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Artem Kononov, Gulibusitan Abulizi, Kejian Qu, Jiaqiang Yan, David Mandrus, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Christian Schönenberger

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[One-Dimensional Edge Transport in Few-Layer WTe<sub>2</sub>](https://mdr.nims.go.jp/datasets/252a3d03-b581-4b5b-9693-0ca8dc48bdb8)

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One-Dimensional Edge Transport in Few-Layer WTe2One-Dimensional Edge Transport in Few-Layer WTe2Artem Kononov,* Gulibusitan Abulizi, Kejian Qu, Jiaqiang Yan, David Mandrus, Kenji Watanabe,Takashi Taniguchi, and Christian Schönenberger*Cite This: Nano Lett. 2020, 20, 4228−4233 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: WTe2 is a layered transitional-metal dichalcogenide(TMD) with a number of intriguing topological properties.Recently, WTe2 has been predicted to be a higher-order topologicalinsulator (HOTI) with topologically protected hinge states alongthe edges. The gapless nature of WTe2 complicates the observationof one-dimensional (1D) topological states in transport due totheir small contribution relative to the bulk. Here, we study thebehavior of the Josephson effect in magnetic field to distinguishedge from bulk transport. The Josephson effect in few-layer WTe2reveals 1D states residing on the edges and steps. Moreover, ourdata demonstrates a combination of Josephson transport propertiesobserved solely in another HOTIbismuth, including Josephsontransport over micrometer distances, extreme robustness in amagnetic field, and nonsinusoidal current-phase relation (CPR). Our observations strongly suggest the topological origin of the 1Dstates and that few-layer WTe2 is a HOTI.KEYWORDS: WTe2, 1D edge states, Josephson effect, nonsinusoidal CPR, higher order topological insulatorsMaterials with nontrivial topology attract a lot of attentiondue to their intriguing properties and the potential toharness them for quantum computing. Nonabelian excitations,occurring when topology meets superconductivity, areespecially interesting for applications.1 Many realizations ofthese excitations have been proposed and implementedrecently, including designing topological superconductivity bycombining spin−orbit interaction and Zeeman effect withnormal s-wave superconductors,2 or by proximity inducingsuperconductivity in topological insulators.3 Recently, it hasalso been demonstrated that one can engineer them in hingestates of a higher-order topological insulator (HOTI)combined with proximity induced superconductivity.4 Thelayered TMD WTe2, which in the form of a 3D crystal is aWeyl semimetal5,6 and a 2D topological insulator in the formof a monolayer,7,8 has been predicted to be a HOTI,9 hostingtopological hinge states on the edges and steps of the crystal.However, the bulk conductivity of WTe2 complicates theobservation of these states. One way to overcome bulkconductivity is to use local measurement techniques such asscanning tunneling spectroscopy.4 Another possibility is toemploy the Josephson effect.10−12 Here, the evolution of thecritical current Ic(B⊥) with a perpendicular magnetic field B⊥ isconnected with the current distribution in the plane by aFourier transform.13 The asymmetry of the critical current canprovide additional information about properties of thesupercurrent carrying states. The asymmetric Josephson effect(AJE) is expected in systems with a nonsinusoidal CPR,14which is often linked with the presence of Andreev boundstates with high transmission.15 The AJE has been previouslyobserved in a 2D topological insulator coupled to asuperconductor.16Here, we reveal 1D states along edges and steps in few-layerWTe2 by studying the Josephson effect in a perpendicularmagnetic field. The superconducting contacts required forJosephson junctions are realized by a lithographically patternedPd film that is in contact with clean WTe2 and inducessuperconductivity therein. We found that a Josephson currentcan be measured over distances up to 3 μm and that itwithstands magnetic fields up to 2 T, suggesting its 1D naturewith a very tight lateral confinement. Moreover, transportthrough these 1D states shows signatures of the asymmetricJosephson effect. We think that the observed behavior can be aresult of Josephson transport