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Register a new userSpin Angular Momentum Transfer and Plasmogalvanic Phenomena
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We introduce the continuity equation for the electromagnetic spin angular momentum (SAM) in matter and discuss the torque associated with the SAM transfer in terms of effective spin forces acting in a material. In plasmonic metal, these spin forces result in plasmogalvanic phenomenon which is pinning the plasmon-induced electromotive force to atomically-thin layer at the metal interface.
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We review the recently discovered spin Hall magnetoresistance (SMR) and related effects from a theoretical point of view. The SMR is observed in bilayers of a magnetic insulator and a metal, in which spin currents aregenerated in the normal metal due to the spin Hall effect. The associated angular momentum transfer to the ferromagnetic layer and thereby the electrical resistance is modulated by the angle between the applied current and the magnetization direction. The SMR provides a convenient tool to non-invasively measure the magnetization direction and spin-transfer torque to an insulator. We introduce the minimal theoretical instruments to calculate the SMR, i.e. spin diffusion theory and quantum mechanical boundary conditions. This leads to a small set of parameters that can be fitted to experiments. We discuss the limitations of the theory as well as alternative mechanisms such as the ferromagnetic proximity effect and Rashba spin-orbit torques, and point out new developments related to the SMR.
Spin and orbital angular momentum of light plays a central role in quantum nanophotonics as well as topological electrodynamics. Here, we show that the thermal radiation from finite-sized bodies comprising of nonreciprocal magneto-optical materials can exert a spin torque even in global thermal equilibrium. Moving beyond the paradigm of near-field heat transfer, we calculate near-field radiative angular momentum transfer between finite-sized nonreciprocal objects by combining Rytovs fluctuational electrodynamics with the theory of optical angular momentum. We prove that a single magneto-optical cubic particle in non-equilibrium with its surroundings experiences a torque in the presence of an applied magnetic field (T-symmetry breaking). Furthermore, even in global thermal equilibrium, two particles with misaligned gyrotropic axes experience equal magnitude torques with opposite signs which tend to align their gyrotropic axes parallel to each other. Our results are universally applicable to semiconductors like InSb (magneto-plasmas) as well as Weyl semi-metals which exhibit the anomalous Hall effect (gyrotropy) at infrared frequencies. Our work paves the way towards near-field angular momentum transfer mediated by thermal fluctuations for nanoscale devices.
Arising from the interplay between charge, spin and orbital of electrons, spin-orbit torque (SOT) has attracted immense interest in the past decade. Despite vast progress, the existing quantification methods of SOT still have their respective restrictions on the magnetic anisotropy, the entanglement between SOT effective fields, and the artifacts from the thermal gradient and the planar Hall effect, etc. Thus, accurately characterizing SOT across diverse samples remains as a critical need. In this work, with the aim of removing the afore-mentioned restrictions, thus enabling the universal SOT quantification, we report the characterization of the sign and amplitude of SOT by angular measurements. We first validate the applicability of our angular characterization in a perpendicularly magnetized Pt/Co-Ni heterostructure by showing excellent agreements to the results of conventional quantification methods. Remarkably, the thermoelectric effects, i.e., the anomalous Nernst effect (ANE) arising from the temperature gradient can be self-consistently disentangled and quantified from the field dependence of the angular characterization. The superiority of this angular characterization has been further demonstrated in a Cu/CoTb/Cu sample with large ANE but negligible SOT, and in a Pt/Co-Ni sample with weak perpendicular magnetic anisotropy (PMA), for which the conventional quantification methods are not applicable and even yield fatal error. By providing a comprehensive and versatile way to characterize SOT and thermoelectric effects in diverse heterostructures, our results pave the important foundation for the spin-orbitronic study as well as the interdisciplinary research of thermal spintronic.
We find that in BaTiO$_3$ the phonon angular momentum is dominantly pointing in directions perpendicular to the electrical polarization. Therefore, external electric field in ferroelectric BaTiO$_3$ does not control only the direction of electrical polarization, but also the direction of phonon angular momentum. This finding opens up the possibility for electric-field control of physical phenomena that rely on phonon angular momentum. We construct an intuitive model, based on our first-principles calculations, that captures the origin of the relationship between phonon angular momentum and electric polarization.
We present an optomechanical device designed to allow optical transduction of orbital angular momentum of light. An optically induced twist imparted on the device by light is detected using an integrated cavity optomechanical system based on a nanobeam slot-mode photonic crystal cavity. This device could allow measurement of the orbital angular momentum of light when photons are absorbed by the mechanical element, or detection of the presence of photons when they are scattered into new orbital angular momentum states by a sub-wavelength grating patterned on the device. Such a system allows detection of a $l = 1$ orbital angular momentum field with an average power of $3.9times10^3$ photons modulated at the mechanical resonance frequency of the device and can be extended to higher order orbital angular momentum states.
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