Spatiotemporal spin transport from first principles

arXiv:2505.07745 · cond-mat.mes-hall, cond-mat.mtrl-sci, physics.comp-ph · Submitted 2025-05-12 · 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: "Spatiotemporal spin transport from first principles".

Mira: Here's the gist of the paper. The gist: This work introduces a computational framework for first-principles density matrix transport within the Wigner function formalism to predict spin transport from first principles and account for electron-phonon scattering at device length scales.

Kai: Framework and Methodology The researchers introduce a computational framework for first-principles density matrix transport within the Wigner function formalism to predict transport of quantum-mechanical degrees of freedom such as spin over long time and length scales This framework facilitates simulation of spin dynamics and transport from first principles, while accounting for electron-phonon scattering at device length scales.

Mira: The approach generalizes first-principles density matrix dynamics simulations to include spatial resolution within the Wigner function formalism.

Kai: Using this framework, they can explicitly account for electron-phonon scattering at device length scales and can be generally applied to any material as it does not require the parameterization necessary for semi-empirical methods.

Paper summary: Kai: So, to summarize this paper, "Spatiotemporal spin transport from first principles" they introduce a computational framework for density matrix transport within the Wigner function formalism. The goal is to predict spin transport over long times and lengths from first principles.

Mira: They claim this framework allows them to facilitate simulations of spin dynamics and transport while explicitly accounting for electron-phonon scattering at device length scales, which is a significant step because it moves beyond simpler models.

Lev: When you say "first principles," what does that actually mean in practice for the simulation? Are they solving the full many-body problem, or just using an approximation?

Kai: They state this framework generalizes first-principles density matrix dynamics simulations to include spatial resolution within the Wigner function formalism. This lets them explicitly account for electron-phonon scattering at device length scales and is generally applicable to any material because it doesn't need the specific parameterization required by semi-empirical methods.

Mira: They use graphene under an electric field as a prototype example to investigate incoherent transport, which they model using a Lindbladian formalism that incorporates first-principles electron-phonon scattering. This led them to identify three distinct regimes in the spin diffusion length: free induction decay, Dyakonov-Perel, and Elliott-Yafet.

Lev: Three regimes is a lot of complexity for real hardware implementation. What's the practical impact on running an error correction code if you're operating in one of these specific transport regimes?

Kai: Most interestingly, they find that in the Dyakonov-Perel regime, the spin diffusion length remains independent of the scattering strength. They show this dependence when plotting the spin diffusion length Ls against the scattering strength.

Mira: That constant Ls as scattering strength changes is a distinct feature compared to what you might see with simpler models like Einstein's estimate, which they mention shows similar qualitative behavior but they don't expect it to match quantitatively.

Lev: So, if we were designing a device for quantum information, knowing that the diffusion length stays fixed in the DP regime regardless of phonon noise would simplify our noise budgeting significantly.

Kai: They also look at coherent transport first; they show ballistic results for different materials with various spin-orbit field profiles, observing dephasing due to transport in the absence of scattering.

Mira: For graphene under an external electric field and graphene-hBN, they observed spin dephasing purely from spatial transport because each spin travels a different path, which is illustrated by Figure one(f) <ref:2505.07745#pg1>.

Lev: That path length difference is what causes the dephasing in coherent transport; it's about the physical separation of those quantum paths.

Kai: They also examine materials like GaN for three dee Rashba and hybrid perovskite materials like (forty-four-DFPD)2PbI4 with a persistent spin helix PSH texture <ref:2505.07745#pg2,4,4-DFPD)2PbI4 with a persistent spin helix PSH>. In the PSH material, they predict weak transport dephasing because the internal magnetic field profile deviates from the ideal PSH form.

Mira: That deviation is important because it shows that even in systems designed to have perfect spin texture, you still get some transport dephasing due to those imperfections in reality.

Lev: So, for our hardware goals, this suggests we can't just rely on idealized material models; we have to account for these structural deviations when calculating the actual coherence time.

Conclusion: Kai: So, looking at this paper, "Spatiotemporal spin transport from first principles," it's basically about giving us a rigorous way to study how spin moves across devices by linking the microscopic physics to the macroscopic transport we observe.

Mira: The authors show that by using this Wigner function approach with electron-phonon scattering included, they can map out exactly when and how spin coherence starts to break down based on the material's properties and environmental noise.

