Nonsymmorphic Dirac semimetal and carrier dynamics in doped spin-orbit-coupled Mott insulator Sr$_2$IrO$_4$

TL;DR

This study reveals a correlation-induced Dirac semimetal state in doped Sr$_2$IrO$_4$, stabilized by nonsymmorphic symmetry, with experimental confirmation of Dirac carriers exhibiting relativistic-like electrodynamics at room temperature.

Contribution

It demonstrates the emergence of a symmetry-protected Dirac semimetal in a correlated spin-orbit Mott insulator, highlighting the role of nonsymmorphic symmetry and many-body effects.

Findings

Nonsymmorphic symmetry stabilizes a Dirac line-node semimetal.

Correlation effects induce a transition to a Dirac point-node semimetal.

Dirac carriers exhibit extremely low scattering rates at room temperature.

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 manybody effects in correlated system. Here, we demonstrate the emergence of correlation-induced symmetry-protected Dirac semimetal state in the lightly-doped spin-orbit-coupled Mott insulator Sr$_2$IrO$_4$. 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)$_2$IrO$_4$. 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.