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arxiv: 2411.17225 · v2 · submitted 2024-11-26 · ❄️ cond-mat.mtrl-sci · cond-mat.mes-hall

Chiral Phonons in 2D Halide Perovskites

Mike Pols , Geert Brocks , Sof\'ia Calero , Shuxia Tao This is my paper

Pith reviewed 2026-05-23 16:44 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci cond-mat.mes-hall
keywords chiral phonons2D halide perovskitesangular momentumtemperature gradientchirality-induced spin selectivityspin Seebeck effectmachine-learning force fields
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The pith

Chiral phonons in 2D halide perovskites carry angular momentum that temperature gradients drive into observable currents.

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

The paper shows that low-energy phonons in chiral 2D halide perovskites arise mainly from the inorganic framework and display circular polarization. These modes produce sizable angular momentum when a temperature gradient is applied, which can couple to electron spin and appear in heat and spin currents. The authors use machine-learning force fields to confirm this behavior across several structures. A reader would care because the finding supplies a concrete phononic route to effects such as chirality-induced spin selectivity and the spin Seebeck effect in a tunable semiconductor family.

Core claim

Chiral phonons exist in 2D halide perovskites formed with chiral organic cations. Low-energy vibrational modes that originate from the inorganic layers are the ones that predominantly exhibit chirality. When a temperature gradient is imposed, these modes generate substantial angular momentum that is large enough to produce experimentally detectable signals through coupling to spin and heat transport.

What carries the argument

Chiral phonons: circularly polarized vibrational modes in a chiral crystal lattice that carry net angular momentum.

If this is right

  • Low-energy phonons from the inorganic framework dominate the chiral contribution rather than the organic cations.
  • A temperature gradient induces a net angular momentum current carried by these chiral phonons.
  • The angular momentum can couple to electron spin, offering a mechanism for observed spin-dependent transport phenomena.
  • The same platform allows joint study of phononic, electronic, and spintronic responses in one material family.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Varying the chiral organic cation might allow systematic tuning of the phonon angular momentum without changing the inorganic lattice.
  • Similar angular-momentum-carrying phonons could be searched for in other layered chiral semiconductors that lack the perovskite chemistry.
  • Time-resolved probes of phonon polarization under controlled heat flow would provide an independent test of the predicted currents.

Load-bearing premise

The on-the-fly machine-learning force fields, trained on density functional theory data, reproduce the phonon dispersion relations, circular polarization, and angular momentum values that would be obtained from direct first-principles calculations on the same atomic structures.

What would settle it

A first-principles phonon calculation or inelastic scattering measurement on the same chiral 2D perovskite structures that shows the low-energy modes lack circular polarization or produce negligible net angular momentum under a temperature gradient.

Figures

Figures reproduced from arXiv: 2411.17225 by Geert Brocks, Mike Pols, Shuxia Tao, Sof\'ia Calero.

Figure 1
Figure 1. Figure 1: a) Phonon density of states (DOS) of (S-MBA)2PbI4 with the (i) low-energy, (ii) intermediate-energy and (iii) high-energy regions in red, yellow, and green colors, re￾spectively. b) A zoom-in of the low-energy region (0 - 25 meV). Gaussian broadenings of 2.0 meV and 0.1 meV were used in the full and detailed DOS, respectively. In heat transport and electron-phonon coupling the low-energy phonons are partic… view at source ↗
Figure 2
Figure 2. Figure 2: a) Unit cell of (S-MBA)2PbI4 with P212121 space group. Hydrogen (H), carbon (C), nitrogen (N), iodine (I), and lead (Pb) are represented by white, gray, blue, purple, darkgray spheres, respectively. Brillouin zone of (S-MBA)2PbI4 , with paths along the b) b1-axis (Γ−X), c) b2-axis (Γ−Y), and d) b3-axis (Γ−Z). Special points are X = ( 1 2 , 0, 0), Y = (0, 1 2 , 0), and Z = (0, 0, 1 2 ), with -X = (- 1 2 , 0… view at source ↗
Figure 3
Figure 3. Figure 3: Phonon dispersion along the a) x-axis (-X−Γ−X), b) y-axis (-Y−Γ−Y), and c) z-axis (-Z−Γ−Z) of (S-MBA)2PbI4 . Phonon branches are color-coded with the circular polarization of the phonon modes. Red, blue, and gray are used to represent right-handed (s α q,σ > 0), left-handed (s α q,σ < 0), and non-polarized (s α q,σ = 0) phonon modes. By examining the chirality of the phonons across the whole spectrum, we f… view at source ↗
Figure 4
Figure 4. Figure 4: a) Phonon dispersion and b) atomic motion in the sele [PITH_FULL_IMAGE:figures/full_fig_p008_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: a) Illustration of the induced angular momentum fro [PITH_FULL_IMAGE:figures/full_fig_p010_5.png] view at source ↗
read the original abstract

