{"id":"ea7384bd-42f5-47f1-b055-f1ae2bf48764","arxiv_id":"1908.03987","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"high","formal_verification":"none","parameter_count":4,"one_line_summary":"Magnetization measurements on HF-etched Nb2C MXene powder show a diamagnetic transition at about 12.5 K, claimed as the highest Tc among MXene superconductors.","lead":"This paper reports a superconducting transition near 12.5 K in powdered Nb2C MXene, made by etching aluminum from the Nb2AlC 'MAX' phase. The evidence is magnetization data only, without transport or specific heat, so the intrinsic origin of the signal is not yet convincing.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The 12.5 K signal is only ~0.1% of a full Meissner response and no shielding fraction is reported, so a trace impurity phase is not excluded and the intrinsic-MXene attribution is the weakest load-bearing link.","rationale":"The reader's verdict is CONDITIONAL, with the same weakest assumption at its core. I agree that the single load-bearing question is whether the observed magnetic transition belongs to the 2D Nb2C MXene phase. Quantitatively, the raw M(T) data (if plotted on the stated scale) correspond to a very small diamagnetic response relative to full Meissner screening, so the signal is consistent with a minority phase. Since the manuscript provides no volume-fraction estimate, transport, or heat-capacity data, the claim cannot be distinguished from a trace NbC-type impurity. The test I propose is a simple re-analysis of the existing ZFC data; if the shielding fraction is low, the paper's central claim would need to be reframed as an observation of a minority superconducting phase, not intrinsic Nb2C MXene superconductivity. This confirms the CONDITIONAL verdict rather than moving it.","tokens_in":7777,"tokens_out":17337,"duration_ms":190098,"concrete_test":"Compute the shielding fraction at 2 K from the reported ZFC data in Fig. 3a at 10 mT, using the sample mass and the ideal Meissner response M_ideal = -H/(4πρ) for Nb2C (ρ≈5.5 g/cm3 from the reported lattice parameters). If the fraction is below about 1%, the 12.5 K signal is a minority contribution and the intrinsic-MXene claim is unsupported; if it is a substantial fraction (e.g., >10%), the impurity concern is largely resolved. A control measurement on the un-etched Nb2AlC MAX powder under identical conditions would further distinguish a pre-existing impurity.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The most load-bearing assumption is that the 12.5 K diamagnetic transition is an intrinsic property of the 2D Nb2C MXene phase. The data do not yet support this. In Fig. 3a, the ZFC magnetization below Tc is about -1.2x10^-3 emu/g at 10 mT. For a fully diamagnetic Nb2C powder (density around 5-6 g/cm3), the ideal Meissner response would be M = -H/(4πρ), roughly -1.5 to -2 emu/g; the measured value therefore corresponds to a shielding fraction of order 0.1%, or a few percent at most after demagnetization corrections. The starting Nb2AlC powder is only 90-95% pure, and the XRD-based exclusion of Nb, NbC, NbAlC, Al, and 3D Nb2C has a detection limit of a few percent, so a trace superconducting impurity could produce exactly this small signal. The manuscript reports no specific heat, no transport, no AC susceptibility, and no quantified Meissner/shielding fraction. The GL fits of Hc1 and Hc2 are derived from a small number of M(H) points after subtracting a high-field 'paramagnetic' background that may itself be part of the reversible mixed-state response. Thus, while an intrinsic superconducting signal is possible, the central claim that the Nb2C MXene is an intrinsic superconductor with Tc=12.5 K is not established.