坂井 徹

In recent studies we find that three kinds of magnetization plateaux appear at one-third of the saturation magnetization in the ground-state magnetization curve of an anisotropic (S, S') = (1, 2) spin-alternating chain, in which S = 1 and S' = 2 spins with on-site anisotropies are alternatingly arranged and coupled antiferromagnetically. One of these plateaux is a quantum plateau, while the other two are classical plateaux. We here show that the first-order phase transition between the two classical one-third plateau states occurs when the on-site anisotropy for the S = 1 spin is of easy-axis type and its magnitude is sufficiently large. We numerically determine the phase transition line for this first-order transition and briefly discuss the origin of its occurrence. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The spin ladder system with the easy-axis anisotropy in magnetic field is theoretically investigated using the numerically exact diagonalization, the density matrix renormalization group (DMRG) and the finite-size scaling analysis. It is found that the field-induced nematic phase appears above the critical field. In addition, there exists another critical field at which the quantum phase transition from the nematic phase to the conventional field-induced Tomonaga-Luttinger liquid (TLL) phase occurs. The phase diagram on the coupling anisotropy versus magnetization plane is given by the numerical diagonalization. Furthermore, a typical magnetization curve calculated by the DMRG is presented. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The distorted diamond chain has attracted a lot of interest, since the compound Cu-3(CO3)(2)(OH)(2), so-called azurite, was revealed to be a good candidate for this system. Although several theoretical studies were performed, the strong anisotropy observed in the magnetization measurement of azurite has not been explained very well. To solve the mechanism of this anisotropy, we introduce the Dzyaloshinsky-Moriya (DM) interaction suitable for the crystal symmetry of azurite. The numerical diagonalization study on this realistic model indicates that the anisotropy in the experimental magnetization curve would be reproduced with sufficiently large DM interaction. We also propose a mechanism of the field-induced incommensurate order, which was observed above the 1/3 magnetization plateau by the NMR measurement.

We numerically investigate the magnetization process of a frustrated S = 1 chain with antiferromagnetic nearest-neighbor (nn) and next-nearest-neighbor (nnn) exchange interactions, which compete with each other, and also with uniaxial single-ion-type anisotropy energies. Special attention is paid to the half magnetization plateau which appears at the half of the saturation magnetization in the ground-state magnetization curve. We determine the parameter region where this plateau appears by employing the level spectroscopy method, and find that the plateau appears even in the absence of the single-ion-type anisotropy, if the ratio of the nnn to the nn interaction constant is in a specified region.

We investigate the elementary excitations and magnetization dynamics of the recently synthesized single-chain magnets, which consist of Mn and Ni ions, and some related systems, using the numerical exact diagonalization of finite-size clusters and the density matrix renormalization group method. It is found that, as the easy-axis anisotropy D increases, a crossover of the lowest-lying elementary excitation should exist between the spin wave and Ising-like excitations. For the realistic trimer single-chain magnet, we estimate several crossover temperatures. In addition we predict that a magnetization plateau at 2/3 of the saturation magnetization would appear in the (s, S) = (1, 2) spin-alternating chain. The phase diagram of the plateau is also presented.

We study the low-energy excitation of the quantum spin tube with the triangular lattice structure, using density matrix renormalization group. Taking account of the edge spin effect, we particularly investigate the spin gap behavior and the low-field magnetization curve near the quantum phase transition point in contrast with the usual free boundary condition. We then find that the bulk behavior of the spin tube can be extracted easier for the single spin termination.

We have measured the specific heat of the S = 1/2 alternating-bond Heisenberg antiferromagnetic chain compound pentafluorophenyl nitronyl nitroxide in magnetic fields using a single crystal and powder. A sharp peak due to field-induced magnetic ordering (FIMO) is observed in both samples. The H-T phase boundary of the FIMO of the single crystal is symmetric with respect to the central field of the gapless field region H-C1 <= H <= H-C2, whereas it is distorted for the powder whose ordering temperatures are lower. We discuss possibility that an effective pressure caused by mixing the powder with grease, which is reported for various organic compounds, plays an important role for the distorted phase boundary. An analysis employing calculations based on the finite temperature density matrix renormalization group suggests that the pressure-induced frustration enhances incommensurate spin correlation leading to the distorted phase boundary for the powder.

