QDev-All meeting (Open to all students) – University of Copenhagen

QDev-All meeting (Open to all students)

On Friday, the 5th of October, our monthly QDev-all meetings will be held in Auditorium 3 at 14.00 with the following program:

 Sabbir Khan

Controlled Synthesis of Large Scale Semiconductor Nanowire Networks

Group III-V compound-based semiconductor nanowire networks are promising material platform for quantum applications and also can be worthwhile for the electronics ranging from photovoltaics to highly efficient memory devices. However, the scalable growth of in-situ high crystal quality nanowire networks is still challenging.

In this talk, I will demonstrate high quality large scale in-situ semiconductor nanowire networks for quantum transport measurement. Here, I will present controlled synthesis of InAs/InSb nanowire networks using molecular beam epitaxy (MBE) from (111)B faceted trenches. 

 

Juan Carlos Estrada Saldana

The ground state of a double quantum dot with superconducting leads

Hybrid nanowire devices -composed of a nanowire in close contact to a superconductor (S)- have emerged as flexible platforms for Majorana bound states generation and detection, holding the promise of topologically-protected quantum computation1. This requires a nanowire with ballistic transport over several micrometers. However, it has been proposed that a chain of quantum dots with superconducting coupling could also map the Majorana Hamiltonian, relaxing the condition of a localization-free material2.

We study transport through the simplest quantum dot chain, namely two dots in series (DQD) electrostatically defined in the channel of a semiconducting InAs nanowire with epitaxial hard-gap superconducting Al contacts3. Yu-Shiba-Rusinov/Andreev bound states -a precursor of Majorana bound states- were described recently in an S-DQD-N geometry4 (N: normal metal) and in an effective S-DQD-N system5 (due to a soft gap). However, a genuine S-DQD-S system remains to be explored. Interestingly, the latter system can support the passage of Cooper pairs through the Josephson effect, as we observe in our device.

We map the supercurrent and the subgap states in the nanowire as a function of the charge occupation in each of the dots and of the coupling to the superconducting leads, finding a rich phase diagram. Changes in the ground state of the system are seen as discontinuities in the critical current of the junction, which can be attributed to 0 to π phase transitions6,7.

Besides its interest in the field of Majorana modes, our two-dot system constitutes a testbed for novel correlated ground states in the superconducting regime.

References 

1 R. M. Lutchyn et alNature Rev. Mat. 3 (2018), 52-68.

J. D. Sau and S. Das Sarma, Nature Comm3 (2012), 964

P. Krogstrup et al, Nature Materials 14 (2015), 400–406 

K. Grove-Rasmussen et alNature Comm. 9(1) (2018), 2376

Z. Su et alNature Comm. 8 (2017), 585

H. I. Jørgensen et alNano Lett. 7(8) (2007), 2441–2445

7 J.C. Estrada Saldaña et al, arXiv:1808.05837 (2018).

To see the schedule and more details about the talks, please visit: https://qdev.nbi.ku.dk/qdev-all-group-meetings/