Axion Electrodynamics Research Articles

Thin layer axion dynamo

We study interacting classical magnetic and pseudoscalar fields in frames of the axion electrodynamics. A large scale pseudoscalar field can be the coherent superposition of axions or axion like particles. We consider the evolution of these fields inside a spherical clump. Decomposing the magnetic field into the poloidal and toroidal components, we take into account their symmetry properties. Within a spherical clump, we use a thin layer approximation in the induction and Klein–Gordon equations, where the dependence of the fields on the latitude is accounted for. Then, we derive the dynamo equations in the low mode approximation. The nonlinear evolution equations for the harmonics of the magnetic and pseudoscalar fields are solved numerically. As an application, we consider a dense axion star embedded in solar plasma. The behavior of the harmonics and their typical oscillations frequencies are obtained. We suggest that such small size axionic objects, containing oscillating magnetic fields, can cause electromagnetic flashes, recently observed in the solar corona, contributing to the corona heating.

Axion‐Like Interactions and CFT in Topological Matter, Anomaly Sum Rules and the Faraday Effect

AbstractFundamental aspects of chiral anomaly‐driven interactions in conformal field theory (CFT) in four spacetime dimensions are discussed. These interactions find application in very general contexts, from early universe plasma to topological condensed matter. The key shared characteristics of these interactions are outlined, specifically addressing the case of chiral anomalies, both for vector currents and gravitons. In the case of topological materials, the gravitational chiral anomaly is generated by thermal gradients via the (Tolman–Ehrenfest) Luttinger relation. In the CFT framework, a nonlocal effective action, derived through perturbation theory, indicates that the interaction is mediated by excitation in the form of an anomaly pole, which appears in the conformal limit of the vertex. To illustrate this, it is demonstrated how conformal Ward identities (CWIs) in momentum space allow to reconstruct the entire chiral anomaly interaction in its longitudinal and transverse sectors just by inclusion of a pole in the longitudinal sector. Both sectors are coupled in amplitudes with an intermediate chiral fermion or a bilinear Chern–Simons current with intermediate photons. In the presence of fermion mass corrections, the pole transforms into a cut, but the absorption amplitude in the axial‐vector channel satisfies mass‐independent sum rules related to the anomaly in any chiral interaction. The detection of an axion‐like/quasiparticle in these materials may rely on a combined investigation of these sum rules, along with the measurement of the angle of rotation of the plane of polarization of incident light when subjected to a chiral perturbation. This phenomenon serves as an analog of a similar one in ordinary axion physics, in the presence of an axion‐like condensate, which is rederived using axion electrodynamics.

Electromagnetic fields and radiation from localized current source interacting with a cosmic axion field in the context of massive axion electrodynamics

Electromagnetic fields from localized current source interacting with an oscillatory cosmic axion field are studied, accounting for the possibility of a finite photon mass. In particular, focus will be on the dipole radiation from static current sources. The Proca theory is generalized to include the axion field and solutions to the modified EM field equations are obtained to first order in the axion coupling parameter. Analysis of the interplay between the two vanishingly small parameters – the axion mass (via the coupling constant and the oscillating frequency) and the photon mass – is carried out with reference to possible future detection of the cosmic axion and/or the photon mass via electromagnetic interaction with the axion.

Axion electrodynamics without Witten effect in metamaterials

Axion electrodynamics without Witten effect in metamaterials

Zero modes of massive fermions delocalize from axion strings

Massless chiral excitations can arise from the interactions between a fermion and an axion string, propagating along the string and allowing it to superconduct. The properties of these excitations, or zero modes, dictate how the string interacts with light and can thus have important phenomenological consequences. In this paper, we add a nowhere-vanishing Dirac mass for the fermion in the usual model of axion electrodynamics. We find that the zero modes exhibit an interesting phase structure in which they delocalize from the string’s core as the mass increases, up until a critical value past which they disappear. We study this structure from an analytic perspective, with explicit numerical solutions, and via anomaly inflow arguments. Finally, we derive the two-dimensional effective theory of the zero mode and its interactions with the four-dimensional gauge field and show how this effective theory breaks down as the zero modes delocalize.