through hinge states due tohigher-order topology in WTe2.Figure 1(a) demonstrates an optical image of our firstdevice. It consists of a few-layer (∼12) thick WTe2 flakecovered with hBN and placed on the prepatterned Pd leads onReceived: February 14, 2020Revised: April 19, 2020Published: May 12, 2020Letterpubs.acs.org/NanoLett© 2020 American Chemical Society4228https://dx.doi.org/10.1021/acs.nanolett.0c00658Nano Lett. 2020, 20, 4228−4233This is an open access article published under a Creative Commons Non-Commercial NoDerivative Works (CC-BY-NC-ND) Attribution License, which permits copying andredistribution of the article, and creation of adaptations, all for non-commercial purposes.Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on June 30, 2021 at 04:54:06 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Artem+Kononov"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Gulibusitan+Abulizi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kejian+Qu"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Jiaqiang+Yan"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="David+Mandrus"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kenji+Watanabe"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takashi+Taniguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takashi+Taniguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Christian+Scho%CC%88nenberger"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.nanolett.0c00658&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/nalefd/20/6?ref=pdfhttps://pubs.acs.org/toc/nalefd/20/6?ref=pdfhttps://pubs.acs.org/toc/nalefd/20/6?ref=pdfhttps://pubs.acs.org/toc/nalefd/20/6?ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://dx.doi.org/10.1021/acs.nanolett.0c00658?ref=pdfhttps://pubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org/NanoLett?ref=pdfhttp://pubs.acs.org/page/policy/authorchoice/index.htmlhttp://pubs.acs.org/page/policy/authorchoice_ccbyncnd_termsofuse.htmlSiO2 substrate. The leads are forming several junctions withdifferent lengths 1−4 μm. We measured the differentialresistances of the junctions in the four-probe setup sketchedin Figure 1(a). Additional details of the fabrication process andthe measurement setup are provided in the SupportingInformation. All measurements were performed in the dilutionrefrigerator with base temperature of 30 mK.Figure 1(b) demonstrates experimental dV/dI(I) depend-encies of three junctions of the device 1. The differentialresistance goes to zero at small currents for the two 1 μm-longjunctions. For the 2 μm junction the differential resistancedoes not go to zero but has a small dip at zero current. Similarresults are obtained in all studied samples. Moreover, theobserved behavior is present only below a certain temperatureand magnetic field. This behavior is typical for Josephsonjunctions, where the proximity effect creates dissipationlesstransport between superconductors connected by a normalmaterial.17 Our experimental data suggest the formation ofsuperconductivity in WTe2 above Pd leads, as sketched inFigure 1(c). These superconducting regions induce a proximityeffect in WTe2 between the leads, leading to the Josephsoneffect in the shorter junctions.The observation of superconductivity may not be surprising,since WTe2 is known to become superconducting at differentconditions, i.e., under pressure,18,19 electron doping,20 orelectrostatic gating.21,22 So, superconductivity can occur inWTe2 on top of Pd due to charge transfer23 or due to flat-bandformation in WTe2, as has recently been reported in anotherWeyl semimetal Cd3As2.24 Another possibility is interdifussionof Pd and Te with the formation of superconductingPdTex25,26 at the interface. To understand the reasons forsuperconductivity is beyond the scope of the current article;only the formation of Josephson junction within our samples isimportant.We can use the observed Josephson effect to obtaininformation about the current distribution in the WTe2devices. The spatial current distribution defines the evolutionof the critical current as a function of the flux through theJosephson junction (JJ). When the supercurrent is uniformlydistributed through the JJ, the critical current Ic(B⊥) as afunction of perpendicular magnetic field B⊥ shows oscillationswith a rapidly decaying