arXiv subjects

Hiroaki T. Ueda

Publications and source records attributed to Hiroaki T. Ueda.

9 recordsLinked to original sources

arXiv2016

Quantum solitons with emergent interactions in a model of cold atoms on the triangular lattice

Cold atoms bring new opportunities to study quantum magnetism, and in particular, to simulate quantum magnets with symmetry greater than $SU(2)$. Here we explore the topological excitations which arise in a model of cold atoms on the triangular lattice with $SU(3)$ symmetry. Using a combination of homotopy analysis and analytic field-theory we identify a new family of solitonic wave functions characterised by integer charge ${\bf Q} = (Q_A, Q_B, Q_C)$, with $Q_A + Q_B + Q_C = 0$. We use a numerical approach, based on a variational wave function, to explore the stability of these solitons on a finite lattice. We find that, while solitons with charge ${\bf Q} = (Q, -Q, 0)$ are stable, wave functions with more general charge spontaneously decay into pairs of solitons with emergent interactions. This result suggests that it could be possible to realise a new class of interacting soliton, with no classical analogue, using cold atoms. It also suggests the possibility of a new form of quantum spin liquid, with gauge--group U(1)$\times$U(1).

cond-mat.str-el↗

arXiv2015

Ghost in the machine: Theory of inelastic neutron scattering in a field-induced spin-nematic state

The spin-nematic state has proved elusive, due to the difficulty of observing the order parameter in experiment. In this article we develop a theory of spin excitations in a field-induced spin-nematic state, and use it to show how a spin-nematic order can be indentified using inelastic neutron scattering. We concentrate on 2-dimensional frustrated ferromagnets, for which a two-sublattice, bond-centered spin-nematic state is predicted to exist over a wide range of parameters. First, to clarify the nature of spin-excitations, we introduce a soluble spin-1 model, and use this to derive a continuum field theory, applicable to any two-sublattice spin-nematic state. We then parameterise this field theory, using diagrammatic calculations for a realistic microscopic model of a spin-1/2 frustrated ferromagnet, and show how it can be used to make predictions for inelastic neutron scattering. As an example, we show quantitative predictions for inelastic scattering of neutrons from BaCdVO(PO_4)_2, a promising candidate to realise a spin-nematic state at an achievable h\sim 4T. We show that in this material it is realistic to expect a ghostly Goldstone mode, signalling spin-nematic order, to be visible in experiment.

cond-mat.str-el↗

arXiv2015

Magnetic phase diagram slightly below the saturation field in the stacked $J_1$-$J_2$ model in the square lattice with the $J_{\text{C}}$ interlayer coupling

We study the effect of adding interlayer coupling to the square lattice, $J_1$-$J_2$ Heisenberg model in high external magnetic field. In particular, we consider a cubic lattice formed from stacked $J_1$-$J_2$ layers, with interlayer exchange coupling $J_{\text{C}}$. For the 2-dimensional model ($J_{\text{C}}=0$) it has been shown that a spin-nematic phase appears close to the saturation magnetic field for the parameter range $-0.4 \lesssim J_2/J_1$ and $J_2>0$. We determine the phase diagram for 3-dimensional model at high magnetic field by representing spin flips out of the saturated state as bosons, considering the dilute boson limit and using the Bethe-Salpeter equation to determine the first instability of the saturated paramagnet. Close to the highly frustrated point $J_2/J_1\sim0.5$, we find that the spin-nematic state is stable even for $|J_{\text{C}}/J_1|\sim 1$. For larger values of $J_2/J_1$, interlayer coupling favors a broad, phase-separated region. Further increase of $|J_{\text{C}}|$ stabilizes a collinear antiferromagnet, which is selected via the order-by-disorder mechanism.

cond-mat.str-el↗

arXiv2014

Spontaneous chiral symmetry breaking on a fully polarized frustrated magnet at finite temperature

Frustration can introduce more-than-two minima in a spin dispersion relation even in a fully polarized magnet under high magnetic field. We generally discuss, on the fully polarized phase, the possibility of the chiral symmetry breaking where the number of magnons pumped by finite temperature deviates to one side of minima. We study this phase by constructing the Ginzburg-Landau energy which is controlled by the external magnetic field and interactions between magnons near the dispersion minima. This chirality breaking phase accompanies, not a magnetization perpendicular to the external field, but the vector chirality $S_m \times S_n$. We also discuss the possibility of the chirality breaking phase on LiCuVO4 slightly above the saturation field.

cond-mat.str-el↗

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

Reflection and refraction process of spinwave in a ferromagnet/frustrated ferromagnet junction system

Frustration introduces a nontrivial dispersion relation of spinwave even in a ferromagnetic phase in a spin system. We study the reflection and refraction process of spinwaves in the ferromagnet/frustrated ferromagnet junction system by using the Holstein-Primakoff spinwave expansion and taking the large-S limit. We discuss the relation between the incident angle and refraction angle of spinwave, namely, the Snell's low of spinwave. As concrete examples of frustrated ferromagnets, we study the fully polarized ferromagnet phases in the J1-J2 chains and the J1-J2 model on the square lattice. The interesting refraction processes, such as the splitting of the incident spinwave and the negative refraction, are discussed. We also study the transmittance and reflectance in these concrete models.

cond-mat.mes-hall↗

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↗

arXiv2012

Active electromagnetic metamaterial based on spin torque oscillators

We propose theoretically an active material for electromagnetic radiation with frequency of GHz by use of spin-torque oscillators. The origin of the amplification is the energy supplied to the magnetization by the injected current. We show that close to a resonance with current-driven magnetization, the imaginary part of magnetic permeability becomes indeed negative for either of the two circular polarizations, resulting in negative imaginary part of refractive index. Besides, the real part of the refractive index is also manipulated by the current. Our system thus realizes an active filter to obtain circular polarized radiation and/or an electromagnetic metamaterial having negative refractive index, both controlled electrically.

cond-mat.mes-hall↗

arXiv2012

Topological charge pumping effect by the magnetization dynamics on the Surface of Three-Dimensional Topological Insulators

We discuss a current dynamics on the surface of a 3-dimensional topological insulator induced by magnetization precession of a ferromagnet attached. It is found that the magnetization dynamics generates a direct charge current when the precession axis is within the surface plane. This rectification effect is due to a quantum anomaly and is topologically protected. The robustness of the rectification effect against first-varying exchange field and impurities is confirmed by explicit calculation.

cond-mat.str-el↗