Photon quantum kinetic equations and collective modes in an axion background

We develop a quantum kinetic theory for photons in the presence of an axion background and in the collisioness limit. In deriving the classical regime of our quantum kinetic equations, we observe that they capture well-known features of axion electrodynamics. By projecting the Wigner function onto a polarization basis, relating the Wigner matrix function with the Stokes parameters, we establish the dispersion relations and transport equations for each polarization space component. Additionally, we investigate how the axion background affects the dispersion relations of photon collective modes within an electron-positron plasma at equilibrium temperature T. While the plasmon remains unaffected, we find that the axion background breaks the degeneracy of transverse collective modes at order egaγT(∂a), where e represents the electron charge, gaγ denotes the photon-axion coupling, and ∂a represents the scale associated with variations in the axion field. Published by the American Physical Society 2024

Dipole-dipole interactions in the presence of a topological insulator stratified sphere: dyadic Green’s function method

Dyadic Green’s functions (DGFs) are developed for calculating electric fields induced by multiple sources in the presence of a topological insulator (TI) stratified sphere within the framework of axion electrodynamics. Generalizing our previous work, these sources can be placed at arbitrary locations rather than be limited outside the TI stratified sphere. Utilizing these DGFs, we explore the topological magneto-electric (TME) effect of the dipole–dipole interaction in the presence of composite structures of an alternating metal-TI stratified sphere, causing some modifications of the energy transfer (ET) enhancement spectrum. Furthermore, for the multipolar resonances of the metal shells, we find the TME-induced red-shifts of each bonding and antibonding mode in the ET enhancement spectrum are independent on the locations and orientations of the two dipoles but only depend on the configuration of these composite structures. These phenomenological findings provide some useful guidance with experimenters to design realistic experiments for exploring possible unique TME signatures via the energy transfer between molecules near/in TI multicoated nanoparticles in the near-infrared region.

Axion Electrodynamics and the Casimir Effect

We present a concise review of selected parts of axion electrodynamics and their application to Casimir physics. We present the general formalism including the boundary conditions at a dielectric surface, derive the dispersion relation in the case where the axion parameter has a constant spatial derivative in the direction normal to the conducting plates, and calculate the Casimir energy for the simple case of scalar electrodynamics using dimensional regularization.

Casimir effect in axion electrodynamics with lattice regularizations

Casimir effect in axion electrodynamics with lattice regularizations

Dipole symmetries from the topology of the phase space and the constraints on the low-energy spectrum

We demonstrate the general existence of a local dipole conservation law in bosonic field theory. The scalar charge density arises from the symplectic form of the system, whereas the tensor current descends from its stress tensor. The algebra of spatial translations becomes centrally extended in presence of field configurations with a finite nonzero charge. Furthermore, when the symplectic form is closed but not exact, the system may, surprisingly, lack a well-defined momentum density. This leads to a theorem for the presence of additional light modes in the system whenever the short-distance physics is governed by a translationally invariant local field theory. We also illustrate this mechanism for axion electrodynamics as an example of a system with Nambu-Goldstone modes of higher-form symmetries.

Radiation of a short linear antenna above a topologically insulating half-space

The topological magnetoelectric effect is a unique macroscopic manifestation of quantum states of matter possessing topological order and it is described by axion electrodynamics. In three-dimensional topological insulators, for instance, the axion coupling is of the order of the fine structure constant, and hence, a perturbative analysis of the field equations is plenty justified. In this paper, we use Green’s function techniques to obtain time-dependent solutions to the axion field equations in the presence of a planar domain wall separating two media with different topological order. We apply our results to investigate the radiation of a short linear antenna near the domain wall.

On the Axion Electrodynamics in a two-dimensional slab and the Casimir effect

We analyze the Axion Electrodynamics in a two-dimensional slab of finite width L containing a homogeneous and isotropic dielectric medium with constant permittivity and permeability. We start from the known decomposition of modes in the nonaxion case and then solve perturbatively the governing equations for the electromagnetic fields to which the axions are also coupled. This is a natural approach, since the finiteness of L destroys the spatial invariance of the theory in the z-direction normal to the plates. In this way, we derive the value of the axion-generated rotation angle of the electric and magnetic fields after their passage through the slab, and use the obtained results to calculate the Casimir force between the two conducting plates. Our calculations make use of the same method as previously outlined by [J. S. Høye and I. Brevik, Eur. Phys. J. Plus 135, 271 (2020)] for the case of Casimir calculations in chiral media and extend former results on the Casimir force in Axion Electrodynamics.

