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2012-08-11
|
Suppression of backward scattering of Dirac fermions in iron pnictides Ba(Fe$_{1-x}$Ru$_x$As)$_2$
|
We report electronic transport of Dirac cones when Fe is replaced by Ru,
which has an isoelectronic electron configuration to Fe, using single crystals
of Ba(Fe$_{1-x}$Ru$_x$As)$_2$. The electronic transport of parabolic bands is
shown to be suppressed by scattering due to the crystal lattice distortion and
the impurity effect of Ru, while that of the Dirac cone is not significantly
reduced due to the intrinsic character of Dirac cones. It is clearly shown from
magnetoresistance and Hall coefficient measurements that the inverse of average
mobility, proportional to cyclotron effective mass, develops as the square root
of the carrier number (n) of the Dirac cones. This is the unique character of
the Dirac cone linear dispersion relationship. Scattering of Ru on the Dirac
cones is discussed in terms of the estimated mean free path using experimental
parameters.
|
1208.2328v2
|
2012-08-13
|
Specific Heat Discontinuity DeltaC vs Tc in Annealed Ba(Fe1-xCox)2As2
|
The low temperature specific heat of annealed single crystal samples of
Ba(Fe1-xCox)2As2 with compositions spanning the entire superconducting phase
diagram was measured. Effort was made to discover the best annealing schedule
to maximize Tc and minimize transition width in these samples. Values of
deltaC/Tc normalized to 100% superconducting volume fractions varied
proportionally to Tc^alpha. Within a rather narrow error bar of +-0.15, the
exponent alpha was the same (approximately 2) over a range of compositions
(0.055 < x < 0.15) around the optimal concentration, x=0.08, where Tc is the
maximum in the phase diagram. Thus, whether the superconductivity was
coexistent with magnetism (underdoped) or not (overdoped) did not affect the
non-BCS variation (alpha nearly 2 instead of 0.8-0.9 for BCS superconductors)
of deltaC/Tc with Tc. The annealed samples in the present work, with increased
deltaC/Tc and Tc values compared to previous results for the Ba(Fe1-xCox)2As2
alloy system, suggest that the intrinsic value of the exponent alpha for the
iron superconductors, when the samples are annealed, with fewer defects, may
indeed be even further from the BCS value of 0.8-0.9 than previously thought.
|
1208.2492v1
|
2012-08-16
|
Filamentary superconductivity across the phase diagram of Ba(Fe,Co)$_2$As$_2$
|
We show magnetotransport results on Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($0.0 \leq
x \leq 0.13$) single crystals. We identify the low temperature resistance step
at 23 K in the parent compound with the onset of filamentary superconductivity
(FLSC), which is suppressed by an applied magnetic field in a similar manner to
the suppression of bulk superconductivity (SC) in doped samples. FLSC is found
to persist across the phase diagram until the long range antiferromagnetic
order is completely suppressed. A significant suppression of FLSC occurs for
$0.02<x<0.04$, the doping concentration where bulk SC emerges. Based on these
results and the recent report of an electronic anisotropy maximum for 0.02
$\leq x \leq$ 0.04 [Science 329, 824 (2010)], we speculate that, besides spin
fluctuations, orbital fluctuations may also play an important role in the
emergence of SC in iron-based superconductors.
|
1208.3352v1
|
2012-08-17
|
Anisotropic resistivity of Na$_{1-δ}$Fe$_{1-x}$Co$_x$As
|
Temperature-dependent resistivity is studied in single crystals of
iron-arsenide superconductor Na$_{1-\delta}$Fe$_{1-x}$Co$_x$As for electrical
current directions along, $\rho_a (T)$, and transverse, $\rho_c (T)$, to the
Fe-As layers. Doping with Co increases stability of this compound to reaction
with the environment and suppresses numerous features in both $\rho_a(T)$ and
$\rho_c(T)$ compared to the stoichiometric NaFeAs. Evolution of $\rho_a (T)$
with $x$ follows a universal trend observed in other pnictide superconductors,
exhibiting a $T$-linear temperature dependence close to the optimal doping and
development of $T^2$ dependence upon further doping. $\rho_c (T)$ in parent
compound shows a non - monotonic behavior with a crossover from non-metallic
resistivity increase on cooling from room temperature down to $\sim$ 80 K to a
metallic decrease below this temperature. Both $\rho_a (T)$ and $\rho_c (T)$
show several correlated crossover - like features at $T>$ 80 K. Despite a
general trend towards more metallic behavior of inter - plane resistivity in
Co-doped samples, the temperature of the crossover from insulating to metallic
behavior (80 K) does not change much with doping.
|
1208.3646v1
|
2012-08-21
|
Contrast of LiFeAs with isostructural, isoelectronic, and non-superconducting MgFeGe
|
Stoichiometric LiFeAs at ambient pressure is an 18 K superconductor while
isoelectronic MgFeGe is not, despite their extremely similar electronic
structures. To investigate possible sources of this distinctively different
superconducting behavior, we quantify the differences using first principles
density functional theory, establishing first that the Fe total 3d occupations
are identical in the two compounds. Individual 3d orbital occupations also
differ very little ($\sim 0.01$). The differences in Fermi surfaces (FSs) do
not seem significant; however a redistribution of bands just above the Fermi
level does represent a possibly significant distinction. Because the bands and
FSs of LiFeAs are less in agreement with experiment than for other
iron-pnictides, we study the effects of additional exchange-correlations
effects beyond GGA (the generalized gradient approximation) by applying the
modified Becke-Johnson potential (mBJ) exchange potential, which gives much
improved bandgaps in insulators compared to GGA and might be useful for
semimetals such as the Fe-based superconductors. Overall, we conclude that the
mBJ corrections do not improve the description of LiFeAs as compared to
experiment.
|
1208.4180v2
|
2012-08-22
|
Chandra grating spectroscopy of three hot white dwarfs
|
High-resolution soft X-ray spectroscopic observations of single hot white
dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have
observed two white dwarfs, one is of spectral type DA (LB 1919) and the other
is a non-DA of spectral type PG1159 (PG 1520+525). The spectra of both stars
are analyzed, together with an archival Chandra spectrum of another DA white
dwarf (GD 246). The soft X-ray spectra of the two DA white dwarfs are
investigated in order to study the effect of gravitational settling and
radiative levitation of metals in their photospheres. LB 1919 is of interest
because it has a significantly lower metallicity than DAs with otherwise
similar atmospheric parameters. GD 246 is the only white dwarf known that shows
identifiable individual iron lines in the soft X-ray range. For the PG1159
star, a precise effective temperature determination is performed in order to
confine the position of the blue edge of the GW Vir instability region in the
HRD. (abridged)
|
1208.4478v1
|
2012-08-24
|
Superconductivity Appears in the Vicinity of an Insulating-Like Behavior in CeO$_{1-x}$F$_{x}$BiS$_{2}$
|
Resistive and magnetization properties have been measured in BiS$_2$-based
samples CeO$_{1-x}$F$_{x}$BiS$_{2}$ with a systematic substitution of O with F
(0 $<$ x $<$ 0.6). In contrast to the band structure calculations, it is found
that the parent phase of CeOBiS$_2$ is a bad metal, instead of an band
insulator. By doping electrons into the system, it is surprising to find that
superconductivity appears together with an insulating normal state. This
evolution is clearly different from the cuprate and the iron pnictide systems,
and is interpreted as approaching the von Hove singularity. Furthermore,
ferromagnetism which may arise from the Ce moments, has been observed in the
low temperature region in all samples, suggesting the co-existence of
superconductivity and ferromagnetism in the superconducting samples.
|
1208.5000v1
|
2012-08-25
|
Glassy Transition in the Vortex Lattice of Ba(Fe0.93Rh0.07)2As2 superconductor, probed by NMR and ac-susceptibility
|
By using Nuclear Magnetic Resonance and ac-susceptibility, the characteristic
correlation times for the vortex dynamics, in an iron-based superconductor,
have been derived. Upon cooling, the vortex dynamics displays a crossover
consistent with a vortex glass transition. The correlation times, in the fast
motions regime, merge onto a universal curve which is fit by the Vogel-Fulcher
law, rather than by an Arrhenius law. Moreover, the pinning barrier shows a
weak dependence on the magnetic field which can be heuristically justified
within a fragile glass scenario. In addition, the glass freezing temperatures
obtained by the two techniques merge onto the de Almeida-Thouless line. Finally
the phase diagram for the mixed phase has been derived.
|
1208.5142v4
|
2012-08-28
|
Alternative route to charge density wave formation in multiband systems
|
Charge and spin density waves, periodic modulations of the electron and
magnetization densities, respectively, are among the most abundant and
non-trivial low-temperature ordered phases in condensed matter. The ordering
direction is widely believed to result from the Fermi surface topology.
However, several recent studies indicate that this common view needs to be
supplemented. Here, we show how an enhanced electron-lattice interaction can
contribute to or even determine the selection of the ordering vector in the
model charge density wave system ErTe3. Our joint experimental and theoretical
study allows us to establish a relation between the selection rules of the
electronic light scattering spectra and the enhanced electron-phonon coupling
in the vicinity of band degeneracy points. This alternative proposal for charge
density wave formation may be of general relevance for driving phase
transitions into other broken-symmetry ground states, particularly in multiband
systems such as the iron based superconductors.
|
1208.5701v2
|
2012-08-31
|
Comparison of Gamow-Teller strengths in the random phase approximation
|
The Gamow-Teller response is astrophysically important for a number of
nuclides, particularly around iron. The random phase approximation (RPA) is an
efficient way to generate strength distributions. In order to better understand
both theoretical systematics and uncertainties, we compare the Gamow-Teller
strength distributions for a suite of nuclides and for a suite of interactions,
including semi-realistic interactions in the $1p$-$0f$ space with the RPA and a
separable multi-shell interaction in the quasi-particle RPA. We also compare
with experimental results for GT$_-$ on ${54}$Fe.
|
1208.6538v2
|
2012-09-03
|
Growth and characterization of n-type electron-induced ferromagnetic semiconductor (In,Fe)As
|
We show that by introducing isoelectronic iron (Fe) magnetic impurities and
Beryllium (Be) double-donor atoms into InAs, it is possible to grow a n-type
ferromagnetic semiconductor (FMS) with the ability to control ferromagnetism by
both Fe and independent carrier doping by low-temperature molecular-beam
epitaxy. We demonstrate that (In,Fe)As doped with electrons behaves as an
n-type electron-induced FMS. This achievement opens the way to realize novel
spin-devices such as spin light-emitting diodes or spin field-effect
transistors, as well as helps understand the mechanism of carrier-mediated
ferromagnetism in FMSs.
|
1209.0250v1
|
2012-09-05
|
A 57Fe Mössbauer Study Of Local Structure And Spin Arrangements In Antiferromagnetic NaFeAs
|
Detailed 57Fe M\"ossbauer spectroscopy measurements on superconducting NaFeAs
powder crystals have been performed. The 57Fe spectra recorded in the
paramagnetic temperature range (T > TN \approx 46 +/- 2 K) were discussed
supposing Fe2+ ions are located in distorted tetrahedral (FeAs4) polyhedra. Our
results reveal that the structural transition (TS \approx 55 K) has a weak
effect on the electronic structure of iron atoms while the development of the
antiferromagnetic order (T =< TN) induces a redistribution of the charge at
57Fe nuclei. In the low-temperature range, the spectra show a diffuse resonance
absorption structure which evidences for the existence of a continuous
distribution of hyperfine fields HFe at 57Fe nuclei. The shape of these spectra
can be related to the formation of a microdomain structure or phase separation.
