One view of the cuprate high-transition temperature (high-T c ) superconductors is that they are conventional superconductors where the pairing occurs between weakly interacting quasiparticles, which stand in one-to-one correspondence with the electrons in ordinary metals -although the theory has to be pushed to its limit [1]. An alternative view is that the electrons organize into collective textures (e.g. charge and spin stripes) which cannot be mapped onto the electrons in ordinary metals. The phase diagram, a complex function of various parameters (temperature, doping and magnetic field), should then be approached using quantum field theories of objects such as textures and strings, rather than point-like electrons [2,3,4,5,6]. In an external magnetic field, magnetic flux penetrates type-II superconductors via vortices, each carrying one flux quantum [7]. The vortices form lattices of resistive material embedded in the non-resistive superconductor and can reveal the nature of the ground state -e.g. a conventional metal or an ordered, striped phase -which would have appeared had superconductivity not intervened. Knowledge of this ground state clearly provides the most appropriate starting point for a pairing theory. Here we report that for one high-T c superconductor, the applied field which imposes the vortex lattice, also induces antiferromagnetic order. Ordinary quasiparticle pictures cannot account for the nearly fieldindependent antiferromagnetic transition temperature revealed by our measurements.La 2-x Sr x CuO 4 , is the simplest high-T c superconductor. The undoped compound is an insulating antiferromagnet, where the spin moments on adjacent Cu 2+ ions are antiparallel [8]. Introduction of charge carriers via Sr doping reduces the ordered moment until it vanishes at x<0.13. In addition, for x>0.05 the commensurate antiferromagnetism is replaced by incommensurate order [2,3,9,10], where the repeat distance for the pattern of ordered moments is substantially larger than the spacing between neighbouring copper ions. La 2-x Sr x CuO 4 becomes a 2 superconductor for Sr dopings of 0.06

The excitations responsible for producing high-temperature superconductivity in the copper oxides have yet to be identified. Two promising candidates are collective spin excitations and phonons 1 . A recent argument against spin excitations is based on their inability to explain structures observed in electronic spectroscopies such as photoemission 2-5 and optical conductivity 6,7 . Here, we use inelastic neutron scattering to demonstrate that collective spin excitations in optimally doped La 2−x Sr x CuO 4 are more structured than previously thought. The excitations have a two-component structure with a lowfrequency component strongest around 18 meV and a broader component peaking near 40-70 meV. The second component carries most of the spectral weight and its energy matches structures observed in photoemission 2-5 in the range 50-90 meV. Our results demonstrate that collective spin excitations can explain features of electronic spectroscopies and are therefore likely to be strongly coupled to the electron quasiparticles.Since their discovery, considerable progress has been made in understanding the properties of the high-critical-temperature, T c , cuprate superconductors. We know, for example, that the superconductivity involves Cooper pairs, but with d-wave rather than the s-wave pairing of conventional Bardeen-CooperSchrieffer (BCS) superconductors. One outstanding issue is the pairing mechanism itself. For conventional superconductors, identifying the bosonic excitations that strongly couple to the electron quasiparticles played a pivotal role in confirming the phonon-mediated pairing mechanism 8,9 . In the case of the copper oxide superconductors, electronic spectroscopies such as angle-resolved photoemission (ARPES) and infrared optical conductivity measurements 6,7 have revealed structures in the lowenergy electronic excitations, which may reflect coupling to bosonic excitations. ARPES measurements on Bi 2 Sr 2 CaCu 2 O 8 , Bi 2 Sr 2 CuO 6 and La 2−x Sr x CuO 4 have shown rapid changes or 'kinks' in the quasiparticle dispersion, E(k), for energies in the range 50-80 meV (refs 2-5). These features in ARPES have been interpreted in terms of coupling to phonon modes 5 . However, the ARPES measurements do not distinguish between coupling to lattice and spin excitations. Identifying phonons as the strongly coupled bosons is not without its difficulties: we must explain what is special about the phonons in the cuprates; interactions with phonons do not naturally explain other important properties of the cuprates, such as the large linear temperature dependence of the normal-state resistivity at optimal doping and the origin of d-wave symmetry of the superconducting gap itself.The interpretation of the kinks and other features in electronic spectroscopies 2-7 in terms of coupling to collective spin excitations 10 has been hampered by the lack of magnetic spectroscopy data. Most neutron scattering data refer to YBa 2 Cu 3 O 6+x , a compound for which ARPES data are scarce. Although ARPES kinks have also been re...

