Room temperature Planar Hall effect in nanostructures of trigonal-PtBi2

I appreciate the authors' response to my technical queries, but I am not convinced by their response to some of the conceptual questions. To my understanding, the main claims of this manuscript include: i) the observation of the planar Hall effect up to room temperature; ii) experimental evidence of the impact of band topology on the electronic transport ("Our results strengthen the topological nature of PtBi2 and the strong influence of quantum geometric effects on the electronic transport...")

Regarding i), another reviewer has already raised some technical concerns that have to be addressed. My main doubts concern the point ii), as follows:

1. Fig. 1 depicting 12 Weyl nodes has been added to the manuscript. However, other Weyl nodes are still prominently mentioned in the discussion. Some of them lie very far away from the Fermi level (-655 meV, -497 meV), and it is very hard to imagine that such features may contribute to the transport, so I am not sure why they are mentioned at all. It gets even more confusing with the last added sentence in the first paragraph of the Discussion: "...these 6 groups are referred to as the field-generated Weyl nodes, as they exist even in the absence of an external magnetic field". It's probably a typo, and the authors imply that these nodes appear in the magnetic field only. But on reading this paragraph several times I am still not quite sure what the intended meaning is. The whole discussion of these "distant" Weyl nodes is very obscure and seemingly irrelevant to the experimental data presented in this work, especially if APHE is excluded.

2. Different calculations of the Fermi surface (Fig. 8c in PRM'2020; Fig. 1c in Nature'2024; Fig. 5 in arXiv:2504.13661) all show large sheets that lie somewhat far away from the anticipated Weyl nodes shown in Fig. 1 of the present work. What is the rationale for discussing electronic transport in the context of these Weyl nodes? What about the large and "conventional" parts of the Fermi surface that should, naively, dominate the transport?

3. A concurrent study of the planar Hall effect in PtBi2 has been published by Zhu et al. [PRB 110, 125148 (2024)]. It appeared in September 2024, about a month before the initial submission and half a year before the resubmission of the present manuscript, yet it has not been mentioned at all. Detailed transport measurements are certainly time-consuming, and the publication of similar data by another group does not compromise the novelty per se. However, it does raise some conceptual questions about the interpretation, because Zhu et al. arrive at a very different conclusion that the properties of the PtBi2 flakes are dominated by anisotropic orbital magnetoresistance. In fact, that study looks more convincing because it juxtaposes transport measurements on the bulk samples and thin flakes, and eventually identifies the possible effects of band topology in the bulk but not in the flakes. It also shows a direct comparison of the RRR of the bulk and thin-flake samples and demonstrates a quite drastic reduction in the sample quality (in terms of RRR) upon exfoliation. While a similar reduction in RRR can be inferred from the authors' response, it does not appear prominently in the manuscript, although it seems crucial for the interpretation, especially with the knowledge that bulk samples show a much stronger resemblance to the expected behavior of a Weyl semi-metal than the thin flakes.

As a peer, I find it confusing and even disconcerting that similar data are used to produce entirely different claims: orbital magnetoresistance by Zhu et al. vs. "quantum geometric effects" in the present work. In my opinion, the authors should either find an interpretation consistent with Zhu et al., or provide clear-cut arguments why their data unambiguously prove the role of band topology in the electronic transport. Otherwise, this work will mainly generate confusion instead of clarifying the physics of the potentially interesting quantum material.

Additionally, I would like to mention that I find it rather disturbing when different pieces of characterization are scattered across different publications. Readers are sent to Ref. 19 to see temperature dependence of the resistivity and RRR, and to the supplemental material of the Nano Lett.'2023 publication to see the effect of the contacts geometry. It is at best inconvenient. Moreover, some of the most exciting results for PtBi2 demonstrate an acute sample dependence, see the very unsystematic superconducting gaps shown in the STM study [Nature Comm. 15, 9895 (2024)] and the absence of superconductivity in arXiv:2503.08841, as opposed to Nature'2024, etc. While I understand that these different Dresden-centered publications on PtBi2 are related to different collaborators, I have to say that there are growing doubts on whether all unconventional and exciting physics of PtBi2 is really intrinsic. Careful tracking of the sample characterization and comparisons between the bulk and thin-flake samples could help to resolve these doubts.

Ask for major revision

The authors have measured the transverse and the longitudinal resistance of three exfoliated samples of t-PtBi2 flakes in a magnetic field rotating the mutual orientation of applied current and magnetic field. The study produces interesting data and beautiful figures. However, this reviewer is not impressed by the analysis and the presentation and believes that it is not appropriate for Scipost. Here is the list of my objections.

i) The main observation is that both longitudinal and transverse resistance oscillate with a periodicity of 180° and these oscillations are phase shifted. But this is a trivial result, expected in any metal. When the magnetic field and the electric current, there is a Lorentz force and when they are parallel, there is no Lorentz force. Independent of microscopic details. When the current and field are parallel to each other, the longitudinal resistance is expected to be lower than when they are perpendicular. When the current and field are parallel to each other, the transverse resistance is expected to be zero, provided that: i) There is no misalignment; ii) The symmetry axes of the crystal and the Fermi surface pockets coincide with each other. There is no discussion of these prosaic possibilities in this paper.

ii) A serious study of transport in any metal (topological or otherwise) would inform the reader about the RRR of the samples, the mean free path (or the mobility) of electrons, the carrier density of the system, the shape and the anisotropy of the Fermi surface, before linking the results to any exotic physics, such as the presence of Weyl nodes. There is no trace of this here. The modest amplitude of the magnetoresistance indicates that low-temperature mobility is not very high.

iii) The issue of reproducibility is not addressed (or rather addressed in a strange way). Yes, all three samples show phase-shifted oscillations of the conductivity. But this is trivial. Do the color figures of Fig.3d-i, become similar when the same angular convention (phi=0 when I//B) is used? Why not plot resistivity (and not resistance)? Is the amplitude of the oscillations in three samples identical in resistivity or conductivity? This looks unlikely. The relative size of oscillations in Rxx is 4e-3 in sample 19e-3 in sample 2, and 22e-3 in sample 3. Since sample 2 and 3 are those which have the most irregular shape, one wonders if the observed signal is not enhanced by them. The irreproducibility in the case of the Hall resistance is even larger. Even the sign of the effect is not the same across the three samples.

iv) There is no information about the in-plane orientation of the applied current respective to the crystal axes. Is it the same in the three samples?

v) The main result appears in the title of the paper. Have the authors observed a planar Hall effect at room temperature? I am not convinced. One needs to exclude misalignment. Since the authors admit that the plane of rotation may not be the (electric field, charge current) plane, then the whole signal may be due to a combination of rotational misalignment and a genuine in-plane anisotropy of Fermi velocity. Only a two-axis rotational set-up can dissipate any ambiguity. The fact that the oscillations keep their phase from 1K to 300 K is actually an argument in favor of misalignment origin.

Reject