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports the observation of superconductivity in two-dimensional Nb2C MXene powder synthesized by HF etching of Nb2AlC MAX phase. The claims are based on magnetization measurements: M(T) under ZFC/FC conditions shows a diamagnetic transition with onset Tc ≈ 12.5 K, M(H) loops exhibit hysteresis characteristic of a type-II superconductor, and the temperature dependences of the lower and upper critical fields are fitted with Ginzburg-Landau expressions. The authors argue that this is the highest superconducting transition temperature reported for any MXene so far, and they attribute it to the intrinsic 2D structure of Nb2C, supported by the proximity of the observed Tc to a DFT-predicted value of 13 K for Mo2C.","tokens_in":8000,"tokens_out":4432,"duration_ms":46470,"significance":"If the observation is correct, the paper would establish a new record transition temperature in MXenes and provide a strong candidate for intrinsic 2D superconductivity in a transition-metal carbide. The paper also reports a coherence length of about 25 nm and critical current densities estimated from Bean's model. However, the evidence rests entirely on magnetization data with a very small diamagnetic signal, no transport or specific-heat confirmation, and no quantification of the superconducting volume fraction. The claimed record is therefore not securely established, and the paper's significance is conditional on additional supporting evidence.","major_comments":[{"comment":"The ZFC magnetization below Tc at 10 mT is approximately -1.2×10^-3 emu/g. For a fully diamagnetic Nb2C powder (density ~5-6 g/cm³), the ideal Meissner response is M ≈ -H/(4πρ) ≈ -1.5 to -2 emu/g, so the measured value corresponds to a shielding fraction of order 0.1% (a few percent at most after demagnetization corrections). The manuscript does not report a Meissner or shielding fraction, and XRD-based impurity exclusion has a detection limit of a few percent. Since the starting Nb2AlC powder is only 90-95% pure, a trace superconducting impurity phase could produce exactly the observed small signal. This leaves the central claim—that the 12.5 K transition is intrinsic to the Nb2C MXene phase—unsubstantiated.","section":"Fig. 3a and the paragraph beginning 'Since our Nb2C MXene samples are in powder form...'"},{"comment":"The paramagnetic background subtraction is described only as removing a linear increase at high fields with Mp = GH, where G is the gradient of the high-field data. The manuscript does not specify the field range over which G is determined, whether G varies with temperature, or whether the subtracted component might include a reversible mixed-state response. This procedure directly determines the Hc1 and Hc2 values that are then fitted to Ginzburg-Landau theory, so the resulting parameters may be artifacts of the subtraction rather than intrinsic superconducting properties.","section":"Paragraph beginning 'It can be seen that the width of M-H loop...' and the description of Mp = GH subtraction"},{"comment":"The argument that the observed Tc ≈ 12.5 K is intrinsic because it is close to the DFT-predicted 13 K for Mo2C is not a valid proof. The theoretical prediction was made for a different material (Mo2C), and no first-principles calculations are performed here for Nb2C. The proximity of the experimental Tc to this prediction does not exclude impurity phases and cannot serve as evidence that the 2D structure of Nb2C is responsible for the observed superconductivity. Similarly, the claim that 'the DOS above the Fermi level will be much higher compared to all other 2D MXenes' is speculative and not supported by any calculation.","section":"Section beginning 'Now, we discuss the relatively higher Tc...'"}],"minor_comments":[{"comment":"There are numerous typographical errors: 'pramagnetic' should be 'paramagnetic', 'yeilds' should be 'yields', 'relativly' should be 'relatively', 'imurity' should be 'impurity', 'supercondutivity' should be 'superconductivity', 'occurence' should be 'occurrence', and 'It is well know' should be 'It is well known'.","section":"Throughout the text"},{"comment":"The reference list contains a duplicate number: [22] is used for both Awana et al. (Solid State Comm. 2006) and Farhadi et al. (J. Nanostruct. Chem. 2013). The references should be renumbered and checked for completeness.","section":"References"},{"comment":"The text states that the fits use Hc1(T) = Hc1(0)[1 - (T/Tc)²] and similarly for Hc2, but the figure axis label reads 'T (K)' rather than 'T/Tc'. The figure should be clarified so the reader can distinguish the temperature axis from the normalized axis used in the fit.","section":"Fig. 3d caption and axis labels"},{"comment":"The sentence 'this is the first study on the superconductivity of as-prepared MXene (Nb2C-powder in present case)' is misleading because the introduction