450 research outputs found
Simulation of the enhanced Curie temperature in Mn_5Ge_3C_x compounds
Mn_5Ge_3C_x films with x>0.5 were experimentally shown to exhibit a strongly
enhanced Curie temperature T_C compared to Mn_5Ge_3. In this letter we present
the results of our first principles calculations within Green's function
approach, focusing on the effect of carbon doping on the electronic and
magnetic properties of the Mn_5Ge_3. The calculated exchange coupling constants
revealed an enhancement of the ferromagnetic Mn-Mn interactions mediated by
carbon. The essentially increased T_C in Mn_5Ge_3C is well reproduced in our
Monte Carlo simulations and together with the decrease of the total
magnetisation is found to be predominantly of an electronic nature
Strain and correlation of self-organized Ge_(1-x)Mn_x nanocolumns embedded in Ge (001)
We report on the structural properties of Ge_(1-x)Mn_x layers grown by
molecular beam epitaxy. In these layers, nanocolumns with a high Mn content are
embedded in an almost-pure Ge matrix. We have used grazing-incidence X-ray
scattering, atomic force and transmission electron microscopy to study the
structural properties of the columns. We demonstrate how the elastic
deformation of the matrix (as calculated using atomistic simulations) around
the columns, as well as the average inter-column distance can account for the
shape of the diffusion around Bragg peaks.Comment: 9 pages, 7 figure
Termination control of electronic phases in oxide thin films and interfaces: LaAlO 3/SrTiO 3(001)
A wealth of intriguing properties emerge in the seemingly simple system composed of the band insulators LaAlO 3 and SrTiO 3 such as a two-dimensional electron gas, superconductivity and magnetism. In this paper, we review the current insight obtained from first principles calculations on the mechanisms governing the behaviour of thin LaAlO 3 films on SrTiO 3(001). In particular, we explore the strong dependence of the electronic properties on the surface and interface termination, the finite film thickness, lattice polarization and defects. A further aspect that is addressed is how the electronic behaviour and functionality can be tuned by an SrTiO 3 capping layer, adsorbates and metallic contacts. Lastly, we discuss recent reports on the coexistence of magnetism and superconductivity in this system for what they might imply about the electronic structure of this system
Constraints on the mass and abundance of black holes in the Galactic halo: the high mass limit
We establish constraints on the mass and abundance of black holes in the
Galactic halo by determining their impact on globular clusters which are
conventionally considered to be little evolved. Using detailed Monte Carlo
simulations and simple analytic estimates, we conclude that, at Galactocentric
radius R~8 kpc, black holes with masses M_bh >~(1-3) x 10^6 M_sun can comprise
no more than a fraction f_bh ~ 0.025-0.05 of the total halo density. This
constraint significantly improves those based on disk heating and dynamical
friction arguments as well as current lensing results. At smaller radius, the
constraint on f_bh strengthens, while, at larger radius, an increased fraction
of black holes is allowed.Comment: 13 pages, 10 figures, revised version, in press, Monthly Notice
Reionization Constraints on the Contribution of Primordial Compact Objects to Dark Matter
Many lines of evidence suggest that nonbaryonic dark matter constitutes
roughly 30% of the critical closure density, but the composition of this dark
matter is unknown. One class of candidates for the dark matter is compact
objects formed in the early universe, with typical masses M between 0.1 and 1
solar masses to correspond to the mass scale of objects found with microlensing
observing projects. Specific candidates of this type include black holes formed
at the epoch of the QCD phase transition, quark stars, and boson stars. Here we
show that accretion onto these objects produces substantial ionization in the
early universe, with an optical depth to Thomson scattering out to z=1100 of
approximately tau=2-4 [f_CO\epsilon_{-1}(M/Msun)]^{1/2} (H_0/65)^{-1}, where
\epsilon_{-1} is the accretion efficiency \epsilon\equiv L/{\dot M}c^2 divided
by 0.1 and f_CO is the fraction of matter in the compact objects. The current
upper limit to the scattering optical depth, based on the anisotropy of the
microwave background, is approximately 0.4. Therefore, if accretion onto these
objects is relatively efficient, they cannot be the main component of
nonbaryonic dark matter.Comment: 12 pages including one figure, uses aaspp4, submitted to Ap
Kick velocity induced by magnetic dipole and quadrupole radiation
We examine the recoil velocity induced by the superposition of the magnetic
dipole and quadrupole radiation from a pulsar/magnetar born with rapid
rotation. The resultant velocity depends on not the magnitude, but rather the
ratio of the two moments and their geometrical configuration. The model does
not necessarily lead to high spatial velocity for a magnetar with a strong
magnetic field, which is consistent with the recent observational upper bound.
The maximum velocity predicted with this model is slightly smaller than that of
observed fast-moving pulsars.Comment: 10 pages, 3 color figure
Radiation from condensed surface of magnetic neutron stars
Recent observations show that the thermal X-ray spectra of many isolated
neutron stars are featureless and in some cases (e.g., RX J1856.5-3754) well
fit by a blackbody. Such a perfect blackbody spectrum is puzzling since
radiative transport through typical neutron star atmospheres causes noticeable
deviation from blackbody. Previous studies have shown that in a strong magnetic
field, the outermost layer of the neutron star may be in a condensed solid or
liquid form because of the greatly enhanced cohesive energy of the condensed
matter. The critical temperature of condensation increases with the magnetic
field strength, and can be as high as 10^6 K (for Fe surface at B \sim 10^{13}
G or H surface at B \sim a few times 10^{14} G). Thus the thermal radiation can
directly emerge from the degenerate metallic condensed surface, without going
through a gaseous atmosphere. Here we calculate the emission properties
(spectrum and polarization) of the condensed Fe and H surfaces of magnetic
neutron stars in the regimes where such condensation may be possible. For a
smooth condensed surface, the overall emission is reduced from the blackbody by
less than a factor of 2. The spectrum exhibits modest deviation from blackbody
across a wide energy range, and shows mild absorption features associated with
the ion cyclotron frequency and the electron plasma frequency in the condensed
matter. The roughness of the solid condensate (in the Fe case) tends to
decrease the reflectivity of the surface, and make the emission spectrum even
closer to blackbody. We discuss the implications of our results for
observations of dim, isolated neutron stars and magnetars.Comment: 12 pages, 11 figures. ApJ, accepted (final version; eq.(3) corrected
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