arXiv subjects

Takanori Sugimoto

Publications and source records attributed to Takanori Sugimoto.

32 records · Page 2Linked to original sources

arXiv2020

Magnetic excitations in magnetization plateaux of a frustrated spin ladder

Magnetization plateaux emerging in quantum spin systems due to spontaneously breaking of translational symmetry have been reported both theoretically and experimentally. The broken symmetry can induce reconstruction of elementary excitations such as Goldstone and Higgs modes, whereas its microscopic mechanism and reconstructed quasi-particle in magnetization-plateau phases have remained unclear so far. Here we theoretically study magnetic excitations in the magnetization-plateau phases of a frustrated spin ladder by using dynamical density-matrix renormalization-group method. Additionally, analytical approaches with perturbation theory are performed to obtain intuitive view of magnetic excitations. Comparison between numerical and analytical results indicates the presence of a reconstructed quasi-particle originating from spontaneously broken translational symmetry, which is realized as a collective mode of spin trimer called trimeron.

cond-mat.str-el↗

arXiv2019

Magnetic orders induced by RKKY interaction in Tsai-type quasicrystalline approximant Au-Al-Gd

Recent experimental study on Tsai-type quasicrystalline approximant Au-Al-Gd has revealed the presence of magnetic orders and phase transitions with changing the Au/Al concentration. Motivated by the experiment, we theoretically investigate whether a successive change of magnetic orders occurs in a minimal magnetic model including the RKKY interaction only. We find that the model induces multifarious magnetic orders depending on the Fermi wavenumber and gives a good starting point for understanding the experimental observation. In addition, we predict the presence of an undiscovered novel magnetic order called cuboc order at large Fermi wavenumber region.

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arXiv2018

Cluster-Based Haldane States in Spin-1/2 Cluster Chains

The Haldane state is a typical quantum and topological state of matter, which exhibits an edge state corresponding to symmetry-protected topological order in a one-dimensional integer spin chain. This edge state can be utilized for a processing unit of quantum computation. Its realization, however, has difficulties with synthesis of integer spin compounds. In contrast, one-half spin systems are more designable due to recent progress on intended synthesis of organic materials, quantum dots, and optical lattices. Here we propose a concept to design the Haldane state with one-half spins by making use of a chain composed of one-half spin clusters. If the clusters contains two spins, the ground state of a chain corresponds to the Affleck--Kennedy--Lieb--Tasaki state. In the case of an odd number of spins in the clusters, we propose a concrete procedure to construct a field-induced Haldane state. We illustrate the procedure with a 5-spin cluster chain.

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arXiv2017

Reentrant Topological Phase Transition in a Bridging Model between Kitaev and Haldane Chains

We present a reentrant phase transition in a bridging model between two different topological models: Kitaev and Haldane chains. This model is activated by introducing a bond alternation into the Kitaev chain [A. Yu Kitaev, Phys.-Usp. {\bf 44} 131 (2001)]. Without the bond alternation, finite pairing potential induces a topological state defined by zero-energy Majorana edge mode, while finite bond alternation without the pairing potential makes a different topological state similar to the Haldane state, which is defined by local Berry phase in the bulk. The topologically-ordered state corresponds to the Su-Schrieffer-Heeger state, which is classified as the same symmetry class. We thus find a phase transition between the two topological phases with a reentrant phenomenon, and extend the phase diagram in the plane of the pairing potential and the bond alternation by using three techniques: recursive equation, fidelity, and Pfaffian. In addition, we find that the phase transition is characterized by both the change of the position of Majorana zero-energy modes from one edge to the other edge and the emergence of a string order in the bulk, and that the reentrance is based on a sublattice U(1) rotation. Consequently, our study and model do not only open a direct way to discuss the bulk and edge topologies, but demonstrate an example of the reentrant topologies.