Analysis of the resulting distributions p(HFe) has shown that the M\"ossbauer
spectra give consistent and physically reasonable 57Fe hyperfine parameters
over the whole temperature range.
|
1209.0924v2
|
2012-09-05
|
High resolution reddening map in the direction of the stellar system Terzan 5
|
We have used optical images acquired with the Hubble Space Telescope to
construct the first high-resolution extinction map in the direction of Terzan
5, a peculiar stellar system in the inner bulge of our Galaxy. The map has a
spatial resolution of 8" X 8", over a total FoV of 200" X 200". The absorption
clouds show a patchy structure on a typical scale of 20" and extinction
variations as large as delta E(B-V) = 0.67 mag, especially in the direction of
the center of the system. These correspond to an absolute color excess ranging
from E(B-V)=2.15 mag, up to 2.82 mag. After the correction for differential
reddening, two distinct red giant branches become clearly visible in the color
magnitude diagram of Terzan 5 and they well correspond to the two
sub-populations with different iron abundances recently discovered in this
system.
|
1209.0990v1
|
2012-09-10
|
The reawakening of the sleeping X-ray pulsar XTE J1946+274
|
We report on a series of outbursts of the high mass X-ray binary XTE
J1946+274 in 2010/2011 as observed with INTEGRAL, RXTE, and Swift. We discuss
possible mechanisms resulting in the extraordinary outburst behavior of this
source. The X-ray spectra can be described by standard phenomenological models,
enhanced by an absorption feature of unknown origin at about 10 keV and a
narrow iron K alpha fluorescence line at 6.4 keV, which are variable in flux
and pulse phase. We find possible evidence for the presence of a cyclotron
resonance scattering feature at about 25 keV at the 93% level. The presence of
a strong cyclotron line at 35 keV seen in data from the source's 1998 outburst
and confirmed by a reanalysis of these data can be excluded. This result
indicates that the cyclotron line feature in XTE J1946+274 is variable between
individual outbursts.
|
1209.1918v1
|
2012-09-11
|
The stratified evolution of a cool star
|
A low mass star usually experiences stratification and abundance anomalies
during its evolution. A 0.95 solar mass star with a metallicity Z = 0.004 is
followed from the main-sequence to the Horizontal Branch (HB). On the
main-sequence the larger effects of stratification may come from accretion as
was suggested in relation to metallicity and planet formation. As it evolves
through the giant branch, stratification appears around the hydrogen burning
shell. It may create hydrodynamic instabilities and be related to abundance
anomalies on the giant branch. After the He flash the star evolves to the HB.
If it loses enough mass, it ends up a hot HB star (or in the field an sdB star)
with effective temperatures larger than 11000 K. All sdB stars are observed to
have an approximately solar iron abundance whatever their original metallicity,
implying overabundances by factors of up to 100. So should the 0.95 solar mass
star. How its internal hydrodynamic properties on the main sequence may
influence its fate on the HB is currently uncertain.
|
1209.2403v1
|
2012-09-18
|
Impurity states in multiband s-wave superconductors: analysis of iron pnictides
|
We examine the effect of a single, non-magnetic impurity in a multiband,
extended s-wave superconductor allowing for anisotropy of the gaps on the Fermi
surfaces. We derive analytic expressions for the Green's functions in the
continuum and analyse the conditions for the existence of sharp
impurity-induced resonant states. Underlying band structure is more relevant
for the multiband than for single band case, and mismatch between the bands
generically makes the formation of the impurity states less likely in the
physical regime of parameters. We confirm these conclusions by numerically
solving the impurity problem in a tight-binding parameterization of the bands
relevant to pnictide superconductors.
|
1209.3835v2
|
2012-09-21
|
Engineering Study of Sector Magnet for the Daedalus Experiment
|
The Daedalus experiment seeks to evaluate neutrino scattering effects that go
beyond the standard model. Modular accelerators are employed to produce 800 MeV
proton beams at the megawatt power level directed toward a target, producing
neutrinos. The Superconducting Ring Cyclotron (SRC) consists of identical
sectors (currently 6) of superconducting dipole magnets with iron return
frames. The Daedalus Collaboration has produced a conceptual design for the
magnet, which, after several iterations, is the current best design that
achieves the physics requirements of the experiment. The Technology and
Engineering Division (T&ED) of the MIT Plasma Science and Fusion Center was
awarded with a contract by the Daedalus team to further develop the magnet
conceptual design. The resulting Engineering Study is reported here.
|
1209.4886v1
|
2012-09-25
|
Study of The Abundance Patterns in The Metal-Poor Stellar Stream
|
The chemical abundances of the metal-poor stars in the stellar stream provide
important information for setting constraints on models of neutron-capture
processes. The study of these stars could give us a better understanding of
r-process nucleosynthesis and chemical composition of the early Galaxy. Using
the updated main r-process and weak r-process patterns, we fit abundances in
the stellar stream stars. The weak r-process component coefficients are almost
constant for the sample stars, including r-rich stars, which means that both
weak r-process and Fe are produced as primary elements from SNeII and their
yields have nearly a constant mass fraction. The difference between the stream
stars and r-rich stars is obvious. For the stream stars, that the increase
trend in the main r-process component coefficients as metallicity increases
means the gradual increase in the production of main r-process elements
relative to iron. This behavior implies that the masses of progenitors for the
main r-process are smaller than those of the weak r-process. Furthermore, we
find metal-poor stream star HD 237846 is a weak r-process star.
|
1209.5505v1
|
2012-09-25
|
Structural phase transition below 250 K in superconducting K$_{0.75}$Fe$_{1.75}$Se$_{2}$
|
Vibrational properties of iron-chalcogenide superconductor
K$_{0.75}$Fe$_{1.75}$Se$_{2}$ with $T_{c}\sim$ 30 K have been measured by Raman
and optical spectroscopies over temperature range of 3-300 K. Sample undergoes
\textit{I4/m} $\to $ \textit{I4} structural phase transition accompanied by
loss of inversion symmetry at $T_{1}$, below 250 K, observed as appearance of
new fully-symmetric Raman mode at $\sim$ 165 cm$^{-1}$. Small vibration mode
anomalies are also observed at $T_{2}\sim$ 160 K. From first-principles
vibrational analysis of antiferromagnetic K$_{0.8}$Fe$_{1.6}$Se$_{2}$ utilizing
pseudopotentials all observed Raman and infrared modes have been assigned and
the displacement patterns of the new Raman mode identified as involving
predominantly the Se atoms.
|
1209.5718v1
|
2012-10-02
|
Graphene as a reversible spin manipulator of molecular magnets
|
One of the primary objectives in molecular nano-spintronics is to manipulate
the spin states of organic molecules with a d-electron center, by suitable
external means. In this letter, we demonstrate by first principles density
functional calculations, as well as second order perturbation thoery, that a
strain induced change of the spin state, from S=1 $\to$ S=2, takes place for an
iron porphyrin (FeP) molecule deposited at a divacancy site in a graphene
lattice. The process is reversible in a sense that the application of tensile
or compressive strains in the graphene lattice can stabilize FeP in different
spin states, each with a unique saturation moment and easy axis orientation.
The effect is brought about by a change in Fe-N bond length in FeP, which
influences the molecular level diagram as well as the interaction between the C
atoms of the graphene layer and the molecular orbitals of FeP.
|
1210.0666v1
|
2012-10-03
|
Atomically Controlled Epitaxial Growth of Single-Crystalline Germanium Films on a Metallic Silicide
|
We demonstrate high-quality epitaxial germanium (Ge) films on a metallic
silicide, Fe3Si, grown directly on a Ge(111) substrate. Using molecular beam
epitaxy techniques, we can obtain an artificially controlled arrangement of
silicon (Si) or iron (Fe) atoms at the surface on Fe3Si(111). The Si-terminated
Fe3Si(111) surface enables us to grow two-dimensional epitaxial Ge films,
whereas the Fe-terminated one causes the three-dimensional epitaxial growth of
Ge films. The high-quality Ge grown on the Si-terminated surface has almost no
strain, meaning that the Ge films are not grown on the low-temperature-grown Si
buffer layer but on the lattice matched metallic Fe3Si. This study will open a
new way for vertical-type Ge-channel transistors with metallic source/drain
contacts.
|
1210.1027v1
|
2012-10-05
|
Thermodynamics of Ising Spins on the Star Lattice
|
There is a new class of two-dimensional magnetic materials polymeric iron
(III) acetate fabricated recently in which Fe ions form a star lattice. We
study the thermodynamics of Ising spins on the star lattice with exact analytic
method and Monte Carlo simulations. Mapping the star lattice to the honeycomb
lattice, we obtain the partition function for the system with asymmetric
interactions. The free energy, internal energy, specific heat, entropy and
susceptibility are presented, which can be used to determine the sign of the
interactions in the real materials. Moreover, we find the rich phase diagrams
of the system as a function of interactions, temperature and external magnetic
field. For frustrated interactions without external field, the ground state is
disordered (spin liquid) with residual entropy 1.522 per unit cell. When a weak
field is applied, the system enters a ferrimagnetic phase with residual entropy
ln4 per unit cell.
|
1210.1675v1
|
2012-10-08
|
Microwave absorption study of pinning regimes in $\bf Ba(Fe_{1-x}Co_x)_2As_2$ single crystals
|
Magnetic field dependent modulated microwave absorption (MMWA) measurements
have been carried out to investigate vortex pinning effects in single crystals
of the iron-based high-Tc superconductor Ba(Fe_{1-x}Co_x)_2As_2 with three
different cobalt doping levels of x = 0.07, 0.09, and 0.11. The dependence of
the MMWA hysteresis loops on temperature, magnetic field, and Co-concentration
have been measured and analyzed using a theoretical model of microwave
absorption in superconductors. The analysis reveals that in an underdoped
crystal (x = 0.07) the so called \dtc-pinning due to magnetically ordered
regions defines the temperature dependence of the critical current density,
while in the optimally doped (x = 0.09) and overdoped (x = 0.11) samples the
pinning is governed by structural imperfections due to inhomogeneous
distribution of cobalt dopant and has the so called \delta-l character.
|
1210.2309v1
|
2012-10-09
|
Spin Wave Excitations in AFe$_{1.5}$Se$_2$ (A=K, Tl): Analytical Study
|
We have analytically solved the spin wave excitations for the intercalated
ternary iron-selenide AFe$_{1.5}$Se$_2$ (A=K, Tl) in the $4\times 2$ collinear
antiferromagnetic order. It is found that there are one acoustic branch
(gapless Goldstone mode) and two gapful optical branches of spin wave
excitations with each in double degeneracy. By examining the non-imaginary
excitation frequency condition, we can determine the corresponding phase
boundary. The exchange couplings between Fe moments in AFe$_{1.5}$Se$_2$ are
derived based on the first-principles total energy calculations. The Fe spin is
found to be $S=3/2$ through computing the antiferromagnetic quantum
fluctuation. And it is further found that a very small spin-orientation
anisotropy can remarkably suppress the antiferromagnetic quantum fluctuation.
