December 2020

Abstracts of the QSIT Lunch Seminar, Thursday, December 3, 2020

Gate-Defined Josephson Junctions in Magic-Angle Twisted Bilayer Graphene

Fokko de Vries – Nanophysics Group (Ensslin group), ETH Zurich

In the past two years, magic-angle twisted bilayer graphene has emerged as a uniquely versatile experimental platform that combines metallic, superconducting, magnetic and insulating phases in a single crystal. In particular the ability to tune the superconducting state with a gate voltage opened up intriguing prospects for novel device functionality. Here we present the first demonstration of a device based on the interplay between two distinct phases in adjustable regions of a single magic-angle twisted bilayer graphene crystal. We electrostatically define the superconducting and insulating regions of a Josephson junction and observe tunable DC and AC Josephson effects. Shapiro steps, a hallmark of the AC Josephson effect and therefore the formation of a Josephson junction, are observed. This work is an initial step towards devices where separate gate-defined correlated states are connected in single-crystal nanostructures.

A Double Quantum Dot Spin Valve

Arunav Bordoloi - Quantum- and Nanoelectronics (Schönenberger group), University of Basel

Arunav Bordoloi¹, Valentina Zannier², Lucia Sorba², Christian Schönenberger^1,3 and Andreas Baumgartner<sup>1,3

1</sup>Department of Physics, University of Basel, Switzerland
²NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, Pisa, Italy
³Swiss Nanoscience Institute, University of Basel, Switzerland
Email:

Keywords: Spintronics, Double Quantum Dot, Nanowires, Ferromagnetic Split Gates, Micro Magnets, Tunneling Magnetoresistance, Spin Polarization, Spin Detection

Abstract: A longstanding fundamental goal in spintronics is to electrically tune highly efficient spin injectors and detectors, preferably compatible with nanoscale electronics and superconducting elements. Semiconducting InAs nanowires (NWs) provide an ideal system for electron spin control due to a large electron g-factor and spin-orbit coupling. Short confined single NW segments forming a quantum dot (QD), combined with a single ferromagnetic split-gate (FSGs), exhibit a strong hysteretic magnetic field dependence of the device resistance (magnetoresistance, MR) [1], due to the stray field of the FSGs.

Here, we demonstrate a double quantum dot spin valve (DQD-SV) with two weakly-coupled QDs in series [2], each independently spin-polarized as well as electrically tuned by individual split-gates, magnetized in parallel (P) or anti-parallel (AP). In tunneling magnetoresistance (TMR) experiments we find a strongly reduced spin valve conductance for the two AP configurations compared to the P configurations, with a single QD polarization of ~27%. This TMR can be significantly improved by a small external field and optimized gate voltages, which results in a continuously electrically tunable TMR between +80% and -90%. A simple model quantitatively reproduces all our findings, which allows us to extract a fully gate tunable QD polarization of ±80%. Such versatile QD spin filters are compatible with superconducting electronic elements, for example in spin projection and correlation experiments in a Cooper pair splitter [3], or to initialize and read-out spin qubits [4].

[1] G. Fabian et al., PRB 94, 195415 (2016)
[2] A. Bordoloi et al., Comm. Phys. 3, 135 (2020)
[3] A. Bordoloi et al., in preparation
[4] M. Pioro-Ladrière et al., Nat. Phys. 4, 776-779 (2008)