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arxiv: 2109.01364 · v1 · submitted 2021-09-03 · ❄️ cond-mat.supr-con

Superconductivity in hole-doped germanium point contacts

N. V. Gamayunova , M. Kuzmiak , P. Szabo , P. Samuely , Yu. G. Naidyuk This is my paper

Pith reviewed 2026-05-24 12:51 UTC · model grok-4.3

classification ❄️ cond-mat.supr-con
keywords superconductivitygermaniumpoint contactsAndreev reflectionhole-dopedBTK modelcritical temperaturecritical field
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The pith

Point-contact spectroscopy reveals superconductivity in heavily hole-doped germanium up to 6 K with an unusually large gap ratio.

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

The paper establishes the presence of superconductivity in heavily hole-doped germanium through point-contact experiments with PtIr tips. Measurements of differential resistance dV/dI(V) show Andreev reflection-like features that disappear above 6 K or in magnetic fields above 1 T. These spectra are fitted to the one-gap Blonder-Tinkham-Klapwijk model, yielding a gap that follows BCS-like temperature dependence but with the ratio 2Δ/kBTc equal to 10 plus or minus 1. The same features are absent in comparably doped n-type germanium, suggesting a role for hole carriers. The magnetic-field response of the gap resembles that seen in certain type-II superconductors.

Core claim

We have observed superconductivity in heavy p-doped Ge by measuring of differential resistance dV/dI(V) of Ge - PtIr point contacts. The superconducting (SC) features disappear above 6 K or above 1 T, what can be taken as the critical temperature and the critical magnetic field, respectively. The observed dV/dI(V) spectrum with Andreev reflection like features was fitted within one-gap Blonder-Tinkham-Klapwijk model. The extracted SC gap demonstrates Bardeen-Cooper-Schrieffer-like behavior with 2 Delta/kBTc = 10+/-1 ratio, which is much higher that expected for conventional superconductors. Magnetic field suppresses Andreev reflection features, but the SC gap moderately decreases in magnetic

What carries the argument

Andreev reflection-like features in differential resistance dV/dI(V) spectra, analyzed with the one-gap Blonder-Tinkham-Klapwijk model.

If this is right

  • The extracted gap follows BCS temperature dependence yet shows a ratio 2Δ/kBTc = 10±1, far above the conventional value.
  • Magnetic field suppresses the Andreev features while the gap decreases only moderately, matching behavior reported for certain type-II superconductors.
  • No superconductivity appears in n-doped germanium at comparable dopant levels, indicating a hole-carrier dependence.
  • The superconductivity is detected only in the point-contact geometry with PtIr tips.

Where Pith is reading between the lines

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

  • Confirmation as bulk superconductivity would establish that heavy hole doping alone can produce superconductivity in an elemental group-IV semiconductor.
  • The contrast with n-doped samples suggests that future work could test whether the pairing is mediated by holes or requires specific band-structure features unique to the valence band.
  • Interface contributions at the metal-semiconductor contact remain possible, so measurements with alternative tip materials or on epitaxial layers could distinguish intrinsic versus contact-induced effects.

Load-bearing premise

The observed dV/dI(V) features with Andreev-reflection-like shape are produced by superconductivity in the Ge rather than by heating, interface states, or other non-superconducting effects at the point contact.

What would settle it

Direct observation of zero resistance or a diamagnetic response in the same p-doped germanium below 6 K in a bulk or different-geometry measurement would confirm the superconductivity claim.

Figures

Figures reproduced from arXiv: 2109.01364 by M. Kuzmiak, N. V. Gamayunova, P. Samuely, P. Szabo, Yu. G. Naidyuk.

Figure 1
Figure 1. Figure 1: (a) dV/dI(V) spectra of p-type Ge (p=1018 см-3 ) – PtIr PCs at T=1.5K. The needle moves down to the bulk material of the sample that results in decrease of the PC resistance. (b) Temperature series of normalized dV/dI(V) spectra of p-type Ge (p=2.8·1017 см-3 ) – PtIr contact [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Temperature (a) and magnetic field (b) series of experimental [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Theoretical fit (solid lines) for the temperature (a) and magnetic field (b) series of normalized dV/dI(V) spectra (dots) from [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
read the original abstract

We have observed superconductivity in heavy p-doped Ge by measuring of differential resistance dV/dI(V) of Ge - PtIr point contacts. The superconducting (SC) features disappear above 6 K or above 1 T, what can be taken as the critical temperature and the critical magnetic field, respectively. The observed dV/dI(V) spectrum with Andreev reflection like features was fitted within one-gap Blonder-Tinkham-Klapwijk model. The extracted SC gap demonstrates Bardeen-Cooper-Schrieffer-like behavior with 2 Delta/kBTc = 10+/-1 ratio, which is much higher that expected for conventional superconductors. Magnetic field suppresses Andreev reflection features, but the SC gap moderately decreases in magnetic field similarly as it was observed previously for the type-II superconductors, including nickel borocarbide and iron-based superconductors. Curiously, we have not yet observed superconductivity in n-doped Ge with a similar dopant concentration.

