Periodic Reporting for period 3 - QUESTDO (Quantum electronic states in delafossite oxides)
Reporting period: 2020-01-01 to 2021-06-30
One key finding of our work has been in identifying and understanding the interactions between the metallic and insulating layers when the latter is a so-called correlated, or Mott, insulator. We have shown how the properties of the two subsystems become delicately intertwined, such that removing an electron from the Mott layer causes a hole to move to and propagate in the metallic layer while retaining memory of the Mott layer’s magnetism. This opens the door to using the non-magnetic probe of angle-resolved photoemission to study correlated magnetism in a wide range of interesting materials.
Another significant finding in our work to date has been in understanding how the bulk electronic properties of delafossites are modified at their surfaces. In general, electronic states can be very different at surfaces as compared to in the bulk of materials. The delafossites host so-called polar surfaces: their layer-by-layer building blocks are charged, with an alternating positive and negative sign. Truncating the crystal on one of these layers causes its charge carrier doping to become strongly modified as compared to the bulk. For the Pd-terminated surface, we have shown how this causes the surface to become magnetic, despite being non-magnetic within the bulk. This is driven by electrostatic effects, and suggests new routes to creating magnetic materials or interfaces between compounds with different magnetic characteristics, and for tuning the interactions between conduction electrons and magnetic excitations. Even more remarkable, for the oxide-terminated surface, we have found that the layers that are insulating within the bulk of the crystal become metallic. Their electrons behave as if they are heavy, due to strong electronic interactions. Yet, they also host effects that are due to relativistic corrections to the standard approximations used in describing the motion of electrons in solids. In this case, it leads to a pronounced separation of the energies of electronic states based upon the spin of the electron (attached image) – a so-called Rashba-like spin splitting, but found here in an unexpected environment where the electrons move with rather low velocities. Indeed, our work opens new routes to maximise this effect, which may have widespread relevance to realising similar effects in other materials.