Phonons in chiral crystal structures can be circularly polarized, making them chiral. Chiral phonons carry angular momentum, which is observable in heat currents, and, via coupling to electron spin, in spin currents. Two-dimensional (2D) halide perovskites, versatile direct band gap semiconductors, can easily form chiral structures by incorporating chiral organic cations. As a result, they exhibit phenomena such as chirality-induced spin selectivity (CISS) and the spin Seebeck effect, although the underlying mechanisms remain unclear. Using on-the-fly machine-learning force fields trained against density functional theory calculations, we confirm the presence of chiral phonons, a potential key factor for these effects. Our analysis reveals that low-energy phonons, originating from the inorganic framework, primarily exhibit chirality. Under a temperature gradient, these chiral phonons generate substantial angular momentum, leading to experimentally observable effects. These findings position chiral 2D perovskites as a promising platform for exploring the interplay between phononic, electronic, spintronic, and thermal properties.

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

2 major / 2 minor

Summary. The manuscript claims that low-energy phonons originating from the inorganic framework in chiral 2D halide perovskites primarily exhibit chirality. Using on-the-fly machine-learning force fields (MLFF) trained against density functional theory (DFT), the authors confirm the presence of these chiral phonons and show that they generate substantial angular momentum under a temperature gradient, potentially explaining observed effects such as chirality-induced spin selectivity (CISS) and the spin Seebeck effect.

Significance. If the MLFF-derived phonon properties prove accurate, the work would provide a mechanistic link between structural chirality, phonons, and spin/thermal transport in these versatile semiconductors, strengthening their candidacy for spintronic and thermoelectric applications. The on-the-fly MLFF approach is a methodological strength for scaling beyond direct DFT limits.

major comments (2)
  1. [Abstract] Abstract: the claim of confirmation via MLFF supplies no quantitative validation metrics, error analysis, or side-by-side comparison to direct DFT phonon calculations (dispersions, eigenvectors, circular polarizations, or mode angular momenta), leaving the central claim without visible supporting evidence.
  2. [Methods] Methods/results sections: the fidelity of on-the-fly MLFFs to direct DFT for the small eigenvector components that determine circular polarization and angular momentum is unbenchmarked; such quantities are sensitive to force-field inaccuracies even when energies and forces are adequately reproduced.
minor comments (2)
  1. Specify the exact chiral organic cations, inorganic frameworks, and supercell sizes employed, along with any convergence tests for the MLFF training.
  2. Clarify how the temperature gradient is applied and how the resulting angular momentum is quantified (e.g., via specific formulas or summation over modes).

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive comments, which highlight important aspects of validation for our MLFF-based phonon analysis. We address each major comment below and will revise the manuscript to incorporate additional benchmarks and clarifications.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the claim of confirmation via MLFF supplies no quantitative validation metrics, error analysis, or side-by-side comparison to direct DFT phonon calculations (dispersions, eigenvectors, circular polarizations, or mode angular momenta), leaving the central claim without visible supporting evidence.

    Authors: We agree that the abstract would benefit from explicit reference to validation. In the revised manuscript we will modify the abstract to note the on-the-fly training protocol and direct the reader to the quantitative error metrics (energy and force RMSE) and phonon dispersion comparisons already present in the Methods and Supplementary Information. We will also add a concise statement on the level of agreement obtained for circular polarization of the low-energy modes. revision: yes

  2. Referee: [Methods] Methods/results sections: the fidelity of on-the-fly MLFFs to direct DFT for the small eigenvector components that determine circular polarization and angular momentum is unbenchmarked; such quantities are sensitive to force-field inaccuracies even when energies and forces are adequately reproduced.

    Authors: This is a valid concern. While the current manuscript reports overall force and energy accuracy during on-the-fly training, it does not include explicit eigenvector-level benchmarks. In the revision we will add a dedicated subsection (or supplementary figure) comparing DFT and MLFF phonon eigenvectors and the resulting circular polarization and mode angular momenta for representative structures, thereby addressing the sensitivity of these derived quantities. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation is self-contained

full rationale

The paper trains on-the-fly ML force fields against DFT data then computes phonon dispersions, circular polarizations, and angular momenta from the resulting dynamical matrices. This is a standard two-stage computational workflow with no equations shown to reduce the reported angular momentum or chirality metrics to fitted parameters by construction. No self-citation chains, uniqueness theorems, or ansatzes imported from prior author work are invoked to force the central claim. The assumption that the MLFF reproduces DFT eigenvectors is an external validation question, not a definitional loop. The provided abstract and reader summary contain no load-bearing steps that match any of the enumerated circularity patterns.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Abstract-only review yields no explicit free parameters or invented entities; the central claim rests on the unstated premise that MLFFs trained on DFT reproduce chiral phonon properties.

axioms (1)
  • domain assumption Density functional theory provides a sufficiently accurate reference for training force fields that capture phonon chirality.
    The training target for the on-the-fly ML force fields is DFT.

pith-pipeline@v0.9.0 · 5723 in / 1185 out tokens · 21936 ms · 2026-05-23T16:44:16.195737+00:00 · methodology

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Reference graph

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