REVIEW 4 major objections 5 minor 48 references
Photoemission Spectroscopic Evidence for the Dirac Nodal Line in Monoclinic Semimetal SrAs$_3$
T0 review · 4 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read This paper reports the first complete spectroscopic trace of the predicted Dirac nodal loop in SrAs3.
desk verdict Direct ARPES evidence for a nodal loop in SrAs3 is plausible and worth publishing, but the paper overstates how unambiguously it maps the loop. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the Dirac nodal loop itself: a one-dimensional locus in momentum space where conduction and valence bands cross, protected here by the combination of spatial-inversion and time-reversal symmetries in the $C2/m$ structure. The experimental machinery is photon-energy-dependent ARPES, whose varying photon energy moves the measurement cut through the Brillouin zone along $k_y$; a free-electron final-state model with an inner potential of $19.3\ \mathrm{eV}$ assigns each photon energy a $k_z$, and the measured $Y$-$M$ dispersions supply the crossing positions. On the theory side, density-functional calculations with a modified exchange-correlation potential and parity and orbital analyses establish the band inversion around $Y$, and a tight-binding model built from localized orbitals yields the calculated loop and surface states.
What would settle it
Measure the same $Y$-$M$ dispersions with an independent $k_z$ assignment, for example by scanning photon energies over more than one Brillouin zone, and check whether the crossings still form one closed loop of the calculated size; alternatively, look for a symmetry-breaking gap at the crossings with energy resolution better than a few millielectronvolts. If the loop fails to close continuously or the crossings split, the nodal-line identification collapses.
Extended reading notes
Core claim
On the paper's own terms, two bands of opposite parity invert near the Fermi energy only around the $Y$ point, and because the material preserves spatial-inversion and time-reversal symmetries, that inversion produces a single closed nodal loop on the $\Gamma$-$Y$-$S$ mirror plane. The loop is elliptical, about $0.2\ \AA^{-1}$ along $k_y$ and $0.15\ \AA^{-1}$ along $k_x$, with no resolvable spin-orbit splitting of the crossings. Photon-energy-dependent ARPES cuts through the loop show the crossing region shrinking and disappearing as the cut leaves $Y$, and the extracted node positions coincide with the calculated loop. The authors therefore state that they have tracked the topological non-trivial nodal line and demonstrated Dirac nodal-line fermions in SrAs$_3$.
Load-bearing premise
The load-bearing premise is that the photon-energy-to-momentum conversion used to locate the crossings, based on a simple model with one fitted inner potential, is reliable, and that the calculated band ordering near the Y point is correct; if either is wrong, the tracked crossings need not be the symmetry-protected topological nodes.
Editorial extensions
If this is right
- SrAs$_3$ becomes the first CaP$_3$-family material with a spectroscopically tracked nodal loop, moving these predictions from calculation to measured band structure.
- Because the loop sits near the Fermi energy with no interfering trivial states, transport and quantum-oscillation experiments on this compound can be interpreted against a single clean nodal-loop band structure.
- The measured loop dimensions, $0.2 \times 0.15\ \AA^{-1}$, give a quantitative benchmark that future calculations of the CaP$_3$ family should reproduce.
- The absence of a resolvable spin-orbit gap at the crossings supports the claim that spin-orbit coupling is negligible for the bulk nodes, so the Dirac description of the low-energy fermions is appropriate.
- The drumhead surface state should be visible on projections along the $k_c$ direction but hidden on the cleavage-plane projection, indicating where future surface-sensitive experiments should look.
Reading between the lines
- Editorial inference: quantum oscillation measurements on these crystals should show a nontrivial $\pi$ Berry phase for orbits that enclose the loop, a testable consequence not reported in this paper.
- Editorial inference: because the loop is protected by inversion and time-reversal symmetries, breaking either symmetry with strain or magnetic doping should open a gap along the loop, allowing its size and existence to be tuned.
