Correlation Effects and Hidden Spin-Orbit Entangled Electronic Order in Parent and Electron-Doped Iridates Sr2IrO4

Abstract

Analogs of the high-T c cuprates have been long sought after in transition metal oxides. Because of the strong spin-orbit coupling, the 5d perovskite iridates Sr 2 IrO 4 exhibit a low-energy electronic structure remarkably similar to the cuprates. Whether a superconducting state exists as in the cuprates requires understanding the correlated spin-orbit entangled electronic states. Recent experiments discovered hidden order in the parent and electron-doped iridates, some with striking analogies to the cuprates, including Fermi surface pockets, Fermi arcs, and pseudogap. Here, we study the correlation and disorder effects in a five-orbital model derived from the band theory. We find that the experimental observations are consistent with a d-wave spin-orbit density wave order that breaks the symmetry of a joint twofold spin-orbital rotation followed by a lattice translation. There is a Berry phase and a plaquette spin flux due to spin procession as electrons hop between Ir atoms, akin to the intersite spin-orbit coupling in quantum spin Hall insulators. The associated staggered circulating J eff ¼ 1=2 spin current can be probed by advanced techniques of spin-current detection in spintronics. This electronic order can emerge spontaneously from the intersite Coulomb interactions between the spatially extended iridium 5d orbitals, turning the metallic state into an electron-doped quasi-2D Dirac semimetal with important implications on the possible superconducting state suggested by recent experiments. . The canting of the inplane magnetic moments tracks the θ ≃ 11°staggered IrO 6 octahedra rotation about the c axis [3][4][5][6] due to the strong spin-orbit coupling (SOC). The AFM insulating state arises from a novel interplay between SOC and electron correlation most easily understood near the atomic limit. Ir 4þ has a 5d 5 configuration. The 5 electrons occupy the lower threefold t 2g orbitals separated from the higher twofold e g orbitals by the cubic crystal field Δ c . The strong atomic SOC λ SOC splits the t 2g orbitals into a low-lying J eff ¼ 3=2 spin-orbit multiplet occupied by 4 electrons and a singly occupied J eff ¼ 1=2 doublet. Assuming λ SOC and Δ c are sufficiently large compared to the relevant bandwidths when Sr 2 IrO 4 crystalizes, a single J eff ¼ 1=2 band is half filled and can be driven by a moderate local Coulomb repulsion U to an AFM Mott insulating state [1,2,7]. The nature of the spin-orbit entangled insulating state has been studied using the localized picture based on the J eff ¼ 1=2 pseudospin anisotropic Heisenberg model [7][8][9][10][11], the three-orbital Hubbard model for the t 2g electrons with SOC [12][13][14][15], and the microscopic correlated density functional theory such as the LDA þ U and GGA þ U [1, [16][17][18]. Moreover, carrier doping the AFM insulating state was proposed to potentially realize a 5d t 2g -electron analog of the 3d e g -electron high-T c cuprate superconductors [8,12,13,19,20].In this work, we study the hidden order in both stoichiometric and electron-doped Sr...

ARPES study of orbital character, symmetry breaking, and pseudogaps in doped and pure Sr2IrO4

“…On the other hand, this is already incompatible with the splitting of 0.2eV at X considered by Ref. 33 as a sign of d-wave spin-orbit density wave order. We note that such a splitting was also unclear in the raw data of Ref.…”

Unidirectional spin density wave state in metallic (Sr1−xLa x )2IrO4

“…Similarly, hole doping into Sr 2 IrO 4 also reveals parallel phenomenology such as the pseudogap and hidden order in cuprates 28–31 . The underlying nature of this pseudogap state is currently debated; however, the onset of short-range magnetic order is proposed as one possible origin.…”

Mott transition and collective charge pinning in electron doped Sr2IrO4

“…ARPES [7] and STM [8] experiments on K-covered Sr 2 IrO 4 further reported evidence for a d-wave gap closing at 30-50 K. In addition * dirk.vandermarel@unige.ch a degeneracy splitting of the bands near (π,0) was found in ARPES experiments [6] and a hidden order parameter has been claimed based on observations with optical second harmonic generation [9]. These observations have lead to different mutually exclusive speculations as to the nature of this state of matter, in particular superconducting fluctuations [7] and a d-wave pseudospin-current ordered state [10]. ARPES and STM probe the single electron spectral function.…”