A ferroelectric is a material with a polar structure whose polarity can be reversed (switched) by applying an electric field. In metals, itinerant electrons screen electrostatic forces between ions, which explains in part why polar metals are very rare. Screening also excludes external electric fields, apparently ruling out the possibility of ferroelectric switching. However, in principle, a thin enough polar metal could be sufficiently penetrated by an electric field to have its polarity switched. Here we show that the topological semimetal WTe provides an embodiment of this principle. Although monolayer WTe is centro-symmetric and thus non-polar, the stacked bulk structure is polar. We find that two- or three-layer WTe exhibits spontaneous out-of-plane electric polarization that can be switched using gate electrodes. We directly detect and quantify the polarization using graphene as an electric-field sensor. Moreover, the polarization states can be differentiated by conductivity and the carrier density can be varied to modify the properties. The temperature at which polarization vanishes is above 350 kelvin, and even when WTe is sandwiched between graphene layers it retains its switching capability at room temperature, demonstrating a robustness suitable for applications in combination with other two-dimensional materials.

A two-dimensional topological insulator (2DTI) is guaranteed to have a helical 1D edge mode 1-11 in which spin is locked to momentum, producing the quantum spin Hall effect and prohibiting elastic backscattering at zero magnetic field. No monolayer material has yet been shown to be a 2DTI, but recently the Weyl semimetal WTe 2 was predicted 12 to become a 2DTI in monolayer form if a bulk gap opens. Here, we report that at temperatures below about 100 K monolayer WTe 2 does become insulating in its interior, while the edges still conduct. The edge conduction is strongly suppressed by in-plane magnetic field and is independent of gate voltage, save for mesoscopic fluctuations that grow on cooling due to a zero-bias anomaly which reduces the linear-response conductance. Bilayer WTe 2 also becomes insulating at low temperatures but does not show edge conduction. Many of these observations are consistent with monolayer WTe 2 being a 2DTI. However, the low temperature edge conductance, for contacts spacings down to 150 nm, is below the quantized value, at odds with the prediction that elastic scattering is completely absent in the helical edge.Experimental work on 2DTIs to date has focused on quantum wells in Hg/CdTe 4-7 and InAs/GaSb 9-11 designed to achieve an inverted band gap. These heterostructures show edge conduction as anticipated 13,14 , but they also present some puzzles. One is that the conductance at low temperatures is not perfectly quantized, becoming small in long edges 13 and showing mesoscopic fluctuations as a function of gate voltage 5,7,10 . This is inconsistent with the predicted absence of elastic backscattering at zero magnetic field, although several possible explanations have been put forward for the discrepancy [15][16][17][18][19][20] . Another is that the edges show signs of conducting even at high magnetic field 21,22 , contrary to expectations that helical modes, protected by timereversal (TR) symmetry at zero field, should Anderson-localize once this symmetry is broken. An additional complication is that non-helical edge conduction may also be present, due for instance to band bending when a gate voltage is applied 23 .Identification of a natural monolayer 2DTI, which lacked some of these discrepancies and which could be probed, manipulated, and coupled with other materials more easily than quantum wells, would be helpful for elucidating and exploiting TI physics. Band structure calculations predict that certain monolayer materials are intrinsically topologically nontrivial 12 . An example is monolayer WTe2, which has the T′ structure illustrated in Fig. 1a. Three-dimensional WTe2, in which such monolayers are stacked in the orthorhombic Td structure, has recently attracted attention as a type-II Weyl semimetal 24,25 that exhibits extreme non-saturating magnetoresistance 26,27 related to the closely balanced electron and hole densities [28][29][30] . Calculations suggest that the monolayer will be likewise a semimetal 12,30 , its Fermi surface comprising two electron pockets (green) an...

The layered semimetal WTe2 has recently been found to be a two-dimensional topological insulator (2D TI) when thinned down to a single monolayer, with conducting helical edge channels. We report here that intrinsic superconductivity can be induced in this monolayer 2D TI by mild electrostatic doping, at temperatures below 1 K. The 2D TI-superconductor transition can be easily driven by applying a just a small gate voltage. This discovery offers new possibilities for gatecontrolled devices combining superconductivity and topology, and could provide a basis for quantum information schemes based on topological protection. Main text:Many of the most important, and fascinating, phenomena in condensed matter emerge from the quantum mechanics of electrons in a lattice. The periodic potential of the lattice gives rise to Bloch energy bands, and so to the physics of semiconductors that underlies all modern-day electronics. On the more exotic side, electrons in a lattice can pair up to act as bosons and condense into a macroscopic quantum state conducting electricity with zero resistance. More recently, it was realized that Bloch wavefunctions can have a non-trivial topology, incorporating twists analogous to a Möbius strip. This led to the discovery of topological insulators-materials that are electrically insulating in their interior but have conducting boundary modes that result from the topological discontinuity between inside and outside(1). The first of these to be studied was the so-called 2D topological insulator (2D TI), in which the one-dimensional helical edge modes (spin locked to momentum) give rise to the quantum spin Hall effect(2-4).Materials that combine non-trivial topology with superconductivity have been the subject of active investigation in recent years. For example, hybrid structures that couple an s-wave superconductor to a 2D TI have also been proposed as platform for Majorana modes(5), whose non-abelian exchange properties might be harnessed for qubits(6) with coherence times far longer than those built on conventional platforms. There are also topological superconductors, in which vortices or boundaries can host Majorana modes(7).Here we report the remarkable finding that monolayer WTe2, recently shown(8-13) to be an intrinsic 2D TI, itself turns superconducting under moderate electrostatic gating. Several other non-topological layered materials superconduct in the monolayer limit, either intrinsically or under heavy doping using ionic liquid gates(14-22). However, the present case constitutes the first instance of a phase transition from a 2D topological insulator to a superconductor, which moreover is readily controlled by a gate voltage. The discovery creates new opportunities for gateable superconducting circuitry, and offers the potential to develop topological superconducting devices in a single material, as opposed to the hybrid constructions currently required.

Contact angle measurements and scanning force microscopy (SFM) have been used to study the morphology of self-assembled monolayers of octadecyltrichlorosilane (OTS) and undecyltrichlorosilane (UTS) on silicon oxide surfaces as a function of annealing temperature. Characterization of the monolayers before annealing indicates that the as-formed monolayers cover the substrate fully and have thicknesses of 2.9 and 1.5 nm, respectively, with the average surface having a roughness of approximately 0.3 nm. Water contact angle results and SFM roughness analysis showed that UTS monolayers annealed for 2 h at temperatures over 125 °C exhibited permanent changes in monolayer structure. Experimental data indicated a higher transition temperature for permanent structural changes for the OTS monolayers. Water contact angle measurements indicated a transition centered at 125 °C, hexadecane contact angle measurements indicated a transition around 130 °C, and SFM indicated a transition centered at 155 °C for OTS monolayers. We have observed that contact angle measurements can be sensitive to surface structure on an angstrom length scale. These results observed after 2 h are not equilibrium results, as evidenced by the further development in the monolayer after an anneal for 5 h. Thermodynamic considerations support the earlier (lower temperature) changes observed for UTS than for OTS monolayers. The observed transition is likely the result of hydrolysis of the organic molecules by water molecules existing at the monolayer-substrate interface.