- Editorial inference: the note added about a conflicting ARPES report on SrAs$_3$ suggests sample-growth details may determine whether the band inversion survives; a systematic comparison of differently grown crystals would separate material-specific effects from the intrinsic topology.
- Editorial inference: if the spin-orbit gap is as small as calculated, increasing the effective spin-orbit coupling could drive SrAs$_3$ toward the predicted strong-topological-insulator phase, making it a candidate for a tunable topological transition.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript reports angle-resolved photoemission spectroscopy (ARPES) measurements and first-principles calculations on the monoclinic semimetal SrAs3, claiming direct spectroscopic evidence for a single Dirac nodal loop on the Γ–Y–S mirror plane near the Fermi energy. The authors characterize high-quality single crystals, compare measured band dispersions along Y–M and Y–N with density functional theory (GGA+MBJ), track the band crossing as a function of photon energy, and extract a loop with height 0.2 Å⁻¹ and width 0.15 Å⁻¹. The paper argues that the nodal loop is protected by inversion and time-reversal symmetries, that spin-orbit coupling effects are negligible, and that SrAs3 provides a clean platform for studying nodal-line fermions. A note added acknowledges a conflicting ARPES report on the same material.
Significance. If the claims hold, this would be the first direct spectroscopic confirmation of a Dirac nodal loop in the CaP3 family and would establish SrAs3 as a relatively clean nodal-line semimetal with simple Fermi surface topology. The paper has notable strengths: the DFT calculations are independent of the ARPES data and provide concrete predictions; the sample characterization (XRD, Laue, EDS) is careful; and the measured Y–M versus Y–N contrast is a meaningful, symmetry-specific test. The drumhead surface-state calculation and the comparison of experimental and theoretical band dispersions at two high-symmetry planes add credibility. However, the quantitative loop reconstruction and the 'unambiguous' phrasing hinge on assumptions that are not fully validated, as detailed in the major comments.
major comments (4)
- [§4, Fig. 3(c) and Fig. 4(c)] The complete-loop claim rests on a single fitted inner potential V0 = 19.3 eV obtained by fitting the periodic modulation of one band feature using the free-electron final-state model. The conversion from photon energy to ky is nonlinear, and the loop's height, width, and closure are direct consequences of this mapping. No independent validation is provided: for example, a dense photon-energy scan across the same feature, a comparison of the fitted V0 with the calculated band structure along kz, or a one-step photoemission calculation. If V0 is off by a few eV, or if free-electron final states are a poor approximation in this layered monoclinic compound, the observed 'shrink and disappear' of the crossing could simply reflect the boundaries of the scanned photon range rather than the closing of a symmetry-protected nodal loop. Please provide a sensitivity analysis of the extracted loop size versus V0, or otherwise verify the kz assignment.
- [Fig. 4(a)–4(c)] The nodal loop is traced by manually placing blue dots on the ARPES dispersions; the extraction criterion is not described. There is no statement of whether the dots correspond to energy-distribution-curve maxima, second-derivative minima, or a fitting procedure, and the quoted uncertainties (0.2±0.02 Å⁻¹ and 0.15±0.03 Å⁻¹) are not defined or propagated from any statistical analysis. Given that the paper claims to determine the 'real size' of the nodal loop quantitatively, please describe the node-position extraction method, provide reproducibility across independent measurements, and report the uncertainty in a way that includes both the photon-energy mapping and the peak-finding errors.
- [Fig. 3(h), 'negligible SOC effect'] The claim that spin-orbit coupling has a negligible effect is based only on the statement that no visible lifting of the node degeneracy was resolved along Y–M. With an energy resolution of about 10 meV, this sets only an upper bound on the SOC gap. Please report the size of the SOC gap obtained in your calculations (the Supplemental Material is referenced but not available to the reader here) and state the experimental upper bound explicitly. Without this, 'negligible' is not quantitatively supported and could be misleading in comparison with other nodal-line materials.