Chirality manipulation of ultrafast phase switches in a correlated CDW-Weyl semimetal

Light engineering of correlated states in topological materials provides a new avenue of achieving exotic topological phases inaccessible by conventional tuning methods. Here we demonstrate a light control of correlation gaps in a model charge-density-wave (CDW) and polaron insulator (TaSe4)2I recently predicted to be an axion insulator. Our ultrafast terahertz photocurrent spectroscopy reveals a two-step, non-thermal melting of polarons and electronic CDW gap via the fluence dependence of a longitudinal circular photogalvanic current. This helicity-dependent photocurrent reveals continuous ultrafast phase switches from the polaronic state to the CDW (axion) phase, and finally to a hidden Weyl phase as the pump fluence increases. Additional distinctive attributes aligning with the light-induced switches include: the mode-selective coupling of coherent phonons to the polaron and CDW modulation, and the emergence of a non-thermal chiral photocurrent above the pump threshold of CDW-related phonons. The demonstrated ultrafast chirality control of correlated topological states here holds large potentials for realizing axion electrodynamics and advancing quantum-computing applications.

Non-standard axion electrodynamics and the dual Witten effect

Standard axion electrodynamics has two closely related features. First, the coupling of a massless axion field to photons is quantized, in units proportional to the electric gauge coupling squared. Second, the equations of motion tell us that a time-dependent axion field in a background magnetic field sources an effective electric current, but a time-dependent axion field in a background electric field has no effect. These properties, which manifestly violate electric-magnetic duality, play a crucial role in experimental searches for axions. Recently, electric-magnetic duality has been used to motivate the possible existence of non-standard axion couplings, which can both violate the usual quantization rule and exchange the roles of electric and magnetic fields in axion electrodynamics. We show that these non-standard couplings can be derived from SL(2,ℤ) duality, but that they come at a substantial cost: in non-standard axion electrodynamics, all electrically charged particles become dyons when the axion traverses its field range, in a dual form of the standard Witten effect monodromy. This implies that there are dyons near the weak scale, leads to a large axion mass induced by Standard Model fermion loops, and dramatically alters Higgs physics. We conclude that non-standard axion electrodynamics, although interesting to consider in abstract quantum field theory, is not phenomenologically viable.

Single-crystal growth, structure and thermal transport properties of the metallic antiferromagnet Zintl-phase β-EuIn2As2.

Zintl-phase materials have attracted significant research interest owing to the interplay of magnetism and strong spin-orbit coupling, providing a prominent material platform for axion electrodynamics. Here, we report the single-crystal growth, structure, magnetic and electrical/thermal transport properties of the antiferromagnet layer Zintl-phase compound β-EuIn2As2. Importantly, the new layered structure of β-EuIn2As2, in rhombohedral (R3̄m) symmetry, contains triangular layers of Eu2+ ions. The in-plane resistivity ρ(H, T) measurements reveal metal behavior with an antiferromagnetic (AFM) transition (TN ∼ 23.5 K), which is consistent with the heat capacity Cp(H, T) and magnetic susceptibility χ(H, T) measurements. Negative MR was observed in the temperature range from 2 K to 20 K with a maximum MR ratio of 0.06. Unique 4f7J = S = 7/2 Eu2+ spins were supposed magnetically order along the c-axis. The Seebeck coefficient shows a maximum thermopower |Smax| of about 40 μV K-1. The kink around 23 K in the Seebeck coefficient originates from the effect of the antiferromagnetic phase on the electron band structure, while the pronounced thermal conductivity peak at around 10 K is attributed to the phonon-phonon Umklapp scattering. The results suggest that the Eu2+ spin arrangement plays an important role in the magnetic, electrical, and thermal transport properties in β-EuIn2As2, which might be helpful for future potential technical applications.