The spin dynamical structure factors are calculated and discussed in
association with neutron inelastic scattering experiment.
|
1210.2668v1
|
2012-10-11
|
Recent developments in the shell model Monte Carlo approach to nuclei
|
The shell model Monte Carlo (SMMC) approach provides a powerful method for
the microscopic calculation of statistical and collective nuclear properties in
model spaces that are many orders of magnitude larger than those that can be
treated by conventional methods. We discuss recent applications of the method
to describe the emergence of collectivity in the framework of the
configuration-interaction shell model and the crossover from vibrational to
rotational collectivity in families of rare-earth nuclei. We have calculated
state densities of these rare-earth nuclei and find their collective
enhancement factors to be correlated with the pairing and shape phase
transitions. We also discuss an accurate method to calculate the ground-state
energy of odd-even and odd-odd nuclei, circumventing the sign problem that
originates in the projection on an odd number of particles. We have applied
this method to calculate pairing gaps in families of isotopes in the iron
region.
|
1210.3102v1
|
2012-10-11
|
Thermopower of the Correlated Narrow Gap Semiconductor FeSi and Comparison to RuSi
|
Iron based narrow gap semiconductors such as FeSi, FeSb2, or FeGa3 have
received a lot of attention because they exhibit a large thermopower, as well
as striking similarities to heavy fermion Kondo insulators. Many proposals have
been advanced, however, lacking quantitative methodologies applied to this
problem, a consensus remained elusive to date. Here, we employ realistic
many-body calculations to elucidate the impact of electronic correlation
effects on FeSi. Our methodology accounts for all substantial anomalies
observed in FeSi: the metallization, the lack of conservation of spectral
weight in optical spectroscopy, and the Curie susceptibility. In particular we
find a very good agreement for the anomalous thermoelectric power. Validated by
this congruence with experiment, we further discuss a new physical picture of
the microscopic nature of the insulator-to-metal crossover. Indeed, we find the
suppression of the Seebeck coefficient to be driven by correlation induced
incoherence. Finally, we compare FeSi to its iso-structural and iso-electronic
homologue RuSi, and predict that partially substituted Fe(1-x)Ru(x)Si will
exhibit an increased thermopower at intermediate temperatures.
|
1210.3379v1
|
2012-10-17
|
Ferromagnetic Quantum Critical Point in Heavy-Fermion Iron Oxypnictide Ce(Ru$_{1-x}$Fe$_x$)PO
|
We have performed ^{31}P-NMR measurements on Ce(Ru_{1-x}Fe_{x})PO in order to
investigate ferromagnetic (FM) quantum criticality, since heavy-fermion (HF)
ferromagnet CeRuPO with a two-dimensional structure turns to a HF paramagnet by
an isovalent Fe-substitution for Ru. We found that Ce(Ru_{0.15}Fe_{0.85})PO
shows critical fluctuations down to \sim 0.3 K, as well as the continuous
suppression of Curie temperature and the ordered moments by the
Fe-substitution. These experimental results suggest the presence of a FM
quantum critical point (QCP) at x \sim 0.86, which is a rare example among
itinerant ferromagnets. In addition, we point out that the critical behaviors
in Ce(Ru_{0.15}Fe_{0.85})PO share the similarity with those in YbRh_{2}Si_{2},
where the local criticality of $f$ electrons has been discussed. We reveal that
Ce(Ru_{1-x}Fe_{x})PO is a new system to study FM quantum criticality in HF
compounds.
|
1210.4604v1
|
2012-10-17
|
The spectrum of Fe II
|
The spectrum of singly-ionized iron (Fe II) has been recorded using
high-resolution Fourier transform and grating spectroscopy over the wavelength
range 900 {\AA} to 5.5 {\mu}m. The spectra were observed in high-current
continuous and pulsed hollow cathode discharges using Fourier transform (FT)
spectrometers at the Kitt Peak National Observatory, Tucson, AZ and Imperial
College, London and with the 10.7 m Normal Incidence Spectrograph at the
National Institute of Standards and Technology. Roughly 12 900 lines were
classified using 1027 energy levels of Fe II that were optimized to measured
wavenumbers. The wavenumber uncertainties of lines in the FT spectra range from
10-4 cm-1 for strong lines around 4 {\mu}m to 0.05 cm-1 for weaker lines around
1500 {\AA}. The wavelength uncertainty of lines in the grating spectra is 0.005
{\AA}. The ionization energy of (130 655.4+-0.4) cm-1 was estimated from the
3d6(5D)5g and 3d6(5D)6h levels.
|
1210.4773v1
|
2012-10-19
|
Unique phase diagram with narrow superconducting dome in EuFe$_2$(As$_{1-x}$P$_x$)$_2$ due to Eu$^{2+}$ local magnetic moments
|
The interplay between superconductivity and Eu$ ^{2+}$ magnetic moments in
EuFe$_2$(As$_{1-x}$P$_x$)$_2$ is studied by electrical resistivity measurements
under hydrostatic pressure on $x=0.13$ and $x=0.18$ single crystals. We can map
hydrostatic pressure to chemical pressure $x$ and show, that superconductivity
is confined to a very narrow range $0.18\leq x \leq 0.23$ in the phase diagram,
beyond which ferromagnetic (FM) Eu ordering suppresses superconductivity. The
change from antiferro- to FM Eu ordering at the latter concentration coincides
with a Lifshitz transition and the complete depression of iron magnetic order.
|
1210.5440v2
|
2012-10-26
|
Bulk properties and electronic structure of PuFeAsO
|
Here we present bulk property measurements and electronic structure
calculations for PuFeAsO, an actinide analogue of the iron-based rare-earth
superconductors RFeAsO. Magnetic susceptibility and heat capacity data suggest
the occurrence of an antiferromagnetic transition at TN=50 K. No further
anomalies have been observed down to 2 K, the minimum temperature that we have
been able to achieve. Structural measurements indicate that PuFeAsO, with its
more localized 5f electrons, bears a stronger resemblance to the RFeAsO
compounds with larger R ions, than NpFeAsO does.
|
1210.7251v1
|
2012-10-28
|
Observation of charge accumulation and onsite Coulomb repulsion at transition metal impurities in the iron pnictides
|
We report a combined valence band photoemission and Auger spectroscopy study
of single crystalline Ca(Fe,Co)2As2 and Ba(Fe,TM)2As2 with TM=Ni or Cu. The
valence band photoemission data show directly that the TM-states move to higher
binding energies with increasing atomic number, contributing less and less to
the states close to the Fermi level. Furthermore, the 3d8 final state of the
LVV Auger decay, which is observed for Ni and Cu, unambiguously reveals the
accumulation of charge at these impurities. We also show that the onsite
Coulomb interaction on the impurity strongly increases when moving from Co over
Ni to Cu. Our results quantify the impurity potentials and imply that the
superconducting state is robust against impurity scattering.
|
1210.7483v1
|
2012-10-30
|
Physical parameters of three field RR Lyrae stars
|
Str\"omgren $uvby-\beta$ photometry of the stars classified as RR Lyrae stars
RU Piscium, SS Piscium and TU Ursae Majoris has been used to estimate their
iron abundance, temperature, gravity and absolute magnitude. The stability of
the pulsating period is discussed. The nature of SS Psc as a RRc or a HADS is
addressed. The reddening of each star is estimated from the Str\"omgren colour
indices and reddening sky maps. The results of three approaches to the
determination of [Fe/H], $T_{\rm eff}$ and $\log(g)$ are discussed: Fourier
light curve decomposition, the Preston $\Delta S$ index and the theoretical
grids on the $(b-y)_o - c_1{_o}$ plane.
|
1210.7886v1
|
2012-10-31
|
Theory and Simulation of the diffusion of kinks on dislocations in bcc metals
|
Isolated kinks on thermally fluctuating (1/2)<111> screw, <100> edge and
(1/2)<111> edge dislocations in bcc iron are simulated under zero stress
conditions using molecular dynamics (MD). Kinks are seen to perform stochastic
motion in a potential landscape that depends on the dislocation character and
geometry, and their motion provides fresh insight into the coupling of
dislocations to a heat bath. The kink formation energy, migration barrier and
friction parameter are deduced from the simulations. A discrete
Frenkel-Kontorova-Langevin (FKL) model is able to reproduce the coarse grained
data from MD at a fraction of the computational cost, without assuming an a
priori temperature dependence beyond the fluctuation-dissipation theorem.
Analytic results reveal that discreteness effects play an essential r\^ole in
thermally activated dislocation glide, revealing the existence of a crucial
intermediate length scale between molecular and dislocation dynamics. The model
is used to investigate dislocation motion under the vanishingly small stress
levels found in the evolution of dislocation microstructures in irradiated
materials.
|
1210.8327v2
|
2012-11-02
|
One-step theory of fcc-bcc martensitic transformation
|
Martensitic transformation in steels is responsible for their very high
strength and has thus been studied for more than one century since the first
works of Martens. However, there is not yet simple physical theory. A rigorous
classification of the crystallographic subgroupoids (packets) of the KS
variants and the continuity between the KS, NW and Pitsch variants are
introduced to represent the crystallographic intricacy associated to the
martensite transformation. From this analysis, a new simple theory based on
Pitsch distortion is proposed. The distortion respects the hard sphere packing
of the iron atoms and implies the existence of a neutral line along the close
packed directions. Its principal strains are 0, -5.8 and +15.5 %, well below
the +12, +12 and -20 % values of the Bain distortion. Martensite variants
nucleate by Pitsch distortion in an austenitic matrix continuously deformed by
the transformation. The martensite variants grow by the same Pitsch distortion;
they are locally in Pitsch orientation, and therefore are gradually oriented
inside the deformation field of austenite leading to the continuum of
orientations including KS and NW. Many observations reported in literature can
now be re-interpreted.
|
1211.0495v1
|
2012-11-02
|
Polarized neutron reflectometry study of Fe16N2 with Giant Saturation Magnetization prepared by N Inter-diffusion in Annealed Fe-N Thin Films
|
We report a synthesis route to grow iron nitride thin films with giant
saturation magnetization (Ms) through an N inter-diffusion process. By post
annealing Fe/Fe-N structured films grown on GaAs(001) substrates, nitrogen
diffuses from the over-doped amorphous-like Fe-N layer into strained
crystalline Fe layer and facilitates the development of metastable Fe16N2
phase. As explored by polarized neutron reflectometry, the depth-dependent Ms
profile can be well described by a model with the presence of a giant Ms up to
2360 emu/cm3 at near-substrate interface, corresponding to the strained regions
of these annealed films. This is much larger than the currently known limit
(Fe65Co35 with Ms \sim 1900 emu/cm3). The present synthesis method can be used
to develop writer materials for future magnetic recording application.
|
1211.0551v1
|
2012-11-06
|
Calculation of the transport critical current density of c-axis textured 122 iron-based superconductors
|
The c-axis textured Sr1-xKxFe2As2 tapes produced by cold rolling and
post-annealing, could carry a high super-current over 2*104 A/cm2. However, the
magnitude is far from its maximum, because of the current obstacles associated
with various defects in the material. To predict the maximal transport critical
current density, we modeled the current paths in a c-axis textured polycrystal
as a three-dimensional flow network, and calculated the maximum flow with the
Ford-Fulkerson algorithm. It indicates that a much higher super-current of
about 2*105 A/cm2 could be achieved in an ideal c-axis textured K-doped 122
polycrystal. The dependences of transport Jc on density, content of invalid
boundary and grain size and shape were also studied. The results imply that,
over 30% of the grain boundaries in the reported c-axis textured Sr1-xKxFe2As2
tapes may act as current obstacles, and the large ratio of width to thickness
was expected to be the most favorable grain shape for high transport Jc in
c-axis textured 122 superconducting tapes.
|
1211.1141v1
|
2012-11-06
|
Anisotropies and Homogeneities of Superconducting Properties in Iron-Platinum-Arsenide Ca10(Pt3As8)(Fe1.79Pt0.21As2)5
|
We report a study on the anisotropy of superconducting properties in single
crystalline Ca10(Pt4-{\delta}As8)(Fe1.79Pt0.21As2)5 with Tc ~13.6 K. Under a
field of 5 Oe, the sample reaches fully-diamagnetic state at ~10 K for H//c,
and ~8 K for H//ab, indicating the presence of slight inhomogeneities. The
magnetization measurements reveal fish-tail effect in the hysteresis loop for
both H//c and H//ab. Averaged critical current densities at low magnetic fields
along the c-axis and ab-plane Jc(H//c) and Jc(H//ab) at 5 K are estimated to be
0.9*10^5 and 0.7*10^5 A/cm2, respectively. Resistive transitions under magnetic
field show broadening, which is consistent with a relatively large anisotropy
of upper critical field. Magneto-optical images reveal homogenous current flow
within the ab-plane.
|
1211.1221v2
|
2012-11-08
|
Effects of nuclear medium and nonisoscalarity in extracting $sin^2θ_W$ using Paschos-Wolfenstein relation
|
We study the nuclear medium effects and nonisoscalarity correction in the
extraction of weak mixing angle sin$^2\theta_W$ using Paschos-Wolfenstein (PW)
relation. The calculations are performed for the iron nucleus. The results are
discussed along with the experimental result inferred by NuTeV collaboration.
