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2009-03-24
|
Timescale Resolved Spectroscopy of Cyg X-1
|
We propose the timescale-resolved spectroscopy (TRS) as a new method to
combine the timing and spectral study. TRS is based on the time domain power
spectrum and reflects the variable amplitudes of spectral components on
different timescales. We produce the TRS with the RXTE PCA data for Cyg X-1 and
studied the spectral parameters (the power law photon index and the equivalent
width of the iron fluorescent line) as a function of timescale. The results of
TRS and frequency-resolved spectra (FRS) have been compared, and similarities
have been found for the two methods with the identical motivations. We also
discover the correspondences between the evolution of photon index with
timescale and the evolution of the equivalent width with timescale. The
observations can be divided into three types according to the correspondences
and different type is connected with different spectral state.
|
0903.4118v1
|
2009-03-25
|
Three- to Two-Dimensional Transition of the Electronic Structure in CaFe2As2 - parent compound for an iron arsenic high temperature superconductor
|
We use angle-resolved photoemission spectroscopy (ARPES) to study the
electronic properties of CaFe2As2 - parent compound of a pnictide
superconductor. We find that the structural and magnetic transition is
accompanied by a three- to two-dimensional (3D-2D) crossover in the electronic
structure. Above the transition temperature (Ts) Fermi surfaces around Gamma
and X points are cylindrical and quasi-2D. Below Ts the former becomes a 3D
ellipsoid, while the latter remains quasi-2D. This finding strongly suggests
that low dimensionality plays an important role in understanding the
superconducting mechanism in pnictides.
|
0903.4388v1
|
2009-03-26
|
Superconductivity induced by ruthenium substitution in an iron arsenide: investigation of SrFe2-xRuxAs2 (0 <= x <= 2)
|
The magnetism in SrFe2As2 can be suppressed by electron doping through a
small substitution of Fe by Co or Ni, giving way to superconductivity. We
demonstrate that a massive substitution of Fe by isovalent ruthenium similarly
suppresses the magnetic ordering in SrFe2-xRuxAs2 and leads to bulk
superconductivity for 0.6 <= x <= 0.8. Magnetization, electrical resistivity,
and specific heat data show Tc up to approx 20K. Detailed structural
investigations reveal a strong decrease of the lattice parameter ratio c/a with
increasing x. DFT band structure calculations are in line with the observation
that the magnetic order in SrFe2-xRuxAs2 is only destabilized for large x.
|
0903.4668v2
|
2009-03-27
|
The electronic structure of LiFeAs and NaFeAs probed by resonant inelastic x-ray scattering spectra
|
Results of resonant inelastic X-ray scattering (RIXS) measurements at Fe
L-edges and electronic structure calculations of LiFeAs and NaFeAs are
presented. Both experiment and theory show that in the vicinity of the Fermi
energy, the density of states is dominated by contributions from Fe 3d-states.
The comparison of Fe L2,3 non-resonant and resonant (excited at L2-threshold)
X-ray emission spectra with spectra of LaOFeAs and CaFe2As2 show a great
similarity in energy and I(L2)/I(L3) intensity ratio. The I(L2)/I(L3) intensity
ratio of all FeAs-based superconductors is found to be more similar to metallic
Fe than to correlated FeO. Basing on these measurements we conclude that
iron-based superconductors are weakly or moderately correlated systems.
|
0903.4901v2
|
2009-03-29
|
Anisotropic spin fluctuations and multiple superconducting gaps in hole-doped Ba_0.7K_0.3Fe_2As_2: NMR in a single crystal
|
We report the first ^{75}As-NMR study on a single crystal of the hole-doped
iron-pnictide superconductor Ba_{0.7}K_{0.3}Fe_2As_{2} (T_c = 31.5 K). We find
that the Fe antiferromagnetic spin fluctuations are anisotropic and are weaker
compared to underdoped copper-oxides or cobalt-oxide superconductors. The spin
lattice relaxation rate 1/T_1 decreases below T_c with no coherence peak and
shows a step-wise variation at low temperatures, which is indicative of
multiple superconducting gaps, as in the electron-doped
Pr(La)FeAsO$_{1-x}$F$_{x}$. Furthermore, no evidence was obtained for a
microscopic coexistence of a long-range magnetic and superconductivity.
|
0903.5098v2
|
2009-04-06
|
Redshift Evolution in the Iron Abundance of the Intracluster Medium
|
Clusters of galaxies provide a closed box within which one can determine the
chemical evolution of the gaseous baryons with cosmic time. We studied this
metallicity evolution in the hot X-ray emitting baryons through an analysis of
XMM-Newton observations of 29 galaxy clusters in the redshift range 0.3 < z <
1.3. Taken alone, this data set does not show evidence for significant
evolution. However, when we also include a comparable sample of 115 clusters
observed with Chandra (Maughan et al. 2008) and a lower redshift sample of 70
clusters observed with XMM at z < 0.3 (Snowden et al. 2008), there is
definitive evidence for a decrease in the metallicity. This decrease is
approximately a factor of two from z = 0 to z \approx 1, over which we find a
least-squares best-fit line Z(z) / Z_{\odot} = (0.46 \pm 0.05) - (0.38 \pm
0.03)z. The greatest uncertainty in the evolution comes from poorly constrained
metallicities in the highest redshift bin.
|
0904.1007v1
|
2009-04-06
|
Is There a Dark Matter Signal in the Galactic Positron Annihilation Radiation?
|
Assuming Galactic positrons do not go far before annhilating, a difference
between the observed 511 keV annihilation flux distribution and that of
positron production, expected from beta-plus decay in Galactic iron
nucleosynthesis, was evoked as evidence of a new source and a signal of dark
matter. We show, however, that the dark mater sources can not account for the
observed positronium fraction without extensive propagation. Yet with such
propagation, standard nucleosynthetic sources can fully account for the spatial
differences and the positronium fraction, leaving no signal for dark mater to
explain.
|
0904.1025v3
|
2009-04-09
|
Evaluating the locality of intrinsic precession damping in transition metals
|
The Landau-Lifshitz-Gilbert damping parameter is typically assumed to be a
local quantity, independent of magnetic configuration. To test the validity of
this assumption we calculate the precession damping rate of small amplitude
non-uniform mode magnons in iron, cobalt, and nickel. At scattering rates
expected near and above room temperature, little change in the damping rate is
found as the magnon wavelength is decreased from infinity to a length shorter
than features probed in recent experiments. This result indicates that
non-local effects due to the presence of weakly non-uniform modes, expected in
real devices, should not appreciably affect the dynamic response of the element
at typical operating temperatures. Conversely, at scattering rates expected in
very pure samples around cryogenic temperatures, non-local effects result in an
order of magnitude decrease in damping rates for magnons with wavelengths
commensurate with domain wall widths. While this low temperature result is
likely of little practical importance, it provides an experimentally testable
prediction of the non-local contribution of the spin-orbit torque-correlation
model of precession damping. None of these results exhibit strong dependence on
the magnon propagation direction.
|
0904.1455v1
|
2009-04-11
|
Three-band s+- Eliashberg theory and the superconducting gaps of iron pnictides
|
The experimental critical temperatures and gap values of the superconducting
pnictides of both the 1111 and 122 families can be simultaneously reproduced
within the Eliashberg theory by using a three-band model where the dominant
role is played by interband interactions and the order parameter undergoes a
sigh reversal between hole and electron bands (s+- wave symmetry). High values
of the electron-boson coupling constants and small typical boson energies (in
agreement with experiments) are necessary to obtain the values of all the gaps
and to correctly reproduce their temperature dependence.
|
0904.1808v2
|
2009-04-14
|
Optimizing the flux coupling between a nanoSQUID and a magnetic particle using atomic force microscope nanolithography
|
We present results of Niobium based SQUID magnetometers for which the
weak-links are engineered by the local oxidation of thin films using an Atomic
Force Microscope (AFM). Firstly, we show that this technique allows the
creation of variable thickness bridges with 10 nm lateral resolution. Precise
control of the weak-link milling is offered by the possibility to realtime
monitor weak-link conductance. Such a process is shown to enhance the magnetic
field modulation hence the sensitivity of the magnetometer. Secondly, AFM
lithography is used to provide a precise alignment of NanoSQUID weak-links with
respect to a ferromagnetic iron dot. The magnetization switching of the
near-field coupled particle is studied as a junction of the applied magnetic
field direction.
|
0904.2100v1
|
2009-04-14
|
The Electronic Phase Diagram of the Iron-based High Tc Superconductor Ba(Fe(1-x)Co(x))2As2 Under Hydrostatic Pressure (0 < x <0.099)
|
We report comprehensive resistivity measurements of single crystalline
samples of the Ba(Fe(1-x)Co(x))2As2 high Tc superconductor under hydrostatic
pressure up to 2.75 GPa and over a broad concentration range, 0 < x < 0.099. We
show that application of pressure progressively suppresses the SDW transition
temperature, T_SDW, in the underdoped regime. There is no sign of
pressure-induced superconductivity in the undoped BaFe2As2 down to 1.8 K, but
applied pressure dramatically enhances Tc in the underdoped regime. The effect
of pressure on Tc is very small in the optimally and overdoped regimes. As a
consequence, the dome of the superconducting phase extends to x 0.02 under
pressure. We discuss the implications of our findings in the context of a
possible quantum phase transition between the SDW and superconducting phases.
|
0904.2215v1
|
2009-04-17
|
Relativistically broadened iron line in the SUZAKU observation of the neutron star X-ray binary 4U 1705-44
|
The X-ray spectra of accreting compact objects often exhibit discrete
emission features associated with fluorescent emission in the accretion disk,
the strongest of which is the Fe Kalpha fluorescence line at 6.4--6.97keV.
These reflection features are amongst the best tools in the study of the inner
region of accretion flow around a compact object. Here we report on three
Suzaku observations of the neutron star X-ray binary 4U1705-44 where a broad,
skewed Fe Kalpha emission line is clearly visible above the continuum. By using
a relativistically-blurred reflection model we find that in 4U1705-44 the inner
disk radius extends down to rin=10.5^{+1.0}_{-1.7} GM/c^2 and is at an angle of
29.8^{+1.1}_{-1.0} degrees to the line of sight. Furthermore, we find that the
level of ionisation in the surface layers of the accretion disk changes by two
orders of magnitude between the three observations, however the inner radius
obtained from the line profile remains stable.
|
0904.2747v1
|
2009-04-19
|
First-principles study of the effect of Fe impurities in MgO at geophysically relevant pressures
|
The self-interaction corrected local spin density (SIC-LSD) formalism and the
standard GGA treatment of the exchange-correlation energy have been applied to
study the collapse of the magnetic moment of Fe impurities in MgO. The system
Mg_{1-x}Fe_xO is believed to be the second most abundant mineral in the Earth's
lower mantle.
