Quantum Criticality in a Layered Iridate

TL;DR

This study provides evidence of a quantum critical point in Co-doped Sr2IrO4, revealing heavy quantum fluctuations and coupling between electronic and phononic excitations near a metal-insulator transition.

Contribution

It demonstrates the realization of a quantum critical point in a layered iridate through Co doping, linking quantum phase transitions with heavy-fermion behavior.

Findings

Linear temperature dependence of electrical conductivity

Presence of heavy quantum critical fluctuations

Coupling between electronic continuum and optical phonon

Abstract

Iridates provide a fertile ground to investigate correlated electrons in the presence of strong spin-orbit coupling. Bringing these systems to the proximity of a metal-insulator quantum phase transition is a challenge that must be met to access quantum critical fluctuations with charge and spin-orbital degrees of freedom. Here, electrical transport and Raman scattering measurements provide evidence that a metal-insulator quantum critical point is effectively reached in 5 % Co-doped Sr$_2$IrO$_4$ with high structural quality. The dc-electrical conductivity shows a linear temperature dependence that is successfully captured by a model involving a Co acceptor level at the Fermi energy that becomes gradually populated at finite temperatures, creating thermally-activated holes in the $J_{\text {eff}}=1/2$ lower Hubbard band. The so-formed quantum critical fluctuations are exceptionally heavy and the resulting electronic continuum couples with an optical phonon at all temperatures. The magnetic order and pseudospin-phonon coupling are preserved under the Co doping. This work brings quantum phase transitions, iridates and heavy-fermion physics to the same arena.