In the search for quantum spin liquids, candidate materials for the Kitaev model and its extensions have been intensively explored during the past decade, as the models realize the exact quantum spin liquids in the ground state. Thus far, insulating magnets in the low-spin d 5 electron configuration under the strong spin-orbit coupling have been studied for realizing the Kitaev-type bond-dependent anisotropic interactions between the spin-orbital entangled Kramers doublets. To extend the candidates, here we investigate the systems in a high-spin d 7 electron configuration, whose ground state is described by the spin-orbital entangled Kramers doublet. By the secondorder perturbation in terms of the t 2g -t 2g and t 2g -e g hoppings, we show that the effective spin model possesses the anisotropic Kitaev interactions as well as the isotropic Heisenberg ones. While the Kitaev interaction is always ferromagnetic, the Heisenberg interaction can become either ferromagnetic or antiferromagnetic depending on the Coulomb interactions and the crystalline electric fields. We also derive the effective model for the low-spin d 5 electron configuration within the same perturbation scheme, in which the Kitaev interaction becomes both ferromagnetic and antiferromagnetic, while the Heisenberg one always ferromagnetic. Referring to the previous study for the Kitaev-Heisenberg model, we find that the quantum spin liquid phase exists in the reasonable parameter region in both d 7 and d 5 cases, while the former has a richer structure of the phase diagram. We discuss the advantages of the d 7 case in comparison with the d 5 case. Our results indicate that the high-spin d 7 state provides another platform for the Kitaev-type quantum spin liquid.

We theoretically propose a family of f -electron based magnets that realizes Kitaev-type bonddependent anisotropic interactions. Based on ab initio calculations, we show that A2PrO3 (A: alkali metals) crystalize in a triclinic structure with honeycomb layers of edge-sharing PrO6 octahedra. Each Pr 4+ cation has a 4f electron in the Γ7 doublet, which comprises a spin-orbital entangled Kramers pair with the effective moment J eff = 1/2. By using the Wannier orbitals from the ab initio calculations, we find that the effective interactions between the J eff = 1/2 moments are predominantly of antiferromagnetic Kitaev type for light alkali metals A=Li and Na, in stark contrast to the ferromagnetic ones in 4d-and 5d-electron based materials. Our finding would provide a playground for the Kitaev spin liquids that is hard to be accessed by the candidates ever discovered. arXiv:1807.01443v1 [cond-mat.str-el]

The Kitaev spin liquid provides a rare example of well-established quantum spin liquids in more than one dimension. It is obtained as the exact ground state of the Kitaev spin model with bond-dependent anisotropic interactions. The peculiar interactions can be yielded by the synergy of spin-orbit coupling and electron correlations for specific electron configuration and lattice geometry, which is known as the Jackeli-Khaliullin mechanism. Based on this mechanism, there has been a fierce race for the materialization of the Kitaev spin liquid over the last decade, but the candidates have been still limited mostly to 4d-and 5d-electron compounds including cations with the low-spin d 5 electron configuration, such as Ir 4+ and Ru 3+ . Here we discuss recent efforts to extend the material perspective beyond the Jackeli-Khaliullin mechanism, by carefully reexamining the two requisites, formation of the j eff = 1/2 doublet and quantum interference between the exchange processes, for not only dbut also f -electron systems. We present three examples: the systems including Co 2+ and Ni 3+ with the high-spin d 7 electron configuration, Pr 4+ with the f 1 -electron configuration, and polar asymmetry in the lattice structure. In particular, the latter two are intriguing since they may realize the antiferromagnetic Kitaev interactions, in contrast to the ferromagnetic ones in the existing candidates. This partial overview would stimulate further material exploration of the Kitaev spin liquids and its topological properties due to fractional excitations.

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Effects of interfacial interactions on the electrocatalytic activity of protein-tethered bilayer lipid membranes (ptBLMs) containing cytochrome c oxidase (CcO) for the oxygen reduction reaction are studied by using protein film electrochemistry and surface-enhanced infrared absorption (SEIRA) spectroscopy. Mammalian CcO was immobilized on a gold electrode via self-assembled monolayers (SAMs) of mixed alkanethiols. The protein orientation on the electrode is controlled by SAM–CcO interactions and is critical to the cytochrome c (cyt c) binding. The CcO–phospholipid and CcO–cyt c interactions modulate the electrocatalytic activity of CcO, and more densely packed ptBLMs show higher electrocatalytic activity. Our study indicates that spectroscopic and electrochemical studies of ptBLMs can provide insights into the effects of relatively weak protein–protein and protein–lipid interactions on the enzymatic activity of transmembrane enzymes.