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Martin Endres, Artem Kononov, Michael Stiefel, Marcus Wyss, Hasitha Suriya Arachchige, 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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[Transparent Josephson junctions in higher-order topological insulator <math>  <msub>    <mi>WTe</mi>    <mn>2</mn>  </msub></math> via Pd diffusion](https://mdr.nims.go.jp/datasets/8a5237ac-5e8a-4add-8615-a41d1c0c2ab5)

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Transparent Josephson junctions in higher-order topological insulator ${\rm WTe}_{2}$ via Pd diffusionPHYSICAL REVIEW MATERIALS 6, L081201 (2022)LetterTransparent Josephson junctions in higher-order topological insulator WTe2 via Pd diffusionMartin Endres ,1,* Artem Kononov ,1 Michael Stiefel,2 Marcus Wyss ,3 Hasitha Suriya Arachchige,4 Jiaqiang Yan ,4,5David Mandrus,4,5,6 Kenji Watanabe ,7 Takashi Taniguchi,7 and Christian Schönenberger 1,3,†1Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland2Laboratory for Nanoscale Material Science, Swiss Federal Laboratories for Material Science and Technology,EMPA, Überlandstrasse 129, 8600 Dübendorf, Switzerland3Swiss Nanoscience Institute, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland4Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA5Material Science and Technology Division, Oak Ridge Laboratory, Oak Ridge, Tennessee 37831, USA6Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA7National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan(Received 30 May 2022; revised 25 July 2022; accepted 8 August 2022; published 29 August 2022)Highly transparent superconducting contacts to a topological insulator (TI) remain a persistent challenge onthe route to engineer topological superconductivity. Recently, the higher-order TI WTe2 was shown to turnsuperconducting when placed on palladium (Pd) bottom contacts, demonstrating a promising material systemin pursuing this goal. Here, we report the diffusion of Pd into WTe2 and the formation of superconducting PdTexas the origin of observed superconductivity. We find an atomically sharp interface in the direction vertical to thevan der Waals layers between the diffusion crystal and its host crystal, forming state-of-the-art superconductingcontacts to a TI. The diffusion is discovered to be nonuniform along the width of the WTe2 crystal, with a greaterextent along the edges compared to the bulk. The potential of this contacting method is highlighted in transportmeasurements on Josephson junctions by employing external superconducting leads.DOI: 10.1103/PhysRevMaterials.6.L081201Introduction. Topological insulators (TIs) are insulatingin the bulk while hosting gapless boundary states in whichthe spin of the electron is locked to its momentum [1].When brought in contact with an s-wave superconductor,a novel pairing mechanism is predicted with Cooper pairsthat resemble an effectively spinless superconductor [2,3].Such topological superconductors could host Majorana boundstates, the elementary building block of fault-tolerant quantumbits [4].Fundamental to this approach is a highly transparent in-terface between the superconductor and topological insulator[5] through which the boundary states are proximitized. Evenwith state-of-the-art nanofabrication it remains challengingto create such pristine material interfaces as oxidation [6–9],contamination, and rough crystal interfaces introduce defectsand therefore decrease contact transparency [10].The van der Waals (vdW) material WTe2 is predicted to bea higher-order TI [11–15], hosting topological edge states atits crystal hinges. It was recently shown that thin crystals ofthe material placed on top of palladium (Pd) bottom contacts*martin.endres@unibas.ch†christian.schoenenberger@unibas.chPublished by the American Physical Society under the terms of theCreative Commons Attribution 4.0 International license. Furtherdistribution of this work must maintain attribution to the author(s)and the published article’s title, journal citation, and DOI.turn superconducting [16], with a pronounced flow of super-current along the edges of a Josephson junction (JJ) formedout of this material system [17].Here, we report