{"id":"22af812f-1a6d-41e8-9e90-400891c2225b","arxiv_id":"2505.12062","paper_version":4,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":6,"one_line_summary":"Bismuth dihydride formed at 140-211 GPa superconducts up to ~70 K with unusually low upper critical fields (12-16 T), placing it among 'soft' hydride superconductors.","lead":"Researchers compressed bismuth and hydrogen in diamond anvil cells to form bismuth dihydride, a superconductor that turns resistance-free near 70 kelvin and is easily suppressed by magnetic fields. The experiments classify BiH2 as a 'soft' hydride superconductor and provide a new benchmark for how electron velocity controls critical fields in high-pressure superconductors.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The 61–70 K transition and 12–16 T Hc2 are assigned to P21/m-BiH2 even though XRD cannot resolve H and the samples contain C2/m-BiH2, BiH4, and >95% bcc-Bi; a phase-specific measurement is needed.","rationale":"The reader identified the same assumption as weakest, and I agree. The paper reports real transport data and detailed calculations, but the phase attribution is the hinge. The authors themselves state 'probably P21/m' and Fig. S8 lists multiple hydride phases plus >95% unreacted Bi. Because H is nearly invisible to XRD and the BiH2 candidates share a Cmcm-like Bi sublattice, diffraction cannot discriminate the polymorph that carries the superconducting current. The theoretical Tc comparison makes the ambiguity concrete: the paper's own Cmcm-BiH2 calculation gives Tc ≈ 85 K, close to the measured 70 K, so C2/m-BiH2 is a credible alternative carrier. The two-gap evidence is also weaker than claimed: ΔS = 1.5 ± 9.4 meV is statistically indistinguishable from zero. These issues do not require rejection — the low Hc2 and soft behavior may survive a phase-specific test — but they do require the CONDITIONAL verdict to remain. The proposed micro-XRD/current-path check would settle the attribution directly.","tokens_in":20398,"tokens_out":6793,"duration_ms":68198,"concrete_test":"Perform spatially resolved synchrotron micro-XRD mapping (beam ≤ 5 µm) of the inter-electrode region of DAC B3 at 172 GPa after the transport and Jc measurements, and compare diffraction from the exact current path with the P21/m-BiH2, C2/m-BiH2, C2/c-BiH4, and bcc-Bi patterns. If the current path contains no P21/m-BiH2 (for example, only C2/m-BiH2 or BiH4), the phase-specific claims fail. Alternatively, synthesize a fresh DAC where the R-T curve shows a single sharp transition and where XRD of the same sample volume shows only P21/m-BiH2; remeasure Tc, Hc2, and Jc. If Hc2 remains ≈ 12–16 T for the single-phase P21/m sample, the phase-attribution concern is resolved.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim — that P21/m-BiH2 is a soft superconductor with Tc ≈ 70 K, μ0Hc2(0) = 12–16 T, and VF ≈ 1.1×10^6 m/s — rests on assigning the resistive transitions and Hc2 data to the P21/m phase. That assignment is not secured. In DAC B1 the XRD shows bcc-Bi plus BiH2 with a heavy-atom sublattice consistent with Im-3m; the authors say the H sublattice is 'probably P21/m' (Section 'X-ray diffraction measurements') and note a third series of reflections that may be C2/m-BiH2. In DAC B3, Fig. S8 explicitly lists P21/m-BiH2, C2/m-BiH2, and C2/c-BiH4, with >95% unreacted bismuth. The C2/m-BiH2 phase has the same (Cmcm-like) Bi sublattice and, according to the paper's own calculations for Cmcm-BiH2, a higher theoretical Tc (~85 K) than P21/m-BiH2 (~59 K). Since XRD cannot locate H atoms, the 61–70 K transition and the 10.5–16 T Hc2 could be dominated by C2/m-BiH2, by BiH4 remnants, or by a grain-boundary/interface channel rather than by P21/m-BiH2. If so, the 'soft molecular superconducting hydride' conclusion has no verified subject. Secondary supporting claims share the same weakness: the small gap is reported as ΔS(0) = 1.5 ± 9.4 meV, so the two-gap interpretation is not statistically supported; and VF = 1.1×10^6 m/s is computed for P21/m-BiH2, not for the actual phase in the current path.