We report a resonant inelastic X-ray scattering study on a single crystal of a non-stoichiometric pyrochlore iridate Tb$_{2+x}$Ir$_{2-x}$O$_{7-y}$ ($x \simeq 0.25$) that magnetically orders at $T_{\rm{N}}\simeq 50$ K. We find that the strength of the spin-orbit coupling and the trigonal distortion of the IrO$_6$ octahedra are comparable with the ones obtained in other pyrochlore iridates. We observe a propagating gapped magnon mode at low energy, and model it using a Hamiltonian consisting of a Heisenberg exchange [$J = 16.2(9)$ meV] and Dzyaloshinskii-Moriya interactions [$D = 5.2(3)$ meV], which shows the robustness of interactions despite Tb-stuffing at the Ir-site. Strikingly, the ratio $D/J = 0.32(3)$ supports possible non-trivial topological magnon band crossing. This material may thus host coexisting fermionic and bosonic topology, with potential for manipulating electronic and magnonic topological bands thanks to the $d-f$ interaction.

We report a comprehensive experimental investigation of the magnetic structure of the cycloidal phase in Ca$_3$Ru$_2$O$_7$, 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 $(\delta,0,1)$ with $\delta \simeq 0.025$ and an ordered moment of about 1 $\mu_{\rm{B}}$, with the eccentricity of the cycloid evolving with temperature. In an applied magnetic field applied parallel to the $b$-axis, the intensity of the $(\delta,0,1)$ satellite peaks decreases continuously up to about $\mu_0 H \simeq 5$ 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 $T\simeq 70$ 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 Ca$_3$Ru$_2$O$_7$ that utilised samples which were more metallic and/or poly-domain.

The layered-ruthenate family of materials possess an intricate interplay of structural, electronic and magnetic degrees of freedom that yields a plethora of delicately balanced ground states. This is exemplified by Ca3Ru2O7, which hosts a coupled transition in which the lattice parameters jump, the Fermi surface partially gaps and the spins undergo a 90∘ in-plane reorientation. Here, we show how the transition is driven by a lattice strain that tunes the electronic bandwidth. We apply uniaxial stress to single crystals of Ca3Ru2O7, using neutron and resonant x-ray scattering to simultaneously probe the structural and magnetic responses. These measurements demonstrate that the transition can be driven by externally induced strain, stimulating the development of a theoretical model in which an internal strain is generated self-consistently to lower the electronic energy. We understand the strain to act by modifying tilts and rotations of the RuO6 octahedra, which directly influences the nearest-neighbour hopping. Our results offer a blueprint for uncovering the driving force behind coupled phase transitions, as well as a route to controlling them.

Although ultrafast manipulation of magnetism holds great promise for new physical phenomena and applications, targeting specific states is held back by our limited understanding of how magnetic correlations evolve on ultrafast timescales. Using ultrafast resonant inelastic x-ray scattering we demonstrate that femtosecond laser pulses can excite transient magnons at large wavevectors in gapped antiferromagnets, and that they persist for several picoseconds which is opposite to what is observed in nearly gapless mag-nets. Our work suggests that materials with isotropic magnetic interactions are preferred to achieve rapid manipulation of magnetism.

P. Alexeev, 2 O. Leupold, I. Sergueev, M. Herlitschke, D.F. McMorrow, R.S. Perry, E.C. Hunter, R. Röhlsberger, 2, ∗ and H.-C. Wille † Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, 22607 Hamburg, Germany The Hamburg Centre for Ultrafast Imaging, Luruper Chaussee 149, 22761 Hamburg, Germany London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom (Dated: February 3, 2021)

YbAgGe contains a magnetic geometrically frustrated kagome-like lattice that features significant local single-ion anisotropy. The electronic state is established by hybridization of 4$f$ and conduction electrons, leading to heavy electronic masses. Competition between these various interactions leads to nontrivial behavior under external magnetic field. Here, the authors present neutron diffraction experiments under magnetic fields up to 8 T in the hexagonal plane, revealing the microscopic nature of the first four subsequent magnetic states, and propose further steps to understand the microscopic interactions.

