Magnetic-Flux Trapping and Self-Heating Phenomenon in Phase-Separated Superconducting Composite—Sn-Pb Solders and New Materials

In this presentation, we show electronic and thermal properties of phase-separated superconducting composites. The most targets are Sn-Pb solders with different Sn/Pb compositions, where Sn and Pb are basically phase-separated perfectly. Pure Sn and Pb are superconductors with transition temperatures (Tc) of 3.7 K and 7.2 K, respectively. Critical fields (Hc) of them are about 300 and 800 Oe for Sn and Pb. In Sn-Pb solders, for example, in Sn10-Pb90, magnetic field of 700 Oe is trapped after field cooling (FC). The high field greater than Hc of Sn is trapped in the Sn regions, and the stable flux trapping is achieved by supercurrent of Pb regions. The Sn regions loose bulk nature of superconductivity. Noticeably, the trapped field is depending on Sn concentration and decreases with increasing Sn concentration.<br/>Using the flux-trapped states of Sn-Pb solders, we achieved nonvolatility of magneto-thermal switching (MTS), which is quite rare functionality useful for thermal control [1]. At the zero-field-cooling state, thermal conductivity is low due to the formation of Cooper pairs. After applying H &gt; Hc (Pb), superconducting states of the solder is suppressed, and thermal conductivity becomes large. When external H is reduced to zero, fluxes are trapped in the Sn regions, and thermal conductivity is still high, which is the nonvolatile-MTS states. In addition, we observed anomalous self-heating phenomenon when trapped-flux amount decreases [2]. The trapped field continuously decreases with increasing temperature, and the self-heating is observed from the lowest temperature to about 4 K. We discuss the possible explanation of the causes of the heating and correlation with the trapped fluxes. Our discussion will include possibilities of the presence of proximity superconducting states, intermediate states, vortices or vortex lattice in the flux-trapped Sn regions. In addition, we show exploration of new phase-separated superconducting composites and flux-related physical properties. Our prospect of phase-separated superconducting composites is the application as quantum sensors where the physical properties are controlled by the trapped fluxes in the superconducting composites.<br/><br/>[1] H. Arima et al., Commun. Mater. 5, 34 (2024).<br/>[2] Y. Mizuguchi et al., arXiv:2405.01850