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Nakano, Hiroki*; Sakai, Toru
Journal of Physics; Conference Series, 868, p.012006_1 - 012006_10, 2017/07
Times Cited Count：3 Percentile：78.46(Physics, Applied)The spin gap issue of the kagome lattice antiferromagnet is investigated by the finite size scaling analysis applied for the energy spectrum of the rhombic clusters calculated by the numerical exact diagonalization. The conclusion is that the kagome lattice antiferromagnet is gapless, as well as the triangular lattice one.
Sakai, Toru; Nakano, Hiroki*
Physics Procedia, 75, p.369 - 375, 2015/12
Times Cited Count：1 Percentile：46.28(Physics, Applied)The S=1/2 three-leg quantum spin tube is investigated using the numerical diagonalization. The study indicated a new quantum phase transition between the 1/3 magnetization plateau phase and the plateauless one, with respect to the spin anisotropy. The phase diagram is also presented.
Sakai, Toru; Nakano, Hiroki*
Physics Procedia, 75, p.821 - 828, 2015/12
Times Cited Count：0 Percentile：0.04(Physics, Applied)The magnetization process of the S=1/2 kagome-lattice antiferromagnet is investigated using the numerical diagonalization up to 42-spin clusters. The critical exponent analysis confirms the unconventional magnetization behavior around the 1/3 magnetization plateau-like anomaly; the field derivative is infinite at the low-field side, while it is zero at the high-field side. We also confirm that a magnetization jump appears at 1/3 of the saturation magnetization in distorted kagome-lattice antiferromagnets.
Nakano, Hiroki*; Hasegawa, Yasumasa*; Sakai, Toru
Journal of the Physical Society of Japan, 84(11), p.114703_1 - 114703_6, 2015/11
Times Cited Count：34 Percentile：83.42(Physics, Multidisciplinary)The magnetization process of the spin-1/2 Heisenberg antiferromagnet on the distorted square-kagome lattice is investigated by the large-scale numerical diagonalization method. It is found that a novel magnetization jump appears at 1/3 of the saturation magnetization.
Sakai, Toru; Kasahara, Toshihiro*; Hijii, Keigo*; Ota, Hitoshi*; Nakano, Hiroki*
Synthetic Metals, 208, p.26 - 28, 2015/10
Times Cited Count：1 Percentile：4.78(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.
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：6.46(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.
Nakano, Hiroki*; Sakai, Toru
Journal of the Physical Society of Japan, 84(6), p.063705_1 - 063705_5, 2015/06
Times Cited Count：28 Percentile：80.43(Physics, Multidisciplinary)The magnetization process of the spin-S Heisenberg antiferromagnet on the kagome lattice is studied by the numerical-diagonalization method. Our numerical-diagonalization data for small finite-size clusters with S=1, 3/2, 2 and 5/2 suggest that a magnetization plateau appears at one-third of the height of the saturation in the magnetization process irrespective of S. We discuss the S dependences of the edge fields and the width of the plateau in comparison with recent results by real-space perturbation theory.
=1/2 Heisenberg antiferromagnet in two dimensionsNakano, Hiroki*; Sakai, Toru
Japanese Journal of Applied Physics, 54(3), p.030305_1 - 030305_4, 2015/03
To clarify the instability of the ferrimagnetism which is the fundamental magnetism of ferrite, numerical diagonalization study for the two-dimensional =1/2 Heisenberg antiferromagnet with frustration. We find that the ferrimagnetic ground state has the spontaneous magnetization in small frustration.
Nakano, Hiroki*; Sakai, Toru
Japanese Journal of Applied Physics, 54(3), p.030305_1 - 030305_4, 2015/03
Times Cited Count：10 Percentile：41.17(Physics, Applied)To clarify the instability of the ferrimagnetism which is the fundamental magnetism of ferrite, numerical-diagonalization study is carried out for the two-dimensional S=1/2 Heisenberg antiferromagnet with frustration. We find that the ferrimagnetic ground state has the spontaneous magnetization in small frustration.
Sakai, Toru; Nakano, Hiroki*
Journal of Physics; Conference Series, 568(4), p.042025_1 - 042025_6, 2014/12
Times Cited Count：0 Percentile：0.05(Physics, Applied)The kagome-lattice antiferromagnets have attracted a lot of interest in the field of the low-temperature physics. It exhibits some exotic field-induced phenomena, like a magnetization plateau, jump etc. Our previous large-scale yumerical exact diagonalization study on the S=1/2 kagome-lattice antiferromagnet revealed that a novel field-induced phenomenon, "the magnetization ramp", occurs at 1/3 of the saturation magnetization. It is characterized by different critical exponents between the lower-field and higher-field sides of the magnetization curve. In order to clarify unconventional properties around the 1/3 magnetization, we considered some extended lattice models; a distorted kagome lattice and distorted triangular lattice etc. Our numerical exact diagonalization study also indicates that the ground state of the kagome-lattice antiferromagnet is gapless quantum spin liquid.
=1/2 kagome-lattice Heisenberg antiferromagnet at one-third height of the saturationNakano, Hiroki*; Sakai, Toru
Journal of the Physical Society of Japan, 83(10), p.104710_1 - 104710_7, 2014/09
Times Cited Count：34 Percentile：83.02(Physics, Multidisciplinary)The magnetization process of the =1/2 Heisenberg antiferromagnet on the kagome lattice is studied by the numerical diagonalization method. We successfully obtain a new result of the magnetization process of a 42-site cluster in the entire range. Our analysis clarifies that the critical behavior around one-third of the height of the saturation is different from the typical behavior of the well-known magnetization plateau in two-dimensional systems.