High-frequency ESR measurements have been performed on the S=12 quasi-one-dimensional antiferromagnet Cu2Cl4 center dot H8C4SO2 in the frequency range up to 360 GHz and the field range up to 12 T. Reflecting the small alternation of exchange interaction along the chain direction, very peculiar temperature dependences of g value and linewidth are observed. Moreover, we have observed the direct transition between the singlet ground state and triplet excited states in Cu2Cl4 center dot H8C4SO2 at T=1.7 K and determined the excitation gap (for c axis) Delta/k(B)=7.9 +/- 0.4 K. Considering the observed anisotropic nature of the direct transition and applying the selection rules of the direct transition proposed by Sakai [J. Phys. Soc. Jpn. 72, Suppl. B 53 (2003)], we have come to the conclusion that the direct transition is caused by the Dzyaloshinsky-Moriya interaction and are able to determine the D vector in Cu2Cl4 center dot H8C4SO2 as D-a(*):D-b:D-c=1.0:0.0:2.3 +/- 0.1.

We study polarization effects of two-component atomic Fermi gases loaded on three-leg triangular optical lattice by using the density-matrix renormalization-group method. Concentrating on a case in which a Mott phase coexists with metallic ones, we observe plateulike structures in the Mott phase polarization as a function of the input population imbalance. The structures are clearly analogous to the magnetization plateau states studied in the triangular Heisenberg lattice model under the magnetic field. In fact, we find that spin patterns peculiar to the plateau states emerge inside the Mott phase. This result suggests that the Mott phase is an excellent stage in systematically investigating frustrated spin systems.

The magnetization process of the distorted diamond chain is theoretically investigated using the numerical exact diagonalization and the density matrix renormalization group calculation. It is found that for some specified parameters the ground-state critical behavior is dominated by the incommensurate spin correlation parallel to the external magnetic field, around the 2/3 magnetization plateau. In the presence of interchain interaction it should be an incommensurate long-range order which possibly leads to a supersolid if coexisting with the usual transverse antiferromagnetic long-range order.

The magnetization process of the S = 1/2 spin ladder system with ferromagnetic rung exchange interaction is investigated by the numerical exact diagonalization of finite-size clusters. It is found that in the presence of an easy-axis anisotropy the system exhibits a two-step field induced phase transition corresponding to the spin flop. Due to large quantum fluctuation, each step is a second-order transition and the second step occurs between two different Tomonaga-Luttinger liquid phases. Some phase diagrams are also presented.

Employing various numerical methods, we determine the ground-state phase diagram of an (S, S') = (1, 2) spin-alternating chain with antiferromagnetic nearest-neighboring exchange interactions and uniaxial single-ion anisotropies. The resulting phase diagram consists of eight kinds of phases including two phases which accompany the spontaneous breaking of the translational symmetry and a ferrimagnetic phase in which the ground-state magnetization varies continuosly with the uniaxial single-ion anisotropy constants for the S = 1 and S' = 2 spinds. The appearance of these three phases is attributed to the competition between the uniaxial single-ion anisotropies of both spins.

We investigate quantum phase transitions of the S=1/2 three-leg antiferromagnetic spin tube with asymmetric interchain (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, in contrast to a recent paper by Nishimoto and Arikawa, [Phys. Rev. B 78, 054421 (2008)]. The numerical calculations indicate that this global phase diagram obtained by use of the effective theories is qualitatively correct. In the gapless phase on the phase diagram, the numerical data are fitted by a finite-size scaling in the c=1 conformal field theory. We argue that all the phase transitions between the gapful and gapless phases belong to the Berezinskii-Kosterlitz-Thouless universality class.

