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arxiv: 2604.02843 · v1 · submitted 2026-04-03 · ❄️ cond-mat.supr-con · cond-mat.str-el

High-energy electronic excitations in La3Ni2O7 by time-resolved optical spectroscopy

Junzhi Zhu , Mengwu Huo , Yubin Wang , Yuxin Zhai , Lili Hu , Haiyun Huang , Xiu Zhang , Baixu Xiang
show 6 more authors
Mengdi Zhang Yusong Gan Zhiyuan An Meng Wang Qihua Xiong Haiyun Liu
This is my paper

Pith reviewed 2026-05-13 18:27 UTC · model grok-4.3

classification ❄️ cond-mat.supr-con cond-mat.str-el
keywords La3Ni2O7density-wave ordertime-resolved optical spectroscopyelectronic excitationsRothwarf-Taylor modelphonon modesinterband transitionselectron-phonon coupling
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The pith

Two high-energy electronic excitations in La3Ni2O7 open distinct density-wave gaps of 54 and 67 meV.

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

In the bilayer nickelate La3Ni2O7, which superconducts under pressure but hosts a density-wave order at ambient pressure below 150 K, time-resolved optical spectroscopy tracks ultrafast responses from 10 K to room temperature. Two separate electronic excitations near 1.8 eV and 2.4 eV are resolved, each tied to its own interband transition. These excitations display different density-wave gaps and relax according to the Rothwarf-Taylor model, while four coherent Raman phonons couple selectively to them. Temperature-dependent phonon softening follows thermal expansion and anharmonic coupling above 100 K, with low-temperature deviations indicating extra electron-phonon contributions. The results map a multi-gap electronic structure and mode-specific lattice interactions linked to the density-wave state.

Core claim

The paper identifies two high-energy electronic excitations at approximately 1.8 eV and 2.4 eV that arise from distinct interband transitions. Each excitation carries its own density-wave gap (54 meV and 67 meV) whose closure and recovery dynamics are described by the Rothwarf-Taylor model. Four Raman-active phonon modes are observed to couple differently to the two electronic channels. Phonon frequency softening with temperature is accounted for by thermal expansion plus anharmonic phonon-phonon coupling between roughly 100 K and room temperature; deviations at cryogenic temperatures are attributed to electron-phonon coupling.

What carries the argument

Rothwarf-Taylor relaxation applied to the recovery of the two density-wave-gapped electronic excitations, combined with temperature-dependent fitting of phonon softening that incorporates thermal expansion, anharmonic coupling, and low-T electron-phonon contributions.

If this is right

  • The density-wave order involves at least two distinct electronic bands or Fermi-surface sheets with separate gaps.
  • Phonon modes interact selectively with different electronic excitations, implying mode-specific contributions to the order.
  • Relaxation dynamics follow the Rothwarf-Taylor bottleneck picture, consistent with a partial gap in the electronic spectrum.
  • Electron-phonon coupling becomes detectable at low temperature in addition to anharmonic effects.
  • The high-energy excitations provide a spectroscopic window onto the density-wave mechanism that competes with superconductivity under pressure.

Where Pith is reading between the lines

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

  • The two-gap structure suggests the density wave may be multi-band, which could allow different bands to respond differently when pressure suppresses the order and superconductivity appears.
  • Selective phonon coupling identifies which lattice vibrations are most relevant for stabilizing the density-wave state.
  • The low-temperature electron-phonon term may influence the superconducting pairing channel once the density wave is removed by pressure.
  • The same optical protocol could be applied to other layered nickelates or related materials to compare gap structures across the family.

Load-bearing premise

Temperature changes in the electronic excitations and phonon frequencies are driven mainly by the density-wave order and the usual thermal-expansion plus anharmonic-coupling model, with low-temperature deviations correctly assigned to electron-phonon coupling alone.

What would settle it

High-pressure optical measurements that continue to detect the same 1.8 eV and 2.4 eV excitations but find no closing of the 54 meV and 67 meV gaps once the density-wave transition is suppressed.

Figures

Figures reproduced from arXiv: 2604.02843 by Baixu Xiang, Haiyun Huang, Haiyun Liu, Junzhi Zhu, Lili Hu, Mengdi Zhang, Meng Wang, Mengwu Huo, Qihua Xiong, Xiu Zhang, Yubin Wang, Yusong Gan, Yuxin Zhai, Zhiyuan An.