We confirm the experimentally found increase of the critical pressure upon
iron concentration. Our calculations using standard GGA for a fixed Fe
concentration show that different arrangements of Fe atoms can remarkably shift
the transition pressure of the high spin (HS) to low spin (LS) transition. This
could explain the experimentally found broad transition regions. Our results
indicate that the HS-LS transition in Mg$_{1-x}Fe_xO is first order. We find
that SIC-LSD fails to predict the divalent Fe configuration as the lowest
energy configuration and discuss possible reasons for it.
|
0904.2901v1
|
2009-04-24
|
Effect of electron-doping on spin excitations of underdoped BaFe$_{1.96}$Ni$_{0.04}$As$_{2}$
|
We use neutron scattering to study magnetic order and spin excitations in
BaFe$_{1.96}$Ni$_{0.04}$As$_{2}$. On cooling, the system first changes the
lattice symmetry from tetragonal to orthoromhbic near $\sim$97 K, and then
orders antiferromagnetically at $T_N=91$ K before developing weak
superconductivity below $\sim$15 K. Although superconductivity appears to
co-exist with static antiferromagnetic order from transport and neutron
diffraction measurement, inelastic neutron scattering experiments reveal that
magnetic excitations do not respond to superconductivity. Instead, the effect
of electron-doping is to reduce the c-axis exchange coupling in BaFe$_2$As$_2$
and induce quasi two-dimensional spin excitations. These results suggest that
transition from three-dimensional spin waves to two-dimensional spin
excitations by electron-doping is important for the separated
structural/magnetic phase transitions and high-temperature superconductivity in
iron arsenides.
|
0904.3775v1
|
2009-04-24
|
Microscopic origin of magnetism and magnetic interactions in ferropnictides
|
One year after their initial discovery, two schools of thought have
crystallized regarding the electronic structure and magnetic properties of
ferropnictide systems. One postulates that these are itinerant weakly
correlated metallic systems that become magnetic by virtue of spin-Peierls type
transition due to near-nesting between the hole and the electron Fermi surface
pockets. The other argues these materials are strongly or at least moderately
correlated, the electrons are considerably localized and close to a
Mott-Hubbard transition, with the local magnetic moments interacting via
short-range superexchange. In this paper we argue that neither picture is fully
correct. The systems are moderately correlated, but with correlations driven by
Hund's rule coupling rather than by the on-site Hubbard repulsion. The iron
moments are largely local, driven by Hund's intra-atomic exchange.
Superexchange is not operative and the interactions between the Fe moments are
considerably long-range and driven mostly by one-electron energies of all
occupied states.
|
0904.3857v1
|
2009-04-24
|
Efficacy of surface error corrections to density functional theory calculations of vacancy formation energy in transition metals
|
We calculate properties like equilibrium lattice parameter, bulk modulus and
monovacancy formation energy for nickel (Ni), iron (Fe) and chromium (Cr) using
Kohn-Sham density functional theory (DFT). We compare relative performance of
local density approximation (LDA) and generalized gradient approximation (GGA)
for predicting such physical properties for these metals. We also make a
relative study between two different flavors of GGA exchange correlation
functional, namely, PW91 and PBE. These calculations show that there is a
discrepancy between DFT calculations and experimental data. In order to
understand this discrepancy in the calculation of vacancy formation energy, we
introduce a correction for the surface intrinsic error corresponding to an
exchange correlation functional using the scheme implemented by Mattsson et al.
[Phys. Rev. B 73, 195123 (2006)] and compare the effectiveness of the
correction scheme for Al and the 3d-transition metals.
|
0904.3859v2
|
2009-04-27
|
The 13C Pocket in Low Mass AGB Stars
|
It is well known that thermally pulsing Asymptotic Giant Branch stars with
low mass play a relevant role in the chemical evolution. They have synthesized
about 30% of the galactic carbon and provide an important contribution to the
nucleosynthesis of heavy elements (A>80). The relevant nucleosynthesis site is
the He-rich intermediate zone (less than 10^{-2} Msun), where
alpha(2alpha,gamma)12C reactions and slow neutron captures on seed nuclei
essentially iron) take place. A key ingredient is the interplay between nuclear
processes and convective mixing. It is the partial overlap of internal and
external convective zones that allows the dredge-up of the material enriched in
C and heavy elements. We review the progresses made in the last 50 years in the
comprehension of the s process in AGB stars, with special attention to the
identification of the main neutron sources and to the particular physical
conditions allowing this important nucleosynthesis.
|
0904.4163v1
|
2009-04-27
|
Fermi liquid state and enhanced electron correlations in the new iron pnictide CaFe$_4$As$_3$
|
The newly discovered CaFe$_4$As$_3$ system displays low-temperature Fermi
liquid behavior, with enhanced electron-electron correlations. At high
temperatures, the magnetic susceptibility shows Curie-Weiss behavior, with a
large temperature-independent contribution. Antiferromagnetic ordering is
observed below T$_N$ = (88.0 $\pm$ 1.0) K, possibly via a spin density wave
(SDW) transition. A remarkably sharp drop in resistivity occurs below T$_2$ =
(26.4 $\pm$ 1.0) K, correlated with a similarly abrupt increase in the
susceptibility, but no visible feature in the specific heat. The electronic
specific heat coefficient $\gamma$ at low temperatures is close to 0.02 J
mol$^{-1}_{Fe}$ K$^{-2}$, but a higher value for $\gamma$ ($\sim$0.08 J
mol$^{-1}_{Fe}$ K$^{-2}$ can be inferred from a linear C$ / $T \textit{vs.}
T$^2$ just above T$_2$. The Kadowaki-Woods ratio A$/\gamma^2$ = 55$*10^{-5}$
$\mu \Omega$cm mol$^2$ K$^2 $mJ$^{-2}$ is nearly two orders of magnitude larger
than that of heavy fermions.
|
0904.4188v1
|
2009-04-29
|
Can mass loss and overshooting prevent the excitation of g-modes in blue supergiants?
|
Thanks to their past history on the main sequence phase, supergiant massive
stars develop a convective shell around the helium core. This intermediate
convective zone (ICZ) plays an essential role in governing which g-modes are
excited. Indeed a strong radiative damping occurs in the high density radiative
core but the ICZ acts as a barrier preventing the propagation of some g-modes
into the core. These g-modes can thus be excited in supergiant stars by the
kappa-mechanism in the superficial layers due to the opacity bump of iron, at
log T=5.2. However massive stars are submitted to various complex phenomena
such as rotation, magnetic fields, semiconvection, mass loss, overshooting.
Each of these phenomena exerts a significant effect on the evolution and some
of them could prevent the onset of the convective zone. We develop a numerical
method which allows us to select the reflected, thus the potentially excited,
modes only. We study different cases in order to show that mass loss and
overshooting, in a large enough amount, reduce the extent of the ICZ and are
unfavourable to the excitation of g-modes.
|
0904.4641v1
|
2009-05-04
|
Low Temperature Crystal Structure and 57Fe Moessbauer Spectroscopy of Sr3Sc2O5Fe2As2
|
The crystal structure of the layered iron arsenide Sr3Sc2O5Fe2As2 was
determined between 300 and 10 K. The lattice parameters of the tetragonal cell
decrease anisotropically according to delta(c)/c : delta(a)/a = 4.2, which
results in a slight flattening of the As-Fe-As bond angle within the FeAs
layers. No indication of a structural instability could be detected. 57Fe
Moessbauer spectroscopic data show a single signal at 4.2, 77, and 298 K,
respectively, subjected to quadrupole splitting. The isomer shift increases
from 0.36(1) mm/s at 298 K to 0.49(1) mm/s at 4.2 K. No indication for magnetic
ordering was found.
|
0905.0337v1
|
2009-05-05
|
Structural and magnetic phase transitions in Na$_{1-δ}$FeAs
|
We use neutron scattering to study the spin and lattice structures of single
crystal and powder samples of Na$_{1-\delta}$FeAs ($T_c = 23$ K). On cooling
from room temperature, the system goes through a series of phase transitions:
first changing the crystal symmetry from tetragonal to orthorhombic at 49 K,
then ordering antiferromagnetically with a spin structure similar to that of
LaFeAsO and a small moment (0.09$\pm$0.04 $\mu_B$), and finally becoming
superconducting below about 23 K. These results confirm that antiferromagnetic
order is ubiquitous for the parent compounds of the iron arsenide
superconductors, and suggest that the separated structural and magnetic phase
transition temperatures are due to the reduction in the c-axis exchange
coupling of the system.
|
0905.0525v1
|
2009-05-08
|
The Multiverse Hierarchy
|
I survey physics theories involving parallel universes, arguing that they
form a natural four-level hierarchy of multiverses allowing progressively
greater diversity. Level I: A generic prediction of inflation is an infinite
ergodic universe, which contains Hubble volumes realizing all initial
conditions -- including an identical copy of you about 10^(10^29)m away. Level
II: In chaotic inflation, other thermalized regions may have different physical
constants, dimensionality and particle content. Level III: In unitary quantum
mechanics, other branches of the wavefunction add nothing qualitatively new,
which is ironic given that this level has historically been the most
controversial. Level IV: Other mathematical structures give different
fundamental equations of physics. The key question is not whether parallel
universes exist (Level I is the uncontroversial cosmological concordance
model), but how many levels there are. I discuss how multiverse models can be
falsified and argue that there is a severe "measure problem" that must be
solved to make testable predictions at levels II-IV.
|
0905.1283v1
|
2009-05-12
|
Capture and Indirect Detection of Inelastic Dark Matter
|
We compute the capture rate for Dark Matter in the Sun for models where the
dominant interaction with nuclei is inelastic -- the Dark Matter up-scatters to
a nearby dark "partner" state with a small splitting of order a 100 keV. Such
models have previously been shown to be compatible with DAMA/LIBRA data, as
well as data from all other direct detection experiments. The kinematics of
inelastic Dark Matter ensures that the dominant contribution to capture occurs
from scattering off of iron. We give a prediction for neutrino rates for
current and future neutrino telescopes based on the results from current direct
detection experiments. Current bounds from Super--Kamiokande and IceCube-22
significantly constrain these models, assuming annihilations are into two-body
Standard Model final states, such as W+W-, t-tbar, b-bbar or tau+tau-.
Annihilations into first and second generation quarks and leptons are generally
allowed, as are annihilations into new force carriers which decay dominantly
into e+e-, mu+mu- and pi+pi-.
|
0905.1847v2
|
2009-05-12
|
Strong-coupling Spin-singlet Superconductivity with Multiple Full Gaps in Hole-doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ Probed by Fe-NMR
|
We present $^{57}$Fe-NMR measurements of the novel normal and
superconducting-state characteristics of the iron-arsenide superconductor
Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ ($T_c$ = 38 K). In the normal state, the
measured Knight shift and nuclear spin-lattice relaxation rate $(1/T_1)$
demonstrate the development of wave-number ($q$)-dependent spin fluctuations,
except at $q$ = 0, which may originate from the nesting across the disconnected
Fermi surfaces. In the superconducting state, the spin component in the
$^{57}$Fe-Knight shift decreases to almost zero at low temperatures, evidencing
a spin-singlet superconducting state. The $^{57}$Fe-$1/T_1$ results are totally
consistent with a $s^\pm$-wave model with multiple full gaps, regardless of
doping with either electrons or holes.
|
0905.1896v1
|
2009-05-16
|
Strong electron-electron correlation and weak localization in CeO_{0.9}F_{0.1}Fe_{1-x}Co_xAs
|
Electron-doping of the semimetal (CeOFeAs) by either fluorine (max Tc ~ 43
K)or cobalt (max Tc ~ 11 K) leads to superconductivity. Here we show the effect
of transition metal (Co) substitution at the iron site on the superconducting
properties of CeO0.9F0.1FeAs (Tc ~38 K)to understand the interplay of charge
carriers in both the rare earth-oxygen and Fe-As layers. Simultaneous doping of
equivalent number of charge carriers in both layers leads to a Tc of 9.8 K
which is lower than the Tc obtained when either the conducting layer (CeAs) or
charge reservoir layer (CeO) is individually doped. This suggests a clear
interplay between the two layers to control the superconductivity. The
resistivity shows a T2 dependence (T >>Tc) which indicates strong
electron-electron correlation. Hall coefficient and thermoelectric power
indicate increased carrier concentration with cobalt doping in CeO0.9F0.1FeAs.
