4,083 research outputs found
Optical conductivity and Raman scattering of iron superconductors
We discuss how to analyze the optical conductivity and Raman spectra of
multi-orbital systems using the velocity and the Raman vertices in a similar
way Raman vertices were used to disentangle nodal and antinodal regions in
cuprates. We apply this method to iron superconductors in the magnetic and
non-magnetic states, studied at the mean field level. We find that the
anisotropy in the optical conductivity at low frequencies reflects the
difference between the magnetic gaps at the X and Y electron pockets. Both gaps
are sampled by Raman spectroscopy. We also show that the Drude weight
anisotropy in the magnetic state is sensitive to small changes in the lattice
structure.Comment: 14 pages, 10 figures, as accepted in Phys. Rev. B,
explanations/discussion added in Secs. II, III and V
Oscillatory dynamics of a superconductor vortex lattice in high amplitude ac magnetic fields
In this work we study by ac susceptibility measurements the evolution of the
solid vortex lattice mobility under oscillating forces. Previous work had
already shown that in YBCO single crystals, below the melting transition, a
temporarily symmetric magnetic ac field (e.g. sinusoidal, square, triangular)
can heal the vortex lattice (VL) and increase its mobility, but a temporarily
asymmetric one (e.g. sawtooth) of the same amplitude can tear the lattice into
a more pinned disordered state. In this work we present evidence that the
mobility of the VL is reduced for large vortex displacements, in agreement with
predictions of recent simulations. We show that with large symmetric
oscillating fields both an initially ordered or an initially disordered VL
configuration evolve towards a less mobile lattice, supporting the scenario of
plastic flow.Comment: 5 pages, 4 figures. To appear in Phys. Rev.
Effect of Coulomb interactions on the optical properties of doped graphene
Recent optical conductivity experiments of doped graphene in the infrared
regime reveal a strong background in the energy region between the intra and
interband transitions difficult to explain within conventional pictures. We
propose a phenomenological model taking into account the marginal Fermi liquid
nature of the quasiparticles in graphene near the neutrality point that can
explain qualitatively the observed features. We also study the electronic Raman
signal and suggest that it will also be anomalous.Comment: 4 pages, 2 figure
Enhanced spin accumulation at room temperature in graphene spin valves with amorphous carbon interfacial layers
We demonstrate a large enhancement of the spin accumulation in monolayer
graphene following electron-beam induced deposition of an amorphous carbon
layer at the ferromagnet-graphene interface. The enhancement is 10^4-fold when
graphene is deposited onto poly(methyl metacrylate) (PMMA) and exposed with
sufficient electron-beam dose to cross-link the PMMA, and 10^3-fold when
graphene is deposited directly onto SiO2 and exposed with identical dose. We
attribute the difference to a more efficient carbon deposition in the former
case due to an increase in the presence of compounds containing carbon, which
are released by the PMMA. The amorphous carbon interface can sustain very large
current densities without degrading, which leads to very large spin
accumulations exceeding 500 microeVs at room temperature
Substructure and halo density profiles in a Warm Dark Matter Cosmology
We performed a series of high-resolution simulations designed to study the
substructure of Milky Way-size galactic halos (host halos) and the density
profiles of halos in a warm dark matter (WDM) scenario with a non-vanishing
cosmological constant. The virial masses of the host halos range from 3.5 x
10^12 to 1.7 x 10^12 solar masses and they have more than 10^5 particles each.
A key feature of the WDM power spectrum is the free-streaming length R_f which
fixes an additional parameter for the model of structure formation. We analyze
the substructure of host halos using three R_f values: 0.2, 0.1, and 0.05 Mpc
and compare results to the predictions of the cold dark matter (CDM) model. We
find that guest halos (satellites) do form in the WDM scenario but are more
easily destroyed by dynamical friction and tidal disruption than their
counterparts in a CDM model. The small number of guest halos that we find
within the virial radii of host halos at z = 0 in the WDM models is the result
of a less efficient halo accretion and a higher satellite destruction rate.
