Epitaxial growth, optical and electrical conductivity and electronic structure of metallic pyrochlore Bi2Ru2O7 heterostructures Marita O'Sullivan 1 , Matthew S. Dyer 2 , Michael W. Gaultois 2,3 , John B. Claridge 2 , Matthew J. Rosseinsky 2 , Jonathan Alaria 1 1 Department of Physics, University of Liverpool, UK, 2 Department of Chemistry, University of Liverpool, UK, 3 Leverhulme Research Centre for Functional Materials Design, University of Liverpool, UK Forthcoming data-hungry technologies including the internet of things, artificial intelligence (AI), big data and ‘The Bitcoin bite’ mean the high-power consumption of existing complementary metal-oxide-semiconductor (CMOS) technology based on inefficient current-controlled switching in metal-oxide-semiconductor field effect transistors is economically and environmentally unsustainable [1] . Advances beyond CMOS, to realise high speed, ultra-low energy computing, with alternative switching mechanisms require new materials systems based on new state variables including the electronic spin and topologically protected surface states. Epitaxial heterostructures composed of complex correlated metal oxides grown along specific crystallographic orientations offer a route to investigating emergent phenomena and unlocking potential functionality such as topological states [2, 3] and spin liquids [4] through geometrical lattice engineering [5] . A 2 Ru 2 O 7 pyrochlore ruthenates in particular exhibit a metal- insulator transition with varying A-cation whose mechanism is not fully understood [6] . We report on the epitaxial growth, structural and electrical properties of metallic pyrochlore bismuth ruthenate heterostructures grown along both the [001] and [111] directions. Ordered pyrochlore thin films were obtained with highly oriented texture along the [001] and [111] crystallographic directions. Density functional theory calculations of the electronic band structure and density of states indicate that B i2 Ru 2 O 7 is semimetallic and that hybridisation of the Ru 4d and Bi 6p orbitals via the anion network at the Fermi energy is responsible for the metallicity. Electrical conductivity measurements confirm that the compound is weakly metallic in agreement with the reported conductivity for the stoichiometric bulk compound. The carrier concentration and mobility of the electrons compare favourably with previous reports on bulk material and indicate strong electron-electron interactions [7] . The measured and computed optical conductivities are found to share coincident spectral features. Comparison of the electrical and optical properties of the two distinct orientations indicates differences that cannot be attributed to differences in crystalline quality or dislocations and may indicate anisotropy in the electronic structure of B i2 Ru 2 O 7 . This study will enable access to the kagome lattice arising naturally in the (111) planes of the pyrochlore B cation sublattice which may be used to uncover emergent topological properties. References 1. Jones N. Nature. 2018;561:163 - 6. 2. Yang B-J, Nagaosa N. Physical Review Letters. 2014;112(24):246402. 3. Song Z-D, Elcoro L, Xu Y-F, Regnault N, Bernevig BA. Physical Review X. 2020;10(3):031001. 4. Bovo L, Moya X, Prabhakaran D, Soh Y-A, Boothroyd AT, Mathur ND, et al. Nature Communications. 2014;5(1):3439. 5. Liu X, Middey S, Cao Y, Kareev M, Chakhalian J. MRS Communications. 2016;6(3):133-44. 6. Gaultois MW, Barton Pt Fau - Birkel CS, Birkel Cs Fau - Misch LM, Misch Lm Fau - Rodriguez EE, Rodriguez Ee Fau - Stucky GD, Stucky Gd Fau - Seshadri R, et al. J Phys Condens Matter. 2013;8(25):186004. 7. Cox PA, Egdell RG, Goodenough JB, Hamnett A, Naish CC. Journal of Physics C: Solid State Physics. 1983;16(32):6221.
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