The nuclear medium effects like Fermi motion, binding, shadowing and
antishadowing corrections and pion and rho meson cloud contributions have been
taken into account. Calculations have been performed in the local density
approximation using a relativistic nuclear spectral function which includes
nucleon correlations. These studies may be useful for the ongoing MINER$\nu$A
experiment as well as for the proposed NuSOnG experiment.
|
1211.1817v2
|
2012-11-08
|
Constraints of the compactness of the isolated neutron stars via X-ray phase-resolved spectroscopy
|
A model with a condensed iron surface and partially ionized hydrogen-thin
atmosphere allows us to fit simultaneously the observed general spectral shape
and the broad absorption feature (observed at 0.3 keV) in different spin phases
of the isolated neutron star RBS 1223. We constrain some physical properties of
the X-ray emitting areas, i.e. the temperatures (Tpole1 ~ 105eV, Tpole2 ~99eV),
magnetic field strengths Bpole1 ~ Bpole2 ~ 8.6x10^13G) at the poles, and their
distribution parameters (a1 ~ 0.61, a2 ~ 0.29, indicating an absence of strong
toroidal magnetic field component). In addition, we are able to place some
constraints on the geometry of the emerging X-ray emission and the
gravitational redshift (z ~0.16+0.03-0.01) of the isolated neutron star RBS
1223.
|
1211.1842v2
|
2012-11-08
|
Closing the Window on Strongly Interacting Dark Matter
|
Constraints are placed on the spin-independent interaction cross section of
dark matter with regular matter by refining two methods. First, dark
matter--cosmic ray interactions are considered, wherein cosmic ray protons
collide with dark matter to contribute to the gamma ray sky. This constraint is
developed using the NFW and Moore dark matter density profiles and new data
from the Fermi gamma ray space telescope. Second, the Earth capture scenario is
considered, wherein particles that are captured self-annihilate at Earth's
center, thus adding to its internal heat flow. The constraint presented here is
developed based on analysis of the drift time of dark matter particles through
Earth, modeled as a core composed of iron and a mantle composed of oxygen with
linear density gradients between layers. An analysis of the cosmic ray
constraint (which rules out dark matter--regular matter interaction cross
sections greater than its value) shows that it overlaps significantly with the
Earth drift time constraint (which rules out cross sections smaller than its
value), closing the window on strongly interacting dark matter particles up to
a mass of about 10^{17} GeV when combined with other exclusions.
|
1211.1951v1
|
2012-11-12
|
An orbital-selective spin liquid in a frustrated heavy fermion spinel LiV$_2$O$_4$
|
The pronounced enhancement of the effective mass is the primary phenomenon
associated with strongly correlated electrons. In the presence of local
moments, the large effective mass is thought to arise from Kondo coupling, the
interaction between itinerant and localised electrons. However, in d electron
systems, the origin is not clear because of the competing Hund's rule coupling.
Here we experimentally address the microscopic origin for the heaviest d
fermion in a vanadium spinel LiV2O4 having geometrical frustration. Utilising
orbital-selective 51V NMR, we elucidate the orbital-dependent local moment that
exhibits no long-range magnetic order despite persistent antiferromagnetic
correlations. A frustrated spin liquid, Hund-coupled to itinerant electrons,
has a crucial role in forming heavy fermions with large residual entropy. Our
method is important for the microscopic observation of the orbital-selective
localisation in a wide range of materials including iron pnictides, cobaltates,
manganites, and ruthnates.
|
1211.2844v1
|
2012-11-14
|
Are There Any Stars Lacking Neutron-Capture Elements? Evidence from Strontium and Barium
|
The cosmic dispersion in the abundances of the heavy elements strontium and
barium in halo stars is well known. Strontium and barium are detected in most
cool, metal-poor giants, but are these elements always detectable? To identify
stars that could be considered probable candidates for lacking these elements,
I examine the stellar abundance data available in the literature for 1148 field
stars and 226 stars in dwarf galaxies, 776 of which have metallicities lower
than [Fe/H]<-2.0. Strontium or barium have been detected in all field, globular
cluster, and dwarf galaxy environments studied. All upper limits are consistent
with the lowest detected ratios of [Sr/H] and [Ba/H]. The frequent appearance
of these elements raises the intriguing prospect that at least one kind of
neutron-capture reaction operates as often as the nucleosynthesis mechanisms
that produce lighter elements, like magnesium, calcium, or iron, although the
yields of heavy elements may be more variable.
|
1211.3427v1
|
2012-11-15
|
Annealing, acid, and alcoholic beverage effects on Fe1+yTe0.6Se0.4
|
We have systematically investigated and compared different methods to induce
superconductivity in iron chalcogenide Fe1+yTe0.6Se0.4 including annealing in
vacuum, N2, O2, I2 atmosphere, and immersing samples into acid and alcoholic
beverages. Vacuum and N2 annealing are proved to be ineffective to induce
superconductivity in Fe1+yTe0.6Se0.4 single crystal. O2 and I2 annealing, acid
and alcoholic beverages can induce superconductivity by oxidizing the excess Fe
in the sample. Superconductivity in O2 annealed sample is in bulk nature, while
I2, acid and alcoholic beverages can only induce superconductivity near the
surface. By comparing different effects of O2, I2, acid and alcoholic
beverages, we propose a scenario to explain how the superconductivity is
induced in the non-superconducting as-grown Fe1+yTe0.6Se0.4.
|
1211.3491v1
|
2012-11-16
|
Bicrystalline Grain Boundary and Hybrid SNS Junctions Based on Ba-122 Thin Films
|
To investigate the properties of iron-based superconductors, we examined
different kinds of Josephson junctions using Co-doped Ba-122 thin films. Based
on bicrystalline SrTiO3 substrates we prepared grain boundary (GB) junctions,
which showed clear Josephson effects. We present electrical measurements at
varying temperatures and magnetic fields. Furthermore, we prepared hybrid
junctions using thin film technique. Confined by a Ba-122 base electrode (S)
and a PbIn counter electrode (S') two different geometries of hybrid junctions
are formed: a planar SNS' junction with a gold barrier (N) in between and an
edge-type junction with an interface-engineered barrier. All three kinds of
junctions showed asymmetric resistively shunted junction-like (RSJ) behavior
with ICRN products of 20.2 /mu V (GB junctions), 18.4 /mu V (planar SNS'
junctions) and 12.3 /mu V (edge-type junctions), respectively. An excess
current could be observed at all junctions.
|
1211.3879v1
|
2012-11-27
|
Theoretical spectroscopy of realistic condensed matter systems
|
This thesis is devoted to ab initio calculations of ground and excited state
properties of different systems within density functional theory and time
dependent density functional theory. From the numerical point of view we
implemented an original method in the DP code to calculate the independent
particle response function. Moreover, we generalized the DP code to the spin
degree of freedom in order to study the magnetic properties of realistic
condensed matter systems. We studied reflectance anisotropy and energy loss
spectra of a clean and oxidized surface, and we performed an analysis of the
origin of the main spectral features. Thanks to the comparison between
experimental and theoretical energy loss spectra, we roule out the p(2x1)
reconstruction for the Si(100) surface. Moreover, in the case of a simple BeH
molecule we evidenced the problem of correctly describing the excitation
spectra for open shell systems within the TDDFT framework. In the second part
of the thesis, we presented the study of the optical properties of magnetic
systems such as FeS2, CoS2 or NiS2, interesting materials for possible
technological applications in the growing field of spintronics. Within this
context we calculated ground state properties and optical condictivity of BCC
bulk iron, for which we found a nice agreement with available experimental
data.
|
1211.6270v1
|
2012-11-28
|
Stability of xenon oxides at high pressures
|
Xenon, which is quite inert under ambient conditions, may become reactive
under pressure. The possibility of formation of stable xenon oxides and
silicates in the interior of the Earth could explain the atmospheric missing
xenon paradox. Using the ab initio evolutionary algorithm, we predict the
thermodynamical stabilization of Xe-O compounds at high pressures (XeO, XeO2
and XeO3 at pressures above 83, 102 and 114 GPa, respectively). Our
calculations indicate large charge transfer in these oxides, suggesting that
large electronegativity difference and pressure are the key factors favoring
the formation of xenon compounds. Xenon compounds in the Earth's mantle,
however, cannot directly explain the missing xenon paradox: xenon oxides are
unstable in equilibrium with metallic iron in the Earth's lower mantle, while
xenon silicates are predicted to spontaneously decompose at all mantle
pressures (<136 GPa). This does not preclude Xe atoms from being retained in
defects of mantle silicates and oxides.
|
1211.6520v2
|
2012-11-28
|
Nonlocal optical generation of spin and charge currents on the surface of magnetic insulators using total absorption and surface plasmons
|
We study the nonlocal spin and charge current generation in a finite metallic
element on the surface of magnetic insulators such as \tcb{yttrium iron garnet}
due to the absorption of the magnetic surface plasmon (MSP). Whereas a surface
plasmon is completely reflected by a metal, \tcb{an} MSP \tcb{can be} absorbed
\tcb{due to the absence of backward states}. The \tcb{injection of} MSP
generates a voltage in the longitudinal direction parallel to the wave vector,
\tcb{with the voltage} proportional to input power. If the metal is a
ferromagnet, a spin current can also be \tcb{induced} in the longitudinal
direction. Our \tcb{results provide a way to improve upon} integrated circuits
of spintronics and spin wave logic devices.
|
1211.6716v1
|
2012-12-02
|
Lateral spin injection and detection through electrodeposited Fe/GaAs contacts
|
Efforts to achieve efficient injection of spin-polarized electrons into a
semiconductor, a key prerequisite for developing electronics that exploit the
electron's spin degree of freedom, have so far met with limited success. Here
we report experimental studies of lateral spin injection and detection through
electrodeposited Fe/GaAs tunnel contacts. We demonstrate spin injection
efficiencies two orders of magnitude higher than for state-of-the-art contacts
fabricated via ultra-high-vacuum methods, including those with MgO or Al2O3
tunnel barriers. To account for this enhancement, we propose that an iron oxide
layer that forms at the Fe/GaAs interface during electrodeposition, being
magnetic, acts as a tunnel barrier with a spin-dependent height, presenting
quantum spin transport calculations for such systems. This serendipitous
discovery of greatly enhanced efficiency of spin injection into GaAs via
electrodeposited contacts introduces a promising new direction for the
development of practical semiconductor spintronic devices.
|
1212.0258v1
|
2012-12-11
|
Red Supergiant Stars as Cosmic Abundance Probes: II. NLTE Effects in J-band Silicon Lines
|
Medium resolution J-band spectroscopy of individual red supergiant stars is a
promising tool to investigate the chemical composition of the young stellar
population in star forming galaxies. As a continuation of recent work on iron
and titanium, detailed non-LTE calculations are presented to investigate the
influence of NLTE on the formation of silicon lines in the J-band spectra of
red supergiants. Substantial effects are found resulting in significantly
stronger absorption lines of neutral silicon in non-LTE. As a consequence,
silicon abundances determined in non-LTE are significantly smaller than in LTE
with the non-LTE abundance corrections varying smoothly between -0.4 dex to
-0.1 dex for effective temperatures between 3400K to 4400K. The effects are
largest at low metallicity. The physical reasons behind the non-LTE effects and
the consequences for extragalactic J-band abundance studies are discussed.
|
1212.2649v1
|
2012-12-14
|
Forward and inverse problems in fundamental and applied magnetohydrodynamics
|
This Minireview summarizes the recent efforts to solve forward and inverse
problems as they occur in different branches of fundamental and applied
magnetohydrodynamics. As for the forward problem, the main focus is on the
numerical treatment of induction processes, including self-excitation of
magnetic fields in non-spherical domains and/or under the influence of
non-homogeneous material parameters. As an important application of the
developed numerical schemes, the functioning of the von-K\'{a}rm\'{a}n-sodium
(VKS) dynamo experiment is shown to depend crucially on the presence of
soft-iron impellers. As for the inverse problem, the main focus is on the
mathematical background and some first practical applications of the
Contactless Inductive Flow Tomography (CIFT), in which flow induced magnetic
field perturbations are utilized for the reconstruction of the velocity field.