- [Note added and ref. [47]] The Note added acknowledges a recent ARPES study of SrAs3 that reports different results, but it does not provide a quantitative comparison. Since the abstract and conclusions use the word 'unambiguously', a mere statement that the samples differ in growth method is insufficient. Please compare the measured band dispersions and extracted loop parameters between the two works as far as the data allow, or moderate the claim to 'our results are consistent with' rather than 'unambiguously identify' until the discrepancy is understood.
minor comments (5)
- [Fig. 4(a)] There is a typo in the sentence describing the ARPES cut: 'the the Y point' should be 'the Y point'.
- [§2, paragraph 3] The phrase 'with both the spatial-inversion and time-reversal symmetries reserved' should be 'preserved' rather than 'reserved'.
- [Abstract and Introduction] The phrase 'would perplex their identification' is informal; consider 'complicate their identification'.
- [Introduction, last sentence] The sentence 'so far direct spectroscopic evidence of such novel band structure on that is still lack' contains grammatical errors; please revise to 'direct spectroscopic evidence for such band structure is still lacking'.
- [Supplemental Material reference [44]] The reference to Supplemental Material does not specify its contents; please list the included measurements (e.g., EDS, X-ray diffraction, SOC calculations) so that readers know what to expect.
Circularity Check
No significant circularity: the DFT prediction and the ARPES measurement are independent, and the only fitted parameter (inner potential) is a standard calibration that does not determine the nodal-loop claim by construction.
full rationale
The paper's derivation chain is: (1) first-principles DFT (WIEN2K with GGA+MBJ, Wannier tight-binding, slab spectral function) predicts a single Dirac nodal loop on the Γ-Y-S plane around Y; (2) ARPES measures band dispersions and tracks the band crossings as a function of photon energy; (3) the crossings are mapped onto the kx-ky plane using the free-electron final-state model with an inner potential V0 = 19.3 eV, fitted to the periodic modulation of a band feature; (4) the extracted loop (height 0.2±0.02 Å−1, width 0.15±0.03 Å−1) is compared with the calculated loop. The comparison is genuinely predictive: the ARPES data could have contradicted the DFT band inversion or crossing topology, and the paper does report a quantitative match rather than a forced agreement. The inner-potential fit is a standard ARPES calibration based on the known crystal periodicity; it is not fitted to the nodal loop itself, so the loop's existence and closed shape are not baked into the fitting procedure. The self-citation to ref. [33] (Quan, Yin, Pickett, with a coauthor overlap) is a prior theoretical prediction, but the present experimental confirmation is independent evidence; the paper does not rely on that citation to justify the ARPES interpretation. The Note added acknowledges a conflicting ARPES report, which is a scientific concern rather than a circularity. No equation or parameter is defined in terms of the claimed result, and no 'prediction' reduces to its own input by construction. Therefore no circular steps are identified.
Assumptions & free parameters
free parameters (1)
- Inner potential V0 =
19.3 eV
assumptions (4)
- domain assumption DFT with GGA+MBJ exchange-correlation potential correctly describes the band ordering and band inversion near Y in SrAs3.
- domain assumption The free-electron final-state model maps ARPES photon energy to perpendicular momentum kz.
- domain assumption ARPES intensity maps directly reflect bulk band dispersions at the probed k points, with negligible surface-state or matrix-element distortions.
- standard math The nodal loop is protected by the combination of spatial-inversion and time-reversal symmetries, as established by the cited theory (ref. 34).
