Mirage Andreev Spectra Generated by Mesoscopic Leads in Nanowire Quantum Dots

Abstract

We study transport mediated by Andreev bound states formed in InSb nanowire quantum dots. Two kinds of superconducting source and drain contacts are used: epitaxial Al/InSb devices exhibit a doubling of tunneling resonances, while in NbTiN/InSb devices Andreev spectra of the dot appear to be replicated multiple times at increasing source-drain bias voltages. In both devices, a mirage of a crowded spectrum is created. To describe the observations a model is developed that combines the effects of a soft induced gap and of additional Andreev bound states both in the quantum dot and in the finite regions of the nanowire adjacent to the quantum dot. Understanding of Andreev spectroscopy is important for the correct interpretation of Majorana experiments done on the same structures.The superconductor-semiconductor hybrid structures are of recent interest due to the possibility of inducing topological superconductivity accompanied by Majorana bound states (MBS) [1][2][3][4]. More generally, when a semiconductor is of finite size, proximity to a superconductor gives rise to subgap quasiparticle excitations, the so-called Andreev bound states (ABS), that appear due to successive Andreev reflections at the interfaces. Single ABS have been demonstrated in a variety of structures including self-assembled quantum dots, semiconductor nanowires, atomic break junctions, carbon nanotubes and graphene [5][6][7][8][9][10][11]. ABS exhibit many similarities to MBS, and therefore ABS can be served as a prototypical system for Majorana studies [12,13]. A powerful experimental method for investigating both MBS and ABS is via tunneling, either from a nanofabricated probe or by scanning tunneling spectroscopy. The latter is typically performed on Yu-Shiba-Rusinov states which are closely analogous to ABS but originate from magnetic impurities in superconductors [14,15].In this paper, we focus on the mesoscopic effects within the tunneling probes. We show that the non-trivial densities of states (DOS) in the probes can drastically affect tunneling characteristics by generating multiple replicas of ABS. To experimentally investigate these effects, we use semiconductor nanowires coupled to superconductors. ABS are induced in a quantum dot by strongly coupling the dot to one superconducting contact. A second superconducting contact and a nanowire segment ad-jacent to it act as a tunneling probe. To explain our observations, we consider the effects of soft induced superconducting gap in the nanowire, and of additional ABS induced in nanowire segments adjacent to the dot. The surprising observation of sub-gap negative differential conductance (NDC) is found to be consistent with a peak in the density of states of the probe at zero chemical potential, which is present even at zero magnetic field. The exact origin of this anomalous density of states remains an open question. Our findings emphasize the importance of understanding the spectral structure of the measuring contacts to interpret tunneling experiments in mesoscopic systems. We expect t...

Two-impurity Yu-Shiba-Rusinov states in coupled quantum dots

“…These indicate that a gapped and peaked density of states in one lead is probing its counterpart in the other lead [15]. Apparent replicas of the lowest pair of peaks may be related to our hybrid nanowire/superconductor leads [15,45,46]. A charge-independent singlet GS in both panels is deduced from the absence of peak crossings [19,30] of the pair of states at lowest bias, in agreement with the I c gate dependence in Fig.…”

“…It also reproduces NDC and the order of magnitude of dI/dV sd . However, the model does not take into account multiple Andreev reflection [49], the AC Josephson effect, hybrid leads [15,45,46], and higher-order processes (the two sides are decoupled), and it is therefore unable to reflect all the details of the measurement.…”

Universal Platform for Scalable Semiconductor‐Superconductor Nanowire Networks

“…Next to combining normal metals with superconductors, this approach can also be used to combine two different superconductors for sensitive tunnel probing of the density of states (see Supporting Information 8). [41,42] Shadow deposition is currently the only known way to create combinations of ultra-high vacuum deposited superconductors without involving processing. Alternatives would require either the ability to selectively etch one superconductor without damaging the other or the use of lift-off processes, leading to potential material contamination.…”

Universal Platform for Scalable Semiconductor-Superconductor Nanowire Networks

“…Next to combining normal metals with superconductors, this approach can also be used to combine two different superconductors for sensitive tunnel probing of the density of states (see Supplementary Information 8). 39,40 Shadow deposition is currently the only known way to create combinations of ultra-high vacuum deposited superconductors without involving processing. Alternatives would require either the ability to selectively etch one superconductor without damaging the other or the use of lift-off processes, leading to potential material contamination.…”