Dispersionless Spin Waves and Underlying Field-Induced Magnetic Order in Gadolinium Gallium Garnet.

We report the results of neutron diffraction and inelastic neutron scattering on a powder sample of Gd_{3}Ga_{5}O_{12} at high magnetic fields. Analysis of the diffraction data shows that in high fields (B≳1.8 T) the spins are not fully aligned, but are canted slightly as a result of the dipolar interaction. The magnetic phase for fields ≲1.8 T is characterized by antiferromagnetic peaks at (210) and an incommensurate wave vector. The dominant contribution to inelastic scattering at large momentum transfers is from a band of almost dispersionless excitations. We show that these correspond to the spin waves localized on ten site rings, expected on the basis of nearest neighbor exchange interaction, and that the spectrum at high fields B≳1.8 T is well described by a spin wave theory.

Patterning of sodium ions and the control of electrons in sodium cobaltate.

Sodium cobaltate (Na(x)CoO2) has emerged as a material of exceptional scientific interest due to the potential for thermoelectric applications, and because the strong interplay between the magnetic and superconducting properties has led to close comparisons with the physics of the superconducting copper oxides. The density x of the sodium in the intercalation layers can be altered electrochemically, directly changing the number of conduction electrons on the triangular Co layers. Recent electron diffraction measurements reveal a kaleidoscope of Na+ ion patterns as a function of concentration. Here we use single-crystal neutron diffraction supported by numerical simulations to determine the long-range three-dimensional superstructures of these ions. We show that the sodium ordering and its associated distortion field are governed by pure electrostatics, and that the organizational principle is the stabilization of charge droplets that order long range at some simple fractional fillings. Our results provide a good starting point to understand the electronic properties in terms of a Hubbard hamiltonian that takes into account the electrostatic potential from the Na superstructures. The resulting depth of potential wells in the Co layer is greater than the single-particle hopping kinetic energy and as a consequence, holes preferentially occupy the lowest potential regions. Thus we conclude that the Na+ ion patterning has a decisive role in the transport and magnetic properties.

Design, calibration, and performance of a uniaxial pressure cell for neutron scattering studies of quantum magnetism.

We provide an overview of a pressure cell designed to apply uniaxial pressure to single crystals for the study, by neutron scattering techniques, of strongly correlated magnetic systems and, in particular, quantum magnets. A detailed overview of the pressure cell components, their requirements, and links to the scientific and technical specifications are presented. The pressure cell is able to accommodate a 200 mm3 single crystal that can be pressurized up to 2 GPa at cryogenic temperatures. The pressure cell is consistent with the requirements of inelastic neutron scattering and, importantly, neutron polarization analysis. A particular strength of the uniaxial pressure cell is the highly uniform and low background for a wide scattering angle of 360° horizontally and ±20° vertically. We show the performance of the uniaxial pressure cell using a relevant neutron scattering instrument, the polarized diffuse scattering instrument, D7. The experiments confirm that the cell complies with the scientific and technical requirements. This uniaxial pressure cell will provide a useful additional tool in the sample environment suite available for the study of quantum magnetism.

CSPEC: The cold chopper spectrometer of the ESS, a detailed overview prior to commissioning.

CSPEC is the cold chopper spectrometer of the European Spallation Source (ESS) and will come online with the ESS beam on the target. CSPEC will be the first cold chopper spectrometer on a long pulsed spallation source, which provides great opportunities in terms of signal to noise and novel measuring schemes. We provide a detailed overview of the instrument, scientific design considerations, and engineering requirements.

Scale-free antiferromagnetic fluctuations in the s = 1/2 kagome antiferromagnet herbertsmithite.

Neutron spectroscopy and diffuse neutron scattering on herbertsmithite [ZnCu(3)(OH)(6)Cl(2)], a near-ideal realization of the s=1/2 kagome antiferromagnet, reveal the hallmark property of a quantum spin liquid: instantaneous short-ranged antiferromagnetic correlations in the absence of a time-averaged ordered moment. These dynamic antiferromagnetic correlations are weakly dependent of neutron-energy transfer and temperature, and persist up to 25 meV and 120 K. At low energy transfers a shift of the magnetic scattering to low Q is observed with increasing temperature, providing evidence of gapless spinons. It is argued that these observations provide important evidence in favor of resonating-valence-bond theories of (doped) Mott insulators.

Disentangling time-focusing from beam divergence: A novel approach for high-flux thermal neutron spectroscopy at continuous and long-pulse sources.

We present the concept of a novel time-focusing technique for neutron spectrometers, which allows us to disentangle time-focusing from beam divergence. The core of this approach is a double rotating-crystal monochromator that can be used to extract a larger wavelength band from a white beam, thus providing a higher flux at the sample compared to standard time-of-flight instruments, yet preserving energy resolution and beam collimation. The performances of a spectrometer based on this approach are quantitatively discussed in terms of possible incident wavelengths, flux at the sample, and (Q, E)-resolution. Analytical estimates suggest flux gains of about one order of magnitude at comparable resolutions in comparison to conventional time-of-flight spectrometers. Moreover, the double monochromator configuration natively shifts the sample away from the source line-of-sight, thus significantly improving the signal-to-noise ratio. The latter, in combination with a system that does not increase the beam divergence, brings the further advantage of a cleaner access to the low-Q region, which is recognized to be of fundamental interest for magnetism and for disordered materials, from glasses to biological systems.