Lev: What does this mean for us building qubits? It means we have a more accurate way to predict the decoherence channels that aren't just due to qubit-qubit interactions but also due to coupling with the environment through phonon scattering.

Kai: Specifically, they’ve mapped out three distinct regimes for incoherent spin transport: free induction decay, Dyakonov-Perel, and Elliott-Yafet regimes.

Mira: The most impactful finding is that in the Dyakonov-Perel regime, the spin diffusion length stays constant even when the scattering strength changes. This tells us there's a specific physical condition where transport noise doesn't immediately kill our spin information.

Lev: That constant diffusion length in that DP region is something we could target if we were trying to engineer a material for robust quantum memory, because the noise doesn't scale with how strong the electron-phonon interaction gets.

Kai: They also show that when you apply magnetic fields in this DP regime, the spin transport becomes sensitive to scattering strength, which lets us probe that internal spin-orbit field directly through asymmetric Hanle curves.

Mira: Overall, this work shows that their first-principles technique is general and not tied to specific models for electronic Hamiltonians or phonon models, allowing them to study a wide range of materials with electron-phonon scattering included.

Lev: The limitation they flag is that the method relies on spatial transport in the local semi-classical limit when deriving the spatial contribution to drho/dt, so it doesn't cover things that are purely non-local in a way.

Kai: So, for future work, they need to push this framework further to handle those more complex transport scenarios where the local approximation might not capture everything about how spin moves across a device.

Department of Materials Science and Engineering, Rensselaer Polytechnic Institute · Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute · nOhm Devices, Inc. · Department of Materials Science and Engineering, University of Wisconsin–Madison

cond-mat.mes-hall, cond-mat.mtrl-sci, physics.comp-ph

Submitted: 2025-05-12

Updated: 2026-10-21

Comments: 7 pages, 4 figures, SI in source archive

Journal ref: Phys. Rev. B 114, 185117 (2026)

DOI: 10.1103/39qf-nq2b

License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/

Importance score: 88/100

The gist: The gist: This work introduces a computational framework for first-principles density matrix transport within the Wigner function formalism to predict spin transport from first principles and account

Key concepts

Density Matrix Quantum Transport: This is a theoretical approach used to track how quantum states, like electron spins, evolve over time and space. It allows researchers to calculate the probability of finding an electron in a certain state while accounting for quantum coherence and interactions.
Wigner Function Formalism: A mathematical tool that bridges quantum mechanics and classical physics. It represents a quantum system using quasi-probability distributions, allowing the simulation of spin dynamics by incorporating spatial information directly into the transport equations.
Dyakonov-Perel (DP) Regime: One of three regimes describing incoherent spin relaxation caused by electron-phonon scattering. A key finding is that in this specific regime, the calculated spin diffusion length does not change even when the strength of the electron-phonon scattering is varied.
Spin Diffusion Length ($L_s$): A characteristic length scale that describes how far a spin can travel before its polarization significantly decays due to scattering events. The study found that in the DP regime, this length remains independent of how strong the electron-phonon scattering is.

Terminology

Electron-Phonon Scattering — 2× in this paper · explained in 2 episodes
Wigner function — 1× in this paper · explained in 2 episodes

Summary

The gist: This work introduces a computational framework for first-principles density matrix transport within the Wigner function formalism to predict spin transport from first principles and account for electron-phonon scattering at device length scales.

Framework and Methodology

The researchers introduce a computational framework for first-principles density matrix transport within the Wigner function formalism to predict transport of quantum-mechanical degrees of freedom such as spin over long time and length scales This framework facilitates simulation of spin dynamics and transport from first principles, while accounting for electron-phonon scattering at device length scales. The approach generalizes first-principles density matrix dynamics simulations to include spatial resolution within the Wigner function formalism. Using this framework, they can explicitly account for electron-phonon scattering at device length scales and can be generally applied to any material as it does not require the parameterization necessary for semi-empirical methods.

Coherent Transport

The study first shows ballistic transport results for different materials having various spin-orbit field profiles, where they observe dephasing due to transport in the absence of scattering. This transport dephasing is due to the different path-lengths traveled by each electron illustrated in Figure 1(f). For graphene under an external electric field and graphene-hBN, they observe spin dephasing due to spatial transport alone. The transport dephasing is due to the different path lengths traveled by each spin, as illustrated in Figure 1(f).

