arXiv subjects

Motohiko Ezawa

Publications and source records attributed to Motohiko Ezawa.

At least 127 records · Page 7Linked to original sources

arXiv2017

Exact solutions in two-dimensional topological superconductors: Hubbard interaction induced spontaneous symmetry breaking

We present an exactly solvable model of a spin-triplet $f$-wave topological superconductor on the honeycomb lattice in the presence of the Hubbard interaction for arbitrary interaction strength. First we show that the Kane-Mele model with the corresponding spin-triplet $f$-wave superconducting pairings becomes a full-gap topological superconductor possessing the time-reversal symmetry. We then introduce the Hubbard interaction. The exactly solvable condition is found to be the emergence of perfect flat bands at zero energy. They generate infinitely many conserved quantities. It is intriguing that the Hubbard interaction breaks the time-reversal symmetry spontaneously. As a result, the system turns into a trivial superconductor. We demonstrate this topological property based on the topological number and by analyzing the edge state in nanoribbon geometry.

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arXiv2017

Exact solutions and topological phase diagram in interacting dimerized Kitaev topological superconductors

It was recently shown that an interacting Kitaev topological superconductor model is exactly solvable based on two-step Jordan-Wigner transformations together with one spin rotation. We generalize this model by including the dimerization, which is shown also to be exactly solvable. We analytically determine the topological phase diagram containing seven distinct phases. It is argued that the system is topological when a fermionic many-body Majorana zero-energy edge state emerges. It is intriguing that there are two tetra-critical points, at each of which four distinct phases touch.

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arXiv2017

Nonlinear spin current generation in noncentrosymmetric spin-orbit coupled systems

Spin current plays a central role in spintronics. In particular, finding more efficient ways to generate spin current has been an important issue and studied actively. For example, representative methods of spin current generation include spin polarized current injections from ferromagnetic metals, spin Hall effect, and spin battery. Here we theoretically propose a new mechanism of spin current generation based on nonlinear phenomena. By using Boltzmann transport theory, we show that a simple application of the electric field $\bf{E}$ induces spin current proportional to $\bf{E^2}$ in noncentrosymmetric spin-orbit coupled systems. We demonstrate that the nonlinear spin current of the proposed mechanism is supported in the surface state of three-dimensional topological insulators and two-dimensional semiconductors with the Rashba and/or Dresselhaus interaction. In the latter case, the angular dependence of the nonlinear spin current can be manipulated by the direction of the electric field and by the ratio of the Rashba and Dresselhaus interactions. We find that the magnitude of the spin current largely exceeds those in the previous methods for a reasonable magnitude of the electric field. Furthermore, we show that application of AC electric fields (e.g. terahertz light) leads to the rectifying effect of the spin current where DC spin current is generated. These findings will pave a new route to manipulate the spin current in noncentrosymmetric crystals.

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arXiv2017

Merging of momentum-space monopoles by controlling magnetic field: From cubic-Dirac to triple-Weyl fermion systems

We analyze a generalized Dirac system, where the dispersion along the $k_{x}$ and $k_{y}$ axes is $N$-th power and linear along the $k_{z}$ axis. When we apply magnetic field, there emerge $N$ monopole-antimonopole pairs beyond a certain critical field in general. As the direction of the magnetic field is rotated toward the $z$ axis, monopoles move to the north pole while antimonopoles move to the south pole. When the magnetic field becomes parallel to the $z$ axis, they merge into one monopole or one antimonopole whose monopole charge is $\pm N$. The resultant system is a multiple-Weyl semimetal. Characteristic properties of such a system are that the anomalous Hall effect and the chiral anomaly are enhanced by $N$ times and that $N$ Fermi arcs appear. These phenomena will be observed experimentally in the cubic-Dirac and triple-Weyl fermion systems ($N=3$).

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arXiv2017

Photoinduced topological phase transition from a crossing-line nodal semimetal to a multiple-Weyl semimetal

We propose a simple scheme to construct a model whose Fermi surface is comprised of crossing-line nodes. The Hamiltonian consists of a normal hopping term and an additional term which is odd under the mirror reflection. The line nodes appear along the mirror-invariant planes, where each line node carries the quantized Berry magnetic flux. We explicitly construct a model with the $N$-fold rotational symmetry, where the $2N$ line nodes merge at the north and south poles. Photoirradiation induces a topological phase transition. When we apply photoirradiation along the $k_z$ axis, there emerge point nodes carrying the monopole charge $\pm N$ at these poles, while all the line nodes disappear. The resultant system describes anisotropic multiple-Weyl fermions.

