Quantum tunnelling with tunable spin geometric phases in van der Waals antiferromagnets

Electron tunnelling in solids, a fundamental quantum phenomenon, lays the foundation for various modern technologies. The emergence of van der Waals magnets presents opportunities for discovering unconventional tunnelling phenomena. Here, we demonstrate quantum tunnelling with tunable spin geometric phases in a multilayer van der Waals antiferromagnet CrPS4. The spin geometric phase of electron tunnelling is controlled by magnetic-field-dependent metamagnetic phase transitions. The square lattice of a CrPS4 monolayer causes strong t2g-orbital delocalization near the conduction band minimum. This creates a one-dimensional spin system with reversed energy ordering between the t2g and eg spin channels, which prohibits both intralayer spin relaxation by means of collective magnon excitations and interlayer spin hopping between the t2g and eg spin channels. The resulting coherent electron transmission shows pronounced tunnel magnetoresistance oscillations, manifesting quantum interference of cyclic quantum evolutions of individual electron Bloch waves by means of the time-reversal symmetrical tunnelling loops. Our results suggest the appearance of Aharonov-Anandan phases that originate from the non-adiabatic generalization of the Berry's phase. It is difficult to control the geometric phase of particles as they undergo quantum tunnelling. Now tuning of the geometric phase of electron spin is demonstrated in tunnelling in a multilayer van der Waals antiferromagnet.