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Spin transport and dynamics in silicon-germanium islands

Final Report Summary - SDYSIGIS (Spin transport and dynamics in silicon-germanium islands)

This project is positioned in the field of nanoelectronics, one of the major branches of nanoscience and nanotechnology, whose goal is to develop electronic devices and circuits from nanometer scale building blocks. In this domain, there is an increasing interest towards the realization of hybrid devices which couple low dimensional semiconductors to electrical leads with different functionalities, such as ferromagnetism (FM) or superconductivity (S). Here we proposed to study hybrid semiconductor quantum dot (QD) devices in two different directions. The first one aimed at achieving spin injection from ferromagnets into QDs, in order to allow for the dynamics of individual spins to be probed, and ultimately, to enable individual spins to be manipulated inside a QD. The second direction involved the investigation of hybrid superconductor-QD devices, a rich system which has been widely explored as tunable Josephson junctions as well as studied as a potential source of entangled electrons from the splitting of individual Cooper pairs.

Our work has mainly addressed the interplay between the Kondo effect and superconductivity, and the formation of intra-gap bound states owing to the exchange interaction between an unpaired electron in a QD with the neighboring superconducting leads. In virtue of the more promising results obtained with the hybrid superconductor-QD devices, combined with the increased interest resulting from theoretical predictions concerning the existence of Majorana fermions in this kind of system, we have concentrated our efforts towards this direction. The most significant scientific achievements in the project can be summarized as follows:

Preliminary results on FM-contacted SiGe QD devices. We have fabricated devices comprised of self-assembled SiGe islands contacted by ferromagnetic NiFe contacts. In particular, we have measured an interesting response of a given device to external magnetic fields. More specifically, we have observed the opening of a conductance gaps at Coulomb resonances whose magnitude was found to increase with the applied magnetic field. Future work will be focused on reproducing this effect and understanding its origin..

Coexistence of zero-bias Kondo peak and superconductivity. We have studied the origin of sub-gap anomalies observed in the Coulomb-blockade regime of S-QD-S devices for odd numbers of confined electrons. In particular, we have found that zero-bias peaks due to the Kondo effect, observed at a finite magnetic field large enough to suppress superconductivity in the leads, persist also below the critical field of the leads themselves. With the aid of numerical calculations, we were able to conclude that the coexistence of the Kondo effect with superconductivity arises from a non-vanishing density of quasiparticle states within the superconducting gap. This conclusions shines new light on earlier experimental results on S-QD-S systems.

Observation of intra-gap Shiba bound states. Devices comprised of a QD coupled to both superconducting and normal metal (N) leads were fabricated. Low temperature transport measurements revealed intra-gap bound states originating from the exchange interaction of the localized spin in the QD with the superconducting leads. By tuning the gate voltages, we were able to observe a transition from a magnetic doublet to a non-magnetic singlet ground state.

Preliminary results on Cooper pair splitting. Hybrid devices with S and N leads were also employed to study the splitting of Cooper pairs from a central superconducting electrode into two normal-type electrodes. In this case, two independent QDs are defined in the nanowire sections between the central S-type lead and the outer N-type contacts. In agreement with previous works, we were able to observe a zero-bias conductance enhancement on a resonant dot when the second dot is also driven into resonance. However, in our case, the resonant states corresponded to intra-gap bound states. Further work will be performed in this direction.