The rf penetration depth both for CeO0.9F0.1Fe0.95Co0.05As and CeO0.9F0.1FeAs
show an exponential temperature dependence with a gap value of ~ 1.6 and 1.9
meV. A resistance minimum is observed in the normal state near Tc which also
shows negative magnetoresistance and provides evidence for the onset of weak
localization.
|
0905.2679v1
|
2009-05-19
|
The fourth Dimension
|
In different passages of his dialogues, Plato showed deep
mathematically-based physical insights. Regrettably most readers overlooked the
respective statements, or they utterly did not understand those hints since
they were full of philological fallacious terms. Respectable translators
misinterpreted such statements and therefore Plato's respective remarks were
not recognized as substantial knowledge. Furthermore, Plato often supplemented
such basic remarks by diffusely veiled and varied allusions that were often
ironically hidden somewhere in his dialogues by inconspicuous double meanings.
However, this mode of intentionally coded discrete communication was generally
not understood because such irony is not to everyone's taste. However, the
attempts to reconstruct Plato's system on the basis of admittedly individually
interpreted double meanings lead to a conclusive mathematical-physical cyclical
system of dimensions. Additionally it was possible to assign Plato's system of
philosophical ideas analogously to this cyclical system. Plato took the
verifiability of the mathematical-physical results as proof of the system of
his ideas and finally as proof of his ethical creed, the unconditional trust in
the 'all surmounting Good.'
|
0905.3048v2
|
2009-05-19
|
Testing astrophysical models for the PAMELA positron excess with cosmic ray nuclei
|
The excess in the positron fraction reported by the PAMELA collaboration has
been interpreted as due to annihilation or decay of dark matter in the Galaxy.
More prosaically, it has been ascribed to direct production of positrons by
nearby pulsars, or due to pion production during stochastic acceleration of
hadronic cosmic rays in nearby sources. We point out that measurements of
secondary nuclei produced by cosmic ray spallation can discriminate between
these possibilities. New data on the titanium-to-iron ratio from the ATIC-2
experiment support the hadronic source model above and enable a prediction to
be made for the boron-to-carbon ratio at energies above 100 GeV. Presently, all
cosmic ray data are consistent with the positron excess being astrophysical in
origin.
|
0905.3152v3
|
2009-05-19
|
Surface Geometric and Electronic Structure of BaFe2As2(001)
|
BaFe2As2 exhibits properties characteristic of the parent compounds of the
newly discovered iron (Fe)-based high-TC superconductors. By combining the real
space imaging of scanning tunneling microscopy/spectroscopy (STM/S) with
momentum space quantitative Low Energy Electron Diffraction (LEED) we have
identified the surface plane of cleaved BaFe2As2 crystals as the As terminated
Fe-As layer - the plane where superconductivity occurs. LEED and STM/S data on
the BaFe2As2(001) surface indicate an ordered arsenic (As) - terminated
metallic surface without reconstruction or lattice distortion. It is surprising
that the STM images the different Fe-As orbitals associated with the
orthorhombic structure, not the As atoms in the surface plane.
|
0905.3194v1
|
2009-05-20
|
Accelerated search kinetics mediated by redox reactions of DNA repair enzymes
|
A Charge Transport (CT) mechanism has been proposed in several papers (e.g.,
Yavin, et al. PNAS, v102 3546 (2005)) to explain the localization of Base
Excision Repair (BER) enzymes to lesions on DNA. The CT mechanism relies on
redox reactions of iron-sulfur cofactors that modify the enzyme's binding
affinity. These redox reactions are mediated by the DNA strand and involve the
exchange of electrons between BER enzymes along DNA. We propose a mathematical
model that incorporates enzyme binding/unbinding, electron transport, and
enzyme diffusion along DNA. Analysis of our model within a range of parameter
values suggests that the redox reactions can increase desorption of BER enzymes
not already bound to their targets, allowing the enzymes to be recycled, thus
accelerating the overall search process. This acceleration mechanism is most
effective when enzyme copy numbers and enzyme diffusivity along the DNA are
small. Under such conditions, we find that CT BER enzymes find their targets
more quickly than simple "passive" enzymes that simply attach to the DNA
without desorbing.
|
0905.3211v1
|
2009-05-20
|
Topological change of the Fermi surface in ternary iron-pnictides with reduced c/a ratio: A dHvA study of CaFe2P2
|
We report a de Haas-van Alphen effect study of the Fermi surface of CaFe2P2
using low temperature torque magnetometry up to 45 T. This system is a close
structural analogue of the collapsed tetragonal non-magnetic phase of CaFe2As2.
We find the Fermi surface of CaFe2P2 to differ from other related ternary
phosphides in that its topology is highly dispersive in the c-axis, being
three-dimensional in character and with identical mass enhancement on both
electron and hole pockets (~1.5). The dramatic change in topology of the Fermi
surface suggests that in a state with reduced (c/a) ratio, when bonding between
pnictogen layers becomes important, the Fermi surface sheets are unlikely to be
nested.
|
0905.3305v1
|
2009-05-20
|
Looking for pulsations in HgMn stars through CoRoT lightcurves
|
HgMn Chemically Peculiar stars are among the quietest stars of the
main-sequence. However, according to theoretical predictions, these stars could
have pulsations related to the very strong overabundances of iron peak
elements, which are produced by atomic diffusion in upper layers. Such
pulsations have never been detected from ground based observations.
Our aim is to search for signatures of pulsations in HgMn stars using the
high quality lightcurves provided by the CoRoT satellite.
We identified three faint stars (V>12), from VLT-GIRAFFE multiobject
spectrograph survey in a field which was planned for observation by CoRoT. They
present the typical characteristics of HgMn stars. They were observed by the
CoRoT satellite during the long run (131 days) which started from the 24th of
October 2007, with the exoplanets CCD's (Additional Programme). In the present
work, we present the analysis of the ground based spectra of these three stars
and the analysis of the corresponding CoRoT lightcurves.
Two of these three HgMn candidates show low amplitude (less than 1.6 mmag)
periodic variations (4.3 and 2.53 days respectively, with harmonics) which are
compatible with periods predicted by theoretical models.
|
0905.3340v1
|
2009-05-29
|
Ferromagnetic resonance linewidth in ultrathin films with perpendicular magnetic anisotropy
|
Transition metal ferromagnetic films with perpendicular magnetic anisotropy
(PMA) have ferromagnetic resonance (FMR) linewidths that are one order of
magnitude larger than soft magnetic materials, such as pure iron (Fe) and
permalloy (NiFe) thin films. A broadband FMR setup has been used to investigate
the origin of the enhanced linewidth in Ni$|$Co multilayer films with PMA. The
FMR linewidth depends linearly on frequency for perpendicular applied fields
and increases significantly when the magnetization is rotated into the film
plane. Irradiation of the film with Helium ions decreases the PMA and the
distribution of PMA parameters. This leads to a great reduction of the FMR
linewidth for in-plane magnetization. These results suggest that fluctuations
in PMA lead to a large two magnon scattering contribution to the linewidth for
in-plane magnetization and establish that the Gilbert damping is enhanced in
such materials ($\alpha \approx 0.04$, compared to $\alpha \approx 0.002$ for
pure Fe).
|
0905.4779v2
|
2009-05-31
|
Nodeless superconducting gap in electron-doped BaFe$_{1.9}$Ni$_{0.1}$As$_2$ probed by quasiparticle heat transport
|
The in-plane thermal conductivity $\kappa$ of electron-doped iron-arsenide
superconductor BaFe$_{1.9}$Ni$_{0.1}$As$_2$ ($T_c$ = 20.3 K) single crystal was
measured down to 70 mK. In zero field, the absence of a residual linear term
$\kappa_0/T$ at $ T \to 0$ is strong evidence for nodeless superconducting gap.
In magnetic field, $\kappa_0/T$ shows a slow field dependence up to $H$ = 14.5
T ($\approx$ 30% $H_{c_2}$). This is consistent with the superconducting gap
structure demonstrated by angle-resolved photoemission spectroscopy experiments
in BaFe$_{1.85}$Co$_{0.15}$As$_2$ ($T_c$ = 25.5 K), where isotropic
superconducting gaps with similar size on hole and electron pockets were
observed.
|
0906.0138v2
|
2009-06-01
|
$π$-junction to probe antiphase s-wave pairing in iron pnictide superconductors
|
Josephson junctions between a $FeAs$-based superconductor with antiphase
s-wave pairing and a conventional s-wave superconductor are studied. The
translational invariance in a planar junction between a single crystal pnictide
and an aluminum metal greatly enhances the relative weight of electron pockets
in the pnictide to the critical current. In a wide doping region of the
pnictide, a planar and a point contact junctions have opposite phases, which
can be used to design a tri-junction ring with $\pi$ phase to probe the
antiphase pairing.
|
0906.0169v3
|
2009-06-04
|
Neutrino deuteron reaction in the heating mechanism of core-collapse supernovae
|
We examine a potential role of the neutrino deuteron reactions in the
mechanism of supernova explosion by evaluating the energy transfer cross
section for the neutrino heating. We calculate the energy loss rate due to the
neutrino absorptions through the charged-current process as well as the
neutrino scattering through the neutral-current process. In so doing, we adopt
a detailed evaluation of cross sections for the neutrino deuteron reactions
with the phenomenological Lagrangian approach. We find the energy transfer
cross section for the deuteron is larger than those for $^{3}$H, $^{3}$He and
$^{4}$He for neutrino temperatures (T$_\nu$ $\sim 4$ MeV) relevant to supernova
core. Because of the low energy threshold for the deuteron breakup, the energy
transfer rate rapidly increases from low temperature, T$_\nu$ $\sim 1$ MeV.
