We are exploring new compounds with transition metal elements in which novel, exotic and/or functional electronic phases are realized. Our main targets included, 5d complex Ir oxides with interplay of electron correlations and strong spin orbit coupling, spin liquids, anti-perovskites with Dirac electrons, and excitonic ground states.
Realization of new spin liquid and Kitaev physics:
Realization of spin liquid, where quantum spins fluctuates at abosolute zero, should be a milestone in the field of quantum spin physics. After a theoretical achievement of the exaclty solvable spin liquid state on a honeycomb lattice, by Alexei Kitaev, a materialization of this Kitaev Honeycomb Model (KHM) has been intensively pursuit. One dimensional spin liquid has been commonly accepted, while in two or three dimensions, typical known frustrated quantum spin liquid materials, like triangular compounds, is not based on an exactly solvable lattice model. We have been focussed on a two-dimensonal honeycomb iridate, H3LiIr2O6, and discovered that H3LiIr2O6 is indeed spin liquid, as the first material of such a liquid, down to 50 mK by specific heat, magnetic susceptibility, and nuclear magnetic resonance experiments. This key result was published in 2018-2019.
The key ingredient to realize KHM is bond-dependent anisotropic Ising-like interactions, and it was suggested that material engineering for spin-orbit coupled Jeff=1/2 quantum pesudo spins of Ir on (hyper-)honeycomb lattice would be a main route. Two kinds of Majorana fermions represent KHM and they are particles on the exaclty solved ground state. Since our discovery is an only spin liquid on Kitaev system, and no report was given to proof two Majorana particles. We will pursuit identification of elementary excitations in H3LiIr2O6 this year. We preliminary succeeded in fabricating a single crystal of this compounds, which should open a way to investigate a pristine thermally/artificially activated Majorana excitations or local excitations near an implanted defects, which can be evalulated rigorously for KHM. We expect that the latter effects can be caught by our NMR spectroscopy technique.
Three-dimensional Dirac electron systems:
We have demonstrated a realization of three-dimensional Dirac electrons in anti-perovskite oxide Sr3PbO, which is evidenced by the quantum-limit characters in the magnetoresistance under high magnetic fields. In addition to this, we have carried out 207Pb NMR experiments on single-crystal samples with different carrier densities to establish Dirac-type dispersions. It was found that the temperature dependence of NMR relaxation rate certainly reflects three-dimensional Dirac-type density of states. This year, we conducted very accurate angle-dependent magnetoresistance measurements to investigate chiral anomaly phenomenon which is peculiar to this quantum-limit physics. Newly developed small two-axis goniometric device was used. A current jetting effect was clearly observed as a negative resistance when an applied megnetic field directed one of elecrodes. Altough reproducibility needs to be examined further, we succeeded in separate the effects from chiral anomaly and current jetting effect.
The semi-metallic AIrO3 (A=Sr,Ca) perovskites are predicted to have three dimensional Dirac-node electrons and heavier holes at the fermi level. We fabricated epitaxially grown AIr1-xSnxO3 (A=Sr,Ca) on SrTiO3(001) substrate to construct phase diagrams consisting of a magnetism phase, Dirac-node semimetal, and unknown physics in between. Substitution of Ir by Sn makes the system more insulating, and as a result, weak ferromagnetism appear. In these systems, namely, a competition between hopping and Coulomb repulsion can be well managed by a diltuion of Ir ions or a distortion of Ir-O-Ir bond. Band structures at each phase and phase boundary are yet to be identified and will be investigated.
