Materials with transition-metals straddle the boundary between covalent, ionic, and metallic bonding, showing many fascinating physical phenomena, such as high-T_c superconductivity in layered cuprates. The understanding of these phenomena can be largely attributed to the complex correlation between electrons in these materials. In this talk, I will present my recent study of the electronic structure of transition-metal oxides and transition-metal phthalocyanines. The electronic structures and magnetic properties of the interesting transition-metal oxides, including CaCrO₃ [1], MnV₂O₄ [2], and Sr₃NiIrO₆ [3] have been studied theoretically and experimentally. In addition, the magnetism in the newly established organic strongly correlated organic compounds, transition-metal phthalocyanines (where transition-metals are isolated by organic rings), including lithium-phthalocyanines (LiPc) [4], cobalt-phthalocyanines [5, 6], chromium-phthalocyanines [7], has been studied theoretically and experimentally. CaCrO₃ exhibits peculiar electronic properties – a co-existence of antiferromagnetism and metallic conductivity. Hybrid-exchange density-functional theory (HDFT) and DFT + U methods have been used to study the electronic structure of CaCrO₃. DFT + U has predicted a conducting state with AFM-C magnetic structure while HDFT has opened a small band gap. MnV₂O₄ spinel is promising for finding orbital-glass and orbital-ice states. The cubic structure of MnV₂O₄ has been studied by using HDFT, suggesting an exotic orbital ordering (OO) structure; ferro-OO along [110] and antiferro-OO along [-110]. In Sr₃NiIrO₆ spin-chain compound, we have found a large spin-excitation gap induced by anisotropic exchange and single-ion anisotropy. The numerical results from a linear spin-wave theory derivation are consistent with the corresponding inelastic neutron scattering experiment. My calculations in CoPc chain showed a large anti-ferromagnetic exchange interaction (approximately 85 K) in the molecular magnets (See Fig.1a), leading to a magnetic transition temperature far above the boiling point of liquid nitrogen. This has inspired my colleagues to perform the related experiments on the magnetic properties of CoPc. In LiPc, we have found a geometry-driven spin-density wave phase. The theoretical study of CrPc show it consists of 1D spin-2 chain, which is a good candidate for testing Haldane's conjecture.

Reference:
[1] Wei Wu, arXiv:1501.00319 (2015).
[2] Wei Wu, Phys. Rev. B 91 (19), 195108 (2015).
[3] W. Wu, D. T. Adroja, S. Toth, S. Rayaprol, and E. V. Sampathkumaran, arXiv:1501.05735 (2015).
[4] Wei Wu, N. M. Harrison, and A. J. Fisher, Phys. Rev. B (Rap. Comm.) 88, 180404(R) (2013).
[5] Wei Wu, N. M. Harrison, and A. J. Fisher, Phys. Rev. B 88, 024426 (2013);
[6] M. Serri, Wei Wu, L. Fleet, N. M. Harrison, C. W. Kay, A. J. Fisher, C. Hirjibehedin, G. Aeppli, S. Heutz, Nat. Commun. 5, 3079 (2014).
[7] Wei Wu, A. J. Fisher, and N. M. Harrison, Phys. Rev. B 88, 224417 (2013).