arXiv subjects

Daigo Oue

Publications and source records attributed to Daigo Oue.

33 records · Page 2Linked to original sources

arXiv2022

Valley transport driven by dynamic lattice distortion

Angular momentum conversion between mechanical rotation and the valley degree of freedom in 2D Dirac materials is investigated theoretically. Coupling between the valley and vorticity of dynamic lattice distortions is derived by applying the k.p method to 2D Dirac materials with an inertial effect. Lattice strain effects are also incorporated. Valley transfer and valley-dependent carrier localization are predicted using the dynamic lattice distortions. The transport properties are found to be controllable, allowing the system to be insulating and to generate pulsed charge current. Our formalism offers a route toward mechanical manipulation of valley dynamics in 2D Dirac materials.

cond-mat.mes-hall↗

arXiv2022

Valley hydrodynamics in gapped graphene

Recent experiments have revealed that novel nonequilibrium states consistent with the hydrodynamic description of electrons are realized in ultrapure graphene, which hosts the valley degrees of freedom. Here, we formulate a theory of electron hydrodynamics including dissipation processes of the valley angular momentum by employing the concept of micropolar fluids. As a result, our theory proposes a novel strategy to generate a valley polarization by the microrotation. We uncover that the rotational viscosity induces longitudinal valley currents which are second order in electric fields.

cond-mat.mes-hall↗

arXiv2022

Dynamical metasurfaces: electromagnetic properties and instabilities

In this thesis, I analyse the electromagnetic properties of dynamical metasurfaces and find two critical phenomena. The first is the Casimir-induced instability of a deformable metallic film. In general, two charge-neutral interfaces attract with or repel each other due to the contribution from the zero-point fluctuation of the electromagnetic field between them, namely, the Casimir effect. The effects of perturbative interface corrugation on the Casimir energy in the film system is studied by the proximity force approximation with dispersion correction. If the corrugation period exceeds a critical value, the Casimir effect dominates the surface tension and brings about structural instability. The second is Čerenkov radiation in the vacuum from a time-varying, corrugated surface. Travelling faster than light brings about electromagnetic shock waves, Čerenkov radiation. Since light is the fastest object in a vacuum, it has been considered that Čerenkov radiation is emitted only in the presence of some refractive index. Here, I propose mimicking a series of particles travelling faster than light in a vacuum by dynamical surface corrugation to find Čerenkov radiation in a vacuum from the surface. The dynamical corrugation induces an effective current source on the surface with an external electrostatic field applied. When the corrugation profile is of travelling wave type, the source can be regarded as a series of dipoles virtually travelling along the surface. If the phase velocity of the travelling wave profile exceeds the speed of light, and so do the dipoles, they emit Čerenkov radiation in a vacuum.

quant-ph↗

arXiv2022

Motion-induced spin transfer

We propose a spin transport induced by inertial motion. Our system is composed of two host media and a narrow vacuum gap in between. One of the hosts is sliding at a constant speed relative to the other. This mechanical motion causes the Doppler effect that shifts the density of states and the nonequilibrium distribution function in the moving medium. Those shifts induce the difference in the distribution function between the two media and result in tunnelling spin current. The spin current is calculated from the Schwinger-Keldysh formalism with a spin tunnelling Hamiltonian. This scheme does not require either temperature difference, voltage or chemical potential.

cond-mat.mes-hall↗

arXiv2021

Einstein--de Haas fluctuation of a nanoparticle in spin polarized gases

We theoretically study angular momentum (AM) transfer from a spin-polarized dilute gas into an nanoparitcle (NP) tightly trapped in optical tweezers. We formulate a microscopic model based on the spin tunneling Hamiltonian method and derive a macroscopic stochastic differential equation (SDE) which governs the AM-transfer-induced rotational motion of the NP. It is shown that the AM transfer rate at the NP surface can be extracted via the inference of the SDE. This work will open the door to the manipulation of nano-spintronic systems in gaseous environments.

