We report a combined experimental and theoretical investigation of the magnetic structure of the honeycomb lattice magnet Na2IrO3, a strong candidate for a realization of a gapless spin-liquid. Using resonant x-ray magnetic scattering at the Ir L3-edge, we find 3D long range antiferromagnetic order below TN =13.3 K. From the azimuthal dependence of the magnetic Bragg peak, the ordered moment is determined to be predominantly along the a-axis. Combining the experimental data with first principles calculations, we propose that the most likely spin structure is a novel "zig-zag" structure.

The electronic structure of the honeycomb lattice iridates Na(2)IrO(3) and Li(2)IrO(3) has been investigated using resonant inelastic x-ray scattering (RIXS). Crystal-field-split d-d excitations are resolved in the high-resolution RIXS spectra. In particular, the splitting due to noncubic crystal fields, derived from the splitting of j(eff)=3/2 states, is much smaller than the typical spin-orbit energy scale in iridates, validating the applicability of j(eff) physics in A(2)IrO(3). We also find excitonic enhancement of the particle-hole excitation gap around 0.4 eV, indicating that the nearest-neighbor Coulomb interaction could be large. These findings suggest that both Na(2)IrO(3) and Li(2)IrO(3) can be described as spin-orbit Mott insulators, similar to the square lattice iridate Sr(2)IrO(4).

We have used a combination of resonant magnetic x-ray scattering (RMXS) and x-ray absorption spectroscopy (XAS) to investigate the properties of the doped spin-orbital Mott insulator Sr2Ir1−xRhxO4 (0.07 ≤ x ≤ 0.70). We show that Sr2Ir1−xRhxO4 represents a unique model system for the study of dilute magnetism in the presence of strong spin-orbit coupling, and provide evidence of a doping-induced change in magnetic structure and a suppression of magnetic order at xc ∼ 0.17. We demonstrate that Rh-doping introduces Rh 3+ /Ir 5+ ions which effectively hole-dope this material. We propose that the magnetic phase diagram for this material can be understood in terms of a novel spin-orbital percolation picture.

We report a Fe Kβ x-ray emission spectroscopy study of local magnetic moments in the rare-earth doped iron pnictide Ca1−xRExFe2As2 (RE=La, Pr, and Nd). In all samples studied the size of the Fe local moment is found to decrease significantly with temperature and goes from ∼0.9µB at T = 300 K to ∼0.45µB at T = 70 K. In the collapsed tetragonal (cT) phase of Nd-and Pr-doped samples (T<70K) the local moment is quenched, while the moment remains unchanged for the La-doped sample, which does not show lattice collapse. Our results show that Ca1−xRExFe2As2 (RE= Pr and Nd) exhibits a spin-state transition and provide direct evidence for a non-magnetic Fe 2+ ion in the cT-phase, as predicted by Yildirim. We argue that the gradual change of the the spin-state over a wide temperature range reveals the importance of multiorbital physics, in particular the competition between the crystal field split Fe 3d orbitals and the Hund's rule coupling.PACS numbers: 74.70. Xa, 75.20.Hr, 78.70.En, 75.30.Wx The interesting orbital physics found in many 3d and 4d transition metal compounds, such as manganites [1,2] and ruthenates [2], seems to play an important role in the iron based superconductors as well [3][4][5][6][7][8][9]. In the iron pnictides, many low-energy probes such as transport [10], scanning tunnelling microscopy [11], inelastic neutron scattering [12], angle-resolved photoemission spectroscopy [13,14], and most recently magnetic torque measurements [15] have reported a strong in-plane anisotropy of electronic properties. These results have spurred a great deal of interest in the orbital physics of the iron pnictides, in particular the possibility of orbital order [3][4][5][6][7][8][9].An important aspect of the orbital physics is the competition between the Hund's rule coupling constant J H and the crystal field splitting, ∆ CF . In the case of LaCoO 3 , the energy scales of ∆ CF and J H are similar, resulting in spin-state transition; Co 3+ ions take on a low-spin state (S=0) at low temperature, but go into thermally excited high/intermediate-spin (S=2 or S=1) states at elevated temperature [16,17] Among the iron based superconductors, CaFe 2 As 2 offers perhaps the best system to investigate the competition between ∆ CF [20] and J H , and its effect on the spin-state. Like many iron pnictides, CaFe 2 As 2 goes from a high temperature tetragonal phase (T-phase) to an orthorhombic and antiferromagnetically (AFM) ordered phase, below T N ≈ 170 K [21]. More importantly, CaFe 2 As 2 takes on yet another structural phase at low temperatures through application of a modest pressure of 0.35 GPa [22] or chemical doping, with rare-earths [23] or phosphorus [24]. Upon entering this phase, known as the collapsed tetragonal phase (cT-phase), the lattice undergoes a ∼ 10% reduction along the c-axis and an ∼2% increase along the a-axis. This is accompanied by a disappearance of the AFM order [22], supression of spin fluctuations [25], and recovery of Fermi liquid behavior [24]. It is thus clear that an unusually dramatic lattice i...

The low energy excitations in Na2IrO3 have been investigated using resonant inelastic x-ray scattering (RIXS). A magnetic excitation branch can be resolved, whose dispersion reaches a maximum energy of about 35 meV at the Γ-point. The momentum dependence of the excitation energy is much larger along the Γ − X direction compared to that along the Γ − Y direction. The observed dispersion relation is consistent with a recent theoretical prediction based on Heisenberg-Kitaev model. At high temperatures, we find large contributions from lattice vibrational modes to our RIXS spectra, suggesting that a strong electron-lattice coupling is present in Na2IrO3.

