Comment on "Perfect Andreev reflection due to the Klein paradox in a topological superconducting state (Nature 570, 344 (2019))"
Pith reviewed 2026-05-24 20:22 UTC · model grok-4.3
The pith
Low-bias conductance doubling in SmB6 point contacts occurs in the thermal transport regime and does not require Klein tunneling or topological superconductivity.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Observing a low-bias conductance enhancement by a factor of 2 is not special for Klein tunneling into topological superconductors; such an enhanced conductance can be observed in point contacts between all types of superconductors and normal metals when the contacts are in the dissipative thermal regime of mesoscopic transport.
What carries the argument
The dissipative thermal regime of point-contact transport, in which local heating produces conductance doubling independent of the superconducting pairing symmetry.
If this is right
- The reported conductance doubling does not constitute unique evidence for topological superconductivity or the Klein paradox.
- Identical conductance doubling must appear in point contacts to conventional superconductors once they enter the thermal regime.
- Any claim of exotic Andreev reflection in SmB6 must first rule out the thermal-regime explanation.
- Transport-regime diagnostics are required before attributing low-bias features to topological states.
Where Pith is reading between the lines
- Varying contact diameter or lattice temperature while monitoring the doubling would test whether the feature tracks thermal-regime predictions.
- The same ambiguity may affect other point-contact reports that invoke perfect Andreev reflection in candidate topological materials.
- Independent mean-free-path data on SmB6 would directly confirm or refute the regime assignment.
- Thermal-regime effects could systematically mimic or obscure signatures sought in point-contact searches for topological superconductivity.
Load-bearing premise
The point contacts in the original experiment sit in the thermal regime because the electron mean free path in SmB6 is very short.
What would settle it
A direct measurement showing that the mean free path in the SmB6 samples is long enough to place the contacts in the ballistic regime, or a demonstration that the same doubling vanishes when contact size or bias is adjusted to exit the thermal regime.
read the original abstract
In a recent publication, Lee $et$ $al.$ discussed experimental observation of Klein tunneling into a proximity-induced topological superconducting state of SmB$_{6}$ through the measurement of Andreev reflection with low-bias conductance doubling across point-contact junctions between sharp Pt-Ir tips and SmB$_{6}$/YB$_{6}$ heterostructures. However, the interpretation of the presented point-contact data is rather ambiguous because observing a low-bias conductance enhancement by a factor of 2 is not special for Klein tunneling into topological superconductors. Such an enhanced conductance can be observed in point contacts between all types of superconductors and normal metals when the contacts are neither in the ballistic nor in the diffusive regimes of mesoscopic transport, but in a dissipative thermal regime. The thermal regime is expected for the point contacts presented in Lee $et$ $al.$ primarily because of the anticipated very short electron mean free path of SmB$_6$ -- we will discuss this in more detail here.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript is a comment on Lee et al. (Nature 570, 344, 2019). It claims that the reported factor-of-2 low-bias conductance enhancement in Pt-Ir/SmB6 point contacts is not diagnostic of Klein tunneling into a proximity-induced topological superconductor. Instead, such doubling arises generically for NS point contacts in the dissipative thermal regime of mesoscopic transport, which the authors argue applies here because of the anticipated short electron mean free path in SmB6.
Significance. If the contacts lie in the thermal regime, the comment supplies a standard-transport alternative that removes the need to invoke Klein tunneling, thereby challenging the central experimental claim of Lee et al. The argument invokes established BTK and Maxwell transport theory without free parameters or circular fitting. Its impact is reduced, however, by the lack of quantitative support for the regime assignment.
major comments (1)
- [Abstract] Abstract: The assertion that the thermal regime applies 'primarily because of the anticipated very short electron mean free path of SmB6' is load-bearing for the central claim yet contains no numerical content. No value or bound is given for the mean free path l, no contact radius a is extracted from the measured junction resistance (via Sharvin or Maxwell formulas), and no reference is made to published resistivity or mobility data on the SmB6/YB6 heterostructures. Without these, the contacts could remain ballistic or diffusive, in which case standard Andreev theory already accounts for the observed conductance and the alternative explanation is unnecessary.
Simulated Author's Rebuttal
We thank the referee for the careful reading of our comment and the constructive criticism. We address the single major comment below and will revise the manuscript to incorporate quantitative support for the regime assignment.
read point-by-point responses
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Referee: [Abstract] Abstract: The assertion that the thermal regime applies 'primarily because of the anticipated very short electron mean free path of SmB6' is load-bearing for the central claim yet contains no numerical content. No value or bound is given for the mean free path l, no contact radius a is extracted from the measured junction resistance (via Sharvin or Maxwell formulas), and no reference is made to published resistivity or mobility data on the SmB6/YB6 heterostructures. Without these, the contacts could remain ballistic or diffusive, in which case standard Andreev theory already accounts for the observed conductance and the alternative explanation is unnecessary.
Authors: We agree that explicit numerical estimates would strengthen the central claim and remove any ambiguity about the transport regime. In the revised version we will (i) extract the contact radius a from the junction resistances reported by Lee et al. using the Sharvin formula, (ii) cite published resistivity and mobility data for SmB6 (and, where available, for the SmB6/YB6 heterostructures) to obtain a bound on the elastic mean free path l, and (iii) compare a and l directly to the established criterion for the thermal (Maxwell) regime. These additions will be placed in the main text immediately after the statement about the short mean free path. revision: yes
Circularity Check
No significant circularity; critique relies on external transport principles
full rationale
The comment paper advances its central claim—that low-bias conductance doubling is generic to the dissipative thermal regime rather than diagnostic of Klein tunneling—by reference to established mesoscopic transport regimes without any internal derivation chain, equations, or self-citations. The expectation that the Pt-Ir/SmB6 contacts lie in the thermal regime is stated as an anticipation based on the material's short mean free path, but this is not derived from or fitted to the paper's own data or prior results; it functions as an external assumption open to independent verification. No load-bearing step reduces the conclusion to the inputs by construction, and the paper remains self-contained against external benchmarks of point-contact spectroscopy.
discussion (0)
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