The promises of CIFT for flow field monitoring in the continuous casting of
steel are substantiated by results obtained at a test rig with a low melting
liquid metal. While CIFT is presently restricted to flows with low magnetic
Reynolds numbers, some selected problems of non-linear inverse dynamo theory,
with possible application to geo- and astrophysics, are also discussed.
|
1212.3447v1
|
2012-12-15
|
Calibration Systems of the ATLAS Tile Calorimeter
|
TileCal is the hadronic calorimeter covering the most central region of the
ATLAS experiment at the LHC. This sampling calorimeter uses iron plates as
absorber and plastic scintillating tiles as the active material. A
multi-faceted calibration system allows to monitor and equalize the calorimeter
response at each stage of the signal production, from scintillation light to
digitization. This calibration system is based on signal generation from
different sources: a Cs radioactive source, laser light, charge injection and
minimum bias events produced in proton-proton collisions. A brief description
of the different TileCal calibration systems is given and the latest results on
their performance in terms of calibration factors, linearity and stability are
presented.
|
1212.3676v1
|
2012-12-16
|
Wetting dynamics on lyophilic solid surfaces patterned by lyophobic islands
|
A theory for wetting of structured solid surfaces is developed, based on the
delta-comb periodic potential. It possesses two matching parameters: the
effective line tension and the friction coefficient on the three-phase contact
line at the surface. The theory is validated on the dynamics of spreading of
liquid zinc droplets on morphologically patterned zinkophilic iron surface by
means of square patterns of zinkophobic aluminum oxide. It is found that the
effective line tension is negative and it has essential contribution to the
dynamics of spreading. Thus, the theoretical analysis shows that the presence
of lyophobic patterns situated on lyophilic surface makes the latter completely
wettable, i.e. no equilibrium contact angle on such surface exists making the
droplet spread completely in form of thin liquid layer on the patterned
surface.
|
1212.3791v4
|
2012-12-18
|
Time Reversal Invariant Topological Superconductivity and Majorana Kramers Pairs
|
We propose a feasible route to engineer one and two dimensional time reversal
invariant (TRI) topological superconductors (SC) via proximity effects between
nodeless extended s wave iron-based SC and semiconductors with large Rashba
spin-orbit interactions. At the boundary of a TRI topological SC, there emerges
a Kramers pair of Majorana edge (bound) states. For a Josephson pi-junction we
predict a Majorana quartet that is protected by mirror symmetry and leads to a
mirror fractional Josephson effect. We analyze the evolution of Majorana pair
in Zeeman fields, as the SC undergoes a symmetry class change as well as
topological phase transitions, providing an experimental signature in tunneling
spectroscopy. We briefly discuss the realization of this mechanism in candidate
materials and the possibility of using s and d wave SC and weak topological
insulators.
|
1212.4232v2
|
2012-12-18
|
Experimental realization of dynamo action: present status and prospects
|
In the last decades, the experimental study of dynamo action has made great
progress. However, after the dynamo experiments in Karlsruhe and Riga, the
von-Karman-Sodium (VKS) dynamo is only the third facility that has been able to
demonstrate fluid flow driven self-generation of magnetic fields in a
laboratory experiment. Further progress in the experimental examination of
dynamo action is expected from the planned precession driven dynamo experiment
that will be designed in the framework of the liquid sodium facility DRESDYN
(DREsden Sodium facility for DYNamo and thermohydraulic studies).
In this paper, we briefly present numerical models of the VKS dynamo that
demonstrate the close relation between the axisymmetric field observed in that
experiment and the soft iron material used for the flow driving impellers. We
further show recent results of preparatory water experiments and design studies
related to the precession dynamo and delineate the scientific prospects for the
final set-up.
|
1212.4331v1
|
2012-12-21
|
Structural and magnetic phase transitions in triclinic Ca10(FeAs)10(Pt3As8)
|
We report the structural and magnetic phase transitions of triclinic
Ca10(FeAs)10(Pt3As8). High-resolution X-ray powder diffraction reveals
splitting of the in-plane (a,b) lattice parameters at Ts ~ 120 K.
Platinum-doping weakens the distortion and shifts the transition temperature to
80 K in Ca10(Fe1-xPt_xAs)10(Pt3As8) with x = 0.03. muSR experiments show the
onset of magnetic order near Ts and a broad magnetic phase transition. No
symmetry breaking is associated to the structural transition in
Ca10(FeAs)10(Pt3As8) in contrast to the other parent compounds of iron arsenide
superconductors.
|
1212.5531v1
|
2012-12-24
|
Fabrication and transport properties of Sr0.6K0.4Fe2As2 multifilamentary superconducting wires
|
Seven-core Ag/Fe sheathed Sr0.6K0.4Fe2As2 (Sr-122) superconducting wires were
produced by the ex situ powder-in-tube (PIT) method. The relationship between
the cold-work deformation process and the superconducting properties of wires
were systematically studied. It was found that flat rolling can efficiently
increase the density of the superconducting core and largely improve the
transport critical current density (Jc) of as-drawn wires. The Jc of the best
sample achieved 21.1 kA/cm^2 at 4.2 K in self field, and showed very weak
magnetic field dependence in high fields. Our result suggested a promising
future of multifilamentary iron-based superconductors in practical application.
|
1212.5873v1
|
2012-12-24
|
Spin-state transition in the Fe-pnictides
|
We report a Fe K\beta x-ray emission spectroscopy study of local magnetic
moments in the rare-earth doped iron pnictide Ca_{1-x}RE_xFe_2As_2 (RE=La, Pr,
and Nd). In all samples studied the size of the Fe local moment is found to
decrease significantly with temperature and goes from ~0.9 \mu_B at T = 300 K
to ~0.45 \mu_B at T = 70 K. In the collapsed tetragonal (cT) phase of Nd- and
Pr-doped samples (T<70K) the local moment is quenched, while the moment remains
unchanged for the La-doped sample, which does not show lattice collapse. Our
results show that Ca_{1-x}RE_xFe_2As_2 (RE= Pr and Nd) exhibits a spin-state
transition and provide direct evidence for a non-magnetic Fe^{2+} ion in the
cT-phase, as predicted by Yildirim. We argue that the gradual change of the the
spin-state over a wide temperature range reveals the importance of multiorbital
physics, in particular the competition between the crystal field split Fe 3d
orbitals and the Hund's rule coupling.
|
1212.5925v1
|
2013-01-02
|
Discovery of the first Super-Lithium rich Beat Cepheid: V371 Per
|
Four high-resolution spectra of the double-mode Cepheid V371 Per, obtained
for the first time, showed the presence of the abnormally strong Li I 6707.76 A
line. Our analysis of the light element abundances indicates that the star did
not go through the evolutionary dredge-up stage. Large distance from the
galactic plane and the low metallicity suggest that V371 Per may belong to the
thick disc (or to the halo) of the Galaxy, which is consistent with its low
metallicity [Fe/H]=-0.42 and the enhancement of the alpha- and s-elements
relative to iron. Line splitting is observed in one of the spectra, which can
be due to the non-radial pulsations.
|
1301.0199v1
|
2013-01-03
|
Pair Breaking Caused by Magnetic Impurities in the High-T$_\text{C}$ Superconductor Bi$_{2.1}$Sr$_{1.9}$Ca(Cu$_{1-x}$Fe$_{x}$)$_{2}$O$_{y}$
|
Conventional superconductivity is robust against the addition of impurities
unless the impurities are magnetic in which case superconductivity is quickly
suppressed. Here we present a study of the cuprate superconductor
Bi$_2$Sr$_2$Ca$_1$Cu$_2$O$_{8+\delta}$ that is intentionally doped with the
magnetic impurity, Fe. Through the use of our Tomographic Density of States
(TDoS) technique, we find that while the superconducting gap magnitude is
essentially unaffected by the inclusion of iron, the onset of
superconductivity, T$_{C}$, and the pair-breaking rate are strongly dependent
and correlated. These findings suggest that, in the cuprates, the pair-breaking
rate is critical to the determination of T$_{C}$ and that magnetic impurities
do not disrupt the strength of pairing but rather the lifetime of the pairs.
|
1301.0521v2
|
2013-01-11
|
Perspectives of the disproportionation driven superconductivity in strongly correlated 3d compounds
|
Disproportionation in 3d compounds can give rise to an unconventional
electron-hole Bose liquid with a very rich phase diagram from a Bose metal,
charge ordering insulator to an inhomogeneous Bose superfluid. Optimal
conditions for the disproportionation driven high-T_c superconductivity are
shown to realize only for several Jahn-Teller d^n configurations that permit
the formation of well defined local composite bosons. These are the high-spin
d^4, low-spin d^7, and d^9 configurations given the octahedral crystal field,
and the d^1, high-spin d^6 configurations given the tetrahedral crystal field.
