arXiv subjects

Yuya Ominato

Publications and source records attributed to Yuya Ominato.

30 records · Page 2Linked to original sources

arXiv2020

Quantum Oscillations of Gilbert Damping in Ferromagnetic/Graphene Bilayer Systems

We study the spin dynamics of a ferromagnetic insulator on which graphene is placed. We show that the Gilbert damping is enhanced by the proximity exchange coupling at the interface. The modulation of the Gilbert damping constant is proportional to the product of the spin-up and spin-down densities of states of graphene. Consequently, the Gilbert damping constant in a strong magnetic field oscillates as a function of the external magnetic field that originates from the Landau level structure of graphene. We find that a measurement of the oscillation period enables the strength of the exchange coupling constant to be determined. The results demonstrate in theory that the ferromagnetic resonance measurements may be used to detect the spin resolved electronic structure of the adjacent materials, which is critically important for future spin device evaluations.

cond-mat.mes-hall↗

arXiv2019

Phase diagram of a magnetic topological nodal semimetal: Stable nodal line in an easy-plane ferromagnet

We study a topological phase diagram of a ferromagnetic topological nodal semimetal. We consider a lattice model for three-dimensional topological insulators with ferromagnetic ordering. The exchange coupling between the magnetization and the electron spin leads to the nodal band structure. The topology of the nodal band structure depends on the direction of the magnetization and both the Weyl points and the nodal line emerge. We find that the nodal line structure is stable under an easy-plane magnetization in appropriate model parameters. In this case, the nodal line phase emerges as a phase boundary between two topologically distinct Weyl semimetal phases.

cond-mat.mes-hall↗

arXiv2019

Spin-orbit crossed susceptibility in topological Dirac semimetals

We theoretically study the spin-orbit crossed susceptibility of topological Dirac semimetals. Because of strong spin-orbit coupling, the orbital motion of electrons is modulated by Zeeman coupling, which contributes to orbital magnetization. We find that the spin-orbit crossed susceptibility is proportional to the separation of the Dirac points and it is highly anisotropic. The orbital magnetization is induced only along the rotational symmetry axis. We also study the conventional spin susceptibility. The spin susceptibility exhibits anisotropy and the spin magnetization is induced only along the perpendicular to the rotational symmetry axis in contrast to the spin-orbit crossed susceptibility. We quantitatively compare the two susceptibilities and find that they can be comparable.

cond-mat.mes-hall↗

arXiv2018

Electric polarization in magnetic topological nodal semimetal thin films

We theoretically study the electric polarization in magnetic topological nodal semimetal thin films. In magnetically doped topological insulators, topological nodal semimetal phases emerge once the exchange coupling overcomes the band gap. Changing the magnetization direction, nodal structure is modulated and the system becomes topological nodal point or line semimetals. We find that nodal line semimetals are characterized by non-linear electric polarization, which is not observed in nodal point semimetals. The non-linear response originates from the existence of the surface states. Screening effect is self consistently included within a mean field approximation and the non-linear electric polarization is observed even in the presence of screening effect.

cond-mat.mes-hall↗

arXiv2018

Helicity-protected domain-wall magnetoresistance in ferromagnetic Weyl semimetal

The magnetotransport properties of disordered ferromagnetic Weyl semimetals are investigated numerically. We found an extraordinarily stable and huge magnetoresistance effect in domain walls of Weyl semimetals. This effect originates from the helicity mismatch of Weyl fermions and is a specific property of Weyl semimetals. Although conventional magnetoresistance effects are strongly suppressed in domain walls where local magnetization varies gradually, the helicity-protected magnetoresistance in Weyl semimetals maintains almost $100\%$ of the magnetoresistance ratio for any kind of thick domain walls, even in the presence of disorder. The contribution of surface Fermi arcs to the magnetoresistance is also discussed.

cond-mat.mes-hall↗

arXiv2018

Spin susceptibility of three-dimensional Dirac semimetals

We theoretically study the spin susceptibility of Dirac semimetals using the linear response theory. The spin susceptibility is decomposed into an intraband contribution and an interband contribution. We obtain analytical expressions for the intraband and interband contributions of massless Dirac fermions. The spin susceptibility is independent of the Fermi energy while it depends on the cutoff energy, which is introduced to regularize the integration. We find that the cutoff energy is appropriately determined by comparing the results for the Wilson-Dirac lattice model, which approximates the massless Dirac Hamiltonian around the Dirac point. We also calculate the spin susceptibility of massive Dirac fermions for the model of topological insulators. We discuss the effect of the band inversion and the strength of spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2016

Anisotropic magnetotransport in Dirac-Weyl magnetic junctions

We theoretically study the anisotropic magnetotransport in Dirac-Weyl magnetic junctions where a doped ferromagnetic Weyl semimetal is sandwiched between doped Dirac semimetals. We calculate the conductance using the Landauer formula and find that the system exhibits extraordinarily large anisotropic magnetoresistance (AMR). The AMR depends on the ratio of the Fermi energy and the strength of the exchange interaction. The origin of the AMR is the shift of the Fermi surface in the Weyl semimetal and the mechanism is completely different from the conventional AMR originating from the spin dependent scattering and the spin-orbit interaction.

