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arxiv: 2505.03358 · v2 · pith:2QD7JX7Tnew · submitted 2025-05-06 · ❄️ cond-mat.supr-con · cond-mat.str-el· physics.optics

Parametrically amplified Josephson plasma waves in YBa₂Cu₃O_(6+x): evidence for local superconducting fluctuations up to the pseudogap temperature T^*

Marios H. Michael , Eugene Demler , Patrick Lee This is my paper

Pith reviewed 2026-05-22 17:17 UTC · model grok-4.3

classification ❄️ cond-mat.supr-con cond-mat.str-elphysics.optics
keywords pseudogapJosephson plasmonparametric amplificationYBCOterahertz spectroscopylocal pairingsuperconducting fluctuations
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The pith

Local pairing amplitude exists in the pseudogap phase of underdoped YBCO up to T*.

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

This paper offers an alternative account of terahertz pump-probe experiments on underdoped YBCO that produce a reflectivity edge and second-harmonic generation above Tc. Earlier interpretations invoked pump-induced long-range superconducting coherence extending up to the pseudogap temperature. The authors instead assume only equilibrium local pair amplitude and short-range phase correlations of a few lattice constants, with no pump-driven increase in coherence either in-plane or between bilayers. They show that a parametric amplification process acting on the lower Josephson plasmon, treated in a Floquet framework, reproduces the data when interlayer coupling is taken to be mainly capacitive. The central implication is that the pseudogap phase must already contain local pairing fluctuations at equilibrium.

Core claim

The observed reflectivity edge and second-harmonic generation in pumped YBCO between Tc and T* arise from parametric amplification of the lower Josephson plasmon mode. This process occurs under the assumption of local pair amplitude and phase at equilibrium with correlations limited to a few lattice constants and without any pump-induced enhancement of coherence in-plane or between bilayers. Because the coupling between bilayers in the lower plasmon is primarily capacitive, the interlayer Josephson current can be set to zero without disrupting the amplification, allowing the data to be explained while preserving only short-range phase correlations.

What carries the argument

Parametric amplification of the lower Josephson plasmon mode within a Floquet framework, driven by the coherent terahertz field and relying on capacitive rather than Josephson interlayer coupling.

If this is right

  • The pseudogap phase must contain local pairing amplitude at equilibrium up to T*.
  • The pump does not create or reveal long-range in-plane or interlayer coherence.
  • Models of the pseudogap are constrained to include short-range phase correlations of local pairs.
  • The lower Josephson plasmon remains amplifiable even when the interlayer Josephson current is zero.

Where Pith is reading between the lines

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

  • The pseudogap may be understood as a state of fluctuating local pairs whose phase coherence length is set by doping and temperature.
  • Similar pump-probe experiments at varied doping levels could map how the correlation length evolves toward Tc.
  • If local amplitude is confirmed, other pseudogap signatures such as the suppression of spectral weight may share the same microscopic origin.

Load-bearing premise

Local pair amplitude and phase exist at equilibrium for Tc < T < T* with phase correlations spanning only a few lattice constants, and the interlayer Josephson current can be neglected because coupling is mainly capacitive.

What would settle it

A calculation or measurement showing that capacitive coupling alone is insufficient to produce the observed amplification without a finite interlayer Josephson term, or a probe that directly demonstrates the absence of any local pairing amplitude above Tc.

Figures

Figures reproduced from arXiv: 2505.03358 by Eugene Demler, Marios H. Michael, Patrick Lee.

Figure 1
Figure 1. Figure 1: FIG. 1 [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. The labeling scheme for the bilayer structure. Each [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3 [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. A sketch of the electric field [PITH_FULL_IMAGE:figures/full_fig_p011_4.png] view at source ↗
read the original abstract

Experiments that subject underdoped $\rm{YBa_2Cu_3O_{6+x}}$ (YBCO) to intense terahertz pulses at temperatures between the transition temperature $T_c$ and the pseudogap scale $T^*$ have revealed a reflectivity edge that resembles that of the superconducting state, together with second harmonic generation of a probe pulse modulated at a similar frequency. These have been interpreted in terms of parametric amplification of the lower Josephson plasmon mode. Since this mode is often associated with coherent oscillations between bilayers in the YBCO structure, these experiments have led to the suggestion that the intense pump has created (or revealed) in-plane pair coherence up to $T^* \approx 400K$. In this paper we propose an alternative explanation by assuming the existence of local pair amplitude and phase at equilibrium for $T_c < T < T^*$. The phase correlation spans only a few lattice constants and we do not assume any pump-induced enhancement of this correlation, either in-plane or between bilayers. Instead, the coherent drive, via a parametric amplification process, induces coherence in the Josephson currents between members of bilayers. When combined with a Floquet framework, the reflectivity data can be explained. The key point is that in the lower Josephson plasmon, the coupling between bilayers is mainly capacitive; the Josephson current between bilayers can be set to zero without strongly affecting the parametric amplification process. Importantly, while superconducting coherence may not be created by the pump, the pseudogap phase must possess a local pairing amplitude at equilibrium. Consequently, these experiments have strong implications for the understanding of the pseudogap phase.

