45238244
Quantum Monte Carlo calculations of light nuclei using chiral potentials.
We present the first Green's function Monte Carlo calculations of light nuclei with nuclear interactions derived from chiral effective field theory up to next-to-next-to-leading order. Up to this order, the interactions can be constructed in a local form and are therefore amenable to quantum Monte Carlo calculations. We demonstrate a systematic improvement with each order for the binding energies of A=3 and A=4 systems. We also carry out the first few-body tests to study perturbative expansions of chiral potentials at different orders, finding that higher-order corrections are more perturbative for softer interactions. Our results confirm the necessity of a three-body force for correct reproduction of experimental binding energies and radii, and pave the way for studying few- and many-nucleon systems using quantum Monte Carlo methods with chiral interactions.
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.
118607911
Acceleration of Relativistic Electrons by Magnetohydrodynamic Turbulence: Implications for Non-thermal Emission from Black Hole Accretion Disks
We use analytic estimates and numerical simulations of test particles interacting with magnetohydrodynamic (MHD) turbulence to show that subsonic MHD turbulence produces efficient second-order Fermi acceleration of relativistic particles. This acceleration is not well described by standard quasi-linear theory but is a consequence of resonance broadening of wave-particle interactions in MHD turbulence. We provide momentum diffusion coefficients that can be used for astrophysical and heliospheric applications and discuss the implications of our results for accretion flows onto black holes. In particular, we show that particle acceleration by subsonic turbulence in radiatively inefficient accretion flows can produce a non-thermal tail in the electron distribution function that is likely important for modeling and interpreting the emission from low-luminosity systems such as Sgr A* and M87.
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.