4,773 research outputs found
Structural, Magnetic and Magneto-caloric studies of Ni50Mn30Sn20Shape Memory Alloy
We have synthesized a nominal composition of Ni50Mn30Sn20 alloy using arc
melting technique. Rietveld refinement confirms the austenite L21 structure in
Fm-3m space group. Electrical resistivity has been found to clearly exhibiting
two different phenomena viz. a magnetic transition from paramagnetic to
ferromagnetic and a structural transition from austenite to martensitic phase.
Thermo-magnetization measurements M(T) confirms ferromagnetic transition
temperature TC at 222 K and martensitic transition starting at 127 K(MS).
Magnetization measurement M(H) at 10 K confirms the ferromagnetic state.
Frequency dependence of ac susceptibility \c{hi}' at low temperature suggests
spin glass behavior in the system. The isothermal magnetic entropy change
values have been found to be 1.14 J/Kg.K, 2.69 J/Kg.K and 3.9 J/Kg.K, with
refrigeration capacities of 19.6 J/kg, 37.8 J/kg and 54.6 J/kg for the field
change of 1, 2 and 3 Tesla respectively at 227 K.Comment: 16 pages text + Figs. Ni50Mn30Sn20 alloy: reasonable refrigeration
capacity tunable to Room
A Study of starless dark cloud LDN 1570: Distance, Dust properties and Magnetic field geometry
We wish to map the magnetic field geometry and to study the dust properties
of the starless cloud, L1570, using multi-wavelength optical polarimetry and
photometry of the stars projected on the cloud. We made R-band imaging
polarimetry of the stars projected on a cloud, L1570, to trace the magnetic
field orientation. We also made multi-wavelength polarimetric and photometric
observations to constrain the properties of dust in L1570. We estimated a
distance of 394 +/- 70 pc to the cloud using 2MASS JHKs colours. Using the
values of the Serkowski parameters namely , ,
{\lambda}max and the position of the stars on near infrared color-color
diagram, we identified 13 stars that could possibly have intrinsic polarization
and/or rotation in their polarization angles. One star, 2MASS
J06075075+1934177, which is a B4Ve spectral type, show the presence of diffuse
interstellar bands in the spectrum apart from showing H{\alpha} line in
emission. There is an indication for the presence of slightly bigger dust
grains towards L1570 on the basis of the dust grain size-indicators such as
{\lambda}max and Rv values. The magnetic field lines are found to be parallel
to the cloud structures seen in the 250{\mu}m images (also in 8{\mu}m and
12{\mu}m shadow images) of L1570. Based on the magnetic field geometry, the
cloud structure and the complex velocity structure, we believe that L1570 is in
the process of formation due to the converging flow material mediated by the
magnetic field lines. Structure function analysis showed that in the L1570
cloud region the large scale magnetic fields are stronger when compared with
the turbulent component of magnetic fields. The estimated magnetic field
strengths suggest that the L1570 cloud region is sub-critical and hence could
be strongly supported by the magnetic field lines.Comment: 26 pages, 22 figures, and 7 tables; Accepted for its publication in
A&
Multiwavelength Study of NGC 281 Region
We present a multiwavelength study of the NGC 281 complex which contains the
young cluster IC 1590 at the center, using deep wide-field optical UBVI_c
photometry, slitless spectroscopy along with archival data sets in the
near-infrared (NIR) and X-ray. The extent of IC 1590 is estimated to be ~6.5
pc. The cluster region shows a relatively small amount of differential
reddening. The majority of the identified young stellar objects (YSOs) are low
mass PMS stars having age <1-2 Myr and mass 0.5-3.5 M_\odot. The slope (\Gamma)
of the mass function for IC 1590, in the mass range 2 < M/M_\odot \le 54, is
found to be -1.11+-0.15. The slope of the K-band luminosity function
(0.37+-0.07) is similar to the average value (~0.4) reported for young
clusters. The distribution of gas and dust obtained from the IRAS, CO and radio
maps indicates clumpy structures around the central cluster. The radial
