811 research outputs found
The Infrared Einstein Ring in the Gravitational Lens MG1131+0456 and the Death of the Dusty Lens Hypothesis
We have obtained and modeled new NICMOS images of the lens system
MG1131+0456, which show that its lens galaxy is an H=18.6 mag, transparent,
early-type galaxy at a redshift of about z_l = 0.85; it has a major axis
effective radius R_e=0.68+/-0.05 arcsec, projected axis ratio b/a=0.77+/-0.02,
and major axis PA=60+/-2 degrees. The lens is the brightest member of a group
of seven galaxies with similar R-I and I-H colors, and the two closest group
members produce sufficient tidal perturbations to explain the ring morphology.
The host galaxy of the MG1131+0456 source is a z_s > 2 ERO (``extremely red
object'') which is lensed into optical and infrared rings of dramatically
different morphologies. These differences imply a strongly wavelength-dependent
source morphology that could be explained by embedding the host in a larger,
dusty disk. At 1.6 micron (H), the ring is spectacularly luminous, with a total
observed flux of H=17.4 mag and a de-magnified flux of 19.3 mag, corresponding
to a 1-2L_* galaxy at the probable source redshift of z_s > 2. Thus, it is
primarily the stellar emission of the radio source host galaxy that produces
the overall colors of two of the reddest radio lenses, MG1131+0456 and
B~1938+666, aided by the suppression of optical AGN emission by dust in the
source galaxy. The dusty lens hypothesis -- that many massive early-type
galaxies with 0.2 < z_l < 1.0 have large, uniform dust opacities -- is ruled
out.Comment: 27 pages, 8 COLOR figures, submitted to ApJ. Black and white version
available at http://cfa-www.harvard.edu/castle
Direct Observation of High-Temperature Polaronic Behavior In Colossal Magnetoresistive Manganites
The temperature dependence of the electronic and atomic structure of the
colossal magnetoresistive oxides (x = 0.3, 0.4) has
been studied using core and valence level photoemission, x-ray absorption and
emission, and extended x-ray absorption fine structure spectroscopy. A dramatic
and reversible change of the electronic structure is observed on crossing the
Curie temperature, including charge localization and spin moment increase of
Mn, together with Jahn-Teller distortions, both signatures of polaron
formation. Our data are also consistent with a phase-separation scenario.Comment: 5 pages, 4 figures, revte
Neutron scattering and scaling behavior in URu2Zn20 and YbFe2Zn20
The dynamic susceptibility chi"(deltaE), measured by inelastic neutron
scattering measurements, shows a broad peak centered at Emax = 16.5 meV for the
cubic actinide compound URu2Zn20 and 7 meV at the (1/2, 1/2, 1/2) zone boundary
for the rare earth counterpart compound YbFe2Zn20. For URu2Zn20, the low
temperature susceptibility and magnetic specific heat coefficient gamma =
Cmag/T take the values chi = 0.011 emu/mole and gamma = 190 mJ/mole-K2 at T = 2
K. These values are roughly three times smaller, and Emax is three times
larger, than recently reported for the related compound UCo2Zn20, so that chi
and gamma scale inversely with the characteristic energy for spin fluctuations,
Tsf = Emax/kB. While chi(T), Cmag(T), and Emax of the 4f compound YbFe2Zn20 are
very well described by the Kondo impurity model, we show that the model works
poorly for URu2Zn20 and UCo2Zn20, suggesting that the scaling behavior of the
actinide compounds arises from spin fluctuations of itinerant 5f electrons.Comment: 7 pages, 5 figure
AC-induced superfluidity
We argue that a system of ultracold bosonic atoms in a tilted optical lattice
can become superfluid in response to resonant AC forcing. Among others, this
allows one to prepare a Bose-Einstein condensate in a state associated with a
negative effective mass. Our reasoning is backed by both exact numerical
simulations for systems consisting of few particles, and by a theoretical
approach based on Floquet-Fock states.Comment: Accepted for publication in Europhysics letters, 6 pages, 4 figures,
Changes in v2: reference 7 replaced by a more recent on
Phonon Density of States of LaFeAsO1-xFx
We have studied the phonon density of states (PDOS) in LaFeAsO1-xFx with
inelastic neutron scattering methods. The PDOS of the parent compound(x=0) is
very similar to the PDOS of samples optimally doped with fluorine to achieve
the maximum Tc (x~0.1). Good agreement is found between the experimental PDOS
and first-principle calculations with the exception of a small difference in Fe
mode frequencies. The PDOS reported here is not consistent with conventional
electron-phonon mediated superconductivity
Critical change in the Fermi surface of iron arsenic superconductors at the onset of superconductivity
The phase diagram of a correlated material is the result of a complex
interplay between several degrees of freedom, providing a map of the material's
behavior. One can understand (and ultimately control) the material's ground
state by associating features and regions of the phase diagram, with specific
physical events or underlying quantum mechanical properties. The phase diagram
of the newly discovered iron arsenic high temperature superconductors is
particularly rich and interesting. In the AE(Fe1-xTx)2As2 class (AE being Ca,
Sr, Ba, T being transition metals), the simultaneous structural/magnetic phase
transition that occurs at elevated temperature in the undoped material, splits
and is suppressed by carrier doping, the suppression being complete around