amplitude (top in Figure 2(a)). Thecentral lobe is twice as wide as compared to the other lobes.This dependence of Ic(B⊥) is known as the Fraunhofer pattern.If, on the other hand, the supercurrent flows only along thesample edges, as indicated in Figure 2(a), Ic(B⊥) displaysslowly decaying oscillations typical for SQUIDs. The period ofoscillations corresponds to a single flux quantum Φ0 = h/2ethrough the area enclosed by the SQUID.17Figure 2(b) shows the measured Ic(B⊥) dependence for thetwo 1 μm long JJs. The critical current oscillates withperpendicular magnetic field. The central peak of Ic has awidth between one and two oscillations periods. Theamplitude of the oscillations is decaying faster at smaller fieldsand slower at larger ones. The measured Ic(B⊥) is acombination of a Fraunhofer pattern creating a peak of criticalcurrent at zero magnetic field and a SQUID-like pattern withmore than 50 visible oscillations. The period of theseoscillations ΔB ∼ 0.27 mT is given by a flux Φ0 through theeffective area of the junction Seff. From Seff we obtain aneffective junction length Leff = Seff/W = 1.75 μm, where W ∼4.3 μm is the sample width. Leff is larger than the length of thejunction L due to the penetration of magnetic field into thesuperconducting leads. A coexistence of the SQUID andFraunhofer behavior indicates the precence of edge and bulksupercurrent. The latter can be carried by the bulk of thecrystal or by Fermi arc surface states.27 A persistence of theSQUID-like oscillations in magnetic field means that the edgesupercurrent is carried by very narrow states.To obtain the spatial distribution of the supercurrent, weperformed a Fourier transform of Ic(B⊥) by following theDynes−Fulton approach.13 This method assumes a sinusoidalCPR and a nearly symmetric supercurrent distribution acrossthe width of the junction. In this case the minima of Ic(B)should approach zero. The result of the Fourier transformshould therefore be more accurate for junction 2 as comparedto junction 1, since the Ic(B) minima are found to be muchcloser to zero in junction 2. Figure 2(c) shows the result of thetransformation for junction 2. The supercurrent peaks are veryFigure 1. (a) Optical image of the device 1 (scale bar = 10 μm) with asketch of the four-terminal measurement setup. (b) Four-terminaldifferential resistances dV/dI of three junctions 1−3 with lengths 1, 1,and 2 μm, respectively. The 1 μm long junctions demonstrate zerodifferential resistance at small currents as a result of the Josephsoneffect. (c) Schematic side view of the sample illustrating its state:above the Pd leads WTe2 is superconducting (red regions), and thecurrent between these regions is mediated by the Josephson effect(dashed lines).Nano Letters pubs.acs.org/NanoLett Letterhttps://dx.doi.org/10.1021/acs.nanolett.0c00658Nano Lett. 2020, 20, 4228−42334229http://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.0c00658/suppl_file/nl0c00658_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.0c00658/suppl_file/nl0c00658_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?fig=fig1&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://dx.doi.org/10.1021/acs.nanolett.0c00658?ref=pdfnarrow, suggesting a strong edge confinement. The width ofthese supercurrent density peaks obtained from the Gaussianfit is below 80 nm.There is another reason, beyond an asymmetric currentdistribution,13 why the oscillations in Figure 2(b) are notreaching zero; this can be caused by a nonsinusoidal CPR28,29of the edge states. We can immediately confirm that the CPR isnonsinusoidal. This is seen as follows. The ratio of the criticalcurrents of the two edge states IcH/IcL is obtained from theratio of the average critical current to the critical currentoscillation amplitude IcH/IcL = (Icmax + Icmin)/(Icmax − Icmin).For junction 1, this ratio is large, ∼7 ≫ 1, hence,corresponding to a highly asymmetric SQUID. In such anasymmetric SQUID, the dependence of Ic(B⊥) on B⊥ mimicsdirectly the CPR of the edge state with the lower criticalcurrent.30 But, Ic(B) for junction 1 is clearly not a sinefunction, as one can see from Figure 2(b) and Figure 3.Additional evidence for a nonsinusoidal CPR can beobtained by looking at the symmetry of the dependenceIc(B⊥) as a function of B⊥. For a conventional Josephsonjunction with a sinusoidal