353 research outputs found

    SmO thin films: a flexible route to correlated flat bands with nontrivial topology

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    Using density functional theory based calculations, we show that the correlated mixed-valent compound SmO is a 3D strongly topological semi-metal as a result of a 4ff-5dd band inversion at the X point. The [001] surface Bloch spectral density reveals two weakly interacting Dirac cones that are quasi-degenerate at the M_bar-point and another single Dirac cone at the Gamma_bar-point. We also show that the topological non-triviality in SmO is very robust and prevails for a wide range of lattice parameters, making it an ideal candidate to investigate topological nontrivial correlated flat bands in thin-film form. Moreover, the electron filling is tunable by strain. In addition, we find conditions for which the inversion is of the 4f-6s type, making SmO to be a rather unique system. The similarities of the crystal symmetry and the lattice constant of SmO to the well studied ferromagnetic semiconductor EuO, makes SmO/EuO thin film interfaces an excellent contender towards realizing the quantum anomalous Hall effect in a strongly correlated electron system.Comment: Paper+supplemen

    Insulating state and the importance of the spin-orbit coupling in Ca3_3CoRhO6_6

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    We have carried out a comparative theoretical study of the electronic structure of the novel one-dimensional Ca3_3CoRhO6_6 and Ca3_3FeRhO6_6 systems. The insulating antiferromagnetic state for the Ca3_3FeRhO6_6 can be well explained by band structure calculations with the closed shell high-spin d5d^5 (Fe3+^{3+}) and low-spin t2g6t_{2g}^{6} (Rh3+^{3+}) configurations. We found for the Ca3_3CoRhO6_6 that the Co has a strong tendency to be d7d^7 (Co2+^{2+}) rather than d6d^6 (Co3+^{3+}), and that there is an orbital degeneracy in the local Co electronic structure. We argue that it is the spin-orbit coupling which will lift this degeneracy thereby enabling local spin density approximation + Hubbard U (LSDA+U) band structure calculations to generate the band gap. We predict that the orbital contribution to the magnetic moment in Ca3_3CoRhO6_6 is substantial, i.e. significantly larger than 1 μB\mu_B per formula unit. Moreover, we propose a model for the contrasting intra-chain magnetism in both materials.Comment: 7 pages, 4 figures, and 1 tabl

    Verwey transition in Fe3_{3}O4_{4} thin films: Influence of oxygen stoichiometry and substrate-induced microstructure

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    We have carried out a systematic experimental investigation to address the question why thin films of Fe3_3O4_4 (magnetite) generally have a very broad Verwey transition with lower transition temperatures as compared to the bulk. We observed using x-ray photoelectron spectroscopy, x-ray diffraction and resistivity measurements that the Verwey transition in thin films is drastically influenced not only by the oxygen stoichiometry but especially also by the substrate-induced microstructure. In particular, we found (1) that the transition temperature, the resistivity jump, and the conductivity gap of fully stoichiometric films greatly depends on the domain size, which increases gradually with increasing film thickness, (2) that the broadness of the transition scales with the width of the domain size distribution, and (3) that the hysteresis width is affected strongly by the presence of antiphase boundaries. Films grown on MgO (001) substrates showed the highest and sharpest transitions, with a 200 nm film having a TV_V of 122K, which is close to the bulk value. Films grown on substrates with large lattice constant mismatch revealed very broad transitions, and yet, all films show a transition with a hysteresis behavior, indicating that the transition is still first order rather than higher order.Comment: 9 pages, 12 figure

    Nature of magnetism in Ca3_3Co2_2O6_6

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    We find using LSDA+U band structure calculations that the novel one-dimensional cobaltate Ca3_3Co2_2O6_6 is not a ferromagnetic half-metal but a Mott insulator. Both the octahedral and the trigonal Co ions are formally trivalent, with the octahedral being in the low-spin and the trigonal in the high-spin state. The inclusion of the spin-orbit coupling leads to the occupation of the minority-spin d2d_{2} orbital for the unusually coordinated trigonal Co, producing a giant orbital moment (1.57 μB\mu_{B}). It also results in an anomalously large magnetocrystalline anisotropy (of order 70 meV), elucidating why the magnetism is highly Ising-like. The role of the oxygen holes, carrying an induced magnetic moment of 0.13 μB\mu_{B} per oxygen, for the exchange interactions is discussed.Comment: 5 pages, 4 figures, and 1 tabl