Cite this review
Pith. "Pith review of Photoemission Spectroscopic Evidence for the Dirac Nodal Line in Monoclinic Semimetal SrAs$_3$." pith.science (2026). https://pith.science/paper/24HLVCNB
@misc{pith2026190904754,
author = {Pith},
title = {Pith review of: Photoemission Spectroscopic Evidence for the Dirac Nodal Line in Monoclinic Semimetal SrAs$_3$},
year = {2026},
howpublished = {\url{https://pith.science/paper/24HLVCNB}},
note = {Machine review of arXiv:1909.04754}
}
abstract
Topological nodal-line semimetals with exotic quantum properties are characterized by symmetry-protected line-contact bulk band crossings in the momentum space. However, in most of identified topological nodal-line compounds, these topological non-trivial nodal lines are enclosed by complicated topological trivial states at the Fermi energy ($E_F$), which would perplex their identification and hinder further applications. Utilizing angle-resolved photoemission spectroscopy and first-principles calculations, we provide compelling evidence for the existence of Dirac nodal-line fermions in the monoclinic semimetal SrAs$_3$, which are close to $E_F$ and away from distraction of complex trivial Fermi surfaces or surface states. Our calculation indicates that two bands with opposite parity are inverted around \emph{Y} near $E_F$, which results in the single nodal loop at the $\Gamma$-\emph{Y}-\emph{S} plane with a negligible spin-orbit coupling effect. We track these band crossings and then unambiguously identify the complete nodal loop quantitatively, which provides a critical experimental support to the prediction of nodal-line fermions in the CaP$_3$ family of materials. Hosting simple topological non-trivial bulk electronic states around $E_F$ and no interfering with surface states on the natural cleavage plane, SrAs$_3$ is expected to be a potential platform for topological quantum state investigation and applications.
Figures
Reference graph
Works this paper leans on
-
[1]
G. E. V olovik, The universe in a helium droplet , V ol. 117 (Ox- ford University Press on Demand, 2003)
work page 2003
-
[2]
Z. Fang, N. Nagaosa, K. S. Takahashi, A. Asamitsu, R. Math- ieu, T. Ogasawara, H. Yamada, M. Kawasaki, Y . Tokura, and K. Terakura, Science 302, 92 (2003)
work page 2003
-
[3]
A. A. Burkov, M. D. Hook, and L. Balents, Phys. Rev. B 84, 235126 (2011)
2011
-
[4]
C.-K. Chiu, J. C. Teo, A. P. Schnyder, and S. Ryu, Reviews of Modern Physics 88, 035005 (2016)
2016
-
[5]
Z. Liu, B. Zhou, Y . Zhang, Z. Wang, H. Weng, D. Prabhakaran, S.-K. Mo, Z. Shen, Z. Fang, X. Dai, et al. , Science 343, 864 (2014)
work page 2014
-
[6]
S. Borisenko, Q. Gibson, D. Evtushinsky, V . Zabolotnyy, B. B¨uchner, and R. J. Cava, Physical review letters113, 027603 (2014)
work page 2014
-
[7]
S.-Y . Xu, I. Belopolski, N. Alidoust, M. Neupane, G. Bian, C. Zhang, R. Sankar, G. Chang, Z. Yuan, C.-C. Lee, et al., Sci- ence 349, 613 (2015)
