Periodic Reporting for period 3 - TOPOQDot (A bottom-up topological superconductor based on quantum dot arrays)
Reporting period: 2020-06-01 to 2021-11-30
While topological superconductors are not readily available in nature, theory predicts that they can be engineered by combining different materials and physical effects. Perhaps the most explored approach for realizing a 1D topological superconductor relates to the hybrid combination of superconductors and semiconductor nanowires with strong spin-orbit interaction in the presence of an external magnetic field. In spite of the great experimental advances reported in the past years, a fully conclusive demonstration of a topological superconductor and Majorana modes is still lacking. TOPOQDot investigates an alternative route towards the realization of a 1D topological superconductor: to controllably assemble it from an array of quantum dots with induced superconductivity. Indeed, theory suggests that the subgap states of a quantum dot coupled to a superconductor, also known as Shiba states, are natural precursors towards this goal. The approach offers the advantage of minimizing the effects of disorder, as the tuning of the quantum dot array can be corrected by means of electrostatic gating and applied magnetic fields. In addition, the entire evolution of the trivial subgap states into Majorana modes can be followed during the tuning, thus providing an unambiguous demonstration of their realization. The main objectives of the project are: to study the hybridization of Shiba states into molecular levels, to obtain robust signatures of Majorana modes in a triple quantum dot geometry, to study the properties of the detected subgap states, and to scale to longer and non-linear arrays.
Another important achievement concerns the sample preparation, which is a crucial part of the project. In order to prepare the envisioned hybrid devices, wherein low-dimensional semiconductors are coupled to superconductors, we employ state-of-the-art nanofabrication techniques, such as e-beam lithography and metal deposition techniques. These are needed to achieve the nanometer-scale features required in our devices. My research group and I have established a full process flow for the fabrication of hybrid devices based on semiconductor nanowires, as well as many of the processes required for the fabrication of devices based on two-dimensional electron gases. These developments put us in a good position to perform the experiments planned in the project.
While setting up the new lab space and the nanofabrication in the host institution, our scientific output benefited from internal and external local collaborations. This includes a published article reporting on a joint theoretical and experimental study of replicas of bound states in nanowire devices, and a review article discussing trivial and topological sub-gap states in hybrid nanowires. The results have been disseminated in international scientific conferences and workshops. In addition, I have participated in the organization of a Summer School on Quantum Transport in Topological Materials. The school was a great success, having been attended by around 100 students and researchers from around the world.