{"id":"64fa73d3-e92b-4269-b7be-cfd4a40ced9c","arxiv_id":"2412.16169","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"Two new sub-Kelvin magnetic phase transitions are observed in optimally doped EuFe2(As0.79P0.21)2, and one in overdoped EuFe2(As0.71P0.29)2, from specific heat and magnetization data.","lead":"Heat capacity measurements on two phosphorus-doped EuFe2As2 crystals reveal new magnetic transitions at temperatures below 1.2 K, inside the superconducting state. The findings suggest that europium spin order or spin reorientation can give rise to multiple low-temperature magnetic phases coexisting with superconductivity.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Sub-Kelvin anomalies could be extrinsic or inhomogeneity-related; internal mismatch between C/T and M transition temperatures (0.2 K lag) and uncharacterized overdoped crystal quality leave the 'intrinsic magnetic orders' claim unproven.","rationale":"The reader's weakest assumption captures the central risk: the low-temperature specific heat and magnetization anomalies are interpreted as intrinsic bulk magnetic transitions of the P-doped EuFe2As2 phase. The stress-test confirms this is the load-bearing point. The paper has genuine strengths: reproducible anomalies in two dopings, systematic field and orientation dependence, matching (though offset) magnetization features, and entropy estimates consistent with Eu2+ spin degrees of freedom. However, the anomalies are small, the overdoped crystal is explicitly of imperfect quality with element vacancies, and the paper itself leaves the impurity/inhomogeneity scenario open. The internal 0.2 K mismatch between heat capacity and magnetization transition temperatures under the same field is an additional red flag that a simple thermodynamic transition would not produce; it makes the extrinsic or inhomogeneous interpretation more plausible, not less. A zero-field local probe such as muSR would directly measure the ordered volume fraction and settle whether the anomalies are bulk magnetic order or minority-phase artifacts. Since this is exactly the condition the reader attached, the verdict should remain CONDITIONAL; no change is needed.","tokens_in":23408,"tokens_out":5363,"duration_ms":62303,"concrete_test":"Run zero-field muon spin rotation (muSR) on the same x = 0.21 batch between 2 K and 0.25 K. If T1 and T2 are bulk magnetic transitions, coherent muon precession with amplitude proportional to the ordered volume fraction should appear at each transition; an impurity phase gives either no coherent precession or a small amplitude matching the impurity fraction. In addition, remeasure C/T and magnetization on a second crystal from a different growth batch and compare anomaly amplitude with EDX element maps and powder XRD impurity peaks; if the anomaly scales with the amount of a minority phase, the extrinsic scenario is confirmed.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The most load-bearing assumption is that the sub-Kelvin C/T jumps (T1, T2, T3) are sharp thermodynamic transitions of the EuFe2(As1-xPx)2 main phase. This is not yet secured. The anomalies are small relative to the background, no error bars or baseline-fit details are given, and the overdoped crystal is non-superconducting, has element vacancies, and the authors explicitly leave sample inhomogeneity as an open scenario (final section). Moreover, under 0.3 T the magnetization features in the optimally doped crystal occur about 0.2 K below the heat capacity features (Tm1* ~ 0.6 K vs T1 ~ 0.81 K; Tm2* ~ 0.43 K vs T2 ~ 0.65 K), a discrepancy the paper notes but does not explain. A true bulk phase transition should appear at the same temperature in C(T) and M(T); a systematic 0.2 K lag suggests either different thermometry, broadened/inhomogeneous transitions, or different microscopic origins of the two signals. If the C/T jumps arise from a minority impurity phase or from P-poor regions, the counting of 'two new orders' and the doping-evolution claim collapse. Local, element-specific probes are therefore required before the discovery claim can be accepted.