Observation of Andreev Bound States at Spin-active Interfaces

We report on high-resolution differential conductance experiments on nanoscale superconductor-ferromagnet tunnel junctions with ultrathin oxide tunnel barriers. We observe subgap conductance features that are symmetric with respect to bias and shift according to the Zeeman energy with an applied magnetic field. These features can be explained by resonant transport via Andreev bound states induced by spin-active scattering at the interface. From the energy and Zeeman shift of the bound states, both the magnitude and sign of the spin-dependent interfacial phase shifts between spin-up and spin-down electrons can be determined. These results contribute to the microscopic insight into the triplet proximity effect at spin-active interfaces.

Figure 1

(a) Scanning electron microscopy image of one of our samples, together with the measurement scheme. (b) High-resolution transmission electron microscopy cross section of a reference structure, showing the atomic structure of the tunnel barrier.

Figure 2

Differential conductance spectra. (a) Differential conductance of one junction as a function of applied magnetic field $B$ at lowest temperature. The solid line is a fit to Eq. (3). (b) Subgap differential conductance of the same contact at $B=0$ (symbols), together with a fit according to Eq. (2) (line).

Figure 3

Density of states and Zeeman shift of the bound states. (a) and (b) Differential subgap conductance of two contacts of the same sample as a function of applied magnetic field $B$. The dotted lines indicate the Zeeman shift of the bound states (shown for positive fields only), and the arrows indicate spin. (c) Spin-resolved density of states due to the bound state shown in panel (a), plotted for the parameters obtained from fits to Eq. (6), and with the sign of the spin-mixing angle, inferred from the Zeeman shift (see text). (d) Data for a third contact at $B=0$ (circles, left conductance scale) and in the normal state at $B=2\mathrm{T}$ (diamonds, right conductance scale). Both scales span $15\mu \mathrm{S}$. The lines are fits to Eq. (2) and the standard model of dynamic Coulomb blockade [22], respectively, (see text).

Figure 4

(a) Normalized bound-state energy ${ϵ}_{+}/\Delta$ for all measured junctions. Sample numbers are given at the bottom. (b) Histogram of the data from panel (a), bin size 0.1.