Incoherent Transport and Regimes

The researchers then take graphene with an applied electric fields as a prototypical example to investigate incoherent transport. Accounting for first-principles electron-phonon scattering in a Lindbladian formalism, they identify three distinct regimes in the spin diffusion length: free induction decay, Dyakonov-Perel (DP) [12] and Elliott-Yafet (EY) [13, 14]. Most interestingly, the spin diffusion length is independent of scattering strength in the DP regime.

Spin Diffusion Length Dependence

In Figure 3(b), they show how the spin diffusion length Ls from the spin transport simulations is plotted against the scattering strength. It is shown that in the DP region, Ls stays fixed as scattering strength changes. The Einstein estimate shows the same qualitative behavior, but they do not expect it to match quantitatively. A distinct feature in the DP region is a constant Ls as the scattering strength changes.

Conclusion

To summarize, they have introduced a first-principles density matrix technique with semi-classical spatial transport which simulates spatio-temporal spin dynamics and transport in realistic device geometries. They have shown that the variation of spin diffusion length with scattering strength exhibits three distinct regimes: free induction decay, DP and EY, with a notable scattering-strength-independent spin diffusion length in the DP regime. They have demonstrated that their ab initio technique is general and not constrained to specific parameterizations of electronic Hamiltonians or phonon models, allowing them to study spin dynamics and transport with first-principles electron-phonon scattering for a wide range of materials.

Acknowledgments

This work is supported by the Computational Chemical Sciences program within the Office of Science at DOE under Grant No. DE-SC0023301. Calculations were carried out at the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility operated under Contract No. DEAC02-05CH11231.

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Spatio-temporal spin transport from first principles

Mayada Fadel,1 Joshua Quinton,2 Mani Chandra,3 Mayank Gupta,4 Yuan Ping,4, ∗ and Ravishankar Sundararaman1, †

1Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180>

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FIG. 1. The normalized average steady-state spin polarization in each direction as a function of the transport direction x for different materials, when x-spins are injected from the left contact. Spin dephasing due to spatial transport is observed in (a) graphene under an external magnetic field in the z-direction, (b) graphene under an external electric field in the z-direction (2D Rashba), (c) graphene-hBN (2D Rashba), (d) GaN (3D Rashba) and (e) hybrid perovskite 4,4-DFPD2PbI4 with a persistent spin helix PSH spin texture. For (a), (b) and (c) the dotted lines compare corresponding analytical predictions. This transport dephasing is due to the different paths traveled by each electron illustrated in (f), where for transport along x-axis, as we initially have spins all in phase, after traveling a distance Lx while precessing, each spin has traveled a different path, and the spins are no longer in phase, leading to decrease in the average spin magnitude.

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Theory–To incorporate spatial transport in real-time dynamics of the density matrix ρ, we use the Wigner function formalism to derive the spatial contribution to dρ/dt in the local semi-classical limit as detailed in SI.I.

Improvements for AI systems

A new simulation framework can predict transport of quantum-mechanical degrees of freedom like spin over long time and length scales using first principles density matrix transport within the Wigner function formalism. This allows for simulating spin dynamics from first principles while accounting for electron-phonon scattering at device length scales.
The improved AI system can elucidate the impact of various spin-orbit field profiles, such as Rashba and persistent spin helix, on coherent spin transport in several materials by analyzing the results of this framework.
The system can account for electron-phonon scattering from first principles at device length scales to capture temperature effects in spintronic devices, which is essential for computational materials design.
The AI can identify three distinct regimes of incoherent spin transport corresponding to the free induction decay, Dyakonov-Perel and Elliott-Yafet regimes of spin relaxation by simulating graphene under an electric field with electron-phonon scattering.
The system can determine the spin diffusion length in the Dyakonov-Perel regime, which is shown to be insensitive to the strength of scattering within that regime, even though the overall dependence agrees qualitatively with the Einstein relation.
The AI can analyze spatial transport dephasing in ballistic transport results without electron-phonon scattering by observing how the spins are no longer in phase due to different path lengths traveled by each electron.
The system can predict whether a beat pattern is present in spin transport based on the direction of transport, as observed when comparing transport along the x-axis versus the y-direction in graphene under an electric field.