5,895 research outputs found
Memory and rejuvenation in a spin glass
The temperature dependence of the magnetisation of a Cu(Mn) spin glass (
57 K) has been investigated using weak probing magnetic fields ( =
0.5 or 0 Oe) and specific thermal protocols. The behaviour of the zero-field
cooled, thermoremanent and isothermal remanent magnetisation on (re-)cooling
the system from a temperature (40 K) where the system has been aged is
investigated. It is observed that the measured magnetisation is formed by two
parts: (i) a temperature- and observation time-dependent thermally activated
relaxational part governed by the age- and temperature-dependent response
function and the (latest) field change made at a lower temperature, superposed
on (ii) a weakly temperature-dependent frozen-in part. Interestingly we observe
that the spin configuration that is imprinted during an elongated halt in the
cooling, if it is accompanied by a field induced magnetisation, also includes a
unidirectional excess magnetisation that is recovered on returning to the
ageing temperature.Comment: EPL style; 7 pages, 5 figure
X rays from old open clusters: M 67 and NGC 188
We have observed the old open clusters M 67 and NGC 188 with the ROSAT PSPC.
In M 67 we detect a variety of X-ray sources. The X-ray emission by a
cataclysmic variable, a single hot white dwarf, two contact binaries, and some
RS CVn systems is as expected. The X-ray emission by two binaries located below
the subgiant branch in the Hertzsprung Russell diagram of the cluster, by a
circular binary with a cool white dwarf, and by two eccentric binaries with
orbital period > 700 d is puzzling. Two members of NGC 188 are detected,
including the FK Com type star D719. Another possible FK Com type star,
probably not a member of NGC 188, is also detected.Comment: 10 pages, 5 figures. Accepted for publication on Astronomy &
Astrophysic
The Progeny of Stellar Dynamics and Stellar Evolution within an N-body model of NGC 188
We present a direct N-body simulation modeling the evolution of the old (7
Gyr) open cluster NGC 188. This is the first N-body open cluster simulation
whose initial binary population is directly defined by observations of a
specific open cluster: M35 (150 Myr). We compare the simulated color-magnitude
diagram at 7 Gyr to that of NGC 188, and discuss the blue stragglers produced
in the simulation. We compare the solar-type main sequence binary period and
eccentricity distributions of the simulation to detailed observations of
similar binaries in NGC 188. These results demonstrate the importance of
detailed observations in guiding N-body open cluster simulations. Finally, we
discuss the implications of a few discrepancies between the NGC 188 model and
observations and suggest a few methods for bringing N-body open cluster
simulations into better agreement with observations.Comment: Contributed talk at IAUS 266 'Star clusters: basic galactic building
blocks', R. de Grijs and J.R.D. Lepine, eds. 6 pages, 4 figure
The bicomplex quantum Coulomb potential problem
Generalizations of the complex number system underlying the mathematical
formulation of quantum mechanics have been known for some time, but the use of
the commutative ring of bicomplex numbers for that purpose is relatively new.
This paper provides an analytical solution of the quantum Coulomb potential
problem formulated in terms of bicomplex numbers. We define the problem by
introducing a bicomplex hamiltonian operator and extending the canonical
commutation relations to the form [X_i,P_k] = i_1 hbar xi delta_{ik}, where xi
is a bicomplex number. Following Pauli's algebraic method, we find the
eigenvalues of the bicomplex hamiltonian. These eigenvalues are also obtained,
along with appropriate eigenfunctions, by solving the extension of
Schrodinger's time-independent differential equation. Examples of solutions are
displayed. There is an orthonormal system of solutions that belongs to a
bicomplex Hilbert space.Comment: Clarifications; some figures removed; version to appear in Can. J.
