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Author: Efremov, D. ×

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    Manifestations of impurity-induced s±⇒s++ transition: Multiband model for dynamical response functions
    (Bristol : IOP, 2013) Efremov, D.; Golubov, A.A.; Dolgov, O.V.
    We investigate the effects of disorder on the density of states, the single-particle response function and optical conductivity in multiband superconductors with s± symmetry of the order parameter, where s± → s++ transition may take place. In the vicinity of the transition, the superconductive gapless regime is realized. It manifests itself in anomalies in the above-mentioned properties. As a result, intrinsically phase-insensitive experimental methods such as angle-resolved photoemission spectroscopy, tunneling and terahertz spectroscopy may be used to reveal information about the underlying order parameter symmetry.
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    Pressure-driven collapse of the relativistic electronic ground state in a honeycomb
    (London : Nature Publishing Group, 2018) Clancy, J.P.; Gretarsson, H.; Sears, J.A.; Singh, Y.; Desgreniers, S.; Mehlawat, K.; Layek, S.; Rozenberg, G.K.; Ding, Y.; Upton, M.H.; Casa, D.; Chen, N.; Im, J.; Lee, Y.; Yadav, R.; Hozoi, L.; Efremov, D.; Van Den Brink, J.; Kim, Y.-J.
    Honeycomb-lattice quantum magnets with strong spin-orbit coupling are promising candidates for realizing a Kitaev quantum spin liquid. Although iridate materials such as Li2IrO3 and Na2IrO3 have been extensively investigated in this context, there is still considerable debate as to whether a localized relativistic wavefunction (J eff = 1/2) provides a suitable description for the electronic ground state of these materials. To address this question, we have studied the evolution of the structural and electronic properties of α-Li2IrO3 as a function of applied hydrostatic pressure using a combination of X-ray diffraction and X-ray spectroscopy techniques. We observe striking changes even under the application of only small hydrostatic pressure (P ≤ 0.1 GPa): A distortion of the Ir honeycomb lattice (via X-ray diffraction), a dramatic decrease in the strength of spin-orbit coupling effects (via X-ray absorption spectroscopy), and a significant increase in non-cubic crystal electric field splitting (via resonant inelastic X-ray scattering). Our data indicate that α-Li2IrO3 is best described by a J eff = 1/2 state at ambient pressure, but demonstrate that this state is extremely fragile and collapses under the influence of applied pressure.