Optical chirality in gyrotropic media: Symmetry approach

We discuss optical chirality in different types of gyrotropic media. Our analysis is based on the formalism of nongeometric symmetries of Maxwell's equations in vacuum generalized to material media with given constituent relations. This approach enables us to directly derive conservation laws related to nongeometric symmetries. For isotropic chiral media, we demonstrate that like a free electromagnetic field, both duality and helicity generators belong to the basis set of nongeometric symmetries that guarantees the conservation of optical chirality. In gyrotropic crystals, which exhibit natural optical activity, the situation is quite different from the case of isotropic media. For light propagating along a certain crystallographic direction, there arises two distinct cases: (1) the duality is broken but the helicity is preserved, or (2) only the duality symmetry survives. We show that the existence of one of these symmetries (duality or helicity) is enough to define optical chirality. In addition, we present examples of low-symmetry media, where optical chirality cannot be defined. © 2017 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.This work was supported by the Government of the Russian Federation Program 02.A03.21.0006 and by RFBR Grant No. 17-52-50013. The authors also acknowledge support by JSPS KAKENHI Grants Nos. 25220803, 17H02923, the JSPS Core-to-Core Program, A. Advanced Research Networks, and the JSPS Bilateral (Japan-Russia) Joint Research Projects. IP acknowledges financial support by Center for Chiral Science, Hiroshima University and by the Ministry of Education and Science of the Russian Federation, Grant No. MK-6230.2016.2

Optical chirality in gyrotropic media: Symmetry approach

We discuss optical chirality in different types of gyrotropic media. Our analysis is based on the formalism of nongeometric symmetries of Maxwell's equations in vacuum generalized to material media with given constituent relations. This approach enables us to directly derive conservation laws related to nongeometric symmetries. For isotropic chiral media, we demonstrate that like a free electromagnetic field, both duality and helicity generators belong to the basis set of nongeometric symmetries that guarantees the conservation of optical chirality. In gyrotropic crystals, which exhibit natural optical activity, the situation is quite different from the case of isotropic media. For light propagating along a certain crystallographic direction, there arises two distinct cases: (1) the duality is broken but the helicity is preserved, or (2) only the duality symmetry survives. We show that the existence of one of these symmetries (duality or helicity) is enough to define optical chirality. In addition, we present examples of low-symmetry media, where optical chirality cannot be defined. © 2017 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.This work was supported by the Government of the Russian Federation Program 02.A03.21.0006 and by RFBR Grant No. 17-52-50013. The authors also acknowledge support by JSPS KAKENHI Grants Nos. 25220803, 17H02923, the JSPS Core-to-Core Program, A. Advanced Research Networks, and the JSPS Bilateral (Japan-Russia) Joint Research Projects. IP acknowledges financial support by Center for Chiral Science, Hiroshima University and by the Ministry of Education and Science of the Russian Federation, Grant No. MK-6230.2016.2

Field-theoretical renormalization group for a flat two-dimensional Fermi surface

We implement an explicit two-loop calculation of the coupling functions and the self-energy of interacting fermions with a two-dimensional flat Fermi surface in the framework of the field theoretical renormalization group (RG) approach. Throughout the calculation both the Fermi surface and the Fermi velocity are assumed to be fixed and unaffected by interactions. We show that in two dimensions, in a weak coupling regime, there is no significant change in the RG flow compared to the well-known one-loop results available in the literature. However, if we extrapolate the flow to a moderate coupling regime there are interesting new features associated with an anisotropic suppression of the quasiparticle weight Z along the Fermi surface, and the vanishing of the renormalized coupling functions for several choices of the external momenta.Comment: 16 pages and 22 figure

Generation of spin motive force in a soliton lattice

The generation of a spin motive force in a chiral helimagnet due to the action of two crossed magnetic fields is considered. The cases of pulsed and periodic magnetic fields directed along the helical axis under a perpendicular dc field are analyzed. It is shown that, in the case of a pulsed field, the spin motive force is related to dissipation, whereas in a periodic field, there is a reactive component that is not related to damping processes. © 2013 Pleiades Publishing, Ltd