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.

We used resonant inelastic x-ray scattering (RIXS) with and without analysis of the scattered photon polarization, to study dispersive spin excitations in the high temperature superconductor YBa2Cu3O6+x over a wide range of doping levels (0.1 ≤ x ≤ 1). The excitation profiles were carefully monitored as the incident photon energy was detuned from the resonant condition, and the spin excitation energy was found to be independent of detuning for all x. These findings demonstrate that the largest fraction of the spin-flip RIXS profiles in doped cuprates arises from magnetic collective modes, rather than from incoherent particle-hole excitations as recently suggested theoretically [Benjamin et al. Phys. Rev. Lett. 112, 247002(2014)]. Implications for the theoretical description of the electron system in the cuprates are discussed. PACS numbers:Electronic spin fluctuations are of central importance for current models of unconventional superconductivity in d-and f -electron compounds [1]. Inelastic neutron scattering (INS) provides comprehensive maps of the spin fluctuation intensity at energies and momenta that are well matched to the intrinsic collective response of correlated-electron systems, and has thus played a pivotal role in motivating and guiding theoretical work on unconventional superconductors [2]. Because of the limited intensity of primary neutron beams, however, INS can only be applied to materials of which large single crystals can be grown, and it is unsuitable as a probe of spin excitations in atomically thin heterostructures of complex materials, which provide perspectives for control -and ultimately design -of unconventional superconductivity [3].Resonant inelastic x-ray scattering (RIXS) at transition-metal L 2,3 -edges has recently emerged as a powerful momentum-resolved spectroscopic probe of collective spin excitations in crystals of sub-millimeter dimensions, and in thin films and multilayers [4,5]. Recent examples of RIXS studies of spin excitations include cuprates [6][7][8][9][10][11][12][13][14][15][16][17], iron-based superconductors [18] or iridates [19], where the intrinsic energy scale of the spin dynamics exceeds 100 meV. Initial RIXS data on the dispersion of magnons in the antiferromagnetic "parent compounds" of the cuprate high-temperature superconductors are fully consistent with prior INS data [6,12]. Remarkably, further RIXS studies revealed that magnon-like collective spin excitations persist in almost undiminished form even in optimally doped and overdoped cuprates, [12][13][14][15][16] where INS data are very limited. This indicates that strong electronic correlations persist even in a regime where Fermi-liquid properties have been well documented [20,21]. Motivated by these results, soft x-ray RIXS spectrometers with greatly enhanced resolution are currently under construction at many synchrotron facilities worldwide.To realize the potential of RIXS as a probe of unconventional superconductors and other correlated-electron systems, it is imperative to develop a quantitative...

We study the structural and magnetic orders in electron-doped BaFe2−xNixAs2 by high-resolution synchrotron X-ray and neutron scatterings. Upon Ni-doping x, the nearly simultaneous tetragonalto-orthorhombic structural (Ts) and antiferromagnetic (TN ) phase transitions in BaFe2As2 are gradually suppressed and separated, resulting in Ts > TN with increasing x as was previously observed. However, the temperature separation between Ts and TN decreases with increasing x for x ≥ 0.065, tending towards a quantum bi-critical point near optimal superconductivity at x ≈ 0.1. The zerotemperature transition is preempted by the formation of a secondary incommensurate magnetic phase in the region 0.088 x 0.104, resulting in a finite value of TN ≈ Tc + 10 K above the superconducting dome around x ≈ 0.1. Our results imply an avoided quantum critical point, which is expected to strongly influence the properties of both the normal and superconducting states. A determination of the structural and magnetic phase diagram in correlated electron materials is important for understanding their underlying electronic excitations. In the iron pnictides, superconductivity arises at the border of both antiferromagnetic (AF) and structural orders [1][2][3][4][5]. This motivates the exploration of quantum critical points, where the transition temepratures for such orders are continuously suppressed to zero by a non-thermal control parameter. For the iron pnictide superconductors derived from electron or hole doping of their parent compounds, the most heavily studied materials are probably the electron-doped BaFe 2−x T x As 2 (where T = Co, Ni) because of the availability of high-quality single crystals [1, 6-15, 17, 18]. In the undoped state, BaFe 2 As 2 exhibits a tetragonal-to-orthorhombic structural transition at temperature T s and an AF phase transitions below nearly the same temperature T N ≈ T s ≈ 138 K [3, 4]. Upon electron-doping of BaFe 2 As 2 via partially replacing Fe by Co or Ni, various experiments, including transport [8,9], neutron [1,[11][12][13][14][15], and highresolution X-ray scattering [4,18] reveal that the structural (T s ) and magnetic (T N ) phase transition temperatures in BaFe 2−x T x As 2 gradually decrease and separate with increasing x, and have T s > T N for all doping levels. In the initial X-ray [10] and neutron [11] scattering work on BaFe 2−x Co x As 2 , it was suggested that the separated T s and T N smoothly extend into the superconducting dome, resulting in distinct structural and magnetic quantum critical points at different x. Subsequent X-ray [18] and neutron [12][13][14] scattering experiments on superconducting BaFe 2−x T x As 2 samples with coexisting AF order revealed that superconductivity actually competes with the static AF order and lattice orthorhombicity. As a consequence, the smoothly decreasing T s and T N are reported to bend back below T c , and the orthorhombic structure above T c for optimally doped sample evolves back to a tetragonal structure well below T c (termed the "re-entrant" tet...