Magnetic-field-tunable anisotropic blackbody radiation and condensation of slow thermal light in dynamical axion insulators

TL;DR

The paper demonstrates that in dynamical axion insulators, a uniform magnetic field induces a strong, anisotropic modification of Planck's blackbody radiation through axion–photon (axion–polariton) coupling. By formulating the problem in thermal field theory and deriving the axion–photon partition function, the authors show that the resulting dispersion relations generate direction-dependent energy densities and spectral fluxes, with a low-temperature crossover to slow-light states. A magnetic-field–tunable condensate of slow thermal photons emerges, representing a many-body, quasi-one-dimensional transport state distinct from conventional photon Bose–Einstein condensation. These findings suggest a controllable, directional mechanism for thermal quantum manipulation in DAIs and hint at analogous behavior in dense QCD phases (MDCDW) relevant to neutron-star cores.

Abstract

Thermal radiation features of dynamical axion insulators, which are characterized by an antiferromagnetic order with simultaneously broken time-reversal and space-inversion symmetries, are investigated. Planck's radiation law is shown to exhibit remarkable anisotropic behavior as a result of the strong dispersion caused by the light-matter interaction. A crossover scenario at low temperature is identified and an associated phase highly populated by slow thermal photons is revealed. We show that the asymmetry degree of the heat radiation and its angular distribution can be controlled via a magnetic field, paving the way toward a directional-tunable mechanism for thermal quantum manipulation and storage. Analogies are drawn with the expected behavior of blackbody radiation in the core of neutron stars.

Figures (5)

• Figure 1: (a) Polarization tensor in AED. (b) Dispersion relations. The dashed line is linked to the ordinary photon $\omega_{\mathrm{o}}$, whereas the gray band shows the maximum gap. While the lower (massless) branch is associated with extraordinary photons $\omega_{-}$, the upper (massive) one is linked to axions $\omega_+$. (c) Blackbody spectrum. The gray band shows the maximum gap at $\mathfrak{b}=5$. The dashed (solid) curves display the ordinary (extraordinary) photon spectrum. The black solid curve in the lower right corner depicts the axion-like spectrum at $T=5\;\mathrm{K}$. (d) Angular distribution of the internal energy density linked to extraordinary photons at $\omega=m$ [$d\Omega_{\mathpzc{v}}\equiv d\varphi d\cos(\vartheta)$]. Curves sharing a color are linked to a common temperature at $\mathfrak{b}=1$ (thick) and $\mathfrak{b}=5$ (thin). The dotted lines show the contributions of the ordinary mode.
• Figure 2: (a) Spectral intensity distributions parallel (solid) and perpendicular (dashed) to $\pmb{B}$ at $\mathfrak{b}=5$ for different temperatures. The gray band shows the maximum frequency gap. Also shows is the spectral intensity (b) perpendicular and (c) parallel to the magnetic-field direction, which can undergo refraction at the DAI-vacuum interface. (d) Dependence of internal energy density $\mathpzc{U}$ on the system's temperature for various magnetic fields. The dashed line gives for comparison a ratio of unity.
• Figure 3: (a) Dependence of the ratio between the thermal de Broglie wavelength and the interparticle distance with temperature for various field strengths. (b) Critical temperature as a function of the magnetic field parameter (red) and the density of extraordinary photons (green). (c) Asymmetry degree of heat radiation in DAI vs temperature for different $\mathfrak{b}$. (d) Behavior of the group velocity component of extraordinary photons along the magnetic field for $\omega \geqslant \gamma m$.
• Figure 4: (a) Condensate fraction of extraordinary photons occupying slow light states as a function of temperature for various $\mathfrak{b}$'s. (b) Dependence of Snell's law with the frequency $\omega$ at $\theta=\theta^\prime$. For the solid curves, the label must be understood as $\Upsilon\equiv \mathpzc{n}_-^\prime\sin(\theta^\prime)$, whereas for the dashed ones, it is $\Upsilon\equiv\mathpzc{n}_-^\prime\cos(\theta^\prime)$.
• Figure 5: Cartesian components of the extraordinary mode's group velocity (a) parallel and (b) perpendicular to the magnetic-field axis as a function of the frequency $\omega$ and the phase angle $\theta$. (c) Critical angle for total internal reflection of the extraordinary mode parallel (solid curves) and perpendicular (green dashed curve) to the magnetic field. The latter is unaffected by the field strength and approaches zero abruptly at $\omega=m$. The horizontal dotted line represents the critical angle related to ordinary radiation.