Using the numerical diagonalization of the finite-cluster t-J model, we investigate the mechanism of the charge stripes based on the ring exchange interaction. We calculate the many-hole correlation functions related with the two-types of charge stripes; the vertical and diagonal types of the charge stripe. As a result, we present a phase diagram including the two stripe phases and a phase separation phase. (C) 2008 Elsevier B.V. All rights reserved.

The phonon effect of the in-plane oxygen breathing vibration in the high-Tc cuprates is investigated by the numerical diagonalization of an extended t-J-Holstein model, including the modulation of t. As a result, it is found that if the modulation of t due to the phonon is sufficiently large, the breathing mode possibly stabilizes the superconductivity.

The Lanczos algorithm is applied to calculate some finite-temperature physical quantities, as well as the ground-state ones. We introduce the finite-temperature Lanczos algorithm and apply it for some current problems in the low-dimensional strongly correlated electron systems; the charge stripe, the pseudogap and phonon-induced anomalous ARPES spectrum in the high-Tc superconducting cuprates.

Employing numerical methods, we study the ground-state magnetization curve of a (S, S′) (1, 2) spin-alternating chain with uniaxial single-ion anisotropies, where S = 1 and S′ = 2 spins are alternating and coupled antiferromagnetically. Special attention is paid to the magnetization plateau, the one-third plateau, which appears at one third of the saturation magnetization in the ground-state magnetization curve. We find that three kinds of one-third plateaux appear, one of which has a quantum nature, while the remaining two have a classical (Ising) nature. We determine the one-third plateau phase diagram on the D 2 versus D 1 plane, where D 1 (D 2) is the ratio of the single-ion anisotropy constant for the S = 1 (S′ = 2) spin to the exchange interaction constant. This phase diagram consists of a phase boundary line between the quantum and classical one-third plateau states as well as three phase boundary lines between the one-third plateau and plateauless states. We emphasize that there is no phase transition between two classical one-third plateau states and both states are regarded as a single phase. Furthermore, the region of the one-third plateauless phase is divided into two parts, that is, the region in which the one-third magnetization state is not realized at any magnetic field as the ground state, and the region in which it is realized but no one-third plateau appears in the ground-state magnetization curve. ©2007 The Physical Society of Japan.

Recently a ferrimagnetic quantum spin chain consisting of Ni and Mn ions has been synthesized to create a new type of magnet with a long life time, based on the Glauber's slow dynamics. Its magnetic properties are well described by the (s, S) = (1, 2) spin alternating chain model, with single-ion anisotropies D-1 and D-2 at Ni and Mn sites, respectively. Thus we investigate the magnetization process of this model, using the numerical exact diagonalization and finite-size scaling technique. It predicts a new field-induced quantum phase transition between two different Tomonaga-Luttinger liquid phases; TLL1 and TLL2 for some negative values of D-1 and D-2. This transition corresponds to a quantum version of the spin flop. Some typical phase diagrams are presented. (C) 2007 Elsevier B.V. All rights reserved.

In the magnetization process of the S = 1/ 2 spin ladder system there are two critical magnetic fields H-c1 and H-c2. The spin gap vanishes at H-c1 and the magnetization is saturated at H-c2. Usually the commensurate antiferromagnetic spin correlation perpendicular to the external field H is dominant and the canted Neel order would be stabilized by interladder interaction for H-c1 H < H-c2. The present theoretical study based on numerical diagonalization indicated the following interesting results. In the presence of the next-nearest-neighbour interaction, the incommensurate spin correlation parallel to H would be possibly stronger than the commensurate one for some intermediate external field. As a result, with interladder interaction, another field- induced transition would occur from the usual canted N eel order to the incommensurate one. The new field- induced transition is based on the so- called.- inversion, namely a change from eta(z) > eta(x) to eta(z) < eta(x), where eta(z) and eta(x) are the critical exponents of the parallel and perpendicular spin correlation functions, respectively. Some phase diagrams including the eta-inversion and the magnetization plateau are presented.