This suggests that the neutrino deuteron reactions may contribute effectively
to the heating mechanism during the dissociation of irons into light elements
and nucleons in the shocked material of supernova core.
|
0906.0856v1
|
2009-06-09
|
Fabrication of the iron-based superconducting wire using Fe(Se, Te)
|
We have fabricated the Fe(Se, Te) superconducting wire by a special process
based on a powder-in-tube method. The pure Fe tube plays the role of not only
the sheath but also the raw material for synthesizing the superconducting
phases. We succeeded in observing zero resistivity current on the
current-voltage measurements for the Fe(Se, Te) wire. Introduction of the
pinning centers and fabricating a multi-core wire will enhance the critical
current density for the next step.
|
0906.1636v2
|
2009-06-09
|
Particle Propagation in the Galactic Center and Spatial Distribution of Non-Thermal X-rays
|
We showed that if the non-thermal emission from the Galactic center in the
range 14-40 keV is due to inverse bremsstrahlung emission of subrelativistic
protons, their interactions with hot and cold fractions of the interstellar
medium are equally important. Our estimation show that about 30% of the total
non-thermal flux from the GC in the range 14-40 keV is generated in regions of
cold gas while the rest is produced by proton interaction with hot plasma. From
the spatial distribution of 6.7 keV iron line we concluded the spatial
distribution of hot plasma is strongly non-uniform that should be taken into
account in analysis of protons propagation in the GC. From the Suzaku data we
got independent estimates for the diffusion coefficient of subrelativistic
protons in the GC, which was in the range $ 10^{26} - 10^{27}$ cm$^2$s$^{-1}$
|
0906.1712v1
|
2009-06-10
|
Superconductivity in SmFe1-xMxAsO (M = Co, Rh, Ir)
|
In this paper we report the comparative study of superconductivity by 3d
(Co), 4d (Rh), 5d (Ir) element doping in SmFeAsO. X-ray diffraction patterns
indicate that the material has formed the ZrCuSiAs-type structure with a space
group P4/nmm. It is found that the antiferromagnetic spin-density-wave (SDW)
order in the parent compounds is rapidly suppressed by Co, Rh, and Ir doping,
and superconductivity emerges. Both electrical resistance and magnetization
measurements show superconductivity up to around 10 K in SmFe1-xMxAsO (M = Co,
Rh, Ir). Co, Rh and Ir locate in the same column in the periodic table of
elements but have different electronic band structure, so comparative study
would add more ingredients to the underlying physics of the iron-based
superconductors.
|
0906.1898v1
|
2009-06-10
|
Spectroscopic Imaging Scanning Tunneling Microscopy as a Probe of Orbital Structures and Ordering
|
Unlike charge and spin, the orbital degree of freedom of electrons in
transition metal oxides is difficult to detect. We present the theoretical
study of a new detection method in metallic orbitally active systems by
analyzing the quasiparticle scattering interference (QPI) pattern of the
spectroscopic imaging scanning tunneling spectroscopy, which is sensitive to
orbital structures and orbital ordering. The QPIs for the $d_{xz}$ and
$d_{yz}$-orbital bands in the $t_{2g}$-orbital systems show a characteristic
stripe-like feature as a consequence of their quasi-one-dimensional nature,
which is robust against orbital hybridization. With the occurrence of orbital
ordering proposed in Sr$_3$Ru$_2$O$_7$ and iron-pnictides, the stripe-like QPI
patterns exhibit nematic distortion breaking the $C_4$-symmetry.
|
0906.1973v3
|
2009-06-10
|
Cosmic-ray origin in OB associations and preferential acceleration of refractory elements: Evidence from abundances of elements 26Fe through 34Se
|
We report abundances of elements from 26Fe to 34Se in the cosmic radiation
measured during fifty days of exposure of the Trans-Iron Galactic Element
Recorder (TIGER) balloon-borne instrument. These observations add support to
the concept that the bulk of cosmic-ray acceleration takes place in OB
associations, and they further support cosmic-ray acceleration models in which
elements present in interstellar grains are accelerated preferentially compared
with those found in interstellar gas.
|
0906.2021v1
|
2009-06-19
|
Homogeneous vs. inhomogeneous coexistence of magnetic order and superconductivity probed by NMR in Co and K doped iron pnictides
|
In Ba(Fe0.95Co0.05)2As2 all of the 75As NMR intensity at the paramagnetic
resonance position vanishes abruptly below Tonset(SDW)=56 K, indicating that
magnetic (spin density wave) order is present in all of the sample volume,
despite bulk superconductivity below Tc=15 K. The two phases thus coexist
homogeneously at the microscopic scale. In Ba0.6K0.4Fe2As2, on the other hand,
the signal loss below Tonset(SDW)~75 K is not complete, revealing that magnetic
order is bound to finite-size areas of the sample, while the remaining NMR
signal shows a clear superconducting response below Tc=37 K. Thus, the two
phases are not homogeneously mixed, at least for this potassium concentration.
For both samples, spatial electronic and/or magnetic inhomogeneity is shown to
characterize the NMR properties in the normal state.
|
0906.3708v1
|
2009-06-26
|
Differential stellar population models: how to reliably measure [Fe/H] and [alpha/Fe] in galaxies
|
We present differential stellar population models, which allow improved
determinations of the ages, iron and alpha-element abundances of old stellar
populations from spectral fitting. These new models are calibrated at solar
abundances using the predictions from classical, semi-empirical stellar
population models. We then use the predictive power of fully synthetic models
to compute predictions for different [Fe/H] and [alpha/Fe]. We show that these
new differential models provide remarkably accurate fits to the integrated
optical spectra of the bulge globular clusters NGC6528 and NGC6553, and that
the inferred [Fe/H] and [alpha/Fe] agree with values derived elsewhere from
stellar photometry and spectroscopy. The analysis of a small sample of SDSS
early-type galaxies further confirms that our alpha-enhanced models provide a
better fit to the spectra of massive ellipticals than the solar-scaled ones.
Our approach opens new opportunities for precision measurements of abundance
ratios in galaxies.
|
0906.5000v1
|
2009-06-29
|
Upper critical fields of the 11-system iron-chalcogenide superconductor FeSe$_{0.25}$Te$_{0.75}$
|
We have performed electrical resistivity measurements of a polycrystalline
sample of FeSe$_{0.25}$Te$_{0.75}$, which exhibits superconductivity at $T_{\rm
c} \sim 14$ K, in magnetic fields up to 55 T to determine the upper critical
field $\mu_{0}H_{\rm c2}$. In this compound, very large slopes of
$\mu_{0}H_{\rm c2}$ at the onset, the mid-point, the zero-resistivity
temperatures on superconductivity are determined to be -13.7, -10.1, and -6.9
T/K, respectively. The observed $\mu_{0}H_{\rm c2}(T)$s of this compound are
considerably smaller than those expected from the Werthamer-Helfand-Hohenberg
model, manifesting the Pauli limiting behavior. These results suggest that this
compound has a large Maki parameter, but it is smaller than that calculated for
a weak-coupling superconductor, indicating a large superconducting gap of this
compound as a strong-coupling superconductor.
|
0906.5248v1
|
2009-06-30
|
Fermi surface topology and low-lying quasiparticle dynamics of parent Fe$_{1+x}$Te/Se Superconductor by orbital-polarization resolved ARPES
|
We report the first photoemission study of Fe$_{1+x}$Te - the host compound
of the newly discovered iron-chalcogenide superconductors (maximum T$_c$ $\sim$
27K). Our results reveal a pair of nearly electron-hole compensated Fermi
pockets, strong Fermi velocity renormalization and an absence of a
spin-density-wave gap. A shadow hole pocket is observed at the X-point of the
Brillouin zone which is consistent with a long-range ordered magneto-structural
groundstate. No signature of Fermi surface nesting instability associated with
Q=($\pi$/2, $\pi$/2) is observed. Our results collectively reveal that the
Fe$_{1+x}$Te series is dramatically different from the high T$_{c}$ pnictides
and likely harbor unusual mechanism for superconductivity and magnetic order.
|
0906.5392v1
|
2009-06-30
|
Disorder-sensitive superconductivity in the iron silicide Lu$_2$Fe$_3$Si$_5$ studied by the Lu-site substitutions
|
We studied effect of non-magnetic and magnetic impurities on
superconductivity in Lu$_2$Fe$_3$Si$_5$ by small amount substitution of the Lu
site, which investigated structural, magnetic, and electrical properties of
non-magnetic (Lu$_{1-x}$Sc$_x$)$_2$Fe$_3$Si$_5$,
(Lu$_{1-x}$Y$_x$)$_2$Fe$_3$Si$_5$, and magnetic
(Lu$_{1-x}$Dy$_x$)$_2$Fe$_3$Si$_5$. The rapid depression of $T_c$ by
non-magnetic impurities in accordance with the increase of residual resistivity
reveals the strong pair breaking dominated by disorder. We provide compelling
evidence for the sign reversal of the superconducting order parameter in
Lu$_2$Fe$_3$Si$_5$.
|
0906.5569v1
|
2009-07-01
|
Chemical Compositions of Kinematically Selected Outer Halo Stars
|
Chemical abundances of 26 metal-poor dwarfs and giants are determined from
high-resolution and high signal-to-noise ratio spectra obtained with
Subaru/HDS. The sample is selected so that most of the objects have outer-halo
kinematics. Self-consistent atmospheric parameters were determined by an
iterative procedure based on spectroscopic analysis. Abundances of 13 elements,
including $\alpha$-elements (Mg, Si, Ca, Ti), odd-Z light elements (Na, Sc),
iron-peak elements (Cr, Mn, Fe, Ni, Zn) and neutron-capture elements (Y, Ba),
are determined by two independent data reduction and LTE analysis procedures,
confirming the consistency of the stellar parameters and abundances results. We
find a decreasing trend of [$\alpha$/Fe] with increasing [Fe/H] for the range
of $-3.5 <$ [Fe/H]$ < -1$, as found by Stephens and Boesgaard (2002). [Zn/Fe]
values of most objects in our sample are slightly lower than the bulk of halo
stars previously studied. These results are discussed as possible chemical
properties of the outer halo in the Galaxy.
|
0907.0076v2
|
2009-07-03
|
Atomically-flat, chemically-stable, superconducting epitaxial thin film of iron-based superconductor, cobalt-doped BaFe$_2$As$_2$
|
Epitaxial growth of Fe-based superconductors such as Co-doped SrFe$_2$As$_2$
(SrFe$_2$As$_2$:Co) was reported recently, but has still insufficient
properties for device application because they have rough surfaces and are
decomposed by reactions with water vapor in an ambient atmosphere. This letter
reports that epitaxial films of Co-doped BaFe$_2$As$_2$ grown at 700 oC show
the onset superconducting transition tempearture of 20 K. The transition is
sharper than those observed on the SrFe$_2$As$_2$:Co films, which would
originate from their improved crystallinity. These films also have
atomically-flat surfaces with steps-and-terraces structures and exhibit
chemical stability against exposure to water vapor.
|
0907.0666v1
|
2009-07-07
|
Phase diagram of the PrFeAsO$_{1-x}$F$_{x}$ superconductor
|
The electronic phase diagram of PrFeAsO$_{1-x}$F$_{x}$ (0$\leq$x$\leq$0.225)
has been determined using synchrotron X-ray powder diffraction, magnetization
and resistivity measurements. The structural transition temperature is
suppressed from 154 K to $\approx$120 K and the magnetic phase transitions of
both iron and praseodymium ions are completely suppressed by x$\approx$0.08
fluorine doping, coinciding with the emergence of superconductivity. The
optimal doping is x$\approx$0.15 when T$_{C}$=47 K, while the maximum
solubility of fluorine in PrFeAsO$_{1-x}$F$_{x}$ is reached around x=0.22. The
structural, magnetic and superconducting phase diagram is presented.
|
0907.1308v1
|
2009-07-15
|
Quark matter under strong magnetic fields in the su(3) Nambu-Jona-Lasinio Model
|
In the present work we use the mean field approximation to investigate quark
matter described by the su(3) Nambu-Jona-Lasinio model subject to a strong
magnetic field. We consider two cases: pure quark matter and quark matter in
beta-equilibrium possibly present in magnetars. The results are compared with
the ones obtained with the su(2) version of the model. The energy per baryon of
magnetized quark matter becomes more bound than nuclear matter made of iron
nuclei, for B around 2x10^(19)G. When the su(3) NJL model is applied to stellar
matter, the maximum mass configurations are always above 1.45 solar masses and
may be as high as 1.86 solar masses for a central magnetic field of 5x10^(18)G.
