Andreev spectroscopy of CrO ₂ thin films on TiO ₂ and Al ₂ O ₃

Abstract

Here we analyse the spectroscopic information gathered at a number of single CrO2/Pb interfaces. We examine thin films requiring additional interfacial layers to generate long-range spin triplet proximity effect superconductivity (CrO2/TiO2) or not (CrO2/Al2O3). We analyse the data using two theoretical models and explore the use of a parameter-free method to determine the agreement between the models and experimental observations, showing the necessary temperature range that would be required to make a definitive statement. The use of the excess current as a further tool to distinguish between models is also examined. The analysis of the spectra demonstrates that the temperature dependence of the normalised zero-bias conductance is independent of the substrate onto which the films are grown. This result has important implications for the engineering of interfaces required for the long-range spin triplet proximity effect.

Spin-orbit coupling as a source of long-range triplet proximity effect in superconductor-ferromagnet hybrid structures

“…16,22 It is commonly believed that in both cases the long-range triplet component is generated due to a magnetic inhomogeneity, either originated at the superconductor/CrO 2 interface (spin-active interface) or in the Ni interlayer. 23 We give here an additional possible explanation for the long-range proximity effect in such lateral structures, based on the presence of SO coupling at the contact region. The existence of a SO coupling in the CrO 2 experiments was suggested in Ref.…”

“…The existence of a SO coupling in the CrO 2 experiments was suggested in Ref. 23 , but not discussed quantitatively due to the lack of a formalism for this. We have now all ingredients to include the SO coupling in the study of the proximity effect, and focus our analysis on the system sketched in Fig.…”

General boundary conditions for quasiclassical theory of superconductivity in the diffusive limit: application to strongly spin-polarized systems

“…The problem of a superconductor in proximity contact with a half-metallic ferromagnet has been studied within the frameworks of Eilenberger equations [11,12,20,50,52,[67][68][69], Bogoliubov-de Gennes equations [70][71][72][73], recursive Green function methods [74], circuit theory [75], within a magnon-assisted tunneling model [76], and in the quantum limit [77]. Various experiments on superconductor/half-metal devices have been reported, both for layered systems involving high-temperature superconductors [78][79][80][81] and in diffusive structures involving conventional superconductors [82][83][84][85][86][87][88]. An important consequence of the new boundary conditions in equation (69a) is that half-metals can now be incorporated in the Usadel equation, which is appropriate to describe the second class of experiments mentioned above, whereas there previously existed no suitable boundary conditions to do so.…”

Interfacial spin-orbit coupling: A platform for superconducting spintronics

“…While this procedure usually relies on point contacts [31], it was also applied for F/I /S tunnel junctions [32] with additional suppression of Andreev reflection due to potential scattering at an insulator. In both cases, it is important to decouple the role of conductance suppression due to spin polarization from that arising from interfacial scattering [33][34][35][36][37]. In contrast to this conventional Andreev reflection, a spin-active interface with interfacial spin-flip scattering can also yield Andreev reflection with an equal spin of electrons and holes [38], which is responsible for a spin-triplet Cooper pair as shown in Fig.…”