An S = (1)/(2) three-leg antiferromagnetic spin nanotube is investigated using the DMRG method and the level spectroscopy analysis combined with the numerical exact diagonalization. It is known that the system has a spin gap, when the three rung couplings are equivalent. In this paper, we see that as one of the three rung exchanges varies, a quantum phase transition occurs at a critical value of the varied rung exchange, and then the system is in a gapless Tomonaga-Luttinger-liquid phase. A ground-state phase diagram containing this phase transition is obtained by the phenomenological renormalization. (c) 2006 Published by Elsevier B.V.

Using numerical methods, we study the finite-field ground state of an S = 1 zigzag ladder. Special attention is paid to the magnetization plateau which appears at a half of the saturation magnetization in the ground-state magnetization curve. (c) 2006 Elsevier B.V. All rights reserved.

We investigate the ground state phase diagram of the S = (1)/(2) distorted diamond type spin chain with the XXZ anisotropy by 2 use of the degenerate perturbation theory and the level spectroscopy analysis of the numerical diagonalization data for the finite-size systems. As the XXZ anisotropy Delta is increased from 0 to infinity, there appears the reentrant quantum phase transition TL-SF double right arrow Neel double right arrow TL-SF double right arrow ferrimagnetic, when the coupling parameters have suitable values. Here TL-SF denotes the Tomonaga-Luttinger spin-fluid state. Further we find that four Berezinskii-Kosterlitz-Thouless lines meet together at one point in the phase diagram on the Delta versus J(3)/J(2) plane. These interesting phenomena are attributed to the interplay between the trimer nature, frustrating interactions and the XXZ anisotropy. Similar phenomena can be found in several systems having these three keywords. (c) 2006 Elsevier B.V. All rights reserved.

The low-energy physics of three-leg frustrated antiferromagnetic spin-S tubes in the vicinity of the upper critical field are studied. Utilizing the effective field theory based on the spin-wave approximation, we argue that in the intermediate-interchain-coupling regime, the ground state exhibits a vector chiral order or an inhomogeneous magnetization for the interchain (rung) direction and the low-energy excitations are described by a one-component Tomonaga-Luttinger liquid (TLL). In both chiral and inhomogeneous phases, the Z(2) parity symmetry along the rung direction is spontaneously broken. It is also predicted that a two-component TLL appears and all the symmetries are restored in the strong-rung-coupling case.

We report the combined measurements of the dc susceptibility chi(0), the ac susceptibility chi('), and the NMR relaxation rate T-1(-1) for the molecular-based heterometallic single-chain magnet [Mn(saltmen)](2)[Ni(pao)(2)(py)(2)](PF6)(2). At low temperatures, this system is well described by a one-dimensional array of effective spin S=3 chains comprising the Mn-III-Ni-II-Mn-III trimers and treated as the S=3 Ising chain with the single-ion term (Blume-Capel model). Using the exact solution of the model and based on the picture that the random motion of the local domain walls dominates the low-temperature spin dynamics, we succeeded in reproducing the experimental results of the dc susceptibility chi(0), the ac susceptibility chi('), and the F-19-NMR relaxation rate T-1(-1) in a consistent manner.

Collective magnetic excitation of the single-chain magnet (SCM) has been observed using high-frequency electron spin resonance (ESR). The observed ESR mode is attributed to the spin wave excitation with strong magnetic anisotropy where its temperature dependence shows the continuous evolution of the short-range spin correlation inside the chain. The uniaxial anisotropy constants and the exchange couplings of the related SCM have been quantitatively deduced from our theoretical analysis. Moreover, the relaxation mechanism of the SCM is found to originate from a collective reversal of spins via the transverse exchange couplings between the molecular units.

Cu-3(CO3)(2)(OH)(2) is a new compound consisting of diamond shaped spin chains. Magnetization measurements revealed a plateau at (3)\(1) of the saturated moment. This feature together with the magnetic excitations observed by high-field electron spin resonance (ESR) measurements can be understood as a model of coupled spin dimer and monomer system. We present a theory for the magnetic excitations of this system based on an extended bond-operator theory. and discuss the field dependence of the observed excitation modes by the ESR measurements. (c) 2006 Elsevier B.V. All rights reserved.