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arXiv2017

Topological Semimetals carrying Arbitrary Hopf Numbers: Hopf-Link, Solomon's-Knot, Trefoil-Knot and Other Semimetals

We propose a new type of Hopf semimetals indexed by a pair of numbers $(p,q)$, where the Hopf number is given by $pq$. The Fermi surface is given by the preimage of the Hopf map, which is nontrivially linked for a nonzero Hopf number. The Fermi surface forms a torus link, whose examples are the Hopf link indexed by $(1,1)$, the Solomon's knot $(2,1)$, the double Hopf-link $(2,2)$ and the double trefoil-knot $(3,2)$. We may choose $p$ or $q$ as a half integer, where torus-knot Fermi surfaces such as the trefoil knot $(3/2,1)$ are realized. It is even possible to make the Hopf number an arbitrary rational number, where a semimetal whose Fermi surface forms open strings is generated.

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arXiv2017

Triplet Fermions and Dirac Fermions in Borophene

Borophene is a monolayer materials made of boron. A perfect planar boropehene called $β_{12}$ borophene has Dirac cones and they are well reproduced by a tight-binding model according to recent experimental and first-principles calculation results. We explicitly derive a Dirac theory for them. Dirac cones are gapless when the inversion symmetry exists, while they are gapped when it is broken. In addition, three-band touching points emerge together with pseudospin triplet fermions when all transfer energy is equal and all on-site energy is equal. The three-band touching is slightly resolved otherwise. We construct effective three-band theories for triplet fermions. We also study the edge states of borophene nanoribbons, which show various behaviors depending on the way of edge terminations.

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arXiv2017

Chiral anomaly enhancement and photo-irradiation effects in multi-band touching fermion systems

Multi-band touchings together with the emergence of fermions exhibiting linear dispersions have recently been predicted and realized in various materials. We first investigate the Adler-Bell-Jackiw chiral anomaly in these multi-band touching semimetals when they are described by the pseudospin operator in high dimensional representation. By evaluating the Chern number, we show that the anomalous Hall effect is enhanced depending on the magnitude of the pseudospin. It is also confirmed by the analysis of the Landau levels when magnetic field is applied. Namely, charge pumping occurs from one multi-band touching point to another through multi-channel Landau levels in the presence of parallel electric and magnetic fields. We also show a pair annihilation of two multi-band touching points by photo-irradiation. Furthermore, we propose generalizations of Dirac semimetals, multiple-Weyl semimetals and loop-nodal semimetals to those composed of fermions carrying pseudospins in high dimensional representation. Finally we investigate the 3-band touching protected by the $% C_{3}$ symmetry. We show that the 3-band touching point is broken into two Weyl points by photo-irradiation.

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arXiv2016

Hourglass Fermion Surface States in Stacked Topological Insulators with Nonsymmorphic Symmetry

Recently a nonsymmorphic topological insulator was predicted, where the characteristic feature is the emergence of a "hourglass fermion" surface state protected by the nonsymmorphic symmetry. Such a state has already been observed experimentally. We propose a simple model possessing the hourglass fermion surface state. The model is constructing by stacking the quantum-spin-Hall insulators with the interlayer coupling introduced so as to preserve the nonsymmorphic symmetry and the time reversal symmetry. The Dirac theory is also derived, whose analytical results reproduce the hourglass fermion surface state remarkably well. Furthermore, we discuss how the hourglass state is destroyed by introducing perturbations based on the symmetry analysis. Our results show that the hourglass fermion surface state is universal in the helical edge system with the nonsymmorphic symmetry.

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arXiv2016

Pseudospin 3/2 Fermions, Type-II Weyl Semimetals and Critical Weyl Semimetals in Tricolor Cubic Lattice

Multi-band touchings such as 3-band, 6-band and 8-band touchings together with the emergence of high pseudospin fermions were predicted recently at high-symmetry points in three-dimensional space. In this paper, we propose a simple cubic model whose unit cell contains three atoms. There are 6 bands in the system due to the spin degrees of freedom. The 4-band and 2-band touchings are realized at high-symmetry points, where we derive low-energy theories, demonstrating the emergence of pseudospin-3/2 fermions and Weyl fermions, respectively. Away from the high-symmetry points, we find critical Weyl fermions present exactly at the boundary between the type-I and type-II Weyl fermions. This critical Weyl fermion transforms into the type-I or type-II Weyl fermion once the magnetic field is applied.

