Cited Count:3 Percentile:17.85(Nanoscience & Nanotechnology)

The magnetization process of the 3-leg quantum spin nanotube is investigated by the numerical diagonalization of finite-size clusters and the density matrix renormalization group method. It is found that the magnetization plateau appears at 1/3 of the saturation magnetization for sufficiently large interchain coupling. In addition we find that in the presence of asymmetric lattice distortion, there are two different mechanisms of the magnetization plateau formation. We also successfully present a phase diagram of the magnetization plateau.

Journal Articles

Quantum spin nanotubes; Frustration, competing orders and criticalities

Sakai, Toru; Sato, Masahiro*; Okamoto, Kiyomi*; Okunishi, Koichi*; Itoi, Chigaku*

Journal of Physics; Condensed Matter, 22(40), p.403201_1 - 403201_13, 2010/10

 Times Cited Count:33 Percentile:40.7(Physics, Condensed Matter)

Recent developments of theoretical studies on spin nanotubes are reviewed, especially focusing on the S=1/2 three-leg spin tube. In contrast to the three-leg spin ladder, the tube has a spin gap in case of the regular-triangle unit cell when the rung interaction is sufficiently large. The effective theory based on the Hubbard Hamiltonian indicates a quantum phase transition to the gapless spin liquid due to the lattice distortion to an isosceles triangle. This is also supported by the numerical diagonalization and the density matrix renormalization group analyses. Furthermore, combining analytical and numerical approaches, we reveal several novel magnetic-field-induced phenomena: N$'e$el, dimer, chiral and/or inhomogeneous orders, new mechanism for the magnetization plateau formation, and others. The recently synthesized spin tube materials are also briefly introduced.

Journal Articles

Field induced exotic phenomena of the S=1/2 three-leg quantum spin nanotube

Sakai, Toru; Okunishi, Koichi*; Okamoto, Kiyomi*; Itoi, Chigaku*; Sato, Masahiro*

Journal of Low Temperature Physics, 159(1-2), p.55 - 59, 2010/04

 Times Cited Count:8 Percentile:40.08(Physics, Applied)

The numerical study on the S=1/2 three-leg spin nanotube using the exact diagonalization and the density matrix renormalization group method indicated that the 1/3 magnetization plateau appears for sufficently large rung interaction caused by two different mechanisms, depending on the asymmetry.

Journal Articles

Quantum phase transitions of the asymmetric three-leg spin tube

Sakai, Toru; Sato, Masahiro*; Okunishi, Koichi*; Otsuka, Yuichi*; Okamoto, Kiyomi*; Itoi, Chigaku*

Physical Review B, 78(18), p.184415_1 - 184415_11, 2008/11

 Times Cited Count:45 Percentile:84.16(Materials Science, Multidisciplinary)

We investigate quantum phase transitions of the S = $$frac{1}{2}$$ three-leg antiferromagnetic spin tube with asymmetric inter-chain (rung) exchange interactions. On the basis of the electron tube system, we propose a useful effective theory to give the global phase diagram of the asymmetric spin tube. In addition, using other effective theories we raise the reliability of the phase diagram. The density-matrix renormalization-group and the numerical diagonalization analyses show that the finite spin gap appears in a narrow region around the rung-symmetric line.

Journal Articles

Longitudinal spin density wave order in a quasi-1D Ising-like quantum antiferromagnet

Kimura, Shojiro*; Matsuda, Masaaki; Masuda, Takatsugu*; Hondo, Suguru*; Kaneko, Koji; Metoki, Naoto; Hagiwara, Masayuki*; Takeuchi, Tetsuya*; Okunishi, Koichi*; He, Z.*; et al.

Physical Review Letters, 101(20), p.207201_1 - 207201_4, 2008/11

 Times Cited Count:41 Percentile:84.31(Physics, Multidisciplinary)

From neutron diffraction measurements on a quasi-1D Ising-like Co$$^{2+}$$ spin compound BaCo$$_2$$V$$_2$$O$$_8$$, we observed an appearance of a novel type of incommensurate ordering in magnetic fields. This ordering is essentially different from the N$'e$el-type ordering, which is expected for the classical system, and the peculiar spin structure is caused by quantum fluctuation inherent in the quantum spin chain. A Tomonaga-Luttinger liquid (TLL) nature characteristic of the gapless quantum 1D system is responsible for the realization of the incommensurate ordering.

