Low-Dimensional Transport Research Unit

Josephson tunneling in twisted bilayer cuprates

Superconductivity is one of the macroscopic quantum phenomena. The superconducting state can be described by quantum mechanical wave functions categorized as s-, p-, d-wave, etc., similar to the wave functions of a hydrogen atom. Conventional superconductors such as tin or aluminum host s-wave pairing symmetry, and although it remains debatable, most researchers believe that copper oxides are d-wave superconductors instead. Theoretically, if two d-wave superconductors are stacked vertically, the Josephson coupling strength starts to change once one superconductor is twisted against the other one. It monotonically drops to zero when the rotation angle increases up to 45 degrees. By contrast, the s-wave superconductors show constant Josephson coupling, irrespective of the rotation angles. Recently, we successfully fabricated high-quality c-axis twisted Josephson junctions with accurate control over the twist angle. Junctions at various twist angles all exhibit a single tunneling branch behavior, suggesting that only the first half of a unit cell on both sides of the twisted flakes is involved in the Josephson tunneling process. Interestingly, we found that when one cuprate flake is rotated by 45° against the other, Josephson coupling still exists, and the coupling strength of 45° and 0° are comparable. This behavior is consistent with s-wave pairing symmetry.

Twisted cuprate bilayer.

Type-II Ising pairing in few-layer stanene

In recent years, two-dimensional non-centrosymmetric superconductors show large in-plane upper critical fields, far exceeding the conventionally expected value. To explain this phenomenon, a so-called Ising pairing mechanism was proposed, in which the paired electrons stem from spin-split bands with the spin orientationslocked out-of-plane. Recently, we observed enhanced in-plane upper critical fieldin a highly symmetric material—few-layer stanene. This is based on our previous discovery of superconductivity in the same material. In the more recent work, we clearly observed an up-turn behavior of the critical magnetic field as the temperature approaches absolute zero. However, since stanene is centrosymmetric, the observed behaviors cannot be explained by the established theory of Ising superconductivity. We therefore proposed a new type of Ising pairing—type-II Ising paring—which results from the combination of spin-orbit coupling and high lattice symmetry of the materials. Ourwork provides strong evidence for the existence of Ising superconductivityand substantially broadens the scope for the realization of Ising superconductors.

Illustration of the stanene lattice with Ising-like Cooper pairs. The background shows the corresponding band structure. Upper right shows the element information of Sn.