Multiband superconductivity in strongly hybridized 1T′−WTe2/NbSe2 heterostructures

Abstract

The interplay of topology and superconductivity has become a subject of intense research in condensed matter physics for the pursuit of topologically non-trivial forms of superconducting pairing. An intrinsically normal-conducting material can inherit superconductivity via electrical contact to a parent superconductor via the proximity effect, usually understood as Andreev reflection at the interface between the distinct electronic structures of two separate conductors. However, at high interface transparency, strong coupling inevitably leads to changes in the band structure, locally, owing to hybridization of electronic states. Here, we investigate such strongly proximity-coupled heterostructures of monolayer 1T'-WTe2, grown on NbSe2 by van-der-Waals epitaxy. The superconducting local density of states (LDOS), resolved in scanning tunneling spectroscopy down to 500 mK, reflects a hybrid electronic structure, well-described by a multi-band framework based on the McMillan equations which captures the multi-band superconductivity inherent to the NbSe2 substrate and that induced by proximity in WTe2, self-consistently. Our material-specific tight-binding model captures the hybridized heterostructure quantitatively, and confirms that strong inter-layer hopping gives rise to a semi-metallic density of states in the 2D WTe2 bulk, even for nominally band-insulating crystals. The model further accurately predicts the measured order parameter ∆ 0.6 meV induced in the WTe2 monolayer bulk, stable beyond a 2 T magnetic field. We believe that our detailed multi-band analysis of the hybrid electronic structure provides a useful tool for sensitive spatial mapping of induced order parameters in proximitized atomically thin topological materials.

Moiré Enhanced Two-Band Superconductivity in a MnTe/NbSe2 Heterojunction

“…We further checked the magnetic response of the two-gap superconductivity by applying a perpendicular magnetic field. As shown in Figure a, we find that the subgap feature gradually becomes dissipated at 0.2 T for MnTe, while the energy positions of coherence peaks only weakly depend on the magnetic field and survive up to 2.3 T for both layers. This indicates that the subgap is extremely fragile far below the upper critical magnetic field of NbSe 2 (∼5 T), qualitatively in agreement with the magnetic signature of two-band superconductivity. , The quick suppression of subgap can be understood from the 3D characters of NbSe 2 band structure.…”

“…Still, the tunneling selectivity effect is completely excluded here, because our experiments are always performed along the nominal c -axis. A third possibility regarding the hybridization between the bands of NbSe 2 and MnTe around E F , as manifested in previous MnSe/NbSe 2 , WTe 2 /NbSe 2 and Fe chains on NbSe 2 , is also unlikely. This lies in the fact that our MnTe film can be only regarded as a tunneling barrier of a 1.8 eV insulating gap.…”

“…It is in contrast to the WTe 2 /NbSe 2 heterojunction, where the electronic structure of WTe 2 is strongly modified from insulating to metallic by the hybridized interface. 45 The preserved band gap naturally questions the possibility of potential magnetic characteristics of MnTe. 38 We thus perform high resolution superconducting dI/dV spectra on both NbSe 2 and MnTe surfaces at 0.4 K, respectively, as displayed in Figure 2a, b.…”

“…This confirms the insulating property of MnTe, excluding the possibility of band hybridization with NbSe 2 around the Fermi surface (FS). It is in contrast to the WTe 2 /NbSe 2 heterojunction, where the electronic structure of WTe 2 is strongly modified from insulating to metallic by the hybridized interface …”

Theory of Glide Symmetry Protected Helical Edge States in WTe$_{2}$ Monolayer

“…Scanning tunneling microscopy/spectroscopy also revealed the existence of states on the boundaries [53,[56][57][58], consistent with a QSH edge state scenario. Furthermore, various spectral features of a bulk gap have been detected such as the Coulomb gap at the Fermi energy [59], NbSe 2 proximity-induced superconducting gap [60,61], straininduced gap [62], and CrI 3 antiferromagnet exchangefield gap [63]. As the electron concentration is increased, transitions to metallic and superconducting bulk states were observed [64,65].…”