High-resolution neutron scattering experiments on optimally doped La2-xSrxCuO4 (x=0.16) reveal that the magnetic excitations are dispersive. The dispersion is the same as in YBa2Cu3O6.85, and is quantitatively related to that observed with charge sensitive probes. The associated velocity in La2-xSrxCuO4 is only weakly dependent on doping with a value close to the spin-wave velocity of the insulating (x=0) parent compound. In contrast with the insulator, the excitations broaden rapidly with increasing energy, forming a continuum at higher energy and bear a remarkable resemblance to multiparticle excitations observed in 1D S=1/2 antiferromagnets. The magnetic correlations are 2D, and so rule out the simplest scenarios where the copper oxide planes are subdivided into weakly interacting 1D magnets.

The magnetic structure and electronic ground state of the layered perovskite Ba(2)IrO(4) have been investigated using x-ray resonant magnetic scattering. Our results are compared with those for Sr(2)IrO(4), for which we provide supplementary data on its magnetic structure. We find that the dominant, long-range antiferromagnetic order is remarkably similar in the two compounds and that the electronic ground state in Ba(2)IrO(4), deduced from an investigation of the x-ray resonant magnetic scattering L(3)/L(2) intensity ratio, is consistent with a J(eff)=1/2 description. The robustness of these two key electronic properties to the considerable structural differences between the Ba and Sr analogues is discussed in terms of the enhanced role of the spin-orbit interaction in 5d transition metal oxides.

Sr2IrO4 is a prototype of the class of Mott insulators in the strong spin-orbit interaction (SOI) limit described by a Jeff = 1/2 ground state. In Sr2IrO4, the strong SOI is predicted to manifest itself in the locking of the canting of the magnetic moments to the correlated rotation by 11.8(1)° of the oxygen octahedra that characterizes its distorted layered perovskite structure. Using x-ray resonant scattering at the Ir L3 edge we have measured accurately the intensities of Bragg peaks arising from different components of the magnetic structure. From a careful comparison of integrated intensities of peaks due to basal-plane antiferromagnetism, with those due to b-axis ferromagnetism, we deduce a canting of the magnetic moments of 12.2(8)°. We thus confirm that in Sr2IrO4 the magnetic moments rigidly follow the rotation of the oxygen octahedra, indicating that, even in the presence of significant non-cubic structural distortions, it is a close realization of the Jeff = 1/2 state.

Rønnow et al. Reply:In a recent Letter we reported on a comprehensive investigation of the magnetic excitation spectrum of CuDCOO 2 4D 2 O, an excellent realization of a 2D quantum (S 1=2) Heisenberg antiferromagnet on a square lattice [1]. We obtained renormalization factors of Z c 1:21 0:05 and Z 0:51 0:04 at low temperature, in good agreement with theory, and discovered a wave-vector dependent quantum renormalization of the excitation energies. By comparing to exact diagonalization and quantum Monte Carlo (QMC) computations, this was shown to be a feature intrinsic to the model. Finally, we studied the temperature dependence of the softening and damping, ÿT, of the magnetic excitations. The former was shown to be consistent with higher-order quantum corrections to spin-wave theory, while the latter was in excellent agreement with QMC. We noticed that the damping of the spin waves is in surprisingly good agreement with the simple relation ÿT v s T=T, where v s T and T are the spin-wave velocity and correlation length, respectively. In their Comment [2], Kopietz and Spremo address this last observation and propose an alternative functional form for ÿT [3].The magnon damping rate shown in Fig. 1 was extracted by fitting a damped harmonic oscillator line shape to the experimentally measured Sk; ! (see [4] for details). Within the statistical accuracy of the measurements, no systematic k dependence of the damping could be observed. Therefore, to ameliorate statistical quality, we presented an average of ÿ k T over 1 4a < jkj < =

