Observation of light-induced Haldane gap in graphene

Fei Wang; Xuanxi Cai; Wanying Chen; Xiao Tang; Jinze Li; Benshu Fan; Tianshuang Sheng; Jinxi Lu; Jiansong Li; Hongyun Zhang; Pu Yu; Peizhe Tang; Wenhui Duan; Angel Rubio; Zhiyuan Sun; Shuyun Zhou

AI summary · arXiv: 2610.42115v1 · cond-mat.mes-hall

Observation of light-induced Haldane gap in graphene

Fei Wang, Xuanxi Cai, Wanying Chen, Xiao Tang, Jinze Li, Benshu Fan, Tianshuang Sheng, Jinxi Lu,

and 8 more authors

Jiansong Li, Hongyun Zhang, Pu Yu, Peizhe Tang, Wenhui Duan, Angel Rubio, Zhiyuan Sun, Shuyun Zhou

Original paper Submitted to arXiv on 30 Oct. 2026 This manuscript was submitted and sent out for review on 2026/09/24 License: NONEXCLUSIVE-DISTRIB 1.0 Also in: fr

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The essentials

Time- and angle-resolved photoemission detects a circular-polarization-dependent Dirac gap in driven graphene, which calculations identify as a light-induced Haldane mass with associated Floquet topology.

Question

Can circularly polarized light dynamically break time-reversal symmetry in initially gapless, non-magnetic graphene and directly open the predicted topological Dirac gap? The study addresses the lack of energy- and momentum-resolved evidence for this gap.

Method

The authors used time- and angle-resolved photoemission spectroscopy on exfoliated monolayer graphene, driving it with 310 meV mid-infrared pulses; typical fluence was 9.0 mJ/cm2, corresponding to an in-plane field of 1.0 × 10^8 V/m. They extracted the gap from energy-distribution curves and compared measurements under right- and left-circular and linear polarization with tight-binding Floquet calculations, including calculations of Berry curvature and topology.

Findings

Result At temporal overlap with circularly polarized driving, the measured Dirac gap was 203 ± 30 meV.
See the passage in the paper
Fitting these peak positions gives the gap value $\Delta_{0}=203\pm 30$ meV (Fig. 3h).
Result The gap appears under both circular helicities but is absent under linear polarization.
See the passage in the paper
A pronounced Dirac gap opens under both RCP and LCP driving but is absent under LP excitation.
Result The gap and Floquet sideband are confined to the pump–probe overlap, unlike the longer-lived photo-excited conduction-band population.
See the passage in the paper
Whereas the photo-excited CB population persists over several hundred femtoseconds before fully relaxing, both the Floquet sideband and the suppression of the spectral weight at the Dirac point are confined only to the pump–probe temporal-overlap region, distinguishing them from long-lived population effects.
Authors’ interpretation The authors identify the observed gap as a light-induced Haldane mass and associate it with low-energy Floquet Chern topology.
See the passage in the paper
Our measurements and calculations therefore establish a light-induced Haldane mass near the Dirac points and the associated low-energy Floquet Chern topology.

Limitations

The high-frequency effective-Hamiltonian description applies near the Dirac points and only at energies well below the pump photon energy.
See the passage in the paper
This low-energy description applies near the Dirac points for electronic energies well below the pump photon energy $\hbar\omega$.
The paper notes that Floquet bands are generally only partially occupied, so an integer Chern number does not imply quantized Hall conductance.
See the passage in the paper
An integer Chern number does not imply a quantized Hall conductance as the Floquet bands are generally only partially occupied[11, 20, 35, 36, 37].
The globally defined topological invariants are sensitive to weak higher-order resonances.
See the passage in the paper
Note that these topological invariants are rigorously defined only globally over the whole Brillouin zone and are therefore sensitive to the weak higher-order resonances[58].