Figure 1
Figure 1. Figure 1: FIG. 1. Temperature-dependent high-energy electronic excitations in La [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Temperature evolution of high energy electronic excitations and DW gaps. (a) and (b) [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Temperature-dependent kinetics of [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Temperature-dependent coherent phonons. (a) FFT 2D images, calculated from Δ [PITH_FULL_IMAGE:figures/full_fig_p007_4.png] view at source ↗
read the original abstract

Recently, high-temperature superconductivity has been established in bilayer La3Ni2O7, which exhibits a density-wave (DW) transition at ~ 150 K under ambient pressure. The DW order is believed to be linked to superconductivity, as it is suppressed upon the emergence of superconductivity at high pressures. Here, we explore the ultrafast dynamics of high-energy electronic excitations from 10 K to room temperature under ambient pressure using time-resolved optical spectroscopy. Two high-energy electronic excitations at ~1.8 and ~ 2.4 eV, arising from distinct interband transitions, are identified. They exhibit different DW gaps of approximately 54 and 67 meV, respectively, along with relaxation dynamics that can be well described by the Rothwarf-Taylor model. In addition, we observe four coherent Raman-active phonon modes that exhibit distinct coupling with different electronic excitations. The phonon softening with increasing temperature can be well described between ~100 K and room temperature by a semi-quantitative model, which includes thermal expansion and anharmonic phonon-phonon coupling. At cryogenic temperatures, deviations from the measured temperature-dependent phonon frequencies and the model fits suggest an additional contribution from electron-phonon coupling. Our study provides direct evidence of the complex gap structure and phonon dynamics in this material, offering critical insights into the DW mechanism and many-body effects.

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 reports time-resolved optical spectroscopy on La3Ni2O7 from 10 K to room temperature, identifying two high-energy electronic excitations at ~1.8 eV and ~2.4 eV from distinct interband transitions. These exhibit density-wave gaps of approximately 54 meV and 67 meV, with relaxation dynamics described by the Rothwarf-Taylor model. Four coherent Raman-active phonon modes are observed with distinct couplings to the electronic excitations. Phonon softening is modeled between ~100 K and room temperature via thermal expansion and anharmonic phonon-phonon coupling, with low-temperature deviations attributed to electron-phonon coupling.

Significance. If the gap assignments, Rothwarf-Taylor fits, and phonon-coupling claims hold, the work supplies direct experimental constraints on the electronic structure and many-body interactions in La3Ni2O7, a bilayer nickelate whose ambient-pressure density-wave order is suppressed under the high-pressure conditions that induce superconductivity. Differentiating the gaps and couplings of separate interband excitations offers a useful probe of the DW mechanism beyond transport or diffraction.

major comments (2)
  1. [phonon temperature dependence] In the section on temperature-dependent phonon frequencies, the semi-quantitative model incorporating thermal expansion and anharmonic coupling reproduces the data from ~100 K to room temperature, yet the residual low-T deviations are assigned to electron-phonon coupling without quantitative comparison to alternative contributions such as magnetic fluctuations near the 150 K DW transition. No magnetic-field-dependent spectra or doping-series data are presented to isolate the e-ph term, rendering the attribution an assumption rather than a tested conclusion.
  2. [electronic excitations and relaxation dynamics] The extraction of DW gaps (54 meV for the 1.8 eV excitation and 67 meV for the 2.4 eV excitation) and the Rothwarf-Taylor model fits to the relaxation dynamics rely on post-processing choices whose robustness cannot be assessed without raw time-resolved spectra, error bars on the fitted parameters, or explicit exclusion criteria for alternative gap values.
minor comments (2)
  1. [figures] Figure captions should explicitly state the pump and probe wavelengths and fluences used for each dataset to allow direct comparison with related optical studies on nickelates.
  2. [results] The notation for the two electronic excitations (e.g., E1 at 1.8 eV and E2 at 2.4 eV) is introduced without a clear table summarizing their fitted gap values, relaxation times, and coupling strengths to each phonon mode.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thorough review and valuable feedback on our manuscript. We have carefully considered each comment and provide detailed responses below. Where appropriate, we have revised the manuscript to address the concerns raised.

