{"id":"faa078e3-8b80-4183-986d-652207f17962","arxiv_id":"2501.09226","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"In FIB-fabricated NbGe2 devices, diode efficiency stays below 2% in the type-I bulk regime and rises to about 50% in the type-II surface regime, tracking the vortex creep rate.","lead":"This paper reports experiments on micro-fabricated devices of the chiral superconductor NbGe2, showing almost no superconducting diode effect at low magnetic fields and a strong diode effect at high fields where vortices move. The authors use this contrast to argue that vortex dynamics, not just symmetry breaking, controls the superconducting diode effect.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Low-field regime assignment rests on an unvalidated Ic estimate; if the amorphous surface carries the low-field current, the paper's vortex-dynamics conclusion loses its foundation.","rationale":"The reader's weakest assumption matches the most load-bearing concern. I independently re-derived the Supplementary Note 6 numbers and they are internally consistent, but the estimate is not a measurement of current partitioning. The paper's own STEM/EELS evidence shows a 10-15 nm amorphous layer with a composition gradient and device Hc2 two orders of magnitude above bulk, so the surface is not a negligible component a priori. Yet the calculation assigns the bulk a 2λ-thick conduction channel; whether that is the correct cross-section is not established. If the low-field Ic is bulk-dominated, the absence of SDE in a type-I chiral superconductor is a solid, novel negative result; if not, the causal narrative is unsubstantiated. The proposed width-scaling and surface-removal tests would settle the issue. The reader's conditional verdict is therefore appropriate and I see no internal contradiction or overreach beyond the identified assumption. The note-added about a similar study [53] and the data-on-request policy are secondary concerns and do not alter the central risk assessment.","tokens_in":18917,"tokens_out":4516,"duration_ms":47860,"concrete_test":"Fabricate a set of NbGe2 lamellae with identical FIB parameters but different widths (e.g., 3, 6, and 12 μm) and measure Ic at B=20 mT and B=100 mT at 0.3 K. If the low-field Ic scales with the full cross-sectional area (width × thickness), the bulk carries the current; if it scales with width alone or with surface perimeter, the amorphous surface or interface dominates. In parallel, use a low-energy Ar-ion etch on one finished device to selectively remove the 10-15 nm amorphous NbGe surface and remeasure; a drop in low-field Ic by more than an order of magnitude, or a shift of the 40 mT onset of Q(B), would show the original regime assignment is not secure.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's central causal claim—that the vanishing diode effect below 40 mT demonstrates the type-I bulk state produces no SDE, and that the high-field SDE arises from activated vortex dynamics—depends entirely on the assertion that the measured Ic in the low-field regime is carried by the NbGe2 bulk rather than by the 10-15 nm amorphous NbGe surface. This assertion rests only on the self-field estimate in Supplementary Note 6, yielding 66 mA (bulk) versus 0.35 mA (surface). That estimate is fragile. First, it uses an effective bulk cross-section of only 2λ (250 nm) of the lamella thickness, not the full ~2 μm; the actual current distribution between bulk and surface is not measured. Second, the λ and ξ values for both layers are inferred from separate bulk and film samples and may not apply to the FIB-damaged interface, where a modified Tc and Hc2 are explicitly documented. Third, the calculation assumes ideal parallel conduction; if the FIB process creates a percolative or locally weak region at the NbGe2/NbGe interface, the low-field Ic could be limited by the surface or by a junction-like path, making the Q<2% data a property of the artificial layer rather than of chiral type-I NbGe2. If so, the Q(B) versus S(B) correlation is a correlation within the surface layer and loses its claim that vortex-free type-I superconductivity is SDE-absent.