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Tsutomu Momoi

Publications and source records attributed to Tsutomu Momoi.

At least 37 records · Page 2Linked to original sources
arXiv2013

Nematic phase and phase separation near saturation field in frustrated ferromagnets

We study the effects of quantum fluctuations in magnetic properties of quantum frustrated ferromagnets in a magnetic field. It is shown that a non-classical phase or a phase separation appears due to quantum fluctuations below the saturation field in a parameter range close to the classical zero-field phase boundary between ferromagnetic and antiferromagnetic phases, for the case that the classical antiferromagnetic state is not an eigenstate of the quantum model. As an example to which this argument is applicable, we study the S=1/2 J1-J2 Heisenberg model with ferromagnetic J1 (J1<0) on the bcc lattice using a dilute Bose gas approach. For -1.50097 < J1/J2 < -1.389, magnons form f-wave two-magnon bound states, leading to a spin nematic phase, and for -1.389 < J1/J2 < -0.48 a canted coplanar antiferromagnetic phase appears accompanied with a phase separation below the saturation field.

cond-mat.str-el↗
arXiv2013

Spin-Nematic and Spin-Density-Wave Orders in Spatially Anisotropic Frustrated Magnets in a Magnetic Field

We develop a microscopic theory of finite-temperature spin-nematic orderings in three-dimensional spatially anisotropic magnets consisting of weakly-coupled frustrated spin-1/2 chains with nearest-neighbor and next-nearest-neighbor couplings in a magnetic field. Combining a field theoretical technique with density-matrix renormalization group results, we complete finite-temperature phase diagrams in a wide magnetic-field range that possess spin-bond-nematic and incommensurate spin-density-wave ordered phases. The effects of a four-spin interaction are also studied. The relevance of our results to quasi-one-dimensional edge-shared cuprate magnets such as LiCuVO4 is discussed.

cond-mat.str-el↗
arXiv2013

Dynamical spin structure factors of quantum spin nematic states

Dynamical spin structure factors of quantum spin nematic states are calculated in a spin-1/2 square-lattice J1-J2 model with ferromagnetic J1 and competing antiferromagnetic J2 interactions. To this end, we use a fermion representation, generalizing it to N flavors. We begin with a spin-triplet pairing state of fermion fields, called Z2 planar state, which is a stable saddle-point solution in the large-N limit in a finite parameter range where the couplings J1 and J2 compete strongly [R. Shindou and T. Momoi, Phys. Rev. B 80, 064410 (2009)]. Using a large-N expansion, we take into account fluctuations around this saddle point up to corrections of order 1/N. The dynamical spin structure factors thus obtained signify the existence of gapless q-linear director-wave (spin-wave) modes at q=(0,0) and gapped `gauge-field' like collective modes at q=(pi,pi), whose spectral weight vanishes as a linear and quadratic function of the momentum respectively. The low-energy collective modes contain fluctuations of nematic-director, spin, and gauge degrees of freedom. Associated with the gapless q-linear modes, we evaluate the temperature dependence of the nuclear spin relaxation rate 1/T1 in the low-temperature regime as 1/T1 \propto T^{2d-1}, where d is the effective spatial dimension.

cond-mat.str-el↗
arXiv2011

Spin nematic order in multiple-spin exchange models on the triangular lattice

We figure out that the ground state of a multiple-spin exchange model applicable to thin films of solid 3He possesses an octahedral spin nematic order. In the presence of magnetic field, it is deformed into an antiferro-quadrupolar order in the perpendicular spin plane, in which lattice Z3 rotational symmetry is also broken. Furthermore, this system shows a narrow magnetization plateau at half, m/m_{sat}=1/2, which resembles recent magnetization measurement [H. Nema et al., Phys. Rev. Lett. 102, 075301 (2009)].

cond-mat.str-el↗
arXiv2011

Phase diagram of the classical Heisenberg antiferromagnet on a triangular lattice in applied magnetic field

