Absolute Superconducting Spin Valve Effects in Superconductors with a Spin-Splitting Field and Spin-Orbit Coupling

At a thin-film superconductor (S) interface with a ferromagnetic insulator (FI), the magnetic proximity effect (MPE) can spin-split the superconducting density of states and suppress the critical temperature (<i>T</i>_c) of S. For S metals with weak spin-orbit coupling (SOC), the spin splitting can translate to several Tesla [1, 2] and the <i>T</i>_c suppression can reach tens of mK [3]. For a FI/S/FI spin switch, the <i>T</i>_c suppression is reduced for antiparallel (AP) FI magnetizations due to a net cancellation effect of the MPE in S; conversely, for parallel (P) magnetizations, the exchange fields add, enhancing the suppression of <i>T</i>_c with the spin switch efficiency defined as Δ<i>T</i>_c=<i>T</i>_c(AP)-<i>T</i>_c(P)&gt;0 [4].<br/><br/>In the presence of SOC, the spin-splitting in S should smear out, reducing Δ<i>T</i>_c. Here, however, we show that in optimized EuS/Nb/EuS (FI/S/FI) structures, Δ<i>T</i>_c can reach 1 K and achieve an absolute spin-valve effect with Δ<i>T</i>_c/<i>T</i>_c close to 1 by inserting a thin layer of Au at a single interface - i.e., EuS/Nb/Au/EuS. Scanning tunneling spectroscopy on Au in Au/Nb and Au/Nb/EuS structures show evidence for a modification of the superconducting state due to the presence of EuS. These results demonstrate new physics in which Δ<i>T</i>_c is boosted by SOC.<br/><br/>[1] X. Hao, J. S. Moodera, and R. Meservey, Phys. Rev. Lett. <b>67</b>, 10 (1991).<br/>[2] E. Strambini, et al., Phys. Rev. Mat. <b>1</b>, 054402 (2017).<br/>[3] B. Li, et al., Phys. Rev. Lett. <b>110</b>, 097001 (2013).<br/>[4] P. G. de Gennes, Phys. Lett. <b>23</b>, 1 (1966).