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports transport measurements on FIB-fabricated microdevices of the chiral type-I superconductor NbGe2, in which focused-ion-beam milling creates a 10–15 nm amorphous NbGe surface layer with higher Tc and type-II behavior. Below about 40 mT, where the NbGe2 bulk is superconducting and vortex dynamics are suppressed, the diode efficiency Q is below 2%; above 40 mT, Q rises to about 50% and coincides with an abrupt increase in the vortex creep rate S extracted from I–V curves. The authors interpret these observations as evidence that vortex dynamics, rather than intrinsic Cooper-pair asymmetry alone, are critical for the superconducting diode effect, and that type-I superconductivity with strongly suppressed vortices does not produce a diode effect.","tokens_in":19184,"tokens_out":4076,"duration_ms":40762,"significance":"If correct, this is an important empirical constraint on SDE mechanisms: it separates a vortex-free type-I regime from a vortex-dominated type-II regime in the same device and shows that the diode effect appears only when vortex dynamics are activated. The paper's strengths include multi-device reproducibility (devices D1–D3), direct structural characterization of the amorphous layer by STEM/EELS, BKT analysis of the 2D surface superconductivity, and explicit acknowledgment of a similar concurrent study. The main caveat is that the central mechanistic separation relies on an auxiliary theoretical estimate of the critical-current distribution between the bulk and surface layers.","major_comments":[{"comment":"The claim that the low-field Ic and hence the negligible Q are properties of the chiral type-I NbGe2 bulk depends entirely on the estimate in Supplementary Note 6 that the bulk self-field depairing current (66 mA) far exceeds the surface value (0.35 mA). This estimate assumes values of lambda and xi for both layers and effective cross-sectional areas (0.25 x 5 um^2 for the bulk and 0.03 x 5 um^2 for the surface) that are not directly measured in the actual devices. If the low-field critical current is carried instead by the amorphous surface, by the FIB-damaged interface, or by a parallel path, then the Q<2% data would not establish the absence of SDE in type-I NbGe2, and the Q(B)-S(B) correlation would lose its mechanistic interpretation. The authors should provide direct evidence for the current path (for example, comparison of measured Ic with the two estimates, thickness-dependent or temperature-dependent tests, or a control device with the surface layer removed) or explicitly reframe the conclusion as conditional on this assumption.","section":"Supplementary Note 6 and main text, 'Absence of SDE in type-I NbGe2 regime'"},{"comment":"The central correlation between Q(B) and S(B) is empirically visible, but the claim of an 'abrupt increase' in S at exactly the same 40 mT threshold is identified post hoc and the plotted S values carry no error bars or fit-quality information. Because N is extracted from power-law fits V proportional to I^N near Ic, the uncertainty in N propagates strongly into S = 1/(N-1) (for example, N = 87 versus N = 5 in Figs. 5c-d), and the slope change in Fig. 5e should be quantified with confidence intervals or alternative threshold criteria to support the coincidence with Hc2 of NbGe2. Without this, the quantitative statement that S and Q switch at the same field is weaker than the text suggests.","section":"Section 'Nonreciprocal transport near Tc' and Fig. 5e; S = 1/(N-1)"},{"comment":"The manuscript's concluding lesson, that 'the ubiquitous vortex effect plays a nonnegligible and even critical role in realizing the SDE,' goes beyond the data unless the regime assignment is validated. The note added (reference [53]) mentions a similar concurrent study on FIB-fabricated NbGe2, but the relationship between that study and the present conclusions is not discussed; this is relevant because the concurrent study could either support or challenge the present interpretation, and its existence strengthens the need for a robust current-path assignment.","section":"Conclusions and Note added"}],"minor_comments":[{"comment":"The sentence 'Although we did not observe observe the SDE in chiral NbGe2' contains a duplicated word; please revise.","section":"Conclusions"},{"comment":"The phrase 'two-fold rotational symmetry (2001)' appears to be a typographical error for a screw-axis or rotation-axis notation; please correct the symmetry symbol.","section":"Section 'Nonreciprocal transport near Tc'"},{"comment":"The caption statement that 'Ic maximum is calculated from the self-field Ic, and the empirical evolution is capped at the measured upper critical field Hc2' is unclear; please specify the model used for the field dependence of Ic.","section":"Fig. 5a caption"},{"comment":"The voltage criterion of 1 