C. D. Dashwood , A. Geondzhian, J. G. Vale, A. C. Pakpour-Tabrizi, C. A. Howard , Q. Faure, L. S. I. Veiga, D. Meyers, S. G. Chiuzbăian , A. Nicolaou , N. Jaouen , R. B. Jackman , A. Nag , M. García-Fernández, Ke-Jin Zhou, A. C. Walters , K. Gilmore , D. F. McMorrow, and M. P. M. Dean 5,† London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London, WC1E 6BT, United Kingdom Max Planck POSTECH/KOREA Research Initiative, 37673 Pohang, South Korea Max Planck Institute for the Structure and Dynamics of Matter, Luruper Chaussee 149, 22761 Hamburg, Germany London Centre for Nanotechnology and Department of Electronic and Electrical Engineering, University College London, London, WC1E 6BT, United Kingdom Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA Department of Physics, Oklahoma State University, Stillwater, Oklahoma 74078, USA Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, B.P. 48, 91192 Gif-sur-Yvette, France Sorbonne Université, CNRS, Laboratoire de Chimie Physique-Matiére et Rayonnement, UMR 7614, 4 place Jussieu, 75252 Paris Cedex 05, France Diamond Light Source, Didcot, Oxfordshire, OX11 0DE, United Kingdom Physics Department and IRIS Adlershof, Humboldt-Universität zu Berlin, Zum Großen Windkanal 2, 12489 Berlin, Germany European Theoretical Spectroscopy Facility (ETSF)

Although ultrafast manipulation of magnetism holds great promise for new physical phenomena and applications, targeting specific states is held back by our limited understanding of how magnetic correlations evolve on ultrafast timescales. Using ultrafast resonant inelastic x-ray scattering we demonstrate that femtosecond laser pulses can excite transient magnons at large wavevectors in gapped antiferromagnets, and that they persist for several picoseconds which is opposite to what is observed in nearly gapless magnets. Our work suggests that materials with isotropic magnetic interactions are preferred to achieve rapid manipulation of magnetism.

We report the synthesis and characterization of polycrystalline Na2RuO3, a layered material in which the Ru4+ (4d(4) configuration) form a honeycomb lattice. The optimal synthesis condition was found to produce a nearly ordered Na2RuO3 (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 [chi(0) similar to 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 (chi(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 approximate to 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 Na2RuO3 is a moderately correlated electron metal. Our results thus stand in contrast to earlier reports that Na2RuO3 is an antiferromagnetic insulator at low temperatures.

We have used neutron spectroscopy to investigate the spin dynamics of the quantum (S = 1/2) antiferromagnetic Ising chains in RbCoCl3. The structure and magnetic interactions in this material conspire to produce two magnetic phase transitions at low temperatures, presenting an ideal opportunity for thermal control of the chain environment. The high-resolution spectra we measure of two-domain-wall excitations therefore characterize precisely both the continuum response of isolated chains and the "Zeeman-ladder" bound states of chains in three different effective staggered fields in one and the same material. We apply an extended Matsubara formalism to obtain a quantitative description of the entire dataset, Monte Carlo simulations to interpret the magnetic order, and finite-temperature density-matrix renormalization-group calculations to fit the spectral features of all three phases.

We show how complex modulated order can spontaneously emerge when magnetic interactions compete in a metal with polar lattice distortions. Combining neutron and resonant x-ray scattering with symmetry analysis, we reveal that the spin reorientation in Ca_3Ru_2O_7 is mediated by a magnetic cycloid whose eccentricity evolves smoothly but rapidly with temperature. We find the cycloid to be highly sensitive to magnetic fields, which appear to continuously generate higher harmonic modulations. Our results provide a unified picture of the rich magnetic phases of this correlated, multi-band polar metal.

We present a combined polarized neutron and x-ray scattering study on two enantiopure langasite single crystals aimed at the determination of their absolute structural and magnetic chiralities and the coupling between them. Our respective data sets unambiguously reveal two samples of opposite structural chirality, where the magnetic handedness is pinned by the structural one. Simple energy considerations of the magnetic exchange and single-ion anisotropy parameters reveal that it is not the Dzyaloshinskii-Moriya interaction but the local single-ion anisotropy on a triangular plaquette which plays a key role in stabilizing one of the two magnetic helices.