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 / 1 minor

Summary. The paper claims to have observed superconductivity in heavily p-doped germanium via dV/dI(V) measurements on Ge-PtIr point contacts. Andreev-reflection-like features are reported to vanish above ~6 K and ~1 T (taken as Tc and Bc), are fitted to the one-gap BTK model, and yield 2Δ/kBTc = 10±1. No analogous features appear in comparably doped n-type Ge.

Significance. If the interpretation holds, the result would be significant as the first report of superconductivity in hole-doped elemental Ge, with an unusually large gap ratio that deviates strongly from BCS expectations and may indicate unconventional pairing. The p- versus n-doping contrast would also be noteworthy for semiconductor-based superconductivity studies.

major comments (2)
  1. [Abstract] The central claim that the observed dV/dI(V) features arise from a superconducting gap in the Ge (rather than heating, Schottky-barrier modulation, or interface states) is load-bearing but not adequately supported. The manuscript notes the features' suppression with T and B and the BTK fit, yet provides no discussion or controls that distinguish these spectra from known non-SC artifacts in metal-semiconductor point contacts.
  2. [Abstract] The reported 2Δ/kBTc = 10±1 is extracted from the BTK fit, but without shown spectra, fit residuals, or error analysis, it is impossible to assess whether the one-gap model is uniquely required or whether the large ratio is robust.
minor comments (1)
  1. [Abstract] The sentence 'measuring of differential resistance dV/dI(V)' contains a grammatical error and should be rephrased for clarity.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading of our manuscript and for highlighting these important points regarding the interpretation of our point-contact data. We address each major comment below and outline the revisions we will make.

read point-by-point responses

  1. Referee: [Abstract] The central claim that the observed dV/dI(V) features arise from a superconducting gap in the Ge (rather than heating, Schottky-barrier modulation, or interface states) is load-bearing but not adequately supported. The manuscript notes the features' suppression with T and B and the BTK fit, yet provides no discussion or controls that distinguish these spectra from known non-SC artifacts in metal-semiconductor point contacts.

    Authors: We agree that an explicit discussion of possible non-superconducting artifacts is needed to strengthen the central claim. In the revised manuscript we will add a new paragraph that systematically considers heating, Schottky-barrier modulation, and interface-state effects in metal-semiconductor point contacts. We will show why each of these alternatives is inconsistent with (i) the simultaneous suppression of the features by both temperature and magnetic field, (ii) the doping-type selectivity (absent in n-Ge at comparable carrier density), and (iii) the quantitative agreement with the BTK model. Relevant literature on point-contact spectroscopy of semiconductors will be cited to place our controls in context. revision: yes

  2. Referee: [Abstract] The reported 2Δ/kBTc = 10±1 is extracted from the BTK fit, but without shown spectra, fit residuals, or error analysis, it is impossible to assess whether the one-gap model is uniquely required or whether the large ratio is robust.

    Authors: The raw spectra and the corresponding one-gap BTK fits are already displayed in the main figures. Nevertheless, we accept that additional documentation of the fitting procedure is required. In the revision we will (i) include the fit residuals as insets or in the supplementary material, (ii) tabulate the fitted parameters together with their uncertainties, and (iii) briefly discuss why a two-gap model does not improve the description. These additions will allow readers to judge the robustness of the extracted gap ratio. revision: yes

Circularity Check

0 steps flagged

No circularity: pure experimental observation and standard fitting

full rationale

The paper reports direct measurements of dV/dI(V) spectra in Ge-PtIr point contacts, notes their disappearance above ~6 K and ~1 T, and performs a standard one-gap BTK fit to extract Δ(T) and the ratio 2Δ/kBTc. No derivation chain exists; the BTK model is an external fitting tool, not derived from the data or self-cited in a load-bearing way. The extracted ratio is a reported fit result, not a 'prediction' that reduces to the input by construction. No self-citation, ansatz smuggling, or renaming of known results is present in the provided text. This is a standard experimental report whose central claim stands or falls on the interpretation of the spectra, not on any internal circular step.

Axiom & Free-Parameter Ledger

1 free parameters · 0 axioms · 0 invented entities

The central claim rests on the interpretation of point-contact spectra as Andreev reflection from bulk Ge superconductivity; no free parameters, axioms, or invented entities are introduced beyond standard BTK fitting.

free parameters (1)
  • fitted gap Delta
    Value extracted from BTK fit to dV/dI spectra

pith-pipeline@v0.9.0 · 5715 in / 965 out tokens · 25795 ms · 2026-05-24T12:51:54.349354+00:00 · methodology

discussion (0)

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Reference graph

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    Previous PC measurements with Ge Main panel of Fig. S1 shows our data from Ref. [10]. Insert shows dV/dI for corresponding curves from the main panel after their integration. We see that dV/dI demonstrate minimum at zero bias, which is more likely connected with superconductivity, but not with a peculiar electron-phonon interaction as it was supposed in R...

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    Additional dV/dI(V) data on germanium PCs -15 -10 -5 0 5 10 151,00 1,04 T(K) 1.4 1.8 3.5 dV/dI (rel.un.) V (mV) Fig. S2. The incomplete temperature series of dV/dI(V) spectra of Ge(p=2.8·1017 см-3) – PtIr point contact which was destroyed at the temperatures above 3.5 K. dV/dI(V) spectra are shifted vertically for 10 a better overview. The distinct zero-b...

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