New correlated electroic states emerging from strong spin-orbit coupling : the case of iridates – SOCRATE

The last decade in solid state physics has been marked by the rediscovery of spin-orbit coupling (SOC) in different fields. So far, most examples occur in conventional semiconductors, e.g. Bi2Te3 in the field of topological insulators or GaAs in spintronic applications. SOC is a general feature of heavy elements but its impact on correlated systems has been much less studied. At the theoretical level, many proposals of new phenomena associating strong correlations and strong SOC have flourished, but their experimental realizations are still elusive in many cases. Among correlated systems with strong SOC, iridates have captured the main attention. They can form many different structures, where Ir usually retains 5 electrons in the 5d shell. This brings the interesting situation where a half-filled state with effective angular momentum Jeff=1/2 is formed, for which correlations and quantum magnetic fluctuations are enhanced.

In this project, we want to explore as thoroughly as possible the impact of strong SOC in iridates. The force of our consortium is to gather expertise in (i) elaboration of new materials, (ii) combination of different experimental techniques and (iii) high level theoretical approaches. We have identified three directions where SOC could result in new phenomena.

New Mott insulators – In Mott insulators, electrons are localized because of the strong Coulomb repulsion between them. Many of them are based on 3d transition metals (V2O3, cuprates, manganites, etc). 5d orbitals are more extended spatially than 3d ones, which reduces this Coulomb repulsion and should destabilize the Mott state. However, many iridates turn out to be insulators, despite having partially filled bands. They could realize a new form of insulators, called spin-orbit Mott insulator. The layered perovskite Sr2IrO4 is certainly the most studied case and is becoming a reference example. We propose to elaborate Sr2IrO4 and related compounds as single crystals, powders and thin films. We will study various aspects of their physical properties, dealing with structural, charge, spin and orbital degrees of freedom, combining experimental techniques like ARPES, transport, NMR, muSR and RIXS. In a second step, we will try and dope them to obtain metallic phases. Very little is known on the properties of the doped phases, but similarities with the cuprates have led theoreticians to predict that high temperature superconductivity could be possible. We will test this proposal and clarify the similarities and differences introduced by strong SOC in this new type of correlated systems.

New spin liquids – Spin liquids are systems where enhanced quantum fluctuations prevent the system from ordering, even at T=0. In dimension d>1, they are usually realized on frustrated lattices, like the kagomé lattice. SOC enriches this search by stabilizing new states with pseudo-spins Jeff = ½, where orbital and spin degrees of freedom are entangled. We will study one of the first discovered example, the ordered spinel Na4Ir3O8 with a three dimensional frustrated “hyperkagomé” lattice. Its hole-doped version Na3Ir3O8 may further realize the long sought doped spin liquid state.

New topological phases – Ever since the isolation of graphene in 2005, the study of Dirac fermions physics has led to two novel research areas, topological insulators and Weyl semi-metals in 3D materials. Both need strong SOC and were proposed in iridates, although not yet firmly evidenced experimentally. We will try and synthesize the multilayer Sr2IrRhO6, which was proposed as a topological insulator. The existence of the Weyl semi-metal was originally predicted for pyrochlore. We will test this proposal through transport and Raman spectroscopy. Theoretically, an open question concerns the stability of such a phase. We will address this issue within the framework of generic tight-binding models.