discusses previous superconductivity studies on Mo2C MXenes; the statement should be rephrased to claim the first observation of superconductivity in Nb2C MXene specifically.","section":"Conclusion"},{"comment":"The abstract states that 'strong-electron phonon interaction and the large density-of-states at Fermi level may cause the emergence of superconductivity', but this is presented as a speculation without supporting evidence; it should be explicitly labeled as a hypothesis.","section":"Abstract"}],"recommendation":"major_revision","confidential_remarks":"The paper reports an exciting claim but the evidence is extremely thin: a single magnetization measurement with a signal lower than 0.1% of a full Meissner response and no shielding fraction. This is far below the standard for establishing a record superconducting transition temperature. The revision should require the authors to quantify the superconducting volume fraction, provide a detailed justification of the background subtraction, and ideally add AC susceptibility or specific-heat data. If such data cannot be provided, the manuscript should be reconsidered for rejection or downgraded to a highly cautious report."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague,\n\nYou should know two things: this is the first claim of superconductivity in Nb2C MXene, with a reported Tc of 12.5 K, and the evidence is a small diamagnetic signal in magnetization only—no transport, no heat capacity, no shielding fraction. The stress-test arithmetic is right: the ZFC moment at 10 mT corresponds to roughly 0.1% of a full Meissner response, so a trace impurity phase is entirely plausible.\n\nWhat's genuinely new: the synthesis is a systematic etch of Nb2AlC to Nb2C, XRD shows the expected delamination (c parameter from 13.8 to 22.7 Å), and the authors take the trouble to list known superconducting impurities (Nb, NbC, NbAlC, Al, 3D Nb2C) and argue from XRD that they are absent. That is honest engagement. The GL fits of Hc1 and Hc2 and the coherence length estimate are standard, though they inherit the background subtraction problem.\n\nSoft spots, in order: (1) The magnetization signal is tiny, and the paper never quantifies the superconducting volume. 0.1% shielding fraction means the intrinsic attribution is unproven, and XRD detection limits are a few percent, so the impurity argument is not conclusive. (2) The paramagnetic background subtraction is described qualitatively; the 'linear increase at higher fields' may include the reversible mixed-state response, so Hc1 and Hc2 could be artifacts. (3) There are no error bars anywhere. (4) The mechanistic discussion—strong electron-phonon coupling and high DOS—is speculative, not computed, and the comparison to the Mo2C theory is suggestive, not evidence for Nb2C. (5) Bean's model on a powder in a glass vial gives a Jc that is probably meaningless as an absolute number.\n\nThe paper is not a hoax and the claim might be real; it just isn't established. The right outcome is a serious peer review with a demand for more evidence: a measured shielding fraction, transport on aligned flakes or a film, specific heat, or at minimum AC susceptibility and a careful impurity search with EDX/magnetometry on known phases. A journal that publishes it as is would be doing the field a disservice; a journal that sends it back for that evidence would be treating it fairly.\n\nRecommendation: send to peer review, but expect major revision. The result, if true, would set a record Tc for MXenes and deserves referee time. I would not cite it yet.","headline":"First claim of superconductivity in Nb2C MXene at 12.5 K, but the evidence is a tiny magnetization signal with no shielding fraction; worth peer review, not yet convincing.","tokens_in":8622,"tokens_out":2697,"would_cite":false,"duration_ms":26933,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"The paper reports that a two-dimensional Nb2C MXene becomes superconducting at 12.5 K, the highest transition temperature yet measured in an MXene.","keywords":["superconductivity","MXene","Nb2C","two-dimensional materials","transition temperature","Meissner effect","type-II superconductor","critical fields"],"falsifier":"Measure the low-temperature specific heat of the same powder: an intrinsic bulk superconductor at 12.5 K should