Observation of a $d$-wave gap in electron-doped Sr$_2$IrO$_4$ (1506.06639v2)

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.

• The paper identifies a d-wave gap in electron-doped Sr₂IrO₄, mirroring the dual-phase behavior seen in cuprate superconductors.
• Employing ARPES and innovative in situ potassium deposition, the study precisely maps the Fermi surface and temperature-dependent gap variations.
• The findings highlight heterogeneous gap profiles at nodes and antinodes, offering insights into the interplay between pseudogap phenomena and potential superconducting correlations.

Observation of a $d$-wave Gap in Electron-Doped Sr$_2$IrO$_4$

The research presented in this paper focuses on the exploration of the $d$-wave gap in electron-doped Sr$_2$IrO$_4$ and its potential implications for high-temperature superconductivity (HTSC). This investigation is particularly notable as it parallels the properties of HTSC cuprates, offering a fresh perspective on the relationship between superconductivity and pseudogap phases.

Employing angle-resolved photoemission spectroscopy (ARPES), the paper investigates the low-temperature phase of electron-doped Sr$_2$IrO$_4$ with electron doping levels around 6-8%. Notably, the research identifies a $d$-wave symmetry in the gap, revealing gapless excitations at four isolated Brillouin zone points—a haLLMark of cuprate phenomenology. This significant finding confirms that Sr$_2$IrO$_4$ can reproduce the dual-phase characteristics observed in cuprates, specifically the transition from Fermi arcs to $d$-wave nodes.

The electron-doped Sr$_2$IrO$_4$ demonstrates a nodal Fermi surface at low temperatures, undetectable from single-electron band structure predictions. This behavior suggests a complex, correlated electronic phase arising from electron doping in Sr$_2$IrO$_4$. The density-functional predictions of a circular Fermi surface at the $\Gamma$ point, consistent with earlier observations at higher temperatures and doping levels, complement these findings.

Temperature-dependent measurements of the gap indicate a $k$-dependence, with the gap magnitude extending up to approximately 22 meV at 10 K, yet absent at the node, contrasting with antinode behavior. The observed nodal metal phase seems dissociated from any phase transition. This aligns with the behavior of superconducting and non-superconducting cuprates, where near-nodal gaps emerge without explicit phase transitions, potentially linked to $d$-wave pairing correlations or a yet unidentified phase.

A crucial finding of this research is the heterogeneity in the gap profile, displaying larger gaps at the antinode compared to the d-wave form-derived near-nodal gaps. This indicates potentially distinct origins for the two gaps, perhaps elucidating the pseudogap's relation with superconducting gaps—a long-standing question in HTSC research.

Methodologically, the paper overcame challenges in achieving clean electron doping by employing in situ potassium deposition for surface electron doping, eschewing the typical challenges associated with chemical doping in Sr$_2$IrO$_4$. Additionally, the research used a material engineering approach by embedding Sr$_2$IrO$_4$ into Sr$_3$Ir$_2$O$_7$, enhancing ARPES measurement capabilities at low temperatures without sample charging issues.

In conclusion, the paper establishes Sr$_2$IrO$_4$ as a promising non-cuprate platform for investigating the connection between pseudogap and $d$-wave superconducting gaps. Future work may focus on chemically doping the sample to verify the potential superconducting origin of these observations or uncover alternative quantum states that emerge, broadening our understanding of HTSC mechanisms in 5$d$ transition-metal oxides. The implications extend to developing a comprehensive theory of HTSC, emphasizing the critical role of $d$-wave symmetry in high-temperature superconductors.