These numbers are within the masses of observed neutron stars.
|
0907.2607v1
|
2009-07-15
|
Driving and damping mechanisms in hybrid pressure-gravity modes pulsators
|
We study the energetic aspects of hybrid pressure-gravity modes pulsations.
The case of hybrid beta Cephei-SPB pulsators is considered with special
attention. In addition to the already known sensitivity of the driving
mechanism to the heavy elements mixture (mainly the iron abundance), we show
that the characteristics of the propagation and evanescent regions play also a
major role, determining the extension of the stable gap in the frequency domain
between the unstable low order pressure and high order gravity modes. Finally,
we consider the case of hybrid delta Sct-gamma Dor pulsators.
|
0907.2636v1
|
2009-07-16
|
Lattice distortion and magnetic quantum phase transition in CeFeAs(1-x)P(x)O
|
We use neutron scattering to show that replacing the larger arsenic with
smaller phosphorus in CeFeAs(1-x)P(x)O simultaneously suppresses the AF order
and orthorhombic distortion near x = 0.4, providing evidence for a magnetic
quantum critical point. Furthermore, we find that the pnictogen height in iron
arsenide is an important controlling parameter for their electronic and
magnetic properties, and may play an important role in electron pairing and
superconductivity.
|
0907.2853v1
|
2009-07-17
|
Phase diagram of CeFeAs$_{1-x}$P$_{x}$O obtained from electric resistivity, magnetization, and specific heat measurements
|
We performed a systematic study on the properties of CeFeAs$_{1-x}$P$_{x}$O
($0\leq x\leq 1$) by electrical resistivity, magnetization and specific heat
measurements. The c-axis lattice constant decreases significantly with
increasing P content, suggesting a remarkable chemical pressure. The Fe-3d
electrons show the enhanced metallic behavior upon P-doping and undergo a
magnetic quantum phase transition around $x \approx 0.4$. Meanwhile, the Ce-4f
electrons develop a ferromagnetic order near the same doping level. The
ferromagnetic order is vanishingly small around $x=0.9$. The data suggest a
heavy-fermion-like behavior as $x\geq 0.95$. No superconductivity is observed
down to 2 K. Our results show the ferromagnetic ordered state as an
intermediate phase intruding between the antiferromagnetic bad metal and the
nonmagnetic heavy fermion metal and support the cerium-containing iron
pnictides as a unique layered Kondo lattice system.
|
0907.2961v2
|
2009-07-21
|
Discrimination of mass hierarchy with atmospheric neutrinos at a magnetized muon detector
|
We have studied the mass hierarchy with atmospheric neutrinos considering the
muon energy and zenith angle of the event at the magnetized iron calorimeter
detector. For $\chi^2$ analysis we have migrated the number of events from
neutrino energy and zenith angle bins to muon energy and zenith angle bins
using the two-dimensional energy-angle correlated resolution functions. The
binning of data is made in two-dimensional grids of $\log_{10} E - L^{0.4}$
plane to get a better reflection of the oscillation pattern in the $\chi^2$
analysis. Then the $\chi^2$ is marginalized considering all possible systematic
uncertainties of the atmospheric neutrino flux and cross section. The effects
of the ranges of oscillation parameters on the marginalization are also
studied. The lower limit of the range of $\theta_{13}$ for marginalization is
found to be very crucial in determining the sensitivity of hierarchy for a
given $\theta_{13}$. Finally, we show that one can discriminate atmospheric
neutrino mass hierarchy at $>$90% C.L. if the lower limit of $\theta_{13} \ge
5^\circ$.
|
0907.3540v3
|
2009-07-21
|
The Dependency of the Cepheid Period-Luminosity Relation on Chemical Composition
|
The dependency of the Cepheid Period-Luminosity Relation on chemical
composition at different wavelengths is assessed via direct detailed abundance
analysis of Galactic and Magellanic Cepheids, as derived from high resolution,
high signal-to-noise spectra. Our measurements span one order of magnitude in
iron content and allow to rule out at the ~ 9 sigma level the universality of
the Period-Luminosity Relation in the V band, with metal rich stars being
fainter than metal poor ones by ~0.3 mag. The dependency is less pronounced in
the K band. Its magnitude and statistical significance decisively depend on
detailed distance measurements to individual stars, as inferred via the
Infrared Surface Brightness Method.
|
0907.3655v2
|
2009-07-23
|
Prospects of measuring the leptonic CP phase with atmospheric neutrinos
|
We have studied the prospects of measuring the CP violating phase with
atmospheric neutrinos at a large magnetized iron calorimeter detector
considering the muons (directly measurable) of the neutrino events generated by
a MonteCarlo event generator Nuance. The effect of $\theta_{13}$ and
$\delta_{CP}$ appears dominantly neither in atmospheric neutrino oscillation
nor in solar neutrino oscillation, but appears as subleading in both cases.
These are observable in range of $E \sim 1$ GeV for atmospheric neutrino, where
solar and atmospheric oscillation couple. In this regime, the quasi-elastic
events dominate and the energy resolution is very good, but the angular
resolution is very poor. Unlike beam experiments this poor angular resolution
acts against its measurements. However, we find that one can be able to
distinguish $\delta_{CP}\approx 0^\circ$ and $180^\circ$ at 90% confidence
level. We find no significant sensitivity for $\delta_{CP}\approx 90^\circ$ or
$270^\circ$.
|
0907.3978v2
|
2009-07-24
|
In-situ fabrication of cobalt-doped SrFe2As2 thin films by using pulsed laser deposition with excimer laser
|
The remarkably high superconducting transition temperature and upper critical
field of iron(Fe)-based layered superconductors, despite ferromagnetic material
base, open the prospect for superconducting electronics. However, success in
superconducting electronics has been limited because of difficulties in
fabricating high-quality thin films. We report the growth of high-quality
c-axis-oriented cobalt(Co)-doped SrFe2As2 thin films with bulk
superconductivity by using an in-situ pulsed laser deposition technique with a
248-nm-wavelength KrF excimer laser and an arsenic(As)-rich phase target. The
temperature and field dependences of the magnetization showing strong
diamagnetism and transport critical current density with superior Jc-H
performance are reported. These results provide necessary information for
practical applications of Fe-based superconductors.
|
0907.4227v1
|
2009-07-31
|
A new surface parameter for composition studies at high energies
|
A new family of parameters intended for composition studies is presented.
They make exclusive use of surface data combining the information from the
total signal at each triggered detector and the array geometry. We perform an
analytical study of these composition estimators in order to assess their
reliability, stability and possible optimization. The influence of the
different slopes of the proton and Iron lateral distribution function on the
discrimination power of the estimators is also studied. Additionally, the
stability of the parameter in face of a possible underestimation of the size of
the muon component by the shower simulation codes, as it is suggested by
experimental evidence, is also studied.
|
0907.5480v1
|
2009-07-31
|
The cleavage surface of the BaFe_(2-x)Co_(x)As_(2) and Fe_(y)Se_(1-x)Te_(x) superconductors: from diversity to simplicity
|
We elucidate the termination surface of cleaved single crystals of the
BaFe_(2-x)Co_(x)As_(2) and Fe_(y)Se_(1-x)Te_(x) families of the high
temperature iron based superconductors. By combining scanning tunneling
microscopic data with low energy electron diffraction we prove that the
termination layer of the Ba122 systems is a remnant of the Ba layer, which
exhibits a complex diversity of ordered and disordered structures. The observed
surface topographies and their accompanying superstructure reflections in
electron diffraction depend on the cleavage temperature. In stark contrast,
Fe_(y)Se_(1-x)Te_(x) possesses only a single termination structure - that of
the tetragonally ordered Se_(1-x)Te_(x) layer.
|
0907.5544v1
|
2009-07-31
|
Magnetism and superconductivity in single crystals $Eu_{1-x}Sr_xFe_{2-y}Co_{y}As_2$
|
We systematically studied the transport properties of single crystals of
$Eu_{1-x}Sr_xFe_{2-y}$Co$_{y}As_2$. Co doping can suppress the spin-density
wave (SDW) ordering and induces a superconducting transition, but a resistivity
reentrance due to the antiferromagnetic ordering of $Eu^{2+}$ spins is
observed, indicating the competition between antiferromagnetism (AFM) and
superconductivity. It is striking that the resistivity reentrance can be
completely suppressed by external magnetic field (H) because a metamagnetic
transition from antiferromagnetism to ferromagnetism for $Eu^{2+}$ spins is
induced by magnetic field. Superconductivity without resistivity reentrance
shows up by partial substitution of Eu$^{2+}$ with non-magnetic Sr$^{2+}$ to
completely destroy the AFM ordering of $Eu^{2+}$ spins. These results suggest
that the antiferromagnetism destroys the superconductivity, while the
ferromagnetism can coexist with the superconductivity in the iron-based
high-$T_c$ superconductors.
|
0907.5547v1
|
2009-08-05
|
^{31}P and ^{75}As NMR evidence for a residual density of states at zero energy in superconducting BaFe_2(As_{0.67}P_{0.33})_2
|
^{31}P and ^{75}As NMR measurements were performed in superconducting
BaFe_2(As_{0.67}P_{0.33})_2 with T_c = 30 K. The nuclear-spin-lattice
relaxation rate T_1^{-1} and the Knight shift in the normal state indicate the
development of antiferromagnetic fluctuations, and T_1^{-1} in the
superconducting (SC) state decreases without a coherence peak just below T_c,
as observed in (Ba_{1-x}K_{x})Fe_2As_2. In contrast to other iron arsenide
superconductors, the T_1^{-1} \propto T behavior is observed below 4K,
indicating the presence of a residual density of states at zero energy. Our
results suggest that strikingly different SC gaps appear in
BaFe_2(As_{1-x}P_{x})_2 despite a comparable T_c value, an analogous phase
diagram, and similar Fermi surfaces to (Ba_{1-x}K_{x})Fe_2As_2.
|
0908.0625v2
|
2009-08-05
|
Krylov implementation of the hybridization expansion impurity solver and application to 5-orbital models
|
We present an implementation of the hybridization expansion impurity solver
which employs sparse matrix exact-diagonalization techniques to compute the
time evolution of the local Hamiltonian. This method avoids computationally
expensive matrix-matrix multiplications and becomes advantageous over the
conventional implementation for models with 5 or more orbitals. In particular,
this method will allow the systematic investigation of 7-orbital systems
(lanthanide and actinide compounds) within single-site dynamical mean field
theory. We illustrate the power and usefulness of our approach with dynamical
mean field results for a 5-orbital model which captures some aspects of the
physics of the iron based superconductors.
|
0908.0681v2
|
2009-08-12
|
The Inner Rim Structures of Protoplanetary Discs
|
The inner boundary of protoplanetary discs is structured by the dramatic
opacity changes at the transition from the dust-containing to a dust-free zone.
