Observation of a $d$-wave gap in electron-doped Sr$_2$IrO$_4$

TL;DR

This study demonstrates that electron-doped Sr$_2$IrO$_4$ exhibits a $d$-wave gap similar to cuprates, linking low-temperature $d$-wave instability with high-temperature Fermi arcs, providing a new platform for understanding high-temperature superconductivity.

Contribution

It is the first non-cuprate material to reproduce the full cuprate phenomenology of a $d$-wave gap and Fermi arcs using spectroscopic methods.

Findings

Observation of a $d$-wave gap in electron-doped Sr$_2$IrO$_4$

Identification of gapless points with $d$-wave symmetry

Connection between low-temperature $d$-wave gap and high-temperature Fermi arcs

Abstract

High temperature superconductivity in cuprates emerges out of a highly enigmatic `pseudogap' metal phase. The mechanism of high temperature superconductivity is likely encrypted in the elusive relationship between the two phases, which spectroscopically is manifested as Fermi arcs---disconnected segments of zero-energy states---collapsing into $d$-wave point nodes upon entering the superconducting phase. Here, we reproduce this distinct cuprate phenomenology in the 5$d$ transition-metal oxide Sr$_2$IrO$_4$. Using angle-resolved photoemission, we show that clean, low-temperature phase of 6-8$\%$ electron-doped Sr$_2$IrO$_4$ has gapless excitations only at four isolated points in the Brillouin zone with a predominant $d$-wave symmetry of the gap. Our work thus establishes a connection between the low-temperature $d$-wave instability and the previously reported high-temperature Fermi arcs in electron-doped Sr$_2$IrO$_4$. Although the physical origin of the $d$-wave gap remains to be understood, Sr$_2$IrO$_4$ is a first non-cuprate material to spectroscopically reproduce the complete phenomenology of the cuprates, thus offering a new material platform to investigate the relationship between the pseudogap and the $d$-wave gap.