cond-mat.mes-hall↗

arXiv2021

Čerenkov radiation in vacuum from a superluminal grating

Nothing can physically travel faster than light in vacuum. This is why it has been considered that there is no Čerenkov radiation (ČR) without an effective refractive index due to some background field. In this Letter, we theoretically predict ČR in vacuum from a spatiotemporally modulated boundary. We consider the modulation of traveling wave type and apply a uniform electrostatic field on the boundary to generate electric dipoles. Since the induced dipoles stick to the interface, they travel at the modulation speed. When the grating travels faster than light, it emits ČR. In order to quantitatively examine this argument, we need to calculate the field scattered at the boundary. We utilise a dynamical differential method, which we developed in the previous paper, to quantitatively evaluate the field distribution in such a situation. We can confirm that all scattered fields are evanescent if the modulation speed is slower than light while some become propagating if the modulation is faster than light.

physics.optics↗

arXiv2021

Calculating spatiotemporally modulated surfaces: a dynamical differential formalism

Electromagnetic waves in a system with a space and time dependent boundary experience both diffraction and Doppler-like frequency conversion. In order to analyse such situations, conventional methods call for either the eigenmodes or the dyadic Green's function in space and time dependent media. Here, we propose a dynamical differential method which does not require either of them. Our method utilises a dynamical coordinate transformation in order to simplify the calculation of the optical response of the space and time dependent system. We reveal that the diffraction symmetry is broken in the presence of traveling-wave type spatiotemporal modulation.

physics.optics↗

arXiv2020

Optically induced electron spin currents in the Kretschmann configuration

We investigate electron spin currents induced optically via plasmonic modes in the Kretschmann configuration. By utilising the scattering matrix formalism, we take the plasmonic mode coupled to external laser drive into consideration and calculate induced magnetisation in the metal. The spatial distribution of the plasmonic mode is inherited by the induced magnetisation, which acts as inhomogeneous effective magnetic field and causes the Stern-Gerlach effect to drive electron spin currents in the metal. We solve spin diffusion equation with a source term to analyse the spin current as a function of the spin diffusion length of the metal, the frequency and the incident angle of the external drive.

cond-mat.mes-hall↗

arXiv2020

Spin Current Generation by a Surface Plasmon Polariton

Surface plasmon polariton (SPP) is an electromagnetic wave which is tightly localised beyond the diffraction limit at metallic surfaces. Recently, it is theoretically proposed that the angular momentum conversion between the SPP and electrons. In this work, we have successfully measured SPP induced spin currents, which proves the fact that the angular momenta are interconverted. Such conversion from light to a spin current can be used as a coupler in a next generation spintronic computing with optical data transfer or storage.

cond-mat.mes-hall↗

arXiv2020

Effects of surface plasmons on spin currents in a thin film system

We propose and analyze surface-plasmon-driven electron spin currents in a thin metallic film. The electron gas in the metal follows the transversally rotating electric fields of the surface plasmons (SPs), which leads to a static magnetization gradient. We consider herein SPs in a thin-film insulator-metal-insulator structure and solve the spin diffusion equation in the presence of a magnetization gradient. The results reveal that the SPs at the metal interfaces generate spin currents in the metallic film. For thinner film, the SPs become strongly hybridized, which increases the magnetization gradient and enhances the spin current. We also discuss how the spin current depends on SP wavelength and the spin-diffusion length of the metal. The polarization of the spin current can be controlled by tuning the wavelength of the SPs and/or the spin diffusion length.

cond-mat.mes-hall↗

arXiv2020

Electron spin transport driven by surface plasmon polariton

We propose a mechanism of angular momentum conversion from optical transverse spin in surface plasmon polaritons (SPPs) to conduction electron spin. Free electrons in the metal follow the transversally spinning electric field of SPP, and the resulting orbital motions create inhomogeneous static magnetisation in the metal. By solving the spin diffusion equation in the SPP, we find that the magnetisation field generates an electron spin current. We show that there exists a resonant condition where the spin current is resonantly enhanced, and the polarisation of the spin current is flipped. Our theory reveals a novel functionality of SPP as a spin current source.