diffusion of Pd into the WTe2 formingsuperconducting PdTex as the origin of superconductivity inthe WTe2/Pd system. The interface between PdTex and WTe2in the direction vertical to the vdW layers is found to beatomically sharp, eliminating crystal-roughness between thesuperconductor and the higher-order TI completely. We fur-ther investigate the formation of PdTex along the width of theWTe2 host crystal and find it to be nonuniform, with a greaterextent along the edges compared to the bulk. The potentialof this novel contacting method to WTe2 is highlighted intransport measurements on JJs that show improved qualitywhen contacted externally by an intrinsic superconductor.Pd diffusion in WTe2. We begin with describing the gen-eral structure of WTe2 Josephson junctions formed with Pd.Fabrication starts with patterning parallel lines of Pd on p-doped Si substrates with 285 nm of SiO2 on top. Next, thevdW materials hexagonal boron nitride (hBN) and WTe2 areexfoliated and afterwards stacked on top of the Pd bottomcontacts, using a standard dry pickup technique [18]. Un-til full encapsulation with hBN, WTe2 is handled inside aninert glovebox atmosphere to protect the material from oxi-dation [6–8]. After the stacking process, the polymer stampis separated from the stacked device by heating the sampleto 155 ◦C for ≈10 min. The remaining polymer residues arechemically dissolved afterwards. Depending on the desiredtransport experiment, contact to WTe2 is made either through2475-9953/2022/6(8)/L081201(7) L081201-1 Published by the American Physical Societyhttps://orcid.org/0000-0001-7749-3585https://orcid.org/0000-0002-3778-8239https://orcid.org/0000-0001-9498-4108https://orcid.org/0000-0001-6625-4706https://orcid.org/0000-0003-3701-8119https://orcid.org/0000-0002-5652-460Xhttp://crossmark.crossref.org/dialog/?doi=10.1103/PhysRevMaterials.6.L081201&domain=pdf&date_stamp=2022-08-29https://doi.org/10.1103/PhysRevMaterials.6.L081201https://creativecommons.org/licenses/by/4.0/MARTIN ENDRES et al. PHYSICAL REVIEW MATERIALS 6, L081201 (2022)FIG. 1. Pd diffusion inside the WTe2 crystal. (a) Left: Opticalimage of the elongated WTe2 flake, covered with hBN on top of Pdcontacts, with additional superconducting contacts (niobium) on top.Right: A schematic cross section of the device in a region of singlePd contact. (b) Illustration of the WTe2 crystal on a single Pd bottomcontact including a superconducting edge contact from the top. Thedirection of the cut lamella for the STEM image is indicated by thedashed line, and the viewing direction of the image is indicated bythe arrow. (c) High-resolution STEM image taken at the edge of thePd bottom contact (indicated by the gray dashed line at the bottomright). A bright diffusion layer at the interface between the Pd bottomcontact and the WTe2 crystal has formed. Black arrows indicate thethickness of the WTe2 crystal, and the white arrow shows the lateralextent of diffusion in WTe2 from the edge of the original Pd contact.(d) Zoom-in STEM image of the interface between the WTe2 crystaland the diffusion layer.the Pd bottom contacts or by etching through the coveringtop hBN and depositing superconducting contacts from thetop. It should be noted that superconductivity is induced intoWTe2 by the Pd contacts alone [16,17] and that additionalsuperconducting contacts are not required to form Josephsonjunctions. Figure 1(a) demonstrates an optical image of oneof the devices. This device was prepared specially for electronmicroscopy, so additional top superconducting contacts do nothave any practical purpose and are placed to replicate realtransport devices. An extended description of the fabricationprocess can be found in the Supplemental Material [19–21].In order to investigate the origin of superconductivity inWTe2 in contact with Pd, we conduct high-resolution scan-ning transmission electron microscopy (STEM) imaging ofthe interface region. The illustration in Fig. 1(b) indicatesthe direction of the extracted lamella by a dashed line andthe viewing direction by a perpendicular arrow. Figure 1(c)presents the STEM image taken with a high-annular angulardark-field detector (HAADF) at the edge of a Pd bottomcontact, reaching into the weak link