Why it matters

The authors argue that optical driving can create a topological phase from an initially gapless, non-magnetic material by making time-reversal symmetry a transient, tunable degree of freedom. They present dynamical time-reversal-symmetry breaking as a route toward controlling chiral quantum matter beyond equilibrium.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: Graphene; Floquet engineering; Circularly polarized light; Dirac-point gap; Haldane phase

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Comprehensive Summary
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Original abstract

The paper asks whether circularly polarized light can dynamically break time-reversal symmetry in graphene and open the Dirac-point gap predicted for a light-induced Haldane phase. Earlier experiments had observed Floquet-dressed states and light-associated transport or photocurrent signals, but had not directly established a topological gap in the electronic structure. The authors set out to measure the gap in energy and momentum while the optical field drove the material. They studied a large, uniform, exfoliated monolayer graphene flake using time- and angle-resolved photoemission spectroscopy (TrARPES). The mid-infrared pump pulses had a photon energy of 310 meV and a duration of 121 femtoseconds; the probe pulses lasted 66 femtoseconds. The typical pump fluence was 9.0 millijoules per square centimetre, corresponding to an estimated in-plane electric field of about 1.0 × 10⁸ volts per metre. The authors compared spectra before, during, and after the pump–probe overlap, and under right- and left-circularly polarized as well as linearly polarized light. They used tight-binding Floquet calculations to interpret the spectra and calculate the driven bands’ Berry curvature and topology. During overlap with circularly polarized light, the spectra show Floquet sidebands and reduced spectral weight at the Dirac point, where the upper and lower branches appear separated. Outside the overlap period, the equilibrium gapless Dirac cone returns. The sidebands and Dirac-point suppression are confined to the overlap period, whereas the photoexcited conduction-band population persists for several hundred femtoseconds. This difference in their time dependence supports the interpretation that the gap signature is a coherent response to the driving field, rather than a consequence of longer-lived excited carriers. Fits to energy-distribution curves yield an estimated Dirac gap of 203 ± 30 meV. The polarization comparison is central to the authors’ interpretation. A gap appears under both right- and left-circularly polarized driving, but not under linear polarization, which instead produces anisotropic avoided crossings at Floquet resonances. The measured gap magnitudes are nearly identical for the two circular helicities. The calculations explain this result: reversing the helicity reverses the sign of the induced mass, but not its magnitude. In the model, virtual photon absorption and emission generate an effective complex next-nearest-neighbour hopping, analogous to the mechanism in the Haldane model. The resulting masses have opposite signs at graphene’s two valleys. Combined with the valleys’ opposite Dirac-cone chiralities, this produces Berry curvature of the same sign at both valleys and a nontrivial low-energy Floquet Chern character. The authors infer this topology from the measured gap in conjunction with theoretical calculations; TrARPES does not directly measure the Berry curvature or Chern numbers. The work provides direct spectroscopic evidence for a transient, light-induced Dirac gap in initially gapless, non-magnetic graphene and supports its identification as a time-reversal-symmetry-breaking Haldane mass. However, the effect is observed under strong, short-lived driving, and the paper does not demonstrate quantized Hall conductance or directly detect chiral edge states. The authors note that Floquet bands are generally only partially occupied, so a Chern number alone does not imply quantized Hall transport. Moreover, exact Floquet topological invariants depend on the full Brillouin zone and can be affected by higher-order resonances; the low-energy Haldane description applies near the Dirac points. Thus, the experiment establishes the gap and supports the associated low-energy topology, while the global topology and transport consequences remain subjects for further study.

Light opens a transient gap in graphene

Graphene is famous for its Dirac cones: energy bands that meet at points where electrons behave as if they have no mass. But what if light could temporarily change that picture? In “Observation of light-induced Haldane gap in graphene,” Fei Wang, Xuanxi Cai, Wanying Chen, Xiao Tang, Jinze Li, Benshu Fan et al. investigate whether circularly polarized light can break time-reversal symmetry in graphene and open a gap at those Dirac points.

The idea connects to the Haldane model, in which a pattern of electron hopping creates a topological state without a net magnetic field. Earlier experiments had found light-dressed electronic states and light-associated transport or photocurrent signals. They had not, however, directly measured the predicted gap in the electronic bands while light was driving the material. This study set out to track that gap in both energy and momentum.

