User and programmers guide to the neutron ray-tracing package McStas, version 1.2

The software package McStas is a tool for writing Monte Carlo ray-tracing simulations of neutron scattering instruments with very high complexity and precision. The simulations can compute all aspects of the performance of instruments and can thus be usedto optimize the use of existing equipment as well as the design of new instrumentation. McStas is based on a unique design where an automatic compilation process translates high-level textual instrument descriptions into efficient ANSI C code. Thisdesign makes it simple to set up typical simulations and also give essentially unlimited freedom to handle more unusual needs. This report constitutes the reference manual for McStas, and contains full documentation for all ascpects of the program. Itcovers the various ways to compile and run simulations; a description of the metalanguage used to define simulations; a full description of all algorithms used to calculate the effects of the various optical components in instruments; and some examplesimulations performed with the program.The software package McStas is a tool for writing Monte Carlo ray-tracing simulations of neutron scattering instruments with very high complexity and precision. The simulations can compute all aspects of the performance of instruments and can thus be used to optimize the use of existing equipment as well as the design of new instrumentation. McStas is based on a unique design where an automatic compilation process translates high-level textual instrument descriptions into efficient ANSI C code. This design makes it simple to set up typical simulations and also give essentially unlimited freedom to handle more unusual needs. This report constitutes the reference manual for McStas, and contains full documentation for all ascpects of the program. It covers the various ways to compile and run simulations; a description of the metalanguage used to define simulations; a full description of all algorithms used to calculate the effects of the various optical components in instruments; and some example simulations performed with the program

Effective Hamiltonian and low-lying energy clustering patterns of four-sublattice antiferromagnets

We study the low-lying energy clustering patterns of quantum antiferromagnets with p sublattices (in particular p=4). We treat each sublattice as a large spin, and using second-order degenerate perturbation theory, we derive the effective (biquadratic) Hamiltonian coupling the p large spins. In order to compare with exact diagonalizations, the Hamiltonian is explicitly written for a finite-size lattice, and it contains information on energies of excited states as well as the ground state. The result is applied to the face-centered-cubic Type I antiferromagnet of spin 1/2, including second-neighbor interactions. A 32-site system is exactly diagonalized, and the energy spectrum of the low-lying singlets follows the analytically predicted clustering pattern.Comment: 17 pages, 4 table

Bi spectral extraction through elliptic neutron guides

In this paper we present the results of investigating a suggested guide extraction system utilizing both a thermal and a cold moderator at the same time, the so called bi spectral extraction. Here, the thermal moderator has line of sight to the sample position, and the neutrons from the cold source are reflected by a supermirror towards the sample. The work is motivated by the construction of the European Spallation Source ESS but the results are general and can be used at any neutron source. Due to the long pulse structure, most instruments at ESS will be long, often exceeding 50 m from moderator to detector. We therefore investigate the performance of bi spectral extraction for instrument lengths of 30 m, 56 m, 81 m and 156 m. In all these cases, our results show that we can utilize both moderators and thus high intensity in a wide wavelength band in the same instrument at a cost of flux of 5 30 for neutrons with wavelength larger than 1 . In general, the divergence distribution is smooth at the sample position for all wavelength

Valence bond glass on an fcc lattice in the double perovskite Ba2YMoO6

Peer reviewedPublisher PD

Spin Susceptibility of the Topological Superconductor UPt3 from Polarized Neutron Diffraction

Experiment and theory indicate that UPt3 is a topological superconductor in an odd-parity state, based in part from temperature independence of the NMR Knight shift. However, quasiparticle spin-flip scattering near a surface, where the Knight shift is measured, might be responsible. We use polarized neutron scattering to measure the bulk susceptibility with H||c, finding consistency with the Knight shift but inconsistent with theory for this field orientation. We infer that neither spin susceptibility nor Knight shift are a reliable indication of odd-parity

Spins in the Vortices of a High Temperature Superconductor

Neutron scattering is used to characterise the magnetism of the vortices for the optimally doped high-temperature superconductor La(2-x)Sr(x)CuO(4) (x=0.163) in an applied magnetic field. As temperature is reduced, low frequency spin fluctuations first disappear with the loss of vortex mobility, but then reappear. We find that the vortex state can be regarded as an inhomogeneous mixture of a superconducting spin fluid and a material containing a nearly ordered antiferromagnet. These experiments show that as for many other properties of cuprate superconductors, the important underlying microscopic forces are magnetic