optimal doping. A dome of superconductivity exists with apparent equal ease in
the orthorhombic / antiferromagnetic (AFM) state as well as in the tetragonal
state with no long range magnetic order. The question then is what determines
the critical doping at which superconductivity emerges, if the AFM order is
fully suppressed only at higher doping values. Here we report evidence from
angle resolved photoemission spectroscopy (ARPES) that critical changes in the
Fermi surface (FS) occur at the doping level that marks the onset of
superconductivity. The presence of the AFM order leads to a reconstruction of
the electronic structure, most significantly the appearance of the small hole
pockets at the Fermi level. These hole pockets vanish, i. e. undergo a Lifshitz
transition, at the onset of superconductivity. Superconductivity and magnetism
are competing states in the iron arsenic superconductors. In the presence of
the hole pockets superconductivity is fully suppressed, while in their absence
the two states can coexist.Comment: Updated version accepted in Nature Physic
Microstructure and pinning properties of hexagonal-disc shaped single crystalline MgB2
We synthesized hexagonal-disc-shaped MgB2 single crystals under high-pressure
conditions and analyzed the microstructure and pinning properties. The lattice
constants and the Laue pattern of the crystals from X-ray micro-diffraction
showed the crystal symmetry of MgB2. A thorough crystallographic mapping within
a single crystal showed that the edge and c-axis of hexagonal-disc shape
exactly matched the (10-10) and the (0001) directions of the MgB2 phase. Thus,
these well-shaped single crystals may be the best candidates for studying the
direction dependences of the physical properties. The magnetization curve and
the magnetic hysteresis for these single crystals showed the existence of a
wide reversible region and weak pinning properties, which supported our single
crystals being very clean.Comment: 5 pages, 3 figures. submitted to Phys. Rev.
Kondo physics in the algebraic spin liquid
We study Kondo physics in the algebraic spin liquid, recently proposed to
describe [Phys. Rev. Lett. {\bf 98}, 117205 (2007)].
Although spin dynamics of the algebraic spin liquid is described by massless
Dirac fermions, this problem differs from the Pseudogap Kondo model, because
the bulk physics in the algebraic spin liquid is governed by an interacting
fixed point where well-defined quasiparticle excitations are not allowed.
Considering an effective bulk model characterized by an anomalous critical
exponent, we derive an effective impurity action in the slave-boson context.
Performing the large- analysis with a spin index , we
find an impurity quantum phase transition from a decoupled local-moment state
to a Kondo-screened phase. We evaluate the impurity spin susceptibility and
specific heat coefficient at zero temperature, and find that such responses
follow power-law dependencies due to the anomalous exponent of the algebraic
spin liquid. Our main finding is that the Wilson's ratio for the magnetic
impurity depends strongly on the critical exponent in the zero temperature
limit. We propose that the Wilson's ratio for the magnetic impurity may be one
possible probe to reveal criticality of the bulk system
Hydrostatic pressure study of pure and doped La1-xRxAgSb2 (R = Ce, Nd) charge-density-wave compounds
The intermetallic compound LaAgSb2 displays two charge-density-wave (CDW)
transitions, which were detected with measurements of electrical resistivity
(rho), magnetic susceptibility, and X-ray scattering; the upper transition
takes place at T1 approx. 210 K, and it is accompanied by a large anomaly in
rho(T), whereas the lower transition is marked by a much more subtle anomaly at
T2 approx. 185 K. We studied the effect of hydrostatic pressure (P) on the
formation of the upper CDW state in pure and doped La1-xRxAgSb2 (R = Ce, Nd)
compounds, by means of measurements of rho(T) for P < 23 kbar. We found that
the hydrostatic pressure, as well as the chemical pressure introduced by the
partial substitution of the smaller Ce and Nd ions for La, result in the
suppression of the CDW ground state, e.g. the reduction of the ordering
temperature T1. The values of dT1/dP are approx. 2-4 times higher for the
Ce-doped samples as compared to pure LaAgSb2, or even La0.75Nd0.25AgSb2
Nd-doped with a comparable T1 (P=0). This increased sensitivity to pressure may
be due to increasing Ce- hybridization under pressure. The magnetic ordering
temperature of the cerium-doped compounds is also reduced by pressure, and the
high pressure behavior of the Ce-doped samples is dominated by Kondo impurity
scattering.Comment: 22 pages, 11 figure
High current-carrying capability in c-axis-oriented superconducting MgB2 thin films
In high-quality c-axis-oriented MgB2 thin films, we observed high critical
current densities (Jc) of 16 MA/cm^2 at 15 K under self fields comparable to
those of cuprate high-temperature superconductors. The extrapolated value of Jc
at 5 K was estimated to be 40 MA/cm^2. For a magnetic field of 5 T, a Jc of 0.1
MA/cm^2 was detected at 15 K, suggesting that this compound would be a very
promising candidate for practical applications at high temperature and lower
power consumption. The vortex-glass phase is considered to be a possible
explanation for the observed high current-carrying capability.Comment: 3 pages and 4 figures, to be published in Physical Review Letter
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