2,200 research outputs found
Universality in the entanglement structure of ferromagnets
Systems of exchange-coupled spins are commonly used to model ferromagnets.
The quantum correlations in such magnets are studied using tools from quantum
information theory. Isotropic ferromagnets are shown to possess a universal
low-temperature density matrix which precludes entanglement between spins, and
the mechanism of entanglement cancellation is investigated, revealing a core of
states resistant to pairwise entanglement cancellation. Numerical studies of
one-, two-, and three-dimensional lattices as well as irregular geometries
showed no entanglement in ferromagnets at any temperature or magnetic field
strength.Comment: 4 pages, 2 figure
Supernova electron capture rates for 55Co and 56Ni
We have calculated the Gamow-Teller strength distributions for the ground
states and first excited states in 55Co and 56Ni. These calculations have been
performed by shell model diagonalization in the pf shell using the KB3
interaction. The Gamow-Teller distributions are used to calculate the electron
capture rates for typical presupernova conditions. Our 55Co rate is noticeably
smaller than the presently adopted rate as it is dominated by weak low-lying
transitions rather than the strong Gamow-Teller (GT) resonance which is located
at a higher excitation energy in the daughter than usually parametrized.
Although our 56Ni rate agrees with the presently adopted rate, we do not
confirm the conventional parametrization of the GT centroid. Our results
support general trends suggested on the basis of shell model Monte Carlo
calculations.Comment: 4 pages, 4 figures, RevTeX 3.1, to appear in Phys. Lett.
QED theory of the nuclear magnetic shielding in hydrogen-like ions
The shielding of the nuclear magnetic moment by the bound electron in
hydrogen-like ions is calculated ab initio with inclusion of relativistic,
nuclear, and quantum electrodynamics (QED) effects. The QED correction is
evaluated to all orders in the nuclear binding strength parameter and,
independently, to the first order in the expansion in this parameter. The
results obtained lay the basis for the high-precision determination of nuclear
magnetic dipole moments from measurements of the g-factor of hydrogen-like
ions.Comment: 4 pages, 2 tables, 2 figure
Kramers-Wannier Approximation for 3D Ising Model
We investigate the Kramers-Wannier approximation for the three-dimensional
(3D) Ising model. The variational state is represented by an effective 2D Ising
model, which contains two variational parameters. We numerically calculate the
variational partition function using the corner transfer matrix renormalization
group (CTMRG) method, and find its maximum with respect to the variational
parameters. The calculated transition point is only 1.5%
less than the true ; the result is better than that obtained by the
corner transfer tensor renormalization group (CTTRG) approach. The calculated
phase transition is mean-field like.Comment: 7 pages, 4 figures, submitted to Prog. Theor. Phy
Equations of motion approach to the spin-1/2 Ising model on the Bethe lattice
We exactly solve the ferromagnetic spin-1/2 Ising model on the Bethe lattice
in the presence of an external magnetic field by means of the equations of
motion method within the Green's function formalism. In particular, such an
approach is applied to an isomorphic model of localized Fermi particles
interacting via an intersite Coulomb interaction. A complete set of
eigenoperators is found together with the corresponding eigenvalues. The
Green's functions and the correlation functions are written in terms of a
finite set of parameters to be self-consistently determined. A procedure is
developed, that allows us to exactly fix the unknown parameters in the case of
a Bethe lattice with any coordination number z. Non-local correlation functions
up to four points are also provided together with a study of the relevant
thermodynamic quantities.Comment: RevTex, 29 pages, 13 figure
Systematics of Gamow-Teller strengths in mid-fp-shell nuclei
We show that the presently available data on the Gamow-Teller (GT) strength
in mid-fp-shell nuclei are proportional to the product of the numbers of
valence protons and neutron holes in the full fp-shell. This observation leads
to important insights into the mechanism for GT quenching and to a simple
parametrization of the Gamow-Teller strengths important for electron capture by
fp-shell nuclei in the early stage of supernovae.Comment: 9 pages + 1 figure, Caltech preprint MAP-16
Collisionless energy absorption in the short-pulse intense laser-cluster interaction
In a previous Letter [Phys. Rev. Lett. 96, 123401 (2006)] we have shown by
means of three-dimensional particle-in-cell simulations and a simple
rigid-sphere model that nonlinear resonance absorption is the dominant
collisionless absorption mechanism in the intense, short-pulse laser cluster
interaction. In this paper we present a more detailed account of the matter. In
particular we show that the absorption efficiency is almost independent of the
laser polarization. In the rigid-sphere model, the absorbed energy increases by
many orders of magnitude at a certain threshold laser intensity. The
particle-in-cell results display maximum fractional absorption around the same
intensity. We calculate the threshold intensity and show that it is
underestimated by the common over-barrier ionization estimate.Comment: 12 pages, 13 figures, RevTeX
Temperature dependence of the nuclear symmetry energy
We have studied the properties of A=54 and A=64 isobars at temperatures T
\leq 2 MeV via Monte Carlo shell model calculations with two different residual
interactions. In accord with empirical indications, we find that the symmetry
energy coefficient, b_{sym}, is independent of temperature to within 0.6 MeV
for T \leq 1 MeV. This is in contrast to a recent suggestion of a 2.5 MeV
increase of b_{sym} for this temperature, which would have significantly
altered the supernova explosion scenario.Comment: 7 pages, including 2 figures, Caltech preprint MAP-17
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