The disproportionation reaction has a peculiar anti-Jahn- Teller character
lifting the bare orbital degeneracy. Superconductivity in the d^4 and d^6
systems at variance with d^1, d^7, and d^9 systems implies an unavoidable
coexistence of the spin-triplet composite bosons and a magnetic lattice. We
argue that unconventional high-T_c superconductivity observed in quasi-2D
cuprates with tetragonally distorted CuO_6 octahedra and iron-based layered
pnictides/chalcogenides with tetrahedrally coordinated Fe^2+ ions can be a key
argument supporting the disproportionation scenario is at work in these
compounds.
|
1301.2395v1
|
2013-01-11
|
Terzan 5: a Fossil Remnant of the Galactic Bulge
|
Terzan 5 is a stellar system located in the Galactic Bulge, at a distance of
5.9 kpc. Recent discoveries show that it hosts two stellar populations with
different iron abundance ({\Delta}[Fe/H]=0.5). Such a large difference has been
measured only in {\omega} Centauri in the Galactic halo. Moreover no
anticorrelation is observed in Terzan 5, hence it is not a genuine globular
cluster. The observed chemical patterns are strikingly similar to those
observed in the Bulge stars. This suggests that Terzan 5 is a remnant fragment
of the Galactic bulge.
|
1301.2436v1
|
2013-01-16
|
NGC5694: another extra-galactic globular cluster
|
We discuss the chemical composition of six giant stars of the outer Halo
globular cluster NGC5694, through the analysis of UVES@FLAMES high-resolution
spectra. The cluster has an average iron content [Fe/H]=--1.83+-0.01,
solar-scaled [alpha/Fe] ratios and a very low Ba abundance
([Ba/Fe]=--0.71+-0.06). These anomalous abundance patterns are different from
those observed in other Halo globular clusters but similar to those of the
metal-poor stars in typical dwarf spheroidal galaxies. These findings suggest
an extra-galactic origin for NGC5694, likely from a dwarf spheroidal galaxy.
|
1301.3606v1
|
2013-01-21
|
Nanoscale Surface Element Identification and Dopant Homogeneity in the High-$T_c$ Superconductor Pr$_x$Ca$_{1-x}$Fe$_2$As$_2$
|
We use scanning tunneling microscopy to determine the surface structure and
dopant distribution in Pr$_x$Ca$_{1-x}$Fe$_2$As$_2$, the highest-$T_c$ member
of the 122 family of iron-based superconductors. We identify the cleaved
surface termination by mapping the local tunneling barrier height, related to
the work function. We image the individual Pr dopants responsible for
superconductivity, and show that they do not cluster, but in fact repel each
other at short length scales. We therefore suggest that the low volume fraction
high-$T_c$ superconducting phase is unlikely to originate from Pr
inhomogeneity.
|
1301.4942v2
|
2013-01-22
|
Quasi-two-dimensional non-collinear magnetism in the Mott insulator Sr$_2$F$_2$Fe$_2$OS$_2$
|
The magnetism of Sr$_2$F$_2$Fe$_2$OS$_2$ was examined through neutron powder
diffraction and thermodynamic and transport measurements. Quasi-two-dimensional
magnetic order develops below $T_N$ = 106(2) K with an in-plane correlation
length exceeding 310 \AA\ and an out-of-plane correlation length of just 17(3)
\AA. The data are consistent with a two-k structure with $k_1$ = (1/2, 0, 1/2)
and $k_2$ = (0, 1/2, 1/2) and an ordered moment of 3.3(1) $\mu_B$ oriented
along the in-plane components of k. This structure is composed of orthogonal
AFM chains intersecting at super-exchange mediating O sites. Density Functional
Theory (DFT) also points to this structure and a narrower Fe 3d band than for
the iron pnictides from which electronic correlations produce a Mott insulator.
|
1301.5056v1
|
2013-01-23
|
Nodal superconductivity in isovalently substituted SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ pnictide superconductor at the optimal doping, $x=$0.35
|
Temperature-dependent London penetration depth, $\lambda(T)$, was measured in
optimally - doped, $x=$0.35, as-grown ($T_c \approx$25 K,
RRR=$\rho(300K)/\rho(T_c)$=4.5) and annealed ($T_c \approx$35 K, RRR=6.4)
single crystals of SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ iron - based superconductor.
Annealing increases the RRR and decreases the absolute value of the London
penetration depth from $\lambda(0) = 300 \pm 10$ nm in as-grown sample to
$\lambda(0) = 275 \pm 10$ nm. At low temperatures, $\lambda(T) \sim T$
indicating superconducting gap with line nodes. Analysis of the full -
temperature range superfluid density is consistent with the line nodes, but
differs from the simple single - gap $d-$wave. The observed behavior is very
similar to that of BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$, showing that isovalently
substituted pnictides are inherently different from the charge - doped
materials.
|
1301.5622v1
|
2013-01-24
|
Superconductivity, Antiferromagnetism, and Neutron Scattering
|
High-temperature superconductivity in both the copper-oxide and the
iron-pnictide/chalcogenide systems occurs in close proximity to
antiferromagnetically ordered states. Neutron scattering has been an essential
technique for characterizing the spin correlations in the antiferromagnetic
phases and for demonstrating how the spin fluctuations persist in the
superconductors. While the nature of the spin correlations in the
superconductors remains controversial, the neutron scattering measurements of
magnetic excitations over broad ranges of energy and momentum transfer provide
important constraints on the theoretical options. We present an overview of the
neutron scattering work on high-temperature superconductors and discuss some of
the outstanding issues.
|
1301.5888v1
|
2013-01-25
|
Upper critical field of isoelectron substituted SrFe$_2$(As$_{1-x}$P$_x$)$_2$
|
The upper critical field $H_{c2}$ of optimally doped iron-based
superconductor SrFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ ($x$ = 0.35, $T_c$ = 25 K)
was measured as a function of temperature down to 1.6 K for two principal
directions of magnetic field $H \parallel c$ and $H \parallel a$. Measurements
were performed in pulsed magnetic fields up to 65 T using a tunnel-diode
resonator technique on as-grown and heavy-ion irradiated single crystals, with
columnar defect density corresponding to a matching field $B\phi$ = 25 T. The
$H_{c2,c}(T)$ is close to $T$-linear, while clear saturation is observed for
$H_{c2,a}(T)$, leading to a strongly temperature dependent anisotropy parameter
$\gamma$. The linear shape of $H_{c2,c}(T)$ is very similar to that observed in
nodal KFe$_2$As$_2$ but very different from full-gap LiFeAs. Irradiation does
not introduce any additional features on $H_{c2}(T)$ line corresponding to the
matching field. Instead, it suppresses uniformly both $T_c$ and $H_{c2}$,
keeping their ratio constant.
|
1301.6142v1
|
2013-01-27
|
Angular-dependent upper critical field of overdoped Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$
|
In-plane resistivity measurements as a function of temperature, magnetic
field and its orientation with respect to the crystallographic $ab-$plane were
used to study the upper critical field, $H_{c2}$, of two overdoped compositions
of the iron-based superconductor Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$, $x=$0.054 and
$x$=0.072. Measurements were performed using precise alignment (with accuracy
less than 0.1$^o$) of magnetic field with respect to the Fe-As-plane. The
dependence of the $H_{c2}$ on angle $\theta$ between the field and the
$ab$-plane was measured in isothermal conditions in a broad temperature range.
We found that the shape of $H_{c2}(\theta)$, clearly deviates from
Ginzburg-Landau functional form.
|
1301.6396v1
|
2013-02-02
|
Rethinking Globular Cluster Formation
|
This paper is aimed at emphasizing some of the main hints, constraints and
difficulties we currently have in trying to understand how globular clusters
formed, along with their multiple stellar generations, an issue that must be
regarded as intimately connected to the formation process itself. Thus, the
topics that are addressed include i) the required mass of the progenitor, ii)
how to form new stars in an environment already crowded by a previous stellar
generation, iii) how photometry and spectroscopy appear to suggest different
formation processes for second generation stars, iv) whether dilution with
pristine material may (or may not) be necessary for the formation of second
generations, v) why the few clusters with multiple iron abundances are after
all not so different from those that are homogeneous, and finally vi) why
special environmental conditions may not be necessary for the formation of
globular clusters with multiple stellar generations.
|
1302.0329v1
|
2013-02-04
|
Ferromagnetic Quantum Critical Point Induced by Tuning the Magnetic Dimensionality of the Heavy-Fermion Iron Oxypnictide Ce(Ru$_{1-x}$Fe$_x$)PO
|
We have performed 31P-NMR measurements of the c-axis-aligned poly-crystal
Ce(Ru1-xFex)PO with a two-dimensional layered structure in order to understand
the origin of TCurie suppression by Fe substitution. The variation in the
magnetic-fluctuation character with respect to x is investigated from the
in-plane and out-of plane fluctuations and the relationship between the static
spin susceptibility and the in-plane fluctuation. It was found that
three-dimensional ferromagnetic (FM) correlations are dominant and give rise to
the FM ordering in CeRuPO. The out-of-plane fluctuations are significantly
suppressed with increasing x, and it was revealed that the FM fluctuations
become two dimensional near a FM quantum critical point (QCP). Our NMR results
strongly suggest that a unique FM QCP in Ce(Ru1-xFex)PO is induced by the
suppression of the magnetic correlations along the c-axis, which is a different
mechanism from that presumed in heavy-fermion compounds.
|
1302.0578v2
|
2013-02-05
|
Type II superconductivity in SrPd2Ge2
|
Previous investigations have shown that SrPd2Ge2, a compound isostructural
with "122" iron pnictides but iron- and pnictogen-free, is a conventional
superconductor with a single s-wave energy gap and a strongly three-dimensional
electronic structure. In this work we reveal the Abrikosov vortex lattice
formed in SrPd2Ge2 when exposed to magnetic field by means of scanning
tunneling microscopy and spectroscopy. Moreover, by examining the differential
conductance spectra across a vortex and estimating the upper and lower critical
magnetic fields by tunneling spectroscopy and local magnetization measurements,
we show that SrPd2Ge2 is a strong type II superconductor with \kappa >>
sqrt(2). Also, we compare the differential conductance spectra in various
magnetic fields to the pair breaking model of Maki - de Gennes for dirty limit
type II superconductor in the gapless region. This way we demonstrate that the
type II superconductivity is induced by the sample being in the dirty limit,
while in the clean limit it would be a type I superconductor with \kappa\ <<
sqrt(2), in concordance with our previous study (T. Kim et al., Phys. Rev. B
85, (2012)).
|
1302.1022v1
|
2013-02-06
|
Gap Symmetry and Stability Analysis in the Multi-Orbital Fe-Based Superconductors
|
The iron-based superconductors allow for a zoo of possible order parameters
due to their orbital degrees of freedom. These order parameters are often
written in an orbital basis, as this allows to distinguish their different
symmetry. Unlike in standard single-band superconductors, where electrons of
the same band - and hence, energy - are paired, a general order parameter in
such a multi-orbital system also contains pairing of electrons belonging to
different bands. As this corresponds to pairing of electrons of different
energy, such order parameters are energetically less stable. Here, we present a
simple criterion for a stability analysis of the orbital part of the gap
function based on the basic principle that electrons of equal energy are paired
in the superconducting state. This not only allows to find the most stable
states, but also to identify the terms in a (tight-binding) Hamiltonian
suppressing any given superconducting state. Our approach thus allows to
identify the minimal Hamiltonian necessary to compare competing instabilities.
|
1302.1468v2
|
2013-02-07
|
Evidence of Strong Correlations and Coherence-Incoherence Crossover in the Iron Pnictide Superconductor KFe2As2
|
Using resistivity, heat-capacity, thermal-expansion, and susceptibility
measurements we study the normal-state behavior of KFe2As2. We find that both
the Sommerfeld coefficient gamma = 103 mJ mol-1 K-2 and the Pauli
susceptibility chi = 4x10-4 are strongly enhanced, which confirm the existence
of heavy quasiparticles inferred from previous de Haas-van Alphen and ARPES
experiments. We discuss this large enhancement using a Gutzwiller slave-boson
mean-field calculation, which reveals the proximity of KFe2As2 to an
orbital-selective Mott transition. The temperature dependence of the magnetic
susceptibility and the thermal expansion provide strong experimental evidence
for the existence of a coherence-incoherence crossover, similar to what is
found in heavy fermion and ruthenate compounds, due to Hund's coupling between
orbitals.
|
1302.1696v1
|
2013-02-13
|
Ligand field parameters and the ground state of Fe(II) phthalocyanine
|
A judicious analysis of previously published experimental data leads one to
conclude that the ground state of iron(II) phthalocyanine is an orbitally
degenerate spin triplet $a_{1g}^2 e_g^{\uparrow\downarrow\uparrow}
b_{2g}^{\uparrow}$ ($^3E_g$). The ligand field parameters, in relation to
Racah's $C$, are approximately as follows: $B_{20}/C=0.84$, $B_{40}/C=0.0074$.
