MASSS-Spin-orbit splitting of Andreev States

Project MASSS – Marie-Sklodowska Curie Action by Leandro Tosi

Spin-orbit splitting of Andreev States revealed by microwave spectrocopy

In the figure below the color encodes the shif of the resonator's frequency (as explained in the previous section) as we send photons of frequency f1 at different phases delta in the weak-link.

The absorption spectrum is very rich. There are many lines that evidence the different transtions that may occur. Among these lines we have highlighted one in red which is a pair transition and a bundle of four lines in green which are single-particle transitions. Pair-transitions correspond to the promotion of two quasiparticles from the ground state to occupy an Andreev state (see absorption spectrum figure 19 in this link). The observation of the single-particle transitions constitutes the main finding of my project (at least for now). These lines that are degenerate at phase 0 and pi, separate elsewhere as a consequence of the spin-splitting of Andreev states.

In the next figure we see another example of these single-particle transitions obtained for a very different gate voltage.

We have played with the spin character of the Andreev states in the weak-link by applying a magnetic field in different directions in the plane of the wire. As it shown in the next picture, we expect that a field perpendicular to wire axis gives rise to an assymetric spectrum around pi and 0; and a field paralel to the wire, a symmetric spectrum where the degeneracy of the four lines is partially lifted.