show a clear specific-heat anomaly at Tc, while a magnetization signal from a minority phase would show little or no anomaly; alternatively, extract the superconducting volume fraction from the zero-field-cooled shielding signal, where a bulk superconductor should give a large diamagnetic fraction.","tokens_in":7529,"feed_emoji":"🧲","tokens_out":6822,"duration_ms":66671,"temperature":0.7,"pith_summary":"This paper reports that chemically etched two-dimensional Nb2C MXene powder becomes superconducting at an onset of 12.5 K, the highest transition temperature reported for any MXene. The claim matters because MXene superconductors are rare and their transition temperatures have lagged behind theoretical predictions; 12.5 K is close to the 13 K predicted for the related Mo2C MXene. The evidence is magnetic: zero-field-cooled and field-cooled magnetization curves show a diamagnetic Meissner transition near 12.5 K, and the lower and upper critical fields follow the expected Ginzburg-Landau temperature dependence of a type-II superconductor. If correct, the result shows that high transition temperatures can arise from the intrinsic two-dimensional structure of a transition-metal carbide MXene.","feed_headline":"2D Nb2C MXene superconducts at 12.5 K, highest for MXenes","feed_subtitle":"The niobium-carbide flakes beat earlier MXene superconductors and approach the 13 K theory predicted for Mo2C.","key_machinery":"The central object is the two-dimensional Nb2C MXene sheet obtained by selective HF etching of the Nb2AlC MAX phase; its identity as a 2D phase is carried by the c-lattice expansion from 13.83 Å to 22.72 Å seen in XRD. The argument for superconductivity is carried by the magnetic response: a clear diamagnetic Meissner transition in ZFC and FC magnetization at 12.5 K, and magnetization loops whose lower and upper critical fields follow the Ginzburg-Landau form Hc(T) = Hc(0)[1-(T/Tc)2]. That fit is the load-bearing quantitative check that the transition is bulk superconducting rather than a spurious magnetic feature, and it yields the 25 nm coherence length quoted in the paper.","core_discovery":"The authors claim that Nb2C MXene synthesized by HF etching of Nb2AlC MAX is an intrinsic superconductor with onset Tc = 12.5 K. They identify the superconducting phase as two-dimensional by the disappearance of the Al-related XRD peak and by the expansion of the c-lattice parameter from 13.83 Å to 22.72 Å, which indicates delamination into sheets. Below Tc, both zero-field-cooled and field-cooled magnetization turn diamagnetic, and the M-H loops show the hysteresis and vortex behavior of a type-II superconductor. The critical fields Hc1(T) and Hc2(T) fit the standard Ginzburg-Landau form, and the upper-critical-field extrapolation gives a coherence length of about 25 nm. The authors argue that the absence of XRD peaks from Nb, NbC, NbAlC, Al, or bulk 3D Nb2C, together with a Tc higher than any of those known phases, makes a minority impurity explanation unlikely and points to the intrinsic 2D MXene structure as the source of the high transition temperature.","pith_inferences":["If confirmed, this result would make the etched powder a test bed for whether surface termination groups such as F, O, and OH left by HF etching enhance or suppress Tc; comparing samples with different terminations would separate surface chemistry from the intrinsic 2D structure.","A decisive check of the intrinsic claim would be to measure the same Nb2C MXene as a continuous film: a much lower Tc there would suggest the powder's value comes from surface or grain-boundary effects rather than from the bare sheet.","Because the paper infers strong electron-phonon coupling from the high Tc without direct transport data, a tunable test would be isotope substitution of carbon or pressure dependence to confirm the phonon mechanism.","The broadening of the magnetic transition could reflect a distribution of Tc across sheets rather than vortex pinning alone; AC susceptibility measurements at different frequencies would separate dynamic pinning from static spread in Tc."],"forward_implications":["If the 12.5 K transition is intrinsic, Nb2C MXene becomes the highest-Tc MXene superconductor known, surpassing the 2D α-Mo2C values of about 3.6–8 K.","The near-agreement with the 13 K predicted