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arXiv2016

Control and manipulation of a magnetic skyrmionium in nanostructures

A magnetic skyrmionium is a nontopological soliton, which has a doughnut-like out-of-plane spin texture in thin films, and can be phenomenologically viewed as a coalition of two topological magnetic skyrmions with opposite topological numbers. Due to its zero topological number ($Q=0$) and doughnut-like structure, the skyrmionium has its distinctive characteristics as compared to the skyrmion with $Q=\pm 1$. Here we systematically study the generation, manipulation and motion of a skyrmionium in ultrathin magnetic nanostructures by applying a magnetic field or a spin-polarized current. It is found that the skyrmionium moves faster than the skyrmion when they are driven by the out-of-plane current, and their velocity difference is proportional to the driving force. However, the skyrmionium and skyrmion exhibit an identical current-velocity relation when they are driven by the in-plane current. It is also found that a moving skyrmionium is less deformed in the current-in-plane geometry compared with the skyrmionum in the current-perpendicular-to-plane geometry. Furthermore we demonstrate the transformation of a skyrmionium with $Q=0$ into two skyrmions with $Q=+1$ in a nanotrack driven by a spin-polarized current, which can be seen as the unzipping process of a skyrmionium. We illustrate the energy and spin structure variations during the skyrmionium unzipping process, where linear relations between the spin structure and energies are found. These results could have technological implications in the emerging field of skyrmionics.

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arXiv2016

Thermally stable magnetic skyrmions in multilayer synthetic antiferromagnetic racetracks

A magnetic skyrmion is a topological magnetization structure with a nanometric size and a well-defined swirling spin distribution, which is anticipated to be an essential building block for novel skyrmion-based device applications. We study the motion of magnetic skyrmions in multilayer synthetic antiferromagnetic (SAF) racetracks as well as in conventional monolayer ferromagnetic (FM) racetracks at finite temperature. There is an odd-even effect of the constituent FM layer number on the skyrmion Hall effect (SkHE). Namely, due to the suppression of the SkHE, the magnetic skyrmion has no transverse motion in multilayer SAF racetracks packed with even FM layers. It is shown that a moving magnetic skyrmion is stable even at room temperature ($T=300$ K) in a bilayer SAF racetrack but it is destructed at $T=100$ K in a monolayer FM racetrack. Our results indicate that the SAF structures are reliable and promising candidates for future applications in skyrmion-electronics and skyrmion-spintronics.

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arXiv2016

Tunable skewed edges in puckered structures

We propose a new type of edges, arising due to the anisotropy inherent in the puckered structure of a honeycomb system such as in phosphorene. Skewed-zigzag and skewed-armchair nanoribbons are semiconducting and metallic, respectively, in contrast to their normal edge counterparts. Their band structures are tunable, and a metal-insulator transition is induced by an electric field. We predict a field-effect transistor based on the edge states in skewed-armchair nanoribbons, where the edge state is gapped by applying arbitrary small electric field $E_z$. A topological argument is presented, revealing the condition for the emergence of such edge states.

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arXiv2016

Antiferromagnetic Skyrmion: Stability, Creation and Manipulation

Magnetic skyrmions are particle-like topological excitations in ferromagnets, which have the topological number $Q=\pm 1$, and hence show the skyrmion Hall effect (SkHE) due to the Magnus force effect originating from the topology. Here, we propose the counterpart of the magnetic skyrmion in the antiferromagnetic (AFM) system, that is, the AFM skyrmion, which is topologically protected but without showing the SkHE. Two approaches for creating the AFM skyrmion have been described based on micromagnetic lattice simulations: (i) by injecting a vertical spin-polarized current to a nanodisk with the AFM ground state; (ii) by converting an AFM domain-wall pair in a nanowire junction. It is demonstrated that the AFM skyrmion, driven by the spin-polarized current, can move straightly over long distance, benefiting from the absence of the SkHE. Our results will open a new strategy on designing the novel spintronic devices based on AFM materials.

