Observation of a d-wave gap in electron-doped Sr2IrO4

Abstract

High-temperature superconductivity in cuprates emerges out of a highly enigmatic 'pseudogap' metal phase. The mechanism of high-temperature superconductivity is probably 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 5d transition-metal oxide Sr 2 IrO 4 . Using angle-resolved photoemission, we show that the 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 (ref. 1). Although the physical origin of the d-wave gap remains to be understood, Sr 2 IrO 4 is the first non-cuprate material to spectroscopically reproduce the complete phenomenology of the cuprates, thus o ering a new material platform to investigate the relationship between the pseudogap and the d-wave gap.Sr 2 IrO 4 is a single-electron-band magnetic insulator with pseudospin-1/2 moments 2,3 on a square lattice 4 . Despite very strong spin-orbit coupling inherent to 5d transition-metal elements, magnetic interactions between iridium pseudospins are predominantly of Heisenberg type 5 , with a large energy scale 6,7 as reflected in their magnon bandwidth of ∼200 meV. This one-to-one correspondence between Sr 2 IrO 4 and high-temperature superconducting (HTSC) cuprates in their lattice, electronic and magnetic structures allows us to present, in Fig. 1a,b, angle-resolved photoemission (ARPES) intensity maps of approximately 7% electron-doped Sr 2 IrO 4 recorded at the Fermi level following the standard notations used in the cuprate literature. A sign difference between the two systems in one of the tight-binding parameters 8 characterizing the hopping of an electron in a quasi-two-dimensional (2D) square lattice renders Fermi surfaces shifted by (π, π) with respect to each other in their non-interacting electron descriptions. This sign of the next-nearest hopping can be reversed by electron-hole conjugation, which means that our results on electron-doped Sr 2 IrO 4 can be directly compared to those of hole-doped cuprates 9 .

Magnetic ground state ofSr2IrO4and implications for second-harmonic generation

“…The formation of a half-filled t 2g doublet by the strong Ir spin-orbit interaction, that is then gapped by correlations, mimics what is seen in the cuprates, making Sr 2 IrO 4 an insulator, despite its Ir 4+ ionic configuration with five occupied t 2g electrons [1,2]. More recent experiments on doped iridates point to the emergence of a pseudogap [3] and at low temperatures a d-wave gap [4], therefore strengthening the analogy with cuprates. Most recently, a new experiment based on second-harmonic generation (SHG) [5] claimed the detection of an oddparity, magnetic hidden order in Sr 2 IrO 4 , suggesting the presence of orbital currents as proposed by Varma for cuprates [6].…”

Crystal growth and intrinsic magnetic behaviour of Sr2IrO4

“…La-doped Sr 2 IrO 4 single crystals have only become available in the last couple of years [21][22][23] , but angle-resolved photoemission studies on them 21,22 do not fully reproduce the observations made via the in situ doping method 12,16 . While it is unclear at the moment what leads to such differences between the two doping methods, we note that a well-defined quasiparticle peak is absent for any doping level in La-doped samples, indicating suppressed coherent motion of doped electrons presumably due to disorder created by chemical doping.…”

“…Most recently, tantalizing indications of d-wave superconductivity have been seen in electron-doped Sr 2 IrO 4 16,17 : a gap of predominantly d-wave symmetry is observed in the low-temperature angle-resolved photoemission spectra 16 , and a particle-hole symmetric density of states is observed in scanning tunneling spectra below a temperature as high as 50 K 17 . Above this temperature, the d-wave gap evolves into a pseudogap, in striking parallel with the cuprates 12,16 .…”

“…Above this temperature, the d-wave gap evolves into a pseudogap, in striking parallel with the cuprates 12,16 . However, because of the inherently surface sensitive nature of the technique used to dope the sample in these studies, involving in situ deposition of alkali metal atoms on the surface of undoped Sr 2 IrO 4 , it remains unclear at this point whether the dwave gap originates from superconductivity without evidence from bulk measurements.…”

Density wave like transport anomalies in surface doped Na2IrO3

“…2, 3 Recently in situ surface electron doping by potassium deposition has been achieved, and it was found that Fermi arcs, pseudogap, and a low-temperature d-wave gap exist for these samples demonstrating properties in complete analogy with the cuprates. [5][6][7] These studies on Sr 2 IrO 4 suggest that properties of other iridates might also be highly tunable. Recently the layered honeycomb lattice Iridates A 2 IrO 3 (A = Na, Li) have been recognized as spin-orbit assisted J eff = 1/2 Mott insulators.…”