Room temperature Planar Hall effect in nanostructures of trigonal-PtBi2

Veyrat et al. report on electrical transport in nanostructures of bulk t-PtBi_2, a Weyl semimetal with multiple Weyl points and potential topological nodal lines in its band structure. The key finding is a planar Hall effect (PHE) linked to the topological nature of its electronic structure. At this point, only few semimetals hosting line nodes have been identified. Hence, the results of this work are of high importance for a better understanding of the fundamental behavior in such materials. I therefore would lean toward publishing this work after a revision.

Questions/remarks:
How do the nanostructured samples compare to the bulk samples? Does the RRR vary for the investigated samples? How was the quality of these samples confirmed?

Have the authors considered high-mobility effects such as current jetting as mentioned in the discussion section? In other words, would the aspect ratio of the measured samples be sufficient for ruling out such effects. One way to check is the local homogeneity of the current distribution. With their multi terminal devices it would be easy to show that for example in sample D2 all 6 Hall pairs exhibit the same magnetoresistance as well as Hall signals. What are the mobilities and charge carrier densities in this material?

It is not immediately clear how the observed transport signature would be related to the band structure. An illustration of the parts of the FS responsible for the observations and a description of the theory behind that would be helpful for a better understanding.

In a previous manuscript (Reference 19) the authors show some of the data presented in this work. Is the newer manuscript a continuation/follow-up/ replacement of the previous work?

Improve the experimental details about the samples.
Provide more evidence about the quality of the samples.
Include more evidence that would prove homogeneous current distribution throughout the samples.
Improve on the theoretical description that links the observed PHE to the electronic structure of the material.

Provide a detailed explanation about the relevance of this work as compared to Reference 19.

Ask for minor revision

This manuscript reports transport measurements on the new topological material t-PtBi2. The main finding is the observation of the planar Hall effect that witnesses the influence of Weyl nodal lines on the electrical transport. This work certainly deserves publication as a very careful experimental study of an interesting new material. In my opinion, the manuscript would potentially meet the acceptance criteria of SciPost Physics with regard to advancing the characterization and understanding of a very non-trivial quantum material, the Weyl semimetal that combines nodal lines with the exotic effect of surface superconductivity. That said, the connection to these phenomena is somewhat indirect, as the present manuscript (in contrast to several other pre-prints and publications from the same group) lacks any explicit theory / ab initio component and analyzes the data on a rather empirical level. The main outcome, beyond the mere observation of the planar Hall effect, appears to be its unusual B^1.24 field dependence, but no further conclusions are derived from this result, and no explicit comparisons to the field dependence in other materials are made.

Several major issues have to be addressed by the authors:

1. The manuscript appears in parallel with arXiv:2410.02353 where at least part of the same data are used to reveal the so-called anomalous planar Hall effect (APHE). For example, Fig. 3a of the present manuscript seems to show almost the same data as Fig. S2A of the APHE preprint (sample D1 at 14 T and 50 vs. 100 K, respectively). However, the fits are different, especially in the case of Rxy where the APHE preprint shows a better fit. This difference can be traced back to the different fitting functions. The manuscript under consideration relies on an assumption that the Rxy signal may be contaminated by Rxx [Eq. (4)], but no such assumption is made in the APHE manuscript [Eq. (S5) in arXiv:2410.02353]. It feels very confusing. It also raises questions on the consistency of the analysis across the two concurrent manuscripts from the same group. A related question is the reasoning behind Eq. (4) vs. the discussion of the discrepancies in page 6. It is argued that a misalignment between the ab-plane of the sample and the rotation plane of the instrument would lead to the 2pi-periodic contribution to Rxy and the pi-periodic contribution to Rxx, but Eq. (4) is written under an assumption that the same kind of misalignment results in the 2pi-contribution to both Rxx and Rxy. Why the difference?

2. The data analysis is certainly complicated by the possible contamination of the Rxy signal with Rxx. It may be a naive question, but why can't one use the 5-probe method and eliminate the longitudinal component?

3. One more question to the Rxx and Rxy data: Fig. 2k shows a visible asymmetry for the Rxx data but not for Rxy. Fig. 2j and Fig. 2l are the opposite: the Rxy data are asymmetric, whereas the Rxx data are not. I assume that this asymmetry is due to the 2pi-periodic contribution, but why does it show up so unsystematically? The data in Figs. 6-9 of the Supplemental Material are somewhat different and always show the 2pi-component in Rxy but not in Rxx. That looks more systematic.

4. Most references to the band structure of t-PtBi2 suggest that it features nodal lines in zero field. Magnetic field converts these lines into Weyl nodes. However, at one point in the discussion section, in the very beginning of page 7, one reads "...stronger than the one originating from the zero-field 12 Weyl nodes". What are these 12 nodes, and how are they related to the nodal line? The text in the same paragraph refers to "the additional Weyl nodes" that presumably lie further away from the Fermi level and can be important yet. However, this is also very hard to understand without an graph showing the band structure. I appreciate that the authors are very familiar with the band structure of t-PtBi2, but potential readers are not.

5. The manuscript would benefit from a dedicated Methods section. The information about the samples and the measurement (vector magnet vs. rotator) is scattered around the text and hard to find, especially when one tries to compare it with other publications. On a related note, readers would benefit from a systematic notation of the samples across the different publications. Sample D3 of the present work seems to be different from sample D3 in the APHE preprint, so they should be given different names, especially if the same D1 and D2 were used in both studies.

6. It is also unclear along which exact direction the current was applied. Trigonal symmetry implies two nonequivalent in-plane directions, such as [100] and [110]. Which of them was used, and does the choice matter?

7. Whereas the samples used in this work are described as "high-quality single crystals of PtBi2", I could not find any examples of their characterization, especially Laue images, in this and in any of the preceding/concurrent manuscripts. As a layered material, trigonal PtBi2 can be tricky, with stacking faults and even intergrowths of the different polymorphs. Are these effects fully excluded?

8. A significant overlap in the figures between this manuscript and the APHE preprint (arXiv:2410.02353) is undesirable: Fig. 1 (PHE) vs. Fig. 1b (APHE), Fig. 3a (PHE) vs. Fig. 2a (APHE), Fig. 4b (PHE) vs. Fig. 2c (APHE), etc. I could not really understand the scientific logic for splitting the analysis of the same data into two separate manuscripts. In my opinion, it makes the data less accessible and imposes unnecessary difficulties on readers who will have to search through two papers if they are interested in details of transport measurements on t-PtBi2. Since SciPost Physics is a flagship journal, I think that its publications should not have such a strong overlap with other journals' publications in terms of both graphics and content. On the other hand, an adequate merger of the PHE and APHE manuscripts should be very welcome in SciPost Physics.

Ask for major revision