The quantum anomalous Hall effect (QAHE) realizes dissipationless longitudinal resistivity and quantized Hall resistance without the need of an external magnetic field. However, when reducing the device dimensions or increasing the current density, an abrupt breakdown of the dissipationless state occurs with a relatively small critical current, limiting the applications of the QAHE. We investigate the mechanism of this breakdown by studying multiterminal devices and identified that the electric field created between opposing chiral edge states lies at the origin. We propose that electric-field-driven percolation of two-dimensional charge puddles in the gapped surface states of compensated topological-insulator films is the most likely cause of the breakdown.

Radioactive ${}^{27}\mathrm{M}\mathrm{g}$ ($t_{1/2}=9.5\min$) was implanted into GaN of different doping types at CERN’s ISOLDE facility and its lattice site determined via ${\beta }^{-}$ emission channeling. Following implantations between room temperature and $800°\mathrm{C}$, the majority of ${}^{27}\mathrm{M}\mathrm{g}$ occupies the substitutional Ga sites; however, below $350°\mathrm{C}$ significant fractions were also found on interstitial positions $\sim 0.6\AA$ from ideal octahedral sites. The interstitial fraction of Mg was correlated with the GaN doping character, being highest (up to 31%) in samples doped $p$ type with $2\times {10}^{19}{\mathrm{cm}}^{-3}$ stable Mg during epilayer growth, and lowest in Si-doped $n$-GaN, thus giving direct evidence for the amphoteric character of Mg. Implanting above $350°\mathrm{C}$ converts interstitial ${}^{27}\mathrm{M}\mathrm{g}$ to substitutional Ga sites, which allows estimating the activation energy for migration of interstitial Mg as between 1.3 and 2.0 eV.

Over the last ten years, signatures of high temperature ferromagnetism have been found in thin films and nanoparticles of various materials which are non-ferromagnetic in bulk, from semiconductors to superconductors. These studies often involve state-of-the-art magnetometers working close to the limits of their sensitivity, where magnetic contaminations and measurement artefacts become non-negligible. Because such spurious effects may be involved, the reliability of magnetometry techniques for the detection of ferromagnetism in these new magnetic nanomaterials has been questioned. In this paper, we present a detailed study on magnetic contamination arising from sample processing and handling, describing how it may occur and how it can be avoided or otherwise removed. We demonstrate that, when proper procedures are followed, extrinsic magnetic signals can be reproducibly kept below 5 × 10⁻⁷ emu (5 × 10⁻¹⁰ A m²). We also give an overview of the expected levels of contamination when such optimum conditions cannot be guaranteed and analyse the characteristics of the resulting magnetic behaviour, discussing which features may or may not be used as criteria to distinguish it from intrinsic ferromagnetism. Although the measurements were performed using superconducting quantum interference device magnetometers, most of what we describe can also be applied when using alternating-gradient force and vibrating-sample magnetometers.

Thin films of ferromagnetic topological insulator materials can host the quantum anomalous Hall effect without the need for an external magnetic field. Inducing Cooper pairing in such a material is a promising way to realize topological superconductivity with the associated chiral Majorana edge states. However, finding evidence of the superconducting proximity effect in such a state has remained a considerable challenge due to inherent experimental difficulties. Here we demonstrate crossed Andreev reflection across a narrow superconducting Nb electrode that is in contact with the chiral edge state of a quantum anomalous Hall insulator. In the crossed Andreev reflection process, an electron injected from one terminal is reflected out as a hole at the other terminal to form a Cooper pair in the superconductor. This is a compelling signature of induced superconducting pair correlation in the chiral edge state. The characteristic length of the crossed Andreev reflection process is found to be much longer than the superconducting coherence length in Nb, which suggests that the crossed Andreev reflection is, indeed, mediated by superconductivity induced on the quantum anomalous Hall insulator surface. Our results will invite future studies of topological superconductivity and Majorana physics, as well as for the search for non-abelian zero modes.