Periodic Reporting for period 4 - QUESTDO (Quantum electronic states in delafossite oxides)
Période du rapport: 2021-07-01 au 2022-12-31
One of the most active challenges of modern solid state physics and chemistry is harnessing the unique and varied physical properties of transition-metal oxides. From improved electrodes for solar cells to loss-less transmission of power, these compounds hold the potential to transform our daily lives. Subtle collective quantum states underpin their diverse properties. These complicate their physical understanding but render them extremely sensitive to their local crystalline environment, offering enormous potential to tune their functional behaviour. To date, the majority of work has focussed on transition-metal oxides based around cubic “perovskite” building blocks. In contrast, the QUESTDO project focussed on the layered traingular network of the delafossite structure. The project brought together and developed new capability for advanced spectroscopic measurements from single-crystals and thin-film samples. Through these investigations, QUESTDO made several surprising discoveries on the interplay of spin-, orbital- and lattice- degrees of freedom in delafossites, providing key new understanding and ultimately helping to establish the materials system as a powerful class of correlated electron materials.
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”.
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”.
Understanding the behaviour of electrons in solids is key to elucidating the electrical and thermodynamic properties of materials. This can be complicated in systems where strong particle-particle interactions are present, driving collective behaviour and new physics to emerge. The results obtained in the QUESTDO project to date have delivered new breakthroughs in how such collective states can be probed and manipulated in solids, and in how known effects resulting from the coupling of spin and orbital degrees of freedom in materials can be driven into new regimes. This is of fundamental interest, and in the long term also of potential technological benefit. Indeed, controlling the motion of electrons in solids is at the heart of electronic devices. Equipping materials with additional control parameters is hoped to pave the way to new generations of energy efficient, fast, and compact technologies that may one day replace today’s semiconductor devices. An example is spintronics, where the spin, rather than charge, of electrons is used as the control parameter. Our work on the surfaces and in thin films of delafossites has shown how the coupling between spin and charge can be maximised, suggesting new strategies for designing spintronic materials of the future.