9,402 research outputs found
Electron-spin beat susceptibility of excitons in semiconductor quantum wells
Recent time-resolved differential transmission and Faraday rotation
measurements of long-lived electron spin coherence in quantum wells displayed
intriguing parametric dependencies. For their understanding we formulate a
microscopic theory of the optical response of a gas of optically incoherent
excitons whose constituent electrons retain spin coherence, under a weak
magnetic field applied in the quantum well's plane. We define a spin beat
susceptibility and evaluate it in linear order of the exciton density. Our
results explain the many-body physics underlying the basic features observed in
the experimental measurements
Ab Initio Calculations of Even Oxygen Isotopes with Chiral Two- Plus Three-Nucleon Interactions
We formulate the In-Medium Similarity Renormalization Group (IM-SRG) for
open-shell nuclei using a multi-reference formalism based on a generalized Wick
theorem introduced in quantum chemistry. The resulting multi-reference IM-SRG
(MR-IM-SRG) is used to perform the first ab initio study of even oxygen
isotopes with chiral NN and 3N Hamiltonians, from the proton to the neutron
drip lines. We obtain an excellent reproduction of experimental ground-state
energies with quantified uncertainties, which is validated by results from the
Importance-Truncated No-Core Shell Model and the Coupled Cluster method. The
agreement between conceptually different many-body approaches and experiment
highlights the predictive power of current chiral two- and three-nucleon
interactions, and establishes the MR-IM-SRG as a promising new tool for ab
initio calculations of medium-mass nuclei far from shell closures.Comment: 5 pages, 4 figures, v2 corresponding to published versio
Pion-less effective field theory for atomic nuclei and lattice nuclei
We compute the medium-mass nuclei O and Ca using pionless
effective field theory (EFT) at next-to-leading order (NLO). The low-energy
coefficients of the EFT Hamiltonian are adjusted to experimantal data for
nuclei with mass numbers and , or alternatively to results from
lattice quantum chromodynamics (QCD) at an unphysical pion mass of 806 MeV. The
EFT is implemented through a discrete variable representation in the harmonic
oscillator basis. This approach ensures rapid convergence with respect to the
size of the model space and facilitates the computation of medium-mass nuclei.
At NLO the nuclei O and Ca are bound with respect to decay into
alpha particles. Binding energies per nucleon are 9-10 MeV and 30-40 MeV at
pion masses of 140 MeV and 806 MeV, respectively.Comment: 26 page
Accelerated Sampling of Boltzmann distributions
The sampling of Boltzmann distributions by stochastic Markov processes, can
be strongly limited by the crossing time of high (free) energy barriers. As a
result, the system may stay trapped in metastable states, and the relaxation
time to the equilibrium Boltzmann distribution may be very large compared to
the available computational time. In this paper, we show how, by a simple
modification of the Hamiltonian, one can dramatically decrease the relaxation
time of the system, while retaining the same equilibrium distribution. The
method is illustrated on the case of the one-dimensional double-well potential
Confinement Effects in Antiferromagnets
Phase equilibrium in confined Ising antiferromagnets was studied as a
function of the coupling (v) and a magnetic field (h) at the surfaces, in the
presence of an external field H. The ground state properties were calculated
exactly for symmetric boundary conditions and nearest-neighbor interactions,
and a full zero-temperature phase diagram in the plane v-h was obtained for
films with symmetry-preserving surface orientations. The ground-state analysis
was extended to the H-T plane using a cluster-variation free energy. The study
of the finite-T properties (as a function of v and h) reveals the close
interdependence between the surface and finite-size effects and, together with
the ground-state phase diagram, provides an integral picture of the confinement
in anisotropic antiferromagnets with surfaces that preserve the symmetry of the
order parameter.Comment: 10 pages, 8 figures, Accepted in Phys. Rev.
Pattern Formation in the Inhomogeneous Cooling State of Granular Fluids
We present results from comprehensive event-driven (ED) simulations of
nonlinear pattern formation in freely-evolving granular gases. In particular,
we focus on the the morphologies of density and velocity fields in the
inhomogeneous cooling state (ICS). We emphasize the strong analogy between the
ICS morphologies and pattern formation in phase ordering systems with a
globally conserved order parameter.Comment: 11 pages, 4 figures. to appear in Europhys. Let
- …