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arXiv2017

Majorana Zero-Energy Mode and Fractal Structure in Fibonacci-Kitaev Chain

We theoretically study a Kitaev chain with a quasiperiodic potential, where the quasiperiodicity is introduced by a Fibonacci sequence. Based on an analysis of the Majorana zero-energy mode, we find the critical $p$-wave superconducting pairing potential separating a topological phase and a non-topological phase. The topological phase diagram with respect to Fibonacci potentials follow a self-similar fractal structure characterized by the box-counting dimension, which is an example of the interplay of fractal and topology like the Hofstadter's butterfly in quantum Hall insulators.

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arXiv2017

Static and Dynamic Magnetic Properties of Spin-1/2 Inequilateral Diamond-Chain Compounds $A_3$Cu$_3$AlO$_2$(SO$_4$)$_4$ ($A$=K, Rb, and Cs)

Spin-1/2 compounds A3Cu3AlO2(SO4)4 (A=K, Rb, and Cs) have one-dimensional (1D) inequilateral diamond chains. We analyze the temperature dependence of the magnetic susceptibility and determine the magnetic exchange interactions. In contrast to the azurite, a dimer is formed on one of the sides of the diamond. From numerical analyses of the proposed model, we find that the dimer together with a nearly isolated 1D Heisenberg chain characterizes magnetic properties including magnetization curve and magnetic excitations. This implies that a dimer-monomer composite chain without frustration is a good starting point for describing these compounds.

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arXiv2016

Phenomenological Magnetic Model in Tsai-Type Approximants

Motivated by recent discovery of canted ferromagnetism in Tsai-type approximants Au-Si-RE (RE = Tb, Dy, Ho), we propose a phenomenological magnetic model reproducing their magnetic structure and thermodynamic quantities. In the model, cubic symmetry ($m\bar{3}$) of the approximately-regular icosahedra plays a key role in the peculiar magnetic structure determined by a neutron diffraction experiment. Our magnetic model does not only explain magnetic behaviors in the quasicrystal approximants, but also provides a good starting point for the possibility of coexistence between magnetic long-range order and aperiodicity in quasicrystals.

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arXiv2015

Quasi-Spin Correlations in a Frustrated Quantum Spin Ladder

The quasi-spin correlations in a frustrated quantum spin ladder with one-half magnetization are theoretically studied by using the density-matrix renormalization-group method and the quasi-spin transformation. In this model, the frustration induces a gapless-to-gapful phase transition with a strong rung coupling. The gapful state is observed as the one-half magnetization plateau in the magnetization curve. In the magnetization-plateau state, we find that the quasi-spin dimer has a large expectation value with long-ranged correlations. This result does not only comes in useful to clarify the magnetization-plateau state, but gives a crucial information to understand the magnetization curve of the real compound BiCu$_2$PO$_6$, whose effective spin model corresponds to ours.

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arXiv2015

Magnetization Plateaux by Reconstructed Quasi-spinons in a Frustrated Two-Leg Spin Ladder under a Magnetic Field

The quantum phase transitions induced by a magnetic field are theoretically studied in a frustrated two-leg spin ladder. Using the density-matrix renormalization-group method, we find some magnetic phase transitions and plateaux in two different cases of strong and weak rung couplings. With the strong rung coupling, the three magnetization plateaux are found at 1/3, 1/2, and 2/3 due to the frustration. Those can be understood in terms of a quasi-spinon reconstructed from the singlet and the triplets of spins on a rung. The plateau at 1/2 corresponds to the valence bond solid of the quasi-spinons, while the plateaux at 1/3 and 2/3 can be associated with the array of quasi-spinons such as soliton lattice. This is different from the usual Bose-Einstein-condensation picture of triplons. Our results will be useful to understand magnetization curves in BiCu$_2$PO$_6$.

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arXiv2014

Lifshitz Transition Induced by Magnetic Field in Frustrated Two-Leg Spin-Ladder Systems

The magnetization curve in a frustrated two-leg spin-ladder system is theoretically studied using both analytical and numerical methods. This spin system is mapped onto a hard-core boson system, which is composed of bond-operators describing the triplon excitation. By the analytical method using the mean-field theory on the bond-operators, we show that cusp singularities emerge in the magnetization curve as a function of a magnetic field due to the strong frustration. This originates from a change of number of Fermi points in the bosonic dispersion relation with the applied field. It is analogous to Lifshitz transition. This singularity is clarified by numerical calculation with the density-matrix renormalization-group method. Our results will be useful to understand the magnetization process observed in the frustrated two-leg spin-ladder compound BiCu2PO6.

