Work in the QUESTDO project focussed predominantly on the study of single-crystal and thin-film samples of the delafossite oxides, including PdCoO2, PtCoO2, PdCrO2, and PdRhO2. These are extremely high-conductivity metals. Nonetheless, their crystal structure can be considered as a natural stacking of good metallic layers with insulating oxide layers.
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. We showed how this can mediate magnetic surfaces, despite the non-magnetic bulk, and how this can create metallic surfaces where the electrons behave as if they are heavy, due to strong electronic interactions, yet also host effects that are due to relativistic corrections to the standard approximations used in describing the motion of electrons in solids. We have developed new approaches to image the real-space variation of these surface-dependent electronic structures, and have probed this in analogue materials where giant spin-orbit and magnetic effects can be combined. We have further realised control of such effects in thin-film geometries, paving the way for stabilising novel surface and interface states in delafossites and related materials “on demand”.