We study an incommensurate long-range order induced by an external magnetic field in a quasi-one-dimensional bond-alternating spin system, F5PNN, focusing on the role of the frustrating interaction which can be enhanced by a high-pressure effect. On the basis of the density matrix renormalization group analysis of a microscopic model for F5PNN, we present several H-T phase diagrams for typical parameters of the frustrating next-nearest-neighbour coupling and the interchain interaction, and then discuss how the field-induced incommensurate order develops by the frustration effect in such phase diagrams. A magnetization plateau at half the saturation moment is also mentioned.

A Reply to the Comment by Bo Gu and Gang Su. © 2006 The American Physical Society.

Using mainly numerical methods, we investigate the ground-state magnetization curve of a mixed spin chain with uniaxial single-ion anisotropies, where S = 1 and S = 2 spins are alternating and coupled antiferromagnetically. Special attention is paid to the magnetization plateau, the 1/3-plateau, which appears at one third of the saturation magnetization in the magnetization curve. It is found that there appear three kinds of 1/3-plateaux, one of which has a quantum nature, while the remaining two have a classical (Ising) nature.

The magnetization process of the S = 1 and S = 2 spin alternating ferrimagnetic chain with the single-ion anisotropies is investigated by the numerical exact diagonalization and the density matrix renormalization group method. It is found that the mechanism of the 2/3 magnetization plateau formation due to quantum effects depends on the anisotropy constants. The obtained phase diagram of the 2/3 plateau is revealed to include three different phases.

We investigate the S=1 frustrated antiferromagnetic quantum spin chain in magnetic field, using numerical exact diagonalization of finite-size clusters. The calculated critical exponents of the spin correlation functions indicate that the incommensurate spin correlation parallel to the external field can be stronger than the transverse antiferromagnetic one for some intermediate fields. When such an incommensurate correlation is dominant, the quasi-1D system with finite interchain interactions should exhibit long-range incommensurate order. We also discuss a possible mechanism for the recently observed field-induced phase of NDMAP within this framework.

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 a first step of theoretical study oil the spin nanotube, we investigate the S = 1/2 three-leg spin tube, which is the simplest one, using numerical analyses of finite clusters and a finite-size scaling technique. The spin gap, which is one of the most interesting quantities reflecting the macroscopic quantum effect, was revealed to be open for ally finite rung exchange couplings, in contrast to the three-leg spin ladder system which is gapless. We also found a quantum phase transition caused by an asymmetric rung interaction. When one of the three rung coupling constants is changed, the spin gap vanishes very rapidly. (c) 2005 Elsevier B.V. All rights reserved.

Recently many interesting magnetic nanostructures have been fabricated and much attention is arising on the rich magnetic properties that originate in the quantum effects eminent in the nanoscale world. One of the peculiar aspects of the quantum effects is the spin excitation gap. In the spin-1/2 low-dimensional systems, the spin gap often appears when the lattice dimerization or the frustration in the spin-spin interaction are introduced. In the present study, we investigate the ground-state property of the spin-1/2 antiferromagnetic spin chiral nanotubes with the spatial modulation in the spin-spin interaction. The ground-state phase diagrams of them are determined by observing the behavior of the expectation value of the Lieb-Schultz-Mattis slow-twist operator calculated by the quantum Monte Carlo method with the continuous-time loop algorithm. We discuss the relation between the characteristic of the topology of the phase diagram and the chiral vector of the nanotubes. (c) 2005 Elsevier B.V. All rights reserved.

Transport properties of one-dimensional quantum systems have. been attracing considerable interest in recent years. Heat transport can directly be observed through thermal conductivity measurements, while spin transport can be deduced from nuclear spin-lattice relaxation-time measurements. Numerous experiments have indeed been performed in an attempt to settle the argument about whether finite-temperature energy transport should be diffusive or could be ballistic. From the theoretical point of view, there is an argument that integrable Hamiltonians may exhibit ballistic transport, whereas nonintegrable ones are generally diffusive, at finite temperatures. In order to reveal the transport properties of spin-gapped antiferromagnets, which are generally described by nonintegrable Hamiltonians and thus almost unexplored theoretically, we take. an interest in organic Haldane-gap antiferromagnets such as Cu(C(14)H(24)N(4))CuCl(4), and (CH(3))(2)CHNH(3)CuCl(3), both of which consist of ferromagnetic and antiferromagnetic bond-alternating spin-1/2 chains. We calculate their level statistics by means of exact numerical diagonalization and finite size scaling. We argue the possibility of diffusive spin transport occurring and propose model experiments on these materials.

