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REVIEW 4 major objections 5 minor 67 references

Probing the temperature dependence of dielectric function of ternary transition metal dichalcogenides: towards thermo-driven ultrathin photonic components

T0 review · 4 major / 5 minor · reviewed 2026-08-01 · deepseek-v4-flash

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Pith's one-line read Ternary TMDs tune their refractive index with temperature from 80 to 670 K

desk verdict Useful first dataset for temperature-dependent optical constants of ternary MoSSe and WSSe, but the central 'Varshni governs' claim is built into the fitting model and the headline NIR numbers are extrapolated beyond the measured range. read the letter →

arxiv 2607.20344 v1 pith:OQ3EVAXN submitted 2026-07-22 physics.optics cond-mat.mtrl-sci

Artsruni Margaryan , Maria Levonyan , Maksim Sargsyan , David Karakhanyan , Meri Hayrapetyan , Kostya S. Novoselov , Davit A. Ghazaryan This is my paper · ORCID
classification physics.opticscond-mat.mtrl-sci PACS 78.20.Ci
keywords ternaryTMDdielectricpermittivitytemperaturedependenceVarshnirelationrefractiveindexultrathinlensMoSSeWSSe
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

The paper measures how the dielectric functions of the ternary transition metal dichalcogenides MoSSe and WSSe change across a wide temperature window, from 80 to 670 K, over 430–1000 nm. It claims that the temperature evolution of these dielectric responses is governed by Varshni's relation in the visible range, which blue-shifts excitonic resonances at low temperatures and red-shifts them at high temperatures. Because of Kramers–Kronig consistency, this shift also raises the real part of the dielectric function in the lossless near-infrared tail, increasing the in-plane refractive index by roughly 0.2 units. The authors exploit this using WSSe's measured dispersion to design a 16-layer plano-convex lens whose focal length continuously shifts from 835 µm at 80 K to 773 µm at 670 K, establishing a route to thermo-driven ultrathin photonic components without moving parts.

What carries the argument

The key machinery is the Tauc–Lorentz oscillator parameterization of the in-plane dielectric function, whose energy and broadening parameters are forced to follow Varshni's relation and Bose–Einstein phonon broadening. This single parametrized model, seeded with room-temperature spectroscopic ellipsometry data, is fitted simultaneously to temperature-dependent differential reflectance spectra over 430–830 nm and then extrapolated to 1000 nm. It is what turns sparse reflectance maps into a continuous temperature-dependent dielectric function dataset, and it is the same dataset that drives the lens focal-length calculations.

What would settle it

Direct spectroscopic ellipsometry on WSSe or MoSSe at 925–1000 nm from 80 to 670 K: if the refractive index does not increase by the predicted ~0.2 (or if absorption emerges), the extrapolated dielectric functions and the lens focal-length shift would be wrong. Additionally, detecting an unexpected new excitonic peak below 80 K would invalidate the fixed-oscillator model and the Varshni claim.

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Extended reading notes

Core claim

The central discovery is that the temperature-dependent optical response of ternary TMDs in the visible is fully captured by Varshni's formalism: each excitonic oscillator's energy follows E(T)=E(0)−αT²/(β+T) and its broadening follows Bose–Einstein phonon statistics, with no new optical transitions appearing down to 80 K. As a direct consequence, the transparency edge redshifts with heating, which, through Kramers–Kronig relations, monotonically increases the refractive index in the extinctionless NIR tail (e.g., for WSSe, n rises from 4.069 to 4.282 at 1000 nm; for MoSSe, from 4.259 to 4.561). The authors demonstrate the practical potential by designing a suspended ultra-thin WSSe lens tha

Load-bearing premise

The entire temperature dependence is captured by a fixed set of Tauc–Lorentz oscillators whose energies and widths are forced to follow Varshni and Bose–Einstein formulas; if new optical transitions appear below 80 K, or if these functional forms are incorrect, the reported Varshni behavior and refractive-index changes are artifacts of the fit.