of the junction. Visibleat first glance is a bright layer that has formed at the interfacebetween the Pd bottom contact and the WTe2 crystal on top.Moreover, the original Pd contact in the bottom right cornerof Fig. 1(c) appears hollow and faded out, suggesting thatthe bright layer in WTe2 is a result of Pd diffusion from thecontact.The presence of the diffusion layer in WTe2 creates apronounced swelling of the crystal, as highlighted by twothickness measurements of the WTe2 flake in Fig. 1(c): insidethe junction and on top of the Pd bottom contact. For pristineWTe2 we extract an interlayer spacing of c ∼ 7.4 Å thatagrees with the literature value [22,23]. Inside the diffusionlayer the perceived layer spacing has doubled to c ∼ 14.8 Å.We connect the change in the layer spacing with the formationof a new crystal structure at the interface of WTe2 and Pd,rather than merely intercalation of the original crystal by Pd.The formation of a new structure is further supported byFig. 1(d), where we see, that the transition between the newlyformed crystal and WTe2 is very sharp and takes place on asingle-layer scale. We also would like to note that the diffusionforming the new structure is quite anisotropic. Laterally, alongthe vdW layers, the diffusion layer extends ∼84 nm whilevertically, perpendicular to the vdW layers, it only reaches∼16 nm at its maximum. The lateral diffusion inside the JJcan diminish the length of the JJ, which could be especiallyprominent for the shorter junctions.In the next section we analyze the atomic compositionof the diffusion layer using energy dispersive x-ray (EDX)analysis. Figure 2(a) on the left shows a STEM image takenat the position of a superconducting niobium (Nb) top contactin this device. For better orientation, the location is illustratedin Fig. 2(b). From the bottom to the top, the faded Pd bot-tom contact, the Pd diffusion layer adjacent to the pristineWTe2 crystal, and the Nb top contact are visible. Towards theright, EDX spectra of the elements in this slab are shown. Nb(turquoise) and the sticking layer for the Pd bottom contactsand titanium (Ti) are at their expected positions. Qualitatively,the concentration of tungsten (W) and tellurium (Te), repre-sented in red and orange, respectively, are maximal in theunchanged WTe2 crystal but reduced in the diffusion layer.Pd (in blue) has diffused through the entire WTe2 crystaland is the dominating element inside the structurally changedlayer. The concentration of Pd at the position of the originalbottom contact is diminished, suggesting that depletion of theavailable material stopped the further growth of the diffusionlayer.A quantitative analysis of the crystal composition is shownin Fig. 2(b), following a trace indicated by the red arrow inthe STEM image in Fig. 2(a). The ratio of W:Te is ∼1:2 andremains the same throughout most of the thickness. For Pd,two distinct concentration levels are visible, a high level of∼60% that coincides with the structurally changed lattice anda second, low level of ∼20% inside the preserved WTe2 crys-tal. The ratio of Pd:Te ∼ 3:1 suggests that the diffusion layer isnot one of the known superconductors PdTe or PdTe2 [24–27].In the unchanged WTe2 crystal above, Pd likely intercalatesthe vdW layers. So possibly, a threshold concentration of Pdis required to trigger the crystallographic change, such thatthe vertical extention of the diffusion layer is determined bythe interplay of available Pd and thermal activation energy.L081201-2TRANSPARENT JOSEPHSON JUNCTIONS IN … PHYSICAL REVIEW MATERIALS 6, L081201 (2022)FIG. 2. EDX analysis of the Pd diffusion. (a) STEM image withthe direction of the line cut in panel (b) indicated by a red arrow.Presented towards the right is the EDX analysis with elements ex-istent in the device. Moving from the bottom to the top, the Pdbottom contact is followed by highly Pd interspersed WTe2 layer thathas structurally changed. Above, the crystal transitions sharply intothe original crystal structure. (b) EDX line cut along the directionindicated in panel (a), with the position of the investigated lamellamarked in the insert. Pd has diffused in the vertical direction throughthe entire WTe2 crystal, with a sharp concentration