[Paper] Imaging Superfluid density in high-quality UTe$_2$

[1] Y. Iguchi et al., Phys. Rev. Lett, 130, 196003 (2023)

UTe$_2$ is a newly discovered odd parity superconductor. It shows a spontaneous time-reversal symmetry breaking and multiple superconducting phase transitions even at ambient pressure, which imply chiral superconductivity, but only in a subset of samples. Recent studies of specific heat measurements in higher-quality samples reported the single-phase transition in UTe$_2$ at ambient pressure near zero magnetic fields. Therefore, the local magnetic imaging of high-quality UTe$_2$ samples is essential to understand the order parameter of UTe$_2$. Still, there has yet to be a scanning magnetic probe microscopy report on UTe$_2$.

We first report scanning magnetic probe microscopy over the cleaved surface of high-quality UTe$_2$ crystals. By examining the scanning SQUID susceptometry, we characterized the samples' homogeneous superfluid density on a micron scale. As expected, they revealed the absence of an additional kink in the superfluid density that would be expected at a second-phase transition. The temperature-dependent superfluid density, determined independent of sample geometry, does not support point nodes along the b-axis but implies deep minima of gaps or point nodes along the a-axis for a quasi-2D Fermi surface in UTe$_2$. We also observed vortices and anti-vortices pinned far from the edges around zero background fields. This result indicates the existence of a local magnetic source, which is likely sample dependent, that induces vortices and antivortices, in spite of the absence of long-range order or strong magnetic sources on the scan plane above the superconducting transition temperature.