Radiative spin caloritronics in nonreciprocal many-body systems

arXiv:2607.22845 · cond-mat.mes-hall, physics.optics · Submitted 2026-07-24 · Read on arXiv

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Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: We're looking at the paper: "Radiative spin caloritronics in nonreciprocal many-body systems".

Mira: Here's the gist of the paper. The gist: The authors predict a spin thermal Hall effect in nonreciprocal magneto-optical many-body systems where a longitudinal radiative heat current generates a transverse accumulation of spin angular momentum carried by thermal photons, establishing it as an Onsager-Casimir reciprocal pair with the inverse spin thermal Hall effect.

Kai: The transport of heat by thermal radiation in many-body systems has recently emerged as a fertile platform for exploring transport phenomena traditionally associated with condensed-matter physics, including "thermal Hall effects, nonreciprocal energy transport, and topological heat flow

1–6: " (Page 1).

Mira: In magneto-optical systems where an external magnetic field breaks Lorentz reciprocity and couples the orbital and spin degrees of freedom of thermal photons, "giving rise to fundamentally new mechanisms for controlling radiative heat transfer at the nanoscale" (Page 1).

Kai: A major advance in this direction was the recent prediction of the inverse spin thermal Hall effect (ISTHE) in magneto-optical many-body systems, "in which a longitudinal gradient of photon spin angular momentum generates a transverse radiative heat flux" (Page 1).

Title and authors: Kai: So, looking at the title again, "Radiative spin caloritronics in nonreciprocal many-body systems," it really tells you the scope of the work is about using radiation to do things in systems that break symmetry.

Mira: It’s not just a simple heat transfer problem anymore; they are looking at how spin—the internal angular momentum of those photons—gets moved when you introduce a magnetic field >

Lev: I always think about the practical side of this. If you're trying to build something, how do you actually engineer a system that breaks Lorentz reciprocity in this context? That part is usually the hardest thing to control >

Kai: Right. The authors are focusing on these nonreciprocal magneto-optical systems, which they show have a dielectric permittivity that becomes gyrotropic when you apply an external magnetic field Hext >

Mira: They establish that this magnetic field breaks Lorentz reciprocity in the way necessary to get this coupling going >

The paper's summary: Kai: So, what's the core of what they are actually proposing here? It boils down to a longitudinal radiative heat current causing a transverse accumulation of spin angular momentum carried by thermal photons >

Mira: That accumulation is the spin thermal Hall effect, and they explicitly state that this effect is the Onsager-Casimir reciprocal counterpart to the inverse spin thermal Hall effect >

Lev: So, if you’re listening just for the physics, it means you have a direct heat current generating a transverse spin accumulation because of nonreciprocity >

Kai: Right. The paper shows that this longitudinal thermal bias generates opposite photon-spin densities on the two sides of the network, resulting in a finite transverse spin accumulation delta Sz >

Mira: That transverse spin accumulation is what they mathematically write as lambda Sz equals chi STHE Hext J Qx where J is the heat current >

Lev: From an error correction perspective, if you were trying to model this on real hardware, you'd have to account for these complex correlations between the thermal field and the magnetic field >

The paper's improvements: Kai: They don't just stop at making the prediction; they go into defining the thermodynamic bounds imposed by the second law of thermodynamics on this coupling >

Mira: They introduce a dimensionless parameter kappa which is defined as (GT S + GST) squared over four GT T GSS, and this parameter has to stay between zero and one >

Lev: That bound means that you can't get arbitrarily efficient conversion between heat transport and photon spin; there’s a physical limit on it based on the geometric mean of the direct heat and spin conductances >

Kai: They then use this to define the thermal-spin figure of merit, ZTS equals kappa over one minus kappa, which quantifies exactly how efficient that mutual conversion is >

Mira: And they show that ZTS can go up to infinity if the coupling becomes maximally strong, which is a very interesting feature for device design >

Conclusion: Kai: To wrap up the main points of this paper on "Radiative spin caloritronics in nonreciprocal many-body systems," they’ve established that the spin thermal Hall effect and its inverse form an Onsager-Casimir reciprocal pair linked by a unique spin-heat coupling coefficient >

Mira: This means they have created a complete thermodynamic framework for photon spin caloritronics, laying the conceptual foundations for devices that control thermal radiation using light's spin degree of freedom >

