942 research outputs found
Anomalous phase of MnP at very low field
Manganese phosphide MnP has been investigated for decades because of its rich
magnetic phase diagram. It is well known that the MnP exhibits the
ferromagnetic phase transition at \Tc=292 K and the helical magnetic phase
below \TN=47 K at zero field. Recently, a novel magnetic phase transition was
observed at K when the magnetic field is lower than 5 Oe. However,
the nature of the new phase has not been illuminated yet. In order to reveal
it, we performed the AC and the DC magnetization measurements for a single
crystal MnP at very low field. A divergent behavior of the real and the
imaginary part of the AC susceptibility and a sharp increase of the DC
magnetization was observed at , indicating the magnetic phase transition
at . Furthermore a peculiar temperature hysteresis was observed: namely,
the magnetization depends on whether cooling sample to the temperature lower
than \TN or not before the measurements. This hysteresis phenomenon suggests
the complicated nature of the new phase and a strong relation between the
magnetic state of the new phase and the helical structure.Comment: 4 pages, 2 figure
Disc-Jet coupling in the LMXB 4U1636-53 from INTEGRAL
We report on the spectral analysis results of the neutron star, atoll type,
low mass X-ray Binary 4U1636-53 observed by INTEGRAL and BeppoSAX satellites.
Spectral behavior in three different epochs corresponding to three different
spectral states has been deeply investigated. Two data set spectra show a
continuum well described by one or two soft blackbody plus a Comptonized
components with changes in the Comptonizing electrons and black body
temperature and the accretion rates, which are typical of the spectral
transitions from high to low state. In one occasion INTEGRAL spectrum shows,
for first time in this source, a hard tail dominating the emission above 30
keV. The total spectrum is fitted as the sum of a Comptonized component similar
to soft state and a power-law component (Gamma=2.76), indicating the presence
of a non thermal electron distribution of velocities. In this case, a
comparison with hard tails detected in soft states from neutron stars systems
and some black hole binaries suggests that a similar mechanism could originate
these components in both cases.Comment: 6 pages, 4 figures, 2 tables. accepted Ap
Possible Spin-triplet Superconductivity in NaxCoO2yH2O - 59Co NMR Study
We report 59Co NMR studies on the magnetically oriented powder samples of
Co-oxide superconductors NaxCoO2yH2O with Tc ~ 4.7 K. From two-dimensional
powder pattern in the NMR spectrum, the ab-plane Knight shift in the normal
state was estimated by the magnetic field dependence of second-order quadrupole
shifts at various temperatures. Below 50 K, the Knight shift shows a
Curie-Weiss-like temperature dependence, similarly to the bulk magnetic
susceptibility chi. From the analysis of so-called K-chi plot, the spin and the
orbital components of K and the positive hyperfine coupling constant were
estimated. The onset temperature of superconducting transition in the Knight
shift does not change so much in an applied magnetic field up to 7 T, which is
consistent with the reported high upper critical field Hc2. The Knight shift at
7 T shows an invariant behavior below Tc. No coherence peak just below Tc was
observed in the temperature dependence of the nuclear spin-lattice relaxation
rate 1/T1 in both cases of NMR and NQR. We conclude that the invariant behavior
of the Knight shift below Tc and unconventional behaviors of 1/T possibly
indicate the spin-triplet superconductivity with p- or f-wave symmetry.Comment: 17 pages, 10 figures. Extended versio
Structural phase transitions in multipole traps
A small number of laser-cooled ions trapped in a linear radiofrequency
multipole trap forms a hollow tube structure. We have studied, by means of
molecular dynamics simulations, the structural transition from a double ring to
a single ring of ions. We show that the single-ring configuration has the
advantage to inhibit the thermal transfer from the rf-excited radial components
of the motion to the axial component, allowing to reach the Doppler limit
temperature along the direction of the trap axis. Once cooled in this
particular configuration, the ions experience an angular dependency of the
confinement if the local adiabaticity parameter exceeds the empirical limit.