The magnetic structures adopted by the Fe and Sm sublattices in SmFeAsO have been investigated using element-specific x-ray resonant and nonresonant magnetic scattering techniques. Between 110 and 5 K, the Sm and Fe moments are aligned along the c and a directions, respectively, according to the same magnetic representation 5 and the same propagation vector (1 0 1 2 ). Below 5 K, the magnetic order of both sublattices changes to a different magnetic structure, and the Sm moments reorder in a magnetic unit cell equal to the chemical unit cell. Modeling of the temperature dependence for the Sm sublattice, as well as a change in the magnetic structure below 5 K, provides clear evidence of a surprisingly strong coupling between the two sublattices, and indicates the need to include anisotropic exchange interactions in models of SmFeAsO and related compounds.

The magnetic excitation spectrum in the two-dimensional (2D) S 1͞2 Heisenberg antiferromagnet copper deuteroformate tetradeuterate has been measured for temperatures up to T ϳ J͞2, where J 6.31 6 0.02 meV is the 2D exchange coupling. For T ø J, a dispersion of the zone boundary energy is observed, which is attributed to a wave vector dependent quantum renormalization. At higher temperatures, spin-wavelike excitations persist, but are found to broaden and soften. By combining our data with numerical calculations, and with existing theoretical work, a consistent description of the behavior of the model system is found over the whole temperature interval investigated. DOI: 10.1103/PhysRevLett.87.037202 PACS numbers: 75.10.Jm, 05.70.Jk, 75.50.Ee While at the atomic level, magnetism is a quantum phenomenon, the collective behavior of magnets can be largely understood using classical concepts. This can even be true for antiferromagnets with low spin and spatial dimensionality, where quantum fluctuations are sizable. A famous example is the 2D quantum ͑S 1͞2͒ Heisenberg antiferromagnet on a square lattice (2DQHAFSL) with nearest-neighbor interactions. Considerable effort has been devoted to this particular model because it describes the parent compounds of the high-T c superconducting cuprates, and also because it once was thought to have a spin fluid rather than a Néel ground state.By now, it is well established that at zero temperature the 2DQHAFSL displays long-range order, albeit with a staggered moment reduced by quantum fluctuations [1]. In addition, harmonic spin-wave (SW) theory, based on a classical image of the spins as coupled precessing tops, gives an excellent account of the spin dynamics up to intermediate frequencies at T 0. At finite temperatures, long-range order is destroyed by thermal fluctuations, and the system possesses short-range order only, characterized by a temperature-dependent correlation length j͑T͒. A combination of low-temperature renormalized classical theory [2], intermediate temperature quantum Monte Carlo (QMC) [3,4] and high-temperature methods [5,6], accounts for the experimental data for j͑T ͒, covering the range J͞5 , T & J [7 -11]. Thus, a coherent picture exists for the thermodynamic properties at all temperatures for S 1͞2, as well as for higher values of the spin [12]. The remaining questions about the 2DQHAFSL concern the intermediate and low frequency spin dynamics at nonzero T, and the high frequency spin dynamics at all T.In both cases one could well expect to see more severe quantum effects because they should be more sensitive than the static properties to the nonlinearities of Heisenberg's equations for a low spin system. The present paper describes the first experiment to confront all of these questions directly over the full energy scale set by J.Though experiments on the dynamics of the 2DQHAFSL are relatively scarce, considerable theoretical work exists. Time dependent information is contained in the dynamical structure factor S͑Q, v͒ R dt e 2ivt P r e iQr ͗S 0...