Kondo Effect in Defect-bound Quantum Dots Coupled to NbSe2_2

We report the fabrication of a van der Waals tunneling device hosting a defect-bound quantum dot coupled to NbSe$_2$. We find that upon application of magnetic field, the device exhibits a zero-bias conductance peak. The peak, which splits at higher fields, is associated with a Kondo effect. At the same time, the junction retains conventional quasiparticle tunneling features at finite bias. Such coexistence of a superconducting gap and a Kondo effect are unusual, and are explained by noting the two-gap nature of the superconducting state of NbSe$_2$, where a magnetic field suppresses the low energy gap associated with the Se band. Our data shows that van der Waals architectures, and defect-bound dots in them, can serve as a novel and effective platform for investigating the interplay of Kondo screening and superconducting pairing in unconventional superconductors

Spectroscopy of bulk and few-layer superconducting NbSe2_2 with van der Waals tunnel junctions

Tunnel junctions, a well-established platform for high-resolution spectroscopy of superconductors, require defect-free insulating barriers with clean engagement to metals on both sides. Extending the range of materials accessible to tunnel junction fabrication, beyond the limited selection which allows high-quality oxide formation, requires the development of alternative fabrication techniques. Here we show that van-der-Waals (vdW) tunnel barriers, fabricated by stacking layered semiconductors on top of the transition metal dichalcogenide (TMD) superconductor NbSe$_2$, sustain a stable, low noise tunneling current, and exhibit strong suppression of sub-gap tunneling. We utilize the technique to measure the spectra of bulk (20 nm) and ultrathin (3- and 4-layer) devices at 70 mK. The spectra exhibit two distinct energy gaps, the larger of which decreases monotonously with thickness and $T_C$, in agreement with BCS theory. The spectra are analyzed using a two-band model modified to account for depairing. We show that in the bulk, the smaller gap exhibits strong depairing in an in-plane magnetic field, consistent with a high Fermi velocity. In the few-layer devices, depairing of the large gap is negligible, consistent with out-of-plane spin-locking due to Ising spin-orbit coupling. Our results demonstrate the utility of vdW tunnel junctions in mapping the intricate spectral evolution of TMD superconductors over a range of magnetic fields.Comment: This submission contains the first part of arxiv:1703.07677 with the addition of spectra taken on this devices. The second part of 1703.07677 will be published separatel

High magnetic field stability in a planar graphene-NbSe2_2 SQUID

Thin NbSe$_2$ retains superconductivity at high in-plane magnetic field up to 30 T. In this work we construct an atomically thin, all van der Waals SQUID, in which current flows between NbSe$_2$ contacts through two parallel graphene weak links. This fully planar device remains uniquely stable at high in-plane field. This enables tracing the evolution of the critical current interference patterns as a function of the field up to 4.5 T, allowing nm-scale sensitivity to deviations from a perfect atomic plane. We present numerical methods to retrieve asymmetric current distributions J$_0$ from measured interference maps, and suggest a new application of the dual junction geometry to probe the current density in the absence of phase information. The interference maps exhibit a striking field-driven transition, indicating a redistribution of supercurrents to narrow channels. Our results suggest the existence of a preferred conductance channel with an exceptional stability to in-plane magnetic field

Tunneling in graphene-topological insulator hybrid devices

Hybrid graphene-topological insulator (TI) devices were fabricated using a mechanical transfer method and studied via electronic transport. Devices consisting of bilayer graphene (BLG) under the TI Bi$_2$Se$_3$ exhibit differential conductance characteristics which appear to be dominated by tunneling, roughly reproducing the Bi$_2$Se$_3$ density of states. Similar results were obtained for BLG on top of Bi$_2$Se$_3$, with 10-fold greater conductance consistent with a larger contact area due to better surface conformity. The devices further show evidence of inelastic phonon-assisted tunneling processes involving both Bi$_2$Se$_3$ and graphene phonons. These processes favor phonons which compensate for momentum mismatch between the TI $\Gamma$ and graphene $K, K'$ points. Finally, the utility of these tunnel junctions is demonstrated on a density-tunable BLG device, where the charge-neutrality point is traced along the energy-density trajectory. This trajectory is used as a measure of the ground-state density of states