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports high-pressure synthesis and transport measurements of bismuth hydride samples made from Bi and ammonia borane in laser-heated diamond anvil cells at pressures of 157–211 GPa. Resistive transitions at 61–70 K are attributed to the low-symmetry P21/m-BiH2 phase, and steady-field (0–16 T) and pulsed-field (0–50 T) measurements yield upper critical fields μ0Hc2(0) = 12–16 T, which the authors identify as unusually low for a hydride superconductor with Tc near 70 K. Using DFT with spin–orbit coupling they compute a high Fermi velocity VF ≈ 1.1×10^6 m/s for P21/m-BiH2 and argue that this high electron velocity, rather than Tc alone, explains the low Hc2 and classifies BiH2 as a soft molecular superconducting hydride with weak vortex pinning. Critical-current density data Jc(T) were measured by a pulsed current technique; a Talantsev–Tallon fit is claimed to show two s-wave gaps, ΔL(0) = 6.9 ± 1.2 meV and ΔS(0) = 1.5 ± 9.4 meV.","tokens_in":20905,"tokens_out":6530,"duration_ms":63444,"significance":"If the phase assignment is correct, this is a valuable counterexample to the usual correlation between high Tc and high Hc2 in hydride superconductors, supporting a soft/hard classification based on Fermi velocity rather than Tc alone. The experiment is technically demanding: pulsed magnetic fields, four-probe van der Pauw transport, and critical-current measurements in laser-heated DACs at megabar pressures. The DFT Fermi velocity is not a fitted parameter, and the low Hc2 is directly measured, so the core 'soft superconductor' classification rests on a testable, non-circular basis. However, the strength of the conclusions depends critically on identifying the phase that actually carries the measured superconducting signal, and that identification is not established from the reported XRD data.","major_comments":[{"comment":"The phase attribution of the superconducting signal is not secured. In DAC B1 the text states that the hydrogen sublattice is 'probably P21/m', and the XRD patterns contain a third series of reflections that 'may correspond' to C2/m-BiH2; in DAC B3, Fig. S8 explicitly lists P21/m-BiH2, C2/m-BiH2, and C2/c-BiH4 alongside more than 95% unreacted bcc-Bi. Since the transport measurements are bulk measurements over the whole sample, the 61–70 K transitions, the 12–16 T Hc2 values, and the Jc(T) data could be dominated by C2/m-BiH2 (whose parent Cmcm structure the authors themselves compute to have Tc ≈ 85 K), by BiH4 remnants, or by grain-boundary/interface channels. This is load-bearing for the central claim that P21/m-BiH2 is a soft superconductor with VF ≈ 1.1×10^6 m/s; please provide phase-specific evidence, for example transport on a region identified as single-phase P21/m-BiH2, or a quantitative multi-phase model showing that the P21/m phase dominates the current path.","section":"X-ray diffraction measurements (main text; Fig. 2; Fig. S8)"},{"comment":"The two-gap s-wave conclusion is not statistically supported. The small gap is reported as ΔS(0) = 1.5 ± 9.4 meV, i.e., consistent with zero within one standard deviation, and the Supporting Information lists Δ(0), λ(0), ΔC/C, and the sample cross-section as refined parameters in the same Talantsev–Tallon fit used to infer the gaps. The claim 'best described only by including a small additional gap' therefore overstates the evidence. Please provide a proper model comparison (e.g., χ², Akaike weights, parameter covariances) or rephrase the conclusion to state that the data are consistent with a single-gap anisotropic s-wave model and that the second gap is not resolved.","section":"Critical current measurements and Supporting Information §4"},{"comment":"The extraction and assignment of the two hydride upper-critical-field values (12 T and 16 T) is not transparent. The Discussion states 'for C2/m and P21/m-BiH2 we obtained 12 and 16 T', but the displayed R(T,H) data in Fig. 3 do