This paper explores the variety and limits of inner rim structures in passively
heated dusty discs. For this study, we implemented detailed sublimation physics
in a fast Monte Carlo radiative transfer code. We show that the inner rim in
dusty discs is not an infinitely sharp wall but a diffuse region which may be
narrow or wide. Furthermore, high surface densities and large silicate grains
as well as iron and corundum grains decrease the rim radius, from a 2.2AU
radius for small silicates around a 47 Solar luminosity Herbig Ae star
typically to 0.4AU and as close as 0.2AU. A passive disc with grain growth and
a diverse dust composition must thus have a small inner rim radius. Finally, an
analytical expression is presented for the rim location as a function of dust,
disc and stellar properties.
|
0908.1692v3
|
2009-08-20
|
Evidence for Nodal superconductivity in Sr$_{2}$ScFePO$_{3}$
|
Point contact Andreev reflection spectra have been taken as a function of
temperature and magnetic field on the polycrystalline form of the newly
discovered iron-based superconductor Sr2ScFePO3. A zero bias conductance peak
which disappears at the superconducting transition temperature, dominates all
of the spectra. Data taken in high magnetic fields show that this feature
survives until 7T at 2K and a flattening of the feature is observed in some
contacts. Here we inspect whether these observations can be interpreted within
a d-wave, or nodal order parameter framework which would be consistent with the
recent theoretical model where the height of the P in the Fe-P-Fe plane is key
to the symmetry of the superconductivity. However, in polycrystalline samples
care must be taken when examining Andreev spectra to eliminate or take into
account artefacts associated with the possible effects of Josephson junctions
and random alignment of grains.
|
0908.2902v2
|
2009-08-21
|
Magnetic structure of EuFe2As2 determined by single crystal neutron diffraction
|
Among various parent compounds of iron pnictide superconductors, EuFe2As2
stands out due to the presence of both spin density wave of Fe and
antiferromagnetic ordering (AFM) of the localized Eu2+ moment. Single crystal
neutron diffraction studies have been carried out to determine the magnetic
structure of this compound and to investigate the coupling of two magnetic
sublattices. Long range AFM ordering of Fe and Eu spins was observed below 190
K and 19 K, respectively. The ordering of Fe2+ moments is associated with the
wave vector k = (1,0,1) and it takes place at the same temperature as the
tetragonal to orthorhombic structural phase transition, which indicates the
strong coupling between structural and magnetic components. The ordering of Eu
moment is associated with the wave vector k = (0,0,1). While both Fe and Eu
spins are aligned along the long a axis as experimentally determined, our
studies suggest a weak coupling between the Fe and Eu magnetism.
|
0908.3142v1
|
2009-08-24
|
The two-dimensional frustrated Heisenberg model on the orthorhombic lattice
|
We discuss new high-field magnetization data recently obtained by Tsirlin et
al. for layered vanadium phosphates in the framework of the square-lattice
model. Our predictions for the saturation fields compare exceptionally well to
the experimental findings, and the strong bending of the curves below
saturation agrees very well with the experimental field dependence. Furthermore
we discuss the remarkably good agreement of the frustrated Heisenberg model on
the square lattice in spite of the fact that the compounds described with this
model actually have a lower crystallographic symmetry. We present results from
our calculations on the thermodynamics of the model on the orthorhombic (i.e.,
rectangular) lattice, in particular the temperature dependence of the magnetic
susceptibility. This analysis also sheds light on the discussion of magnetic
frustration and anisotropy of a class of iron pnictide parent compounds, where
several alternative suggestions for the magnetic exchange models were proposed.
|
0908.3442v1
|
2009-09-02
|
A study of nuclear effect in $F_3$ structure function in the deep inelastic $ν(\barν)$ reactions in nuclei
|
We study nuclear effect in the $F^A_3(x)$ structure function in the deep
inelastic neutrino reactions on iron by taking into account Fermi motion,
binding, target mass correction, shadowing and anti-shadowing corrections.
Calculations have been done in a local density approximation using relativistic
nuclear spectral functions which include nucleon correlations for nuclear
matter. Results for $F^A_3(x)$ have been compared with the results reported at
NuTeV and also with some of the older experiments reported in the literature.
|
0909.0360v1
|
2009-09-02
|
Nuclear EMC Effect in a Statistical Model
|
A simple statistical model in terms of light-front kinematic variables is
used to explain the nuclear EMC effect in the range $x \in [0.2,~0.7]$, which
was constructed by us previously to calculate the parton distribution functions
(PDFs) of the nucleon. Here, we treat the temperature $T$ as a parameter of the
atomic number $A$, and get reasonable results in agreement with the
experimental data. Our results show that the larger $A$, the lower $T$ thus the
bigger volume $V$, and these features are consistent with other models.
Moreover, we give the predictions of the quark distribution ratios,
\emph{i.e.}, $q^A(x) / q^D(x)$, $\bar{q}^A(x) / \bar{q}^D(x)$, and $s^A(x) /
s^D(x)$, and also the gluon ratio $g^A(x) / g^D(x)$ for iron as an example. The
predictions are different from those by other models, thus experiments aiming
at measuring the parton ratios of antiquarks, strange quarks, and gluons can
provide a discrimination of different models.
|
0909.0454v1
|
2009-09-03
|
Observation of Dirac Cone Electronic Dispersion in BaFe2As2
|
We performed an angle-resolved photoemission spectroscopy study of BaFe2As2,
which is the parent compound of the so-called 122 phase of the iron-pnictide
high-temperature superconductors. We reveal the existence of a Dirac cone in
the electronic structure of this material below the spin-density-wave
temperature, which is responsible for small spots of high photoemission
intensity at the Fermi level. Our analysis suggests that the cone is slightly
anisotropic and its apex is located very near the Fermi level, leading to tiny
Fermi surface pockets. Moreover, the bands forming the cone show an anisotropic
leading edge gap away from the cone that suggests a nodal spin-density-wave
description.
|
0909.0574v1
|
2009-09-04
|
Unconventional electronic reconstruction in undoped (Ba,Sr)Fe$_2$As$_2$ across the spin density wave transition
|
Through a systematic high resolution angle-resolved photoemission study of
the iron pnictide compounds (Ba,Sr)Fe$_2$As$_2$, we show that the electronic
structures of these compounds are significantly reconstructed across the spin
density wave ordering, which cannot be described by a simple folding scenario
of conventional density wave ordering. Moreover, we find that LDA calculations
with an incorporated suppressed magnetic moment of 0.5$\mu_{\tiny{\textrm{B}}}$
can match well the details in the reconstructed electronic structure,
suggesting that the nature of magnetism in the pnictides is more itinerant than
local, while the origin of suppressed magnetic moment remains an important
issue for future investigations.
|
0909.0831v1
|
2009-09-04
|
Inelastic X-ray Scattering Study of superconducting SmFeAs(O$_{1-x}$F$_{y}$) Single Crystals: Evidence for Strong Momentum-Dependent Doping-Induced Renormalizations of Optical Phonons
|
We report inelastic x-ray scattering experiments on the lattice dynamics in
SmFeAsO and superconducting SmFeAsO$_{0.6}$F$_{0.35}$ single crystals.
Particular attention was paid to the dispersions along the [100] direction of
three optical modes close to 23 meV, polarized out of the FeAs planes.
Remarkably, two of these modes are strongly renormalized upon fluorine doping.
These results provide significant insight into the energy and momentum
dependence of the coupling of the lattice to the electron system and underline
the importance of spin-phonon coupling in the superconducting iron-pnictides.
|
0909.0913v2
|
2009-09-04
|
Intrinsic pinning on structural domains in underdoped single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$
|
Critical current density was studied in single crystals of
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ for the values of $x$ spanning the entire doping
phase diagram. A noticeable enhancement was found for slightly underdoped
crystals with the peak at $x = 0.058$. Using a combination of polarized-light
imaging, x-ray diffraction and magnetic measurements we associate this behavior
with the intrinsic pinning on structural domains in the orthorhombic phase.
Domain walls extend throughout the sample thickness in the direction of
vortices and act as extended pinning centers. With the increasing $x$ domain
structure becomes more intertwined and fine due to a decrease of the
orthorhombic distortion. This results in the energy landscape with maze-like
spatial modulations favorable for pinning. This finding shows that iron-based
pnictide superconductors, characterized by high values of the transition
temperature, high upper critical fields, and low anisotropy may intrinsically
have relatively high critical current densities.
|
0909.0923v3
|
2009-09-04
|
X-raying the Winds of Luminous Active Galaxies
|
We briefly describe some recent observational results, mainly at X-ray
wavelengths, on the winds of luminous active galactic nuclei (AGNs). These
winds likely play a significant role in galaxy feedback. Topics covered include
(1) Relations between X-ray and UV absorption in Broad Absorption Line (BAL)
and mini-BAL quasars; (2) X-ray absorption in radio-loud BAL quasars; and (3)
Evidence for relativistic iron K BALs in the X-ray spectra of a few bright
quasars. We also mention some key outstanding problems and prospects for future
advances; e.g., with the International X-ray Observatory (IXO).
|
0909.0958v1
|
2009-09-11
|
The double life of electrons in magnetic iron pnictides, as revealed by NMR
|
We present a phenomenological, two-fluid approach to understanding the
magnetic excitations in Fe pnictides, in which a paramagnetic fluid with
gapless, incoherent particle-hole excitations coexists with an
antiferromagnetic fluid with gapped, coherent spin wave excitations. We show
that this two-fluid phenomenology provides an excellent quantitative
description of NMR data for magnetic "122" pnictides, and argue that it finds a
natural justification in LSDA and spin density wave calculations. We further
use this phenomenology to estimate the maximum renormalisation of the ordered
moment that can follow from low-energy spin fluctuations in Fe pnictides. We
find that this is too small to account for the discrepancy between ab intio
calculations and neutron scattering measurements.
|
0909.2207v4
|
2009-09-12
|
Nuclear effects in neutrino-nucleus DIS
|
We explain the current status of nuclear parton distribution functions in
connection with neutrino-nucleus interactions. Neutrino deep inelastic
scattering (DIS) measurements have been done for heavy nuclear targets such as
iron and lead. In order to extract structure functions of the nucleon, one
needs to remove nuclear effects from the data. However, recent studies indicate
that there are inconsistencies in nuclear modifications between charged-lepton
and neutrino scattering measurements. Nuclear medium effects could be also an
origin for the NuTeV anomaly in the weak-mixing angle. In addition, the
modifications could affect neutrino-oscillation experiments because some DIS
events of neutrino-oxygen nucleus interactions are contained. On the other
hand, the nuclear medium effects themselves are interesting and important for
describing nuclei in terms of quark and gluon degrees of freedom.