work page 2015
-
[8]
S.-M. Huang, S.-Y . Xu, I. Belopolski, C.-C. Lee, G. Chang, B. Wang, N. Alidoust, G. Bian, M. Neupane, C. Zhang, et al., Nature communications 6, 7373 (2015)
work page 2015
Show all 48 references
-
[9]
Huang, L
X. Huang, L. Zhao, Y . Long, P. Wang, D. Chen, Z. Yang, H. Liang, M. Xue, H. Weng, Z. Fang,et al., Physical Review X 5, 031023 (2015)
2015
-
[10]
B. Lv, H. Weng, B. Fu, X. Wang, H. Miao, J. Ma, P. Richard, X. Huang, L. Zhao, G. Chen, et al. , Physical Review X 5, 031013 (2015)
2015
-
[11]
H. Weng, C. Fang, Z. Fang, B. A. Bernevig, and X. Dai, Phys- ical Review X 5, 011029 (2015)
2015
-
[12]
N. Xu, H. Weng, B. Lv, C. E. Matt, J. Park, F. Bisti, V . N. Stro- cov, D. Gawryluk, E. Pomjakushina, K. Conder, et al., Nature communications 7, 11006 (2016)
2016
-
[13]
S.-Y . Xu, N. Alidoust, I. Belopolski, Z. Yuan, G. Bian, T.-R. Chang, H. Zheng, V . N. Strocov, D. S. Sanchez, G. Chang, et al., Nature Physics 11, 748 (2015)
2015
-
[14]
L. Yang, Z. Liu, Y . Sun, H. Peng, H. Yang, T. Zhang, B. Zhou, Y . Zhang, Y . Guo, M. Rahn, et al. , Nature physics 11, 728 (2015)
2015
-
[15]
Lv, Z.-L
B. Lv, Z.-L. Feng, Q.-N. Xu, X. Gao, J.-Z. Ma, L.-Y . Kong, P. Richard, Y .-B. Huang, V . Strocov, C. Fang, et al. , Nature 546, 627 (2017)
2017
-
[16]
Z. Zhu, G. W. Winkler, Q. Wu, J. Li, and A. A. Soluyanov, Physical Review X 6, 031003 (2016)
2016
-
[17]
Bzduˇsek, Q
T. Bzduˇsek, Q. Wu, A. R¨uegg, M. Sigrist, and A. A. Soluyanov, Nature 538, 75 (2016)
2016
-
[18]
C. Fang, Y . Chen, H.-Y . Kee, and L. Fu, Physical Review B92, 081201 (2015)
2015
-
[19]
C. Fang, H. Weng, X. Dai, and Z. Fang, Chinese Physics B 25, 117106 (2016)
2016
-
[20]
Award for Outstanding Member in Youth Innovation Promotion Asso- ciation CAS
and result in drumhead-like surface states [19, 21]. To date, nodal-line states have been theoretically proposed and ∗ Equal contributions † yinzhiping@bnu.edu.cn ‡ lzh17@mail.sim.ac.cn § dwshen@mail.sim.ac.cn then experimentally confirmed in several compounds, includ- ing CaAg...
1909 arXiv
-
[21]
Z. Yan, R. Bi, H. Shen, L. Lu, S.-C. Zhang, and Z. Wang, Physical Review B 96, 041103 (2017)
2017
-
[22]
R. Yu, Z. Fang, X. Dai, and H. Weng, Frontiers of Physics 12, 127202 (2017)
2017
-
[23]
Wang, X.-M
X.-B. Wang, X.-M. Ma, E. Emmanouilidou, B. Shen, C.-H. Hsu, C.-S. Zhou, Y . Zuo, R.-R. Song, S.-Y . Xu, G. Wang,et al., Physical Review B 96, 161112 (2017)
2017
-
[24]
Yamakage, Y
A. Yamakage, Y . Yamakawa, Y . Tanaka, and Y . Okamoto, Jour- nal of the Physical Society of Japan 85, 013708 (2015)
2015
-
[25]
Bian, T.-R
G. Bian, T.-R. Chang, R. Sankar, S.-Y . Xu, H. Zheng, T. Neu- pert, C.-K. Chiu, S.-M. Huang, G. Chang, I. Belopolski, et al., Nature communications 7, 10556 (2016)
2016
-
[26]
M. M. Hosen, K. Dimitri, I. Belopolski, P. Maldonado, R. Sankar, N. Dhakal, G. Dhakal, T. Cole, P. M. Oppeneer, D. Kaczorowski, et al., Physical Review B 95, 161101 (2017)
