Vortex Pinning of Ba 0.62 K 0.38 BiO 3 Investigated by Magneto-Optical Kerr-Effect and Magnetization Measurements

Vortex pinning plays a crucial role in determining properties of type-II superconductors. For example, it governs the irreversible magnetic response as well as dissipation caused by vortex motion. Here, we study vortex pinning in the three-dimensional oxide superconductor Ba 1− x K x BiO 3 using high-sensitivity magneto-optical Kerr-effect (MOKE) and detailed magnetization measurements. We find that the zero-field MOKE signal in the superconducting state exhibits a pronounced magnetic-history dependence. This behavior closely resembles the remanent magnetization caused by trapped vortices. Furthermore, we demonstrate that the evolution of the MOKE signals is well described by Bean’s critical-state model for trapped vortices. Our results establish MOKE as a viable optical and mesoscopic probe of vortex pinning in type-II superconductors, providing a new complementary approach to investigate mixed-state phenomena. We also find that the training-field dependence of the MOKE response is linear near zero training field, without any detectable anomalies indicative of spontaneous time-reversal-symmetry breaking in an unconventional superconducting state under the present experimental conditions. Our study defines a clear protocol to distinguish vortex-induced MOKE responses from those associated with a time-reversal-symmetry broken superconducting order parameter.