distribution of the young stellar objects, their ages, \Delta(H-K) NIR-excess,
and the fraction of classical T Tauri stars suggest triggered star formation at
the periphery of the cluster region. However, deeper optical, NIR and MIR
observations are needed to have a conclusive view of star formation scenario in
the region. The properties of the Class 0/I and Class II sources detected by
using the Spitzer mid-infrared observations indicate that a majority of the
Class II sources are X-ray emitting stars, whereas X-ray emission is absent
from the Class 0/I sources. The spatial distribution of Class 0/I and Class II
sources reveals the presence of three sub-clusters in the NGC 281 West region.Comment: 29 pages, 21 figures and 11 tables, Accepted for the publication in
PAS
Optical and Near-infrared survey of the stellar contents associated with the star-forming Complex Sh2-252
We present the analyses of the stellar contents associated with the HII
region Sh2-252 using UBVRI photometry, slit and slitless spectroscopy along
with the NIR data from 2MASS for an area ~1 degree x 1 degree. We studied the
sub-regions of Sh2-252 which includes four compact-HII (CHII) regions, namely
A, B, C and E and two clusters NGC 2175s and Teutsch 136 (Teu 136). Of the
fifteen spectroscopically observed bright stars, eight have been identified as
massive members of spectral class earlier than B3. From the spectro-photometric
analyses, we derived the average distance of the region as 2.4+/-0.2 kpc and
the reddening of the massive members is found to vary between 0.35 to 2.1 mag.
We found that NGC 2175s and Teu 136, located towards the eastern edge of the
complex are the sub-clusters of Sh2-252. The stellar surface density
distribution in K-band shows clustering associated with the regions A, C, E,
NGC 2175s and Teu 136. We have also identified the candidate ionizing sources
of the CHII regions. 61 H_alpha emission sources are identified using slitless
spectroscopy. The distribution of the H_alpha emission sources and candidate
YSOs with IR excess on the V/(V-I) CMD shows that a majority of them have
approximate ages between 0.1 - 5 Myr and masses in the range of 0.3 - 2.5
M_sun. The CMDs of the candidate YSOs in the individual regions also show an
age spread of 0.1 - 5 Myr for each of them. We calculated the KLFs for the
sub-regions A, C, E, NGC 2175s and Teu 136. Within errors, the KLFs for all the
sub-regions are found to be similar and comparable to that of young clusters of
age < 5 Myr. We also estimated the mass functions (MFs) of the PMS sample of
the individual regions in the mass range of 0.3 - 2.5 M_sun. In general, the
slopes of the MFs of all the sub-regions are found comparable to the Salpeter
value.Comment: published in MNRA
Band mass anisotropy and the intrinsic metric of fractional quantum Hall systems
It was recently pointed out that topological liquid phases arising in the
fractional quantum Hall effect (FQHE) are not required to be rotationally
invariant, as most variational wavefunctions proposed to date have been.
Instead, they possess a geometric degree of freedom corresponding to a shear
deformation that acts like an intrinsic metric. We apply this idea to a system
with an anisotropic band mass, as is intrinsically the case in many-valley
semiconductors such as AlAs and Si, or in isotropic systems like GaAs in the
presence of a tilted magnetic field, which breaks the rotational invariance. We
perform exact diagonalization calculations with periodic boundary conditions
(torus geometry) for various filling fractions in the lowest, first and second
Landau levels. In the lowest Landau level, we demonstrate that FQHE states
generally survive the breakdown of rotational invariance by moderate values of
the band mass anisotropy. At 1/3 filling, we generate a variational family of
Laughlin wavefunctions parametrized by the metric degree of freedom. We show
that the intrinsic metric of the Laughlin state adjusts as the band mass
anisotropy or the dielectric tensor are varied, while the phase remains robust.
