Unconventional Geometric Phase in Twisted Junction of Cuprates

Originally introduced in optics, the Pancharatnam-Berry phase is a general concept of geometric phase defined for any two interfering polarization states. In electronic systems, however, its counterpart has long been overlooked due to the absence of electron polarization. Here, using large-scale first-principles calculations, we investigate the electronic structure of twisted bilayer Bi2Sr2CaCu2O8. We find spontaneous spin polarization and the emergence of hidden flat bands at the interface between atomic layers. Most notably, we discover an unconventional geometric phase analogous to the Pancharatnam-Berry phase in optics. This electronic geometric phase exerts opposite effects on superconducting currents of opposite chirality, enabling twisted cuprates to act as a filter for chiral superconducting current-even if the ground state itself is non-chiral.