Bunching of the ion structures can then be observed and an analytic expression
is proposed to take into account for this behaviour
Non-Detection of Gravitationally Redshifted Absorption Lines in the X-ray Burst Spectra of GS 1826-24
During a 200 ks observation with the XMM-Newton Reflection Grating
Spectrometer, we detected 16 type-I X-ray bursts from GS 1826-24. We combined
the burst spectra in an attempt to measure the gravitational redshifts from the
surface of the neutron star. We divided the composite GS 1826-24 burst spectrum
into three groups based on the blackbody temperature during the bursts. The
spectra do not show any obvious discrete absorption lines. We compare our
observations with those of EXO 0748-676.Comment: 4 pages, 4 figures; accepted for publication in ApJ
Atomic Spectral Features During Thermonuclear Flashes on Neutron Stars
The gravitational redshift measured by Cottam, Paerels and Mendez for the
neutron star (NS) in the low-mass X-ray binary EXO 0748-676 depends on the
identification of an absorption line during a type I burst as the H
line from hydrogenic Fe. We show that Fe is present above the photosphere as
long as during the burst. In
this limit, the total Fe column is for incident material of solar abundances and only depends on the
nuclear physics of the proton spallation. The Fe destruction creates many heavy
elements with which may imprint photo-ionization edges on the NS spectra
during a radius expansion event or in a burst cooling tail. Detecting these
features in concert with those from Fe would confirm a redshift measurement. We
also begin to address the radiative transfer problem, and find that a
concentrated Fe layer with and column (depending on the line depth) above the hotter
continuum photosphere is required to create the H line of the observed
strength. This estimate must be refined by considerations of non-LTE effects as
well as resonant line transport. Until these are carried out, we cannot say
whether the Fe column from accretion and spallation is in conflict with the
observations. We also show that hydrogenic Fe might remain in the photosphere
due to radiative levitation from the high burst flux.Comment: Substantially revised version, to appear in Ap J Letter
Certification of Bounds of Non-linear Functions: the Templates Method
The aim of this work is to certify lower bounds for real-valued multivariate
functions, defined by semialgebraic or transcendental expressions. The
certificate must be, eventually, formally provable in a proof system such as
Coq. The application range for such a tool is widespread; for instance Hales'
proof of Kepler's conjecture yields thousands of inequalities. We introduce an
approximation algorithm, which combines ideas of the max-plus basis method (in
optimal control) and of the linear templates method developed by Manna et al.
(in static analysis). This algorithm consists in bounding some of the
constituents of the function by suprema of quadratic forms with a well chosen
curvature. This leads to semialgebraic optimization problems, solved by
sum-of-squares relaxations. Templates limit the blow up of these relaxations at
the price of coarsening the approximation. We illustrate the efficiency of our
framework with various examples from the literature and discuss the interfacing
with Coq.Comment: 16 pages, 3 figures, 2 table
Interplay between quantum criticality and geometrical frustration in Fe3Mo3N with stella quadrangula lattice
In the eta-carbide-type correlated-electron metal Fe3Mo3N, ferromagnetism is
abruptly induced from a nonmagnetic non-Fermi-liquid ground state either when a
magnetic field (~14 T) applied to it or when it is doped with a slight amount
of impurity (~5% Co). We observed a peak in the paramagnetic neutron scattering
intensity at finite wave vectors, revealing the presence of the
antiferromagnetic (AF) correlation hidden in the magnetic measurements. It
causes a new type of geometrical frustration in the stellla quadrangula lattice
of the Fe sublattice. We propose that the frustrated AF correlation suppresses
the F correlation to its marginal point and is therfore responsible for the
origin of the ferromagnetic (F) quantum critical behavior in pure Fe3Mo3N
Local spin and charge properties of beta-Ag0.33V2O5 studied by 51V NMR
Local spin and charge properties were studied on beta-Ag0.33V2O5, a
pressure-induced superconductor, at ambient pressure using 51V-NMR and
zero-field-resonance (ZFR) techniques. Three inequivalent Vi sites (i=1, 2, and
3) were identified from 51V-NMR spectra and the principal axes of the
electric-field-gradient (EFG) tensor were determined in a metallic phase and
the following charge-ordering phase. We found from the EFG analysis that the V1
sites are in a similar local environment to the V3 sites. This was also
observed in ZFR spectra as pairs of signals closely located with each other.
These results are well explained by a charge-sharing model where a 3d1 electron
is shared within a rung in both V1-V3 and V2-V2 two-leg ladders.Comment: 12pages, 16figure
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