Unconventional superconductivity by strain engineering design

According to Fermi-Dirac statistics, the pair correlation function within a superconductor must be anti-symmetric, and the combined odd permutation of spin, parity, orbital index, and time (known as the SPOT criterion) results in a sign reversal of the pairing amplitude. When the crystal structure hosting superconductivity lacks inversion symmetry, parity is not a ‘good’ quantum number, which allows for a mixed superconducting order parameter and, thereby, gives rise to a variety of unusual behaviors. Here we show, for the first time, the stabilization of a 2D non-centrosymmetric orthorhombic superconductor through in-plane strain engineering. We observe various signatures of 2D superconductivity including a Berezinskii-Kosterlitz-Thouless transition, a large Ginzburg-Landau coherence length compared to the film thickness, and an in-plane enhancement of the critical field. All these results point to an unconventional FFLO-type pairing. Remarkably, we find evidence of a field-free superconducting diode effect (SDE) for the first time in a 2D non-centrosymmetric superconductor, indicating intrinsic time reversal symmetry breaking. While field induced SDEs are common across various superconducting and Josephson junctions, the field free analogue is rare. We attribute the field-free nature of the SDE to a mixed paired superconducting state resulting from the non-centrosymmetric structure and strong strain gradients that are likely generated by long-range edge dislocations and twin boundaries. These findings highlight the potential of strain engineering in producing new superconducting states with intrinsic time-reversal symmetry breaking, thereby, opening up new possibilities for future applications.