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A hybrid approach to consider the problem of forecasting a stationary time series using Baylesian functions is proposed.

When the transition temperature of a continuous phase transition is tuned to absolute zero, new ordered phases and physical behaviour emerge in the vicinity of the resulting quantum critical point. Sr3Ru2O7 can be tuned through quantum criticality with magnetic field at low temperature. Near its critical field Bc it displays the hallmark T-linear resistivity and a T log (1 / T) electronic heat capacity behaviour of strange metals. However, these behaviours have not been related to any critical fluctuations. Here we use inelastic neutron scattering to reveal the presence of collective spin fluctuations whose relaxation time and strength show a nearly singular variation with magnetic field as Bc is approached. The large increase in the electronic heat capacity and entropy near Bc can be understood quantitatively in terms of the scattering of conduction electrons by these spin-fluctuations. On entering the spin-density-wave ordered phase present near Bc, the fluctuations become stronger suggesting that the order is stabilised through an "order-by-disorder" mechanism. Sr3Ru2O7 exhibits a quantum critical point tunable by magnetic field and has been widely used in the study of criticality. Here, by using inelastic neutron scattering, the authors measure collective magnetic excitations near the quantum critical point and relate them to thermodynamic properties and spin density wave order. [ABSTRACT FROM AUTHOR]

High incident energy inelastic neutron scattering, magnetization and x-ray powder diffraction measurements are used for examining the effects of diluting Gd with Y on the magnetic properties of the giant magnetocaloric effect material Gd5Si2Ge2. Gd4YSi1.9Ge2.1 orders ferromagnetically at 149 K with an effective paramagnetic moment of 7.87 muB and hence its ordering temperature is about half that of Gd5Si2Ge2.

A new type of high intensity and high resolution time-of-flight near backscattering neutron spectrometer is presented which is capable of an energy resolution of 4.0 μeV with a data collection rate an order of magnitude larger than current near backscattering spectrometers capable of this resolution. This design challenges two accepted norms of high resolution indirect neutron spectrometers: it uses large mosaic pyrolytic graphite (002) crystals instead of nearly perfect silicon (111) crystals to select the final neutron energy, and it operates well away from the normal backscattering condition 2φ ≈ 180° instead using 2φ = 160°. This paper describes the methodology behind this spectrometer along with ray-tracing simulations of the instrument using the McStas program. [ABSTRACT FROM AUTHOR]

The MAPS direct geometry time-of-flight chopper spectrometer at the ISIS pulsed neutron and muon source has been in operation since 1999, and its novel use of a large array of position-sensitive neutron detectors paved the way for a later generations of chopper spectrometers around the world. Almost two decades of experience of user operations on MAPS, together with lessons learned from the operation of new generation instruments, led to a decision to perform three parallel upgrades to the instrument. These were to replace the primary beamline collimation with supermirror neutron guides, to install a disk chopper, and to modify the geometry of the poisoning in the water moderator viewed by MAPS. Together, these upgrades were expected to increase the neutron flux substantially, to allow more flexible use of repetition rate multiplication and to reduce some sources of background. Here, we report the details of these upgrades and compare the performance of the instrument before and after their installation as well as to Monte Carlo simulations. These illustrate that the instrument is performing in line with, and in some respects in excess of, expectations. It is anticipated that the improvement in performance will have a significant impact on the capabilities of the instrument. A few examples of scientific commissioning are presented to illustrate some of the possibilities. [ABSTRACT FROM AUTHOR]

The effects of diluting Gd with Y on the magnetic properties of the giant magnetocaloric effect material Gd5Si2Ge2 have been investigated by high incident energy inelastic neutron scattering, magnetization, and x-ray powder diffraction measurements. The dynamic response for Gd5Si2Ge2 at 15 and 150 K consists of two magnetic excitations extending up to 40 meV. From the x-ray powder diffraction data, Gd4YSi1.9Ge2.1 is monoclinic at room temperature, space group P1121/a with the 4e site at x=-0.0092(3), y=0.1005(2), z=0.1808(4) almost exclusively occupied by Gd (97%), while the remaining four Gd/Y sites are randomly occupied by Gd and Y. The magnetization measurements show that the ordering temperature of Gd4YSi1.9Ge2.1 is about half that of Gd5Si2Ge2, as the former orders ferromagnetically at 149 K with an effective paramagnetic moment of 7.87μB. The observed magnetic dynamic response for the Gd4YSi1.9Ge2.1 at 7 K is also different from the nondiluted compound, whereby the positions and intensities of the magnetic excitations have decreased markedly with respect to Gd5Si2Ge2. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The conduction electrons in a metal experience competing interactions with each other and the atomic nuclei. This competition can lead to many types of magnetic order in metals. For example, in chromium the electrons order to form a spin-density-wave (SDW) antiferromagnetic state. A magnetic field may be used to perturb or tune materials with delicately balanced electronic interactions. Here, we show that the application of a magnetic field can induce SDW magnetic order in a quasi-2D metamagnetic metal, where none exists in the absence of the field. We use magnetic neutron scattering to show that the application of a large (B ≈ 8 T) magnetic field to the perovskite metal Sr3Ru2O7 (refs , , , , ) can be used to tune the material through two magnetically ordered SDW states. The ordered states exist over relatively small ranges in field (≲0.4 T), suggesting that their origin is due to a new mechanism related to the electronic fine structure near the Fermi energy, possibly combined with the stabilizing effect of magnetic fluctuations. The magnetic field direction is shown to control the SDW domain populations, which naturally explains the strong resistivity anisotropy or 'electronic nematic' behaviour observed in this material. [ABSTRACT FROM AUTHOR]