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports heat capacity, resistivity, and magnetization measurements on two single crystals of EuFe2(As1-xPx)2 with x = 0.21 and x = 0.29. Two anomalies in C/T at T1 ~ 0.9 K and T2 ~ 0.6 K are observed in the superconducting optimally doped crystal, and one anomaly at T3 ~ 0.9 K in the overdoped non-superconducting crystal. The anomalies move systematically with applied field and show angular dependence, leading the authors to propose new low-temperature magnetic orders from Eu2+ spins or spin reorientation and to construct a B-T phase diagram.","tokens_in":23617,"tokens_out":4293,"duration_ms":40223,"significance":"If the anomalies are intrinsic bulk transitions, this is a potentially valuable finding for the coexistence of superconductivity and magnetism in iron pnictides. The paper's strengths are the systematic field- and orientation-dependent specific heat measurements, the reproducible jumps at distinct temperatures that evolve with field, and the entropy analysis reaching ~83-95% of Rln8 for the Eu2+ moment at TN. However, the intrinsic origin of the sub-Kelvin anomalies is not yet established, and the systematic C(T)/M(T) temperature mismatch needs explanation before the discovery claim can be accepted.","major_comments":[{"comment":"The assignment of the sub-Kelvin heat capacity anomalies to intrinsic magnetic orders of the main EuFe2(As1-xPx)2 phase is not secured. In the final section the authors explicitly leave 'some unconsidered situations' and the impurity scenario open, and the overdoped crystal is described as having 'a small amount of elements vacancies' and being non-superconducting (p. 2, Fig. 1(d)). Because the anomalies shown in Figs. 2(c)-(g) are small relative to the background, they could in principle arise from a minority impurity phase or from P-poor regions in the crystal. The authors should provide evidence that the anomalies scale with the main-phase volume fraction (e.g., measurements on a second batch, a different piece, or deliberately co-synthesized impurity-phase candidates) and ideally element-specific or local probes (muSR, neutron scattering) to confirm magnetic order.","section":"Final section and Fig. 1(d)"},{"comment":"The systematic mismatch between the heat capacity transition temperatures and the magnetization anomalies is acknowledged but not resolved. For the optimally doped crystal under 0.3 T, the heat capacity gives T1 ~ 0.81 K and T2 ~ 0.65 K, while the magnetization shows Tm1* ~ 0.6 K and Tm2* ~ 0.43 K (inset of Fig. 2(a)); under 1 T, T2 ~ 0.76 K versus Tm2 ~ 0.66 K; for the overdoped crystal under 0.3 T, T3 ~ 0.84 K versus Tm3 ~ 0.72 K. A true bulk thermodynamic transition should appear at the same temperature in C(T) and M(T) measured on the same sample. The systematic lag of ~0.1-0.2 K must be addressed before the heat capacity jumps can be identified with magnetic ordering; it suggests thermometry offsets, broadened/inhomogeneous transitions, or distinct physical origins of the two signals.","section":"Comparison of C(T) and M(T), Fig. 2 and Fig. S6"},{"comment":"The determination of the transition temperatures in Fig. 4 by 'intersection of two extrapolated red lines' is not reproducible without error bars or a quantitative criterion. The anomalies are small and sit on a rapidly varying background (including the nuclear Schottky contribution at low T and the phonon background at higher T), and no standard deviation from repeated measurements or from different crystals is given. Please provide the raw analysis method (e.g., equal-area construction on C/T after subtracting a smooth background), state the uncertainty in each T1/T2/T3 point, and include the same for the B* values used in the phase diagram.","section":"Fig. 4 and Fig. 5"},{"comment":"The doping-evolution conclusion that 'one of the magnetic orders becomes absent' in the overdoped crystal is weakened by the imperfect nature of the overdoped reference: it is non-superconducting, has element vacancies, and is the only representative of x = 0.29. The absence of T1 in this single sample could be a consequence of sample quality rather than the P content. Measuring at least one additional composition (e.g., x = 0.25 or a second x = 0.29 crystal of better quality) would substantially strengthen the claim.","section":"Doping evolution claim, x = 0.29 sample"}],"minor_comments":[{"comment":"Reference [26] is cited as 'self-flux method [26 ? ]' with a placeholder question mark; supply the complete reference.","section":"Reference [26]"},{"comment":"The phrase 'see the inset of Figs. 1(a) and 1(b)' is incorrect; the Faraday magnetization data are shown in the insets of Figs. 2(a) and 2(b).","section":"Text after Fig. 2"},{"comment":"The SI caption of Fig. S1 uses overlapping labels: '(a)–(e)' for the first sample and '(e)–(j)' for the second; relabel the second set to avoid