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arxiv: 2607.00809 · v1 · pith:PIWP3GVCnew · submitted 2026-07-01 · ❄️ cond-mat.mtrl-sci · quant-ph

Vanadium superconducting microwave resonators on silicon wafers

Y. Fujita , Y. Urade , Y. Hibino , M. Tsujimoto , K. Inomata , G. Fujii , W. Mizubayashi This is my paper

Pith reviewed 2026-07-02 10:02 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci quant-ph
keywords vanadiumsuperconducting resonatorsmicrowave lossesnon-TLS lossestantalum cappingniobium buffersilicon substratesquantum devices
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The pith

Losses in vanadium superconducting resonators are dominated by non-TLS mechanisms at the surface that a tantalum capping layer can reduce.

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

The authors fabricate coplanar waveguide resonators from vanadium films grown on silicon in four configurations using niobium buffers and tantalum caps. They measure internal quality factors as a function of average photon number and separate the contributions from two-level systems and other losses. The results show that non-TLS losses at the vanadium surface limit performance and are reduced by the tantalum cap. Films grown on niobium buffers show worse performance than those grown directly on silicon despite better crystal orientation, which the authors link to oxides or hydrides detected by spectroscopy.

Core claim

Loss at the V surface is dominated by ⟨n_ph⟩-independent non-two-level-system (non-TLS) losses, which can be mitigated by introducing the Ta capping layer. The V films on the Nb buffer layers exhibit lower Q_int and higher non-TLS loss than that directly grown on Si wafers, even though the former has higher lattice-orientation uniformity. Origins of these trends might be relevant to V oxides and/or V hydrides at surfaces and grain boundaries.

What carries the argument

The photon-number dependence of internal quality factor Q_int in resonators made from four different V-film stacks (Nb/V/Ta, Nb/V, V/Ta, V), used to separate TLS and non-TLS loss contributions.

If this is right

  • Introducing a Ta capping layer mitigates non-TLS losses in V resonators.
  • Nb buffer layers increase non-TLS loss in V films compared to direct growth on Si.
  • Better lattice orientation uniformity does not guarantee lower losses in these V films.
  • V oxides or hydrides at surfaces and grain boundaries are likely sources of the non-TLS losses.
  • Mitigation strategies should target surface chemistry rather than just improving crystallinity.

Where Pith is reading between the lines

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

  • Vanadium could serve as an alternative material for quantum circuits if surface losses are controlled.
  • Similar capping strategies might apply to other superconducting films prone to oxide formation.
  • Direct growth without buffers may be preferable for V on Si to minimize additional loss channels.
  • Further studies could test hydride formation by varying processing conditions.

Load-bearing premise

The performance differences between film stacks are caused by V oxides or hydrides at surfaces and grain boundaries rather than by differences in interface quality or measurement artifacts.

What would settle it

Measuring resonators from V films where EDX and XPS show no oxides or hydrides but still finding the same differences in Q_int and non-TLS loss between Nb-buffered and direct-growth samples would falsify the proposed origin.

Figures

Figures reproduced from arXiv: 2607.00809 by G. Fujii, K. Inomata, M. Tsujimoto, W. Mizubayashi, Y. Fujita, Y. Hibino, Y. Urade.

Figure 1
Figure 1. Figure 1: (a) shows the out-of-plane XRD profiles of ω-2θ scans from the ⟨001⟩ directions of the samples with Si/Nb/V and Si/V structures. The diffraction peaks of Nb 011, V 011, and V 022 were clearly observed in the Si/Nb/V structure, suggesting that the Nb and V layers were grown on the Si(001) substrate in an orientation such that (011)V∥(011)Nb∥(001)Si. In addition to V 011 and V 022 peaks, a small V 001 peak w… view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. AFM topographic images of the surfaces of (a) Si/Nb/V, (b) Si/V, (c) Si/Nb/V/Ta, and [PITH_FULL_IMAGE:figures/full_fig_p006_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. (a) [PITH_FULL_IMAGE:figures/full_fig_p008_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. (a) Optical micrograph of a CPW resonator chip with a Si/Nb/V/Ta structure. (b) Cross [PITH_FULL_IMAGE:figures/full_fig_p010_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Cross-sectional TEM images of center conductors (left) and EDS elemental maps (middle) [PITH_FULL_IMAGE:figures/full_fig_p011_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. V 2 [PITH_FULL_IMAGE:figures/full_fig_p013_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7 [PITH_FULL_IMAGE:figures/full_fig_p015_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8. (a) Box-and-whisker plots of [PITH_FULL_IMAGE:figures/full_fig_p017_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9. Peak area percentage of surface oxides of V surfaces (left) and Ta surfaces (right) before [PITH_FULL_IMAGE:figures/full_fig_p024_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: FIG. 10. Measurement setup with a representative optical micrograph of an Al-wire-bonded [PITH_FULL_IMAGE:figures/full_fig_p025_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: shows Qint as functions of ⟨nph⟩ for Resonator 2-4 of the four V-film structures. As with the Resonator 1 data shown in [PITH_FULL_IMAGE:figures/full_fig_p026_11.png] view at source ↗
read the original abstract