The Heisenberg antiferromagnet on a two-dimensional triangular lattice is a paradigmatic problem in frustrated magnetism. Even in the classical limit, its properties are far from simple. The "120 degree" ground state favoured by the frustrated antiferromagnetic interactions contains a hidden chiral symmetry, and supports two distinct types of excitation. And famously, three distinct phases, including a collinear one-third magnetisation plateau, are stabilised by thermal fluctuations in applied magnetic field. The questions of symmetry-breaking raised by this model are deep and subtle, and after more than thirty years of study, many of the details of its phase diagram remain surprisingly obscure. In this paper we use modern Monte Carlo simulation techniques to determine the finite-temperature phase diagram of the classical Heisenberg antiferromagnet on a triangular lattice in applied magnetic field. At low to intermediate values of magnetic field, we find evidence for a continuous phase transition from the paramagnet into the collinear one-third magnetisation plateau, belonging to the three-state Potts universality class. We also find evidence for conventional Berezinskii-Kosterlitz-Thouless transitions from the one-third magnetisation plateau into the canted "Y-state", and into the 2:1 canted phase found at high fields. However, the phase transition from the paramagnet into the 2:1 canted phase, while continuous, does not appear to fall into any conventional universality class. We argue that this, like the chiral phase transition discussed in zero field, deserves further study as an interesting example of a finite-temperature phase transition with compound order-parameter symmetry. We comment on the relevance of these results for experiments on magnetic materials with a triangular lattice.

cond-mat.str-el↗
arXiv2011

NMR relaxation rate in the field-induced octupolar liquid phase of spin-1/2 J1-J2 frustrated chains

In the spin-1/2 frustrated chain with nearest-neighbor ferromagnetic exchange J1 and next-nearest-neighbor antiferromagnetic exchange J2 under magnetic field, magnetic multipolar-liquid (quadrupolar, octupolar, and hexadecapolar) phases are widely expanded from the saturation down to a low-field regime. Recently, we have clarified characteristic temperature and field dependence of the NMR relaxation rate 1/T_1 in the quadrupolar phase. In this paper, we examine those of 1/T_1 in the octupolar phase combining field theoretical method with numerical data. The relevance of the results to quasi one-dimensional J1-J2 magnets such as PbCuSO4(OH)2, Rb2Cu2Mo3O12 and Li2ZrCuO4 is shortly discussed.

cond-mat.str-el↗
arXiv2011

Projective studies of spin nematics in a quantum frustrated ferromagnet

We study the ground state properties of the spin-1/2 frustrated ferromagnetic J1-J2 Heisenberg model on the square lattice, employing projected BCS wavefunctions with spin-triplet pairings of the spinon fields as trial wavefunctions. Based on the variational Monte Carlo analysis, we argue that, in the competing coupling regime, a certain type of the projected BCS wavefunction, dubbed the projected Z2 planar state, achieves the best optimal energy among the other competing states such as the ferromagnetic state and collinear antiferromagnetic state. Like in quantum spin liquids, the projected Z2 planar state preserves the translational symmetry of the square lattice. However, it is also accompanied by a d-wave ordering of the quadrupole moments, breaking the spin rotational symmetry. The state thus describes a quantum spin analogue of the nematic liquid crystals. The calculated static correlation functions also reveal that the projected Z2 planar state has a strong collinear antiferromagnetic fluctuation.

cond-mat.str-el↗
arXiv2011

Field and temperature dependence of the NMR relaxation rate in the magnetic quadrupolar liquid phase of spin-1/2 frustrated ferromagnetic chains

It is generally difficult to experimentally distinguish magnetic multipolar orders in spin systems. Recently, it was proposed that the temperature dependence of the nuclear magnetic resonance relaxation rate 1/T_1 can involve an indirect, but clear signature of the field-induced spin nematic or multipolar Tomonaga-Luttinger (TL) liquid phase [Phys. Rev. B79, 060406(R) (2009)]. In this paper, we evaluate accurately the field and temperature dependence of 1/T_1 in spin-1/2 frustrated J1-J2 chains combining field-theoretical techniques with numerical data. Our results demonstrate that isotherms of 1/T_1 as a function of magnetic field also exhibit distinctive non-monotonic behavior in spin nematic TL liquid, in contrast with the standard TL liquid in the spin-1/2 Heisenberg chain. The relevance of our results to quasi one-dimensional edge-sharing cuprate magnets, such as LiCuVO4, is discussed.