Editorial extensions

If this is right

  • The dielectric permittivity of MoSSe and WSSe obeys Varshni's relation across the full 80–670 K temperature window.
  • The in-plane refractive index in the lossless NIR tail increases monotonically with temperature, providing a ~0.2 tunable index swing at 1000 nm.
  • A suspended 16-layer WSSe plano-convex lens shows a continuous focal-length shift from 835.4 µm at 80 K to 773 µm at 670 K.
  • The hybrid ellipsometry-plus-microreflectance approach can be extended to a broader class of van der Waals crystals, including biaxial ones.
  • Excitonic A and B states remain the most robust optical transitions across the temperature range, while higher-energy states show stronger broadening, indicating stronger electron–phonon coupling.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Because the Varshni functional form is imposed in the fit, the reported 'Varshni behavior' is partly an artifact of the model choice; a direct, fit-independent measurement of exciton energies (e.g., photoluminescence peaks) would strengthen the conclusion.
  • The NIR behavior beyond 830 nm was not directly measured but extrapolated from the fitted model to 1000 nm; direct ellipsometry or transmittance measurements at a few temperatures in the 900–1000 nm window would verify the lossless tunability claim.
  • The same temperature-resolved dielectric dataset could be used to design thermally tunable metasurfaces, waveplates, or absorption modulators in the NIR, not just lenses, potentially enabling reconfigurable flat optics without mechanical motion.
  • The excitonic blue-shift with cooling may be partly due to lattice contraction rather than pure electron-phonon interaction; disentangling these contributions would make the physical model more transferable to other TMD systems.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 5 minor

Summary. The paper reports room-temperature (RT) in-plane and out-of-plane dielectric functions of ternary MoSSe and WSSe, benchmarked against parent binary TMDs using spectroscopic micro-ellipsometry over 360–1000 nm. It then uses temperature-dependent micro-reflectance differential reflectance contrast (DRC) over 430–830 nm and 80–670 K, analysed with Tauc–Lorentz oscillators whose energies are constrained to Varshni's relation and widths to a Bose–Einstein broadening model, to extract temperature-dependent dielectric functions. From these it claims that the temperature evolution of the Vis dielectric response is 'governed by Varshni's formalism,' that the NIR lossless refractive index is thermally tunable (e.g., n 4.069→4.282 for WSSe at 1000 nm), and that a WSSe plano-convex ultrathin lens shows a focal-length shift from 835.4 µm at 80 K to 773 µm at 670 K.

Significance. If the reported temperature-dependent optical constants are reliable, the dataset would be a useful reference for ternary TMDs over a wide temperature range, and the lens demonstration would illustrate a route to thermo-driven ultrathin photonics. The room-temperature ellipsometry appears well validated by comparison with parent compounds and by micro-reflectance agreement. However, the Varshni-governed claim is currently an input to, not an output of, the fitting model, and the quantitative NIR and lens predictions rest on an extrapolation beyond the measured spectral range without uncertainties. These issues must be resolved before the central claims can be accepted.

major comments (4)
  1. [Results: 'The temperature dependence of dielectric functions of ternary TMDs'] The central claim that the temperature evolution is 'governed by Varshni's formalism' is circular. The authors state that 'the oscillator energies were described by Varshni's relation' before fitting, and the extracted Eo(T) in Fig. 3(c,d,g,h) therefore reproduce the imposed form. The DRC maps in Fig. 2 directly demonstrate only that the critical points shift monotonically with temperature; they do not establish a Varshni functional form. The claim should be reworded as a modelling assumption, or the paper should include a test of alternative forms (e.g., unconstrained fits of Eo(T) at each temperature, or comparison with Bose–Einstein or O'Donnell–Chen gap expressions) and report residuals.
  2. [Methods: 'Temperature dependent micro-reflectance spectroscopy'] The 1000 nm values—including the reported refractive-index changes and the lens focal-length shift—are not measured. Methods states that the dielectric functions were 'extrapolated up to 1000 nm from 830 nm using the fitted model parameters.' The temperature-dependent DRC data span 430–830 nm. Thus the claims of lossless NIR tunability and the 925 nm lens performance are model extrapolations. Please label them as such throughout, and ideally validate with temperature-dependent measurements extending into the NIR or provide a sensitivity analysis of the extrapolation.
  3. [Results and Methods: temperature-dependent fitting] No uncertainties are reported for any extracted parameter. The temperature-dependent extraction is a multi-oscillator simultaneous fit with many free parameters (amplitudes, Varshni coefficients, broadening parameters, ε∞, and a high-energy tail correction), and strong correlations are likely. Without confidence intervals or a residual analysis, the quantitative claims—e.g., WSSe focal length from 835.4 µm to 773 µm, Δn at 1000 nm—cannot be evaluated. Add error bars, bootstrapped confidence regions, or at least a covariance matrix for the fit parameters.
  4. [Results: 'The temperature dependence of dielectric functions of ternary TMDs'] The extraction assumes that no new optical transitions appear down to 80 K and that the oscillator set is identical to the room-temperature model. This assumption is stated as a conclusion ('suggesting main effect to be governed by modification of Tauc–Lorentz oscillator parameters rather than the emergence of new optical transitions'). If new transitions or changes in oscillator strengths/line shapes occur, the imposed Varshni parameters would absorb them. The paper should test the assumption, e.g., by fitting each temperature independently and comparing residuals, or by including an additional oscillator and showing it is not required.
minor comments (5)
  1. [Fig. 1 caption] The caption says dashed curves represent out-of-plane components 'Re[εz] and Im[εxy]'; the second should presumably be Im[εz].
  2. [Abstract and Results] The phrase 'across 430-1000 nm spectral region' is ambiguous. Room-temperature ellipsometry covers 360–1000 nm, but temperature-dependent DRC covers 430–830 nm and the NIR part is extrapolated. Please state this distinction explicitly in the abstract and main text.
  3. [Results: lens section] The phrase 'infirmly absorbing' should be 'weakly absorbing'.
  4. [Methods: 'Room temperature spectroscopic micro-ellipsometry'] In Eq. (3), N is not defined. Specify that it is the number of Tauc–Lorentz oscillators used in the fit.
  5. [Introduction] The term 'super-mossian characteristics' is used without definition or context; a brief explanation or reference would help the reader.

