arXiv subjects

Keisuke Totsuka

Publications and source records attributed to Keisuke Totsuka.

30 records · Page 2Linked to original sources

arXiv2014

Stability and magnetization curve of spin-nematic phase slightly below saturation field

We discuss the magnetization process slightly below the saturation field in frustrated magnets. A condensation of bound magnons on the spin-polarized state induces either a spin nematic phase or a state with phase separation. The (effective) interaction between the bound magnon pairs not only is crucial to the stability of the nematic phase, but also determines the slope of the magnetization curve near saturation. We generally derive the expression of this interaction by using the perturbative scattering theory. By applying the method to coupled zigzag chains LiCuVO4, we find the positive pair-pair interaction implying the stability of the spin nematic phase. We also point out that the magnetization curve of LiCuVO4 is almost vertical (i.e. very large dM/dH) near the saturation exhibiting one-dimensional feature despite non-negligible interchain couplings.

cond-mat.str-el↗

arXiv2013

Topological Many-Body States in Quantum Antiferromagnets via Fuzzy Super-Geometry

Recent vigorous investigations of topological order have not only discovered new topological states of matter but also shed new light to "already known" topological states. One established example with topological order is the valence bond solid (VBS) states in quantum antiferromagnets. The VBS states are disordered spin liquids with no spontaneous symmetry breaking but most typically manifest topological order known as hidden string order on 1D chain. Interestingly, the VBS models are based on mathematics analogous to fuzzy geometry. We review applications of the mathematics of fuzzy super-geometry in the construction of supersymmetric versions of VBS (SVBS) states, and give a pedagogical introduction of SVBS models and their properties [arXiv:0809.4885, 1105.3529, 1210.0299]. As concrete examples, we present detail analysis of supersymmetric versions of SU(2) and SO(5) VBS states, i.e. UOSp(N|2) and UOSp(N|4) SVBS states whose mathematics are closely related to fuzzy two- and four-superspheres. The SVBS states are physically interpreted as hole-doped VBS states with superconducting property that interpolate various VBS states depending on value of a hole-doping parameter. The parent Hamiltonians for SVBS states are explicitly constructed, and their gapped excitations are derived within the single-mode approximation on 1D SVBS chains. Prominent features of the SVBS chains are discussed in detail, such as a generalized string order parameter and entanglement spectra. It is realized that the entanglement spectra are at least doubly degenerate regardless of the parity of bulk (super)spins. Stability of topological phase with supersymmetry is discussed with emphasis on its relation to particular edge (super)spin states.

hep-th↗

arXiv2013

Quantum Entanglement and Topological Order in Hole-Doped Valence Bond Solid States

We present a detailed analysis of topological properties of the valence bond solid (VBS) states doped with fermionic holes. As concrete examples, we consider the supersymmetric extension of the SU(2)- and the SO(5) VBS states, dubbed UOSp(1|2) and UOSp(1|4) supersymmetric VBS states, respectively. Specifically, we investigate the string-order parameters and the entanglement spectra of these states to find that, even when the parent states (bosonic VBS states) do not support the string order, they recover it when holes are doped and the fermionic sector appears in the entanglement spectrum. These peculiar properties are discussed in light of the symmetry-protected topological order. To this end, we characterize a few typical classes of symmetry-protected topological orders in terms of supermatrix-product states (SMPS). From this, we see that the topological order in the bulk manifests itself in the transformation properties of the SMPS in question and thereby affects the structure of the entanglement spectrum. Then, we explicitly relate the existence of the string order and the structure of the entanglement spectrum to explain the recovery and the stabilization of the string order in the supersymmetric systems.

cond-mat.str-el↗

arXiv2011

Hidden Order and Dynamics in Supersymmetric Valence Bond Solid States -- Super-Matrix Product State Formalism

Supersymmetric valence bond solid models are extensions of the VBS model, a paradigmatic model of `solvable' gapped quantum antiferromagnets, to the case with doped fermionic holes. In this paper, we present a detailed analysis of physical properties of the models. For systematic studies, a supersymmetric version of the matrix product formalism is developed. On 1D chains, we exactly evaluate the hole-doping behavior of various physical quantities, such as the spin/charge excitation spectrum, superconducting order parameter. A generalized hidden order is proposed, and the corresponding string non-local order parameter is also calculated. The behavior of the string order parameter is discussed in the light of the entanglement spectrum.

cond-mat.str-el↗

arXiv2011

Dimer Smectic in Extended Quantum Dimer Model -approach from pseudo S=1 Hamiltonian-

We extend Quantum Dimer Model (QDM) introduced by Rokhsar and Kivelson in such a way that the model includes resonance processes on larger loops. The strategy is to first construct a pseudo spin Hamiltonian which is defined not by the S =1/2 but by the S=1 representation and then establish the correspondence between the pseudo-spin-1 configurations and those of dimers. The standard QDM keeps only the lowest resonance processes for pairs of parallel dimers and its validity in describing the realistic spin systems is not obvious. Our extended QDM (EQDM) improves this point and contains a novel resonance term, which is equivalent to two successive actions of the familiar parallel dimer resonance. For a certain choice of the coupling constants, our model exhibits three exotic valence-bond phases (herringbone, checkerboard and 'dimer smectic') in the ground state, which meet at a triple point. The checkerboard phase has four-fold rotational symmetry, while the dimer smectic is characterized by partial ordering only in one of the two directions. In the dimer smectic phase, we investigate the dimer-dimer correlation by using an analogy to random walks.

