Comment on "Topography of Fermi arcs in t-PtBi$_2$ using high-resolution angle-resolved photoemission spectroscopy" arXiv:2503.08841 (cond-mat)

In the quantum world of solids, electrons usually behave like a crowded dance floor, filling up every available spot from the bottom of the energy scale to a specific ceiling known as the Fermi level. In most materials, this ceiling is a solid wall; electrons cannot exist in the gap above it. However, in a special class of materials called Weyl semimetals, the rules change. Here, the surface of the crystal hosts unique electronic states that form open, incomplete loops in momentum space, resembling a bridge that starts and stops without connecting to anything else. These are called Fermi arcs. Scientists are intensely interested in these arcs because they might carry electricity without resistance, a phenomenon known as superconductivity, which could revolutionize how we transmit energy and build computers. The question is whether these fragile surface bridges can indeed become superconducting, or if they remain ordinary conductors.

A recent debate has centered on a specific material, a crystal of platinum and bismuth, where one group of researchers claimed to see clear signs of superconductivity on the surface, while another group, using a different machine and different samples, concluded that no such superconductivity existed above a very low temperature of 3 Kelvin. The second group argued that the surface states simply crossed the energy level without opening a gap, which is the hallmark of a superconductor. This disagreement was significant because the data from the second group was publicly available, yet their interpretation suggested the first group had been mistaken. A new analysis by Sergey Borisenko has now revisited the raw data from the second group, not by building new theories or models, but by simply looking at the numbers and shapes of the spectral lines exactly as they were recorded. The goal was to see if the evidence for superconductivity was hidden in plain sight, obscured by how the data was presented rather than by the data itself.

The new analysis begins by examining the shape of the electron energy curves at the surface. In a normal metal, the energy of an electron changes smoothly as its momentum changes. In a superconductor, however, the electrons pair up, and this pairing forces the energy curve to bend backward near the Fermi level, creating a distinct U-shape or a dip that prevents the electrons from reaching the lowest energy state. This back-bending is a direct signature of superconductivity. When Borisenko traced the peaks of the spectral lines in the original data from the second group, he found that this back-bending was indeed present. It was not a smooth, straight line as the previous authors had concluded, but a curve that turned around, exactly as expected for a superconducting state. Furthermore, the size of this bend was not the same in all directions; it was very small in one specific direction but grew to about 3 to 5 millielectronvolts in others. This pattern of anisotropy, where the effect changes depending on the angle, matched perfectly with the results from the first group, despite the fact that the two groups used different light sources and different crystals.

The researchers also looked at the edges of the spectral lines, which indicate the precise energy where electrons stop being occupied. In a superconductor, this edge shifts away from the normal level, creating a gap where no electrons can exist. The analysis showed that at the points where the surface arcs cross the Fermi level, this edge had shifted by amounts ranging from 0.8 to 2 millielectronvolts. This shift was consistent across the different cuts of the data, confirming that a gap had opened. The fact that this gap was anisotropic, meaning it was larger in some directions than others, explained why it might have been missed in a quick glance; if one only looked at the direction where the gap was smallest, it would appear almost non-existent. The new work confirmed that the gap was real and followed the same angular pattern reported by the first group, proving that the two independent experiments were actually seeing the same physical phenomenon.

A major part of the confusion in the previous study arose from two critical oversights in how the temperature-dependent data was handled. First, the measurements used to argue against superconductivity at higher temperatures were not taken from the same surface layer as the gap analysis; they came from a different surface termination entirely, meaning the researchers were effectively comparing apples to oranges. Second, the way these datasets were aligned was fundamentally flawed. The original authors had shifted the momentum and energy scales differently for each temperature without a consistent reference point. Specifically, the momentum increment per pixel increased by approximately 8.1% at 11 Kelvin and 11.3% at 19 Kelvin relative to the 3 Kelvin dataset, yet these rescalings were not properly accounted for. When the new analysis re-aligned these datasets using a stable, bulk metallic feature that exists in the material as a reliable ruler, a different picture emerged. Once properly aligned, the data showed that the gap did not disappear at higher temperatures. Instead, the suppression of low-energy electrons persisted up to 19 Kelvin, suggesting that the material enters a state where the gap is still present but perhaps not yet fully coherent, a behavior known as a pseudogap.

The study concludes that the data from the second group, when re-examined with careful attention to how the measurements were registered and which surface was being probed, actually supports the existence of superconductivity on the surface of this material. The apparent contradiction was not due to a lack of superconductivity, but rather to the fact that the data was processed and compared using inconsistent scales and from different surface layers. By using the raw, unsmoothed numbers and checking the positions of the spectral peaks and edges directly, the new work demonstrates that the surface states do exhibit the characteristic back-bending and energy gaps of a superconductor. This finding reinforces the idea that these exotic surface bridges can indeed conduct electricity without resistance, and it highlights the importance of rigorous data alignment when comparing measurements taken at different temperatures. The evidence suggests that the anisotropic gap is a robust feature of the material, persisting well above the temperature where full superconductivity sets in, offering a clearer view into the complex quantum behavior of these unique crystals.