Unconventional magnetization in the multiphase superconductor PdBi$_2$

Abstract

Unconventional superconductors have specific signatures in their magnetic properties, such as intrinsic magnetization at interfaces and around defects and fractional and multiquanta vortices, with much attention focused on heavy fermion and high-$T_c$ superconductors. Here, we report the observation of highly anomalous magnetization in $\beta$-PdBi$_2$, a layered superconductor previously shown to exhibit a magnetic field-induced transition from s-wave to nodal p-wave superconductivity at a transition field $H^*\sim$0.1 T ($H_{C1}<H^*<H_{C2}$). This transition is driven by the coupling between spin-polarized electronic bands and the in-plane magnetic field. In the unconventional phase (above $H^*$) we observe three striking features: strictly linear and non-hysteretic dc magnetization, a sharp drop in the ac susceptibility when the field is applied parallel to the ab-plane, and a pronounced anisotropy in the magnetic response between parallel and perpendicular field orientations. We show that these features are directly correlated with the expected transition to the nodal p-wave state and propose that the unusual magnetization behavior can be explained by a transition from a conventional vortex lattice in the low-field s-wave phase to a domain structure corresponding to spatial phase separation into superconducting (p-wave) and normal domains above $H^*$. Our work identifies new experimental signatures of unconventional multiphase superconductivity, offering an insight into the magnetic-field response of nodal states.