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.

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.