The ability of multiple CF3 -substituted arenes to act as acceptors for anions is investigated. The results of quantum-chemical calculations show that a high degree of trifluoromethyl substitution at the aromatic ring results in a positive quadrupole moment. However, depending on the polarizability of the anion and on the substitution at the arene, three different modes of interaction, namely Meisenheimer complex, side-on hydrogen bonding, or anion-π interaction, can occur. Experimentally, the side-on as well as a η(2) -type π-complex are observed in the crystal, whereas in solution only side-on binding is found.

Using density functional theory calculations including an on-site Coulomb term, we explore electronic and possibly topologically nontrivial phases in 3d transition metal oxide honeycomb layers confined in the corundum structure (α-Al2O3) along the [0001] direction. In most cases the ground state is a trivial antiferromagnetic Mott insulator, often with distinct orbital or spin states compared to the bulk phases. With imposed symmetry of the two sublattices the ferromagnetic phases of Ti, Mn, Co and Ni exhibit a characteristic set of four bands, two relatively flat and two with a Dirac crossing at K, associated with the single electron occupation of e g (Ti) or eg (Mn, Co, Ni) orbitals. Our results indicate that the Dirac point can be tuned to the Fermi level using strain. Applying spin-orbit coupling (SOC) leads to a substantial anomalous Hall conductivity with values up to 0.94 e 2 /h. Moreover, at a Al 2 O 3 = 4.81Å we identify a particularly strong effect of SOC with out-of-plane easy axis for (T i2O3)1/(Al2O3)5(0001) which stabilizes dynamically the system. Due to the unusually high orbital moment of -0.88µB that nearly compensates the spin moment of 1.01 µB, this system emerges as a candidate for the realization of the topological Haldane model of spinless fermions. Parallels to the perovskite analogs (LaXO3)2/(LaAlO3)4(111) are discussed.

Quantum anomalous Hall insulators, which display robust boundary charge and spin currents categorized in terms of a bulk topological invariant known as the Chern number (Thouless et al Phys. Rev. Lett. 49, 405-408 (1982)), provide the quantum Hall anomalous effect without an applied magnetic field. Chern insulators are attracting interest both as a novel electronic phase and for their novel and potentially useful boundary charge and spin currents. Honeycomb lattice systems such as we discuss here, occupied by heavy transition-metal ions, have been proposed as Chern insulators, but finding a concrete example has been challenging due to an assortment of broken symmetry phases that thwart the topological character. Building on accumulated knowledge of the behavior of the 3d series, we tune spin-orbit and interaction strength together with strain to design two Chern insulator systems with bandgaps up to 130 meV and Chern numbers C = −1 and C = 2. We find, in this class, that a trade-off between larger spin-orbit coupling and strong interactions leads to a larger gap, whereas the stronger spin-orbit coupling correlates with the larger magnitude of the Hall conductivity. Symmetry lowering in the course of structural relaxation hampers obtaining quantum anomalous Hall character, as pointed out previously; there is only mild structural symmetry breaking of the bilayer in these robust Chern phases. Recent growth of insulating, magnetic phases in closely related materials with this orientation supports the likelihood that synthesis and exploitation will follow. npj Quantum Materials (2017) 2:4 ; doi:10.1038/s41535-016-0007-2 INTRODUCTIONThe honeycomb lattice, 1,2 particularly in conjunction with its Dirac points and two-valley nature in graphene, 3 has provided the basic platform for a great number of explorations into new phases of matter and new phenomena. Yet the single band, uncorrelated case of graphene is only the simplest level of what can be realized on honeycomb lattices. The recognition that a perovskite (111) bilayer of LaXO 3 encased in LaAlO 3 (we use the notation 2LXO for an X bilayer, X=transition metal [TM]) provides a honeycomb lattice that led to a call for engineering ( i.e., design) of a Chern insulator in such systems. 4,5 The origin of the buckled honeycomb lattice is depicted in Fig. 1. A number of model 6-9 and materialspecific studies [10][11][12][13][14][15][16] have probed the possibilities that such systems may offer. The advent of (111)-oriented growth of oxides 17-25 provides a new platform for design of new materials, and especially Chern insulators.The degree of generalization from graphene is huge. Graphene has a hopping amplitude, or equivalently a velocity, that sets the energy scale, and a two-valley degree of freedom. 2LXO, on the other hand, presents a multi-orbital system with a number of additional degrees of freedom: intraatomic Coulomb repulsion U and Hund's rule spin interaction J H , cubic crystal field splitting Δ cf , trigonal crystal field splitting δ cf , spin-orbit coupling...

Using density functional theory calculations with a Hubbard U , we explore topologically nontrivial phases in X 2 O 3 honeycomb layers with X = 4d and 5d cation inserted in the band insulator α-Al 2 O 3 along the [0001]direction. Several promising candidates for quantum anomalous Hall insulators (QAHI) are identified. In particular, for X = Tc and Pt spin-orbit coupling (SOC) opens a gap of 54 and 59 meV, respectively, leading to Chern insulators (CI) with C = -2 and -1. The nature of different Chern numbers is related to the corresponding spin textures. The Chern insulating phase is sensitive to the Coulomb repulsion strength: X = Tc undergoes a transition from a CI to a trivial metallic state beyond a critical strength of U c = 2.5 eV. A comparison between the isoelectronic metastable FM phases of X = Pd and Pt emphasizes the intricate balance between electronic correlations and SOC: while the former is a trivial insulator, the latter is a Chern insulator. In addition, X = Os turns out to be a FM Mott insulator with an unpaired electron in the t 2g manifold where SOC induces an unusually high orbital moment of 0.34 µ B along the z-axis. Parallels to the 3d honeycomb corundum cases are discussed.

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