|
0909.2329v1
|
2009-09-15
|
Theory of Andreev reflection in a two-orbital model of iron-pnictide superconductors
|
A recently developed theory for the problem of Andreev reflection between a
normal metal (N) and a multiband superconductor (MBS) assumes that the incident
wave from the normal metal is coherently transmitted through several bands
inside the superconductor. Such splitting of the probability amplitude into
several channels is the analogue of a quantum waveguide. Thus, the appropriate
matching conditions for the wave function at the N/MBS interface are derived
from an extension of quantum waveguide theory. Interference effects between the
transmitted waves inside the superconductor manifest themselves in the
conductance. We provide results for a FeAs superconductor, in the framework of
a recently proposed effective two-band model and two recently proposed gap
symmetries: in the sign-reversed s-wave ($\Delta\cos(k_x)\cos(k_y)$) scenario
resonant transmission through surface Andreev bound states (ABS) at nonzero
energy is found as well as destructive interference effects that produce zeros
in the conductance; in the extended s-wave ($\Delta[\cos(k_x)+\cos(k_y)]$)
scenario no ABS at finite energy are found.
|
0909.2826v1
|
2009-09-16
|
Electronic band structure, phonon spectrum, and elastic properties of LaOFeAs
|
We present results of ab-initio calculations of the electronic band
structure, lattice dynamical properties, and elastic constants of LaOFeAs, the
parent compound of the recently discovered superconducting iron-oxypnictide
(LaO1-xFxFeAs). The total and partial electronic density of states (EDOS) of
the undoped LaOFeAs (in the insulating or metallic state) are extracted from
the electronic band structure. The phonon dispersion and the phonon density of
states (PDOS) are also studied. Possible implications of the band structure,
EDOS, and PDOS of LaOFeAs on the eventual appearance of high-Tc
superconductivity upon carrier doping are discussed. The values of various
independent elastic constants for both insulating and metallic states are
estimated and discussed.
|
0909.2914v2
|
2009-09-16
|
Calculation of stellar electron-capture cross sections on nuclei based on microscopic Skyrme functionals
|
A fully self-consistent microscopic framework for evaluation of nuclear
weak-interaction rates at finite temperature is introduced, based on Skyrme
functionals. The single-nucleon basis and the corresponding thermal occupation
factors of the initial nuclear state are determined in the finite-temperature
Skyrme Hartree-Fock model, and charge-exchange transitions to excited states
are computed using the finite-temperature RPA. Effective interactions are
implemented self-consistently: both the finite-temperature single-nucleon
Hartree-Fock equations and the matrix equations of RPA are based on the same
Skyrme energy density functional. Using a representative set of Skyrme
functionals, the model is applied in the calculation of stellar
electron-capture cross sections for selected nuclei in the iron mass group and
for neutron-rich Ge isotopes.
|
0909.3070v1
|
2009-09-17
|
Bulk electronic structure of optimally doped Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$
|
We report high-resolution, bulk Compton scattering measurements unveiling the
Fermi surface of an optimally-doped iron-arsenide superconductor,
Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$. Our measurements are in agreement with
first-principles calculations of the electronic structure, revealing both the
$X$-centered electron pockets and the $\Gamma$-centered hole pockets. Moreover,
our data are consistent with the strong three-dimensionality of one of these
sheets that has been predicted by electronic structure calculations at the
local-density-approximation-minimum As position. Complementary calculations of
the noninteracting susceptibility, $\chi_0({\bf q}, \omega)$, suggest that the
broad peak that develops due to interband Fermi-surface nesting, and which has
motivated several theories of superconductivity in this class of material,
survives the measured three dimensionality of the Fermi surface in this family.
|
0909.3194v2
|
2009-09-25
|
Modelling hybrid Beta Cephei/SPB pulsations: Gamma Pegasi
|
Recent photometric and spectroscopic observations of the hybrid variable
Gamma Pegasi (Handler et al. 2009, Handler 2009) revealed 6 frequencies of the
SPB type and 8 of the Beta Cep type pulsations. Standard seismic models, which
have been constructed with OPAL (Iglesias & Rogers 1996) and OP (Seaton 2005)
opacities by fitting three frequencies (those of the radial fundamental and two
dipole modes), do not reproduce the frequency range of observed pulsations and
do not fit the observed individual frequencies with a satisfactory accuracy. We
argue that better fitting can be achieved with opacity enhancements, over the
OP data, by about 20-50 percent around the opacity bumps produced by excited
ions of the iron-group elements at temperatures of about 200 000 K (Z bump) and
2 million K (Deep Opacity Bump).
|
0909.4643v1
|
2009-09-29
|
Weak anisotropy of the superconducting upper critical field in Fe1.11Te0.6Se0.4 single crystals
|
We have determined the resistive upper critical field Hc2 for single crystals
of the superconductor Fe1.11Te0.6Se0.4 using pulsed magnetic fields of up to
60T. A rather high zero-temperature upper critical field of mu0Hc2(0) approx
47T is obtained, in spite of the relatively low superconducting transition
temperature (Tc approx 14K). Moreover, Hc2 follows an unusual temperature
dependence, becoming almost independent of the magnetic field orientation as
the temperature T=0. We suggest that the isotropic superconductivity in
Fe1.11Te0.6Se0.4 is a consequence of its three-dimensional Fermi-surface
topology. An analogous result was obtained for (Ba,K)Fe2As2, indicating that
all layered iron-based superconductors exhibit generic behavior that is
significantly different from that of the high-Tc cuprates.
|
0909.5328v1
|
2009-09-30
|
He and Ne ages of large presolar silicon carbide grains: Solving the recoil problem
|
Knowledge about the age of presolar grains provides important insights into
Galactic chemical evolution and the dynamics of grain formation and destruction
processes in the Galaxy. Determination from the abundance of cosmic ray
interaction products is straightforward, but in the past has suffered from
uncertainties in correcting for recoil losses of spallation products. The
problem is less serious in a class of large (tens of micrometer) grains. We
describe the correction procedure and summarise results for He and Ne ages of
presolar SiC "Jumbo" grains that range from close to zero to ~850 Myr, with the
majority being less than 200 Myr. We also discuss the possibility of extending
our approach to the majority of smaller SiC grains and explore possible
contributions from trapping of cosmic rays.
|
0909.5532v1
|
2009-10-01
|
Pressure-driven phase transitions in correlated systems
|
Motivated by recent experimental measurements on pressure-driven phase
transitions in Mott-insulators as well as the new iron pnictide
superconductors, we show that first principles Car-Parrinello molecular
dynamics calculations are a powerful method to describe the microscopic origin
of such transitions. We present results for (i) the pressure-induced insulator
to metal phase transition in the prototypical Mott insulator TiOCl as well as
(ii) the pressure-induced structural and magnetic phase transitions in the
family of correlated metals $A$Fe$_2$As$_2$ ($A$=Ca,Sr,Ba). Comparison of our
predictions with existing experimental results yields very good agreement.
|
0910.0158v1
|
2009-10-05
|
Isotope shifts and hyperfine structure of the Fe I 372 nm resonance line
|
We report measurements of the isotope shifts of the $3d^64s^2 a ^5D_4 -
3d^64s4p z ^5F_5^o$ Fe I resonance line at 372 nm between all four stable
isotopes $^{54}$Fe, $^{56}$Fe, $^{57}$Fe, and $^{58}$Fe, as well as the
complete hyperfine structure of that line for $^{57}$Fe, the only stable
isotope having a non-zero nuclear spin. The field and specific mass shift
coefficients of the transition have been derived from the data, as well as the
experimental value for the hyperfine structure magnetic dipole coupling
constant $A$ of the excited state of the transition in $^{57}$Fe: $A(3d^64s4p z
^5F_5^o) = 81.69(86)$ MHz. The measurements were done by means of Doppler-free
laser saturated-absorption spectroscopy in a Fe-Ar hollow cathode using both
natural and enriched iron samples. The measured isotope shifts and hyperfine
constants are reported with uncertainties at the percent level.
|
0910.0479v1
|
2009-10-05
|
First-principles thermal equation of state and thermoelasticity of hcp Fe at high pressures
|
We investigate the equation of state and elastic properties of hcp iron at
high pressures and high temperatures using first principles linear response
linear-muffin-tin-orbital method in the generalized-gradient approximation. We
calculate the Helmholtz free energy as a function of volume, temperature, and
volume-conserving strains, including the electronic excitation contributions
from band structures and lattice vibrational contributions from quasi-harmonic
lattice dynamics. We perform detailed investigations on the behavior of elastic
moduli and equation of state properties as functions of temperature and
pressure, including the pressure-volume equation of state, bulk modulus, the
thermal expansion coefficient, the Gruneisen ratio, and the shock Hugoniot.
Detailed comparison has been made with available experimental measurements and
theoretical predictions.
|
0910.0884v1
|
2009-10-06
|
Band and momentum dependent electron dynamics in superconducting ${\rm Ba(Fe_{1-x}Co_{x})_2As_2}$ as seen via electronic Raman scattering
|
We present details of carrier properties in high quality ${\rm
Ba(Fe_{1-x}Co_{x})_2As_2}$ single crystals obtained from electronic Raman
scattering. The experiments indicate a strong band and momentum anisotropy of
the electron dynamics above and below the superconducting transition
highlighting the importance of complex band-dependent interactions. The
presence of low energy spectral weight deep in the superconducting state
suggests a gap with accidental nodes which may be lifted by doping and/or
impurity scattering. When combined with other measurements, our observation of
band and momentum dependent carrier dynamics indicate that the iron arsenides
may have several competing superconducting ground states.
|
0910.0898v2
|
2009-10-09
|
On the influence of high energy electron populations on metal abundance estimates in galaxy groups and clusters
|
Spectral line emissivities have usually been calculated for a Maxwellian
electron distribution. But many theoretical works on galaxy groups and clusters
and on the solar corona suggest to consider modified Maxwellian electron
distribution functions to fit observed X-ray spectra. Here we examine the
influence of high energy electron populations on measurements of metal
abundances. A generalized approach which was proposed in the paper by Prokhorov
et al. (2009) is used to calculate the line emissivities for a modified
Maxwellian distribution. We study metal abundances in galaxy groups and
clusters where hard X-ray excess emission was observed. We found that for
modified Maxwellian distributions the argon abundance decreases for the HCG 62
group, the iron abundance decreases for the Centaurus cluster and the oxygen
abundance decreases for the solar corona with respect to the case of a
Maxwellian distribution. Therefore, metal abundance measurements are a
promising tool to test the presence of high energy electron populations.
|
0910.1640v1
|
2009-10-14
|
Electron correlation and spin density wave order in iron pnictides
|
We study the correlation effects on the electronic structure and spin density
wave order in Fe-pnictides. Using the multiorbital Hubbard model and Gutzwiller
projection, we show that nonperturbative correlation effects are essential to
stabilize the metallic spin density wave phase for the intermediate correlation
strengths appropriate for pnictides. We find that the ordered moments depend
sensitively on the Hund's rule coupling $J$ but weakly on the intraorbital
Coulomb repulsion $U$, varying from $0.3\mu_B$ to $1.5\mu_B$ in the range
$J=0.3\sim0.8$ eV for $U=3\sim4$ eV. We obtain the phase diagram and discuss
the effects of orbital order and electron doping, the evolution of the Fermi
surface topology with the ordered moment, and compare to recent experiments.
|
0910.2707v2
|
2009-10-15
|
Single vortex structure in two models of iron pnictide $s^\pm$ superconductivity
|
The structure of a single vortex in a FeAs superconductor is studied in the
framework of two formulations of superconductivity for the recently proposed
sign-reversed $s$ wave ($s^\pm$) scenario: {\it (i)} a continuum model taking
into account the existence of an electron and a hole band with a repulsive
local interaction between the two; {\it (ii)} a lattice tight-binding model
with two orbitals per unit cell and a next-nearest-neighbour attractive
interaction. In the first model, the local density of states (LDOS) at the
vortex centre, as a function of energy, exhibits a peak at the Fermi level,
while in the second model such LDOS peak is deviated from the Fermi level and
its energy depends on band filling. An impurity located outside the vortex core
has little effect on the LDOS peak, but an impurity close to the vortex core
can almost suppress it and modify its position.
|
0910.2879v1
|
2009-10-19
|
New approach to primary mass composition analysis with simultaneous use of ground and fluorescence detectors data
|
We study the possibility to reconstruct primary mass composition with the use
of combinations of basic shower characteristics, measured in hybrid
experiments, such as depth of shower maximum from fluorescence side and signal
in water Cherenkov tanks or in plastic scintillators from the ground side. To
optimize discrimination performance of shower observables combinations we apply
Fisher's discriminant analysis and give statistical estimates of separation of
the obtained distributions on Fisher variables for proton and iron primaries.