ambiguity.","section":"Fig. S1 caption"},{"comment":"There are several typos: 'capacitiy' in the Fig. S3 caption, 'anomalies heat capacity' in the main text, and 'The inset show' in the Fig. 1(b) caption.","section":"Typos"},{"comment":"In Fig. S6 the normalization M/M(1.3 K) is used, but data extend only to 0.2 K; state the reference temperature and field consistently in the caption and text.","section":"Fig. S6"}],"recommendation":"major_revision","confidential_remarks":"The manuscript contains extensive data and supplementary material, but the principal claim of new magnetic orders needs the additional evidence outlined in the major comments. The paper appears within the scope of the journal; the self-citations are numerous but relevant."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Xu — quick take on 2412.16169. The paper reports reproducible C/T jumps at 0.9 K and 0.6 K in optimally doped EuFe2(As0.79P0.21)2 and one at 0.9 K in overdoped x = 0.29, with systematic field shifts and matching magnetization anomalies. That is a genuinely new measurement on a well-studied family, and the authors deserve credit for the field- and angle-resolved heat-capacity work and for keeping the interpretation cautious.\n\nWhat's new is the sub-Kelvin phase diagram. Nothing in their citation list suggests these transitions were seen before. The experiments look carefully done: two doping levels help, and the field-dependent evolution of the anomalies is consistent with magnetic ordering rather than, say, a superconducting artifact. The entropy analysis is a reasonable check, not a derivation, and the transition temperatures don't depend on the Debye-Einstein subtraction.\n\nSoft spots, in order of weight. First, the overdoped crystal is admitted to have element vacancies and is non-superconducting; the paper explicitly leaves sample inhomogeneity open as a scenario. That weakens the doping-evolution claim. Second, no error bars or baseline details are given for the small C/T jumps, so I can't judge how sharp the transitions are. Third, under 0.3 T the magnetization features sit about 0.2 K below the heat-capacity features in the optimally doped crystal; the authors note it but don't explain it. A true bulk transition should appear at the same temperature in both. That discrepancy is not fatal—it could be thermometry or broadened transitions—but it's exactly what a referee should push on. Fourth, there is no microscopic probe. Muon spin rotation or neutron scattering is the obvious next step, and without it 'magnetic order' remains an inference from thermodynamics, not a determination.\n\nThe circularity burden is low; the anomaly positions are purely experimental. Self-citations are to prior work on the same material, which is appropriate.\n\nNet: the observation is probably real and worth publishing, but the 'multiple magnetic orders' wording is stronger than the evidence. I'd send it to a serious referee with the expectation of major revision: error analysis, a better overdoped sample, and ideally μSR. The paper is for the iron-pnictide and Eu-intermetallic community; it doesn't reorganize the field, but it adds a new low-temperature corner to a heavily studied phase diagram.","headline":"Sub-Kelvin heat-capacity anomalies in P-doped EuFe2As2 look real and reproducible, but the 'magnetic orders' framing outruns the evidence until the overdoped crystal is cleaned up and a microscopic probe lands.","tokens_in":24287,"tokens_out":1719,"would_cite":true,"duration_ms":19306,"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 two previously unknown magnetic transitions occur inside the superconducting state of optimally doped EuFe2(As0.79P0.21)2, at about 0.9 K and 0.6 K, with one corresponding transition at about 0.9 K in the overdoped…","keywords":["iron pnictide","EuFe2As2","phosphorus doping","superconducting state","magnetic phase transition","specific heat","low-temperature magnetism","field-angle-resolved heat capacity"],"falsifier":"Neutron diffraction on a large single crystal of EuFe2(As0.79P0.21)2 in zero field below 0.4 K would settle it: observation of new magnetic Bragg peaks entering at $T_1$ and $T_2$ confirms the bulk magnetic-order claim, while their absence in a crystal that still shows both heat-capacity jumps rules it out.","tokens_in":23170,"feed_emoji":"🧲","tokens_out":17422,"duration_ms":120059,"temperature":0.7,"pith_summary":"This paper is trying to establish that the iron-pnictide superconductor EuFe2(As0.79P0.21)2 hosts