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Quantum electronic states in delafossite oxides

Periodic Reporting for period 2 - QUESTDO (Quantum electronic states in delafossite oxides)

Reporting period: 2018-07-01 to 2019-12-31

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 vast majority of work has focussed on transition-metal oxides based around cubic “perovskite” building blocks. In contrast, exploiting the layered traingular network of the delafossite structure, the QUESTDO project aims to establish delafossite oxides as a novel class of interacting electron system with properties and potential not known in more established systems. The project brings together advanced spectroscopic measurement with precise materials fabrication. Through these studies, QUESTDO aims to uncover critical new insight on the quantum many-body problem in solids, and to advance our understanding and demonstrate atomic-scale control of the physical properties of delafossites. Ultimately, it seeks to establish new design methodologies for controlling quantum electronic states in this little-studied family of transition-metal oxides, paving the route for their further study and ultimate application.
Work in the QUESTDO project to date has focussed predominantly on the study of the electronic structure of single-crystal samples of the delafossite oxides 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. 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. 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.
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 of delafossite oxides has shown how the coupling between spin and charge can be maximised, suggesting new strategies for designing spintronic materials of the future.
Fermi surface of spin-split surface states in PdCoO2