increase in thestructurally changed area that coincides with the STEM image.The remaining Pd concentration above the PdTex layerquickly decays in the direction parallel to the vdW layersand extends ∼50 nm laterally beyond the structurally changeddiffusion layer, as shown in Ref. [19].Diffusion along the edges. Further, we investigate the uni-formity of the PdTex diffusion layer along the width of theJosephson junction. For this, we have analyzed several lamel-las that are oriented perpendicular to the direction of thecurrent in the JJ. The regions near the physical edges of WTe2are of particular interest, since additional Pd is available theredue to the Pd bottom contacts extending beyond the crystal.The first lamella was cut out through the middle of thebottom Pd contact in a sample with additional, this timemolybdenum-rhenium (MoRe), top contacts, as illustrated byposition 1 in Fig. 3(c). Figure 3(a) presents two EDX spectraobtained at the two edges marked by � and • in Fig. 3(c).Outside of the WTe2 flake we observe a layer of Pd withuniform thickness sandwiched between the MoRe top layerand the Ti bottom layer. Interestingly, inside WTe2 near theedges, the thickness h of the PdTex diffusion layer increaseswithin a region of ∼100 nm away from the edge, as markedin the right spectrum of Fig. 3(a). Further from the edges,Pd is evenly distributed throughout the WTe2 crystal. The1,2, 2,1,FIG. 3. Enhanced Pd diffusion along the edges of the WTe2crystal. (a) EDX analysis of a cross section taken on the Pd contactalong position 1, as indicated in the schematics in panel (c). Thegiven sample was equipped with a superconducting MoRe contact,evidenced by the Mo EDX signal in red. The left and right imagescorrespond to the crystal edges marked by � and • in the schematics,respectively. Visible is the swelling of WTe2 to a thickness of ∼2h atthe edge, compared to the bulk thickness of ∼h, indicated in the rightimage. (b) EDX signal and extracted intensity (Int.) profile takentowards the inside of the junction, indicated by position 2 in panel(c). The left and right images correspond to positions � and • of theWTe2 crystal, respectively. Visible in the EDX data is an increasedintensity of the Pd signal at the edges of the crystal compared to thebulk. The increased Pd concentration is also visible by the enhancedEDX signal in the line cuts taken along the direction pointed out bythe horizontal arrow. (c) Illustration of the inhomogeneous diffusionprofile of PdTex inside the WTe2 host crystal. The self-formed PdTexlayer is drawn in blue inside the host crystal. (d) Cross-sectional cutsthrough the illustration along positions 1 and 2 in panel (c). Theedges are marked by � and • for orientation.difference of PdTex thickness on the edges and in the middleof WTe2 reaches a factor of ∼2.The increase in thickness of PdTex near the edges canbe intuitively explained taking into account the fabricationprocedure. During the last step of stacking, when the substrateis heated up to 155 ◦C to release the hBN/WTe2 stack, theformation of the PdTex takes place. In WTe2 far away from theedges this process stops before reaching the full thickness ofL081201-3MARTIN ENDRES et al. PHYSICAL REVIEW MATERIALS 6, L081201 (2022)the flake due to the depletion of available Pd. Near the edges,due to the availability of additional Pd, this process contin-ues potentially even through the whole thickness of WTe2.Afterwards, the top WTe2 layers, not transformed to PdTex,are etched away during CHF3/O2 plasma etching of hBNprior to the deposition of the superconductor. This explanationis further corroborated by the uniform Pd concentration inFig. 