Supercurrent flow between two superconductors with different order parameters, a phenomenon known as the Josephson effect, can be achieved by inserting a non-superconducting material between two superconductors to decouple their wavefunctions. These Josephson junctions have been employed in fields ranging from digital to quantum electronics, yet their functionality is limited by the interface quality and use of non-superconducting material. Here we show that by exfoliating a layered dichalcogenide (NbSe2) superconductor, the van der Waals (vdW) contact between the cleaved surfaces can instead be used to construct a Josephson junction. This is made possible by recent advances in vdW heterostructure technology, with an atomically flat vdW interface free of oxidation and inter-diffusion achieved by eliminating all heat treatment during junction preparation. Here we demonstrate that this artificially created vdW interface provides sufficient decoupling of the wavefunctions of the two NbSe2 crystals, with the vdW Josephson junction exhibiting a high supercurrent transparency.
We demonstrate a vertical field-effect transistor based on a graphene/MoSe 2 van der Waals (vdW) heterostructure. The vdW interface between the graphene and MoSe 2 exhibits a Schottky barrier with an ideality factor of around 1.3, suggesting a high-quality interface. Owing to the low density of states in graphene, the position of the Fermi level in the graphene can be strongly modulated by an external electric field. Therefore, the Schottky barrier height at the graphene/MoSe 2 vdW interface is also modulated. We demonstrate a large current ON-OFF ratio of 10 5 . These results point to the potential high performance of the graphene/MoSe 2 vdW heterostructure for electronics applications. *E-mail: moriyar@iis.u-tokyo.ac.jp; tmachida@iis.u-tokyo.ac.jp 2 Heterostructures that are held together by the van der Waals (vdW) force between the layered materials have been the subject of considerable interest in the fields of materials science and opto-electronics [1]. To date, several layered materials have been developed and extensively studied. Such materials include graphene, black phosphorous, transition metal dichalcogenides (TMD), and hexagonal boron nitride. Among these materials, heterostructures based on graphene and TMD have been found to exhibit functions that were previously not possible, including those of a vertical field-effect transistor [2][3][4][5], photocurrent generation [6,7], a spin-orbit proximity effect [8], and the ability to fabricate flexible devices [9]. Particularly, vertical field-effect transistors based on graphene/MoS 2 /metal heterostructures have been the subject of considerable attention mainly due to their large current ON-OFF ratio (10 3 to 10 5 ) combined with a large ON current density (>10 3 A/cm 2 ) [3,4]. This level of performance would attract very high demand for electronics applications. Given that the large current ON-OFF ratio is a result of the electric field modulation of the Schottky barrier height at the graphene/MoS 2 interface, the precise tuning of the band alignment at the graphene/TMD interface is crucial in determining the level of performance. To date, such devices have been fabricated using MoS 2 which has an indirect band gap of around 1.3 eV in its bulk form [10]. Changing the chalcogen atom from sulfur to selenium significantly reduces the band gap (the indirect gap of MoSe 2 is around 1.1 eV) [11] and also changes the electron affinity [12,13]. Furthermore, MoSe 2 exhibits better optical properties than MoS 2 [11,14].These results point to MoSe 2 being the better option for opto-electronics applications. In addition, since MoSe 2 has a smaller electron affinity than MoS 2 , under the assumption of Schottky-Mott rule, the Schottky barrier height at graphene/MoSe 2 interface is expected 3 to be larger than that of graphene/MoS 2 . Therefore a comparison of the Schottky barrier property of different TMD materials provides an insight into the vdW interface properties of a graphene/TMD heterostructure. In this study, we fabricated a graphene/MoSe 2 /Ti vertical...
We investigate the micromechanical exfoliation and van der Waals (vdW) assembly of ferromagnetic layered dichalcogenide Fe 0.25 TaS 2 . The vdW interlayer coupling at the Fe-intercalated plane of Fe 0.25 TaS 2 allows exfoliation of flakes. A vdW junction between the cleaved crystal surfaces is constructed by dry transfer method. We observe tunnel magnetoresistance in the resulting junction under an external magnetic field applied perpendicular to the plane, demonstrating spin-polarized tunneling between the ferromagnetic layered material through the vdW junction.*E-mail: moriyar@iis.u-tokyo.ac.jp; tmachida@iis.u-tokyo.ac.jp 2 Recently, studies on two-dimensional (2D) materials such as graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDs) have received considerable attention [1,2]. These materials exhibit layered structure in the bulk form, and the van der Waals (vdW) force connects each of the layers. Thus, these crystals can easily be exfoliated down to monolayer and used to construct heterostructures of different 2D materials connected by vdW force [3]. Metal, semiconductor, and insulator 2D materials have been studied, and various functional electronics and opto-electronics devices have been demonstrated with these materials [4][5][6][7]. Further, these 2D materials could be potentially applied in the field of spintronics [8]; for example, long-distance spin transport has been demonstrated in graphene/h-BN heterostructures [9], and spin polarized tunneling has been demonstrated through graphene [10] and h-BN [11,12]. In these experiments, the source for the spin-polarized electrons is a ferromagnetic metal fabricated by the evaporation technique; thus, the interface between the ferromagnetic and non-magnetic materials involves chemical bonding. On the other hand, the vdW interface does not require any chemical bonding at the junction, in principle. Therefore, layered ferromagnetic 2D materials could possibly be used for constructing vdW heterostructures with spintronic functions. Moreover, the vdW hetrostructure could provide another degree of freedom that has not been possible in the conventional spintronics device; such as controlling the interlayer twist [13,14] In Fig. 1(c), optical micrographs of the exfoliated flake and their vdW junction are shown. The area of vdW junction is 67.8 µm 2 . The quality of the vdW interface is analyzed using cross-sectional transmission electron microscopy (TEM), as shown in Fig. 1(d). The TEM image revealed the layered structure of each Fe 0.25 TaS 2 flake with a stacking period of 0.61 nm; this period is close to the reported value for the bulk material [18]. The vdW contact between the flakes is clearly visible in the TEM image [ Fig. 1(d)].The TEM reveals that vdW junction displays different TEM contrast from the flake; this behavior is attributed to the presence of a native oxide layer existing at the surface of cleaved Fe 0.25 TaS 2 . The presence of Ta 2 O 5 oxide layer in the topmost layer of the exfoliated Fe 0.25 TaS 2 surface i...
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