Understanding the correlation between material properties and microwave losses in superconducting films is a crucial subject for developing low-loss materials for quantum circuits. We focus on vanadium (V) as a novel material for superconducting quantum devices and discuss loss in V films in relation to their structural properties. Using a sputtering method, we grow four V-film structures on (001)-oriented Si wafers, employing Nb and Ta as the buffer and capping layer materials, respectively: Nb/V/Ta, Nb/V, V/Ta, and V. X-ray diffraction and atomic force microscopy reveal that the V films grown on the Nb buffer layers have higher uniformity of lattice orientation and smaller grain size than that directly grown on the Si wafer. Coplanar waveguide resonators are fabricated from the four V-film structures, and averaged photon number ($\langle n_{\rm ph} \rangle$) dependences of internal quality factor ($Q_{\rm int}$) are obtained by performing microwave measurements. By analyzing the obtained $Q_{\rm int}$ vs $\langle n_{\rm ph} \rangle$, it is found that loss at the V surface is dominated by $\langle n_{\rm ph} \rangle$-independent non-two-level-system (non-TLS) losses, which can be mitigated by introducing the Ta capping layer. Furthermore, the V films on the Nb buffer layers exhibit lower $Q_{\rm int}$ in the $\langle n_{\rm ph} \rangle$ range from 10$^{0}$ to 10$^{6}$ and higher non-TLS loss than that directly grown on Si wafers, even though the former has higher lattice-orientation uniformity than the latter. Origins of these trends might be relevant to V oxides, of which presence at surfaces and grain boundaries in bulk regions in the V resonators is suggested by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy, and/or V hydrides.

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

3 major / 2 minor

Summary. The manuscript reports fabrication and microwave characterization of coplanar waveguide resonators from four vanadium film structures (Nb/V/Ta, Nb/V, V/Ta, V) sputtered on Si(001). XRD and AFM show Nb-buffered films have superior lattice-orientation uniformity and smaller grains. Q_int versus <n_ph> measurements indicate that V-surface losses are dominated by <n_ph>-independent non-TLS mechanisms that are reduced by Ta capping; Nb-buffered films nevertheless show lower Q_int (10^0 to 10^6 photons) and higher non-TLS loss than direct V/Si films, with EDX/XPS suggesting V oxides or hydrides at surfaces and grain boundaries as a possible origin.

Significance. If the non-TLS dominance and Ta-mitigation claims are confirmed with quantitative controls, the work would supply concrete guidance on surface-chemistry engineering for low-loss vanadium resonators in quantum circuits, separating structural uniformity from chemical-loss contributions.

major comments (3)
  1. [Abstract / Q_int vs <n_ph> analysis] Abstract and results on Q_int(<n_ph>) analysis: the claim that non-TLS losses dominate and are higher in Nb-buffered films rests on post-measurement fitting whose details (functional form, error bars, number of resonators per structure, exclusion criteria for outlier devices) are not supplied, preventing independent evaluation of the extracted non-TLS loss coefficients.
  2. [Discussion of origins of trends] Discussion of Nb-buffer versus direct-growth contrast: the inference that elevated non-TLS loss originates from V oxides/hydrides (detected by EDX/XPS) lacks any quantitative correlation between oxide/hydride areal density or depth profile and the reported non-TLS loss rates; no controls isolating this from Nb/V interface scattering, film stress, or post-etch contamination are described.
  3. [Results / loss-parameter table] Table or figure presenting Q_int and non-TLS loss values: without tabulated values, uncertainties, or statistical tests for the four structures, the statement that Nb/V films exhibit “higher non-TLS loss” cannot be assessed for magnitude or significance.
minor comments (2)
  1. Notation for average photon number is inconsistent between <n_ph> and ⟨n_ph⟩; standardize throughout.
  2. EDX and XPS spectra or depth profiles should be shown with quantitative elemental percentages rather than qualitative “presence suggested.”