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arXiv2016

Direct Band Gaps in Group IV-VI Monolayer Materials: Binary Counterparts of Phosphorene

We perform systematic investigation on the geometric, energetic and electronic properties of group IV-VI binary monolayers (XY), which are the counterparts of phosphorene, by employing density functional theory based electronic structure calculations. For this purpose, we choose the binary systems XY consisting of equal numbers of group IV (X = C, Si, Ge, Sn) and group VI elements (Y = O, S, Se, Te) in three geometrical configurations, the puckered, buckled and planar structures. The results of binding energy calculations show that all the binary systems studied are energetically stable. It is observed that, the puckered structure, similar to that of phosphorene, is the energetically most stable geometric configuration. Our results of electronic band structure predict that puckered SiO and CSe are direct band semiconductors with gaps of 1.449 and 0.905 eV, respectively. Band structure of CSe closely resembles that of phosphorene. Remaining group IV-VI binary monolayers in the puckered configuration and all the buckled monolayers are also semiconductors, but with indirect band gaps. Importantly, we find that the difference between indirect and direct band gaps is very small for many puckered monolayers. Thus, there is a possibility of making these systems undergo transition from indirect to direct band gap semiconducting state by a suitable external influence. Indeed, we show in the present work that seven binary monolayers namely SnS, SiSe, GeSe, SnSe, SiTe, GeTe and SnTe become direct band gap semiconductors when they are subjected to a small mechanical strain (<= 3 %). This makes nine out of sixteen binary monolayers studied in the present work direct band gap semiconductors. Thus, there is a possibility of utilizing these binary counterparts of phosphorene in future light-emitting diodes and solar cells.

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arXiv2016

High-topological-number magnetic skyrmions and topologically protected dissipative structure

The magnetic skyrmion with the topological number of unity ($Q=1$) is a well-known nanometric swirling spin structure in the nonlinear $σ$ model with the Dzyaloshinskii-Moriya interaction. Here, we show that magnetic skyrmion with the topological number of two ($Q=2$) can be created and stabilized by applying vertical spin-polarized current though it cannot exist as a static stable excitation. Magnetic skyrmion with $Q=2$ is a nonequilibrium dynamic object, subsisting on a balance between the energy injection from the current and the energy dissipation by the Gilbert damping. Once it is created, it becomes a topologically protected object against fluctuations of various variables including the injected current itself. Hence, we may call it a topologically protected dissipative structure. We also elucidate the nucleation and destruction mechanisms of the magnetic skyrmion with $Q=2$ by studying the evolutions of the magnetization distribution, the topological charge density as well as the energy density. Our results will be useful for the study of the nontrivial topology of magnetic skyrmions with higher topological numbers.

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arXiv2015

Purely electrical detection of a skyrmion in constricted geometry

How to detect the skyrmion position is a crucial problem in future skyrmionics since it corresponds to the reading process of information. We propose a method to detect the skyrmion position purely electrically by measuring the Hall conductance in a constricted geometry. The Hall conductance becomes maximum when a skyrmion is at the lead position. It is possible to detect the skyrmion position even at room temperature. We find an optimized width of the sample determined by the skyrmion radius. We also investigate the effects of elastic and inelastic scatterings, and finite temperature. We find that the local density of states become minimum at the skyrmion position. Our results will be a basis of future skyrmion electronics.

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arXiv2015

Loop-nodal and Point-nodal Semimetals in Three-dimensional Honeycomb Lattices

Honeycomb structure has a natural extension to the three dimensions. Simple examples are hyperhoneycomb and stripy-honeycomb lattices, which are realized in $β$-Li$_{2}$IrO$_{3}$ and $γ$-Li$_{2}$IrO$_{3}$, respectively. We propose a wide class of three-dimensional (3D) honeycomb lattices which are loop-nodal semimetals. Their edge states have intriguing properties similar to the two-dimensional honeycomb lattice in spite of dimensional difference. Partial flat bands emerge at the zigzag or beard edge of the 3D honeycomb lattice, whose boundary is given by the Fermi loop in the bulk spectrum. Analytic solutions are explicitly constructed for them. On the other hand, perfect flat bands emerge in the zigzag-beard edge or when the anisotropy is large. All these 3D honeycomb lattices become strong topological insulators with the inclusion of the spin-orbit interaction. Furthermore, point-nodal semimetals may be realized in the presence of both the antiferromagnetic order and the spin-orbit interaction.

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