    Strong spin-orbit coupling effects on the Fermi surface of Sr2RuO4 and Sr2RhO4

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    We present a first-principle study of spin-orbit coupling effects on the Fermi surface of Sr2RuO4 and Sr2RhO4. For nearly degenerate bands, spin-orbit coupling leads to a dramatic change of the Fermi surface with respect to non-relativistic calculations; as evidenced by the comparison with experiments on Sr2RhO4, it cannot be disregarded. For Sr2RuO4, the Fermi surface modifications are more subtle but equally dramatic in the detail: spin-orbit coupling induces a strong momentum dependence, normal to the RuO2 planes, for both orbital and spin character of the low-energy electronic states. These findings have profound implications for the understanding of unconventional superconductivity in Sr2RuO4.Comment: A high-resolution version can be found at http://www.physics.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/SO_Sr2RuO4.pd

    Hybridization gap and Fano resonance in SmB6{_6}

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    We present results of Scanning Tunneling Microscopy and Spectroscopy (STS) measurements on the "Kondo insulator" SmB6_6. The vast majority of surface areas investigated was reconstructed but, infrequently, also patches of varying size of non-reconstructed, Sm- or B-terminated surfaces were found. On the smallest patches, clear indications for the hybridization gap and inter-multiplet transitions were observed. On non-reconstructed surface areas large enough for coherent co-tunneling we were able to observe clear-cut Fano resonances. Our locally resolved STS indicated considerable finite conductance on all surfaces independent of their structure.Comment: 5 pages, 4 figure

    Importance of tetrahedral coordination for high-valent transition metal oxides: YCrO4_4 as a model system

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    We have investigated the electronic structure of the high oxidation state material YCrO4_4 within the framework of the Zaanen-Sawatzky-Allen phase diagram. While Cr4+^{4+}-based compounds like SrCrO3_3/CaCrO3_3 and CrO2_2 can be classified as small-gap or metallic negative-charge-transfer systems, we find using photoelectron spectroscopy that YCrO4_4 is a robust insulator despite the fact that its Cr ions have an even higher formal valence state of 5+. We reveal using band structure calculations that the tetrahedral coordination of the Cr5+^{5+} ions in YCrO4_4 plays a decisive role, namely to diminish the bonding of the Cr 3d3d states with the top of the O 2p2p valence band. This finding not only explains why the charge-transfer energy remains effectively positive and the material stable, but also opens up a new route to create doped carriers with symmetries different from those of other transition-metal ions.Comment: 6 pages, 6 figure

    Electronic and magnetic properties of the kagome systems YBaCo4O7 and YBaCo3MO7 (M=Al, Fe)

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    We present a combined experimental and theoretical x-ray absorption spectroscopy (XAS) study of the new class of cobaltates YBaCo4O7 and YBaCo3MO7 (M= Al, Fe). The focus is on the local electronic and magnetic properties of the transition metal ions in these geometrically frustrated kagome compounds. For the mixed valence cobaltate YBaCo4O7, both the Co2+ and Co3+ are found to be in the high spin state. The stability of these high spin states in tetrahedral coordination is compared with those in the more studied case of octahedral coordination. For the new compound YBaCo3FeO7, we find exclusively Co2+ and Fe3+ as charge states

    Disorder-driven electronic localization and phase separation in superconducting Fe1+yTe0.5Se0.5 single crystals

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    We have investigated the influence of Fe-excess on the electrical transport and magnetism of Fe1+yTe0.5Se0.5 (y=0.04 and 0.09) single crystals. Both compositions exhibit resistively determined superconducting transitions (Tc) with an onset temperature of about 15 K. From the width of the superconducting transition and the magnitude of the lower critical field Hc1, it is inferred that excess of Fe suppresses superconductivity. The linear and non-linear responses of the ac-susceptibility show that the superconducting state for these compositions is inhomogeneous. A possible origin of this phase separation is a magnetic coupling between Fe-excess occupying interstitial sites in the chalcogen planes and those in the Fe-square lattice. The temperature derivative of the resistivity drho/dT in the temperature range Tc < T < Ta with Ta being the temperature of a magnetic anomaly, changes from positive to negative with increasing Fe. A log 1/T divergence of the resistivity above Tc in the sample with higher amount of Fe suggests a disorder driven electronic localization.Comment: 7 page
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