CPR, Ic(B⊥) should be symmetricalwith respect to current reversal Ic+(B⊥) = Ic−(B⊥) and magneticfield reversal Ic(−B⊥) = Ic(B⊥). Two requirements to breakthese symmetries are a nonsinusoidal CPR and an asymmetryin the current distribution.14 However, the time-reversalsymmetry conserves Ic upon simultaneous reversal of themagnetic field and the current Ic±(B⊥) = Ic∓(−B⊥).As is apparent from Figure 3(a), Ic±(B⊥) breaks thesymmetries both with current and field reversal. The symmetryis restored when the current and magnetic field are reversedsimultaneously, as illustrated in Figure 3(b). The time-reversalsymmetry allows us to exclude flux trapping in the JJ31 as areason for the observed asymmetries. The asymmetries inFigure 3 match the prediction of AJE14 and require anonsinusoidal CPR and an asymmetry in current distribution.We have found before that the supercurrent in few-layerWTe2 is of 1D nature, flowing predominately along the edgesand has a nonsinusoidal CPR. With the next sample wedemonstrate that 1D conducting states can also reside at stepedges of WTe2, and they are remarkably robust. Device 2,shown in Figure 4(a), is as before a hBN-covered few-layerFigure 2. (a) Expected dependencies of the critical current of a 2D Josephson junction on B⊥ for two different supercurrent distributions, i.e., for auniform current distribution, Ic(B⊥) shows rapidly decaying oscillations (Fraunhofer behavior), whereas for two narrow edge states, the Ic(B⊥)oscillations do not (or only weakly) decay in amplitude (SQUID behavior). (b) Critical current Ic(B⊥) of junctions 1 and 2 as a function of B⊥. Acombination of a SQUID- and Fraunhofer-like behavior is observed, indicating a significant amount of edge supercurrent. (c) Supercurrent densitydistribution of junction 2 extracted from Ic(B⊥). Two distinctive edge states, each having a width of ∼75 nm, are observed.Figure 3. (a) Critical currents of two 1 μm long junctions of device 1as a function of perpendicular magnetic field for positive I1,2+ andnegative I1,2− currents. Ic(B⊥) lacks the symmetry to the change ofcurrent direction. (b) Same data as in (a) but with a reversedmagnetic field for negative currents. Symmetry is preserved when bothcurrent and magnetic field are reversed.Nano Letters pubs.acs.org/NanoLett Letterhttps://dx.doi.org/10.1021/acs.nanolett.0c00658Nano Lett. 2020, 20, 4228−42334230https://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?fig=fig3&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://dx.doi.org/10.1021/acs.nanolett.0c00658?ref=pdfWTe2 flake placed on top of Pd leads. The main differencefrom the previous device is nonuniform thickness, where themiddle part is five layers thick and the outer parts are bilayers.The low temperature conductivity in WTe2 diminishes withdecrease in number of layers with the bilayer being aninsulator.7,32Figure 4(b) shows dV/dI(I) traces for different junctionsnormalized by the length of the junction. The differentialresistance goes to zero for 1−3 μm long junctions, indicatingthe presence of Josephson current. The normal state resistanceper unit length is comparable for all junctions, yielding ∼100 Ωμm−1. For this sample, the product IcRN ∼ 150−380 μV,depending on the junction and the way the normal stateresistance RN is defined. This value is close to the theoreticalprediction for a short ballistic Josephson junction: IcRN = πΔ/e∼ 540 μV.33 Here, we estimate the energy gap following theformula Δ(T = 0) = 1.76kBTc17 with Tc = 1.1 K defined as themaximal temperature where signs of superconductivity in thesamples are still present. The agreement between the IcRNproduct and the theoretical value implies that there is a strongproximity effect and the JJs are close to the short ballistic limit.The Josephson current for all junctions survives magneticfields above 1 T, see Figure 4(c). This is inconsistent with auniform supercurrent, since even for the shortest junction itwould correspond to BS/Φ0 ∼ 2000 flux quantum through theJJ area. A robust large field supercurrent implies highlylocalized 1D channels that carry the supercurrent. The onlypossible place for these states are the steps from the five-layerpart to bilayers, since the bilayer itself does not conduct. At acloser look, oscillations of Ic(B⊥) are visible for the 2 μm longjunction, see the inset