Circularity Check

2 steps flagged · score 8.0 of 10

Varshni temperature dependence is imposed as a fitting constraint and then reported as a finding; NIR tunability and lens focal-length shift are extrapolated model outputs.

  1. self definitional [Results, 'The temperature dependence of dielectric functions of ternary TMDs' (Fig. 3 discussion); Methods, 'Temperature dependent micro-reflectance spectroscopy']
    "Here, the oscillator energies were described by Varshni’s relation Eo(T) = Eo(0) – αT2/(β+T), where Eo(0) is the transition energy at 0 K, and α and β are constants describing the temperature-induced shifts of optical bandgaps ... and Γ (T) = Γ (0) + γLO/(exp(Θ/T)-1) stands for broadening parameter ... The extracted in-plane components ... exhibit ... temperature evolution consistent with the imposed parametrization, following dependencies prescribed by Varshni’s relations."

    Eo(T) and Γ(T) are not extracted freely from the DRC maps; the simultaneous fit constrains oscillator energies to Varshni's relation and broadenings to the Bose-Einstein form before fitting. The extracted curves must therefore reproduce those functional forms regardless of what the data alone would support. Saying that the temperature evolution 'is governed by Varshni's formalism' or 'follows Varshni' is restating the imposed parametrization, not an independent finding. The raw DRC maps demonstrate only that existing critical points shift with temperature; they do not determine the functional form of the shift.

  2. fitted input called prediction [Methods, 'Temperature dependent micro-reflectance spectroscopy'; Results, 'The temperature dependence of dielectric functions of ternary TMDs' (1000 nm values); Results, 'The effect of temperature in]
    "The presented dielectric permittivity functions of ternary MoSSe and WSSe were extrapolated up to 1000 nm from 830 nm using the fitted model parameters ... at 1000 nm wavelength, the real part of dielectric permittivity increases from 18.14 (and 16.55) to 20.8 (and 18.34) ... allowing a lossless window of the in-plane refractive index tunability ... Our studies display a continuous shift of the focal length from 835.4 µm at 80 K to 773 µm at 670 K."

    The 1000 nm values and the temperature-dependent NIR refractive index are not measured: the model is fit to 430–830 nm DRC data and then 'extrapolated up to 1000 nm from 830 nm using the fitted model parameters.' The reported increase of Re[ε] and in-plane n with temperature at 1000 nm, and the focal-length shift from 835.4 µm at 80 K to 773 µm at 670 K, are numerical consequences of that same Varshni-constrained model. They therefore cannot serve as independent confirmation of the claimed NIR tunability; they are model outputs presented as results.

full rationale

The paper has an independent core at room temperature: ellipsometric spectra of parent TMDs agree with literature (ref 18), and the parent/ternary optical constants are measured, not derived from the temperature model. The temperature-dependent part is where circularity enters. In the simultaneous fit of DRC spectra, oscillator energies are explicitly 'described by Varshni’s relation' and broadenings by the Bose-Einstein form; the extracted Eo(T), Γ(T), and hence the bandgap shift, are therefore forced to follow the imposed laws. The paper even states the result is 'consistent with the imposed parametrization.' The subsequent NIR (1000 nm) refractive-index variation and the WSSe lens focal-length shift are obtained by extrapolating the same fitted model beyond the measured 830 nm range, so they inherit the imposed temperature dependence and provide no independent check. No self-citation chain is load-bearing here; the problem is that the central temperature-law claim reduces to a fitting constraint. Score 8.

