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[en] The magnetic excitations of an antiferromagnetic spin dimer system, (VO)HPO₄· 1/2 H₂O, are examined using inelastic neutron scattering. A dispersionless mode is found, consistent with expectations for a dimer excitation. The intensity variation of the mode reveals a V⁴⁺ - V⁴⁺ dimer separation of 4.43 angstrom, almost 50% larger than the originally expected length

[en] High-resolution inelastic neutron scattering is used to explore the magnetic excitations in Cs₂CuCl₄, a quasi-2D spin-1/2 frustrated Heisenberg antiferromagnet on an anisotropic triangular lattice. In the spin liquid phase above the 3D ordering transition a broad continuum of excited states is observed, characteristic of excitations of pairs of spin-1/2 spinons of a resonating-valence-bond state. In the ordered phase stabilized at low temperatures by the weak inter-layer couplings a sharp spin-wave mode emerges below the continuum lower boundary

[en] Magnetic excitations in an array of (VO)₂P₂O₇ single crystals have been measured using inelastic neutron scattering. Until now, (VO)₂P₂O₇ has been thought of as a two-leg antiferromagnetic Heisenberg spin ladder with chains running in the a-direction. The present results show unequivocally that (VO)₂P₂O₇ is best described as an alternating spin-chain directed along the crystallographic b-direction. In addition to the expected magnon with magnetic zone-center energy gap Δ = 3.1 meV, a second excitation is observed at an energy just below 2Δ. The higher mode may be a triplet two-magnon bound state. Numerical results in support of bound modes are presented

[en] The concept of confinement states that in certain systems the constituent particles are bound together by an interaction for which the strength increases with increasing particle separation. One of the consequences of this is that these individual particles cannot be observed directly. The most famous example of confinement is found in particle physics where baryons and mesons are produced by the confinement of quarks. However, similar phenomena can occur in condensed-matter physics systems such as spin ladders that consist of two spin-1/2 antiferromagnetic chains coupled together by spin exchange interactions. Excitations of individual chains (spinons) carrying spin S=1/2, are confined by even an infinitesimal interchain coupling. Most ladders studied so far cannot illustrate this process because the large strength of their interchain coupling suppresses the spinon excitations at all energy scales. Here we present neutron scattering experiments for a weakly coupled ladder material. At high energies the behavior of this system approaches that of individual chains, but at low energies it is dominated by the integral spin excitations of strongly coupled chains.

[en] The effect of thermal fluctuations on the dynamics of a gapped quantum magnet is studied using inelastic neutron scattering on copper nitrate, a model material for the spin-1/2, one-dimensional (1D) bond alternating Heisenberg chain. A large, highly deuterated, single-crystal sample of copper nitrate is produced using a solution growth method and measurements are made using the high-resolution backscattering spectrometer OSIRIS at the ISIS Facility. Theoretical calculations and numerical analysis are combined to interpret the physical origin of the thermal effects observed in the magnetic spectra. The primary observations are (1) a thermally induced central peak due to intraband scattering, which is similar to Villain scattering familiar from soliton systems in 1D, and (2) the one-magnon quasiparticle pole is seen to develop with temperature into an asymmetric continuum of scattering. We relate this asymmetric line broadening to a thermal strongly correlated state caused by hard-core constraints and quasiparticle interactions. These findings are a counter example to recent assertions of the universality of line broadening in 1D systems and are applicable to a broad range of quantum systems.