The uniqueness of this result is demonstrated by means of a special diagram in
the $B_{20}/C-B_{40}/C$ plane (under additional conditions that
$B_{44}/B_{40}=35/3$ and $B/C=0.227$). The system is in a strong-ligand-field
regime, which enables the use of single-determinant techniques corrected for
correlations within the 3d shell of Fe.
|
1302.3023v2
|
2013-02-13
|
Nucleosynthesis in the gamma-ray burst accretion disks and associated outflows
|
We investigate nucleosynthesis inside the gamma-ray burst (GRB) accretion
disks formed by the Type II collapsars and outflows launched from these disks.
We deal with accretion disks having relatively low accretion rates: 0.001 M_sun
s^{-1} <~ Mdot <~ 0.01 M_sun s^{-1} and hence they are predominantly advection
dominated. We report the synthesis of several unusual nuclei like 31P, 39K,
43Sc, 35Cl and various isotopes of titanium, vanadium, chromium, manganese and
copper in the disk. We also confirm that isotopes of iron, cobalt, nickel,
argon, calcium, sulphur and silicon get synthesized in the disk, as shown by
previous authors. Much of these heavy elements thus synthesized are ejected
from the disk and survive in the outflows. Indeed, emission lines of many of
these heavy elements have been observed in the X-ray afterglows of several
GRBs.
|
1302.3067v1
|
2013-02-13
|
Unconventional Superconductivity
|
A brief review of unconventional superconductivity is given, stretching from
the halcyon days of helium-3 to the modern world of Majorana fermions. Along
the way, we will encounter such strange beasts as heavy fermion
superconductors, cuprates, and their iron-based cousins. Emphasis will be put
on the fact that in almost all cases, an accepted microscopic theory has yet to
emerge. This is attributed to the difficulty of constructing a theory of
superconductivity outside the Migdal-Eliashberg framework.
|
1302.3176v2
|
2013-02-15
|
PARSEC: A Parametrized Simulation Engine for Ultra-High Energy Cosmic Ray Protons
|
We present a new simulation engine for fast generation of ultra-high energy
cosmic ray data based on parametrizations of common assumptions of UHECR origin
and propagation. Implemented are deflections in unstructured turbulent
extragalactic fields, energy losses for protons due to photo-pion production
and electron-pair production, as well as effects from the expansion of the
universe. Additionally, a simple model to estimate propagation effects from
iron nuclei is included. Deflections in galactic magnetic fields are included
using a matrix approach with precalculated lenses generated from backtracked
cosmic rays. The PARSEC program is based on object oriented programming
paradigms enabling users to extend the implemented models and is steerable with
a graphical user interface.
|
1302.3761v2
|
2013-02-19
|
Effect of magnetism on kinetics of "gamma"-"alpha" transformation and pattern formation in iron
|
Kinetics of polymorphous "gamma"-"alfa" transformation in Fe is studied
numerically within a model taking into account both lattice and magnetic
degrees of freedom, based on first-principle calculations of the total energy
for different magnetic states. It is shown that magnetoelastic phenomena,
namely, a strong sensitivity of the potential relief along the Bain deformation
path to the magnetic state, are crucial for the picture of the transformation.
With the temperature increase, a scenario of the phase transformation evolves
from a homogeneous lattice instability at T<Ms (Ms is the temperature of the
beginning of the martensitic transformation) to the growth and nucleation of
embryos of the new phase at T>Ms. In the latter case, a stage of formation of a
tweed-like structure occurs, with a strong short-range order and slow
interphase fluctuations.
|
1302.4643v2
|
2013-02-20
|
Non-metallic, non-Fermi-liquid resistivity of FeCrAs from 0 to 17 GPa
|
An unusual, non-metallic resistivity of the 111 iron-pnictide compound FeCrAs
is shown to be relatively unchanged under pressures of up to 17 GPa. Combined
with our previous finding that this non-metallic behaviour persists from at
least 80 mK to 800 K, this shows that the non-metallic phase is exceptionally
robust. Antiferromagnetic order, with a Neel temperature T_N ~ 125 K at ambient
pressure, is suppressed at a rate of 7.1 +/- 0.1 K/GPa, falling to below 50 K
at 10 GPa. We conclude that formation of a spin-density wave gap at T_N does
not play an important role in the non-metallic resistivity of FeCrAs at low
temperatures.
|
1302.4791v1
|
2013-02-20
|
Transport signatures of spatially modulated electronic nematic phases
|
Electronic nematic phases are broadly characterized by spontaneously broken
rotational symmetry. Although they have been widely recognized in the context
of high temperature cuprates, bilayer ruthenates, and iron-based
superconductors, the focus so far has been exclusively on the uniform nematic
phase. Recently, however, it was proposed that on a square lattice a nematic
instability in the d-wave charge channel could lead to a spatially modulated
nematic state, where the modulation vector q is determined by the relative
location of the Fermi level to the van Hove singularity. Interestingly, this
finite-q nematic phase has also been identified as an additional leading
instability that is as strong as the superconducting instability near the onset
of spin density wave order. Here we study the electrical conductivity tensor in
the modulated nematic phase for a general modulation vector. Our results can be
used to identify modulated nematic phases in correlated materials.
|
1302.5117v1
|
2013-02-21
|
Superconductivity induced by U-doping in the SmFeAsO system
|
Through partial substitution of Sm by U in SmFeAsO, a different member of the
family of iron-based superconductors was successfully synthesized. X-ray
diffraction measurements show that the lattice constants along the a and c axes
are both squeezed through U doping, indicating a successful substitution of U
at the Sm site. The parent compound shows a strong resistivity anomaly near 150
K, associated with spin-density-wave instability.U doping suppresses this
instability and leads to a transition to the superconducting state at
temperatures up to 49 K. Magnetic measurements confirm the bulk
superconductivity in this system. For the sample with a doping level of x =
0.2, the external magnetic field suppresses the onset temperature very slowly,
indicating a rather high upper critical field. In addition, the Hall effect
measurements show that U clearly dopes electrons into the material.
|
1302.5155v1
|
2013-02-28
|
Synthesis of New Layered Oxypnictides Sr2CrO2(FeAs)2
|
We successfully synthesized new layered iron oxypnictide Sr2CrO2(FeAs)2 by
solid-state-reaction techniques. This is the first report on the Fe-substituted
Sr2Mn3As2O2-type oxypnictides to our knowledge. A pellet of stoichiometric
mixtures of SrO, Cr and FeAs wrapped with tantalum foil was sintered at
900{\deg}C for 96 h. Results of x-ray diffraction measurement and Rietveld
refinements show that the structure of the sample belongs to the space group of
I4/mmm with the lattice constants of a = 3.9948(1) {\AA} and c = 18.447(1)
{\AA}. Temperature dependence of the resistivity for Sr2CrO2(FeAs)2 shows
semiconductor-like behavior. Additionally, the sample does not exhibit
superconducting transition in measurements of resistivity and magnetization
even in the low temperature down to 3 K.
|
1302.7196v1
|
2013-03-01
|
Expanded Iron UTA spectra -- probing the thermal stability limits in AGN clouds
|
The Fe unresolved transition array (UTAs) produce prominent features in the
15-17?A wavelength range in the spectra of Active Galactic Nuclei (AGN). Here
we present new calculations of the energies and oscillator strengths of inner-
shell lines from Fe XIV, Fe XV, and Fe XVI. These are crucial ions since they
are dominant at inflection points in the gas thermal stability curve, and UTA
excitation followed by autoionization is an important ionization mechanism for
these species. We incorporate these, and data reported in previous papers, into
the plasma simulation code Cloudy. This updated physics is subsequently
employed to reconsider the thermally stable phases in absorbing media in Active
Galactic Nuclei. We show how the absorption profile of the Fe XIV UTA depends
on density, due to the changing populations of levels within the ground
configuration.
|
1303.0316v1
|
2013-03-07
|
Longitudinal magnetoresistance in Co-doped BaFe2As2 and LiFeAs single crystals: Interplay between spin fluctuations and charge transport in iron-pnictides
|
The longitudinal in-plane magnetoresistance (LMR) has been measured in
different Ba(Fe_(1-x)Co_x)2As2 single crystals and in LiFeAs. For all these
compounds, we find a negative LMR in the paramagnetic phase whose magnitude
increases as H^2. We show that this negative LMR can be readily explained in
terms of suppression of the spin fluctuations by the magnetic field. In the
Co-doped samples, the absolute value of the LMR coefficient is found to
decrease with doping content in the paramagnetic phase. The analysis of its T
dependence in an itinerant nearly antiferromagnetic Fermi liquid model
evidences that the LMR displays a qualitative change of T variation with
increasing Co content. The latter occurs at optimal doping for which the
antiferromagnetic ground state is suppressed. The same type of analysis for the
negative LMR measured in LiFeAs suggests that this compound is on the verge of
magnetism.
|
1303.1677v2
|
2013-03-07
|
Studies of homogeneous precipitation in very dilute iron-coper alloys using kinetic Monte Carlo simulations and statistical theory of nucleation
|
Kinetics of homogeneous nucleation and growth of copper precipitates under
electron irradiation of Fe_{1-x}Cu_x alloys at concentrations x from 0.06 at.%
to 0.4 at.% and temperatures T from 290 to 450C is studied using the kinetic
Monte Carlo (KMC) simulations and the statistical theory of nucleation (STN).
The conventional assumption about the similarity of mechanisms of precipitation
under electron irradiation and during thermal aging is adopted. The
earlier-developed ab initio model of interactions in Fe-Cu alloys is used for
both the KMC simulations and the STN calculations. Values of the nucleation
barrier F_c and the prefactor J_0 in the Zeldovich-Volmer relation for the
nucleation rate J are calculated for a number of concentrations and
temperatures. For the dilute alloys with x less then 0.2%, the STN and the KMC
results for the nucleation barrier F_c do virtually coincide with each other,
which seems to confirm a high reliability of the STN for this problem. The STN
calculations are also used to estimate the temperature dependencies of
concentrations which correspond to the homogeneous or the heterogeneous
precipitation limit and both dependencies are found to be rather sharp.
|
1303.1753v1
|
2013-03-08
|
Heat capacity jump at Tc and pressure derivatives of superconducting transition temperature in the Ba1-xKxFe2As2 (0.2 < x < 1.0) series
|
We present the evolution of the initial (up to ~ 10 kbar) hydrostatic,
pressure dependencies of Tc and of the ambient pressure jump in the heat
capacity associated with the superconducting transition as a function of K -
doping in the Ba1-xKxFe2As2 family of iron-based superconductors. The pressure
derivatives show weak but distinct anomaly near x ~ 0.7. In the same
concentration region Delta Cp at Tc deviates from the Delta Cp ~ T^3 scaling
found for most BaFe2As2 - based superconductors. These results are consistent
with a Lifshitz transition, and possible significant modification of the
superconducting state, occurring near x ~ 0.7.
|
1303.2010v1
|
2013-03-11
|
The Reach of INO for Atmospheric Neutrino Oscillation Parameters
|
The India-based Neutrino Observatory (INO) will host a 50 kt magnetized iron
calorimeter (ICAL@INO) for the study of atmospheric neutrinos. Using the
detector resolutions and efficiencies obtained by the INO collaboration from a
full-detector GEANT4-based simulation, we determine the reach of this
experiment for the measurement of the atmospheric neutrino mixing parameters
($\sin^2 \theta_{23}$ and $|\Delta m_{32}^2 |$). We also explore the
sensitivity of this experiment to the deviation of $\theta_{23}$ from maximal
mixing, and its octant.
