A rare combination of strong spin–orbit coupling and electron–electron correlations makes the iridate Mott insulator Sr2IrO4 a promising host for novel electronic phases of matter1, 2. The resemblance of its crystallographic, magnetic and electronic structures1, 2, 3, 4, 5, 6 to La2CuO4, as well as the emergence on doping of a pseudogap region7, 8, 9 and a low-temperature d-wave gap10, 11, has particularly strengthened analogies to cuprate high-Tc superconductors12. However, unlike the cuprate phase diagram, which features a plethora of broken symmetry phases13 in a pseudogap region that includes charge density wave, stripe, nematic and possibly intra-unit-cell loop-current orders, no broken symmetry phases proximate to the parent antiferromagnetic Mott insulating phase in Sr2IrO4 have been observed so far, making the comparison of iridate to cuprate phenomenology incomplete. Using optical second-harmonic generation, we report evidence of a hidden non-dipolar magnetic order in Sr2IrO4 that breaks both the spatial inversion and rotational symmetries of the underlying tetragonal lattice. Four distinct domain types corresponding to discrete 90°-rotated orientations of a pseudovector order parameter are identified using nonlinear optical microscopy, which is expected from an electronic phase that possesses the symmetries of a magneto-electric loop-current order14, 15, 16, 17, 18. The onset temperature of this phase is monotonically suppressed with bulk hole doping, albeit much more weakly than the Néel temperature, revealing an extended region of the phase diagram with purely hidden order. Driving this hidden phase to its quantum critical point may be a path to realizing superconductivity in Sr2IrO4.
At a glance
- Novel Jeff = 1/2 Mott state induced by relativistic spin–orbit coupling in Sr2IrO4. Phys. Rev. Lett. 101, 076402 (2008). et al.
- Phase-sensitive observation of a spin-orbital Mott state in Sr2IrO4. Science 323, 1329–1332 (2009). et al.
- Neutron scattering study of correlated phase behavior in Sr2IrO4. Phys. Rev. B 87, 144405 (2013). et al.
- Magnetic and crystal structures of Sr2IrO4: A neutron diffraction study. Phys. Rev. B 87, 140406 (2013). et al.
- Locking of iridium magnetic moments to the correlated rotation of oxygen octahedra in Sr2IrO4 revealed by X-ray resonant scattering. J. Phys. Condens. Matter 25, 422202 (2013). et al.
- Structural distortion induced magneto-elastic locking in Sr2IrO4 revealed through nonlinear optical harmonic generation. Phys. Rev. Lett. 114, 096404 (2015). et al.
- Fermi arcs in a doped pseudospin-1/2 Heisenberg antiferromagnet. Science 345, 187–190 (2014). et al.
- /2 Mott insulator Sr2IrO4. Preprint at http://arXiv.org/abs/1406.4978 (2014). et al. Hallmarks of the Mott-Metal Crossover in the Hole Doped J = 1
- http://arXiv.org/abs/1506.00616 (2015). et al. Collapse of the Mott gap and emergence of a nodal liquid in lightly doped Sr2IrO4. Preprint at
- http://arXiv.org/abs/1506.06639 (2015). , , & Observation of a d-wave gap in electron-doped Sr2IrO4. Preprint at
- http://arXiv.org/abs/1506.06557 (2015). et al. Signature of high temperature superconductivity in electron doped Sr2IrO4. Preprint at
- Twisted Hubbard model for Sr2IrO4: Magnetism and possible high temperature superconductivity. Phys. Rev. Lett. 106, 136402 (2011). &
- From quantum matter to high-temperature superconductivity in copper oxides. Nature 518, 179–186 (2015). , , , &
- Non-Fermi-liquid states and pairing instability of a general model of copper oxide metals. Phys. Rev. B 55, 14554–14580 (1997).
- Orbital currents in extended Hubbard models of high-Tc cuprate superconductors. Phys. Rev. Lett. 102, 017005 (2009). , , &
- Numerical exploration of spontaneous broken symmetries in multiorbital Hubbard models. Phys. Rev. B 90, 224507 (2014). et al.
- Optical nonreciprocity in magnetic structures related to high-Tc superconductors. Phys. Rev. Lett. 107, 067002 (2011).
- Tilted loop currents in cuprate superconductors. Physica B 460, 159–164 (2015).
- Second-harmonic generation as a tool for studying electronic and magnetic structures of crystals: Review. J. Opt. Soc. Am. B 22, 96–118 (2005). , &
- Symmetry considerations for the detection of second-harmonic generation in cuprates in the pseudogap phase. Phys. Rev. B 67, 054511 (2003). &
- Giant magnetoelectric effect in the Jeff = 1/2 Mott insulator Sr2IrO4. Phys. Rev. B 80, 140407 (2009). et al.
- Non-trivial order parameter in Sr2IrO4. Phys. Rev. B 91, 020404(R) (2015). , &
- Strange magnetic multipoles and neutron diffraction by an iridate perovskite (Sr2IrO4). J. Phys. Condens. Matter 26, 322201 (2014). &
- Mean-field analysis of intra-unit-cell order in the Emery model of the CuO2 plane. Phys. Rev. B 84, 144502 (2011). &
- Observation of orbital currents in CuO. Science 332, 696–698 (2011). et al.
- Orbital currents, anapoles, and magnetic quadrupoles in CuO. Phys. Rev. B 85, 235143 (2012). &
- Spin–orbit tuned metal-insulator transitions in single-crystal Sr2Ir1−xRhxO4 (0 ≤ x ≤ 1). Phys. Rev. B 86, 125105 (2012). et al.
- Dilute magnetism and spin-orbital percolation effects in Sr2Ir1−xRhxO4. Phys. Rev. B 89, 054409 (2014). et al.
- Monte Carlo study of an unconventional superconducting phase in iridium oxide Jeff = 1/2 Mott insulators induced by carrier doping. Phys. Rev. Lett. 110, 027002 (2013). , &
- Odd-parity triplet superconducting phase in multiorbital materials with a strong spin–orbit coupling: Application to doped Sr2IrO4. Phys. Rev. Lett. 113, 177003 (2014). , &
- A low temperature nonlinear optical rotational anisotropy spectrometer for the determination of crystallographic and electronic symmetries. Rev. Sci. Instrum. 85, 083102 (2014). , , , &
- Electronic structures of layered perovskite Sr2MO4 (M = Ru, Rh, and Ir). Phys. Rev. B 74, 113104 (2006). et al.
- Supplementary information (1,655,867 KB)