Zero Casimir force in axion electrodynamics and the search for a new force

Zero Casimir force in axion electrodynamics and the search for a new force

Electromagnetic radiation from a spherical static current source coupled to harmonic axion field

The electromagnetic fields generated from a static current source on a spherical surface are calculated in the framework of axion electrodynamics to first order in the coupling parameter. Comparisons of the results are made with reference to various results obtained in conventional Maxwell electrodynamics, as well as previous results obtained for point magnetic dipole source coupled to harmonic axion fields. Distinct features from the results so obtained are highlighted for possible experimental probing of the axions via electromagnetic interactions. In particular, electromagnetic radiation from sources with strong magnetic field is studied which may enable the detection of a cosmic axion field from its interaction with objects like neutron stars.

Axion electrodynamics: Green’s functions, zero-point energy and optical activity

Starting from the theory of Axion Electrodynamics, we work out the axionic modifications to the electromagnetic Casimir energy using the Green’s function method, both when the axion field is initially assumed purely time-dependent and when the axion field configuration is a static domain wall, so purely space-dependent. For the first case it means that the oscillating axion background is taken to resemble an axion fluid at rest in a conventional Casimir setup with two infinite parallel conducting plates, while in the second case we evaluate the radiation pressure acting on an axion domain wall. We extend previous theories in order to include finite temperatures. Various applications are discussed. (i) We review the theory of Axion Electrodynamics and particularly the energy–momentum conservation in a linear dielectric and magnetic material. We treat this last aspect by extending former results by Brevik and Chaichian (2022) and Patkos (2022). (ii) Adopting the model of the oscillating axion background we discuss the axion-induced modifications to the Casimir force between two parallel plates by using the Green’s function method. (iii) We calculate the radiation pressure acting on an axion domain wall at finite temperature T. Our results for an oscillating axion field and a domain wall are also useful for condensed matter physics, where some topological materials, “axionic topological insulators”, interact with the electromagnetic field with a Chern–Simons interaction, like the one in Axion Electrodynamics, and there are experimental systems analogous to time-dependent axion fields and domain walls as the ones showed e.g. by Jiang and Wilczek (2019) and Fukushima et al. (2019). (iv) We compare our results, where we assume time-dependent or space-dependent axion configurations, with the discussion of the optical activity of Axion Electrodynamics by Sikivie (2021) and Carrol et al. (1990). We also make comparisons with the properties of known materials, such as optically active, chiral media and the Faraday effect.

Macroscopic Quantum Tunneling of a Topological Ferromagnet.

The recent advent of topological states of matter spawned many significant discoveries. The quantum anomalous Hall (QAH) effectis a prime example due to its potential for applications in quantum metrology, as well as its influence on fundamental research into the underlying topological and magnetic states and into axion electrodynamics. Here, electronic transport studies on a (V,Bi,Sb)2 Te3 ferromagnetic topological insulator nanostructure in the QAH regime are presented. This allows access to the dynamics of an individual ferromagnetic domain. The domain size is estimated to be in the 50-100 nm range. Telegraph noise resulting from the magnetization fluctuations of this domain is observed in the Hall signal. Careful analysis of the influence of temperature and external magnetic field on the domain switching statistics provides evidence for quantum tunneling (QT) of magnetization in a macrospin state. This ferromagnetic macrospin is not only the largest magnetic object in which QT is observed, but also the first observation of the effect in a topological state ofmatter.

Understanding unconventional magnetic order in a candidate axion insulator by resonant elastic x-ray scattering

Magnetic topological insulators and semimetals are a class of crystalline solids whose properties are strongly influenced by the coupling between non-trivial electronic topology and magnetic spin configurations. Such materials can host exotic electromagnetic responses. Among these are topological insulators with certain types of antiferromagnetic order which are predicted to realize axion electrodynamics. Here we investigate the highly unusual helimagnetic phases recently reported in EuIn2As2, which has been identified as a candidate for an axion insulator. Using resonant elastic x-ray scattering we show that the two types of magnetic order observed in EuIn2As2 are spatially uniform phases with commensurate chiral magnetic structures, ruling out a possible phase-separation scenario, and we propose that entropy associated with low energy spin fluctuations plays a significant role in driving the phase transition between them. Our results establish that the magnetic order in EuIn2As2 satisfies the symmetry requirements for an axion insulator.