two new magnetic phase transitions at $T_1\\simeq 0.9\\,$K and $T_2\\simeq 0.6\\,$K, deep inside its superconducting state, and that only one of them survives in the overdoped, non-superconducting crystal EuFe2(As0.71P0.29)2. The evidence is thermodynamic: sharp specific-heat jumps confirmed by low-temperature magnetization anomalies measured down to 0.2 K. The authors propose that these sub-kelvin orders come from the localized Eu2+ moments, either a new spin order or a reorientation of the already ordered europium spins. If true, this gives a previously unseen setting to study how magnetism and superconductivity coexist, because the new magnetic order is not destroying superconductivity but living inside it.","feed_headline":"Two new magnetic orders found inside a superconductor","feed_subtitle":"Specific heat and magnetization place them at 0.9 K and 0.6 K in optimally doped EuFe2(As,P)2, inside the superconducting state.","key_machinery":"The central probe is the heat capacity $C(T)/T$ measured down to 0.4 K in a rotating $^3$He refrigerator, which reveals sharp jumps at $T_1$, $T_2$, and $T_3$ and lets the authors track how these transitions move in a magnetic field. The supporting measurements are the Capacitance-Faraday magnetization, which confirms that the specific-heat anomalies are accompanied by changes in the dc magnetization, and angle-resolved specific heat, whose evolving twofold-to-fourfold symmetry provides a fingerprint of the underlying spin order. The paper's phase diagram assembles these characteristic temperatures and fields, and its entropy analysis—$S_{\\rm mag}$ reaching roughly 83%–95% of $R\\ln 8$ at the 18.5 K Eu transition—is used to argue that the low-temperature jumps release too little entropy to be a full ordering of the $S = 7/2$ Eu moments.","core_discovery":"On the authors' own terms, the core discovery is that the specific heat of optimally doped EuFe2(As0.79P0.21)2 shows two distinct $\\lambda$-anomalies at $T_1 \\simeq 0.9\\,\\text{K}$ and $T_2 \\simeq 0.6\\,\\text{K}$, below the superconducting transition, and that these anomalies are magnetic in origin: they have counterparts in dc magnetization measured by a Capacitance-Faraday method, they evolve strongly with applied field and orientation, and the magnetic entropy released at the low-temperature transitions is tiny, consistent with a spin reorientation rather than a full melting of the $S = 7/2$ Eu moments. In the overdoped, non-superconducting EuFe2(As0.71P0.29)2 crystal only one such transition survives, at $T_3 \\simeq 0.9\\,\\text{K}$, and its field behavior matches $T_2$, suggesting a common microscopic origin. The paper argues that these new phases point to a localized Eu$^{2+}$ spin order or a spin reorientation.","pith_inferences":["A testable extension would be to measure $C(\\phi)$ in a detwinned single crystal at fixed field while sweeping through $T_1$ and $T_2$; if the twofold-to-fourfold evolution of the heat capacity is tied to the magnetic order rather than to the superconducting gap, the angular periodicity should follow the magnetic field direction, not the crystal axes.","The paper's reservations about impurity phases suggest a control experiment: growing overdoped crystals with different fluxes and lower vacancy concentrations, then checking whether the $T_3$ anomaly persists; if it disappears with improved stoichiometry, the vacancy scenario would win.","Because the proposed origin is a reorientation of the existing Eu$^{2+}$ order, a testable consequence is that the sub-kelvin transitions should shift or split in a magnetic field applied along different crystallographic directions; such a measurement would distinguish a simple spin flop from a more complex multi-axis reorientation."],"forward_implications":["Below $T_c$, the superconducting state of optimally doped EuFe2(As,P)2 contains two further ordered phases, so transport, penetration-depth, and vortex-matter measurements below 1 K should show signatures of these orders.","The disappearance of one transition when going from $x = 0.21$ to $x = 0.29$ ties the new order to the same doping axis that suppresses superconductivity, giving a control parameter to separate the two transitions.","The strong field- and orientation-dependence of the heat capacity implies an anisotropic magnetic response of the low-temperature phase, so torque magnetometry and angular-resolved neutron scattering could map its symmetry directly.","The small entropy released at the jumps suggests a