One of the simplest quantum many-body systems is the spin-1/2 Heisenberg antiferromagnetic chain, a linear array of interacting magnetic moments. Its exact ground state is a macroscopic singlet entangling all spins in the chain. Its elementary excitations, called spinons, are fractional spin-1/2 quasiparticles created and detected in pairs by neutron scattering. Theoretical predictions show that two-spinon states exhaust only 71% of the spectral weight and higher-order spinon states, yet to be experimentally located, are predicted to participate in the remaining. Here, by accurate absolute normalization of our inelastic neutron scattering data on a spin-1/2 Heisenberg antiferromagnetic chain compound, we account for the full spectral weight to within 99(8)%. Our data thus establish and quantify the existence of higher-order spinon states. The observation that, within error bars, the experimental line shape resembles a rescaled two-spinon one with similar boundaries allows us to develop a simple picture for understanding multi-spinon excitations.O ne hundred years ago, Max von Laue and co-workers discovered X-ray diffraction 1 , thereby giving birth to the field of crystallography to which we owe much of our understanding of materials on the atomic scale. The very first diffraction image was recorded from a single crystal of copper sulphate pentahydrate 1,2 . In addition to vast practical use including herbicide, wood impregnation and algae control in swimming pools, copper sulphate also carries great educational importance. Generations of school children have been inspired in chemistry classes across the globe by growing from evaporating solution beautiful blue crystals of copper sulphate (in 2008, artist Roger Hiorns created an installation called Seizure 3 covering an entire apartment in copper sulphate crystals). When cooled close to absolute zero temperature, copper sulphate has even more fascinating lessons to teach-it becomes a quantum spin liquid. Moreover, it materializes one of the simplest models hosting complex quantum many-body physics, the one-dimensional spin-1/2 Heisenberg antiferromagnet, for which there exists an exact analytic solution-namely the Bethe ansatz 4 . Quantum spin liquid ground states entangle a macroscopic number of spins and give rise to astonishing and counterintuitive phenomena. Quantum spin liquids occur in a variety of contexts ranging from the quantum spin Hall effect 5,6 and high-T c superconductivity 7-9 to confined ultracold gases and carbon nanotubes 10 . A particularly clear form of a gapless algebraic quantum spin liquid is realized in a one-dimensional array of spins-1/2 that are coupled by nearest-neighbour isotropic exchange, the spin-1/2 Heisenberg antiferromagnetic chain. At zero temperature, this spin liquid is critical with respect to long-range antiferromagnetic order as well as with respect to dimerization 11,12 . Its emerging gapless fractionalized excitations are called spinons 13 . The concept of fractional excitations has been applied to magnetic monopoles ...

Resonant x-ray scattering experiments at the vanadium K edge demonstrate the existence of orbital ordering in V 2 O 3 . Bragg peaks due to the long-range order of 3d orbitals occupancy are observed when the photon energy is tuned to the threshold of the vanadium 3d bands. The azimuthal dependence of the resonant intensities confirms that the resonance arises from the ordering of the vanadium orbital occupancy. The observed orbital structure accounts for the complex magnetic structure of V 2 O 3 . The measured magnetic and orbital responses have the same critical temperature T N .[S0031-9007 (99)09287-X] PACS numbers: 78.70.Ck, 71.30. + h, 75.50.EeTwenty years ago, Castellani et al. [1] proposed that long-range order in the occupancy of the vanadium 3d orbitals was responsible for the complex magnetic properties of V 2 O 3 . Upon doping with Cr and/or under the application of hydrostatic pressure [2,3] V 2 O 3 exhibits both insulating and metallic phases with peculiar magnetic correlations [4][5][6]. It was suggested [1] that the spatial ordering of the occupancy of degenerate electronic orbitals accounts for the anisotropic exchange integrals found in the antiferromagnetic insulator phase (AFI) [5]. Furthermore, fluctuations in the orbital occupancy have been invoked to explain the evolution of the magnetic correlations in various phases of the V 2 O 3 system [6]. It appears that orbital occupancy plays a central role in the physics of V 2 O 3 , but no direct proof for orbital order could be produced experimentally since the original proposal in the late 1970s.In this Letter we present resonant x-ray scattering (RXS) experiments at the K edge of vanadium that demonstrate unambiguously the existence of orbital order in V 2 O 3 and provide information on the type of ordering. RXS is sensitive to the occupancy of electronic orbitals because it probes the symmetry of vacant electronic states through resonant multipole electric transitions; the variation of the orbital resonant scattering cross section with the direction of the incident polarization (azimuthal angle F) reflects the spatial symmetry of ordered orbitals. Furthermore, RXS may be tuned to probe selectively the electronic shells where orbital order takes place. In the case of V 2 O 3 , theoretical calculations [7] have shown that the resonance at the vanadium K edge provides observable cross sections arising from the order of the 3d vanadium states.RXS experiments were performed at the ID20 magnetic scattering undulator beam line at the European Synchrotron Radiation Facility [8]. A double crystal, Si(111), monochromator located between two focusing mirrors defined a narrow energy band around the vanadium K edge (FHWM 0.8 eV) with a high degree of linear s polarization. The x-ray beam was diffracted by the sample onto a pyrolitic graphite crystal analyzer [(004) reflection] to separate the s and p components of the scattered radiation. The sample was mounted with beeswax in a closed cycle refrigerator which could be rotated about the scattering vector to p...

We report neutron scattering measurements of the dynamic structure factor S(Q, omega ) of liquid 4He in the collective excitation regime. The use of the IN6 time-of-flight spectrometer at the Institut Laue-Langevin has enabled us to cover the wavevector region from 0.3 to 2.1 AA-1 in a continuous manner with an accurately calibrated energy scale and an energy resolution of approximately 0.1 meV; temperatures between 1.24 and 4.95 K were examined at saturated vapour pressure. In the wavevector region investigated, sharp excitations are seen that are unique to superfluid 4He; it is the purpose of the present paper to examine their temperature dependence and to discuss measurements of the multiphonon continuum scattering at higher energies. At small wavevectors in the 'phonon' region, the excitations are relatively unaffected by the lambda transition, whereas at larger Q a sharp component disappears at or close to Tlambda . The temperature variation of S(Q, omega ) is much more rapid in the superfluid phase than in the normal phase. In paper II of this series, the results for S(Q, omega ) will be compared with several models for the temperature variation of the 4He excitation spectrum.

We have revisited the magnetic structure of manganese phosphorus trisulfide MnPS3 using neutron diffraction and polarimetry. MnPS3 undergoes a transition towards a collinear antiferromagnetic order at 78 K. The resulting magnetic point group breaks both the time reversal and the space inversion thus allowing a linear magnetoelectric coupling. Neutron polarimetry was subsequently used to prove that this coupling provides a way to manipulate the antiferromagnetic domains simply by cooling the sample under crossed magnetic and electrical fields, in agreement with the non-diagonal form of the magnetoelectric tensor. In addition, this tensor has in principle an antisymmetric part that results in a toroidic moment and provides with a pure ferrotoroidic compound.

The beamline, which is situated on a bending magnet at ESRF, comprises a unique combination of instrumentation for high-resolution and magnetic single-crystal diffraction. White-beam operation is possible, as well as focused and unfocused monochromatic modes. In addition to an eleven-axis Huber diffractometer, which facilitates simple operation in both vertical and horizontal scattering geometries, there is an in-vacuum polarization analyser and slit system, mirrors for harmonic rejection, sub 4.2 K and 1 Tesla magnetic field sample environment, plus a diamond phase plate for polarization conditioning. The instrumentation developed specifically for this beamline is described, and its use illustrated by recent scientific results.