Ubiquity of rotational symmetry breaking in superconducting films, from Fe(Te,Se)/Bi$_2$Te$_3$ to Nb, and the effect of measurement geometry

TL;DR

This study reveals that apparent nematicity in superconducting films, observed via in-plane magneto-transport measurements, is likely an artifact caused by vortex responses to magnetic forces, questioning previous claims of intrinsic nematicity.

Contribution

The paper demonstrates that the commonly used measurement scheme for detecting nematicity in superconductors inherently produces two-fold symmetry, challenging prior interpretations of nematic order.

Findings

Ubiquitous two-fold symmetry observed in different superconducting films.

Vortex response to magnetic Lorentz force causes measurement artifacts.

Previous reports of nematicity may need reinterpretation.

Abstract

FeTe$_{0.5}$Se$_{0.5}$/Bi$_2$Te$_3$ heterostructure is a promising new platform in the journey toward topological quantum computation, considering that first, FeTe$_{0.5}$Se$_{0.5}$ is itself known to be a topological superconductor (TSC) and second, the heterostructure has topological interface states that can be proximitized into TSC even if FTS fails to become TSC on its own. Here, we show that this system exhibits quasi-2D superconductivity, and utilizing the standard in-plane magneto-transport measurements, we discover two-fold anisotropy (a.k.a nematicity) in R$_{xx}$ and I$_c$ measurement, even though the system exhibits globally 12-fold symmetry. Then, we carried out similar measurements on a polycrystalline niobium (Nb) thin film, a well-known s-wave elemental superconductor, and found a similar two-fold symmetry even for this Nb system. This implies either that nematic behavior is ubiquitous or that the in-plane magneto-transport measurement scheme routinely used to detect nematicity is not a reliable method to probe nematicity. We show that the angle-dependent response of vortices in the superconducting regime to the magnetic Lorentz force is very likely the main cause behind the ubiquitous nematic behaviors of this measurement scheme. In other words, this measurement scheme is intrinsically two-fold, and is therefore not suitable to detect the nematicity. Accordingly, all the previous reports of nematicity based on similar measurement practices, reported on various samples, including thin films, bulk crystals, and exfoliated flakes, need to be reinterpreted.