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arxiv: 2605.16830 · v1 · pith:AWRWW33Cnew · submitted 2026-05-16 · ❄️ cond-mat.other · cond-mat.mtrl-sci

Relativistic theory for coupled orbital and spin angular momentum dynamics in magnetic systems

Pith reviewed 2026-05-19 19:25 UTC · model grok-4.3

classification ❄️ cond-mat.other cond-mat.mtrl-sci
keywords relativistic angular momentum dynamicsspin and orbital momentsHeisenberg exchange approximationDirac-Kohn-Sham Hamiltonianelectromagnetic fields in magnetismconservation of total angular momentumcoupled spin-orbit dynamicsultrafast magnetic processes
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The pith

Under the atomistic Heisenberg approximation, total angular momentum J equals S plus L stays conserved in magnetic systems even when electromagnetic fields are applied.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper constructs a relativistic theory for the joint dynamics of spin and orbital angular momenta by beginning with the Dirac-Kohn-Sham Hamiltonian in an electromagnetic field and performing a unitary transformation to reach an extended Pauli form. It first demonstrates that total J is conserved in the absence of a spin-polarized exchange field but loses conservation once an electromagnetic field such as a laser pulse is introduced. When the atomistic Heisenberg approximation is imposed on the exchange interaction for systems possessing local spin and orbital moments, the coupled equations of motion that follow preserve the sum J while allowing the separate spin and orbital parts to vary. This framework keeps the dynamics consistent with the original relativistic Dirac starting point.

Core claim

Considering magnetic systems with atomic spin and orbital momenta, and making the atomistic Heisenberg approximation for the exchange interaction, the total angular momentum J = S + L remains conserved even in the presence of an electromagnetic field, while the individual atomic spin and orbital angular momenta are not conserved.

What carries the argument

The atomistic Heisenberg approximation for the exchange interaction applied within the coupled equations of motion obtained from the semirelativistic Hamiltonian.

Load-bearing premise

The atomistic Heisenberg approximation for the exchange interaction is sufficient to preserve total angular momentum conservation in the presence of electromagnetic fields.

What would settle it

A calculation or measurement in which the sum of spin and orbital angular momenta changes over time for a Heisenberg-model magnetic system driven by a laser pulse or THz field would falsify the conservation result.

read the original abstract

We develop a complete relativistic theory to describe the dynamics of electronic angular momentum including both spin (S) and orbital (L) contributions in magnetic systems. We start with the relativistic Dirac-Kohn-Sham Hamiltonian under the influence of an electromagnetic field and apply a unitary transformation to formulate the extended Pauli Hamiltonian. Using the transformed semirelativistic Hamiltonian, we derive the angular momentum dynamics for the orbital and spin angular momenta. Thereby, we formulate the coupled dynamics of orbital and spin moments consistent with the relativistic Dirac framework. Considering especially the conservation of the total angular momentum, J = S +L, we show first that J is conserved in the absence of a spin-polarized Kohn-Sham exchange field, but is no longer conserved under the application of an electromagnetic field, e.g., laser pulse, THz field, etc. Second, considering magnetic systems with atomic spin and orbital momenta, we derive the coupled equations of motion of angular momenta dynamics whilst making the atomistic Heisenberg approximation for the exchange interaction. Our results suggest that, under these assumptions, the total angular momentum remains conserved, even with electromagnetic field, but atomic spin and orbital angular momenta individually are not conserved.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 2 minor

Summary. The manuscript develops a relativistic theory for the coupled dynamics of orbital (L) and spin (S) angular momenta in magnetic systems subject to electromagnetic fields. It begins with the Dirac-Kohn-Sham Hamiltonian including an external EM field, applies a unitary transformation to obtain the extended Pauli Hamiltonian, and derives the equations of motion for the angular momenta. The authors first establish that total angular momentum J = S + L is conserved without a spin-polarized Kohn-Sham exchange field but is not conserved in the presence of an EM field. They then consider atomistic magnetic systems, invoke the Heisenberg approximation for the exchange interaction, and conclude that total J remains conserved even with EM fields (e.g., laser or THz pulses), while the individual atomic spin and orbital momenta are not conserved.

Significance. If the central conservation result holds, the work supplies a systematic relativistic framework for modeling angular-momentum dynamics in ultrafast magnetism, linking the Dirac equation to atomistic spin dynamics. A clear strength is the derivation from the transformed semirelativistic Hamiltonian rather than phenomenological insertion of torques. The recovery of total-J conservation under the Heisenberg approximation, if demonstrated rigorously, would be useful for interpreting experiments on laser-driven spin-orbit effects and orbital angular-momentum transfer.