cond-mat.mes-hall↗

arXiv2016

Magnetotransport in the Weyl semimetal in the quantum limit - the role of the topological surface states

We theoretically study the magnetoconductivity of the Weyl semimetal having a surface boundary under $E$ || $B$ geometry and demonstrate that the topological surface state plays an essential role in the magnetotransport. In the long-range disorder limit where the scattering between the two Weyl nodes vanishes, the conductivity diverges in the bulk model (i.e., periodic boundary condition) as usually expected, since the direct inter-node relaxation is absent. In the presence of the surface, however, the inter-node relaxation always takes place through the mediation by the surface states, and that prevents the conductivity divergence. The magnetic-field dependence becomes also quite different between the two cases, where the conductivity linearly increases in $B$ in the surface boundary case, in contrast to $B$-independent behavior in the bulk-periodic case. This is an interesting example in which the same system exhibits completely different properties in the surface boundary condition and the periodic boundary condition even in the macroscopic size limit. In the short-range regime where the direct intervalley scattering is dominant, the surface states are irrelevant and the conductivity approaches that of the bulk periodic model.

cond-mat.mes-hall↗

arXiv2014

Quantum transport in three-dimensional Weyl electron system -- in the presence of charged impurity scattering

We theoretically study the quantum transport in three-dimensional Weyl electron system in the presence of the charged impurity scattering using a self-consistent Born approximation (SCBA). The scattering strength is characterized by the effective fine structure constant $α$, which depends on the dielectric constant and the Fermi velocity of the linear band. We find that the Boltzmann theory fails at the band touching point, where the conductivity takes a nearly constant value almost independent of $α$, even though the density of states linearly increases with $α$. There the magnitude of the conductivity only depends on the impurity density. The qualitative behavior is quite different from the case of the Gaussian impurities, where the minimum conductivity vanishes below a certain critical impurity strength.

cond-mat.dis-nn↗

arXiv2014

Quantum transport in three-dimensional Weyl electron system

Quantum transport in three-dimensional massless Dirac electron system with long-range Gaussian impurities is studied theoretically using a self-consistent Born approximation (SCBA). We find that the conductivity significantly changes the behavior at a certain scattering strength which separates the weak and strong disorder regimes. In the weak disorder regime, the SCBA conductivity mostly agrees with the Boltzmann conductivity, while the agreement fails near the Dirac point where the SCBA conductivity drops to zero linearly to the Fermi energy. In the strong disorder regime, the conductivity is smooth and finite near the Dirac point, and the minimum conductivity becomes larger in increasing the disorder potential, contrary to the usual metallic behavior. The theory applies to three dimensional gapless band structures, including Weyl semimetals.

cond-mat.dis-nn↗

arXiv2013

Orbital magnetism of graphene flakes

Orbital magnetism is studied for graphene flakes with various shapes and edge configurations using the tight-binding approximation. In the low-temperature regime where the thermal energy is much smaller than to the energy level spacing, the susceptibility rapidly changes between diamagnetism and paramagnetism as a function of Fermi energy, in accordance with the energy level structure. The susceptibility at charge neutral point is generally larger in armchair flake than in zigzag flake, and larger in hexagonal flake than in triangular flake. As the temperature increases, the discrete structures due to the quantum confinement are all gone, and the susceptibility approximates the bulk limit independently of the atomic configuration. The diamagnetic current circulates entirely on the graphene flake at zero temperature, while in increasing temperature it is localized near the edge with the characteristic depth proportional to 1/T. We predict that the diamagnetism of graphene can be observed using the alignment of graphene flakes in a feasible range of magnetic field.

cond-mat.mes-hall↗

arXiv2012

Orbital magnetic susceptibility of finite-sized graphene

We study the orbital magnetism of graphene ribbon in the effective-mass approximation, to figure out the finite-size effect on the singular susceptibility known in the bulk limit. We find that the susceptibility at T = 0 oscillates between diamagnetism and paramagnetism as a function of Fermi energy, in accordance with the subband structure formed by quantum confinement. In increasing T, the oscillation rapidly disappears once the thermal broadening energy exceeds the subband spacing, and the susceptibility approaches the bulk limit i.e., a thermally broadened diamagnetic peak centered at zero energy point. The electric current supporting the diamagnetism is found to flow near the edge with a depth which proportional to reciprocal of T, with v being the band velocity, while at T = 0 the current distribution spreads entirely in the sample reflecting the absence of the characteristic wavelength in graphene. The result is applied to estimate the three-dimensional random-stacked multilayer graphene, where we show that the external magnetic field is significantly screened inside the sample in low temperatures, in a much stronger manner than in graphite.

cond-mat.mes-hall↗