Simulated Author's Rebuttal

3 responses · 1 unresolved

We thank the referee for the careful reading and constructive comments. We address each major point below. Where details were omitted, we will add them in revision; where inferences are suggestive rather than quantitative, we will clarify the limitations of the data.

read point-by-point responses

  1. Referee: [Abstract / Q_int vs <n_ph> analysis] Abstract and results on Q_int(<n_ph>) analysis: the claim that non-TLS losses dominate and are higher in Nb-buffered films rests on post-measurement fitting whose details (functional form, error bars, number of resonators per structure, exclusion criteria for outlier devices) are not supplied, preventing independent evaluation of the extracted non-TLS loss coefficients.

    Authors: We agree that the fitting procedure and supporting statistics were not described in sufficient detail. In the revised manuscript we will specify the functional form used to separate TLS and non-TLS contributions, include error bars on all Q_int data points, state the number of resonators measured for each film structure, and document the criteria applied to exclude outlier devices. These additions will enable independent assessment of the extracted non-TLS coefficients. revision: yes

  2. Referee: [Discussion of origins of trends] Discussion of Nb-buffer versus direct-growth contrast: the inference that elevated non-TLS loss originates from V oxides/hydrides (detected by EDX/XPS) lacks any quantitative correlation between oxide/hydride areal density or depth profile and the reported non-TLS loss rates; no controls isolating this from Nb/V interface scattering, film stress, or post-etch contamination are described.

    Authors: The manuscript already qualifies the link as suggestive (“might be relevant … suggested by”). We will revise the discussion to state explicitly that no quantitative correlation between oxide/hydride density and loss rates has been established, and that other possible contributions (interface scattering, stress, contamination) have not been isolated by dedicated controls. This change will prevent over-interpretation while retaining the EDX/XPS observations as one plausible origin. revision: yes

  3. Referee: [Results / loss-parameter table] Table or figure presenting Q_int and non-TLS loss values: without tabulated values, uncertainties, or statistical tests for the four structures, the statement that Nb/V films exhibit “higher non-TLS loss” cannot be assessed for magnitude or significance.

    Authors: We will add a table (or expanded figure caption) that reports representative Q_int values at selected photon numbers, the fitted non-TLS loss coefficients with uncertainties, and basic statistical information (mean and standard deviation across devices) for each of the four film structures. This will allow direct evaluation of the magnitude and significance of the reported differences. revision: yes

standing simulated objections not resolved
  • We do not possess additional EDX/XPS data that would allow quantitative areal-density or depth-profile correlations with the measured non-TLS loss rates; the existing spectra were acquired for qualitative identification only.

Circularity Check

0 steps flagged

No circularity: experimental measurements and post-hoc interpretation

full rationale

The paper reports fabrication of four V-film structures, XRD/AFM characterization, resonator fabrication, and direct microwave measurements of Q_int vs ⟨n_ph⟩. Loss analysis separates TLS and non-TLS contributions via standard fitting to measured curves; attributions to oxides/hydrides rest on qualitative EDX/XPS observations rather than any equation that reduces the reported loss rates to fitted inputs by construction. No self-citations, uniqueness theorems, or ansatzes are invoked to force the central claims. The chain consists of independent experimental data and standard loss modeling, making the result self-contained.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The work rests on standard thin-film deposition, structural characterization, and microwave resonator loss analysis techniques. The separation of TLS versus non-TLS contributions requires an assumed functional form for photon-number dependence that is treated as known from prior literature.

free parameters (1)
  • non-TLS loss coefficient
    The analysis of Q_int versus ⟨n_ph⟩ requires fitting or extraction of a photon-number-independent loss term whose value is determined from the data.
axioms (1)
  • domain assumption The standard model form for two-level-system loss in resonators (power-law or saturating dependence on photon number) applies to these vanadium devices.
    Invoked when decomposing measured Q_int(⟨n_ph⟩) into TLS and non-TLS components.

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discussion (0)

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