Theory of standing spin waves in finite-size chiral spin soliton lattice

We present a theory of standing spin wave (SSW) in a monoaxial chiral helimagnet. Motivated by experimental findings on the magnetic field-dependence of the resonance frequency in thin films of Cr${}$Nb$_{3}$S${}_{6}$[Goncalves et al., Phys. Rev. B95, 104415 (2017)], we examine the SSW over a chiral soliton lattice (CSL) excited by an ac magnetic field applied parallel and perpendicular to the chiral axis. For this purpose, we generalize Kittel-Pincus theories of the SSW in ferromagnetic thin films to the case of non-collinear helimagnet with the surface end spins which are softly pinned by an anisotropy field. Consequently, we found there appear two types of modes. One is a Pincus mode which is composed of a long-period Bloch wave and a short-period ripple originated from the periodic structure of the CSL. Another is a short-period Kittel ripple excited by space-periodic perturbation which exists only in the case where the ac field is applied perpendicular the chiral axis. We demonstrate that the existence of the Pincus mode and the Kittel ripple is consistent with experimentally found double resonance profile.Comment: 17 pages, 14 figure

Adiabatic and Nonadiabatic Spin-Transfer Torques in the Current-Driven Magnetic Domain Wall Motion

A consistent theory to describe the correlated dynamics of quantum-mechanical itinerant spins and semiclassical local magnetization is given. We consider the itinerant spins as quantum-mechanical operators, whereas local moments are considered within classical Lagrangian formalism. By appropriately treating fluctuation space spanned by basis functions, including a zero-mode wave function, we construct coupled equations of motion for the collective coordinate of the center-of-mass motion and the localized zero-mode coordinate perpendicular to the domain wall plane. By solving them, we demonstrate that the correlated dynamics is understood through a hierarchy of two time scales: Boltzmann relaxation time τel, when a nonadiabatic part of the spin-transfer torque appears, and Gilbert damping time τDW, when adiabatic part comes up. © 2010 The American Physical Society.J.K. acknowledges Grant-in-Aid for Scientific Research © (Grant No. 19540371) from the Ministry of Education, Culture, Sports, Science and Technology, Japan

Theory of Spin Current in Chiral Helimagnets

We give detailed description of the transport spin current in the chiral helimagnet. under the static magnetic field applied perpendicular to the helical axis, the magnetic kink crystal (chiral soliton lattice) is formed. Once the kink crystal begins to move under the Galilean boost, the spin-density accumulation occurs inside each kink and there emerges periodic arrays of the induced magnetic dipoles carrying the transport spin current. The coherent motion of the kink crystal dynamically generates the spontaneous demagnetization field. To describe the kink crystal motion, we took account of not only the tangential φ fluctuations but the longitudinal θ fluctuations around the helimagnetic configuration. Based on the collective coordinate method and the Dirac canonical formulation for the singular Lagrangian system, we derived the closed formulas for the mass, spin current, and induced magnetic-dipole moment accompanied with the kink crystal motion. To materialize the theoretical model presented here, symmetry-adapted material synthesis would be required, where the interplay of crystallographic and magnetic chirality plays a key role there. © 2008 The American Physical Society

Renormalization Group Technique Applied to the Pairing Interaction of the Quasi-One-Dimensional Superconductivity

A mechanism of the quasi-one-dimensional (q1d) superconductivity is investigated by applying the renormalization group techniques to the pairing interaction. With the obtained renormalized pairing interaction, the transition temperature Tc and corresponding gap function are calculated by solving the linearized gap equation. For reasonable sets of parameters, Tc of p-wave triplet pairing is higher than that of d-wave singlet pairing due to the one-dimensionality of interaction. These results can qualitatively explain the superconducting properties of q1d organic conductor (TMTSF)2PF6 and the ladder compound Sr2Ca12Cu24O41.Comment: 18 pages, 9 figures, submitted to J. Phys. Soc. Jp

Magnetic Soliton Transport Over Topological Spin Texture in Chiral Helimagnet with Strong Easy-plane Anisotropy

We show the existence of an isolated soliton excitation over the topological ground-state configuration in chiral helimagnet with the Dzyaloshinskii-Moryia exchange and the strong easy-plane anisotropy. The magnetic field perpendicular to the helical axis stabilizes the kink crystal state which plays a role of "topological protectorate" for the traveling soliton with a definite handedness. To find new soliton solution, we use the Bäcklund transformation technique. It is pointed out that the traveling soliton carries the magnon density and a magnetic soliton transport may be realized. © 2009 The American Physical Society.J.K. acknowledges Grant-in-Aid for Scientific Research (A) (Contract No. 18205023) and (C) (Contract No. 19540371) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan