We demonstrate the possibility to rectify the nanoscale radiative heat flux between two nanoparticles by coupling them with the nonreciprocal surface modes of a magneto-optical substrate in a Voigt configuration. When the non-reciprocal medium supports a surface wave in the spectral window where heat exchanges take place the rectification coefficient can reach large values opening so the way to the design of true thermal diodes.

In the present Letter we investigate the heat and momentum fluxes radiated by a hot magnetooptical nanoparticle in its surrounding under the action of an external magnetic field. We show that the flux lines circulate in a confined region at nanometric distance from the particle around the axis of magnetic field in a vortex-like configuration. Moreover we prove that the spatial orientation of these vortices (clockwise or counterclockwise) is associated to the contribution of optical resonances with topological charges m = +1 or m = −1 to the thermal emission. This work paves the way to a geometric description of heat and momentum transport in lattices of magneto-optical particles. Moreover it could have important applications in the field of energy storage as well as in thermal management at nanoscale.Unconventional topological magnetic textures such as magnetic spin vortex (magnetic skyrmions [1-3]) has given rise to radically new effects in solids state physics such as the spin Hall effect [4]). In particular, these topological defects have found important applications in spintronics [5] by allowing for the developpment of novel devices for information storage or logic treatment at nanoscale. In this Letter we investigate a thermal analog of these topological defects obtained by texturing the heat and momentum flux around hot bodies when they radiate in their surrounding environment. We show that magneto-optical (MO) nanoparticles in out of thermal equilibrium situation give rise, under the presence of an external magnetic field, to swirling structures of heat and momentum fluxes. Moreover we demonstrate that the topological number associated to these structures can be tuned either by changing the magnitude of magnetic field or by heating/cooling down the particle. Finally we infer that this geometric behavior could explain several thermo-magnetic effects such as the thermal radiative Hall effect, persistent currents, and giant magneto-resistance [6][7][8][9] To start, we consider the system sketched in Fig. 1(a). It consists in a spherical nanoparticle of radius R made of a MO material. The particle is supposed to be held at temperature T , immersed in a transparent surrounding medium at temperature T a = 0 K and subjected to a constant magnetic field B = B e z pointing in z direction. The thermal field radiated by this particle can generally be determined in the framework of Rytov's fluctuational electrodynamics [13] by modelling the nanoparticle as a point dipole with a thermally fluctuating dipole moment p which has a zero mean value, i.e. p = 0. The correlation function can be determined by the fluctuationdissipation theorem givingwith the mean energy of a harmonic oscillator Θ(ω, T ) = ω/ exp( ω/k B T ) − 1 ; k B and stand for the Boltzmann and reduced Planck constant. Here the matrix α stands for the polarizability of the nanoparticle. For an anisotropic particle the quasi-static expression has the form [10]The permittivity tensor describes as usual the optical properties of the nanoparticle. For a MO nan...

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We study the rectification of near-field radiative heat transfer between two InSb nano-particles due to the presence of non-reciprocal surface modes in a nearby InSb sample when an external magnetic field is applied and its dependence on the magnetic field strength. We reveal the spin-spin coupling mechanism of the localized particle resonances and the surface mode resonances which is substantiated by the directional heat flux in the given setup. We discuss further the interplay of the frequency shift, the propagation length, and local density of states on the strength and directionality of the rectification as well as the non-reciprocal heating effect of the nanoparticles. arXiv:2002.08752v1 [cond-mat.mes-hall]