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(searched for: doi:10.1103/physrevb.105.075119)
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Ling-Fang Lin, Rahul Soni, , Shang Gao, Adriana Moreo, Gonzalo Alvarez, Andrew D. Christianson, , Elbio Dagotto
Physical Review B, Volume 105; https://doi.org/10.1103/physrevb.105.245113

Abstract:
Spin-1/2 chains with alternating antiferromagnetic (AFM) and ferromagnetic (FM) couplings have attracted considerable interest due to the topological character of their spin excitations. Here, using density functional theory and density-matrix renormalization-group (DMRG) methods, we have systematically studied the dimerized chain system Na2Cu2TeO6 with a d9 electronic configuration. Near the Fermi level, in the nonmagnetic phase the dominant states are mainly contributed by the Cu 3dx2y2 orbitals highly hybridized with the O 2p orbitals, leading to an “effective” single-orbital low-energy model. By calculating the relevant hoping amplitudes, we explain the size and sign of the exchange interactions in Na2Cu2TeO6. In addition, a single-orbital Hubbard model is constructed for this dimerized chain system where the quantum fluctuations are taken into account. Both AFM and FM couplings (leading to an state) along the chain were found in our DMRG and Lanczos calculations, in agreement with density functional theory and neutron-scattering results. The hole pairing binding energy ΔE is predicted to be negative at Hubbard U11eV, suggesting incipient pairing tendencies.
, Ling-Fang Lin, Adriana Moreo, Thomas A. Maier, Gonzalo Alvarez, Elbio Dagotto
Physical Review B, Volume 105; https://doi.org/10.1103/physrevb.105.174410

Abstract:
The van der Waals oxide dichlorides MOX2 (M=V, Ta, Nb, Ru, and Os; X=halogen element), with different electronic densities, are attracting considerable attention. Ferroelectricity, spin-singlet formation, and orbital-selective Peierls phases were reported in this family with d1 or d2 electronic configurations, all believed to be caused by the strongly anisotropic electronic orbital degree of freedom. Here, using density functional theory and density matrix renormalization group methods, we investigate the electronic and magnetic properties of RuOCl2 and OsOCl2 with d4 electronic configurations. Different from a previous study using VOI2 with d1 configuration, these systems with 4d4 or 5d4 do not exhibit a ferroelectric instability along the a axis. Due to the fully occupied dxy orbital in RuOCl2 and OsOCl2, the Peierls instability distortion disappears along the b axis, leading to an undistorted Immm phase (No. 71). Furthermore, we observe strongly anisotropic electronic and magnetic structures along the a axis. For this reason, the materials of our focus can be regarded as “effective one-dimensional” systems even when they apparently have a dominant two-dimensional lattice geometry. The large crystal-field splitting energy (between dxz/yz and dxy orbitals) and large hopping between nearest-neighbor Ru and Os atoms suppresses the J=0 singlet state in MOCl2 (M=Ru or Os) with electronic density n=4, resulting in a spin-1 system. Moreover, we find staggered antiferromagnetic order with π wave vector along the M-O chain direction (a axis) while the magnetic coupling along the b axis is weak. Based on Wannier functions from first-principles calculations, we calculated the relevant hopping amplitudes and crystal-field splitting energies of the t2g orbitals for the Os atoms to construct a multiorbital Hubbard model for the M-O chains. Staggered AFM with spin structure dominates in our density matrix renormalization group calculations, in agreement with density functional theory calculations. Our results for RuOCl2 and OsOCl2 provide guidance to experimentalists and theorists working on this interesting family of oxide dichlorides.
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