We have studied the magnetic excitations of electron-doped Sr_{2-x}La_{x}IrO_{4} (0≤x≤0.10) using resonant inelastic x-ray scattering at the Ir L_{3} edge. The long-range magnetic order is rapidly lost with increasing x, but two-dimensional short-range order (SRO) and dispersive magnon excitations with nearly undiminished spectral weight persist well into the metallic part of the phase diagram. The magnons in the SRO phase are heavily damped and exhibit anisotropic softening. Their dispersions are well described by a pseudospin-1/2 Heisenberg model with exchange interactions whose spatial range increases with doping. We also find a doping-independent high-energy magnetic continuum, which is not described by this model. The spin-orbit excitons arising from the pseudospin-3/2 manifold of the Ir ions broaden substantially in the SRO phase, but remain largely separated from the low-energy magnons. Pseudospin-1/2 models are therefore a good starting point for the theoretical description of the low-energy magnetic dynamics of doped iridates.

Spin-orbit entangled magnetic dipoles, often referred to as pseudospins, provide a new avenue to explore novel magnetism inconceivable in the weak spin-orbit coupling limit, but the nature of their low-energy interactions remains to be understood. We present a comprehensive study of the static magnetism and low-energy pseudospin dynamics in the archetypal spin-orbit Mott insulator Sr2IrO4. We find that in order to understand even basic magnetization measurements, a formerly overlooked in-plane anisotropy is fundamental. In addition to magnetometry, we use neutron diffraction, inelastic neutron scattering and resonant elastic and inelastic x-ray scattering to identify and quantify the interactions that determine the global symmetry of the system and govern the linear responses of pseudospins to external magnetic fields and their low-energy dynamics. We find that a pseudospin-only Hamiltonian is insufficient for an accurate description of the magnetism in Sr2IrO4 and that pseudospin-lattice coupling is essential. This finding should be generally applicable to other pseudospin systems with sizable orbital moments sensitive to anisotropic crystalline environments.

Over the last few years, Sr 2 IrO 4 , a single-layer member of the Ruddlesden-Popper series iridates, has received much attention as a close analog of cuprate high-temperature superconductors.Although there is not yet firm evidence for superconductivity, a remarkable range of cuprate phenomenology has been reproduced in electron-and hole-doped iridates including pseudogaps, Fermi arcs, and d-wave gaps. Further, a number of symmetry breaking orders reminiscent of those decorating the cuprate phase diagram have been reported using various experimental probes. We discuss how the electronic structures of Sr 2 IrO 4 through strong spin-orbit coupling leads to the low-energy physics that had long been unique to cuprates, what the similarities and differences between cuprates and iridates are, and how these advance the field of high-temperature superconductivity by isolating essential ingredients of superconductivity from a rich array of phenomena that surround it. Finally, we comment on the prospect of finding a new high-temperature superconductor based on the iridate series. b c a Sr O Ir a b c pseudospin-1/2 J a b FIG. 1. Crystal and magnetic structures of Sr 2 IrO 4 . (a) The crystal structure is based on Ref. [26] according to which the space group is I4 1 /acd. However, recent studies indicate that the symmetry is lower and most probably is I4 1 /a [27-29]. (b) Quasi-2D network of Ir and O. (c) The magnetic structure from Ref. [2]. II. MAPPING ONTO CUPRATE PHYSICSSoon after the discovery of HTSC in the cuprates, many complex oxides with a K 2 NiF 4 structure (isostructural to La 2 CuO 4 ) and its variants were searched for signs of superconductivity. Among these were Sr 2 RhO 4 and Sr 2 IrO 4 [30], which are "one-hole" systems, albeit with a t 2g -hole in the low-spin d 5 configuration as opposed to an e g -hole in cuprates. However, it was quickly apparent that 4d and 5d systems tend to be rather weakly correlated. In fact, Sr 2 RhO 4 is a Fermi liquid metal [31, 32] with a Fermi surface understood qualitatively in terms of a band structure calculated using density functional theory within the local density approximation (LDA) [33, 34]. Interestingly, the Fermi surfaces of Sr 2 RhO 4 and Sr 2 IrO 4 , calculated without SOC, are near indistinguishable due to their almost identical crystal structures with less than 1% difference in their lattice parameters [30]. A closer inspection, however, reveals that LDA does not accurately replicate the measured FermiX M X E-μ (eV) E-μ (eV) E-μ (eV) M X Γ Γ Γ Γ Γ Γ U ζ ζ μ wide band metal narrow J eff =1/2 band J eff =1/2 Mott state LDA+SOC LDA LDA+SOC+U t 2g band J eff =1/2 band J eff =3/2 band J eff =3/2 band J eff =1/2 UHB J eff =1/2 LHB FIG. 2. Illustration of the SOC driven Mott transition. Introduction of SOC splits off a narrow band near the Fermi level (orange solid lines), for which a moderate Coulomb interaction U ∼2 eV is sufficient to open a gap.observation of a significant gap reduction across T N , a typical Slater behavior, supports this viewpoint [42,43]. On th...

The electric-current stabilized semi-metallic state in the quasi-two-dimensional Mott insulator Ca 2 RuO 4 exhibits an exceptionally strong diamagnetism. Through a comprehensive study using neutron and X-ray diffraction, we show that this non-equilibrium phase assumes a crystal structure distinct from those of equilibrium metallic phases realized in the ruthenates by chemical doping, high pressure and epitaxial strain, which in turn leads to a distinct electronic band structure. Dynamical mean field theory calculations based on the crystallographically refined atomic coordinates and realistic Coulomb repulsion parameters indicate a semi-metallic state with partially gapped Fermi surface. Our neutron diffraction data show that the non-equilibrium behavior is homogeneous, with antiferromagnetic long-range order completely suppressed. These results provide a new basis for theoretical work on the origin of the unusual non-equilibrium diamagnetism in Ca 2 RuO 4 .

We report on neutron diffraction, thermal expansion, magnetostriction, dielectric, and specific heat measurements on polycrystalline FeCr2S4 in external magnetic fields. The ferrimagnetic ordering temperatures TC ≈ 170 K and the transition at TOO ≈ 10 K, which has been associated with orbital ordering, are only weakly shifted in magnetic fields up to 9 T. The cubic lattice parameter is found to decrease when entering the state below TOO. The magnetic moments of the Cr- and Fe-ions are reduced from the spin-only values throughout the magnetically ordered regime, but approach the spin-only values for fields >5.5 T. Thermal expansion in magnetic fields and magnetostriction experiments indicate a contraction of the sample below about 60 K. Below TOO this contraction is followed by a moderate expansion of the sample for fields larger than ~4.5 T. The transition at TOO is accompanied by an anomaly in the dielectric constant. The dielectric constant depends on both the strength and orientation of the external magnetic field with respect to the applied electric field for T < TOO. A linear correlation of the magnetic-field-induced change of the dielectric constant and the magnetic-field dependent magnetization is observed. This behaviour is consistent with the existence of a ferroelectric polarization and a multiferroic ground state below 10 K.