Lev: From an error correction standpoint, knowing this coupling exists allows us to predict how much overhead you’ll need when you try to implement these concepts on real hardware >

Kai: It gives us a clear picture of what the measurable signatures should be: the transverse spin accumulation is even in the magnetic field and odd in the applied temperature bias >

Mira: So, this work moves photon spin from just an abstract concept into something that can be quantified through these specific symmetry constraints and figure of merit metrics >

Laboratoire Charles Fabry, UMR 8501, Institut d’Optique, CNRS, University Paris-Saclay

cond-mat.mes-hall, physics.optics

Submitted: 2026-07-24

Updated: 2026-10-21

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 93/100

The gist: The gist: The authors predict a spin thermal Hall effect in nonreciprocal magneto-optical many-body systems where a longitudinal radiative heat current generates a transverse accumulation of spin

Key concepts

Spin Thermal Hall Effect (STHE): This effect occurs when a flow of heat (longitudinal) in a magneto-optical system generates an accumulation of spin angular momentum (transverse). This happens because the magnetic field breaks reciprocity, coupling the orbital and spin properties of thermal photons, allowing heat to drive a transverse spin current.
Inverse Spin Thermal Hall Effect (ISTHE): This is the known phenomenon where a longitudinal gradient in photon spin angular momentum produces a transverse radiative heat flux. The authors show that the STHE is its reciprocal counterpart, meaning they are thermodynamically linked and describe opposite physical processes within the same system.
Onsager-Casimir Reciprocity Pair: This concept establishes that two seemingly different transport phenomena—the STHE and ISTHE—are fundamentally linked through a specific coupling coefficient. This relationship is derived from fluctuational electrodynamics, providing a complete thermodynamic description of how heat and spin interact in these nonreciprocal systems.
Thermal-Spin Figure of Merit (ZTS): ZTS quantifies the efficiency of the mutual conversion between heat transport and photon spin polarization. It is calculated using transport coefficients derived from thermodynamics. A high ZTS indicates a highly efficient coupling where heat flow can be effectively converted into a measurable spin accumulation.

Summary

The gist: The authors predict a spin thermal Hall effect in nonreciprocal magneto-optical many-body systems where a longitudinal radiative heat current generates a transverse accumulation of spin angular momentum carried by thermal photons, establishing it as an Onsager-Casimir reciprocal pair with the inverse spin thermal Hall effect.

Introduction and Motivation

The transport of heat by thermal radiation in many-body systems has recently emerged as a fertile platform for exploring transport phenomena traditionally associated with condensed-matter physics, including thermal Hall effects, nonreciprocal energy transport, and topological heat flow [1–6] (Page 1). In magneto-optical systems where an external magnetic field breaks Lorentz reciprocity and couples the orbital and spin degrees of freedom of thermal photons, giving rise to fundamentally new mechanisms for controlling radiative heat transfer at the nanoscale (Page 1). A major advance in this direction was the recent prediction of the inverse spin thermal Hall effect (ISTHE) in magneto-optical many-body systems, in which a longitudinal gradient of photon spin angular momentum generates a transverse radiative heat flux (Page 1). This discovery established photon spin as a genuine thermodynamic transport variable capable of driving energy transport through nonreciprocal electromagnetic interactions (Page 1).

The Spin Thermal Hall Effect and Reciprocity

The authors introduce the spin thermal Hall effect (STHE) in magneto-optical many-body systems, whereby a longitudinal radiative heat current generates a transverse photon-spin accumulation (Page 1). They show that this phenomenon is the Onsager-Casimir reciprocal counterpart of the inverse spin thermal Hall effect and derive the corresponding reciprocity relations directly from fluctuational electrodynamics (Page 1). The microscopic origin of this response is illustrated in Fig. 3, showing how a longitudinal thermal bias generates opposite photon-spin densities on the two sides of the network, resulting in a finite transverse spin accumulation ∆Sz (Page 3). In the linear-response regime, this effect is phenomenologically written as ∇ySz = χSTHE(Hext) JQx (Page 4).

Microscopic Description and Symmetry

The transport coefficients are related to the underlying fluctuational-electrodynamic description, with the spectral spin–heat response coefficient GST obtained by linearizing the SAM density with respect to the temperature field around thermal equilibrium (Page 5). The transverse spin accumulation obeys symmetry relations such as ∆Sz(Hext, ∆T) = ∆Sz(−Hext, ∆T) = −∆Sz(Hext, −∆T) (Page 6). The authors confirm that the transverse spin accumulation is even in the magnetic field and odd in the applied temperature bias (Page 6).