3(a) in contrast with the step in concentration in Fig. 2(b).The increased Pd availability on the edges of the WTe2 hasthe potential not only to increase the PdTex thickness, but alsoto provide further diffusion inside the Josephson junction. Tocheck this, we made a second lamella from another sample,which is cut inside the Josephson junction close to the end ofPd bottom contact, as shown as position 2 in Fig. 3(c). Visiblein the EDX spectrum in Fig. 3(b) is an elevated intensity of Pdcompared to the bulk at both ends of the crystal, highlightedby line cuts through the spectra along the horizontal arrows.This indicates that PdTex is indeed penetrating further insidethe junction along the edges of WTe2 as visualized in Fig. 3(c).We can roughly estimate the extent of the PdTex diffusionalong the edges, assuming that an increase by a factor of 2in thickness h of PdTex on the edges, as compared to thebulk [see Fig. 3(d)], yields the same increase in the diffusioninside the JJ along the edge. Taking from Fig. 1(c) that thePdTex layer extends ∼85 nm inside the junction in the bulk,we would expect it to extend ∼170 nm along the edges. Thisdiffusion could generate signatures of “artificial” edge super-currents not due to a topological state. Nonetheless, evidenceof topological hinge states in WTe2 has been observed incombination with superconducting niobium contacts [15,28],where no edge diffusion is expected.Josephson junction with fully superconducting contacts.During the formation of PdTex in WTe2 the majority of thePd from the bottom contacts is depleted (see Figs. 1–3),thus creating a low-quality interface between bottom contactsand the newly formed Josephson junction. In this section wedemonstrate a method to harness the full potential of the high-quality Josephson junction formed in WTe2 with Pd diffusionby employing additional superconducting contacts from thetop.The fabrication process follows the description in Sec. II.After obtaining the stack, superconducting leads are patternedvia standard e-beam lithography and sputtered onto the sam-ple after etching through the top hBN with CHF3/O2 plasma.Prior to the deposition of MoRe superconducting leads, weperform a short Ar milling inside the sputtering chamber toremove the oxide layer from WTe2. In order to avoid degrada-tion of the JJs due to etching, the superconducting top contactsare separated by a distance of lPd ∼ 0.5 μm from the edgeof the Pd bottom contacts, as indicated in the schematics inFig. 4(a). Figure 4(a) shows a finished device with top MoReleads and its fabricated layer sequence to the right.Measurement of the device is performed in a quasi-four-terminal setup, illustrated in Fig. 4(a). In this configuration,the measured differential resistance includes the contributionfrom the Josephson junction and the resistances of the in-terfaces between the MoRe and superconducting PdTex, butexcludes the line resistances in the cryostat. Figures 4(b)and 4(c) show the dV/dI (I ) and V (I ) dependencies, mea-sured on a 1-μm-long Josephson junction, with their behaviorbeing representative for a number of samples we have stud-ied. The curves reveal several abrupt transitions with current.Steps at I ∼ ±11 μA have minimal hysteresis and corre-spond to the switching of superconducting PdTex to thenormal state or alternatively to a Josephson junction that haspotentially formed at the interface of the vdW stack withMoRe [29].The observed vanishing resistance at low bias currents byitself is not sufficient to ensure that the fabricated device per-forms indeed as a JJ. Potentially, the inhomogeneous diffusionof PdTex could lead to a closed superconducting path throughthe weak link. In order to rule out this option, we study thedependence of dV/dI on the bias current I and the perpen-dicular magnetic field B, as shown in Fig. 4(d). Visible is aperiodic “Fraunhofer”-like interference pattern that is a keysignature of the Josephson effect. The oscillation periodicityδB = 0.13 mT is connected to a flux quantum �0 threadingthe effective junction area Aeff = w × �eff , with w = 4.3 μmbeing the width of the junction and �eff being the effectivelength. The calculated �eff ∼ 3.8 μm exceeds the physicaljunction length of ∼1 μm. However, it can be explainedby the contact geometry, assuming that half of the magneticflux through the superconducting contacts is screened into thejunction [30]. Additionally, a close look at the amplitude ofconsecutive lobes reveals a nonmonotonous behavior, remi-niscent of an even-odd effect. A nonsinusoidal current phaserelation of the junction [17] or an inhomogeneous current dis-tribution [30], originating from the diffusion profile of PdTex,can create this feature.Having established the Josephson effect through WTe2, wetake a closer look at the lower-current behavior observed inFigs. 