Spins in the Vortices of a High Temperature Superconductor

Neutron scattering is used to characterise the magnetism of the vortices for the optimally doped high-temperature superconductor La(2-x)Sr(x)CuO(4) (x=0.163) in an applied magnetic field. As temperature is reduced, low frequency spin fluctuations first disappear with the loss of vortex mobility, but then reappear. We find that the vortex state can be regarded as an inhomogeneous mixture of a superconducting spin fluid and a material containing a nearly ordered antiferromagnet. These experiments show that as for many other properties of cuprate superconductors, the important underlying microscopic forces are magnetic

The magnetic structure and field dependence of the cycloid phas mediating the spin reorientation transition in Ca₃Ru₂O₇

We report a comprehensive experimental investigation of the magnetic structure of the cycloidal phase in Ca3Ru2O7, which mediates the spin reorientation transition, and establishes its magnetic phase diagram. In zero applied field, single-crystal neutron diffraction data confirms the scenario deduced from an earlier resonant x-ray scattering study: between 46.7~K <T<49.0~K the magnetic moments form a cycloid in the a−b plane with a propagation wavevector of (δ,0,1) with δ≃0.025 and an ordered moment of about 1 μB, with the eccentricity of the cycloid evolving with temperature. In an applied magnetic field applied parallel to the b-axis, the intensity of the (δ,0,1) satellite peaks decreases continuously up to about μ0H≃5 T, above which field the system becomes field polarised. Both the eccentricity of the cycloid and the wavevector increase with field, the latter suggesting an enhancement of the anti−symmetric Dzyaloshinskii−Moriya interaction via magnetostriction effects. Transitions between the various low-temperature magnetic phases have been carefully mapped out using magnetometry and resistivity. The resulting phase diagram reveals that the cycloid phase exists in a temperature window that expands rapidly with increasing field, before transitioning to a polarised paramagnetic state at 5 T. High-field magnetoresistance measurements show that below T≃70 K the resistivity increases continuously with decreasing temperature, indicating the inherent insulating nature at low temperatures of our high-quality, untwinned, single-crystals. We discuss our results with reference to previous reports of the magnetic phase diagram of Ca3Ru2O7 that utilised samples which were more metallic and/or poly-domain

Magnetic structure and field dependence of the cycloid phase mediating the spin reorientation transition in Ca3Ru2 O7

We report a comprehensive experimental investigation of the magnetic structure of the cycloidal phase in Ca3Ru2O7, which mediates the spin reorientation transition and establishes its magnetic phase diagram. In zero applied field, single-crystal neutron diffraction data confirm the scenario deduced from an earlier resonant x-ray scattering study: For 46.7K <T< 49.0 K the magnetic moments form a cycloid in the a-b plane with a propagation wave vector of (δ,0,1) with δ≃0.025 and an ordered moment of about 1μB, with the eccentricity of the cycloid evolving with temperature. In an applied magnetic field applied parallel to the b axis, the intensity of the (δ,0,1) satellite peaks decreases continuously up to about μ0H≃5T, above which field the system becomes field polarized. Both the eccentricity of the cycloid and the wave vector increase with field, the latter suggesting an enhancement of the antisymmetric Dzyaloshinskii-Moriya interaction over the symmetric exchange interactions via magnetostriction effects. Transitions between the various low-temperature magnetic phases have been carefully mapped out using magnetometry and resistivity. The resulting phase diagram reveals that the cycloid phase exists in a temperature window that expands rapidly with increasing field, before transitioning to a polarized paramagnetic state at 5 T. High-field magnetoresistance measurements show that below T≃70K the resistivity increases continuously with decreasing temperature, indicating the inherent insulating nature at low temperatures of our high-quality, untwinned, single crystals. We discuss our results with reference to previous reports of the magnetic phase diagram of Ca3Ru2O7 that utilized samples which were more metallic and/or polydomain

All-in all-out magnetic order and propagating spin-waves in Sm2Ir2O7

Using resonant magnetic x-ray scattering we address the unresolved nature of the magnetic groundstate and the low-energy effective Hamiltonian of Sm2Ir2O7, a prototypical pyrochlore iridate with a finite temperature metal-insulator transition. Through a combination of elastic and inelastic measurements, we show that the magnetic ground state is an all-in all-out (AIAO) antiferromagnet. The magnon dispersion indicates significant electronic correlations and can be well-described by a minimal Hamiltonian that includes Heisenberg exchange (J = 27:3(6) meV) and Dzyaloshinskii- Moriya interaction (D = 4:9(3) meV), which provides a consistent description of the magnetic order and excitations. In establishing that Sm2Ir2O7 has the requisite inversion symmetry preserv- ing AIAO magnetic groundstate, our results support the notion that pyrochlore iridates may host correlated Weyl semimetals