Oral presentation

Field-induced new phases in the S=1/2 three-leg spin tube

Sakai, Toru; Sato, Masahiro; Otsuka, Yuichi*; Okunishi, Koichi*; Okamoto, Kiyomi*

no journal, , 

The S=1/2 three-leg frustrated antiferromagnetic spin tube is investigated using the numerical diagonalization, density matrix renormalization group, and effective field theory techniques. Varing one of the three rung couplings, a quantum phase transition between the spin gap and gapless phases was predicted in our previous work. The present phenomenological renormalization approach gives a phase diagram of the spin gap. In addition we study the magnetization process of the regular triangle spin tube. Our present effective field theory between the lower critical field where the spin gap vanishes, and the upper critical field where the magnetization is saturated, predicts two field-induced new phases appear; the one has the vector chiral order and the other has an inhomogeneous distribution of the magnetization. These two phases coexist with the Tomonaga-Luttinger liquid phase. The prediction is confirmed by the numerical diagonalization and finite-size scaling analyses.

Oral presentation

Magnetization process of 3-leg spin tubes

Sakai, Toru; Otsuka, Yuichi*; Sato, Masahiro*; Okunishi, Koichi*; Okamoto, Kiyomi*

no journal, , 

The magnetization process of the 3-leg spin tube is investigated by some numerical simulations and field-theoretical approaches. As a result, we successfully predict the quantum phase transitions at which the 1/3 magnetization plateau appears, and at which the chiral order occurs. More quantitative analyses on these quantum phase transitions give the critical points and lead to a proposal of some new experiments to discover them.

Oral presentation

Quantum phase transition of spin nanotube

Sakai, Toru; Okunishi, Koichi*; Otsuka, Yuichi*; Okamoto, Kiyomi*; Sato, Masahiro*; Itoi, Chigaku*

no journal, , 

The spin nanotube was theoretically investigated by the numerical exact diagonalization and the density matrix renormalization group method. It was found that the three-leg spin tube exhibits a quantum phase transition between the spin gap and gapless phases, as a lattice distortion occurs. The superconductivity induced by the spin gap is also discussed.

Oral presentation

Quantum phase transition of the asymmetric three-leg spin tube

Sakai, Toru; Sato, Masahiro*; Okunishi, Koichi*; Otsuka, Yuichi*; Okamoto, Kiyomi*; Itoi, Chigaku*

no journal, , 

We investigate quantum phase transitions of the S=1/2 three-leg antiferromagnetic spin tube with asymmetric inter-chain (rung) exchange interactions. On the basis of the electron tube system, we propose a useful effective theory to give the global phase diagram of the asymmetric spin tube. In addition, using other effective theories we raise the reliability of the phase diagram. The density-matrix renormalization-group and the numerical diagonalization analyses show that the finite spin gap appears in a narrow region around the rung-symmetric line.

Oral presentation

Quantum phase transitions of the three-leg spin tub

Sakai, Toru; Okunishi, Koichi*; Okamoto, Kiyomi*

no journal, , 

The S=1/2 three-leg quantum spin tube is one of strongly frustrated low-dimensional magnets. Using the numerical diagonalization and the DMRG calculation, we revealed new quantum phase transitions induced by an asymmetric interchain interactions. We also found some anomalous field-induced phenomena; the magnetization plateau, cusp, and chiral order in spin liquid phases.

Oral presentation

1/3 magnetization plateau of the 3-leg spin tube

Sakai, Toru; Okunishi, Koichi*; Okamoto, Kiyomi*; Itoi, Chigaku*; Sato, Masahiro*

no journal, , 

no abstracts in English

Oral presentation

Field-induced quantum phase transitions of the asymmetric three-leg spin tube

Sakai, Toru; Sato, Masahiro*; Okunishi, Koichi*; Okamoto, Kiyomi*; Itoi, Chigaku*

no journal, , 

The S=1/2 three-leg spin tube has a spin gap due to the strong frustration. Using the numerical dyagonalization and the density matrix renormalization group (DMRG) calculation, we revealed new quantum phase transitions between the spin-gap and gapless phases induced by an asymmetric interchain interactions. Under high magnetic field this system also exhibits some interesting phenomena. The same numerical analysis on the magnetization process indicates that the 1/3 magnetization plateau appears for sufficiently large rung interaction caused by two different mechanisms, depending on the asymmetry.