The exact diagonalization study on the magnetization process of the S = 1/2 spin ladder system revealed that the incommensurate spin correlation parallel to H would be possibly dominant around half the saturation magnetization, with the ring exchange. In such a situation, an interladder coupling would yield the field-induced long-range incommensurate order. Several phase diagrams including the magnetization plateau are also presented. (c) 2005 Elsevier Ltd. All rights reserved.

The magnetic susceptibility, high field magnetization, and specific heat measurements of Cu-3(CO3)(2)(OH)(2), which is a model substance for the frustrating diamond spin chain model, have been performed using single crystals. Two broad peaks are observed at around 20 and 5 K in both magnetic susceptibility and specific heat results. The magnetization curve has a clear plateau at one third of the saturation magnetization. The experimental results are examined in terms of theoretical expectations based on exact diagonalization and density matrix renormalization group methods. An origin of magnetic anisotropy is also discussed.

We investigate, analytically and numerically, the ground state of the S = 1/2 distorted diamond type spin chain having three kinds of couplings, where one or two types of the couplings are ferromagnetic. We obtained the phase diagram at zero-magnetic field and also at M-s/3, where M-s is the saturation magnetization. We also discuss the plateau at (2/3)M-s.

We investigate the elementary excitations of the recently synthesized single-chain magnets, which consist of Mn and Ni ions, and some related systems using the numerical exact diagonalization of finite-size clusters. It is found that, as the easy-axis anisotropy D increases, a crossover of the lowest-lying elementary excitation should exist between the spin wave and Ising-like excitations. In the realistic materials both excitations are revealed to be comparably significant.

The collective excitations from the spin density wave pinned by some impurities are investigated with the self-consistent harmonic approximation applied for the phase Hamiltonian. It is found that the pinning-depinning transition due to the quantum fluctuation in the ground state cannot occur in two-dimensional nor three-dimensional isotropic system. In contrast, it has already been revealed that the quantum depinning (melting) can occur in one dimension. It suggests that the critical dimension of the quantum localization-delocalization phase transition is one, within the present approximation. The present result is also available for the charge density wave pinned by some impurities.

The magnetization process of the S = 1 spin ladder system with bond-alternation due to the static lattice distortion is investigated by the numerical diagonalization of finite clusters and the level spectroscopy method. We focus our attention mainly on the magnetization plateau caused by field-induced spin gap. The plateau phase diagram is presented at M-s/4, where M-s is the saturated magnetization. In addition we discuss the magnetic curves and the mechanisms of the plateau at M-s/4 observed in the high field magnetization measurement of organic S = 1 ladder compound BIP-TENO.

The transport properties of quantum systems are one of the most interesting current topics in statistical physics. Heat transport can directly be observed through thermal conductivity measurements, while spin transport can be extracted from nuclear spin-lattice relaxation-time measurements. Numerous experiments have indeed been performed in an attempt to settle the argument about whether finite-temperature energy transport should be diffusive or ballistic. From the theoretical point of view, there is an argument that integrable Hamiltonians exhibit ballistic transport, whereas nonintegrable ones are diffusive or insulators. Since nonintegrable systems have much less been studied in this context so far, we investigate spin-12 ferromagnetic-antiferromagnetic bond-alternating chains, which exhibit common features of Haldane-gap antiferromagnets. Employing the Jordan-Wigner spinless-fermion representation, their level statistics is investigated by the exact-diagonalization method. We argue a possibility of the diffusive transport occurring in the system.

