MPI-SSR Stuttgart and Dept. Physics, University of Tokyo
The exploration of novel phases of interacting electrons (correlated electrons) has long been a major stream of condensed-matter research. Many-body interactions among electrons give rise to a huge variety of phases, grouped into electron-solid, -liquid-crystal, -liquid and -gas states. The wealth of possibilities arises from a complicated interplay of lattice geometry, quantum effects and the multiple degrees of freedom of the electron (charge, spin and orbital). In the past, the two dominant areas of exploration have been the 3d transition-metal (TM) oxides and the 4f intermetallic compounds but recently 5d TM oxides and related compounds have emerged as the next arena of correlated-electron physics. Significant new physics is expected due to the presence of a large spin-orbit coupling in heavy 5d elements, tying together the otherwise independent spin and orbital degrees of freedom. This can be of order 0.5eV and is often larger than the crystal-field splitting of the orbital states, resulting in a spin-orbital-entangled state of correlated electrons. The nature of the spin-orbital entanglement depends significantly on the d-electron number and the chemical bonding, and it is anticipated that, in combination with electron correlations, a rich variety of novel electronic phases are waiting to be discovered. In this talk, I will present the concept of such phases and our effort in materializing them.
- Spin-orbital Mott insulator 
- Correlated topological semimetals 
- Spin-orbital quantum liquid on honeycomb lattice 
- Excitonic magnetism 
- Multipolar ordering 
 J. Matsuno et al. , Phys. Rev. Lett. 114 247209 (2015).
 H. Takagi, T. Takayama, G. Jackeli, G. Khaliullin, and S. N. Nagler, Nature Reviews Physics 1, 264 – 280 (2019).
 G. Khaliullin, Phys. Rev. Lett. 111, 197201 (2013).
 H. Ishikawa, T. Takayama, R. Kremer, J. Nuss, R. Dinnebier, K. Kitagawa, K. Ishii and H. Takagi, Phys. Rev. B100, 045142 (2019)