Under the assumption that each guest halo hosts a luminous galaxy, we find that
the observed circular velocity function of satellites around the Milky Way and
Andromeda is well described by the R_f = 0.1 Mpc WDM model. In the R_f =
0.1-0.2 Mpc models, the surviving subhalos at z=0 have an average concentration
parameter c_1/5 which is approximately twice smaller than that of the
corresponding CDM subhalos. This difference, very likely, produces the higher
satellite destruction rate found in the WDM models. The density profile of host
halos is well described by the NFW fit whereas guest halos show a wide variety
of density profiles (abridged).Comment: Uses emulateapj.sty: 10 pages, 4 figures, ApJ accepted. Some changes
have been introduced as suggested by the referee: (1) the description of the
numerical simulations was sligthly modified to make it clearer, (2) the
ellipticities of the host halos are now measured, and (3) the discussion
section was divided in two subsections and enlarge
On the Structure of Dark Matter Halos at the Damping Scale of the Power Spectrum with and without Relict Velocities
We report a series of high-resolution cosmological N-body simulations
designed to explore the formation and properties of dark matter halos with
masses close to the damping scale of the primordial power spectrum of density
fluctuations. We further investigate the effect that the addition of a random
component, v_rms, into the particle velocity field has on the structure of
halos. We adopted as a fiducial model the Lambda Warm Dark Matter cosmology
with a non-thermal sterile neutrino mass of 0.5 keV. The filtering mass
corresponds then to M_f = 2.6x10^12 M_sun/h. Halos of masses close to M_f were
simulated with several million of particles. The results show that, on one
hand, the inner density slope of these halos (at radii <~0.02 the virial radius
Rvir) is systematically steeper than the one corresponding to the NFW fit or to
the CDM counterpart. On the other hand, the overall density profile (radii
larger than 0.02Rvir) is less curved and less concentrated than the NFW fit,
with an outer slope shallower than -3. For simulations with v_rms, the inner
halo density profiles flatten significantly at radii smaller than 2-3 kpc/h
(<~0.010-0.015Rvir). A constant density core is not detected in our
simulations, with the exception of one halo for which the flat core radius is
~1 kpc/h. Nevertheless, if ``cored'' density profiles are used to fit the halo
profiles, the inferred core radii are ~0.1-0.8 kpc/h, in rough agreement with
theoretical predictions based on phase-space constrains, and on dynamical
models of warm gravitational collapse. A reduction of v_rms by a factor of 3
produces a modest decrease in core radii, less than a factor of 1.5. We discuss
the extension of our results into several contexts, for example, to the
structure of the cold DM micro-halos at the damping scale of this model.Comment: 13 pages, 6 figures, accepted for publication in The Astrophysical
Journa
Forming Disk Galaxies in Lambda CDM Simulations
We used fully cosmological, high resolution N-body + SPH simulations to
follow the formation of disk galaxies with rotational velocities between 135
and 270 km/sec in a Lambda CDM universe. The simulations include gas cooling,
star formation, the effects of a uniform UV background and a physically
motivated description of feedback from supernovae. The host dark matter halos
have a spin and last major merger redshift typical of galaxy sized halos as
measured in recent large scale N--Body simulations. The simulated galaxies form
rotationally supported disks with realistic exponential scale lengths and fall
on both the I-band and baryonic Tully Fisher relations. An extended stellar
disk forms inside the Milky Way sized halo immediately after the last major
merger. The combination of UV background and SN feedback drastically reduces
the number of visible satellites orbiting inside a Milky Way sized halo,
bringing it in fair agreement with observations. Our simulations predict that
the average age of a primary galaxy's stellar population decreases with mass,
because feedback delays star formation in less massive galaxies. Galaxies have
stellar masses and current star formation rates as a function of total mass
that are in good agreement with observational data. We discuss how both high
mass and force resolution and a realistic description of star formation and
feedback are important ingredients to match the observed properties of
galaxies.Comment: Revised version after the referee's comments. Conclusions unchanged.
2 new plots. MNRAS in press. 20 plots. 21 page
Exponentially Modified QCD Coupling
We present a specific class of models for an infrared-finite analytic QCD
coupling, such that at large space-like energy scales the coupling differs from
the perturbative one by less than any inverse power of the energy scale. This
condition is motivated by the ITEP Operator Product Expansion philosophy.
Allowed by the ambiguity in the analytization of the perturbative coupling, the
proposed class of couplings has three parameters. In the intermediate energy
region, the proposed coupling has low loop-level and renormalization scheme
dependence. The present modification of perturbative QCD must be considered as
a phenomenological attempt, with the aim of enlarging the applicability range
of the theory of the strong interactions at low energies.Comment: two references adde
Close encounters involving RAVE stars beyond the 47 Tucanae tidal radius
The most accurate 6D phase-space information from the Radial Velocity
Experiment (RAVE) was used to integrate the orbits of 105 stars around the
galactic globular cluster 47 Tucanae, to look for close encounters between them
in the past, with a minimum distance approach less than the cluster tidal
radius. The stars are currently over the distance range 3.0 kpc d 5.5
kpc. Using the uncertainties in the current position and velocity vector for
both, star and cluster, 105 pairs of star-cluster orbits were generated in a
Monte Carlo numerical scheme, integrated over 2 Gyr and considering an
axisymmetric and non-axisymmetric Milky-Way-like Galactic potential,
respectively. In this scheme, we identified 20 potential cluster members that
had close encounters with the globular cluster 47 Tucanae, all of which have a
relative velocity distribution (V) less than 200 km s at the
minimum distance approach. Among these potential members, 9 had close
encounters with the cluster with velocities less than the escape velocity of 47
Tucanae, therefore a scenario of tidal stripping seems likely. These stars have
been classified with a 93\% confidence level, leading to the identification of
extratidal cluster stars. For the other 11 stars, V exceeds the escape
velocity of the cluster, therefore they were likely ejected or are unassociated
interlopers.Comment: 10 pages, 6 figures, 2 table, Accepted for publication in MNRA
- …