cond-mat.str-el↗
arXiv2010

SU(2) slave-boson formulation of spin nematic states in S=1/2 frustrated ferromagnets

An SU(2) slave boson formulation of bond-type spin nematic orders is developed in frustrated ferromagnets, where the spin nematic states are described as the resonating spin-triplet valence bond (RVB) states. The d-vectors of spin-triplet pairing ansatzes play the role of the directors in the bond-type spin quadrupolar states. The low-energy excitations around such spin-triplet RVB ansatzes generally comprise the (potentially massless) gauge bosons, massless Goldstone bosons, and spinon individual excitations. Extending the projective symmetry group argument to the spin-triplet ansatzes, we show how to identify the number of massless gauge bosons efficiently. Applying this formulation, we next (i) enumerate possible mean field solutions for the S=1/2 ferromagnetic J1-J2 Heisenberg model on the square lattice, with ferromagnetic nearest neighbor J1 and competing antiferromagnetic next-nearest neighbor J2, and (ii) argue their stability against small gauge fluctuations. As a result, two stable spin-triplet RVB ansatzes are found in the intermediate coupling regime around J1:J2 \simeq 1:0.4. One is the Z_2 Balian-Werthamer (BW) state stabilized by the Higgs mechanism and the other is the SU(2) chiral p-wave (Anderson-Brinkman-Morel) state stabilized by the Chern-Simon mechanism. The former Z_2 BW state in fact shows the same bond-type spin quadrupolar order as found in the previous exact diagonalization study [N. Shannon et al., Phys. Rev. Lett. 96, 027213 (2006)].

cond-mat.str-el↗
arXiv2009

Phase diagram of the spin-1/2 $J_1$-$J_2$-$J_3$ Heisenberg model on the square lattice

We presents the results of an extensive numerical study of the phase diagram of the spin-1/2, \protect{$J_1$-$J_2$-$J_3$} Heisenberg model on a square lattice, for both ferromagnetic and antiferromagnetic nearest-neighbor interactions $J_1$, using exact diagonalization with periodic and twisted boundary conditions. Comparison is made with published spin wave calculations. We show that quantum fluctuations play a very important role, changing both the extent and the wave vector of classical spiral phases, and leading to new quantum phases where the classical spiral states have a high degeneracy. These include a new phase with small or vanishing spin-stiffness, in addition to known valence-bond-solid and bond-nematic phases.

cond-mat.str-el↗
arXiv2009

NMR relaxation rate and dynamical structure factors in nematic and multipolar liquids of frustrated spin chains under magnetic fields

Recently, it has been shown that spin nematic (quadrupolar) or higher multipolar correlation functions exhibit a quasi long-range order in the wide region of the field-induced Tomonaga-Luttinger-liquid (TLL) phase in spin-1/2 zigzag chains. In this paper, we point out that the temperature dependence of the NMR relaxation rate 1/T_1 in these multipolar TLLs is qualitatively different from that in more conventional TLLs of one-dimensional quantum magnets (e.g., the spin-1/2 Heisenberg chain); 1/T_1 decreases with lowering temperature in multipolar TLL. We also discuss low-energy features in spin dynamical structure factors which are characteristic of the multipolar TLL phases.

cond-mat.str-el↗
arXiv2008

Finite-Temperature Mott Transition in Two-Dimensional Frustrated Hubbard Models

We investigate the Hubbard model on two typical frustrated lattices in two dimensions, the kagome lattice and the anisotropic triangular lattice, by means of the cellular dynamical mean field theory. We show that the metallic phase is stabilized up to fairly large Hubbard interactions under strong geometrical frustration in both cases, which results in heavy fermion behavior and several anomalous properties around the Mott transition point. In particular, for the anisotropic triangular lattice, we find novel reentrant behavior in the Mott transition in the moderately frustrated parameter regime, which is caused by the competition between Fermi-liquid formation and magnetic correlations. It is demonstrated that the reentrant behavior is a generic feature inherent in the Mott transition with intermediate geometrical frustration, and indeed in accordance with recent experimental findings for organic materials.

