Robust and tunable coreless vortices and fractional vortices in chiral $d$-wave superconductors

Abstract: Chiral $d$-wave superconductivity has recently been proposed in a wide range of materials based on both experiment and theoretical works. Chiral superconductors host a finite Chern number set by the winding of the superconducting order parameter and associated topologically protected chiral edge modes. However, the chiral edge currents and orbital angular momentum (OAM) generated by the edge modes are not topologically protected and another, more robust, experimental probe is therefore needed to facilitate experimental verification of chiral $d$-wave superconductors. We have recently shown the appearance of quadruply quantized coreless vortices (CVs) in chiral $d$-wave superconductors, consisting of a closed domain wall decorated with eight fractional vortices, and generating a smoking-gun signature of the Chern number, chirality, and the superconducting pairing symmetry [P. Holmvall and A. M. Black-Schaffer, arXiv:2212.08156 (2023)]. Specifically, the CV spontaneously breaks axial symmetry for parallel chirality and vorticity, with a signature appearing directly in the local density of states (LDOS) measurable with scanning-tunneling spectroscopy (STS). In this work, we first demonstrate a strong tunability of the CV size and shape directly reflected in the LDOS and then show that the LDOS signature is robust in the presence of regular Abrikosov vortices, strong confinement, system and normal-state anisotropy, different Fermi surfaces (FSs), non-degenerate order parameters, and even non-magnetic impurities. In conclusion, our work establishes CVs as a tunable and robust signature of chiral $d$-wave superconductivity.