Nonsymmorphic Dirac semimetal and carrier dynamics in the doped spin-orbit-coupled Mott insulator Sr2IrO4

Abstract

A Dirac fermion emerges as a result of interplay between symmetry and topology in condensed matter. Current research moves towards investigating the Dirac fermions in the presence of many-body effects in correlated systems. Here, we demonstrate the emergence of a correlation-induced symmetry-protected Dirac semimetal state in the lightly doped spin-orbit-coupled Mott insulator Sr2IrO4. We find that the nonsymmorphic crystalline symmetry stabilizes a Dirac line-node semimetal and that the correlation-induced symmetry-breaking electronic order further leads to a phase transition from the Dirac line-node to a Dirac point-node semimetal. The latter state is experimentally confirmed by angle-resolved photoemission spectroscopy and terahertz spectroscopy on Sr-2(Ir, Tb)O-4 and (Sr, La)(2)IrO4. Remarkably, the electrodynamics of the massless Dirac carriers is governed by the extremely small scattering rate of about 6 cm(-1) even at room temperature, which is iconic behavior of relativistic quasiparticles. Temperature-dependent changes in electrodynamic parameters are also consistently explained based on the Dirac point-node semimetal state.