[en] We explore the magnetic excitations as a function of applied field in the quasi 1D s=((1)/(2)) XY-like magnet Cs₂CoCl₄. A transition from spin-flop antiferromagnetic order to a quantum paramagnet occurs at the critical field B_C=2.15 T and fields higher than 2.5 T fully align the spins along the field. In the spin-flop phase gapped scattering continua dominate, identified with the creation of pairs of quantum domain walls. In the saturated phase sharp peaks are observed, as expected for magnons in a ferromagnetically ordered ground state

[en] The magnetic excitations of the quasi-2D spin-1/2 frustrated Heisenberg antiferromagnet Cs₂CuCl₄ are explored throughout the 2D Brillouin zone using high-resolution time-of-flight inelastic neutron scattering. Measurements are made both in the magnetically ordered phase, stabilized at low temperatures by the weak interlayer couplings, as well as in the spin liquid phase above the ordering temperature T_N, when the 2D magnetic layers are decoupled. In the spin liquid phase the dynamical correlations are dominated by highly dispersive excitation continua, a characteristic signature of fractionalization of S = 1 spin waves into pairs of deconfined S = 1/2 spinons and the hallmark of a resonating-valence-bond (RVB) state. The boundaries of the excitation continua have strong 2D-modulated incommensurate dispersion relations. Upon cooling below T_N magnetic order in an incommensurate spiral forms due to the 2D frustrated couplings. In this phase sharp magnons carrying a small part of the total scattering weight are observed at low energies, but the dominant continuum scattering which occurs at medium to high energies is essentially unchanged compared to the spin liquid phase. Linear spin-wave theory including one- and two-magnon processes can describe the sharp magnon excitation, but not the dominant continuum scattering, which instead is well described by a parametrized two-spinon cross section. Those results suggest a crossover in the nature of the excitations from S = 1 spin waves at low energies to deconfined S = 1/2 spinons at medium to high energies, which could be understood if Cs₂CuCl₄ was in the close proximity of transition between a fractional RVB spin liquid and a magnetically ordered state. A large renormalization factor of the excitation energies (R = 1.63(5)), indicating strong quantum fluctuations in the ground state, is obtained using the exchange couplings determined from saturation-field measurements. We provide an independent consistency check of this quantum renormalization factor using measurements of the second moment of the paramagnetic scattering.

[en] In this paper we present the results of a detailed experimental investigation of the magnetic excitations in the spin-1/2 Heisenberg antiferromagnet CuWO₄. Inelastic neutron scattering measurements made using a triple-axis spectrometer were performed in the low-temperature ordered phase and also briefly in the high-temperature phase at Brookhaven National Laboratory. The excitations in the ordered phase are compared to a spin-wave model and two possible sets of exchange paths are deduced, both of which provide good fits to the data. In both models CuWO₄ consists of weakly coupled alternating one-dimensional chains running in the [2,-1,0] direction. A calculation of the spin-wave intensities was also performed using the fitted exchange constants and agreement was found with the observed intensities. In the ordered phase, CuWO₄ has an energy gap at the zone centre and the excitations are well defined. Above the transition temperature constant-wavevector scans at the antiferromagnetic lattice points suggest the existence of a continuum of excitations. (author)

[en] A theory is developed to model the excitations in a dimerized, spin-1/2 system with a magnetically ordered ground state and where the dimer exchange constant is antiferromagnetic. This method starts by considering the energy levels of a single dimer in the effective, staggered magnetic field due to the mean-field ordering of the surrounding dimers. Pseudo-boson operators are introduced which create and annihilate these excitations, and the Hamiltonian of the magnetic system can be rewritten in terms of these operators and then diagonalized to yield one doubly degenerate transverse mode and a longitudinal singlet mode for each non-equivalent dimer in the magnetic unit cell. The dimer theory has been used to model the measured dispersion relations in the anti ferromagnetically ordered phase of the alternating-chain compound CuWO₄. It provides a good fit to the data and is as successful as spin-wave theory in accounting for the transverse excitations although with different values of the exchange constants. In addition the transition temperature and the size of the reduced moment at T 0 K calculated in the dimer theory are closer to the experimental values of CuWO₄ than those calculated by spin-wave theory. An important difference between these two models lies in their predictions of the longitudinal excitations: whereas in spin-wave theory these are regarded as two-magnon events resulting in a continuum of scattering, in the dimer theory one well defined mode is expected. An experimental measurement of the longitudinal excitations should distinguish between these models. (author)