2017
-
[27]
Lou, J.-Z
R. Lou, J.-Z. Ma, Q.-N. Xu, B.-B. Fu, L.-Y . Kong, Y .-G. Shi, P. Richard, H.-M. Weng, Z. Fang, S.-S. Sun, et al. , Physical Review B 93, 241104 (2016)
2016
-
[28]
L. M. Schoop, M. N. Ali, C. Straßer, A. Topp, A. Varykhalov, D. Marchenko, V . Duppel, S. S. Parkin, B. V . Lotsch, and C. R. Ast, Nature communications 7, 11696 (2016)
2016
-
[29]
Takane, Z
D. Takane, Z. Wang, S. Souma, K. Nakayama, C. Trang, T. Sato, T. Takahashi, and Y . Ando, Physical Review B 94, 121108 (2016)
2016
-
[30]
X. Feng, C. Yue, Z. Song, Q. Wu, and B. Wen, Physical Review Materials 2, 014202 (2018)
2018
-
[31]
Z. Liu, R. Lou, P. Guo, Q. Wang, S. Sun, C. Li, S. Thirupatha- iah, A. Fedorov, D. Shen, K. Liu, et al., Physical Review X 8, 031044 (2018)
2018
-
[32]
R. Lou, P. Guo, M. Li, Q. Wang, Z. Liu, S. Sun, C. Li, X. Wu, Z. Wang, Z. Sun, et al., npj Quantum Materials 3, 50 (2018)
2018
-
[33]
Zhang, Z.-M
X. Zhang, Z.-M. Yu, X.-L. Sheng, H. Y . Yang, and S. A. Yang, Physical Review B 95, 235116 (2017)
2017
-
[34]
Y . Quan, Z. Yin, and W. Pickett, Physical review letters 118, 176402 (2017)
2017
-
[35]
Q. Xu, R. Yu, Z. Fang, X. Dai, and H. Weng, Physical Review B 95, 045136 (2017)
2017
-
[36]
S. Li, Z. Guo, D. Fu, X.-C. Pan, J. Wang, K. Ran, S. Bao, Z. Ma, Z. Cai, R. Wang, et al., Science Bulletin 63, 535 (2018)
2018
-
[37]
L. An, X. Zhu, W. Gao, M. Wu, W. Ning, and M. Tian, Physical Review B 99, 045143 (2019)
2019
-
[38]
Blaha, K
P. Blaha, K. Schwarz, G. K. Madsen, D. Kvasnicka, and J. Luitz, An augmented plane wave+ local orbitals program for calculating crystal properties (2001)
2001
-
[39]
J. P. Perdew, K. Burke, and M. Ernzerhof, Physical review let- ters 77, 3865 (1996)
1996
-
[40]
Tran and P
F. Tran and P. Blaha, Physical review letters 102, 226401 (2009)
2009
-
[41]
Marzari, A
N. Marzari, A. A. Mostofi, J. R. Yates, I. Souza, and D. Van- derbilt, Reviews of Modern Physics 84, 1419 (2012)
2012
-
[42]
A. A. Mostofi, J. R. Yates, Y .-S. Lee, I. Souza, D. Vanderbilt, and N. Marzari, Computer physics communications 178, 685 (2008)
2008
-
[43]
Kune ˇs, R
J. Kune ˇs, R. Arita, P. Wissgott, A. Toschi, H. Ikeda, and K. Held, Computer Physics Communications181, 1888 (2010)
2010
-
[44]
Q. Wu, S. Zhang, H.-F. Song, M. Troyer, and A. A. Soluyanov, Computer Physics Communications 224, 405 (2018). 6
2018
-
[45]
See Supplemental Material for more details on experiments, data analysis and calculations
-
[46]
Bauhofer, M
W. Bauhofer, M. Wittmann, and H. Schnering, Journal of Physics and Chemistry of Solids 42, 687 (1981)
1981
-
[47]
Liebowitz and N
D. Liebowitz and N. Shevchik, Physical Review B 17, 3825 (1978)
1978
-
[48]
M. M. Hosen, B. Wang, G. Dhakal, K. Dimitri, F. Kabir, C. Sims, S. Regmi, T. Durakiewicz, D. Kaczorowski, A. Bansil, et al., arXiv preprint arXiv:1812.06365 (2018)
2018 arXiv
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