to Figure 4(c). The oscillations areclearly of a SQUID character with a period ΔB ∼ 0.33 mT.This period yields a smaller area S = Φ0/ΔB ∼ 6.1 μm2 thanthe relevant junction’s area of 9 μm2. This mismatch is likely aconsequence of the sample geometry and discussed in moredetail in the Supporting Information.The measurement of Ic(B⊥) of the 2 μm long junction showsadditional oscillations with a larger period of δB ∼ 0.3 T (redarrows in Figure 4(c)). Similar oscillations were previouslyobserved for topological hinge states in bismuth and werelinked to a difference in wavevectors of electrons and holesforming the Andreev pairs.34 The observed period ofoscillations is in agreement with the expected value δB ∼2πℏvF/geffμBL ∼ 0.15−0.7 T, where L = 2 μm is the length ofthe junction, vF ∼ 2 × 105 ms−135 the Fermi velocity, and geff ∼10−5036 the Lande ́ g-factor. Alternatively, a slower oscillationcould reflect the presence of multiple states on terraces fromthe five layers to the bilayer, as illustrated in Figure 4(d). Thewidth d of this region can be estimated from the ratio of theperiods of the slow δB ∼ 0.3 T and fast oscillations ΔB ∼ 0.33mT and the width of the junction W ∼ 4.5 μm, where d ∼WδB/ΔB ∼ 5 nm. This value is an upper estimate for thewidth of the edge states.The observation of strong Josephson coupling through 1Dedge states with nonsinusoidal CPR suggests a topologicalorigin of these states.15 The only predicted 1D topologicalstates in few-layer WTe2 are hinge states of a HOTI.9 We thinkthat this is very plausible, since our data reproduces manyfeatures previously observed in bismuth, which is a HOTI.10However, there are still some open questions. Currently wecannot resolve if the states are indeed residing on oppositehinges as expected in a HOTI. Also, the critical current valuesare higher than expected for a single ballistic channel Ic1D =πΔ/eRk= eΔ/2ℏ ∼ 20 nA. This discrepancy is also present inbismuth and can be accounted by multiple states at severalterraces on the edges and degeneracy of edge states due tomultiple orbitals.10In conclusion, we present an experimental study ofJosephson transport in encapsulated few-layer WTe2 samples.Our data strongly suggest the presence of 1D states residing onsteps and edges of WTe2. The Josephson currents in these 1Dstates are extremely robust. They survive magnetic fields up to2 T and extend over distances up to 3 μm. Moreover, thesupercurrent demonstrates signs of nonsinusoidal CPR. Ourfindings fit well with the recent prediction of higher-ordertopological insulator states in WTe29 and demonstrate manyFigure 4. (a) Optical image of device 2 (scale bar = 10 μm) with a sketch of the measurement setup for a single junction. Each Pd lead has a 100nm gap in the middle, which is located below the thicker part of the WTe2 flake. The gaps split every Pd lead into two independent normal contactsto the common superconducting region. (b) Four-terminal dV/dI of junctions with different lengths divided by the length of the correspondingjunctions in micrometers. The Josephson effect is present in junctions that are up to 3 μm long. (c) Critical current Ic(B⊥) as a function ofperpendicular magnetic field B⊥ (note: Ic is here multiplied by the length of the corresponding junctions in micrometers). The arrows highlight theperiodic low-frequency modulation of Ic for the 2 μm long junction. Inset: Ic(B⊥) for the 2 μm long junction zoomed in to the small magnetic fieldregion. A fast periodic oscillation with an amplitude of ∼1% is clearly discerned. (d) Sketch of a cross section of the sample near the step from 5L to2L, illustrating the possibility that multiple 1D channels along the step appear.Nano Letters pubs.acs.org/NanoLett Letterhttps://dx.doi.org/10.1021/acs.nanolett.0c00658Nano Lett. 2020, 20, 4228−42334231http://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.0c00658/suppl_file/nl0c00658_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658?fig=fig4&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://dx.doi.org/10.1021/acs.nanolett.0c00658?ref=pdffeatures previously observed only in another HOTI, i.e.,bismuth.10,34Note. During the preparation of this manuscript we becameaware of two recent preprints37,38 demonstrating edgetransport in WTe2 obtained by the proximity effect fromsuperconducting Nb leads. The experimental results in thesepreprints are in good agreement with our conclusions. Incomparison to the former, our samples are in the thin limit andthey additionally demonstrate a stronger Josephson couplingover longer distances. They thereby provide a more compellingevidence for Josephson coupling through highly localizednarrow 1D states residing on the steps of WTe2.