Topological semimetals are a class of quantum materials characterized by band crossings near the Fermi level, which give rise to massless quasiparticles exhibiting linear energy-momentum dispersion. Based on the degeneracy and dimensionality of these crossings, they are classified into Dirac semimetals (fourfold degenerate points), Weyl semimetals (twofold degenerate points with chirality), and nodal-line semimetals (closed loops). Among them, type-II Dirac semimetals, such as NiTe₂, feature strongly tilted Dirac cones that break Lorentz invariance, leading to anisotropic transport properties and exotic surface states. These unique electronic properties make topological semimetals promising candidates for exploring exotic superconducting phenomena when coupled with a superconductor.
When a topological semimetal is placed in intimate contact with a superconductor, the superconducting proximity effect comes into play. This effect occurs when a non-superconducting material acquires superconducting properties over a characteristic length scale. Specifically, Cooper pairs from the superconductor diffuse into the adjacent semimetal, inducing a finite superconducting order parameter and enabling supercurrent to flow through the otherwise non-superconducting region. Leveraging this proximity effect, we fabricate Al/NiTe₂/Al Josephson junctions using the type-II Dirac semimetal NiTe₂ and systematically investigate their superconducting transport properties at millikelvin temperatures.
The fabrication of high-quality devices relies on advanced micro/nano-fabrication techniques. In particular, electron-beam lithography combined with high-precision alignment is employed to define the junction regions with precisely controllable spacing on mechanically exfoliated NiTe₂ flakes. Subsequently, Al electrodes of 80 nm thickness are deposited on both sides of the junction regions using electron-beam evaporation, forming SNS Josephson junctions. To ensure clean and high-quality interfaces, an in-situ argon ion milling step is performed prior to Al deposition to remove surface oxides and contaminants, thereby promoting efficient superconducting proximity coupling between the Al electrodes and the NiTe₂ flakes.
At millikelvin temperatures, a clear Josephson supercurrent is observed, as evidenced by the direct current (DC) Josephson effect. The measured current-voltage characteristics exhibit well-defined zero-resistance supercurrent branches. When an external perpendicular magnetic field is applied, the critical supercurrent evolves into a Fraunhofer-like diffraction pattern, indicating excellent spatial coherence of the supercurrent across the junction. Notably, the positive and negative branches of the critical supercurrent display asymmetric evolution with the magnetic field, suggesting the emergence of a Josephson diode effect. The origin of this diode effect requires further investigation and may be related to the topological surface states of NiTe₂.
Under microwave irradiation, well-defined Shapiro steps are resolved in the current-voltage characteristics. The step heights appear at integer multiples of hf/2e, confirming the alternating current (AC) Josephson effect and demonstrating robust phase coherence under non-equilibrium conditions. The persistence of these Shapiro steps over a range of microwave powers further attests to the high quality of the Al/NiTe₂ interfaces.
Collectively, these results establish the Al/NiTe₂/Al Josephson junction as an ideal platform for exploring superconducting transport phenomena in topological materials. Specifically, this device provides a promising avenue for verifying the existence of supercurrents carried by topological surface states and higher-order hinge states. Through carefully designed junction geometries and phase-sensitive transport measurements, it may become possible to distinguish bulk contributions from boundary contributions to the Josephson supercurrent. Furthermore, this platform enables detailed investigations into whether the current-phase relationship deviates from the conventional sinusoidal form expected for ordinary SNS junctions, as any such deviation could arise from the nontrivial band topology. Thus, beyond the observations reported in this work, the Al/NiTe₂/Al Josephson junction may serve as a versatile testbed for addressing key unresolved questions regarding topological superconductivity, including the role of boundary states and the nature of the current-phase relation, potentially offering implications for future applications in topological quantum computing and superconducting electronics.