N. Qureshi,1,* A. Bombardi,2 S. Picozzi,3 P. Barone ,3 E. Lelièvre-Berna ,1 X. Xu,4 C. Stock,5,6 D. F. McMorrow,7 A. Hearmon ,8 F. Fabrizi,8 P. G. Radaelli,8 S.-W. Cheong,4 and L. C. Chapon1,2 1Institut Laue-Langevin, 71 avenue des Martyrs, CS 20156, 38042 Grenoble Cedex 9, France 2Diamond Light Source, Chilton, Didcot, Oxfordshire OX11 0QX, United Kingdom 3Consiglio Nazionale Delle Ricerche, Istituto SPIN, UOS l’Aquila, Sede di Lavoro CNR-SPIN c/o Universitá G. d’Annunzio, 66100 Chieti, Italy 4Rutgers Center for Emergent Materials and Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA 5Centre for Science at Extreme Conditions, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom 6School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom 7London Centre for Nanotechnology and Department of Physics and Astronomy, University College London (UCL), Gower Street, London WC1E 6BT, United Kingdom 8Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom

High-resolution resonant inelastic x-ray scattering measurements (Delta E = 46 meV) have been performed on Cd2Os2O7 through the metal-to-insulator transition (MIT). A magnetic excitation at 125 meV evolves continuously through the MIT, in agreement with recent Raman-scattering results, and provides further confirmation for an all-in all-out magnetic ground state. Asymmetry of this feature is likely a result of coupling between the electronic and the magnetic degrees of freedom. We also observe a broad continuum of interband excitations centered at 0.3 eV energy loss. This is indicative of significant hybridization between Os 5d and O 2p states and the concurrent itinerant nature of the system. In turn, this suggests a possible breakdown of the free-ion model for Cd2Os2O7.

We report neutron diffraction measurements of the magnetic structures in two pyrochlore iridates, Yb2Ir2O7 and Lu2Ir2O7. Both samples exhibit the all-in-all-out magnetic structure on the Ir4+ sites below T-N similar or equal to 150 K, with a low temperature moment of around 0.45 mu(B)/Ir. Below 2 K, the Yb moments in Yb2Ir2O7 begin to order ferromagnetically. However, even at 40 mK the ordered moment is only 0.57(2) mu(B)/Yb, well below the saturated moment of the ground state doublet of Yb3+ (1.9 mu(B)/Yb), deduced from magnetization measurements and from a refined model of the crystal field environment, and also significantly smaller than the ordered moment of Yb in Yb2Ti2O7 (0.9 mu(B)/Yb). A mean-field analysis shows that the reduced moment on Yb is a consequence of enhanced phase competition caused by coupling to the all-in-all-out magnetic order on the Ir sublattice.

L. S. I. Veiga ,1,2 M. Etter,3 E. Cappelli,4 H. Jacobsen ,5,6 J. G. Vale,1 C. D. Dashwood,1 D. Le ,7 F. Baumberger,4,8 D. F. McMorrow,1 and R. S. Perry9 1London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom 2Diamond Light Source Ltd., Harwell Science & Innovation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom 3Deutsches Elektronen-Synchrotron (DESY), Hamburg 22607, Germany 4Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland 5Clarendon Laboratory, Department of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom 6Paul Scherrer Institute, Laboratory for Neutron Scattering and Imaging, 5232 Villigen, Switzerland 7ISIS Facility, Rutherford Appleton Laboratory, STFC, Chilton, Didcot OX11 0QX, United Kingdom 8Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen, Switzerland 9London Centre for Nanotechnology and Institute for Materials Discovery, University College London, Gower Street, London WC1E 6BT, United Kingdom