At the final stage we apply Multiparametric Topological Analysis to these
distributions to extract composition from prepared mixtures with known
fractions of showers from different primary particles. It is shown, that due to
high sensitivity of water tanks to muons, combination of signal in them with
$\xmax$ looks especially promising for mass composition analysis, provided the
energy is determined from longitudinal shower profile.
|
0910.3520v1
|
2009-10-22
|
About the strength of correlation effects in the electronic structure of iron
|
The strength of electronic correlation effects in the spin-dependent
electronic structure of ferromagnetic bcc Fe(110) has been investigated by
means of spin and angle-resolved photoemission spectroscopy. The experimental
results are compared to theoretical calculations within the three-body
scattering approximation and within the dynamical mean-field theory, together
with one-step model calculations of the photoemission process. This comparison
indicates that the present state of the art many-body calculations, although
improving the description of correlation effects in Fe, give too small mass
renormalizations and scattering rates thus demanding more refined many-body
theories including non-local fluctuations.
|
0910.4360v1
|
2009-10-29
|
Surprising Metallicity of a Newly Discovered M79 Post-AGB Star
|
A detailed chemical composition analysis based on a high-resolution
(R=35,000) CCD spectrum is presented for a newly discovered post-AGB star in
the globular cluster M79 for the first time. The elemental abundance results of
M79 Post-AGB star are found to be [C/Fe]=-0.7, [O/Fe]=+1.4, [alpha-
process/Fe]=0.5, and [s-process/Fe]=-0.1. The surprising result is that the
iron abundance of the star is apparently about 0.6 dex less than that of the
cluster's red giants as reported by published studies including a recent
high-resolution spectroscopic analysis by Carretta and colleagues.
|
0910.5567v1
|
2009-10-29
|
Simulation of a flux emergence event and comparison with observations by Hinode
|
We study the observational signature of flux emergence in the photosphere
using synthetic data from a 3D MHD simulation of the emergence of a twisted
flux tube. Several stages in the emergence process are considered. At every
stage we compute synthetic Stokes spectra of the two iron lines Fe I 6301.5
{\AA} and Fe I 6302.5 {\AA} and degrade the data to the spatial and spectral
resolution of Hinode's SOT/SP. Then, following observational practice, we apply
Milne-Eddington-type inversions to the synthetic spectra in order to retrieve
various atmospheric parameters and compare the results with recent Hinode
observations. During the emergence sequence, the spectral lines sample
different parts of the rising flux tube, revealing its twisted structure. The
horizontal component of the magnetic field retrieved from the simulations is
close to the observed values. The flattening of the flux tube in the
photosphere is caused by radiative cooling, which slows down the ascent of the
tube to the upper solar atmosphere. Consistent with the observations, the
rising magnetized plasma produces a blue shift of the spectral lines during a
large part of the emergence sequence.
|
0910.5737v1
|
2009-11-04
|
Solar-like oscillations in massive main-sequence stars. I. Asteroseismic signatures of the driving and damping regions
|
Motivated by the recent detection of stochastically excited modes in the
massive star V1449 Aql (Belkacem et al., 2009b), already known to be a $\beta$
Cephei, we theoretically investigate the driving by turbulent convection. By
using a full non-adiabatic computation of the damping rates, together with a
computation of the energy injection rates, we provide an estimate of the
amplitudes of modes excited by both the convective region induced by the iron
opacity bump and the convective core. Despite uncertainties in the dynamical
properties of such convective regions, we demonstrate that both are able to
efficiently excite $p$ modes above the CoRoT observational threshold and the
solar amplitudes. In addition, we emphasise the potential asteroseismic
diagnostics provided by each convective region, which we hope will help to
identify the one responsible for solar-like oscillations, and to give
constraints on this convective zone. A forthcoming work will be dedicated to an
extended investigation of the likelihood of solar-like oscillations across the
Hertzsprung-Russell diagram.
|
0911.0908v1
|
2009-11-05
|
Ultra-High Energy Cosmic-Ray Acceleration in the Jet of Centaurus A
|
We evaluate the achievable maximum energy of nuclei diffusively accelerated
by shock wave in the jet of Cen A, based on an updated model involving the
stochastic magnetic fields that are responsible for recent synchrotron X-ray
measurements. For the maximum energy analysis, conceivable energy constraints
from spatiotemporal scales are systematically considered for the jet-wide
including discrete X-ray knots. We find that in the inner region within ~1
arcmin from galactic core, which includes knots AX and BX, proton and iron
nucleus can be accelerated to 10^{19}-10^{20} and 10^{21} eV (10-100 EeV and
ZeV) ranges, respectively. The upper cutoff energy of the very energetic
neutrinos produced via photopion interaction is also provided. These are
essential for identifying the acceleration site of the ultra-high energy cosmic
ray detected in the Pierre Auger Observatory, which signifies the arrival from
nearby galaxies including Cen A.
|
0911.0921v1
|
2009-11-09
|
Role of dipole-dipole interactions in multiple quantum transitions in magnetic nanoparticles
|
In order to better understand the origin of multiple quantum transitions
observed in superparamagnetic nanoparticles, electron magnetic resonance (EMR)
studies have been performed on iron oxide nanoparticles assembled inside the
anodic alumina membrane. The positions of both the main resonance and
"forbidden" (double-quantum, 2Q) transitions observed at the half-field
demonstrate the characteristic angular dependence with the line shifts
proportional to 3cos2q-1, where q is the angle between the channel axis and
external magnetic field B. This result can be attributed to the interparticle
dipole-dipole interactions within elongated aggregates inside the channels. The
angular dependence of the 2Q intensity is found to be proportional to
sin2qcos2q, that is consistent with the predictions of quantum-mechanical
calculations with the account for the mixing of states by non-secular
inter-particle dipole-dipole interactions. Good agreement is demonstrated
between different kinds of measurements (magnetization curves, line shifts and
2Q intensity), evidencing applicability of the quantum approach to the
magnetization dynamics of superparamagnetic objects.
|
0911.1752v1
|
2009-11-11
|
Gravitational Lensing by Black Holes
|
We review the theoretical aspects of gravitational lensing by black holes,
and discuss the perspectives for realistic observations. We will first treat
lensing by spherically symmetric black holes, in which the formation of
infinite sequences of higher order images emerges in the clearest way. We will
then consider the effects of the spin of the black hole, with the formation of
giant higher order caustics and multiple images. Finally, we will consider the
perspectives for observations of black hole lensing, from the detection of
secondary images of stellar sources and spots on the accretion disk to the
interpretation of iron K-lines and direct imaging of the shadow of the black
hole.
|
0911.2187v2
|
2009-11-13
|
An Unusually Fast-Evolving Supernova
|
Analyses of supernovae (SNe) have revealed two main types of progenitors:
exploding white dwarfs and collapsing massive stars. We present SN2002bj, which
stands out as different from any SN reported to date. Its light curve rises and
declines very rapidly, yet reaches a peak intrinsic brightness greater than -18
mag. A spectrum obtained 7 days after discovery shows the presence of helium
and intermediate-mass elements, yet no clear hydrogen or iron-peak elements.
The spectrum only barely resembles that of a Type Ia supernova, with added
carbon and helium. Its properties suggest that SN2002bj may be representative
of a class of progenitors that previously has been only hypothesized: a helium
detonation on a white dwarf, ejecting a small envelope of material. New surveys
should find many such objects, despite their scarcity.
|
0911.2699v1
|
2009-11-16
|
Effects of nematic fluctuations on the elastic properties of iron arsenide superconductors
|
We demonstrate that the changes in the elastic properties of the FeAs
systems, as seen in our resonant ultrasound spectroscopy data, can be naturally
understood in terms of fluctuations of emerging nematic degrees of freedom.
Both the softening of the lattice in the normal, tetragonal phase as well as
its hardening in the superconducting phase are consistently described by our
model. Our results confirm the view that structural order is induced by
magnetic fluctuations.
|
0911.3084v2
|
2009-11-18
|
Thermonuclear explosions of rapidly rotating white dwarfs - II. Detonations
|
Context: Superluminous type Ia supernovae (SNe Ia) may be explained by
super-Chandrasekhar-mass explosions of rapidly rotating white dwarfs (WDs). In
a preceding paper, we showed that the deflagration scenario applied to rapidly
rotating WDs generates explosions that cannot explain the majority of SNe Ia.
Aims: Rotation of the progenitor star allows super-Chandrasekhar-mass WDs to
form that have a shallower density stratification. We use simple estimates of
the production of intermediate and iron group elements in pure detonations of
rapidly rotating WDs to assess their viability in explaining rare SNe Ia.
Methods: We numerically construct WDs in hydrostatic equilibrium that rotate
according to a variety of rotation laws. The explosion products are estimated
by considering the density stratification and by evaluating the result of
hydrodynamics simulations. Results: We show that a significant amount of
intermediate mass elements is produced for theoretically motivated rotation
laws, even for prompt detonations of WDs. Conclusions: Rapidly rotating WDs
that detonate may provide an explanation of rare superluminous SNe Ia in terms
of both burning species and explosion kinematics.
|
0911.3545v1
|
2009-11-18
|
Suzaku broadband spectroscopy of Swift J1753.5-0127 in the Low-Hard State
|
We present Suzaku observations of the Galactic black hole candidate Swift
J1753.5-0127 in the low-hard state. The broadband coverage of Suzaku enables us
to detect the source over the energy range 0.6 -- 250 keV. The broadband
spectrum (2 -- 250 keV) is found to be consistent with a simple power-law
(gamma \sim 1.63). In agreement with previous observations of this system, a
significant excess of soft X-ray flux is detected consistent with the presence
of a cool accretion disc. Estimates of the disc inner radius infer a value
consistent with the ISCO (R_{in} \lesssim 6 R_g, for certain values of, e.g.
N_H, i), although we cannot conclusively rule out the presence of an accretion
disc truncated at larger radii (R_{in} \sim 10 - 50 R_g). A weak,
relativistically-broadened iron line is also detected, in addition to disc
reflection at higher energy. However, the iron-K line profile favours an inner
radius larger than the ISCO (R _{in} \sim 10 - 20 R_g). The implications of
these observations for models of the accretion flow in the low-hard state are
discussed.
|
0911.3642v1
|
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