microvolt used to extract Ic is stated only in the Figure 5 caption; please define the criterion and the fitting range in the Methods section.","section":"Fig. 5 caption"},{"comment":"The relation S ~ 1/(N-1) is introduced without derivation or a reference to the creep model; a brief justification would improve readability and help the reader assess the uncertainty in S.","section":"Section 'Nonreciprocal transport near Tc'"}],"recommendation":"major_revision","confidential_remarks":"The manuscript is within the scope of a superconductivity journal and the empirical data are likely of broad interest. The main risk is that the regime assignment rests on an order-of-magnitude estimate; the authors should be encouraged to provide direct current-path evidence or substantially soften the mechanistic claim. I do not see grounds for rejection, but the revision should be substantive."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Dear Colleague,\n\nThis paper is worth your time. It reports a clean, systematic transport study on FIB-fabricated NbGe2 devices, where an amorphous NbGe surface layer with higher Tc and Hc2 forms naturally. The central result is a field-resolved comparison of diode efficiency Q and vortex creep rate S: below the bulk Hc2 (~40 mT) Q stays below 2%, and above it Q rises to ~50% while S jumps abruptly. The same threshold appears in three devices and in both polar and chiral current configurations. That correlation is the real, new finding, and it is a useful data point for the ongoing debate about whether vortices are essential to the SDE.\n\nThe paper does several things well. The BKT analysis of the artificial surface is careful and reproducible across devices. The authors are honest: they disclose a similar FIB-NbGe2 study (ref 53) in a note added, and they explicitly say they cannot rule out exotic intrinsic mechanisms. That's the right tone.\n\nThe soft spots are about interpretation, not execution. The claim that the low-field Ic is carried by the NbGe2 bulk rests on a theoretical estimate in Supplementary Note 6, using assumed λ, ξ and cross-sections. If the amorphous surface were actually carrying the low-field current, the 'absence of SDE in type-I NbGe2' would not be established. I think the estimate is physically reasonable—the bulk has much lower kinetic inductance and larger cross-section—but it is not directly measured, and the paper would be stronger if it said so more clearly. The same note applies to the causal language: Q and S track each other, but correlation is not causation. The authors do hedge ('highlight the critical role'), but the abstract says 'unambiguously highlight', which overstates it.\n\nTwo more minor points: Q and S have no error bars, and the data are on request. Both are fixable in revision.\n\nOverall, this is a solid, honest paper that deserves a serious referee. I would send it to review with a request to soften the causal claims, add error bars, and either deposit the data or include representative raw I-V curves. The stress-test worry about the Ic estimate is real but not fatal; it should be a major comment, not a rejection.","headline":"A clean, honest experiment showing SDE absence tracks the type-I regime and emergence tracks vortex creep; the correlation is real, the causal wording is a bit strong.","tokens_in":19847,"tokens_out":3392,"would_cite":true,"duration_ms":34592,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"The superconducting diode effect in NbGe2 is controlled by vortex dynamics: diode efficiency stays below 2% in the vortex-free type-I regime and jumps to about 50% when vortex creep activates above 40 mT.","keywords":["superconducting diode effect","chiral superconductor","type-I superconductivity","NbGe2","vortex dynamics","nonreciprocal transport","focused ion beam","BKT transition"],"falsifier":"Strip or electrically bypass the amorphous NbGe surface in an otherwise identical NbGe2 device and remeasure Q(B): if the diode efficiency still stays below 2% up to the bulk upper critical field, the vortex-dynamics explanation is supported; if a significant Q appears while no vortices are present, the central claim fails.","tokens_in":18679,"feed_emoji":"🧲","tokens_out":8392,"duration_ms":73627,"temperature":0.7,"pith_summary":"This paper tries to settle a disputed question about the superconducting diode effect (SDE): is the asymmetry in critical current an intrinsic property of the superconductor, or does it come from vortices? Using the chiral type-I