We investigate, at T = 0, the magnetization process of the S = 1 frustrated two-leg spin ladder having the leg interaction (J), the rung interaction (J(0)) and the diagonal interaction (J(d)), especially focusing on the appearance and the disappearance of the magnetization plateaux. We use the degenerate perturbation theory, the level spectroscopy analysis of the numerical diagonalization data and the DMRG. When J(d) = 0, the M-s/2 plateau (M-s is the saturation magnetization) appears for J(0)/J > 1.463. Then there is no M-s/2 plateau for the so-called isotropic case J = J(0). In case of J(d) > 0, J(d) helps the M-s/2 plateau. And also, the M-s/4 and (3/4) Ms plateaux appear when Jd is sufficiently large. These two plateaux are associated with the spontaneous breaking of the translation symmetry along the leg direction. We show the plateau phase diagram on the J(d) versus J plane, and also the magnetic phase diagram on the H versus J(d) plane for fixed J.

We study field-induced phase transitions and long-range orders in the S = 1/2 spin bond-alternating chain with frustrating interaction. By using the inter-chain mean field theory combined with the finite temperature density matrix renormalization group, we investigate properties of field-induced long-range orders and phase transitions. We find that the inter-chain interaction strongly affects the realized phases of this system.

A mechanism of the magnetization plateau observed in the high-field magnetizatiom measurement of the organic S = 1 spin ladder system BIP-TENO is investigated. Assuming possible lattice distortion, we propose a possible scenario of the plateau formation on the basis of the bond-alternating ladder. Some unsolved problems are also discussed.

Azurite (Cu-3(CO3)(2)(OH)(2)) is a model substance of a diamond spin chain, one of frustrated antiferromagnetic quantum spin chains. The high field magnetization up to 60 T, magnetic susceptibility, magnetic entropy, muon spin relaxation and H-1-NMR of azurite have been measured using single crystals. A distinct 1/3 magnetization plateau is confirmed to be present in the magnetization curve. Two resonance peaks of H-1-NMR were observed. By analyzing temperature dependencies of their hyperfine shift, we found two different local susceptibilities which correspond to susceptibility of dimer and monomer spins, respectively. New field induced phase transition was found to occur at a critical field where the magnetic plateau onsets. A short range magnetic ordering developed in a peculiar two-stage process, which was revealed by temperature dependence of the magnetic specific heat and susceptibility.

We investigate, using numerical methods, the finite-field ground-state properties of an S = 1 anisotropic antiferromagnetic chain described by the Hamiltonian H=Sigma(l){S-l(x) S-l+1(x) + S-l(y) S-l+1(y) + Delta S-l(z) Sl+1} + d Sigma(l)(S-l(z)). It is found that the magnetization jump anomaly (a first-order phase transition) appears in the ground-state magnetization curve when Delta=5.0 and -1.50 less than or similar to D less than or similar to 1.05; in a certain region of D, the jump starts from a finite value of the magnetization m per spin. We also find that the a second-order phase transition between commensurate and incommensurate gapless phases takes place in the finite-field ground state. The critical point of this transition as well as the values of m at the onset and end of the magnetization jump is summarized in the m versus D phase diagram.

In this paper, we investigate the low-energy properties of the S = 1/2 anriferromagnetic quantum spin tube with the triangular lattice structure. Using the density matrix renormalization group (DMRG) method, we calculate the singlet-triplet excitation gap up to 144 triangles along the leg direction. Analyzing the size dependence of the gap, we find that the system is gapful for alpha &lt; 1.2. A similar analysis of the low-field edge of the magnetization curve suggests that the system is gapless for alpha &gt; 1.2.

The frustration-induced enhancement of the incommensurate correlation for a bond-alternating spin chain in a magnetic field, associated with a quasi-one-dimensional organic compound F5PNN was studied. The temperature dependence of the staggered susceptibilities were investigated using the density matrix renormalization group. It was found that the incommensurate correlation becomes dominant in a certain range of the magnetic field. The mechanism of this enhancement were discussed on the basis of mapping to the effective S=1/2 XYZ chain and a possibility of the field-induced incommensurate long-range order.