118,302 research outputs found
Parameter dependence of magnetized CMB observables
Pre-decoupling magnetic fields affect the scalar modes of the geometry and
produce observable effects which can be constrained also through the use of
current (as opposed to forthcoming) data stemming from the Cosmic Microwave
Background observations. The dependence of the temperature and polarization
angular power spectra upon the parameters of an ambient magnetic field is
encoded in the scaling properties of a set of basic integrals whose derivation
is simplified in the limit of small angular scales. The magnetically-induced
distortions patterns of the relevant observables can be computed analytically
by employing scaling considerations which are corroborated by numerical
results.Comment: 48 pages, 11 figures; corrected minor typos; discussions added; to
appear in Physical Revie
Cosmic polarimetry in magnetoactive plasmas
Polarimetry of the Cosmic Microwave Background (CMB) represents one of the
possible diagnostics aimed at testing large-scale magnetism at the epoch of the
photon decoupling. The propagation of electromagnetic disturbances in a
magnetized plasma leads naturally to a B-mode polarization whose angular power
spectrum is hereby computed both analytically and numerically. Combined
analyses of all the publicly available data on the B-mode polarization are
presented, for the first time, in the light of the magnetized CDM
scenario. Novel constraints on pre-equality magnetism are also derived in view
of the current and expected sensitivities to the B-mode polarization.Comment: 34 pages, 13 figure
Ab initio calculation of the shift photocurrent by Wannier interpolation
We describe and implement a first-principles algorithm based on
maximally-localized Wannier functions for calculating the shift-current
response of piezoelectric crystals in the independent-particle approximation.
The proposed algorithm presents several advantages over existing ones,
including full gauge invariance, low computational cost, and a correct
treatment of the optical matrix elements with nonlocal pseudopotentials.
Band-truncation errors are avoided by a careful formulation of
perturbation theory within the subspace of wannierized bands. The needed
ingredients are the matrix elements of the Hamiltonian and of the position
operator in the Wannier basis, which are readily available at the end of the
wannierization step. If the off-diagonal matrix elements of the position
operator are discarded, our expressions reduce to the ones that have been used
in recent tight-binding calculations of the shift current. We find that this
`diagonal' approximation can introduce sizeable errors, highlighting the
importance of carefully embedding the tight-binding model in real space for an
accurate description of the charge transfer that gives rise to the shift
current.Comment: 13 pages, 7 figure
Thermodynamics of hexagonal-close-packed iron under Earth’s core conditions
The free energy and other thermodynamic properties of hexagonal-close-packed iron are calculated by direct ab initio methods over a wide range of pressures and temperatures relevant to the Earth’s core. The ab initio calculations are based on density-functional theory in the generalized-gradient approximation, and are performed using the projector augmented wave approach. Thermal excitation of electrons is fully included. The Helmholtz free energy consists of three parts, associated with the rigid perfect lattice, harmonic lattice vibrations, and anharmonic contributions, and the technical problems of calculating these parts to high precision are investigated. The harmonic part is obtained by computing the phonon frequencies over the entire Brillouin zone, and by summation of the free-energy contributions associated with the phonon modes. The anharmonic part is computed by the technique of thermodynamic integration using carefully designed reference systems. Detailed results are presented for the pressure, specific heat, bulk modulus, expansion coefficient and Grüneisen parameter, and comparisons are made with values obtained from diamond-anvil-cell and shock experiments
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