superconductor NbGe2, the authors show that even with both space-inversion and time-reversal symmetry broken, the diode efficiency stays below 2% at low fields, where vortices are absent. Above 40 mT, when the type-I bulk is suppressed and an artificially induced type-II NbGe surface takes over, vortex creep switches on and the diode efficiency jumps to about 50%. The authors conclude that vortex dynamics is a critical, not optional, ingredient for a significant SDE, beyond the established symmetry rules.","feed_headline":"Vortex motion, not symmetry alone, turns on the diode effect","feed_subtitle":"NbGe2 diodes stay below 2% efficiency until vortices start creeping, pinning the effect's mechanism.","key_machinery":"The argument is carried by two measured quantities: the diode efficiency Q = |Ic+ - Ic-| / ((Ic+ + Ic-) / 2), which quantifies the difference between positive and negative critical currents, and the vortex creep rate S = 1/(N-1), extracted from the power-law exponent N of the current-voltage curves near Ic. The physical switch between the two regimes is the FIB-induced amorphous NbGe surface layer: it is type-II superconducting with Tc ≈ 2.95 K, so vortices can move in it, while the type-I NbGe2 bulk (Tc ≈ 2 K) is vortex-free until its superconductivity is suppressed by field. The matching field dependence of Q and S, with both jumping at about 40 mT, is the evidence that vortex dynamics controls the SDE.","core_discovery":"The central claim is that the superconducting diode effect requires more than broken symmetries: in NbGe2, a chiral type-I superconductor in which space-inversion and time-reversal symmetries are both broken, the diode efficiency Q stays below 2% at low magnetic fields. The effect appears only above the bulk upper critical field of about 40 mT, where superconductivity is carried by the FIB-induced amorphous NbGe surface, a type-II superconductor with a higher critical temperature; at that same field the vortex creep rate S rises abruptly. The paper therefore concludes that vortex dynamics is the decisive ingredient that turns the symmetry-allowed nonreciprocity into a real diode effect.","pith_inferences":["If vortex dynamics is the controlling factor, then many reported SDEs in thin films and interfaces may be dominated by vortex depinning rather than by intrinsic finite-momentum pairing; the paper urges caution, and this is the natural generalization.","A quantitative prediction follows that the paper does not state: in the vortex-active regime, Q should scale with the creep rate S as temperature, field, or pinning strength is varied, so the two should never decouple.","The same FIB 'surface toggle' could be used as a controlled experiment in other materials: comparing identical devices with and without the amorphous layer would isolate the vortex contribution from the intrinsic symmetry contribution.","If this is right, strengthening vortex pinning (for example by irradiation or nanostructuring) should suppress the diode effect even though symmetry breaking remains, a testable design rule for practical superconducting diodes."],"forward_implications":["In a superconductor where vortices cannot move, the diode effect should remain negligible even when all symmetry requirements are met.","The field at which a diode effect appears should coincide with the onset of vortex creep, not just with the symmetry-breaking conditions.","Focused-ion-beam surfaces can turn a type-I or non-superconducting material into a diode-active device, extending SDE functionality to materials that do not naturally host it.","Diode polarity switched by reversing the magnetic field, observed in the vortex-active regime, supports asymmetric vortex pinning as the rectification mechanism in this device."],"supporting_citations":[{"why":"Establishes that NbGe2 is a type-I noncentrosymmetric superconductor, the basis for expecting vortex-free behavior at low fields.","marker":"[18]"},{"why":"Shows that focused-ion-beam milling induces superconductivity in non-superconducting NbAs via an amorphous niobium surface, supporting the same mechanism in NbGe2 devices.","marker":"[36]"},{"why":"Provides the superconducting parameters of amorphous Nb1-xGex films used to identify the surface layer and compare Hc2.","marker":"[35]"},{"why":"Supplies the self-field critical current formulas used to estimate that the NbGe2 bulk carries the low-field critical current.","marker":"[58]"},{"why":"Documents the FIB microstructuring method and the amorphous