Watching the bands while light is on

The researchers studied a large, uniform monolayer of graphene using time- and angle-resolved photoemission spectroscopy, or TrARPES. The technique measures electrons emitted from a material, revealing how its electronic bands are arranged and how they change over time.

They drove the graphene with mid-infrared pulses, each carrying photons with an energy of 310 millielectronvolts and lasting 121 femtoseconds. The probe pulses lasted 66 femtoseconds. At the typical pump fluence of 9.0 millijoules per square centimetre, the estimated electric field in the graphene plane was about 1.0 × 10⁸ volts per metre.

Measurements were made before, during, and after the pump and probe pulses overlapped. The team also compared right-circularly polarized, left-circularly polarized, and linearly polarized light. Tight-binding Floquet calculations helped interpret the spectra and assess the driven bands’ Berry curvature and topology.

A gap that exists only during the drive

During overlap with circularly polarized light, the spectra show Floquet sidebands—replicas of the electronic bands created by interaction with the periodic light field. They also show reduced spectral weight at the Dirac point, where the upper and lower branches appear separated. Fits to energy-distribution curves give an estimated gap of 203 ± 30 millielectronvolts.

The timing is important. The sidebands and the suppressed signal at the Dirac point are confined to the pump–probe overlap period. Once the drive ends, graphene’s familiar gapless Dirac cone returns. By contrast, the population of photoexcited electrons in the conduction band persists for several hundred femtoseconds.

This difference supports the authors’ interpretation that the gap is a coherent response to the driving light, rather than a by-product of longer-lived excited carriers. The material’s bands change while the optical field is present, then return to their equilibrium form after the field passes.

Why polarization matters

The comparison between light polarizations is central to the result. A gap appears under both right- and left-circularly polarized driving, but not under linear polarization. Instead, linear light produces anisotropic avoided crossings at Floquet resonances.

The measured gap sizes are nearly identical for the two circular helicities. The calculations explain why: switching the helicity reverses the sign of the induced mass, but leaves its magnitude essentially unchanged. In the model, virtual photon absorption and emission generate an effective complex next-nearest-neighbour hopping, analogous to the mechanism in the Haldane model.

This mechanism gives the two graphene valleys masses with opposite signs. Because the valleys also have opposite Dirac-cone chiralities, their Berry curvature has the same sign. The authors therefore interpret the observed gap, together with the calculations, as evidence for a nontrivial low-energy Floquet Chern character associated with a light-induced Haldane mass.

What the experiment does—and does not—establish

TrARPES directly measures the transient gap and the light-dressed spectra. It does not directly measure Berry curvature or Chern numbers; those aspects of the interpretation come from combining the measurements with theoretical calculations. The work provides direct spectroscopic evidence for a transient gap in initially gapless, non-magnetic graphene and supports the proposed time-reversal-symmetry-breaking explanation.

Important questions remain. The effect requires strong, short-lived driving, and the study does not demonstrate quantized Hall conductance or directly detect chiral edge states. Floquet bands are generally only partially occupied, so a Chern number by itself does not guarantee quantized Hall transport. Furthermore, exact Floquet topological invariants depend on the full Brillouin zone and can be influenced by higher-order resonances; the Haldane description used here applies near the Dirac points.

The result is a significant step toward controlling graphene’s electronic structure with light: it shows that a strong circular drive can temporarily open the long-predicted Dirac gap. Whether that transient change can lead to clear topological transport signatures remains an open challenge for future work.

Created on 02 Nov. 2026

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Weekly letter Presented in "Physics of matter and light this week" of 2 November 2026. Physics of matter and light this week: Hidden order, from atomic bonds to distant clocks

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Observation of light-induced Haldane gap in graphene

The essentials

Question

Method

Findings

Limitations

Why it matters

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Definitions

Light opens a transient gap in graphene

Watching the bands while light is on

A gap that exists only during the drive

Why polarization matters

What the experiment does—and does not—establish

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