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.nanolett.0c00658.Methods; superconductivity induced in WTe2 by normalleads; superconductivity and Josephson effect in a few-layer WTe2 device in a Hall bar geometry; supercurrentdistribution in the device 2; and differential resistance asa function of magnetic field and current for the device 2(PDF)Accession CodesAll data in this publication are available in numerical form inthe Zenodo repository at 10.5281/zenodo.3526560.■ AUTHOR INFORMATIONCorresponding AuthorsArtem Kononov − Department of Physics, University of Basel,CH-4056 Basel, Switzerland; Institute of Solid State Physics ofthe Russian Academy of Sciences - Chernogolovka,Chernogolovka 142432, Russia; orcid.org/0000-0002-3778-8239; Email: Artem.Kononov@unibas.chChristian Scho ̈nenberger − Department of Physics and SwissNanoscience Institute, University of Basel, CH-4056 Basel,Switzerland; orcid.org/0000-0002-5652-460X;Email: Christian.Schoenenberger@unibas.chAuthorsGulibusitan Abulizi − Department of Physics, University ofBasel, CH-4056 Basel, SwitzerlandKejian Qu − Department of Materials Science and Engineering,University of Tennessee, Knoxville, Tennessee 37996, UnitedStatesJiaqiang Yan − Materials Science and Technology Division, OakRidge National Laboratory, Oak Ridge, Tennessee 37831,United States; Department of Materials Science andEngineering, University of Tennessee, Knoxville, Tennessee37996, United States; orcid.org/0000-0001-6625-4706David Mandrus − Department of Materials Science andEngineering, University of Tennessee, Knoxville, Tennessee37996, United States; Materials Science and TechnologyDivision, Oak Ridge National Laboratory, Oak Ridge,Tennessee 37831, United StatesKenji Watanabe − National Institute for Material Science,Tsukuba 305-0044, Japan; orcid.org/0000-0003-3701-8119Takashi Taniguchi − National Institute for Material Science,Tsukuba 305-0044, Japan; orcid.org/0000-0002-1467-3105Complete contact information is available at:https://pubs.acs.org/10.1021/acs.nanolett.0c00658Author ContributionsA.K. fabricated the devices 1 and 2, performed the measure-ments, and analyzed the data. G.A. optimized the fabricationrecipe, developed the thickness determination method byoptical contrast, and together with A.K. fabricated andmeasured device S1. K.Q., J.Y., and D.M. provided WTe2crystals. K.W. and T.T. provided hBN crystals. A.K preparedthe manuscript. C.S. initiated and supervised the project andparticipated in all discussions. All authors contributed to themanuscript.NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSWe thank D. Indolese for the help with measurements of thecritical current and fruitful discussions, M. Endres for his helpwith the exfoliation and identification of WTe2 flakes, M.Joodaki for her contribution to the optical identification ofWTe2 flakes thickness, and A. Baumgartner for fruitfuldiscussions. A.K. was supported by the Georg H. Endressfoundation. This project has received further funding from theEuropean Research Council (ERC) under the EuropeanUnion’s Horizon 2020 research and innovation programme(grant agreement no. 787414 TopSupra), from the SwissNational Science Foundation through the National Centre ofCompetence in Research Quantum Science and Technology(QSIT), and from the Swiss Nanoscience Institute (SNI).K.W. and T.T. acknowledge support from the ElementalStrategy Initiative conducted by MEXT, Japan, and CREST(JPMJCR15F3), JST. D.M. and J.Y. acknowledge support fromthe U.S. Department of Energy (U.S. DOE), Office of Science,Basic Energy Sciences (BES), Materials Sciences and Engineer-ing Division.■ REFERENCES(1) Nayak, C.; Simon, S. H.; Stern, A.; Freedman, M.; Das Sarma, S.Non-Abelian anyons and topological quantum computation. Rev.Mod. Phys. 2008, 80, 1083.(2) Kitaev, A.Yu. Unpaired Majorana fermions in quantum wires.Phys.-Usp. 2001, 44, 131.(3) Fu, L.; Kane, C. L. 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