not clearly resolve two distinct hydride transitions, and the text does not specify which data sets or transition criteria give which value. Please clarify how the 12 T and 16 T values are separated, which phase each is assigned to, and how cross-contamination from the coexisting phases is excluded.","section":"Electrical transport measurements under strong magnetic fields (Fig. 3a,b; Discussion)"}],"minor_comments":[{"comment":"The abstract gives ΔS(0) ~ 1.5 meV without the uncertainty, while the body reports ΔS(0) = 1.5 ± 9.4 meV; please include the uncertainty or soften the statement in the abstract.","section":"Abstract and Critical current measurements"},{"comment":"The caption says the panels show BiH2 in 'DAC B1 and B2', but the main text states that steady-field measurements were performed on DACs B1 and B3; please correct the inconsistency.","section":"Caption of Fig. 3"},{"comment":"The phrase 'The R-H Measurements under pulsed magnetic fields' should read 'Resistance–magnetic field (R–H) measurements' or similar.","section":"Introduction"},{"comment":"The acronym WHH is used without definition; please expand it at first use as Werthamer–Helfand–Hohenberg.","section":"Electrical transport measurements under strong magnetic fields"},{"comment":"In Eq. (1), the symbols τ, le, and m are not all defined at the point of use; please define the electron mean free path, scattering time, and electron mass explicitly.","section":"Discussion (Eq. 1)"},{"comment":"The sentence 'This results in broader superconducting transitions and typically higher µ0Hc2(0) values for metal hydrides' has an unclear antecedent for 'This'; please rephrase to specify the cause (e.g., strong vortex pinning from impurities/defects).","section":"Discussion"}],"recommendation":"major_revision","confidential_remarks":"The paper reports a technically impressive transport experiment and the soft-superconductor classification is a plausible and testable contribution. However, the phase attribution is the central risk: with multiple hydride phases and >95% unreacted Bi in the samples, the measured transitions and Hc2 values cannot be uniquely assigned to P21/m-BiH2 from the presented XRD data. I would ask the editor to require the authors to either provide phase-specific evidence (e.g., transport on a region with confirmed single-phase P21/m-BiH2) or substantially weaken the phase-specific conclusions. The two-gap claim should be removed or heavily qualified given the reported uncertainty."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague, here's my read.\n\nThe paper is a solid experimental addition to the hydride superconductor pile. What's actually new: transport measurements of BiH2 (whatever its exact structure) that give Tc up to 70 K, μ0Hc2(0)=12–16 T from steady and pulsed fields, critical current density up to 10 kA/mm2, and a DFT Fermi velocity around 1.1×10^6 m/s for P21/m-BiH2 with SOC. The observation that this hydride has a low Hc2/Tc ratio (~0.16 T/K) compared with clathrate hydrides is a useful data point, and the Fermi velocity argument is a plausible explanation. The authors are unusually candid about the synthesis mess: they show >95% unreacted bcc-Bi in the XRD, list P21/m-BiH2, C2/m-BiH2, and C2/c-BiH4 in the sample, and admit the H sublattice cannot be resolved by XRD ('probably P21/m').\n\nThat candor is also where the trouble sits. The central claim — that this is P21/m-BiH2 with Tc 70 K and soft Hc2 — is not secured. The XRD cannot distinguish the hydrogen ordering, and the authors' own calculations give Cmcm-BiH2 (which has the same Bi sublattice as C2/m) a higher Tc (85 K) than P21/m (~59 K). So the measured transition could be from C2/m-BiH2 or even a remnant BiH4 channel. The 'soft superconductor' classification may survive, but the specific phase attribution is load-bearing and it is weak.