Low-energy electrodynamics of Dirac semimetal phases in the doped Mott insulator Sr$_2$IrO$_4$

TL;DR

This study investigates the low-energy electrodynamics of Dirac semimetal phases in doped Sr$_2$IrO$_4$, revealing how electronic order and doping influence optical properties and transport, with results aligning with experimental observations.

Contribution

It provides a systematic analysis of electrodynamic properties in doped Sr$_2$IrO$_4$ using a realistic model and compares different Dirac semimetal phases, highlighting temperature effects and screening behaviors.

Findings

Strong temperature dependence in Tb-doped system's electrodynamics.

Weak temperature dependence in La-doped and line-node systems.

Consistent experimental data alignment for optical and transport properties.

Abstract

Correlated Dirac semimetal phases emerge in lightly doped (Tb- or La-doped) Mott insulator Sr$_2$IrO$_4$, where a d-wave symmetry-breaking order underlying a pseudogap plays a crucial role in determining the nature of Dirac degeneracy, i.e., whether it is a Dirac line node or Dirac point node. Here, using a realistic five-orbital tight-binding model with a Hubbard U and a semiclassical Boltzmann transport theory, we systematically study the low-energy electrodynamic properties of the Dirac semimetal phases in the paramagnetic lightly doped Sr$_2$IrO$_4$. We investigate the effects of the d-wave electronic order and electron doping concentration on the electronic band structures and optical properties of various Dirac semimetal phases. We calculate the intraband optical conductivity and obtain electrodynamic parameters of dc conductivity, scattering rate, and Drude weight for three Dirac semimetal phases: two are Dirac point-node states observed in the 3% Tb-doped and 5% La-doped Sr$_2$IrO$_4$, and the other is a Dirac line-node state. Our results show that the temperature dependence of the electrodynamic parameters is strong in the Tb-doped system while weak in the La-doped and Dirac line-node systems, which are consistent with available experimental data. Moreover, using the low-energy effective theory, we also compare the temperature-dependent screening effect in the Tb- and La-doped systems using graphene as a reference. Our paper provides valuable insight for understanding the transport and optical properties of correlated Dirac semimetal phases in the doped Sr$_2$IrO$_4$.