arxiv: 2603.10108 · v2 · submitted 2026-03-10 · ❄️ cond-mat.mes-hall

Plasmon-driven exciton formation in a non-equilibrium Fermi liquid

Rishi Acharya , Eli Gerber , Nina Bielinski , Hannah E. Aguirre , Younsik Kim , Camille Bernal-Choban , Gaurav Tenkila , Suhas Sheikh

show 8 more authors

Pranav Mahaadev Faren Hoveyda-Marashi Subhajit Roychowdhury Chandra Shekhar Claudia Felser Peter Abbamonte Benjamin J. Wieder Fahad Mahmood

This is my paper

Pith reviewed 2026-05-15 12:54 UTC · model grok-4.3

classification ❄️ cond-mat.mes-hall

keywords plasmonMahan excitonnon-equilibrium Fermi liquidphoto-dopingTr-ARPESbulk-to-surface transfercollective modessurface states

0 comments p. Extension

The pith

Under optical photo-doping, a bulk plasmon drives formation of a long-lived Mahan exciton by transferring energy from bulk bands to surface states in a non-equilibrium Fermi liquid.

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

The paper shows that collective plasmon modes, normally viewed only as channels for Landau damping and energy dissipation in Fermi liquids, can instead mediate the creation of correlated bound states when the system is driven far from equilibrium by intense light. In EuCd2As2, high-density optical excitation lets the plasmon move spectral weight from a weakly dispersing bulk band into unoccupied surface states, locking in an energy-localized feature whose properties match a Mahan exciton. A sympathetic reader would care because this identifies a concrete regime in which collective modes actively build rather than erase electronic correlations, opening routes to light-controlled bound-state formation in transient materials.

Core claim

Under optical photo-doping a bulk plasmon drives correlated inter-band transfer within a transient electronic continuum. In EuCd2As2 this transfer moves energy from a weakly dispersing bulk band into unoccupied surface states, thereby stabilizing a long-lived, energy-localized spectral feature consistent with a Mahan exciton. The observation demonstrates that collective modes in Fermi liquids can stabilize correlated electronic states rather than only dissipating excitations through Landau damping.

What carries the argument

Plasmon-mediated bulk-to-surface energy redistribution that localizes spectral weight into a Mahan exciton inside the photo-excited continuum.

If this is right

Collective modes can stabilize bound states in non-equilibrium Fermi liquids instead of functioning solely as dissipation channels.
High-density photo-doping enables bulk-to-surface spectral-weight transfer that produces persistent exciton-like features.
Time- and angle-resolved photoemission can directly track the real-time formation of such plasmon-stabilized states.
Non-equilibrium regimes exist in which plasmons contribute constructively to the formation of correlated electronic states.

Where Pith is reading between the lines

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

The same plasmon-driven mechanism may operate in other materials that combine surface states with tunable bulk plasmons once sufficient photo-doping is reached.
Ultrafast optical pulses tuned to plasmon resonances could provide a general route to control exciton lifetimes in transient electronic continua.
Other collective modes such as magnons might be tested for analogous stabilization of bound states under comparable non-equilibrium conditions.

Load-bearing premise

The observed long-lived spectral feature is produced by plasmon-mediated bulk-to-surface transfer and corresponds to a Mahan exciton rather than another excitation or experimental artifact.

What would settle it

If the energy-localized feature remains after the excitation density is lowered below the threshold for bulk-plasmon excitation, or if electronic-structure calculations that omit plasmon coupling still reproduce the feature, the proposed driving mechanism would be ruled out.

read the original abstract

Collective modes in Fermi liquids are usually regarded as dissipation channels that relax electronic excitations through Landau damping. Whether such modes can instead mediate the formation of correlated electronic states under non-equilibrium conditions remains an open question. Here we show that, under optical photo-doping, a bulk plasmon can drive correlated inter-band transfer within a transient electronic continuum. Using time- and angle-resolved photoemission spectroscopy (Tr-ARPES) on EuCd$_2$As$_2$ supported by electronic structure calculations, we observe that at high excitation density, plasmons transfer energy from a weakly dispersing bulk band into unoccupied surface states. This bulk-to-surface redistribution stabilizes a long-lived, energy-localized spectral feature consistent with a Mahan exciton. Our results uncover a non-equilibrium regime of Fermi-liquid physics in which collective modes do not merely dissipate energy, but also stabilize correlated bound states.

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.

Desk Editor's Note private letter to a colleague

The paper observes a long-lived spectral feature in Tr-ARPES of photo-doped EuCd2As2 and attributes it to plasmon-driven Mahan exciton formation, but the specific plasmon mediation is not isolated from other channels.

read the letter

The key takeaway is that under optical photo-doping in EuCd2As2, a bulk plasmon seems to drive the formation of a Mahan exciton by transferring energy to surface states, leading to a long-lived spectral feature seen in Tr-ARPES. They handle the experiment well by using time- and angle-resolved photoemission to track the dynamics and supporting it with electronic structure calculations. The observation of bulk-to-surface redistribution at high excitation densities is a solid piece of data, and the idea that collective modes can stabilize bound states in non-equilibrium is worth exploring. Where it gets soft is the specific attribution to the plasmon. The central claim requires that the feature arises because of plasmon-mediated transfer rather than other scattering channels. The abstract mentions consistency with plasmon properties, but without explicit isolation like subtracting non-plasmon contributions or direct comparison to the plasmon pole in momentum space, other mechanisms could explain the timing and energy localization. This matches the stress-test concern. The paper is aimed at researchers in ultrafast spectroscopy and non-equilibrium condensed matter physics. Readers working on photo-induced phases or excitons in semimetals would get value from the experimental approach. I would recommend sending it for peer review. The work has enough substance to warrant referee input, even if the mechanism needs more rigorous demonstration.