Assumptions & free parameters 5 free parameters · 6 assumptions · 0 invented entities

The central quantitative results depend on a heavily parametrized oscillator fit; the Varshni and broadening forms are assumed, not derived, and the NIR lens behavior is a model extrapolation.

free parameters (5)
  • Tauc-Lorentz oscillator amplitude A_i(T) for each exciton (A-D)
    Fitted at room temperature by ellipsometry and treated as free parameters at each temperature in the DRC fit (Methods: 'the oscillator amplitudes were treated as free fitting parameters').
  • Varshni parameters Eo(0), α, β for each oscillator
    Fitted to the temperature-dependent DRC spectra; these constrain the peak energies to follow Varshni's relation (Tables S5-S6).
  • Broadening parameters Γ(0), γLO, Θ for each oscillator
    Fitted to temperature-dependent peak widths using Bose-Einstein phonon model (Tables S5-S6).
  • ε∞ (high-frequency dielectric constant)
    Fitted in the ellipsometric model, Eq. (3).
  • High-energy Kramers-Kronig tail correction parameter
    Used in fast KK transformation of reflectance-derived imaginary part (Methods).
assumptions (6)
  • domain assumption Tauc-Lorentz oscillator model describes the interband/excitonic absorption of these TMDs
    Used in Eq. (1) and throughout fitting; standard for semiconductors but not exact.
  • standard math Kramers-Kronig relations connect real and imaginary parts of the dielectric function
    Eq. (2) and used for real part extraction.
  • domain assumption Varshni's relation describes the temperature shift of oscillator energies
    Imposed in the fit; this is the empirical relation being tested, so using it as a constraint can be circular.
  • domain assumption Bose-Einstein phonon model describes temperature broadening
    Used for Γ(T); standard but not independently validated for these materials.
  • domain assumption No new optical transitions appear in 80-670 K range; the same oscillator set remains valid
    Asserted in Results; needed for the fixed oscillator model.
  • domain assumption Fused silica substrate reflectance measured at 270 K is valid at all temperatures due to negligible thermo-optic coefficient
    Methods; supported by ref 67 but introduces small systematic error.

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Pith. "Pith review of Probing the temperature dependence of dielectric function of ternary transition metal dichalcogenides: towards thermo-driven ultrathin photonic components." pith.science (2026). https://pith.science/paper/OQ3EVAXN

@misc{pith2026260720344,
  author       = {Pith},
  title        = {Pith review of: Probing the temperature dependence of dielectric function of ternary transition metal dichalcogenides: towards thermo-driven ultrathin photonic components},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/OQ3EVAXN}},
  note         = {Machine review of arXiv:2607.20344}
}
read the original abstract

Transition metal dichalcogenides (TMDs), along with their ternary derivatives, have attracted considerable attention mostly due to pronounced excitonic resonances emerging in visible (Vis) and near-infrared (NIR) spectral regions, enabling strong light-matter interaction. Nevertheless, a comprehensive insight of the temperature-dependent optical dispersions for the most of representatives of the family remains yet unrevealed. Here, we report on systematic studies of dielectric permittivity functions of uniaxial ternary MoSSe and WSSe across 430-1000 nm spectral region over a broad temperature window of 80-670 K. We show that the temperature evolution of their dielectric responses is governed by Varshni's formalism in Vis spectral region further affecting their high refractive index properties at the lossless NIR spectral tails. Furthermore, we exploit the measured optical dispersion of ternary WSSe designing ultrathin plano-convex NIR photonic lenses that demonstrate continuous modulation of performance with temperature variation. Our work provides critical insights for the creation of next-generation thermo-driven nanophotonic and optoelectronic devices.

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Pith tools

Reviewed August 1, 2026 · model on record in the stance chip above.