cond-mat.str-el↗

arXiv2011

The effect of Dzyaloshinskii-Moriya interactions on the phase diagram and magnetic excitations of SrCu2(BO3)2

The orthogonal dimer structure in the SrCu2(BO3)2 spin-1/2 magnet provides a realization of the Shastry-Sutherland model. Using a dimer-product variational wave function, we map out the phase diagram of the Shastry-Sutherland model including anisotropies. Based on the variational solution, we construct a bond-wave approach to obtain the excitation spectra as a function of magnetic field. The characteristic features of the experimentally measured neutron and ESR spectra are reproduced, like the anisotropy induced zero field splittings and the persistent gap at higher fields.

cond-mat.str-el↗

arXiv2011

Extended Quantum Dimer Model and novel valence-bond phases

We extend the quantum dimer model (QDM) introduced by Rokhsar and Kivelson so as to construct a concrete example of the model which exhibits the first-order phase transition between different valence-bond solids suggested recently by Batista and Trugman and look for the possibility of other exotic dimer states. We show that our model contains three exotic valence-bond phases (herringbone, checkerboard and dimer smectic) in the ground-state phase diagram and that it realizes the phase transition from the staggered valence-bond solid to the herringbone one. The checkerboard phase has four-fold rotational symmetry, while the dimer smectic, in the absence of quantum fluctuations, has massive degeneracy originating from partial ordering only in one of the two spatial directions. A resonance process involving three dimers resolves this massive degeneracy and dimer smectic gets ordered (order from disorder).

cond-mat.str-el↗

arXiv2008

Geometric phases and the magnetization process in quantum antiferromagnets

The physics underlying the magnetization process of quantum antiferromagnets is revisited from the viewpoint of geometric phases. A continuum variant of the Lieb-Schultz-Mattis-type approach to the problem is put forth, where the commensurability condition of Oshikawa {\it et al} derives from a Berry connection formulation of the system's crystal momentum. %, similar to that developed by Haldane for ferromagnets. %Building on the physical picture which arises, We then go on to formulate an effective field theory which can deal with higher dimensional cases as well. We find that a topological term, whose principle function is to assign Berry phase factors to space-time vortex objects, ultimately controls the magnetic behavior of the system. We further show how our effective action maps into a ${\bf Z}_2$ gauge theory under certain conditions, which in turn allows for the occurrence of a fractionalized phase with topological order.

cond-mat.stat-mech↗

arXiv2008

Spontaneous trimerization in a bilinear-biquadratic S=1 zig-zag chain

Recent theoretical studies raised the possibility of a realization of spin nematic states in the S=1 triangular lattice compound NiGa_2S_4. We study the bilinear-biquadratic spin 1 chain in a zig-zag geometry by means of the density matrix renormalization group method and exact diagonalization. We present the phase diagram focusing on antiferromagnetic interactions. Adjacent to the known Haldane-double Haldane and the extended critical phase with dominant spin nematic correlations we find a trimerized phase with a nonvanishing energy gap. We discuss results for different order parameters, energy gaps, correlation functions, and the central charge, and make connection to field theoretical predictions for the phase diagram.

cond-mat.str-el↗

arXiv2001

Field-Induced gap due to four-spin exchange in a spin ladder

The effect of the four-spin cyclic exchange interaction at each plaquette in the $S=1/2$ two-leg spin ladder is investigated at T=0, especially focusing on the field-induced gap. The strong rung coupling approximation suggests that it yields a plateau at half of the saturation moment ($m=1/2$) in the magnetization curve, which corresponds to a field-induced spin gap with a spontaneous breaking of the translational symmetry. A precise phase diagram at $m=1/2$ is also presented based on the level spectroscopy analysis of the numerical data obtained by Lanczos method. The boundary between the gapless and plateau phases is confirmed to be of the Kosterlitz-Thouless (KT) universality class.

cond-mat.str-el↗

arXiv2000

Magnetization plateaus of the Shastry-Sutherland model for SrCu_2(BO_3)_2: SDW, supersolid and bound states

We study the Heisenberg antiferromagnet on the Shastry-Sutherland lattice under magnetic fields, to clarify the magnetic properties of SrCu_2(BO_3)_2. Treating magnetic excitations promoted by the field as Bose particles and using strong coupling expansion, we derive an effective Hamiltonian for the effective magnetic particles. Anisotropic repulsive interactions between effective particles induce `insulating' states with a stripe SDW structure at magnetization m/m_{sat}=1/3 and a checkerboard structure at 1/2, and thereby form magnetization plateaus. Supersolid phases appear around insulating SDW phases by changing the magnetic field. Nature of these supersolid phases is discussed in detail. We also demonstrate how the geometry of the Shastry-Sutherland lattice affects dynamical properties of magnetic excitations significantly and makes a novel type of quintuplet (S=2) boundstates condense for very small magnetization.

cond-mat.str-el↗

arXiv2000

Magnetization plateaus as insulator-superfluid transitions in quantum spin systems

We study the magnetization process in two-dimensional S=1/2 spin systems, to discuss the appearance of a plateau structure. The following three cases are considered: (1) the Heisenberg antiferromagnet and multiple-spin exchange model on the triangular lattice, (2) Shastry-Sutherland type lattice, [which is a possible model for SrCu2(BO3)2,] (3) 1/5-depleted lattice (for CaV4O9). We find in these systems that magnetization plateaus can appear owing to a transition from superfluid to a Mott insulator of magnetic excitations. The plateau states have CDW order of the excitations. The magnetizations of the plateaus depend on components of the magnetic excitations, range of the repulsive interaction, and the geometry of the lattice.

cond-mat.str-el↗