cond-mat.mes-hall↗

arXiv2020

Casimir induced instabilities at metallic surfaces and interfaces

Surface plasmons subject to a surface distortion split asymmetrically in energy resulting in a net lowering of zero-point energy. This is because surface plasmon eigenvalues are the square of frequencies, a statement generally true for electromagnetic excitations. We utilize the method based on conformal mapping to demonstrate asymmetric splitting under surface corrugations leading to a decrease in zero-point energy of a single corrugated metallic surface contributing to surface reconstructions but too small on its own to drive the reconstruction. However, by introducing a second metallic surface more significant lowering of energy is seen sufficient to drive the instability of a mercury thin film.

cond-mat.mes-hall↗

arXiv2019

Electromagnetism at finite temperature: a density operator approach

In order to analyse classical electromagnetism in a medium at finite temperature we introduce `an optical density operator', and reformulate Maxwell's equations with the operator, starting from the Dirac-equation-like formulation of electromagnetism. We find the thermal state of electromagnetic field in the medium from the `optical Dirac Hamiltonian', which is the effective Hamiltonian in the Dirac-like formulation. In the thermal state, the two transverse modes (left-handed and right-handed circular polarisation) of electromagnetic fields exist at the same ratio. We also analyse the asymptotics of the thermal state. At the low temperature limit, there is correlation between the electric field and the magnetic field. This means that there exists an electromagnetic wave at the thermal equilibrium, and this recovers Maxwell's classical electromagnetism. In contrast, the correlation vanishes at the high temperature limit. This means that electromagnetic waves are unsustainable and only independent electric fields and magnetic fields exist at the high temperature limit.

physics.optics↗

arXiv2019

Dissipation Effect on Optical Force and Torque near Interfaces

The Fresnel-Snell law, which is one of the fundamental laws in optics and gives insights on the behaviour of light at interfaces, is violated if there exists dissipation in the transmitting media. In order to overcome this problem, we extend the angle of refraction from a real number to a complex number. We use this complex-angle approach to analyse the behaviour of light at interfaces between lossy media and lossless media. We reveal that dissipation makes the wavenumber of the light exceed the maximum allowed at lossless interfaces. This is surprising because, in general, dielectric loss only change the intensity profiles of the light, so this excess wavenumber cannot be produced in the bulk even if there exists dielectric loss. Additionally, anomalous circular polarisation emerges with dissipation. The direction of the anomalous circular polarisation is transverse, whereas without dissipation the direction of circular polarisation has to be longitudinal. We also discuss how the excess wavenumber can increase optical force and how the anomalous circular polarisation can generate optical transverse torque. This novel state of light produced by dissipation will pave the way for a new generation of optical trapping and manipulation.

physics.optics↗

arXiv2018

Complex-angle analysis of electromagnetic waves on interfaces

Electromagnetic wave is reflected and refracted at interfaces, satisfying Fresnel-Snell law which is required by conservations of energy and momentum. If the incident angle is lower than the critical angle, we can use this Fresnel-Snell law, but the Fresnel-Snell law is modified in the case of existence of dissipation ($\tilde{n} = n + iκ, κ> 0$) or in the condition of total internal reflection. In the cases, we have to extend the angle of refraction from real number to complex number ($θ\rightarrow \tildeθ = θ+ iψ, ψ\neq 0$). In this paper, by using complex-angle approach, we analyse the behaviour of electromagnetic waves in various kind of interfaces: dielectric - dielectric system, dissipative dielectric - dielectric system, and metal - dielectric system. We show that iso-frequency curves in wavenumber space is opened in the case where $n, κ> 0$, and closed either when $n \rightarrow 0$ or when $κ\rightarrow 0$ ("Lifshitz transition" of electromagnetic waves). Excess momentum (wavenumber) and anomalous spin (circular polarization) emerging with the transition are also discussed.

physics.optics↗