\n\nThe two-gap claim is worse. The small gap is reported as ΔS(0)=1.5±9.4 meV — the uncertainty swallows the value. And the gaps are fitted parameters in a Talantsev-Tallon model applied to the same Jc(T) data they are used to explain; that is fitting, not evidence. The single-gap or even d-wave fits are shown, and the two-gap model wins, but with that uncertainty in ΔS the discrimination is not meaningful.\n\nThe DFT part is better: CIFs are given, the Fermi surface table is computed with IFermi and SOC, and the Fermi velocity is a parameter-free output (modulo pseudopotential choices). That part is reproducible and is a fair contribution.\n\nNet: this is an incremental but potentially useful data point for hydride physics, if the phase question is fixed. For peer review, I would send it out — the measurements are hard and worth recording — but with a request for major revision: either get phase-specific evidence (micro-XRD or transport on a region with known composition) or tone the title and abstract down to 'bismuth dihydride' without the P21/m assignment, and drop or heavily caveat the two-gap statement.","headline":"New low-Hc2 hydride transport data worth knowing, but the P21/m-BiH2 phase assignment is not secured and the two-gap evidence collapses under its own uncertainty.","tokens_in":21518,"tokens_out":3129,"would_cite":false,"duration_ms":31476,"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 claims that covalent bismuth dihydride BiH$_2$ superconducts up to 70 K yet stays 'soft': an unusually high Fermi velocity of about $1.1 \\times 10^6$ m/s keeps its upper critical field at only 12–16 T.","keywords":["bismuth dihydride","hydride superconductors","upper critical field","Fermi velocity","soft superconductor","high pressure synthesis","two-gap superconductivity","WHH theory"],"falsifier":"A decisive test would be a single-phase, hydrogen-sublattice-resolved sample: neutron or very-high-resolution synchrotron diffraction of deuterated BiD$_2$ that identifies the sublattice as $P2_1/m$, together with a single sharp resistive drop at 61–70 K and $\\mu_0 H_{c2}(0) \\approx 12$–16 T. The phase assignment would fail if the 70 K step survives with an upper critical field above 30 T once BiH$_4$ and $C2/m$-BiH$_2$ contributions are excluded, or if specific-heat or tunneling measurements find no gap near the predicted $\\Delta_L(0) = 6.9$ meV.","tokens_in":20234,"feed_emoji":"🧲","tokens_out":17481,"duration_ms":128259,"temperature":0.7,"pith_summary":"Under pressures of 157–211 GPa, bismuth reacts with hydrogen to form the covalent dihydride BiH$_2$, whose superconducting transition reaches 70 K — the highest $T_c$ of any MH$_2$-type hydride apart from H$_2$S. The paper's central claim is that this superconductor is 'soft': its upper critical field $\\mu_0 H_{c2}(0) = 12$–16 T is several times smaller than that of clathrate-like metal hydrides (LaH$_{10}$, CaH$_6$) with comparable or even lower transition temperatures. The explanation offered is an unexpectedly high Fermi velocity, $v_F \\approx 1.1 \\times 10^6$ m/s at 155 GPa, three to five times the value typical of hydrogen-dominated hydrides, which suppresses the critical field through the WHH/Maki relation even though $T_c$ is high. If correct, this decouples the 'softness' of a hydride superconductor from its transition temperature and makes BiH$_2$ a test case for how the electronic structure of the heavy-element sublattice governs the response to magnetic fields. Pulsed critical-current measurements additionally indicate two $s$-wave superconducting gaps, with the large gap $\\Delta_L(0) = 6.9 \\pm 1.2$ meV.","feed_headline":"70 K superconductor that dies at only 16 tesla","feed_subtitle":"High Fermi velocity, not high Tc, sets the critical field — redrawing how hydride superconductors are classified.","key_machinery":"The load-bearing mechanism is the Fermi velocity $v_F \\approx 1.1 \\times 10^6$ m/s entering the Maki parameter $\\alpha = 3\\hbar/(2 m l_e v_F)$ of WHH theory: with bismuth $p$ orbitals dominating the density of states at the Fermi level instead of hydrogen $s$ orbitals, $v_F$ is three to five times larger than the $2.5$–$3.8 \\times 10^5$ m/s baseline of hydrogen-dominated polyhydrides, and the larger velocity suppresses $-\\mathrm{d}H_{c2}/\\mathrm{d}T$ at $T_c$ and hence $H_{c2}(0)$. A second mechanism carries the gap analysis: the self-field critical-current model converts the measured $J_c(T)$ curve into a penetration depth and fits it against single-gap, two-gap $s$-wave, and $d$-wave forms, yielding the two-gap result $\\Delta_L(0) = 6.9 \\pm 1.2$ meV and $\\Delta_S(0) \\approx 1.5$ meV. The structural object is the low-symmetry $P2_1/m$ phase whose hydrogen sublattice is an ordered array of H$_2$ molecules; the calculated $T_c$ of this phase ($\\approx 59$ K at 150 GPa) brackets the measured 61–70 K, whereas dynamically unstable chain-like Cmcm-BiH$_2$ would give $\\approx 85$ K and disordered molecular variants only $\\approx 34$ K.","core_discovery":"The paper claims that the low-symmetry molecular phase $P2_1/m$-BiH$_2$, synthesized by laser-heating bismuth with ammonia borane at 157–211 GPa, superconducts with a maximum $T_c$ of 70 K around 159 GPa. Transport measurements in steady fields up to 16 T and pulsed fields up to 50 T give $\\mu_0 H_{c2}(0) = 12$–16 T for the BiH$_2$-bearing samples, with 10.5 T measured directly at 2 K in the pulsed experiment, corresponding to $\\mu_0 H_{c2}/T_c \\approx 0.16$ T/K. Using the WHH framework, the authors trace this small critical field to the Maki parameter: because the Fermi level in BiH$_2$ is dominated by bismuth $p$ orbitals rather than hydrogen states, the Fermi velocity reaches $v_F \\approx 1.1 \\times 10^6$ m/s at 155 GPa, three to five times the value in most polyhydrides, and a high $v_F$ drives down both the slope $\\mathrm{d}H_{c2}/\\mathrm{d}T$ at $T_c$ and the zero-temperature critical field. On this basis the paper classifies BiH$_2$ as a 'soft' molecular superconducting hydride with relatively weak vortex pinning, in contrast to hard clathrate hydrides of similar $T_c$ such as Lu$_4$H$_{23}$ or CeH$_9$. Pulsed-mode voltage–current characteristics add a critical current density $J_c(0) \\approx 10$ kA/mm$^2$ whose temperature dependence is best described by a two-gap $s$-wave model with $\\Delta_L(0) = 6.9 \\pm 1.2$ meV and $\\Delta_S(0) \\approx 1.5$ meV.","pith_inferences":["Editorial inference: applied to the whole p-block family (SnH$_4$, SbH$_4$, P–H systems), the same Fermi-velocity argument predicts persistently low $H_{c2}/T_c$ ratios, so the soft-versus-hard split offers a cheap screening rule — compute $v_F$, not just $T_c$, when ranking candidate megabar superconductors.","Editorial inference: the Fermi surface of $P2_1/m$-BiH$_2$ is dominated by quasi-two-dimensional sheets with velocities near $1.0$–$1.4 \\times 10^6$ m/s, so at low fields and high current densities the sample may exhibit two-dimensional fluctuation or depairing physics that this paper does not address.","Editorial inference: because bismuth states, not hydrogen, dominate the Fermi level, pairing should depend mainly on the heavy-atom sublattice; a deuteration experiment (BiD$_2$) offers a sharp test, since a small isotope shift in $T_c$ or $H_{c2}$ would confirm the marginal role of hydrogen phonons."],"forward_implications":["BiH$_2$ becomes the softest high-$T_c$ hydride known: with $T_c \\approx 70$ K but $\\mu_0 H_{c2}(0) = 12$–16 T, moderate laboratory fields fully suppress its superconductivity, and the ratio $\\mu_0 H_{c2}/T_c \\approx 0.16$ T/K sits far below the 0.5–1.2 T/K of clathrate hydrides such as LaH$_{10}$ and CaH$_6$.","Fermi velocity, not $T_c$ alone, sets the critical field: p-element covalent hydrides with high $v_F$ should systematically behave as soft superconductors, giving a concrete electronic-structure criterion for predicting $H_{c2}$ in future hydride searches.","BiH$_2$ is a multiband superconductor with two $s$-wave gaps, $\\Delta_L(0) = 6.9 \\pm 1.2$ meV and $\\Delta_S(0) \\approx 1.5$ meV, a structure that specific-heat, tunneling, and lower-critical-field experiments could independently verify.","The