surface damage layer that creates the type-II NbGe shell.","marker":"[24]"},{"why":"Gives the theory of BKT-enhanced nonreciprocal transport in two-dimensional superconductors, linking vortex motion to the nonreciprocal coefficient.","marker":"[41]"},{"why":"Provides the practical viewpoint that measured critical currents are vortex-depinning currents and that self-fields excite vortex-antivortex pairs, a key alternative to intrinsic depairing theory.","marker":"[12]"},{"why":"Reports a ubiquitous superconducting diode effect in thin films with a linear field dependence of Q, the comparison data for the anomalous Q(B) observed here.","marker":"[13]"},{"why":"Reports the first observation of the superconducting diode effect and demonstrates polarity switching, providing the standard baseline for SDE behavior.","marker":"[4]"},{"why":"Gives the Halperin-Nelson formula used to fit the resistive transition and prove the two-dimensional BKT superconducting nature of the NbGe surface.","marker":"[34]"}],"fun_headline_variants":["NbGe2 diode needs vortices, not just broken symmetry","Vortex creep triggers diode effect in chiral superconductor","Symmetry isn't enough: vortices drive superconducting diode","Diode effect appears only when vortices move in NbGe2"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The interpretation assumes that, below 40 mT, the measured critical current flows through the type-I NbGe2 bulk rather than through the amorphous NbGe surface; that assumption rests on an estimate that the bulk depairing current (66 mA) exceeds the surface value (0.35 mA) using assumed penetration depths, coherence lengths, and cross-sectional areas.","fun_headline_variants_meta":{"raw":{"variants":["NbGe2 diode needs vortices, not just broken symmetry","Vortex creep triggers diode effect in chiral superconductor","Symmetry isn't enough: vortices drive superconducting diode","Diode effect appears only when vortices move in NbGe2"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000708,"raw_usage":{"total_tokens":3145,"prompt_tokens":857,"completion_tokens":2288,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":473,"completion_tokens_details":{"reasoning_tokens":2218}},"tokens_in":473,"tokens_out":2288,"duration_ms":15716,"temperature":1.0,"reasoning_tokens":2218,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T20:09:44.255722+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Strip or electrically bypass the amorphous NbGe surface in an otherwise identical NbGe2 device and remeasure Q(B): if the diode efficiency still stays below 2% up to the bulk upper critical field, the vortex-dynamics explanation is supported; if a significant Q appears while no vortices are present, the central claim fails.","supporting_citations":[{"cited_title":"Lv et al., Type-I superconductivity in noncentrosymmetric NbGe2, Phys","cited_arxiv_id":null,"evidence_quote":"Establishes that NbGe2 is a type-I noncentrosymmetric superconductor, the basis for expecting vortex-free behavior at low fields."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Documents the FIB microstructuring method and the amorphous surface damage layer that creates the type-II NbGe shell."},{"cited_title":"Hoshino, R","cited_arxiv_id":null,"evidence_quote":"Gives the theory of BKT-enhanced nonreciprocal transport in two-dimensional superconductors, linking vortex motion to the nonreciprocal coefficient."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the practical viewpoint that measured critical currents are vortex-depinning currents and that self-fields excite vortex-antivortex pairs, a key alternative to intrinsic depairing theory."},{"cited_title":"Hou et al., Ubiquitous Superconducting Diode Effect in Superconductor Thin Films, Phys Rev Lett 131, 027001, (2023)","cited_arxiv_id":null,"evidence_quote":"Reports a ubiquitous superconducting diode effect in thin films with a linear field dependence of Q, the comparison data for the anomalous Q(B) observed here."},{"cited_title":"Ando et al., Observation of superconducting diode effect, Nature 584, 373, (2020)","cited_arxiv_id":null,"evidence_quote":"Reports the first observation of the superconducting diode effect and demonstrates polarity switching, providing the standard baseline for SDE behavior."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Gives the Halperin-Nelson formula used to fit the resistive transition and prove the two-dimensional BKT superconducting nature of the NbGe surface."}],"review_version":1}