Pt as a possible spin-orbit coupling source for equal-spin triplets

There is some controversy on the role of spin-orbit coupling (SOC) in the generation of equal-spin triplets, a pairing state that realizes the superconducting long-range proximity in ferromagnets. In recent years, theoretical descriptions have been developed of superconductor/ferromagnet with SOC systems that predict a long-range triplet component (LRTC) due to SOC. The collection of experimental research on the subject, however, currently does not provide an unambiguous answer, and conclusive evidence of the SOC-generated LRTC remains lacking. This work adds to the body of experimental explorations of SOC as a source for equal-spin triplets. Our approach is twofold: we probe the proximity effect in bilayer films of Nb with Pt/Co, Pt, and Co, and we measure the properties of lateral Nb/Pt/Co/Pt/Nb Josephson junctions where the supercurrent is forced through the Co layer by a trench that we cut with focused ion beam milling. In our films, we find a striking difference in the critical...Show moreThere is some controversy on the role of spin-orbit coupling (SOC) in the generation of equal-spin triplets, a pairing state that realizes the superconducting long-range proximity in ferromagnets. In recent years, theoretical descriptions have been developed of superconductor/ferromagnet with SOC systems that predict a long-range triplet component (LRTC) due to SOC. The collection of experimental research on the subject, however, currently does not provide an unambiguous answer, and conclusive evidence of the SOC-generated LRTC remains lacking. This work adds to the body of experimental explorations of SOC as a source for equal-spin triplets. Our approach is twofold: we probe the proximity effect in bilayer films of Nb with Pt/Co, Pt, and Co, and we measure the properties of lateral Nb/Pt/Co/Pt/Nb Josephson junctions where the supercurrent is forced through the Co layer by a trench that we cut with focused ion beam milling. In our films, we find a striking difference in the critical temperatures and zero-temperature coherence lengths of the Nb/Pt/Co sample on the one hand and the Nb/Pt and Nb/Co samples on the other hand. Our junction devices all become fully superconducting which would provide direct evidence for the SOC-generated LRTC, if not for a lack of knowledge about the exact structure of the junction that could be either Nb/Pt/Co/Pt/Nb, Nb/Pt/Nb, or a constriction junction, depending on the trench depth. Interestingly, the temperature dependence of the critical current of these devices is anomalous in the case of a short Nb/Pt/Nb or constriction junction, while triplet transport in junctions with half-metallic ferromagnets theoretically has the same relation. Overall, this work does not end the controversy of the SOC-generated LRTC although it adds some unexpected results that may find their origin in the LRTC.Show less

Leiden University Student Repository

Documents

In Collections

Pt as a possible spin-orbit coupling source for equal-spin triplets