spin reorientation rather than a new long-range moment, in which case the transition is a rotation of the Eu$^{2+}$ moment direction that should appear as a change of the magnetic easy axis."],"supporting_citations":[{"why":"Establishes the coexistence of superconductivity and strong ferromagnetic Eu ordering in the EuFe2As2 family, the context that makes new magnetic orders inside the SC state significant.","marker":"[13]"},{"why":"Shows that P substitution suppresses the Fe spin-density wave and induces superconductivity, placing the crystals on the doping axis.","marker":"[24]"},{"why":"Reports the overdoped non-superconducting regime and its sample-quality problems, which the paper must distinguish from its new anomalies.","marker":"[25]"},{"why":"Supplies the self-flux crystal-growth method used to synthesize the two P-doped single crystals.","marker":"[26]"},{"why":"Provides the rotating 3He refrigerator and angle-resolved specific-heat technique with 0.01-degree resolution that underpin the field-orientation data.","marker":"[27–29]"},{"why":"Documents the doping-dependent Eu2+ magnetic order in EuFe2(As,P)2, the localized spin system the authors propose as the origin.","marker":"[14]"},{"why":"Gives the EuPd2As2 analogue with two Eu2+ magnetic transitions and low-temperature entropy behavior that supports the spin-reorientation interpretation.","marker":"[38]"},{"why":"Provides c-axis lattice parameters used as a composition check for the actual P content of the crystals.","marker":"[30]"}],"fun_headline_variants":["Two magnetic orders at 0.9K and 0.6K inside a superconductor","Sub-kelvin magnetic orders discovered in superconducting EuFe2(As,P)2","Magnetic transitions found below superconducting Tc in EuFe2(As,P)2","Two new magnetic orders emerge inside superconductor at sub-kelvin"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The sub-kelvin specific-heat jumps are intrinsic phase transitions of the doped EuFe2As2 crystals, not artifacts of impurity phases, element vacancies, or sample inhomogeneity.","fun_headline_variants_meta":{"raw":{"variants":["Two magnetic orders at 0.9K and 0.6K inside a superconductor","Sub-kelvin magnetic orders discovered in superconducting EuFe2(As,P)2","Magnetic transitions found below superconducting Tc in EuFe2(As,P)2","Two new magnetic orders emerge inside superconductor at sub-kelvin"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000792,"raw_usage":{"total_tokens":3552,"prompt_tokens":1071,"completion_tokens":2481,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":687,"completion_tokens_details":{"reasoning_tokens":2391}},"tokens_in":687,"tokens_out":2481,"duration_ms":119607,"temperature":1.0,"reasoning_tokens":2391,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-11T20:57:19.013134+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Neutron diffraction on a large single crystal of EuFe2(As0.79P0.21)2 in zero field below 0.4 K would settle it: observation of new magnetic Bragg peaks entering at $T_1$ and $T_2$ confirms the bulk magnetic-order claim, while their absence in a crystal that still shows both heat-capacity jumps rules it out.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Establishes the coexistence of superconductivity and strong ferromagnetic Eu ordering in the EuFe2As2 family, the context that makes new magnetic orders inside the SC state significant."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Shows that P substitution suppresses the Fe spin-density wave and induces superconductivity, placing the crystals on the doping axis."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Reports the overdoped non-superconducting regime and its sample-quality problems, which the paper must distinguish from its new anomalies."},{"cited_title":"Veshchunov, L","cited_arxiv_id":null,"evidence_quote":"Supplies the self-flux crystal-growth method used to synthesize the two P-doped single crystals."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Documents the doping-dependent Eu2+ magnetic order in EuFe2(As,P)2, the localized spin system the authors propose as the origin."},{"cited_title":"Zhang, H","cited_arxiv_id":null,"evidence_quote":"Gives the EuPd2As2 analogue with two Eu2+ magnetic transitions and low-temperature entropy behavior that supports the spin-reorientation interpretation."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides c-axis lattice parameters used as a composition check for the actual P content of the crystals."}],"review_version":1}