for Mo2C suggests that the same electron-phonon mechanism can raise Tc in n=1 MXene carbides, a generalization the paper explicitly proposes.","A powder superconductor with Tc = 12.5 K and a critical current density extracted from Bean-model analysis is directly relevant for applications that want superconducting properties in a solution-processable 2D material.","The type-II character, with upper critical field extrapolated from Ginzburg-Landau fits and a coherence length around 25 nm, sets a concrete magnetic phase diagram for future device-oriented studies."],"supporting_citations":[{"why":"First report of superconductivity in 2D α-Mo2C MXene with Tc around 3.6 K; it supplies the baseline record that this paper claims to surpass.","marker":"[6]"},{"why":"These reports give the higher Mo2C value of about 8 K and attribute it partly to substrate and strain effects, the comparison point for the intrinsic claim.","marker":"[7,8]"},{"why":"Theoretical prediction of 13 K for a Mo2C MXene monolayer, which this paper's 12.5 K result is said to approach.","marker":"[9]"},{"why":"Source of the Ginzburg-Landau expression used to fit Hc1(T) and Hc2(T), the central quantitative validation of the superconducting transition.","marker":"[16]"},{"why":"Reports Tc = 9.18 K for 3D hexagonal Nb2C, used as the known non-MXene baseline that the higher 12.5 K value must be distinguished from.","marker":"[18]"},{"why":"Established synthesis protocol for HF etching of MAX phases into MXenes, from which the paper's optimized etching procedure is adapted.","marker":"[24]"}],"fun_headline_variants":["Nb2C MXene sets MXene record with 12.5 K superconductivity","2D Nb2C MXene superconducts at 12.5 K, highest for MXenes","Record 12.5 K superconducting transition in Nb2C MXene","Nb2C MXene hits 12.5 K, best superconductivity in MXenes","Highest Tc yet in MXenes: 2D Nb2C at 12.5 K"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The result depends on the measured diamagnetic signal coming from the two-dimensional Nb2C MXene itself, not from a minor impurity or unetched phase that XRD cannot detect.","fun_headline_variants_meta":{"raw":{"variants":["Nb2C MXene sets MXene record with 12.5 K superconductivity","2D Nb2C MXene superconducts at 12.5 K, highest for MXenes","Record 12.5 K superconducting transition in Nb2C MXene","Nb2C MXene hits 12.5 K, best superconductivity in MXenes","Highest Tc yet in MXenes: 2D Nb2C at 12.5 K"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000193,"raw_usage":{"total_tokens":1392,"prompt_tokens":1028,"completion_tokens":364,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":644,"completion_tokens_details":{"reasoning_tokens":250}},"tokens_in":644,"tokens_out":364,"duration_ms":4224,"temperature":1.0,"reasoning_tokens":250,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T13:55:00.260092+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the low-temperature specific heat of the same powder: an intrinsic bulk superconductor at 12.5 K should show a clear specific-heat anomaly at Tc, while a magnetization signal from a minority phase would show little or no anomaly; alternatively, extract the superconducting volume fraction from the zero-field-cooled shielding signal, where a bulk superconductor should give a large diamagnetic fraction.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"First report of superconductivity in 2D α-Mo2C MXene with Tc around 3.6 K; it supplies the baseline record that this paper claims to surpass."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Theoretical prediction of 13 K for a Mo2C MXene monolayer, which this paper's 12.5 K result is said to approach."},{"cited_title":"and Awana, V.P.S., Superconductor Sci","cited_arxiv_id":null,"evidence_quote":"Source of the Ginzburg-Landau expression used to fit Hc1(T) and Hc2(T), the central quantitative validation of the superconducting transition."},{"cited_title":"Hulm, J.K","cited_arxiv_id":null,"evidence_quote":"Reports Tc = 9.18 K for 3D hexagonal Nb2C, used as the known non-MXene baseline that the higher 12.5 K value must be distinguished from."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Established synthesis protocol for HF etching of MAX phases into MXenes, from which the paper's optimized etching procedure is adapted."}],"review_version":1}