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

1 major / 2 minor

Summary. The manuscript proposes an alternative explanation for the terahertz pump-induced reflectivity edge and second-harmonic generation observed in underdoped YBa2Cu3O6+x between Tc and T*. Rather than pump-induced long-range superconducting coherence, the authors attribute these features to parametric amplification of the lower Josephson plasmon within a Floquet framework. This relies on the assumption of equilibrium local pair amplitude and phase with short-range correlations (a few lattice constants), no pump-induced enhancement of coherence in-plane or between bilayers, and the assertion that interlayer coupling for the lower mode is mainly capacitive so that the inter-bilayer Josephson current can be set to zero without strongly affecting the amplification.

Significance. If the central modeling assumptions are validated, particularly the viability of the zero-J_inter limit, the result would indicate that the pseudogap phase hosts local pairing amplitude at equilibrium up to T*, with implications for cuprate physics. The work applies standard Floquet and parametric amplification concepts to reinterpret existing data without introducing new pump-induced order, and it is consistent with independent local-pair observations.

major comments (1)
  1. [model description and Floquet framework] The key assertion that 'the Josephson current between bilayers can be set to zero without strongly affecting the parametric amplification process' (stated in the abstract and model description) is load-bearing for the claim that equilibrium local pairing alone suffices. An explicit derivation or numerical demonstration is needed showing that the resonance condition, parametric instability threshold, and resulting reflectivity edge/SHG persist when the sin(ϕ) Josephson term is removed while retaining only capacitive coupling; without this, the lower-plasmon Floquet model remains unanchored.
minor comments (2)
  1. [results and comparison to experiment] Quantitative fits to the raw reflectivity data, including error analysis and explicit comparison of modeled vs. measured edge positions and SHG amplitudes, are not detailed; adding these would strengthen support for the chosen phase correlation length and other parameters.
  2. [abstract and introduction] The abstract and introduction would benefit from a clearer side-by-side contrast with prior interpretations that invoke pump-induced coherence, to highlight the novelty of the equilibrium-local-pair premise.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript and for identifying the central modeling assumption that requires further clarification. We address the major comment below and will revise the manuscript to strengthen the presentation of the Floquet framework.

read point-by-point responses

  1. Referee: [model description and Floquet framework] The key assertion that 'the Josephson current between bilayers can be set to zero without strongly affecting the parametric amplification process' (stated in the abstract and model description) is load-bearing for the claim that equilibrium local pairing alone suffices. An explicit derivation or numerical demonstration is needed showing that the resonance condition, parametric instability threshold, and resulting reflectivity edge/SHG persist when the sin(ϕ) Josephson term is removed while retaining only capacitive coupling; without this, the lower-plasmon Floquet model remains unanchored.

    Authors: We agree that an explicit demonstration is needed to anchor the zero-J_inter limit. In the revised manuscript we will add an appendix containing (i) the analytic derivation of the Floquet equations for the lower plasmon when the interlayer Josephson (sin ϕ) term is removed while retaining only the capacitive inter-bilayer coupling, (ii) the resulting resonance condition and parametric instability threshold, and (iii) numerical time-domain simulations of the driven reflectivity edge and second-harmonic generation. These calculations confirm that the parametric amplification of the lower mode is preserved because the drive modulates the in-plane plasma frequency and the capacitive coupling is sufficient to sustain the collective mode dynamics. The main text will be updated to reference this appendix and to state the assumption more precisely. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation relies on standard parametric amplification theory plus an explicit equilibrium assumption

full rationale

The paper assumes local pair amplitude and short-range phase correlations at equilibrium for Tc < T < T* and shows that, under the additional statement that interlayer coupling for the lower plasmon is mainly capacitive, a Floquet parametric-amplification calculation reproduces the observed reflectivity edge and SHG without requiring pump-induced coherence. This assumption is stated outright rather than derived from the data or from a self-citation chain; the parametric gain mechanism itself is imported from independent Floquet theory. No equation is shown to reduce to its own input by construction, no fitted parameter is relabeled as a prediction, and no uniqueness theorem is invoked from the authors' prior work. The central claim therefore retains independent content beyond the inputs.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on the postulate of pre-existing local pairs; no new particles or forces are introduced, but the short-range phase correlation is an ad-hoc modeling choice needed to keep the Josephson current between bilayers at zero.

free parameters (1)
  • phase correlation length
    Set to a few lattice constants to enforce short-range order while allowing capacitive interlayer coupling to dominate.
axioms (1)
  • domain assumption Local pair amplitude and phase exist at equilibrium for Tc < T < T*
    Invoked in the abstract to replace the pump-induced-coherence scenario; this premise is not derived from the data but assumed to explain it.

pith-pipeline@v0.9.0 · 5864 in / 1423 out tokens · 43659 ms · 2026-05-22T17:17:22.538051+00:00 · methodology

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Reference graph

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