Nakano, Hiroki*; Sakai, Toru; Hasegawa, Yasumasa*
Journal of the Physical Society of Japan, 83(8), p.084709_1 - 084709_7, 2014/08
Motivated by a recent finding of a spin-flop phenomenon in a system without anisotropy in spin space reported in the = 1/2 Heisenberg antiferromagnet on the square-kagome lattice, we study the = 1/2 Heisenberg antiferromagnets on two other lattice composed of vertex-sharing triangles using the numerical diagonalization method. One is a novel lattice including a 
shape with four teeth; the other is a distorted kagome lattice, which includes a shape with six teeth. We find in the magnetization processes of these systems that a magnetization jump accompanied by a spin-flop phenomenon occurs at the higher-field-side edge of the magnetization plateau at one-third the height of saturation.
Nakano, Hiroki*; Sakai, Toru; Hasegawa, Yasumasa*
Journal of the Physical Society of Japan, 83(8), p.084709_1 - 084709_7, 2014/08
Times Cited Count：24 Percentile：76.81(Physics, Multidisciplinary)Some two-dimensional frustrated antiferromagnets are investigated by the numerical diagonalization. As a result, it was found that several frustrated antiferromagnets exhibit a spin flop phenomenon even without anisotropy in the spin space.
Isoda, Makoto*; Nakano, Hiroki*; Sakai, Toru
Journal of the Physical Society of Japan, 83(8), p.084710_1 - 084710_7, 2014/08
Times Cited Count：21 Percentile：74.2(Physics, Multidisciplinary)The spin-1/2 Heisenberg antiferromagnet on the Cairo pentagon lattice is investigated by the numerical diagonalization. As a result, it was found that an orthogonal dimer, the disorder, and the ferrimagnetic states appear depending on the ratio of the two exchange interactions.
Nakano, Hiroki*; Sakai, Toru
JPS Conference Proceedings (Internet), 3, p.014003_1 - 014003_6, 2014/06
The magnetization processes of the S=1/2 isotropic Heisenberg quantum antiferromagnets on the kagome and triangular lattices are studied. We find that for the kagome-lattice antiferromagnet, the behavior of the derivative around the 1/3 height of the magnetization saturation is quite different from the cases of typical magnetization plateaux. This new phenomenon is called the "magnetization ramp". In order to clarify the difference between the magnetization ramp and the conventional magnetization plateau, we study a generalized anisotropic triangular-lattice model including the regular-triangular- and the kagome-lattice antiferromagnets in the parameter space. It revealed a quantum phase transition between the triangular- and kagome-lattices at 1/3 of the saturation magnetization.
= 1/2 Heisenberg antiferromagnet on the Cairo pentagon latticeNakano, Hiroki*; Isoda, Makoto*; Sakai, Toru
Journal of the Physical Society of Japan, 83(5), p.053702_1 - 053702_4, 2014/04
Times Cited Count：27 Percentile：79.12(Physics, Multidisciplinary)We study the = 1/2 Heisenberg antiferromagnet on the Cairo pentagon lattice by the numerical diagonalization. We tune the ratio of two antiferromagnetic interactions coming from two kinds of in equivalent sites in this lattice, examining the magnetization process; particular attention are focused on the one-third height of the saturation. We find that a quantum phase transition occurs at a specific value of the ratio and that a magnetization plateau appears in the vicinity of the transition.
Watanabe, Ken*; Kawamura, Hikaru*; Nakano, Hiroki*; Sakai, Toru
Journal of the Physical Society of Japan, 83(3), p.034714_1 - 034714_6, 2014/03
Times Cited Count：116 Percentile：96.08(Physics, Multidisciplinary)The spin-1/2 random Heisenberg antiferromagnet on the triangular lattice is investigated by the numerical diagonalization. As a result, in the case of a large randomness the system exhibits a quantum spin liquid like behavior at low temperatures.
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：76.5(Physics, Applied)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.
Sakai, Toru; Nakano, Hiroki*
Journal of the Korean Physical Society, 63(2), p.601 - 604, 2013/08
Times Cited Count：2 Percentile：19.96(Physics, Multidisciplinary)The kagome-lattice antiferromagnet is investigated using the numerical exact diagonalization up to 42-spin clusters. From our investigation of the magnetization process, we found a novel field-induced quantum phase transition, called the "magnetization ramp", at 1/3 of the saturation magnetization. The finite-size scaling analysis of the critical exponents indicated a significant difference of the magnetization ramp from the magnetization plateau observed in the triangular-lattice antiferromagnet.
lattice; A Lattice composed of vertex-sharing trianglesNakano, Hiroki*; Sakai, Toru
Journal of the Physical Society of Japan, 82(8), p.083709_1 - 083709_5, 2013/08
Times Cited Count：44 Percentile：85.32(Physics, Multidisciplinary)By means of the numerical diagonalization method, we study the S=1/2 Heisenberg antiferromagnet on a two-dimensional lattice composed of vertex-sharing triangles which we called the lattice. This lattice is similar to the kagome lattice but different from it. We examine the ground-state properties and the magnetization process of this model. We find that at the one-third height of the saturation, a magnetization plateau appears. The plateau is accompanied by a magnetization jump at the edge of the higher-field side.
