Low-energy Window Research Articles

Dynamical properties of Ni2MnGa determined from density functional calculations

The phonon spectrum for Ni2MnGa in the cubic L21 structure has been calculated by using density functional theory. Several vibrational anomalies have been found at particular wave vectors, which are related to experimentally observed phase transformations leading to different modulated structures. The cubic L21 structure has been found to be unstable at zero temperature with respect to a particular rearrangement of the atoms which can be related to the TA2 phonon mode. The dispersion of optical phonons of the Ni atoms lies in a relatively low energy window, which leads to a hybridization with acoustic phonons emphasizing the role of a strong electron–phonon coupling in Ni2MnGa.

Thermodynamical observables in a finite temperature window from the Monte Carlo Hamiltonian

The Monte Carlo (MC) Hamiltonian is a new stochastic method to solve many-body problems. The MC Hamiltonian represents an effective Hamiltonian in a finite energy window. We construct it from the classical action via Monte Carlo with importance sampling. The MC Hamiltonian yields the energy spectrum and corresponding wave functions in a low energy window. This allows to compute thermodynamical observables in a low temperature window. We show the working of the MC Hamiltonian by an example from lattice field theory (Klein–Gordon model).

Ion screening effects and stellar collapse

During the collapse of a massive star{close_quote}s stellar core Coulomb effects maintain the ions in a highly correlated state. This has an important consequence: Neutrino-nucleus elastic scattering, which dominates the neutrino opacity, is substantially reduced for low-energy neutrinos. This results from phase interference effects that occur when the neutrino wavelength becomes larger than the interion spacing, and is analogous to a crystal becoming transparent to x rays when the change in wave number from scattering is smaller than the reciprocal lattice spacing. This reduction in the neutrino-nucleus elastic scattering cross section, referred to as {open_quotes}ion screening,{close_quotes} has been calculated most recently by Horowitz. Using his correction, we investigate its effect on stellar core collapse. Our numerical results show that {nu}{sub e} downscattering with electrons is sufficiently rapid to fill the low-energy neutrino window created by ion screening, but the window width is insufficient for ion screening to have a significant effect on core deleptonization. In particular, inclusion of ion screening lowers the trapped lepton fraction by only 0.015 in both our 15M{sub {circle_dot}} and a 25M{sub {circle_dot}} models. We confirm this with an analytic model that elucidates ion screening{close_quote}s essential effect. For the sake of comparison, we also investigate themore » effect on core deleptonization of turning neutrino-nucleus elastic scattering off completely, and of turning off all semileptonic neutral-current neutrino scattering. These latter neutrino opacity modifications have substantially greater effects on core deleptonization than the ion-screening correction. {copyright} {ital 1997} {ital The American Physical Society}« less

Production and inelastic scattering of neutrinos by nuclei at extreme temperatures

The role of neutrino interactions with nuclear states is investigated at high excitation energies during stellar collapse. Cross sections for inelastic scattering from nuclei are determined. These cross sections remain finite as the neutrino energy goes to zero, and for sufficiently high temperatures, the usual low energy window becomes totally closed. The rate of neutrino production from highly excited nuclear states is calculated. The resultant cooling from this process limits the temperature to approximately 10 to the 10th K in the transparent regions during collapse. During collapse this process is dominant in producing neutrinos other than electron neutrinos. After neutrino trapping, since electron-neutrino production is diminished, it becomes the dominant neutrino-production mechanism.

Effects of neutrino degeneracy and of downscatter on neutrino radiation from dense stellar cores

view Abstract Citations (8) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Effects of neutrino degeneracy and of downscatter of neutrino radiation from dense stellar cores. Lichtenstadt, I. ; Ron, A. ; Sack, N. ; Wagschal, J. J. ; Bludman, S. A. Abstract A simplified model is presented for several stages in the development of a neutronization shell in the inner core of a collapsing star. Neutrino degeneracy severely reduces neutrino emission and downscatter in energy, so that for all but the thinnest shell sources, surface emission of an approximately Fermi-Dirac neutrino spectrum obtains. The neutrino spectrum departs from exact Fermi-Dirac form only because of the outstreaming of low-energy neutrinos. Downscatter by electrons is helped by neutron scatterers present, but except for reducing the peak neutrino energy by about 30%, electrons play no dramatic part. The neutrino degeneracy inhibits downscatter so that the low-energy window in the Fermi-Dirac distribution leads to little neutrino loss. A simple equilibrium radiation picture emerges in which neutrinos are LTE thermally emitted in the neutronization shell and isotropically coherently scattered by neutrons and by nuclei on the way out of the overlying mantle. The Fermi statistics limit on neutrino flux is probably reached in practice, but is, in most cases, still insufficient for mantle blow-off. Publication: The Astrophysical Journal Pub Date: November 1978 DOI: 10.1086/156601 Bibcode: 1978ApJ...226..222L Keywords: Fermi-Dirac Statistics; Gravitational Collapse; Neutrinos; Stellar Cores; Stellar Radiation; Absorption Cross Sections; Degeneration; Scattering Cross Sections; Astrophysics; Collapsing Stars:Neutrinos; Neutrinos:Stellar Interiors full text sources ADS |

The Effects of Fermi Statistics on Neutrino Transport in Supernova Models

The neutrino Boltzmann transport equation is solved numerically and neutrino-electron Compton scattering is handled accurately. It is shown that a correct treatment of the (Fermi) statistics in the scattering of neutrinos essentially closes the low-energy window over a large fraction of the core. The low-energy occupation probabilities of the neutrinos are found to be very sensitive to a proper handling of the Pauli inhibition terms, whereas the stress on and the energy transfer to matter are not sensitive to it.

Pauli constriction of the low-energy window in neutrino supernova models

Pauli constriction of the low-energy window in neutrino supernova models