4(b) and 4(c). First, in the superconducting branch of theJJ, the differential resistance is zero, implying that there is nomeasurable contribution of the MoRe/PdTex interfaces. Sec-ond, in contrast to the previously studied devices with solelyPd leads [17], the switching behavior is highly hysteretic. Thetransition from the superconducting to the resistive branch,denoted by the switching current Ic, takes place at absolutecurrent values higher than those of the transition in the op-posite sweep direction, denoted by the retrapping current Ir ,highlighted in Fig. 4(b).Even though the hysteretic switching of the Josephsonjunction is most commonly explained by the junction beingin the underdamped regime [31], we would argue that in ourcase overheating [32,33] plays the dominating role. Startingfrom the superconducting branch, no heat is dissipated in theJosephson junction before switching to the resistive branch.In contrast, lowering the bias current from the resistive branchincludes dissipation of heat in the normal weak link, leadingto a higher electron temperature. This explanation is corrob-orated by the temperature dependence of Ic and Ir shown inFig. 4(e). Moving from high towards low temperatures, Ic andIr both increase continuously down to T ∼ 220 mK, when Irbegins to saturate while Ic remains increasing. Furthermore,this explanation is additionally supported by the devices withonly Pd contacts. There, due to the normal Pd contacts remain-ing dissipative at all times, Ic saturates at low temperatures, asshown in Fig. 4(f).Next, we characterize the quality of the PdTex/WTe2 in-terface. Compared to conventional superconducting contactsL081201-4TRANSPARENT JOSEPHSON JUNCTIONS IN … PHYSICAL REVIEW MATERIALS 6, L081201 (2022)FIG. 4. Switching characteristics of Josephson junctions with superconducting or normal leads. (a) Optical image of the device with anillustration of the quasi-four-terminal measurement setup. The fabricated layer sequence is shown on the right. The scale bar is 10 μm.(b) dV/dI curves at the zero magnetic field of a JJ with superconducting contacts for two different sweep directions of the bias current. Thehysteretic switching current depending on the sweep direction is visible from the shift of the critical current Ic and the retrapping current Ir .(c) V (I ) curves corresponding to the data in panel (b). The excess current Ie is evaluated from the intersection of the extended V (I ) curve tozero voltage. (d) dV/dI as a function of the bias current I and the perpendicular magnetic field B, following the “Fraunhofer” pattern expectedfor a Josephson junction. The insert shows the full range of the central lobe around B = 0. (e) Temperature dependence of the switching andretrapping currents Ic and Ir , respectively, of the same device as in panel (b). The data Ic(T ) are fitted in the scope of a diffusive long junction,plotted in light green. (f) Switching current Ic as a function of temperature T for two different JJs that are contacted only through Pd bottomcontacts.to WTe2, the Josephson effect in junctions formed by Pdinterdiffusion is found to be more robust in terms of junc-tion length and magnetic field resilience [17]. Additionally,due to reduced heating effects, the here proposed devicessupport a critical current density twice as large compared toconventional contacts [28] at four times the junction length.The 1-μm-long junction presented in Fig. 4(b) maintains theJosephson effect with a critical current density jc of up to jc >108 Am−2 , while comparable junctions with an even shorterlength of up to 230 nm and conventional superconductingcontacts are found to be limited by jc ∼ 1 × 107 Am−2 –5 × 107 Am−2 [28].Further, from Fig. 4(e) we see that Ic is suppressed at 0.6 K,which is lower than the critical temperature Tc = 1.2 K [16]of the formed PdTex. We connect this reduction with the greatlength of the junction and fit for this reason Ic(T ) with anexpression for a long diffusive junction [32,34]Ic = ηaETheRN[1 − b exp( −aETh3.2kBT)]. (1)Here, a and b are constants equal to 10.82 and 1.30, respec-tively, kB is the Boltzmann constant, and ETh is the Thoulessenergy. The empirical prefactor