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The type-II terminated 1T-TaS2 surface of a 3D 1T-TaS2 bulk material realizes the effective spin-1/2 degree of freedom on each David star cluster such that the time-reversal symmetry is realized anomalously, despite the fact that bulk 3D 1T-TaS2 material has an even number of electrons per unit cell. This surface is effectively viewed as a spin-1/2 triangular lattice magnet, except with a fully gapped topological bulk. We further propose this surface termination realizes a spinon Fermi surface spin liquid with the surface fractionalization but with a nonexotic 3D bulk.
In this second part, we are motivated by the experiments on the triangular lattice bilayer colbaltate K2Co2(SeO3)3, and consider an extended XXZ model to explore the underlying physics from a couple observation. The model is composed of interacting Co2+ dimers on the triangular lattice, where the Co2+ ion provides an effective spin-1/2 local moment via the spin-orbit coupling and the crystal field effect. The intra-dimer interaction is dominant and would simply favor the local spin singlet, and the inter-dimer interactions compete with the inter-dimer interaction, leading to rich behaviors. With the easy-axis anisotropy, it is shown that, in the ground state manifold of the intra-dimer Ising interaction, the system realizes an effective quantum Ising model, where the ground state is either a 3-sublattice order with a mixture of antiferromagnetic Ising order and a valence-bond spin singlet or Ising disordered. The finite temperature regime realizes the Berezinskii-Kosterlitz-Thouless physics. To explore the full excitations, we incorporate the excited state manifold of the intra-dimer Ising interaction and establish the emergent Kugel-Khomskii physics.
Prof Gang Chen is currently Boya distinguished professor of physics at the international center for quantum materials of Peking University. Previously, he was a professor at the University of Hong Kong. He received his PhD degree at University of California Santa Barbara, working with Leon Balents. He is mainly interested in strongly correlation physics, and has made some contributions to the fields of quantum magnetism, correlated electrons, itinerant frustration, topological materials, mesoscopic physics as well as ultracold atoms. He was awarded 2017 Daniel Tsui fellowship and 2018 Qiushi outstanding youth scientist.
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