ordering of H$_2$ molecules in the sublattice is a strong $T_c$ lever — from about 85 K for unstable chain-like Cmcm down to about 34 K for disordered molecular variants — so the observed $T_c$ reads out the degree of hydrogen-sublattice order, not just stoichiometry.","Weak vortex pinning means currents near 0.14–0.16 A quench superconductivity in BiH$_2$, a practical constraint on any future transport measurement in molecular hydride samples."],"supporting_citations":[{"why":"Supplies the predicted $P2_1/m$-BiH$_2$ crystal structure with a $T_c$ of 59 K at 150 GPa, the phase the experiment claims to realize and whose predicted $T_c$ brackets the measured 61–70 K.","marker":"(25)"},{"why":"Supplies the WHH theory and the Maki parameter through which the high Fermi velocity is converted into a low extrapolated $\\mu_0 H_{c2}(0)$.","marker":"(44)"},{"why":"Establishes the 2.5–$3.8 \\times 10^5$ m/s Fermi-velocity baseline for hydride superconductors against which BiH$_2$'s higher $v_F$ is measured.","marker":"(45)"},{"why":"Gives the self-field critical-current model that converts $J_c(T)$ into penetration depth and gap parameters, the basis of the two-gap fit.","marker":"(40)"},{"why":"Supplies the universal self-field critical-current formula for thin-film superconductors used in the $J_c(T)$ analysis.","marker":"(41)"},{"why":"Reports the coexisting molecular hydride BiH$_4$ with $T_c$ near 91 K at 170 GPa, the main competing phase in the sample mixture and a comparison point for the BiH$_2$ assignment.","marker":"(19)"},{"why":"Provides the hard-clathrate benchmark LaH$_{10}$ with $T_c$ = 250 K and $\\mu_0 H_{c2}/T_c$ = 0.54 T/K against which BiH$_2$'s 0.16 T/K softness is contrasted.","marker":"(2)"},{"why":"Reports H$_3$S and H$_2$S superconductivity, the reference soft covalent hydrides that frame 70 K as the highest MH$_2$-type $T_c$ after H$_2$S.","marker":"(15)"}],"fun_headline_variants":["BiH2's 70 K superconductivity is soft: critical field only 12–16 T","High Fermi velocity explains the weak critical field in 70 K BiH2","Covalent BiH2: a 70 K superconductor that can't handle 16 T","Soft hydride: fast electrons cap BiH2's upper critical field at 16 T"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The measured 61–70 K resistive transition and its 12–16 T upper critical field are assigned to the low-symmetry $P2_1/m$-BiH$_2$ phase, even though X-ray diffraction shows the samples contain more than 95% unreacted bismuth together with a mixture of $P2_1/m$-BiH$_2$, $C2/m$-BiH$_2$, and BiH$_4$, and the hydrogen sublattice itself ('probably $P2_1/m$') cannot be resolved by XRD.","fun_headline_variants_meta":{"raw":{"variants":["BiH2's 70 K superconductivity is soft: critical field only 12–16 T","High Fermi velocity explains the weak critical field in 70 K BiH2","Covalent BiH2: a 70 K superconductor that can't handle 16 T","Soft hydride: fast electrons cap BiH2's upper critical field at 16 T"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000284,"raw_usage":{"total_tokens":1853,"prompt_tokens":1304,"completion_tokens":549,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":920,"completion_tokens_details":{"reasoning_tokens":453}},"tokens_in":920,"tokens_out":549,"duration_ms":5268,"temperature":1.0,"reasoning_tokens":453,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-15T20:41:53.690552+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A decisive test would be a single-phase, hydrogen-sublattice-resolved sample: neutron or very-high-resolution synchrotron diffraction of deuterated BiD$_2$ that identifies the sublattice as $P2_1/m$, together with a single sharp resistive drop at 61–70 K and $\\mu_0 H_{c2}(0) \\approx 12$–16 T. The phase assignment would fail if the 70 K step survives with an upper critical field above 30 T once BiH$_4$ and $C2/m$-BiH$_2$ contributions are excluded, or if specific-heat or tunneling measurements find no gap near the predicted $\\Delta_L(0) = 6.9$ meV.","supporting_citations":[],"review_version":1}