Correlations between electrons in transition-metal oxides and transition-metal phthalocyanines
Dr. Wei Wu
London Centre for Nanotechnology, University College London, UK

Materials with transition-metals straddle the boundary between covalent, ionic, and metallic bonding, showing many fascinating physical phenomena, such as high-Tc 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.


[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).

About the Speaker
2016-01-12 2:00 PM
Room: A403 Meeting Room
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