XXXVI Reunión Bienal de la Real Sociedad Española de Física

Andreev levels in hybrid superconductor-semiconductor nanowire quantum dots: Energy scaling and spin texture

19 Jul 2017, 15:15

30m

Aula Química Inorgánica (Facultad de Química (USC))

Quantum Materials and Technologies (GEFES) Quantum Materials and Technologies (GEFES) II

Description

The interaction of a magnetic impurity and a superconductor yields localized states known as Andreev levels or Yu-Shiba-Rusinov (YSR) states. Recently, there has been a growing interest in this type of system, in a large part, due to theoretical work that suggests that Andreev levels are precursors of Majorana zero modes. Accordingly, it has been proposed that chains of such impurities could be engineered, under appropriate conditions, into a topological superconductor [1-9]. A semiconductor quantum dot coupled to a superconductor constitutes a versatile platform to investigate, in a controllable and quantitative manner, the physics of the corresponding single-impurity limit. Here, we have employed single quantum dots defined in an InAs nanowire, coupled strongly to a superconductor and weakly to a normal metal probe, to study the Andreev levels by tunneling spectroscopy. First, we exploit the ability to tune the hybridization of the quantum dot and the superconductor to quantitatively investigate the energy scaling of Andreev levels. We demonstrate that the energy of such sub-gap excitations scales with the ratio of the Kondo temperature and the superconducting gap. We further leverage the electrical control over device parameters to obtain an experimental phase diagram of the possible ground states: a spin singlet or a magnetic doublet. Our experimental results show remarkable quantitative agreement with numerical renormalization group calculations [10]. In parallel, we have studied the spin texture of the Andreev levels in the presence of an external magnetic field. We demonstrate that the Zeeman effect results in a splitting of the sub-gap states only when the ground state is a spin singlet. In this case, the applied magnetic field can also lead to a quantum phase transition to a spin-polarized ground state [11]. The herein demonstrated electrical tuning of Andreev levels as well as their spin-polarization could be harnessed to pursue proposals of realizing a topological superconductor using quantum dot arrays [7-9]. [1] S. Nadj-Perge et al., Phys. Rev. B 88, 020407(R) (2013). [2] K. Klinovaja et al., Phys. Rev. Lett. 111, 186805 (2013). [3] M. M. Vazifeh and M. Franz, Phys. Rev. Lett. 111, 206802 (2013). [4] B. Braunecker and P. Simon, Phys. Rev. Lett. 111, 147202 (2013). [5] F. Pientka et al., Phys. Rev. B 88, 155420 (2013). [6] S. Nadj-Perge et al., Science 346, 602 (2014) [7] J. D. Sau and S. Das Sarma, Nat. Commun. 3, 964 (2012). [8] M. Leijnse and K. Flensberg, Phys. Rev. B 86, 134528 (2012) [9] I. C. Fulga et al., New J. Phys. 15, 045020 (2013). [10] Lee et al., to appear in Phys. Rev. B – Rapid Commun. [11] Lee et al., Nat. Nanotechnol. 9, 79 (2014).