Phy
Testing Protoplanetary Disk Alignment In Young Binaries
We present K-band (2.2 mum) imaging polarimetry that resolves 19 T Tauri binary and multiple systems in the Taurus-Auriga and Scorpius-Ophiuchus star-forming regions. We observed systems with projected separations 1. 5 - 7. 2 (similar to200 - 1000 AU) in order to determine the relative orientation of the circumstellar disks in each binary system. Scattered light from these disks is polarized, allowing us to deduce the position angle of the disk on the sky from the position angle of polarization even though our observations do not resolve the disks themselves. We detected measurable polarization ( typically 0.5% - 2%, with typical uncertainty 0.1%) from both stars in 14 of the systems observed. In eight of the nine binary systems, the two stars\u27 polarization position angles are within 30degrees of each other, inconsistent with random orientations. In contrast, the five triple and quadruple systems appear to have random disk orientations when comparing the polarization position angles of the widest pair in the system; the close pairs are unresolved in all but one system. Our observations suggest that disks in wide ( 200 - 1000 AU) binaries are aligned with each other within less than or similar to20degrees but not perfectly coplanar. However, we cannot conclusively rule out random relative disk orientations if the observed polarizations are significantly contaminated by interstellar polarization. Even in the presence of interstellar polarization our observations securely exclude coplanar disks. These results provide constraints on possible binary formation mechanisms if the observed orientations are primordial. On the other hand, models of disk-binary interactions indicate that the disks may have had time to decrease their relative inclinations since formation. If the common orientation of the disks in these binaries is a tracer of the binary orbital plane, then our results also have significance for the stability of planetary orbits, suggesting that planetary systems in wide binaries should be stable over 10(9) yr timescales
The nanoscale phase separation in hole-doped manganites
A macroscopic phase separation, in which ferromagnetic clusters are observed
in an insulating matrix, is sometimes observed, and believed to be essential to
the colossal magnetoresistive (CMR) properties of manganese oxides. The
application of a magnetic field may indeed trigger large magnetoresistance
effects due to the percolation between clusters allowing the movement of the
charge carriers. However, this macroscopic phase separation is mainly related
to extrinsic defects or impurities, which hinder the long-ranged charge-orbital
order of the system. We show in the present article that rather than the
macroscopic phase separation, an homogeneous short-ranged charge-orbital order
accompanied by a spin glass state occurs, as an intrinsic result of the
uniformity of the random potential perturbation induced by the solid solution
of the cations on the -sites of the structure of these materials. Hence the
phase separation does occur, but in a more subtle and interesting nanoscopic
form, here referred as ``homogeneous''. Remarkably, this ``nanoscale phase
separation'' alone is able to bring forth the colossal magnetoresistance in the
perovskite manganites, and is potentially relevant to a wide variety of other
magnetic and/or electrical properties of manganites, as well as many other
transition metal oxides, in bulk or thin film form as we exemplify throughout
the article.Comment: jpsj2 TeX style (J. Phys. Soc. Jpn); 18 pages, 7 figure
Isothermal remanent magnetization and the spin dimensionality of spin glasses
The isothermal remanent magnetization is used to investigate dynamical
magnetic properties of spatially three dimensional spin glasses with different
spin dimensionality (Ising, XY, Heisenberg). The isothermal remanent
magnetization is recorded vs. temperature after intermittent application of a
weak magnetic field at a constant temperature . We observe that in the
case of the Heisenberg spin glasses, the equilibrated spin structure and the
direction of the excess moment are recovered at . The isothermal remanent
magnetization thus reflects the directional character of the
Dzyaloshinsky-Moriya interaction present in Heisenberg systems.Comment: tPHL2e style; 7 page, 3 figure
Discovery of Par 1802 as a Low-Mass, Pre-Main-Sequence Eclipsing Binary in the Orion Star-Forming Region
We report the discovery of a pre-main-sequence, low-mass, double-lined,
spectroscopic, eclipsing binary in the Orion star-forming region. We present
our observations including radial velocities derived from optical
high-resolution spectroscopy, and present an orbit solution that permits the
determination of precise empirical masses for both components of the system. We
measure that Par 1802 is composed of two equal mass (0.39+-0.03, 0.40+-0.03
Msun) stars in a circular, 4.7 day orbit. There is strong evidence, such as the
system exhibiting strong Li lines and a center-of-mass velocity consistent with
cluster membership, that this system is a member of the Orion star-forming
region and quite possibly the Orion Nebula Cluster, and therefore has an age of
only a few million years. As there are currently only a few empirical mass and
radius measurements for low-mass, PMS stars, this system presents an
interesting test for the predictions of current theoretical models of pre-main
sequence stellar evolution.Comment: 21 pages, 6 figures, 2 tables; Fig 2 caption edite
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