Choppers to optimise the repetition rate multiplication technique on a direct geometry neutron chopper spectrometer.

In recent years the use of repetition rate multiplication (RRM) on direct geometry neutron spectrometers has been established and is the common mode of operation on a growing number of instruments. However, the chopper configurations are not ideally optimised for RRM with a resultant 100 fold flux difference across a broad wavelength band. This paper presents chopper configurations that will produce a relative constant (RC) energy resolution and a relative variable (RV) energy resolution for optimised use of RRM. The RC configuration provides an almost uniform ΔE/E for all incident wavelengths and enables an efficient use of time as the entire dynamic range is probed with equivalent statistics, ideal for single shot measurements of transient phenomena. The RV energy configuration provides an almost uniform opening time at the sample for all incident wavelengths with three orders of magnitude in time resolution probed for a single European Spallation Source (ESS) period, which is ideal to probe complex relaxational behaviour. These two chopper configurations have been simulated for the Versatile Optimal Resolution direct geometry spectrometer, VOR, that will be built at ESS.

Generalization of the classical xyz-polarization analysis technique to out-of-plane and inelastic scattering.

The technique of longitudinal ("xyz") polarization analysis has been used successfully for many years to study disordered magnetic materials in thermal and cold neutron diffraction experiments. The technique allows the simultaneous and unambiguous separation of the nuclear, magnetic, and nuclear spin-incoherent contributions to the scattering. The technical advances seen in recent years, such as the availability of polarized (3)He analyzer cells to cover a large detector solid angle, the ability to detect out-of-plane scattering in a multi-detector, and a significant increase of the usable beam divergence, call for a generalization of the method. A general treatment of the formalism for carrying out neutron polarization analysis will be given in this paper, which describes a possible method of usage at a future, modern diffractometer or inelastic spectrometer with large area multi-detector coverage.

Neutron scattering and µSR investigations of the low temperature state of LuCuGaO4.

LuCuGaO4 has magnetic Cu(2+) and diamagnetic Ga(3+) ions distributed on a triangular bilayer and is suggested to undergo a spin glass transition at Tg ∼ 0.4 K. Using µSR (muon spin rotation) and neutron scattering measurements, we show that at low temperature the spins form a short range correlated state with spin fluctuations detectable over a wide range of timescales: at 0.05 K magnetic fluctuations can be detected in both the µSR time window and also extending beyond 7 meV in the inelastic neutron scattering response, indicating magnetic fluctuations spanning timescales between ∼10(-5) and ∼10(-10) s. The dynamical susceptibility scales according to the form χ″(ω)T(α), with α = 1, throughout the measured temperature range (0.05-50 K). These effects are associated with quantum fluctuations and some degree of structural disorder in ostensibly quite different materials, including certain heavy fermion alloys, kagome spin liquids, quantum spin glasses, and valence bond glasses. We therefore suggest that LuCuGaO4 is an interesting model compound for the further examination of disorder and quantum magnetism.

Magnetic Coulomb phase in the spin ice Ho2Ti2O7.

Spin-ice materials are magnetic substances in which the spin directions map onto hydrogen positions in water ice. Their low-temperature magnetic state has been predicted to be a phase that obeys a Gauss' law and supports magnetic monopole excitations: in short, a Coulomb phase. We used polarized neutron scattering to show that the spin-ice material Ho2Ti2O7 exhibits an almost perfect Coulomb phase. Our result proves the existence of such phases in magnetic materials and strongly supports the magnetic monopole theory of spin ice.

Neutron scattering investigation of a diluted blend of poly(ethylene oxide) in polyethersulfone.

By using quasielastic neutron scattering (QENS) with isotopic labeling we have investigated the component dynamics in a miscible blend of polyethersulfone (PES) and poly(ethylene oxide) (PEO) with 75% content in weight of PES. Due to the large difference in the glass-transition temperatures, T(g)'s, of the two polymers (T(g) (PEO) approximately equal to 220 K, T(g) (PES) approximately equal to 382 K) the dynamic asymmetry in the system dramatically increases when approaching the average T(g) of the blend, . For the fast (PEO) component, this leads to a behavior which hints a crossover from typical glass-forming liquidlike dynamics at high temperatures to confined dynamics close to induced by the freezing of the segmental motions of the slow PES. The features of the confined PEO motion observed by QENS are similar to those of the secondary gamma-relaxation detected for pure (semicrystalline) PEO. A neutron diffraction study of the short-range order of the homopolymers and the blend suggests that this coincidence could be due to similarities in the intermolecular packing of PEO and PES polymers.

ID20: a beamline for magnetic and resonant X-ray scattering investigations under extreme conditions.

A new experimental station at ESRF beamline ID20 is presented which allows magnetic and resonant X-ray scattering experiments in the energy range 3-25 keV to be performed under extreme conditions. High magnetic field up to 10 T, high pressure up to 30 kbar combined with low temperatures down to 1.5 K are available and experiments can be performed at the M-edges of actinide elements, L-edges of lanthanides and K-edges of transition metals.