It is well established that in the low-temperature limit, the two-dimensional quantum Heisenberg antiferromagnet on a square lattice (2DQHAFSL) exhibits an anomaly in its spectrum at short-wavelengths on the zone-boundary. In the vicinity of the point the pole in the one-magnon response exhibits a downward dispersion, is heavily damped and attenuated, giving way to an isotropic continuum of excitations extending to high energies. The origin of the anomaly and the presence of the continuum are of current theoretical interest, with suggestions focused around the idea that the latter evidences the existence of spinons in a two-dimensional system. Here we present the results of neutron inelastic scattering experiments and Quantum Monte Carlo calculations on the metallo-organic compound Cu(DCOO)D2O (CFTD), an excellent physical realisation of the 2DQHAFSL, designed to investigate how the anomaly at evolves up to finite temperatures . Our data reveal that on warming the anomaly survives the loss of long-range, three-dimensional order, and that it is thus a robust feature of the two-dimensional system. With further increase of temperature the zone-boundary response gradually softens and broadens, washing out the anomaly. This is confirmed by a comparison of our data with the results of finite-temperature Quantum Monte Carlo simulations where the two are found to be in good accord. In the vicinity of the antiferromagnetic zone centre, there was no significant softening of the magnetic excitations over the range of temperatures investigated.

J. G. Vale ,1,2,* S. Calder,3,† N. A. Bogdanov ,4,5 C. Donnerer,1 M. Moretti Sala,6,‡ N. R. Davies,7 D. Mandrus,8,9 J. van den Brink,4 A. D. Christianson,3,9,10 and D. F. McMorrow1 1London Centre for Nanotechnology, University College London (UCL), Gower Street, London WC1E 6BT, United Kingdom 2Laboratory for Quantum Magnetism, École Polytechnique Fédérale de Lausanne (EPFL) CH-1015, Switzerland 3Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 4Institute for Theoretical Solid State Physics, IFW Dresden, D01171 Dresden, Germany 5Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany 6European Synchrotron Radiation Facility (ESRF), CS 40220, F-38043 Grenoble Cedex, France 7Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom 8Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA 9Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 10Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA

Received 24 March 2020Accepted 24 March 2020DOI:https://doi.org/10.1103/PhysRevB.101.169901©2020 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAntiferromagnetismMagnetic orderMagnetic phase transitionsOrder parametersPhase diagramsPhysical SystemsAntiferromagnetsPolycrystalline materialsPyrochloresTechniquesNeutron diffractionSusceptibility measurementsCondensed Matter, Materials & Applied Physics

A pressure-induced collapse of magnetic ordering in $\beta$-Li$_2$IrO$_3$ at $P_m\sim1.5- 2$ GPa has previously been interpreted as evidence for possible emergence of spin liquid states in this hyperhoneycomb iridate, raising prospects for experimental realizations of the Kitaev model. Based on structural data obtained at \emph{room temperature}, this magnetic transition is believed to originate in small lattice perturbations that preserve crystal symmetry, and related changes in bond-directional anisotropic exchange interactions. Here we report on the evolution of the crystal structure of $\beta$-Li$_2$IrO$_3$ under pressure at low temperatures ($T\leq50$ K) and show that the suppression of magnetism coincides with a change in lattice symmetry involving Ir-Ir dimerization. The critical pressure for dimerization shifts from 4.4(2) GPa at room temperature to $\sim1.5-2$ GPa below 50 K. While a direct $Fddd \rightarrow C2/c$ transition is observed at room temperature, the low temperature transitions involve new as well as coexisting dimerized phases. Further investigation of the Ir ($L_3$/$L_2$) isotropic branching ratio in x-ray absorption spectra indicates that the previously reported departure of the electronic ground state from a $J_{\rm{eff}}=1/2$ state is closely related to the onset of dimerized phases. In essence, our results suggest that the predominant mechanism driving the collapse of magnetism in $\beta$-Li$_2$IrO$_3$ is the pressure-induced formation of Ir$_2$ dimers in the hyperhoneycomb network. The results further confirm the instability of the $J_{\rm{eff}}=1/2$ moments and related non-collinear spiral magnetic ordering against formation of dimers in the low-temperature phase of compressed $\beta$-Li$_2$IrO$_3$.