read point-by-point responses

  1. Referee: In the section on temperature-dependent phonon frequencies, the semi-quantitative model incorporating thermal expansion and anharmonic coupling reproduces the data from ~100 K to room temperature, yet the residual low-T deviations are assigned to electron-phonon coupling without quantitative comparison to alternative contributions such as magnetic fluctuations near the 150 K DW transition. No magnetic-field-dependent spectra or doping-series data are presented to isolate the e-ph term, rendering the attribution an assumption rather than a tested conclusion.

    Authors: We appreciate the referee highlighting this important point. The model based on thermal expansion and anharmonic phonon-phonon coupling indeed accounts for the observed phonon softening from ~100 K up to room temperature with high fidelity. The low-temperature deviations, which become apparent below ~100 K, are attributed to electron-phonon coupling because they show a temperature dependence that continues to increase as T decreases, unlike the behavior expected from magnetic fluctuations which typically peak around the DW transition temperature of 150 K. Nevertheless, we concur that without additional experimental controls such as magnetic field dependence or doping variation, this remains an interpretation rather than a fully isolated conclusion. In the revised version, we have expanded the discussion to explicitly consider magnetic fluctuations as a possible alternative contribution and have softened the language to indicate that the deviations suggest an additional contribution from electron-phonon coupling, while noting the need for future studies to confirm this. revision: partial

  2. Referee: The extraction of DW gaps (54 meV for the 1.8 eV excitation and 67 meV for the 2.4 eV excitation) and the Rothwarf-Taylor model fits to the relaxation dynamics rely on post-processing choices whose robustness cannot be assessed without raw time-resolved spectra, error bars on the fitted parameters, or explicit exclusion criteria for alternative gap values.

    Authors: We thank the referee for this constructive criticism. To enhance the transparency and reproducibility of our analysis, we have included the raw time-resolved spectra in the Supplementary Information. Additionally, we have added error bars to all fitted parameters in the main text figures and provided a detailed description of the fitting procedure, including the criteria for selecting the gap values (such as consistency with the temperature dependence and goodness-of-fit metrics). We have also discussed why alternative gap values were excluded based on physical consistency with the observed dynamics. These additions should allow readers to assess the robustness of the DW gap extractions and Rothwarf-Taylor model fits. revision: yes

Circularity Check

0 steps flagged

No significant circularity; results are data-driven from spectra and standard models

full rationale

The paper extracts two high-energy excitations (~1.8 and 2.4 eV) and their DW gaps (~54 and 67 meV) directly from time-resolved optical spectra across 10 K to room temperature. Relaxation dynamics are fitted to the established Rothwarf-Taylor model, and four coherent phonon modes are identified with couplings assigned from data. Phonon softening is modeled from ~100 K to RT using thermal expansion plus anharmonic phonon-phonon coupling, with cryogenic residuals interpreted as electron-phonon coupling. No derivation step reduces by construction to its own inputs, no self-citations are load-bearing for central claims, and no ansatz or uniqueness theorem is smuggled in. The analysis remains self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

The analysis rests on applying the Rothwarf-Taylor model to relaxation dynamics and a semi-quantitative phonon model that combines thermal expansion with anharmonic coupling; the gap sizes are obtained by fitting temperature-dependent data.

free parameters (2)
  • DW gap for 1.8 eV excitation = 54 meV
    Extracted from the temperature dependence of the excitation amplitude or relaxation time.
  • DW gap for 2.4 eV excitation = 67 meV
    Extracted similarly from the second excitation channel.
axioms (2)
  • domain assumption The Rothwarf-Taylor model accurately captures the recombination dynamics of the high-energy electronic excitations
    Invoked to describe the observed relaxation after photoexcitation.
  • domain assumption Phonon frequency shifts above ~100 K arise only from thermal expansion and anharmonic phonon-phonon interactions
    Used as the basis for the semi-quantitative model fit to the temperature-dependent phonon frequencies.

pith-pipeline@v0.9.0 · 5588 in / 1722 out tokens · 46128 ms · 2026-05-13T18:27:26.374798+00:00 · methodology

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