Seminarium "Teoria i Modelowanie Nanostruktur"

Peculiar quantum phenomena arising in transition metal oxides withpartially filled 3d shells, as e.g. in the manganites (colossalmagnetoresistance) or cuprates (high temperature superconductivity),owetheir existence to the interplay between the kinetic energy of theelectrons, strong electron-electron repulsion, and electron-phononinteraction.In the oxides with partially filled 5d shells, this interplay is evenmore interesting -- the on-site (relativistic) spin-orbit coupling isfar larger than in the 3d systems and can no longer be neglected [1].However, unlike in the 3d case, so far the electron-phonon couplinghasbeen assumed to be weak and has not been included in the studies ofthese systems.Nevertheless, we show that even relatively weak electron-phononcouplingmay lead to qualitatively different physics of the 5d oxides than theone discussed so far. Thus, we derive the effective interactionbetweenthe j `spin-orbit' coupled isospins which follows from theorbital-onlyinteraction induced by the cooperative Jahn-Teller effect. Next, weshowthat such interaction may, inter alia, lead to a novel type ofpropagation of the j=3/2 spin-orbit exciton in the ordered j=1/2antiferromagnet which, unlike in the pure superexchange model, doesnotrequire coupling to the j=1/2 'magnon' excitations [2]. The influenceofJahn-Teller effect on collective behaviour of electrons such asspin-orbital exciton in the $5d^5$ materials is a topic of a greatinterest and can lead to a better understanding of recent experimentalworks, e.g. RIXS (cf. [2, 3]).[1] G. K. G. Jackeli, Phys. Rev. Lett 102, 017205 (2009).[2] J. Kim et al., Phys. Rev. Lett. 108, 177003 (2012).[3] J. Kim et al., Nat. Commun. 5, 4453 (2014).