Oral presentation

Coexistence of the staggered magnetization, dimerization, and chiral order in the magnetization plateau of the spin tube

Sakai, Toru; Sato, Masahiro*; Okunishi, Koichi*; Okamoto, Kiyomi*; Itoi, Chigaku*

no journal, , 

no abstracts in English

Oral presentation

Quantum phase transitions of the spin nanotubes

Sakai, Toru; Sato, Masahiro*; Okunishi, Koichi*; Okamoto, Kiyomi*; Itoi, Chigaku*

no journal, , 

Recently some quantum spin systems on tube lattices, so called spin nanotubes, have been synthesized. They are expected to be interesting low-dimensional systems like the carbon nanotubes. As the first step of theoretical study on the spin nanotube, we investigate the S=1/2 three-leg spin tube, which is the simplest one, using the density matrix renormalization group (DMRG) and the numerical exact diagonalization (ED), combined with a precise finite-size scaling analysis named level spectroscopy.

Oral presentation

Magnetization ramp and XXZ anisotropy of kagome lattice antiferromagnets

Sakai, Toru; Nakano, Hiroki*; Isoda, Makoto*; Okunishi, Koichi*

no journal, , 

The previous study indicated that a magnetization plateau-like behavior at 1/3 of saturation magnetization is a new phenomena, so-called a magnetization ramp. In the present work we investigate a change of the anomalous behavior under the XXZ anisotropy. As a result, we found that as the XXZ anisotropy is varied from the Ising and XY sides, the magnetization plateau vanishes at a critical point.

Oral presentation

Exotic quantum phase transitions in the S=1/2 three-leg spin tube

Sakai, Toru; Sato, Masahiro*; Okunishi, Koichi*; Okamoto, Kiyomi*; Itoi, Chigaku*

no journal, , 

Recently some quantum spin systems on tube lattices, so called spin nanotubes, have been synthesized. They are expected to be interesting low-dimensional systems like the carbon nanotubes. As the first step of theoretical study on the spin nanotube, we investigate the S=1/2 three-leg spin tube, which is the simplest one, using the density matrix renormalization group (DMRG) and the numerical exact diagonalization (ED), combined with a finite-size scaling analysis. Particularly, we introduce the lattice distortion from the regular triangle unit to the isosceles one. We revealed that the S=1/2 isosceles triangle spin tube exhibits various exotic quantum phase transitions.

Oral presentation

Density matrix renormalization group and numerical diagonalization study on the quantum spin nanotube in magnetic field

Sakai, Toru; Sato, Masahiro*; Okunishi, Koichi*; Okamoto, Kiyomi*; Itoi, Chigaku*

no journal, , 

The spin tube has attracted a lot of interest as a magnet with a nanotube structure. Various applicable functions of the spin tube are expected to be induced by high magnetic field. The S=1/2 three-leg spin tube has a spin gap due to the strong frustration. Using the numerical dyagonalization and the density matrix renormalization group (DMRG) calculation, we revealed new quantum phase transitions between the spin-gap and gapless phases induced by an asymmetric interchain interactions. Under high magnetic field this system also exhibits some interesting phenomena. The same numerical analysis on the magnetization process indicates that the 1/3 magnetization plateau appears for sufficiently large rung interaction caused by two different mechanisms, depending on the asymmetry. A field induced symmetry breaking accompanied by the chiral order is also revealed to occur.

Oral presentation

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

Sakai, Toru; Nakano, Hiroki*; Okunishi, Koichi*

no journal, , 

The S=1/2 three-leg spin tube is investigated using the large-scale numerical diagonalization up to 42 spins and the level spectroscopy method. The critical ratio of the rung and leg exchange interactions where a quantum phase transition occurs between the gapless and gapped phases is estimated precisely.

22 (Records 1-20 displayed on this page)