η ∈ [0, 1] can be interpretedas a measure for the interface quality, scaling the maximum Ic.The data in Fig. 4(e) are well described by the model, whichyields η = 0.5 and ETh = 3.87 μeV. A similar fit procedurefor the data in Fig. 4(f), obtained from a junction with onlyPd contacts, is not reliable as Ic (T < 400 mK) is limitedby heating effects and deviates strongly from the theoreticalprediction.The evaluation of the interface transparency is corrob-orated by the excess current Ie, extracted from the V (I )curve in Fig. 4(c). We extrapolate Ie after the transition fromthe superconducting to the resistive branch and obtain [35]IeRN/� ∼ 0.03, using � = 1.76kBTc = 182 μeV [16]. In theframework of the Octavio-Tinkham-Blonder-Klapwijk theory[36,37], this relates to a junction transparency of T = 1/(1 +Z2) ∼ 0.5, with Z ∼ 1.1.At this point we would like to comment on the role ofRN in the two analytical models of the preceding analysisyielding a transparency of ∼0.5. The bulk conductivity ofWTe2 increases with flake thickness [38]. These additionalbulk modes in the normal state shunt RN , but do not participatein superconducting transport of the long junction due to theirfast decay Ic,bulk ∝ �2mfp/L3w, compared to ballistic edge modesIc,edge ∝ 1/Lw [39], with �mfp being the electronic mean freepath. This phenomenon has also been reported in JJs formedof the topological material Bi2Se3 [40]. The extracted trans-parency is therefore systematically underestimated and servesonly as a lower bound to the real value.L081201-5MARTIN ENDRES et al. PHYSICAL REVIEW MATERIALS 6, L081201 (2022)Conclusion. We have demonstrated a robust method toform atomically sharp superconducting contacts to WTe2 me-diated by Pd diffusion during stacking. Josephson junctionsformed by these contacts are highly transparent. Given recentreports of similar processes in BiSbTe [35,41] this methodcould be a promising approach for other topological candi-dates based on Te compounds. We have further demonstratedthat the diffusion inside the host crystal could be nonuni-form, generating false signatures of superconducting edgecurrents. Therefore, caution has to be exercised in the evalua-tion of a diffusion-driven Josephson junction when assigningit to a topological superconductor. Furthermore, we have pro-posed a method to avoid overheating in transport throughPd-diffusion-mediated Josephson junctions by employing ad-ditionally superconducting leads.Note added. Recently we became aware of a publication[42] that also demonstrates the interdiffusion of Pd into WTe2,with the formation of PdTe leading to superconductivity.All data in this publication are available in numerical formin the Zenodo repository [43].Acknowledgments. We thank Paritosh Karnatak for fruitfuldiscussions. This project has received funding from the Eu-ropean Research Council (ERC) under the European UnionsHorizon 2020 research and innovation programme, Grant No.787414 TopSupra; from the Swiss National Science Founda-tion through the National Centre of Competence in ResearchQuantum Science and Technology (QSIT); and from theSwiss Nanoscience Institute (SNI). A.K. was supported by theGeorg H. Endress Foundation. D.M. and J.Y. acknowledgesupport from the U.S. Department of Energy (U.S. DOE),Office of Science, Basic Energy Sciences (BES), MaterialsSciences and Engineering Division. H.S.A. was supported bythe Gordon and Betty Moore Foundation’s EPiQS Initiativethrough Grant No. GBMF9069 and the Shull Wollan CenterGraduate Research Fellowship. D.M. acknowledges supportfrom the Gordon and Betty Moore Foundation’s EPiQS Ini-tiative, Grant No. GBMF9069. K.W. and T.T. acknowledgesupport from the Elemental Strategy Initiative conducted byMEXT, Japan, and the CREST (Grant No. JPMJCR15F3),JST.M.E. has fabricated the devices. M.E. and A.K. mea-sured the devices in transport. M.S. and M.W. performed theSTEM and EDX imaging. H.S.A., J.Y., and D.M. providedthe WTe2 crystals. K.W. and T.T. provided hBN crystals.M.E., A.K., and C.S. analyzed the data and wrote themanuscript.The authors declare no competing interests.[1] L. Fu and C. L. Kane, Topological insulators with inversionsymmetry, Phys. Rev. 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