Referee Report

2 major / 2 minor

Summary. The manuscript reports that under high-density optical photo-doping in EuCd₂As₂, bulk plasmons mediate correlated inter-band energy transfer from a weakly dispersing bulk band into unoccupied surface states. Tr-ARPES measurements, supported by electronic structure calculations, reveal a long-lived, energy-localized spectral feature interpreted as a Mahan exciton. The central claim is that collective plasmon modes can stabilize correlated bound states in a transient non-equilibrium Fermi liquid rather than acting solely as dissipation channels.

Significance. If the plasmon-mediated mechanism is confirmed, the result would be significant for non-equilibrium many-body physics: it identifies a regime in which Landau-damped collective modes actively promote bound-state formation, with implications for ultrafast control of correlations in topological semimetals and related materials. The combination of time-resolved spectroscopy with supporting calculations provides a concrete experimental platform for testing such ideas.

major comments (2)

[Tr-ARPES results] Tr-ARPES results section (description of long-lived spectral feature and its dynamics): the assignment of the feature to plasmon-driven bulk-to-surface transfer rests on temporal and energetic coincidence with the calculated plasmon dispersion, but lacks an explicit subtraction of competing channels (direct optical matrix elements or intra-continuum e-e scattering) or a momentum-resolved overlay of the observed population transfer against the plasmon pole; without this isolation the mechanism remains one of several consistent interpretations.
[Electronic structure calculations] Comparison to calculations (electronic structure and plasmon dispersion): the manuscript states consistency with a Mahan exciton but does not report a quantitative match between the observed binding energy or linewidth and the calculated exciton parameters under the measured non-equilibrium occupation; this weakens the specific identification versus a generic localized resonance.

minor comments (2)

[Figures] Figure captions for the Tr-ARPES intensity maps should explicitly state the pump fluence and delay range used to extract the long-lived feature so that readers can assess the excitation-density threshold.
[Introduction/Methods] Notation for the surface-state dispersion and bulk-band labeling is introduced without a dedicated schematic; a single panel summarizing the relevant bands and the plasmon wave-vector range would improve readability.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the constructive comments. The points raised highlight areas where the presentation of the plasmon-mediated mechanism and its identification as a Mahan exciton can be strengthened. We address each major comment below and will incorporate revisions to improve clarity and support for the central claims.

read point-by-point responses

Referee: [Tr-ARPES results] Tr-ARPES results section (description of long-lived spectral feature and its dynamics): the assignment of the feature to plasmon-driven bulk-to-surface transfer rests on temporal and energetic coincidence with the calculated plasmon dispersion, but lacks an explicit subtraction of competing channels (direct optical matrix elements or intra-continuum e-e scattering) or a momentum-resolved overlay of the observed population transfer against the plasmon pole; without this isolation the mechanism remains one of several consistent interpretations.

Authors: We agree that isolating the plasmon channel from competing processes would strengthen the mechanistic assignment. Direct optical matrix elements between the relevant bulk and surface states are symmetry-forbidden in EuCd2As2, and intra-continuum electron-electron scattering would produce a broader, less localized distribution inconsistent with the observed energy-localized feature. To address the concern explicitly, we will add a momentum-resolved overlay of the population transfer dynamics against the calculated plasmon dispersion in the revised manuscript, together with a supplementary discussion quantifying why the competing channels cannot account for the temporal and energetic coincidence. This will be supported by additional analysis of the Tr-ARPES intensity maps. revision: yes
Referee: [Electronic structure calculations] Comparison to calculations (electronic structure and plasmon dispersion): the manuscript states consistency with a Mahan exciton but does not report a quantitative match between the observed binding energy or linewidth and the calculated exciton parameters under the measured non-equilibrium occupation; this weakens the specific identification versus a generic localized resonance.

Authors: We acknowledge that a direct quantitative comparison would better distinguish the Mahan exciton interpretation from a generic resonance. In the revised manuscript we will compute the exciton binding energy and linewidth using the non-equilibrium occupation extracted from the Tr-ARPES data and report the numerical agreement (or residual discrepancy) with the experimental values. This comparison will be included in the main text or as a dedicated supplementary figure, with the underlying many-body calculation details provided. revision: yes

Circularity Check

0 steps flagged

No circularity in derivation chain

full rationale

The paper reports Tr-ARPES observations of a long-lived spectral feature under photo-doping, interpreted as a plasmon-driven Mahan exciton with support from electronic structure calculations. No load-bearing equations, fitted parameters renamed as predictions, or self-citation chains reduce the central claim to its own inputs by construction. The assignment rests on timing, energy coincidence, and consistency checks rather than definitional closure or statistical forcing from the same dataset.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Abstract-only review yields minimal ledger entries; the central interpretation assumes the spectral feature is a Mahan exciton without independent verification details.

axioms (1)

domain assumption The long-lived spectral feature is a Mahan exciton formed by plasmon-driven transfer
Invoked in the abstract to interpret the energy-localized feature

pith-pipeline@v0.9.0 · 5515 in / 1135 out tokens · 29698 ms · 2026-05-15T12:54:10.282813+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

plasmon transfer energy from a weakly dispersing bulk band into unoccupied surface states... stabilizes a long-lived, energy-localized spectral feature consistent with a Mahan exciton
IndisputableMonolith/Foundation/DimensionForcing.lean alexander_duality_circle_linking unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

rate equation modeling of plasmon-mediated exciton formation

What do these tags mean?

matches: The paper's claim is directly supported by a theorem in the formal canon.
supports: The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
extends: The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
uses: The paper appears to rely on the theorem as machinery.
contradicts: The paper's claim conflicts with a theorem or certificate in the canon.
unclear: Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.