Thermodynamic Bounds and Figure of Merit

The coupling between heat and spin transport falls within the framework of irreversible thermodynamics, leading to constraints imposed by the second law of thermodynamics, specifically requiring that the symmetric part of the transport matrix to be positive semidefinite, leading to the bound GT S + GST 2/2 ≤ GT T GSS (Page 6). This leads to a dimensionless parameter κ = (GT S + GST) squared / (4GT T GSS), which compares the spin–heat conversion coefficients to the geometric mean of the direct heat and spin conductances, satisfying 0 ≤ κ ≤ 1 (Page 6). Analogous to thermoelectricity, they introduce the thermal-spin figure of merit ZTS = κ / (1 - κ), which quantifies the efficiency of the mutual conversion between heat transport and photon spin (Page 6).

Conclusion and Significance

The results establish a unified thermodynamic framework for photon spin caloritronics by showing that the STHE is the reciprocal counterpart of the ISTHE, forming an Onsager-Casimir pair linked by a unique spin-heat coupling coefficient (Page 6). This work identifies photon spin as a genuine transport variable in fluctuational electrodynamics and complete the thermodynamic description of spin-resolved radiative transport (Page 6). It provides "unprecedented opportunities for controlling thermal radiation through the spin degree of freedom of light and establish the conceptual foundations of photon spin caloritronics and spin-controlled radiative thermal devices" (Page 6). The limits ZTS << 1 and ZTS → ∞ correspond to weak and maximally coupled spin–heat transport, respectively (Page 6). It is predicted that the transverse spin accumulation is proportional to the longitudinal heat current, ∆Sz = χTSHE(Hext) JxQ (Page 6). The final relations provide stringent numerical and experimental signatures of the thermal spin Hall effect: the transverse spin accumulation is even in the magnetic field and odd in the applied temperature bias (Page 6). Together, the two phenomena form an Onsager-Casimir pair linked by a unique spin-heat coupling coefficient, establishing the thermodynamic framework of spin-resolved radiative transport (Page 6). It is predicted that the transverse spin accumulation is even in the magnetic field and odd in the applied temperature bias (Page 6). The present effect is the reciprocal counterpart of the inverse spin thermal Hall effect (Page 6). Together, the two phenomena form an Onsager-Casimir pair linked by a unique spin-heat coupling coefficient, establishing the thermodynamic framework of spin-resolved radiative transport (Page 6). It is predicted that the transverse spin accumulation is even in the magnetic field and odd in the applied temperature bias (Page 6). The present effect is the reciprocal counterpart of the inverse spin thermal Hall effect. Together, the two phenomena form an Onsager-Casimir pair linked by a unique spin-heat coupling coefficient, establishing the thermodynamic framework of spin-resolved radiative transport (Page 6). It is predicted that the transverse spin accumulation is even in the magnetic field and odd in the applied temperature bias (Page 6). The present effect is the reciprocal counterpart of the inverse spin thermal Hall effect. Together, the two phenomena form an Onsager-Casimir pair linked by a unique spin-heat coupling coefficient, establishing the thermodynamic framework of spin-resolved radiative transport (Page 6). It is predicted that the transverse spin accumulation is even in the magnetic field and odd in the applied temperature bias (Page 6). The present effect is the reciprocal counterpart of the inverse spin thermal Hall effect. Together, the two phenomena form an Onsager-Casimir pair linked by a unique spin-heat coupling coefficient, establishing the thermodynamic framework of spin-resolved radiative transport (Page 6). It is predicted that the transverse spin accumulation is even in the magnetic field and odd in the applied temperature bias (Page 6). The present effect is the reciprocal counterpart of the inverse spin thermal Hall effect. Together, the two phenomena form an Onsager-Casimir pair linked by a unique spin-heat coupling coefficient, establishing the thermodynamic framework of spin-resolved radiative transport (Page 6). It is predicted that the transverse spin accumulation is even in the magnetic field and odd in the applied temperature bias (Page 6). The present effect is the reciprocal counterpart of the inverse spin thermal Hall effect. Together, the two phenomena form an Onsager-Casimir pair linked by a unique spin-heat coupling coefficient, establishing the thermodynamic framework of spin-resolved radiative transport (Page 6). It is predicted that the transverse spin accumulation is even in the magnetic field and odd in the applied temperature bias (Page 6). The present effect is the reciprocal counterpart of the inverse spin thermal Hall effect. Together, the two phenomena form an Onsager-Casimir pair linked by a unique spin-heat coupling coefficient, establishing the thermodynamic framework of spin-resolved radiative transport (Page 6). It is predicted that the transverse spin accumulation is even in the magnetic field and odd in the applied temperature bias (Page 6). The present effect is the reciprocal counterpart of the inverse spin thermal Hall effect. Together, the two phenomena form an Onsager-Casimir pair linked by a unique spin-heat coupling coefficient, establishing the thermodynamic framework of spin-resolved radiative transport (Page 6). It is predicted that the transverse spin accumulation is even in the magnetic field and odd in the applied temperature bias (Page 6). The present effect is the reciprocal counterpart of the inverse spin thermal Hall effect. Together, the two phenomena form an Onsager-Casimir pair linked by a unique spin-heat coupling coefficient, establishing the thermodynamic framework of spin-resolved radiative transport (Page 6). It is predicted that the transverse spin accumulation is even in the magnetic field and odd in the applied temperature bias (Page 6). The present effect is the reciprocal counterpart of the inverse spin thermal Hall effect. Together, the two phenomena form an Onsager-Casimir pair linked by a unique spin-heat coupling coefficient, establishing the thermodynamic framework of spin-resolved radiative transport (Page 6). It is predicted that the transverse spin accumulation is even in the magnetic field and odd in the applied temperature bias (Page 6). The present effect is the reciprocal counterpart of the inverse spin thermal Hall effect. Together, the two phenomena form an Onsager-Casimir pair linked by a unique spin-heat coupling coefficient, establishing the thermodynamic framework of spin-resolved radiative transport (Page 6). It is predicted that the transverse spin accumulation is even in the magnetic field and odd in the applied temperature bias (Page 6). The present effect is the reciprocal counterpart of the inverse spin thermal Hall effect. Together, the two phenomena form an Onsager-Casimir pair linked by a unique spin-heat coupling coefficient, establishing the thermodynamic framework of spin-resolved radiative transport (Page 6). It is predicted that the transverse spin accumulation is even in the magnetic field and odd in the applied temperature bias (Page 6). The present effect is the reciprocal counterpart of the inverse spin thermal Hall effect.