Correlated electron metal properties of the honeycomb ruthenate Na₂RuO₃

We report the synthesis and characterization of polycrystalline Na_{2}RuO_{3}, a layered material in which the Ru^{4+} (4d^{4} configuration) form a honeycomb lattice. The optimal synthesis condition was found to produce a nearly ordered Na_{2}RuO_{3} (C2/c phase), as assessed from the refinement of the time-of-flight neutron powder diffraction. Magnetic susceptibility measurements reveal a large temperature-independent Pauli paramagnetism [x_{0} ~ 1.42(2) x 10^{-3} emu/mol Oe] with no evidence of magnetic ordering down to 1.5 K, and with an absence of dynamic magnetic correlations, as evidenced by neutron scattering spectroscopy. The intrinsic susceptibility (x_{0}) together with the Sommerfeld coefficient of gamma = 11.7(2) mJ/Ru mol K^{2} estimated from heat capacity measurements gives an enhanced Wilson ratio of R_{w} ≈ 8.9(1), suggesting that magnetic correlations may be present in this material. While transport measurements on pressed pellets show nonmetallic behavior, photoemission spectroscopy indicates a small but finite density of states at the Fermi energy, suggesting that the bulk material is metallic. Except for resistivity measurements, which may have been compromised by near-surface and interface effects, all other probes indicate that Na_{2}RuO_{3} is a moderately correlated electron metal. Our results thus stand in contrast to earlier reports that Na_{2}RuO_{3} is an antiferromagnetic insulator at low temperatures

Critical fluctuations in the spin-orbit Mott Insulator Sr₃Ir₂O₇

X-ray magnetic critical scattering measurements and specific heat measurements were performed on the perovskite iridate Sr₃Ir₂O₇. We find that the magnetic interactions close to the N´eel temperature T_{N} = 283.4(2) K are threedimensional. This contrasts with previous studies which suggest two-dimensional behaviour like Sr₃IrO₄. Violation of the Harris criterion (dν > 2) means that weak disorder becomes relevant. This leads a rounding of the antiferromagnetic phase transition at T_{N}, and modifies the critical exponents relative to the clean system. Specifically, we determine that the critical behaviour of Sr₃Ir₂O₇ is representative of the diluted 3D Ising universality class

Circularly Polarized X Rays as a Probe of Noncollinear Magnetic Order in Multiferroic TbMnO3 (vol 102, 237205, 2009)

Erratum: Circularly Polarized X Rays as a Probe of Noncollinear Magnetic Order in Multiferroic TbMnO3 [Phys. Rev. Lett. 102, 237205 (2009)

Tuning of the Ru4+ ground-state orbital population in the 4d(4) Mott insulator Ca2RuO4 achieved by La doping

The ground-state orbital occupancy of the Ru4+ ion in Ca2−xLaxRuO4[x = 0, 0.05(1), 0.07(1), and 0.12(1)] was investigated by performing x-ray absorption spectroscopy (XAS) in the vicinity of the O K edge as a function of the angle between the incident beam and the surface of the single-crystal samples. A minimal model of the hybridization between the O 2p states probed at the K edge and the Ru 4d orbitals was used to analyze the XAS data, allowing the ratio of hole occupancies nxy/nyz,zx to be determined as a function of doping and temperature. For the samples displaying a low-temperature insulating ground state (x 0.07), nxy/nyz,zx is found to increase significantly with increasing doping, with increasing temperature acting to further enhance nxy/nyz,zx . For the x = 0.12 sample, which has a metallic ground state, the XAS spectra are found to be independent of temperature and not to be describable by the minimal hybridization model, while being qualitatively similar to the spectra displayed by the x 0.07 samples above their insulating to metallic transitions. To understand the origin of the evolution of the electronic structure of Ca2−xLaxRuO4 across its phase diagram, we have performed theoretical calculations based on a model Hamiltonian, comprising electron-electron correlations, crystal field , and spin-orbit coupling λ, of a Ru-O-Ru cluster, with realistic values used to parametrize the various interactions taken from the literature. Our calculations of the Ru hole occupancy as a function of /λ provide an excellent description of the general trends displayed by the data. In particular they establish that the enhancement of nxy/nyz,zx is driven by significant modifications to the crystal field as the tetragonal distortion of the RuO6 octahedral changes from compressive to tensile with La doping. We have also used our model to show that the hole occupancy of the O 2p and Ru 4d orbitals displays the same general trend as a function of /λ, thus validating the minimal hybridization model used to analyze the data. In essence, our results suggest that the predominant mechanism driving the emergence of the low-temperature metallic phase in La-doped Ca2RuO4 is the structurally induced redistribution of holes within the t2g orbitals, rather than the injection of free carriers

Nuclear resonant scattering from 193Ir as a probe of the electronic and magnetic properties of iridates

The high brilliance of modern synchrotron radiation sources facilitates experiments with high-energy x-rays across a range of disciplines, including the study of the electronic and magnetic correlations using elastic and inelastic scattering techniques. Here we report on Nuclear Resonance Scattering at the 73 keV nuclear level in 193Ir. The transitions between the hyperfine split levels show an untypically high E2/M1 multi-polarity mixing ratio combined with an increased sensitivity to certain changes in the hyperfine field direction compared to non-mixing transitions. The method opens a new way for probing local magnetic and electronic properties of correlated materials containing iridium and provides novel insights into anisotropic magnetism in iridates. In particular, unexpected out-of-plane components of magnetic hyperfine fields and non-zero electric field gradients in Sr2IrO4 have been detected and attributed to the strong spin-orbit interaction in this iridate. Due to the high, 62% natural abundance of the 193Ir isotope, no isotopic enrichment of the samples is required, qualifying the method for a broad range of applications