The effect of uncertainty in chopper phasing (jitter) has been investigated for the high-resolution time-of-flight spectrometer LET at the ISIS second target station. The investigation is carried out using virtual experiments, with the neutron simulation package McStas, where the chopper jitter is found to cause a Lorentzian tail in the resolution function. We find that jitter even up to the unrealistically high value of 2 μs can be tolerated without any noticeable degradation of resolution or incident intensity. The results are supported by simple analytical estimates and are believed to be general for chopper spectrometers. [ABSTRACT FROM AUTHOR]

Antiferromagnetism is relevant to high-temperature (high-Tc) superconductivity because copper oxide and iron arsenide superconductors arise from electron- or hole-doping of their antiferromagnetic parent compounds. There are two broad classes of explanation for antiferromagnetism: in the ‘local moment’ picture, appropriate for the insulating copper oxides, antiferromagnetic interactions are well described by a Heisenberg Hamiltonian; whereas in the ‘itinerant model’, suitable for metallic chromium, antiferromagnetic order arises from quasiparticle excitations of a nested Fermi surface. There has been contradictory evidence regarding the microscopic origin of the antiferromagnetic order in iron arsenide materials, with some favouring a localized picture and others supporting an itinerant point of view. More importantly, there has not even been agreement about the simplest effective ground-state Hamiltonian necessary to describe the antiferromagnetic order. Here, we use inelastic neutron scattering to map spin-wave excitations in CaFe2As2 (refs 26, 27), a parent compound of the iron arsenide family of superconductors. We find that the spin waves in the entire Brillouin zone can be described by an effective three-dimensional local-moment Heisenberg Hamiltonian, but the large in-plane anisotropy cannot. Therefore, magnetism in the parent compounds of iron arsenide superconductors is neither purely local nor purely itinerant, rather it is a complicated mix of the two. [ABSTRACT FROM AUTHOR]

The density of phonon states for the YbB12 Kondo insulator is calculated from the inelastic neutron scattering spectra of this compound. It is established that thermal vibrations of rare-earth atoms predominantly occur in the low-energy range. These atoms are most weakly bound in the crystal structure of the YbB12 Kondo insulator. The high-energy part of the vibrational spectrum is determined by thermal vibrations of the boron atoms forming a rigidly connected structure of the compound. It is revealed that the temperature dependence of the intensity of the phonon peak attributed to thermal vibrations of the ytterbium atoms exhibits an anomalous behavior. This circumstance suggests that the magnetoelastic coupling occurring in the structure of the YbB12 Kondo insulator is relatively strong and can contribute to the magnetic excitation spectrum of this compound. [ABSTRACT FROM AUTHOR]

The dynamic magnetic response of the intermediate-valence compound EuCu2Si2 has been studied using inelastic neutron scattering. At low temperatures, strong renormalization of the 7 F 0 → 7 F 1 spin-orbit transition energy is detected; it is likely to be related to partial delocalization of the f electrons of Eu. An increase in the temperature increases the valence instability of europium and results in further changes in the magnetic excitation spectrum parameters and the appearance of an intense quasi-elastic component. [ABSTRACT FROM AUTHOR]

The phonon dispersion in RB12 ( R = Yb, Lu) systems is studied in detail using inelastic neutron scattering over a wide range of energies (up to 55 meV). It is demonstrated that the main features of the lattice dynamics in the RB12 crystals can be described in terms of a simple model of force constants with a strong hierarchy of interatomic interactions in the systems: B-B ≫ B- R ≫ R- R. An interesting feature of the low-energy portion of the phonon spectrum is the weakly dispersive optical mode. According to the model calculations, this mode is associated with the relative displacements and “breathing” vibrations of the boron nanoclusters B12. [ABSTRACT FROM AUTHOR]

The multipliers for linear dynamic models have been incorrectly expressed in certain textbooks. This note points out the source of the error and demonstrates the correct formulation of these multipliers. It will be shown that the error implicitly arises from a confusion in the definition of a multiplier. This article therefore, defines a multiplier to be the effect of a unit sustained change from equilibrium in the exogenous variables on the endogenous variables. Therefore, the k-period dynamic multiplier of an sth order linear model is given by the recursion relations. In order to compare the correct formulation of a multiplier with the incorrect version, it can easily be shown that a more general expression for a k-period dynamic multiplier is given by substitution of into. The implication of using the incorrect expression is that a serious misrepresentation of the dynamics of the system will be made if any variable appears in first difference form since those coefficients can bear no influence on the multipliers.