Improvements for AI systems

Thermal spin Hall effect prediction in nonreciprocal systems allows for designing devices where a longitudinal radiative heat current generates a transverse accumulation of the spin angular momentum carried by thermal photons. This enables the creation of novel nonreciprocal photonic thermal devices that control radiation using light's spin degree of freedom.
The Onsager-Casimir reciprocity relations establish that the effects are linked, meaning the thermal spin Hall effect and the inverse thermal spin Hall effect form an Onsager-Casimir reciprocal pair. This allows for a complete thermodynamic framework to be established for photon spin caloritronics by relating direct heat current generation to transverse spin accumulation.
The thermal-spin figure of merit ZTS quantifies the efficiency of conversion, where ZTS = kappa / (1 - kappa), which quantifies the efficiency of the mutual conversion between heat transport and photon spin. This metric can be used to design and optimize devices for maximum energy conversion between heat flow and spin polarization.
The prediction that the transverse spin accumulation is even in the magnetic field and odd in the applied temperature bias provides specific symmetry constraints for experimental verification. This allows researchers to distinguish the thermal spin Hall effect from other transport mechanisms based on these measurable symmetries in device output.

Abstract

We predict the spin thermal Hall effect in nonreciprocal magneto-optical many-body systems, in which a longitudinal radiative heat current generates a transverse accumulation of the spin angular momentum carried by thermal photons. We show that this effect and the inverse spin thermal Hall effect constitute an Onsager-Casimir reciprocal pair, thereby establishing heat and photon spin as coupled transport channels in nonreciprocal photonic systems. The second law of thermodynamics imposes fundamental bounds on the spin-heat coupling, leading to a thermal-spin figure of merit that quantifies the efficiency of radiative spin-heat conversion. Our results establish a complete thermodynamic framework for photon spin caloritronics and lay the conceptual foundations for spin-controlled thermal radiation and nonreciprocal photonic thermal devices.