11160793
Neutron scattering, magnetometry, and quantum Monte Carlo study of the randomly diluted spin-1/2 square-lattice Heisenberg antiferromagnet
Abstract We have successfully grown sizable single crystals of La 2 Cu 1− z (Zn,Mg) z O 4 with up to nearly half of the magnetic Cu sites replaced by non-magnetic Zn and Mg. Neutron scattering, SQUID magnetometry, and complementary quantum Monte Carlo (QMC) simulations demonstrate that this material is an excellent model system for the study of site percolation of the square-lattice Heisenberg antiferromagnet (SLHAF) in the quantum-spin limit S =1/2. Carefully oxygen-reduced samples exhibit Neel order up to the percolation threshold for site dilution, z p ≈40.7%. For z >10%, the material exhibits a low-temperature tetragonal (LTT) structural phase, with a transition temperature that increases linearly with doping. Above z ≈25%, Neel order occurs in the LTT phase. Up to at least z =35%, the Neel temperature T N ( z ) of the experimental system corresponds to the temperature at which QMC indicates that the spin correlations for the nearest-neighbor S =1/2 SLHAF have grown to approximately 100 lattice constants. Neutron scattering measurements of the static structure factor in the paramagnetic regime allow the determination of the two-dimensional spin correlations, which are found to be in excellent quantitative agreement with QMC over a wide common temperature and doping range. Neutron scattering and QMC results for the temperature dependence of the static structure factor amplitude S (π,π) are in good agreement as well. As the concentration of non-magnetic sites is increased, the magnetic correlation length ξ ( T , z ) crosses over from an exponential dependence on ρ s / T to power-law behavior in the temperature regime studied. Fits to a heuristic cross-over form for ξ ( T , z ) allow an estimate of the spin stiffness, ρ s = ρ s ( z ), which approaches zero at z = z p . The combined experimental and numerical data presented here provide valuable quantitative information for tests of theories of the randomly diluted S =1/2 SLHAF.
55286238
Production of ultra-cold neutrons using Doppler-shifted Bragg scattering and an intense pulsed neutron spallation source
We present an analytic and a computer generated simulation of the production of ultra-cold neutrons (UCN) using Bragg scattering from a moving crystal to Doppler-shift higher velocity neutrons into the UCN region. The calculation was carried out with a view toward its application at the Intense Pulsed Neutron Source (IPNS) now under construction at Argonne National Laboratory. This method for the production of UCN appears well matched to a pulsed source, and we show that the UCN can be stored in a neutron bottle at the peak flux which can potentially be much higher than at the present high flux reactors. The predicted density of stored UCN indicates that a highly precise measurement of the neutron electric dipole moment (EDM) will be possible within the next few years.
118523771
RESONANCE BROADENING AND HEATING OF CHARGED PARTICLES IN MAGNETOHYDRODYNAMIC TURBULENCE
The heating, acceleration, and pitch-angle scattering of charged particles by magnetohydrodynamic (MHD) turbulence are important in a wide range of astrophysical environments, including the solar wind, accreting black holes, and galaxy clusters. We simulate the interaction of high-gyrofrequency test particles with fully dynamical simulations of subsonic MHD turbulence, focusing on the parameter regime with β ~ 1, where β is the ratio of gas to magnetic pressure. We use the simulation results to calibrate analytical expressions for test particle velocity-space diffusion coefficients and provide simple fits that can be used in other work. The test particle velocity diffusion in our simulations is due to a combination of two processes: interactions between particles and magnetic compressions in the turbulence (as in linear transit-time damping; TTD) and what we refer to as Fermi Type-B (FTB) interactions, in which charged particles moving on field lines may be thought of as beads sliding along moving wires. We show that test particle heating rates are consistent with a TTD resonance that is broadened according to a decorrelation prescription that is Gaussian in time (but inconsistent with Lorentzian broadening due to an exponential decorrelation function, a prescription widely used in the literature). TTD dominates the heating for vs vA (e.g., electrons), where vs is the thermal speed of species s and vA is the Alfven speed, while FTB dominates for vs vA (e.g., minor ions). Proton heating rates for β ~ 1 are comparable to the turbulent cascade rate. Finally, we show that velocity diffusion of collisionless, large gyrofrequency particles due to large-scale MHD turbulence does not produce a power-law distribution function.
119202923
Real-Space Imaginary-Time Propagators for Non-Local Nucleon-Nucleon Potentials
Nuclear structure quantum Monte Carlo methods such as Green's function or auxiliary field diffusion Monte Carlo have used phenomenological local real-space potentials containing as few derivatives as possible, such as the Argonne-Urbana family of interactions, to make sampling simple and efficient. Basis set methods such as no-core shell model and coupled-cluster techniques typically use softer non-local potentials because of their more rapid convergence with basis set size. These non-local potentials are usually defined in momentum space and are often based on effective field theory. Comparisons of the results of the two types of methods can be difficult when different potentials are used. We show methods for evaluating the real-space imaginary-time propagators needed to perform quantum Monte Carlo calculations using such non-local potentials. We explore the universality of the large imaginary time propagators for different potentials and discuss how non-local potentials can be used in quantum Monte Carlo calculations.