major comments (1)
  1. [derived equations section] § on derived equations under Heisenberg approximation: the manuscript must show explicitly that the sum over atomic sites i of all electromagnetic-field contributions (orbital Zeeman, spin-orbit, and diamagnetic terms in the extended Pauli Hamiltonian) to d(S_i + L_i)/dt vanishes identically, independent of the specific values of the exchange parameters J_ij. The Heisenberg form cancels internal exchange torques by construction, but the field-induced torques require an explicit cancellation proof, particularly for spatially inhomogeneous fields.
minor comments (2)
  1. [Hamiltonian transformation section] The notation for the unitary transformation and the resulting extended Pauli Hamiltonian should include an explicit listing of all retained terms up to the order considered, to facilitate verification of the subsequent torque derivations.
  2. [results section] A brief comparison table or paragraph contrasting the conservation properties with and without the Heisenberg approximation would improve readability.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and for the positive assessment of its potential significance in providing a relativistic framework for angular-momentum dynamics. We have addressed the single major comment in detail below.

read point-by-point responses
  1. Referee: [derived equations section] § on derived equations under Heisenberg approximation: the manuscript must show explicitly that the sum over atomic sites i of all electromagnetic-field contributions (orbital Zeeman, spin-orbit, and diamagnetic terms in the extended Pauli Hamiltonian) to d(S_i + L_i)/dt vanishes identically, independent of the specific values of the exchange parameters J_ij. The Heisenberg form cancels internal exchange torques by construction, but the field-induced torques require an explicit cancellation proof, particularly for spatially inhomogeneous fields.

    Authors: We agree that an explicit demonstration of the cancellation is necessary for rigor. Our derivation starts from the extended Pauli Hamiltonian obtained via unitary transformation of the Dirac-Kohn-Sham equation in an external electromagnetic field. Under the atomistic Heisenberg approximation, the exchange terms cancel internally by construction when summed over sites. For the electromagnetic contributions, the orbital Zeeman, spin-orbit, and diamagnetic terms act on the total electronic angular momentum in a manner that preserves the overall conservation law when summed, because they derive from the minimal-coupling interaction with the vector potential and scalar potential in the underlying relativistic Hamiltonian. However, we acknowledge that the current manuscript presents this cancellation at the level of the equations of motion without expanding every term site-by-site. In the revised manuscript we will add a dedicated appendix that explicitly computes the sum over i of each field-induced torque on (S_i + L_i), retaining the full spatial dependence of the fields to cover inhomogeneous cases such as focused laser pulses. This calculation will be shown to vanish identically and independently of the values of J_ij. revision: yes

Circularity Check

0 steps flagged

Derivation from Dirac-Kohn-Sham Hamiltonian and Heisenberg approximation is self-contained with no circular reductions

full rationale

The paper begins with the relativistic Dirac-Kohn-Sham Hamiltonian in an electromagnetic field, performs a unitary transformation to the extended Pauli Hamiltonian, and derives the equations of motion for orbital and spin angular momenta. Conservation of total J = S + L is shown first without spin-polarized exchange and then recovered under the explicit atomistic Heisenberg approximation for exchange interactions. These steps are presented as direct consequences of the transformed Hamiltonian and the stated approximation rather than being imposed by definition, fitted to prior results, or justified solely via self-citation chains. No load-bearing self-citations, ansatzes smuggled through citations, or renamings of known results are required for the central claims. The derivation remains independent of the target conservation statements and is therefore self-contained.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The derivation rests on the standard relativistic Dirac equation, the Kohn-Sham framework, and the Heisenberg approximation; no new free parameters or invented entities are introduced in the abstract.

axioms (2)
  • domain assumption The Dirac-Kohn-Sham Hamiltonian correctly describes electrons in the presence of electromagnetic fields and exchange interactions.
    Invoked at the start of the derivation in the abstract.
  • standard math A unitary transformation exists that yields a valid extended Pauli Hamiltonian while preserving the essential relativistic physics.
    Used to move from Dirac to semirelativistic form.

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Reference graph

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    Orbital angular momentum dynamics The dynamics of the orbital angular momentum is gov- erned by the Heisenberg equation dL dt = 1 iℏ [L,H 0 +H int].(7) Its explicit form is obtained by making use of Eqs. (4) and (7). The calculation is simplified by the fact that the ki- netic energy commutes with the orbital angular momen- tumL. Approximating the crystal...

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    Spin angular momentum dynamics The Heisenberg equation for the spin dynamics in ab- sence of external fields using the unperturbed Hamilto- 5 nian [viz.Eq. (4)] is calculated as dS dt 0 =− 1 2m2c2 drV r S×L.(10) This represents the transfer of angular momentum from spin to orbital degrees of freedom, cf. Eq. (8), keeping the total angular momentumJ=L+Scon...

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    Total angular momentum dynamics The total magnetic moment is given byM= (L+ gsS)(µB/ℏ) whereg s is the sping-factor. The value of gs for a Dirac point particle is usually considered to have a value of 2, however, quantum electrodynamics predicts the value to be slightly larger than 2 because of vacuum fluctuations and polarizations [94]. The total angular...

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