Steinberg​Lab

Van-der-Waals Tunneling

The vdW transfer technique offers a unique opportunity to expand the range of materials available for tunneling devices. TMDs and hBN can be exfoliated to few-layer thickness, and deployed at will on a material of choice. The resulting system is a tunneling device of amazing versatility. We set up a transfer apparatus in a glove-box, which allows work with materials which tend to oxidize in ambient conditions. We fabricate tunneling devices involving superconductors (NbSe2), topological insulators, and graphene. 
Our recent results show that all-TMD tunnel devices exhibit a hard gap, with the zero-bias signal suppressed by a factor of 500 from the normal conductance. This property attests to the high quality of the junction and will be crucial for future experiments probing sub-gap states. Indeed, when the perpendicular magnetic field is applied, we observe a "V" shaped dI/dV signal, consistent with the onset tunneling into vortex-bound states. 

Dot-assisted tunneling

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We use single defect sin a tunnel barriers as highly sensitive sensors. The experimental system is based on a van-der-Waals (vdW) tunnel junction consisting of a few-layer barrier. The barrier could be hexagonal Boron Nitride (hBN), or a TMD. The key element in our method is the use of defects, hosted in the barrier, as atomic-sized spectral probes. We show that dot-assisted transport provides sharp, stable spectral features found when the dot energy is resonant with the energies in the source and drain.

Defect dots are atomically sized and reside about approximately 1 nm away from the source and drain layers. As a result, the dot tunnel-couples to local regions. At the same time, unlike metallic tips used in STM, the dot cannot undergo spatial charge rearrangements and hence does not screen interactions on the local scale. In this sense, the dot can be considered as a “minimally invasive” probe.

We use this technique for probing graphene – see e.g. our recent paper, where we measure the graphene Landau level spectra up to 33 T and find a clear signature degeneracy lifting.

​We also use the same method for probing the spectrum of NbSe2 (coming soon) where we find that defect-bound dots are highly sensitive to the low-energy gap.

2D Josephson Junctions

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We have developed the two-dimensional Josephson junction (2DJJ) – an all vdW device -consisting of both normal and superconductor exfoliated vdW materials. Devices are made by seeking NbSe2 flakes which present cracks of 200-500 nm width. We subsequently transfer these flakes on top of graphene flakes, which give rise to well-behaved Josephson junctions. A key feature in the functionality of this devices is the reliance on the Ising spin-orbit protection of the thin NbSe2, which can sustain high in-plane magnetic fields with minimal changes to the superconducting properties. As a result, we are able to measure the device at very high in-plane magnetic fields.

The immediate consequence of the use of thin NbSe2 is that the 2DJJ devices can sustain supercurrent up to an in-plane magnetic field of 8.5 T. At such high fields, superconductivity is limited by the combined effects of spin, and of minimal deviations from a planar geometry. In our recent publication, we show that in some cases, critical current exhibits a suppression-recovery feature. We discuss how this feature is related to possible 0- transition, or, alternatively, to effects ripples. The effect of ripples is also discussed in the work of the Linder group.