Superconductivity in Van der Waals (vdW) heterostructures has emerged as a rich field of study due to the unique electronic properties arising from the weak interlayer coupling and the ability to engineer hybrid systems with tailored functionalities. These heterostructures, formed by stacking atomically thin layers of two-dimensional materials, exhibit superconducting transitions that can be modulated through proximity effects, electrostatic gating, and mechanical strain. Key phenomena include proximity-induced superconductivity and the formation of Josephson junctions, which enable the exploration of unconventional pairing mechanisms and critical temperature modulation.

Proximity-induced superconductivity occurs when a non-superconducting material acquires superconducting properties due to its close contact with a superconductor. In vdW heterostructures, this effect is particularly pronounced because the atomically sharp interfaces minimize disorder and allow for clean coupling between layers. For instance, when graphene is placed in contact with a superconducting material like niobium diselenide (NbSe2), the superconducting correlations from NbSe2 can penetrate into graphene, inducing a finite superconducting gap. The induced gap is highly sensitive to the interface quality, the Fermi level alignment, and the thickness of the non-superconducting layer. Experiments have shown that the proximity effect can extend several hundred nanometers in graphene, depending on the temperature and carrier density. The critical temperature of the proximitized system is typically lower than that of the parent superconductor but can be tuned by adjusting the carrier density via electrostatic gating.

Josephson junctions in vdW heterostructures are another hallmark of superconducting behavior, where two superconductors are weakly coupled through a non-superconducting barrier. The weak interlayer coupling in vdW systems allows for the realization of high-quality Josephson junctions with tunable properties. For example, a junction formed by two NbSe2 layers separated by a hexagonal boron nitride (hBN) spacer exhibits a supercurrent that can be modulated by the thickness of the hBN layer and the applied gate voltage. The Josephson effect in these systems is governed by the Andreev bound states, which form at the interface and mediate the supercurrent. The critical current of the junction, which is the maximum supercurrent it can sustain without dissipation, depends on the transparency of the barrier and the superconducting phase difference across the junction. Measurements have shown that the critical current in vdW Josephson junctions can range from nanoamperes to microamperes, depending on the materials and device geometry.

Critical temperature modulation in vdW heterostructures is a key area of research, as it provides insights into the pairing mechanisms and the role of dimensionality in superconductivity. The critical temperature can be tuned by several means, including electrostatic doping, mechanical strain, and interlayer twist angle. For instance, in magic-angle twisted bilayer graphene, superconductivity emerges at a critical temperature of around 1.7 Kelvin, which is highly sensitive to the twist angle and the carrier density. Similarly, in transition metal dichalcogenides like MoS2, the critical temperature can be enhanced by electrostatic doping, with reported values reaching up to 10 Kelvin in heavily doped samples. The modulation of the critical temperature is often attributed to changes in the density of states at the Fermi level, electron-phonon coupling strength, and the emergence of new electronic phases such as charge density waves.

Pairing mechanisms in vdW superconductors are diverse and can involve conventional electron-phonon coupling as well as more exotic interactions. In conventional superconductors like NbSe2, the pairing is mediated by phonons, and the superconducting gap follows the BCS theory. However, in systems like twisted bilayer graphene or certain transition metal dichalcogenides, the pairing mechanism may involve electronic correlations or plasmon-mediated interactions. The symmetry of the superconducting order parameter can also vary, with some systems exhibiting s-wave pairing while others show evidence of p-wave or d-wave symmetry. For example, measurements of the superconducting gap in NbSe2 reveal a conventional s-wave structure, whereas in certain doped TMDs, the gap anisotropy suggests unconventional pairing.

The interplay between superconductivity and other electronic phases in vdW heterostructures further enriches the physics of these systems. For instance, in some materials, superconductivity coexists with charge density waves or ferromagnetism, leading to complex phase diagrams. The competition or cooperation between these phases can influence the superconducting properties, such as the critical temperature and the upper critical field. Experiments have demonstrated that applying a magnetic field can suppress superconductivity and reveal underlying correlated phases, providing a pathway to study the interplay between different quantum states.

The ability to engineer vdW heterostructures with atomic precision opens new avenues for exploring superconducting phenomena in reduced dimensions. The weak interlayer coupling allows for the isolation of individual electronic states and the creation of artificial superlattices with tailored properties. Moreover, the flexibility of these systems enables the integration of superconductors with other functional materials, such as topological insulators or ferromagnets, to realize novel hybrid devices. For example, combining a superconductor with a topological insulator can lead to the emergence of Majorana bound states, although this topic falls under the broader category of topological superconductors and is not discussed here.

In summary, superconducting transitions in vdW heterostructures offer a versatile platform for studying proximity-induced superconductivity, Josephson junctions, and critical temperature modulation. The clean interfaces and tunable properties of these systems provide unique insights into pairing mechanisms and the role of dimensionality in superconductivity. Future research will likely focus on exploring new material combinations, optimizing device geometries, and uncovering novel quantum phenomena in these artificially engineered structures. The continued advancement in fabrication techniques and characterization methods will further enhance our understanding of superconductivity in vdW heterostructures and their potential applications in quantum technologies.