The quantum spin Hall (QSH) state was recently demonstrated in monolayers of the transition metal dichalcogenide 1T'-WTe 2 and is characterized by a band gap in the two-dimensional (2D) interior and helical onedimensional (1D) edge states [1][2][3]. Inducing superconductivity in the helical edge states would result in a 1D topological superconductor, a highly sought-after state of matter [4]. In the present study, we use a novel dry-transfer flip technique to place atomically-thin layers of WTe 2 on a van der Waals superconductor, NbSe 2 . Using scanning tunneling microscopy and spectroscopy (STM/STS), we demonstrate atomically clean surfaces and interfaces and the presence of a proximity-induced superconducting gap in the WTe 2 for thicknesses from a monolayer up to 7 crystalline layers. At the edge of the WTe 2 monolayer, we show that the superconducting gap coexists with the characteristic spectroscopic signature of the QSH edge state. Taken together, these observations provide conclusive evidence for proximity-induced superconductivity in the QSH edge state in WTe 2 , a crucial step towards realizing 1D topological superconductivity and Majorana bound states in this van der Waals material platform.Contemporary interest in topological superconductors has been driven by potential applications of their gapless boundary excitations, which are thought to be emergent Majorana quasiparticles with non-abelian statistics [5][6][7][8]. One path toward topological superconductivity is to realize an intrinsic spinless p-wave superconductor [9]. A powerful alternative is by using a conventional s-wave superconductor to induce Cooper pairing in topologically non-trivial states via the superconducting proximity effect, resulting in an effective pwave pairing [10]. This approach has recently been employed to engineer 2D topological superconductivity in epitaxial three-dimensional topological insulator films grown on a superconducting substrate [11,12], and 1D topological superconductivity by proximitizing a 2D QSH system in buried epitaxial semiconductor quantum wells [13,14]. While such demonstrations mark important milestones, there are clear advantages for exploring topological superconductivity in the van der Waals material platform. Using layered 2D materials allows the 2D QSH edge to be proximitized in vertical heterostructures, circumventing the length restrictions of lateral proximity-effect geometries. Furthermore, the surfaces and edges are readily available for surface probes, allowing detection and fundamental study of signatures of the topological superconducting state. Following recent theoretical predictions [15], an intrinsic QSH state was demonstrated in a monolayer (ML) of 1T'-WTe 2 [1][2][3][16][17][18]. WTe 2 is attractive for studying the QSH edge modes because it can be readily incorporated in van der Waals heterostructures and has shown quantized edge conductance up to 100 K [3]. Furthermore, ML WTe 2 was recently also shown to host intrinsic superconducting behavior below ∼1 K when electrostatically g...

In scanning tunneling microscopy, we witness in recent years a paradigm shift from "just imaging" to detailed spectroscopic measurements at the nanoscale and multi-tip scanning tunneling microscope (STM) is a technique following this trend. It is capable of performing nanoscale charge transport measurements like a "multimeter at the nanoscale." Distance-dependent four-point measurements, the acquisition of nanoscale potential maps at current carrying nanostructures and surfaces, as well as the acquisition of I − V curves of nanoelectronic devices are examples of the capabilities of the multi-tip STM technique. In this review, we focus on two aspects: How to perform the multi-tip STM measurements and how to analyze the acquired data in order to gain insight into nanoscale charge transport processes for a variety of samples. We further discuss specifics of the electronics for multi-tip STM and the properties of tips for multi-tip STM, and present methods for a tip approach to nanostructures on insulating substrates. We introduce methods on how to extract the conductivity/resistivity for mixed 2D/3D systems from four-point measurements, how to measure the conductivity of 2D sheets, and how to introduce scanning tunneling potentiometry measurements with a multi-tip setup. For the example of multi-tip measurements at freestanding vapor liquid solid grown nanowires, we discuss contact resistances as well as the influence of the presence of the probing tips on the four point measurements.

We present a multi-tip scanning tunneling potentiometry technique that can be implemented into existing multi-tip scanning tunneling microscopes without installation of additional hardware. The resulting setup allows flexible in situ contacting of samples under UHV conditions and subsequent measurement of the sample topography and local electric potential with resolution down to Å and μV, respectively. The performance of the potentiometry feedback is demonstrated by thermovoltage measurements on the Ag/Si(111)−(3×3)R30∘ surface by resolving a standing wave pattern. Subsequently, the ability to map the local transport field as a result of a lateral current through the sample surface is shown on Ag/Si(111)−(3×3)R30∘ and Si(111) − (7 × 7) surfaces.