cond-mat.str-el↗
arXiv2008

Vector chiral and multipolar orders in the spin-1/2 frustrated ferromagnetic chain in magnetic field

We study the one-dimensional spin-1/2 Heisenberg chain with competing ferromagnetic nearest-neighbor J_1 and antiferromagnetic next-nearest-neighbor J_2 exchange couplings in the presence of magnetic field. We use both numerical approaches (the density matrix renormalization group method and exact diagonalization) and effective field-theory approach, and obtain the ground-state phase diagram for wide parameter range of the coupling ratio J_1/J_2. The phase diagram is rich and has a variety of phases, including the vector chiral phase, the nematic phase, and other multipolar phases. In the vector chiral phase, which appears in relatively weak magnetic field, the ground state exhibits long-range order (LRO) of vector chirality which spontaneously breaks a parity symmetry. The nematic phase shows a quasi-LRO of antiferro-nematic spin correlation, and arises as a result of formation of two-magnon bound states in high magnetic fields. Similarly, the higher multipolar phases, such as triatic (p=3) and quartic (p=4) phases, are formed through binding of p magnons near the saturation fields, showing quasi-LRO of antiferro-multipolar spin correlations. The multipolar phases cross over to spin density wave phases as the magnetic field is decreased, before encountering a phase transition to the vector chiral phase at a lower field. The implications of our results to quasi-one-dimensional frustrated magnets (e.g., LiCuVO_4) are discussed.

cond-mat.str-el↗
arXiv2007

Finite temperature Mott transition in Hubbard model on anisotropic triangular lattice

We investigate the Hubbard model on the anisotropic triangular lattice by means of the cellular dynamical mean field theory. The phase diagram determined in the Hubbard interaction versus temperature plane shows novel reentrant behavior in the Mott transition due to the competition between Fermi-liquid formation and magnetic correlations under geometrical frustration. We demonstrate that the reentrant behavior is characteristic of the Mott transition with intermediate geometrical frustration and indeed consistent with recent experimental results of organic materials.

cond-mat.str-el↗
arXiv2007

Multi-magnon bound states in the frustrated ferromagnetic 1D chain

We study a one-dimensional Heisenberg chain with competing ferromagnetic nearest-neighbor and antiferromagnetic next-nearest neighbor interactions in magnetic field. Starting from the fully polarized high-field state, we calculate the dispersions of the lowest-lying $n$-magnon excitations and the saturation field (n=2,3,4). We show that the lowest-lying excitations are always bound multi-magnon states with a total momentum of $π$ except for a small parameter range. We argue that bose condensation of the bound $n$ magnons leads to novel Tomonaga-Luttinger liquids with multi-polar correlations; nematic and triatic ordered liquids correspond to n=2 and n=3.

cond-mat.other↗
arXiv2007

Octupolar order in the multiple spin exchange model on a triangular lattice

We show how a gapless spin liquid with hidden octupolar order arises in an applied magnetic field, in a model applicable to thin films of 3He with competing ferromagnetic and antiferromagnetic (cyclic) exchange interactions. Evidence is also presented for nematic -- i.e., quadrupolar -- correlations bordering on ferromagnetism in the absence of a magnetic field.

cond-mat.str-el↗
arXiv2005

Nematic order in square lattice frustrated ferromagnets

We present a new scenario for the breakdown of ferromagnetic order in a two-dimensional quantum magnet with competing ferromagnetic and antiferromagnetic interactions. In this, dynamical effects lead to the formation of two-magnon bound states, which undergo Bose-Einstein condensation, giving rise to bond-centered nematic order. This scenario is explored in some detail for an extended Heisenberg model on a square lattice. In particular, we present numerical evidence confirming the existence of a state with d-wave nematic correlations but no long range magnetic order, lying between the saturated ferromagnetic and collinear antiferromagnetic phases of the ferromagnetic J1-J2 model. We argue by continuity of spectra that this phase is also present in a model with 4-spin cyclic exchange.

cond-mat.str-el↗