Category Archives: 2016

Kitaev magnetism in honeycomb RuCl3 with intermediate spin-orbit coupling

Intensive studies of the interplay between spin-orbit coupling (SOC) and electronic correlations in transition metal compounds have recently been undertaken. In particular, jeff = 1/2 bands on a honeycomb lattice provide a pathway to realize Kitaev’s exactly solvable spin model. However, since current wisdom requires strong atomic SOC to make jeff=1/2 bands, studies have been limited to iridium oxides. Contrary to this expectation, we demonstrate how Kitaev interactions arise in 4d-orbital honeycomb α-RuCl3, despite having significantly weaker SOC than the iridium oxides, via assistance from electron correlations. A strong coupling spin model for these correlation-assisted jeff = 1/2 bands is derived, in which large antiferromagnetic Kitaev interactions emerge along with ferromagnetic Heisenberg interactions. Our analyses suggest that the ground state is a zigzag-ordered phase lying close to the antiferromagnetic Kitaev spin liquid. Experimental implications for angle resolved photoemission spectroscopy, neutron scattering, and optical conductivities are discussed.

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Topological nodal line semimetals with and without spin-orbital coupling

We theoretically study three-dimensional topological semimetals (TSMs) with nodal lines protected by crystalline symmetries. Compared with TSMs with point nodes, e.g., Weyl semimetals and Dirac semimetals, where the conduction and the valence bands touch at discrete points, in these new TSMs the two bands cross at closed lines in the Brillouin zone. We propose two new classes of symmetry protected nodal lines in the absence and in the presence of spin-orbital coupling (SOC), respectively. In the former, we discuss nodal lines that are protected by the combination of inversion symmetry and time-reversal symmetry; yet unlike any previously studied nodal lines in the same symmetry class, each nodal line has a Z2 monopole charge and can only be created (annihilated) in pairs. In the second class, with SOC, we show that a nonsymmorphic symmetry (screw axis) protects a four-band crossing nodal line in systems having both inversion and time-reversal symmetries.

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Spin-Orbit Physics Giving Rise to Novel Phases in Correlated Systems: Iridates and Related Materials

Recently, the effects of spin-orbit coupling (SOC) in correlated materials have become one of the most actively studied subjects in condensed matter physics, as correlations and SOC together can lead to the discovery of new phases. Examples include unconventional magnetism, spin liquids, and strongly correlated topological phases such as topological superconductivity. Among candidate materials, iridium oxides (iridates) have been an excellent playground to uncover such novel phenomena. In this review, we discuss recent progress in iridates and related materials, focusing on the basic concepts, relevant microscopic Hamiltonians, and unusual properties of iridates in perovskite- and honeycomb-based structures. Perspectives on SOC and correlation physics beyond iridates are also discussed.

Jeffrey G. Rau, Eric Kin-Ho Lee, Hae-Young Kee

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