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arXiv2013

Spin-Spin Correlation Enhanced by Impurities in a Frustrated Two-leg Spin Ladder

We theoretically study a spin-spin correlation enhanced by non-magnetic impurities in a frustrated two-leg spin ladder. The frustration is introduced by the next-nearest-neighbor antiferromagnetic interaction in the leg direction in the antiferromagnetic two-leg spin ladder. The spin-spin correlation function around impurity site is calculated by the density-matrix renormalization-group method. We find that the spin-spin correlation is enhanced around impurity site with the wavenumber reflecting the frustration. As increasing the frustration, the wavenumber is shifted from commensurate to incommensurate. We discuss several experimental results on BiCu2PO6 in the light of our theory.

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arXiv2013

Frustration effect on magnetic excitations in a two-leg spin-ladder system

We theoretically study the magnetic excitations in a frustrated two-leg spin-ladder system, in which antiferromagnetic exchange interactions act on the nearest-neighbor and next-nearestneighbor bonds in the leg direction, and on the nearest-neighbor bonds in the rung direction. A dynamical spin correlation function at zero temperature is calculated by using the dynamical density-matrix renormalization-group method for possible magnetic phases, i.e., columnar-dimer and rung-singlet phases. The columnar-dimer phase is characterized by multi-spinon excitations with spin gap, while the rung-singlet phase is dominated by the triplet excitation in the rung direction. It is found that a major difference between these two phases appears in the spectral weight of magnetic excitations, in particular, of the bonding and anti-bonding modes in the rung direction. Therefore, we can distinguish one phase from the other by the difference of the spectral weight. Furthermore, we examine frustration effect on both modes in the rung-singlet phase with a perturbation theory from the strong coupling limit. It is shown that the anti-bonding mode is stable against the frustration, and a wavenumber with minimum excitation energy is shifted from the commensurate to incommensurate ones. On the other hand, the bonding mode is merged into the continuum excitation of multiple triplet excitations by increasing the frustration. By comparing our results with inelastic neutron scattering experiments for BiCu$_2$PO$_6$, the magnitude of the magnetic exchange interactions and the ground state will be determined.

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arXiv2011

Spin Excitation Assisted by Non-Softening Phonon for Spin-Peierls Model

We study spin dynamics of a spin-Peierls chain with nearest-neighbor and next-nearest-neighbor Heisenberg spin exchange interactions together with a gapped and dispersionless phonon. The dynamical spin correlation function and phonon excitation spectrum are calculated at zero temperature by using dynamical density-matrix renormalization-group method. We find a new spin excitation assisted by non-softening phonon. The excitation is located above phonon in energy and shows a dispersive feature with strong intensity near the momentum $π$. The phonon excitation spectrum is also influenced by the spin-phonon interaction. We discuss the possibility of observing the spin-phonon coupled features in inorganic spin-Peierls compound CuGeO$_3$.

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arXiv2010

Chirality in spin-1/2 zigzag XY chain: Low-temperature density-matrix renormalization group study

We study spin chirality for a spin-1/2 zigzag XY chain at low temperature by applying a low-temperature density-matrix renormalization group technique. We calculate temperature dependence of dynamical and static correlations of the chirality. In a chiral phase, chiral long-range order at zero temperature disappears at finite temperature, consistent with the fact that there is no long-range order at finite temperature in one-dimensional systems with short-range interactions. In a dimer phase next to the chiral phase, we find an enhancement of static chiral correlation as well as spin correlation with increasing temperature. The enhancement corresponds to the increase of spectral weight inside a gap in the dynamical chiral-correlation function. This temperature-induced chiral correlation is a demonstration of the presence of a chiral state in excited states.

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