Tunneling into the Vortex State of NbSe₂ with van der Waals Junctions

Abstract

We have performed device-based tunnelling spectroscopy of NbSe 2 in the vortex state with a magnetic field applied both parallel and perpendicular to the a − b plane. Our devices consist of layered semiconductors placed on top of exfoliated NbSe 2 using the van der Waals transfer technique. At zero field, the spectrum exhibits a hard gap, and the quasiparticle peak is split into low and high energy features. The two features, associated with the effective two-band nature of superconductivity in NbSe 2 , exhibit markedly distinct responses to the application of magnetic field, suggesting an order-of-magnitude difference in the spatial extent of the vortex cores of the two bands. At energies below the superconducting gap, the hard gap gives way to vortex-bound Caroli-de Gennes-Matricon states, allowing the detection of individual vortices as they enter and exit the junction. Analysis of the sub-gap spectra upon application of parallel magnetic field allows us to track the process of vortex surface formation and spatial rearrangement in the bulk.

Zeeman Tunability of Andreev Bound States in van der Waals Tunnel Barriers

“…22 To overcome this issue, we recall that few layer NbSe 2 has strong spin-orbit coupling, thus application of in-plane magnetic fields has negligible effect on the spectrum of such superconductors. 21 Figure 2b This observation lends support to the claim that the zero field ground state of the proximitized dot is the singlet state. This was previously observed in quantum dots formed at the edges of nano-wires and carbon nano-tubes, with careful control over E 0 using dielectric gate.…”

FeTe0.55Se0.45 van der Waals tunneling devices

“…A planar junction measures a spatial average of the spectrum over the region of the junction, which may encompass a number of vortices which should increase with field. In NbSe 2 , where ∆/E F 1, and CdGM states are densely spaced, the spatially averaged spectrum over a unit cell of the Abrikosov lattice exhibits a sub-gap spectrum which changes from a 'U' shape to a 'V' shape with increasing field [34,20]. In our measurements, in contrast, a small hard gap persists to high B in both orientations.…”

“…Refining exfoliation-based methods is of particular interest, as flakes can be assembled into a variety of sophisticated devices using the van-der-Waals (vdW) technology [15,16,17]. When applied to proximitized graphene and to layered superconductors, stacking of vdW flakes has allowed the study of planar tunnel junctions [18,19,20,9,21], Josephson devices [22,23], and graphene-superconductor hybrids [24,25,26]. Furthermore, in thin flakes density can be tuned by ionic gating [27,28].…”

Van Der Waals Heterostructures with Spin‐Orbit Coupling

“…For a range of angles, , around 0° (and multiples of 60°) the Dirac points intersect the K (or ) Fermi pocket of NbSe 2 , and for a range of angles, , around 21.9° (and multiples of 60°) the Dirac points intersect the Γ Fermi pocket of NbSe 2 . As a consequence, graphene can be used to probe the differences between the electronic states of the different Fermi pockets of NbSe 2 , including properties of the superconducting pairing …”

“…As a consequence, graphene can be used to probe the differences between the electronic states of the different Fermi pockets of NbSe 2 , including properties of the superconducting pairing. [206] Ab initio calculations [205] show that when placed on NbSe 2 graphene becomes hole doped, Figure 10b, so that the Fermi energy in the graphene layer is ≈ 400 meV below the Dirac point. These calculations also show [205] that the interlayer tunneling between the two systems is of the order of 20 meV and that this value, and the amount of charge transfer do not depend significantly on the twist angle.…”