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We established the theory of the critical magnetic field for a novel class of two-dimensional superconductors with strong spin-orbit coupling. These materials have interesting properties for potential applications in spintronics
Transition metal dichalcogenide (TMDC) monolayers are recently discovered two-dimensional (2D) semiconductors with a hexagonal lattice structure similar to graphene, but with two inequivalent sites in the unit cell. Like graphene, these materials exhibit a valley degree of freedom and have minima/maxima of conduction/valence bands at the corners K and K’=−K of the Brillouin zone. Unlike graphene, however, the absence of inversion symmetry allows for a large, direct band gap, making them promising candidates for a new generation of transistors. Due to the heavy constituent atoms, TMDC monolayers exhibit a very large intrinsic spin-orbit coupling (SOC), often called Ising SOC, which acts as an effective Zeeman field perpendicular to the plane of the material with opposite orientations in the two valleys (Figure 1). As a consequence, a large valley-dependent spin-splitting occurs, opening the door for novel applications in spintronics and so-called valleytronics.
The coupling between the spin and valley degrees of freedom has remarkable repercussions also for the superconducting properties of doped TMDCs. As shown in recent experiments, the in-plane upper critical field greatly surpasses the so-called Pauli limit. Researchers from INAC/PHELIQS established the theory of the upper critical field for Ising superconductors at arbitrary disorder strength, assuming a conventional s-wave Cooper pairing. They showed that Ising superconductivity results in an unusually large enhancement of the critical field. In the absence of intervalley scattering, the critical field diverges at low temperature (Figure 2). When intervalley scattering is at play, it provides an effective spin-flip scattering mechanism and leads to the saturation of the critical field, consistent with the experimental findings.
Understanding the role of impurities is important for future applications of TMDCs and their incorporation into van der Waals heterostructures. The doping techniques used to prepare the superconducting samples, as well as the defects in the crystal lattice could be the source of significant disorder. These theoretical results therefore contribute to assess their potential for applications such as in superconducting spintronics.
CEA is a French government-funded technological research organisation in four main areas: low-carbon energies, defense and security, information technologies and health technologies. A prominent player in the European Research Area, it is involved in setting up collaborative projects with many partners around the world.