In the n=1 Landau level, mass anisotropy drives transitions between
incompressible liquids and compressible states with charge density wave
ordering. In n>=2 Landau levels, mass anisotropy selects and enhances stripe
ordering with compatible wave vectors at partial 1/3 and 1/2 fillings.Comment: 9 pages, 8 figure
Phase Transition in the Three-Dimensional Ising Spin Glass
We have studied the three-dimensional Ising spin glass with a
distribution by Monte Carlo simulations. Using larger sizes and much better
statistics than in earlier work, a finite size scaling analysis shows quite
strong evidence for a finite transition temperature, , with ordering below
. Our estimate of the transition temperature is rather lower than in
earlier work, and the value of the correlation length exponent, , is
somewhat higher. Because there may be (unknown) corrections to finite size
scaling, we do not completely rule out the possibility that or that
is finite but with no order below . However, from our data, these
possibilities seem less likely.Comment: Postscript file compressed using uufiles. The postscript file is also
available by anonymous ftp at ftp://chopin.ucsc.edu/pub/sg3d.p
Two dimensional Ising spin glasses with non-zero ordering temperatures
We demonstrate numerically that for Ising spins on square lattices with
ferromagnetic second neighbour interactions and random near neighbour
interactions, two dimensional Ising spin glass order with a non-zero freezing
temperature can occur. We compare some of the physical properties of these spin
glasses with those of standard spin glasses in higher dimensions.Comment: 9 page latex file and 9 ps figures. To appear in Phys. Rev. Let
The COMPLETE Nature of the Warm Dust Ring in Perseus
The Perseus molecular cloud complex is a ~30pc long chain of molecular clouds
most well-known for the two star-forming clusters NGC1333 and IC348 and the
well-studied outflow source in B5. However, when studied at mid- to
far-infrared wavelengths the region is dominated by a ~10pc diameter shell of
warm dust, likely generated by an HII region caused by the early B-star
HD278942. Using a revised calibration technique the COMPLETE team has produced
high-sensitivity temperature and column-density maps of the Perseus region from
IRAS Sky Survey Atlas (ISSA) 60 and 100um data. In this paper, we combine the
ISSA based dust-emission maps with other observations collected as part of the
COMPLETE Survey, along with archival H-alpha and MSX observations. Molecular
line observations from FCRAO and extinction maps constructed by applying the
NICER method to the 2MASS catalog provide independent estimates of the ``true''
column-density of the shell. H-alpha emission in the region of the shell
confirms that it is most likely an HII region located behind the cloud complex,
and 8um data from MSX indicates that the shell may be interacting with the
cloud. Finally, the two polarisation components previously seen towards
background stars in the region can be explained by the association of the
stronger component with the shell. If confirmed, this would be the first
observation of a parsec-scale swept-up magnetic field.Comment: Accepted by ApJ. Figures have been compressed - full resolution
version available at http://cfa-www.harvard.edu/COMPLETE/results.htm
Bound Magnetic Polaron Interactions in Insulating Doped Diluted Magnetic Semiconductors
The magnetic behavior of insulating doped diluted magnetic semiconductors
(DMS) is characterized by the interaction of large collective spins known as
bound magnetic polarons. Experimental measurements of the susceptibility of
these materials have suggested that the polaron-polaron interaction is
ferromagnetic, in contrast to the antiferromagnetic carrier-carrier
interactions that are characteristic of nonmagnetic semiconductors. To explain
this behavior, a model has been developed in which polarons interact via both
the standard direct carrier-carrier exchange interaction (due to virtual
carrier hopping) and an indirect carrier-ion-carrier exchange interaction (due
to the interactions of polarons with magnetic ions in an interstitial region).
Using a variational procedure, the optimal values of the model parameters were
determined as a function of temperature. At temperatures of interest, the
parameters describing polaron-polaron interactions were found to be nearly
temperature-independent. For reasonable values of these constant parameters, we
find that indirect ferromagnetic interactions can dominate the direct
antiferromagnetic interactions and cause the polarons to align. This result
supports the experimental evidence for ferromagnetism in insulating doped DMS.Comment: 11 pages, 7 figure
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