|
1303.2534v2
|
2013-03-11
|
Individual Alpha Elements, C, N, and Ba in Early Type Galaxies
|
Spectral data on early type galaxies is analyzed for chemical abundance with
an emphasis on obtaining detailed abundances for the elements O and Si in
addition to C, N, Na, Mg, Ca, Fe, and Ba. The abundance trends with velocity
dispersion fit preconceptions based upon previous Mg conclusions, namely that
larger galaxies have a higher alpha element to iron peak ratio indicative of a
higher ratio of Type II to Type Ia supernova products. The heaviest alpha
elements, Ca and Ti, do not participate in this trend, although this fact does
not necessarily alter the basic picture given the uncertainties in
nucleosynthetic yields. Elements that likely have significant contributions
from intermediate-mass stars, namely C, N, and Ba, also gain ground relative to
Fe in massive galaxies at a modest level, with the Ba conclusion uncertain from
our data alone.
|
1303.2603v1
|
2013-03-13
|
A Ni-Fe Layered Double Hydroxide-Carbon Nanotube Complex for Water Oxidation
|
Highly active, durable and cost-effective electrocatalysts for water
oxidation to evolve oxygen gas hold a key to a range of renewable energy
solutions including water splitting and rechargeable metal-air batteries. Here,
we report the synthesis of ultrathin nickel iron layered double hydroxide
nanoplates on mildly oxidized multi-walled carbon nanotubes. Incorporation of
Fe into the nickel hydroxide induced the formation of NiFe-layered double
hydroxide. The nanoplates were covalently attached to a network of nanotubes,
affording excellent electrical wiring to the nanoplates. The ultra-thin Ni-Fe
layered double hydroxide nanoplates/carbon nanotube complex was found to
exhibit unusually high electro-catalytic activity and stability for oxygen
evolution and outperformed commercial precious metal Ir catalysts.
|
1303.3308v2
|
2013-03-14
|
Quasiparticle mass enhancement close to the quantum critical point in BaFe$_2$(As$_{1-x}$P$_x$)$_2$
|
We report a combined study of the specific heat and de Haas-van Alphen effect
in the iron-pnictide superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$. Our data
when combined with results for the magnetic penetration depth give compelling
evidence for the existence of a quantum critical point (QCP) close to $x=0.30$
which affects the majority of the Fermi surface by enhancing the quasiparticle
mass. The results show that the sharp peak in the inverse superfluid density
seen in this system results from a strong increase in the quasiparticle mass at
the QCP.
|
1303.3396v2
|
2013-03-14
|
Investigation of charge states and multiferroicity in Fe-doped h-YMnO3
|
Polycrystalline YMn1-xFexO3 (YMFO_x) (0 <= x <= 0.1) compounds have been
prepared in single phase and characterized by synchrotron X-ray diffraction,
X-ray absorption near edge spectroscopy, magnetization, and dielectric
measurements. Iron-substitution in hexagonal YMnO3 causes intra-lattice changes
exceeding those of the lattice cell. XANES provide mixed-valence Mn3+/Mn4+ and
Fe4+ charge states in these manganites, consistent with the observed decrease
of the effective magnetic moment with Fe-doping. Magnetization M(T) evidence
antiferromagnetic (AFM) ordering of the specimens with little weak
ferromagnetism, and the metrices of exchange interaction suppress with
Fe-doping, attributed to the lengthening of the Mn-O planar bond lengths.
Dielectric {\epsilon}'(T) results showing highly doping-dependent anomaly at
T_N indicate linear magneto-electric coupling.
|
1303.3398v2
|
2013-03-15
|
Phase diagram and superconductivity at 58.1 K in α-FeAs free SmFeAsO1-xFx
|
The Phase diagram of SmFeAsO1-xFx in terms of x is exhibited in this study.
SmFeAsO1-xFx from x = 0 to x = 0.3 were prepared by low temperature sintering
with slow cooling. The low temperature sintering suppresses the formation of
the amorphous FeAs, which is inevitably produced as an impurity by using high
temperature sintering. Moreover, slow cooling is effective to obtain the high
fluorine concentration. The compositional change from feedstock composition is
quite small after this synthesis. We can reproducibly observe a record
superconducting transition for an iron based superconductor at 58.1 K. This
achievement of a high superconducting transition is due to the success in a
large amount of fluorine substitution. A shrinking of the a lattice parameter
caused by fluorine substitution is observed and the substitutional rate of
fluorine changes at x =0.16.
|
1303.3725v2
|
2013-03-17
|
Two-dimensional magnetic interactions in LaFeAsO
|
Inelastic neutron scattering measurements demonstrate that the magnetic
interactions in antiferromagnetic LaFeAsO are two-dimensional. Spin wave
velocities within the Fe layer and the magnitude of the spin gap are similar to
the \textit{A}Fe$_2$As$_2$ based materials. However, the ratio of interlayer
and intralayer exchange is found to be less than $\sim 10^{-4}$ in LaFeAsO,
very similar to the cuprates, and $\sim$ 100 times smaller than that found in
\textit{A}Fe$_2$As$_2$ compounds. The results suggest that the effective
dimensionality of the magnetic system is highly variable in the parent
compounds of the iron arsenides and weak 3-D interactions may limit the maximum
attainable superconducting $T_{c}$.
|
1303.4033v1
|
2013-03-19
|
<3D> NLTE line formation in the atmospheres of red supergiants
|
Red supergiants with their enormous brightness at J-band are ideal probes of
cosmic chemical composition. It is therefore crucial to have realistic models
of radiative transfer in their atmospheres, which will permit determination of
abundances accurate to 0.15 dex, the precision attainable with future telescope
facilities in galaxies as distant as tens of Mpc. Here, we study the effects of
non-local thermodynamic equilibrium (NLTE) on the formation of iron, titanium,
and silicon lines, which dominate J-band spectra of red supergiants. It is
shown that the NLTE radiative transfer models enable accurate derivation of
metallicity and effective temperature in the J-band. We also discuss
consequences for RSG spectrum synthesis in different spectral windows,
including the heavily TiO-blanketed optical region, and atmospheric structure.
We then touch upon challenges of NLTE integration with new generation of 3D
hydrodynamical RSG models and present the first calculations of NLTE spectra
with the mean 3D model of Betelgeuse.
|
1303.4768v1
|
2013-03-24
|
Determination of $sin^{2}θ_W$ using $ν(\barν)$-Nucleus scattering
|
We have studied nonisoscalarity and medium effects in the extraction of weak
mixing angle using Paschos and Wolfenstein relation in the iron nucleus.
Paschos and Wolfenstein(PW) relation is valid for an isoscalar target. We have
modified the PW relation for nonisoscalar target as well as incorporated the
medium effects like Pauli blocking, Fermi motion, nuclear binding energy and
pion rho cloud contributions. In our calculations we have used the relativistic
nuclear spectral function which includes nucleon correlations. Finally local
density approximation is applied to translate the numerical results to the
finite nuclei. We have studied the dependence of $sin^{2}\theta_W$ on Bjorken
variables $x$ and $y$, four momentum transfer square ($Q^2$), energy of the
neutrino and antineutrino, and effect of excess neutrons over protons in the
nuclear target.
|
1303.5928v1
|
2013-03-29
|
An analytically tractable model of bad metals
|
We discuss a model Kondo-type Hamiltonian representing an analytically
tractable version of the model used by Yin {\it et.al.}, Phys. Rev. B{\bf 86},
2399 (2012) to explain the non-Fermi liquid behavior of iron chalcogenides and
ruthenates in an intermediate energy range. We consider a regime where a
complete screening of the local degrees of freedom proceeds in two stages
described by two characteristic energy scales $T_K^{orb} >> E_0$. The first
scale marks a screening of the orbital degrees of freedom and the second one
marks a crossover to the regime with coherent propagation of quasiparticles. We
present analytical results for the specific heat and magnetic susceptibility at
$T << T_K^{orb}$.
|
1303.7448v2
|
2013-04-04
|
On Euler's "Misleading Induction", Andrews' "Fix", and How to Fully Automate them
|
One of the greatest experimental mathematicians of all time was also one of
the greatest mathematicians of all time, the great Leonhard Euler. Usually he
had an uncanny intuition on how many "special cases" one needs before one can
formulate a plausible conjecture, but one time he was "almost fooled", only to
find out that his conjecture was premature.
In 1990, George Andrews found a way to "correct" Euler. Here we show how to
generate, AUTOMATICALLY, rigorously-proved Euler-Andrews Style formulas, that
enables one to generate Euler-style "cautionary tales" about the "danger" of
using naive empirical induction. Ironically, the way we prove the Andrews-style
corrections is empirical! But in order to turn the empirical proof into a
full-fledged rigorous proof, we must make sure that we check sufficiently many
(but still not that many!) special cases.
|
1304.1226v1
|
2013-04-04
|
Ab-initio prediction of magnetoelectricity in infinite-layer CaFeO2 and MgFeO2
|
Density functional based simulations are employed to explore magnetoelectric
effects in iron-based oxides, showing a unique layered structure. We
theoretically predict CaFeO2 to be a promising magnetoelectric, showing
magnetically-controlled large electric polarization, possibly even above room
temperature. The cross coupling between magnetic and dipolar degrees of freedom
needs, as main ingredients, Fe-site spin-orbit coupling and a spin-dependent O
p - Fe d hybridization, along with structural constraints related to the
non-centrosymmetric point group and the peculiar geometry characterized by
"flattened" FeO4 tetrahedrons. In order to enhance magnetoelectric effects, we
performed a materials-design leading to a novel and optimized system, MgFeO2,
where the larger O4 tetrahedral distortion leads to a stronger polarization.
|
1304.1228v2
|
2013-04-04
|
Revisiting the hardening of the cosmic-ray energy spectrum at TeV energies
|
Measurements of cosmic rays by experiments such as ATIC, CREAM, and PAMELA
indicate a hardening of the cosmic-ray energy spectrum at TeV energies. In our
recent work (Thoudam & H\"orandel 2012a), we showed that the hardening can be
due to the effect of nearby supernova remnants. We showed it for the case of
proton and helium. In this paper, we present an improved and detailed version
of our previous work, and extend our study to heavier cosmic-ray species such
as boron, carbon, oxygen, and iron nuclei. Unlike our previous study, the
present work involves a detailed calculation of the background cosmic rays and
follows a consistent treatment of cosmic-ray source parameters between the
background and the nearby components. Moreover, we also present a detailed
comparison of our results on the secondary-to-primary ratios, secondary
spectra, and the diffuse gamma-ray spectrum with the results expected from
other existing models, which can be checked by future measurements at high
energies.
|
1304.1400v2
|
2013-04-09
|
Upper critical field of high quality single crystals of KFe$_2$As$_2$
|
Measurements of temperature-dependent in-plane resistivity, $\rho(T)$, were
used to determine the upper critical field and its anisotropy in high quality
single crystals of stoichiometric iron arsenide superconductor KFe$_2$As$_2$.
The crystals were characterized by residual resistivity ratio,
$\rho(300K)/\rho(0)$ up to 3000 and resistive transition midpoint temperature,
$T_c$=3.8 K, significantly higher than in previous studies on the same
material. We find increased $H_{c2}(T)$ for both directions of the magnetic
field, which scale with the increased $T_c$. This unusual linear $H_{c2}(T_c)$
scaling is not expected for orbital limiting mechanism of the upper critical
field in clean materials.
|
1304.2689v1
|
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