457 research outputs found
Development of Absolute Frequency Reference at 1064 nm for Coherent Transmission and Laser Drift Measurement
Monte Carlo simulations of , a classical Heisenberg antiferromagnet in two-dimensions with dipolar interaction
We study the phase diagram of a quasi-two dimensional magnetic system with Monte Carlo simulations of a classical Heisenberg spin
Hamiltonian which includes the dipolar interactions between
spins. Our simulations reveal an Ising-like antiferromagnetic phase at low
magnetic fields and an XY phase at high magnetic fields. The boundary between
Ising and XY phases is analyzed with a recently proposed finite size scaling
technique and found to be consistent with a bicritical point at T=0. We discuss
the computational techniques used to handle the weak dipolar interaction and
the difference between our phase diagram and the experimental results.Comment: 13 pages 18 figure
Hidden zero-temperature bicritical point in the two-dimensional anisotropic Heisenberg model: Monte Carlo simulations and proper finite-size scaling
By considering the appropriate finite-size effect, we explain the connection
between Monte Carlo simulations of two-dimensional anisotropic Heisenberg
antiferromagnet in a field and the early renormalization group calculation for
the bicritical point in dimensions. We found that the long length
scale physics of the Monte Carlo simulations is indeed captured by the
anisotropic nonlinear model. Our Monte Carlo data and analysis confirm
that the bicritical point in two dimensions is Heisenberg-like and occurs at
T=0, therefore the uncertainty in the phase diagram of this model is removed.Comment: 10 pages, 11 figure
The Pulsed Neutron Beam EDM Experiment
We report on the Beam EDM experiment, which aims to employ a pulsed cold
neutron beam to search for an electric dipole moment instead of the established
use of storable ultracold neutrons. We present a brief overview of the basic
measurement concept and the current status of our proof-of-principle Ramsey
apparatus
Induced four fold anisotropy and bias in compensated NiFe/FeMn double layers
A vector spin model is used to show how frustrations within a multisublattice
antiferromagnet such as FeMn can lead to four-fold magnetic anisotropies acting
on an exchange coupled ferromagnetic film. Possibilities for the existence of
exchange bias are examined and shown to exist for the case of weak chemical
disorder at the interface in an otherwise perfect structure. A sensitive
dependence on interlayer exchange is found for anisotropies acting on the
ferromagnet through the exchange coupling, and we show that a wide range of
anisotropies can appear even for a perfect crystalline structure with an
ideally flat interface.Comment: 7 pages, 7 figure
Long-term Dynamics of the Electron-nuclear Spin System of a Semiconductor Quantum Dot
A quasi-classical theoretical description of polarization and relaxation of
nuclear spins in a quantum dot with one resident electron is developed for
arbitrary mechanisms of electron spin polarization. The dependence of the
electron-nuclear spin dynamics on the correlation time of electron
spin precession, with frequency , in the nuclear hyperfine field is
analyzed. It is demonstrated that the highest nuclear polarization is achieved
for a correlation time close to the period of electron spin precession in the
nuclear field. For these and larger correlation times, the indirect hyperfine
field, which acts on nuclear spins, also reaches a maximum. This maximum is of
the order of the dipole-dipole magnetic field that nuclei create on each other.
This value is non-zero even if the average electron polarization vanishes. It
is shown that the transition from short correlation time to
does not affect the general structure of the equation for nuclear spin
temperature and nuclear polarization in the Knight field, but changes the
values of parameters, which now become functions of . For
correlation times larger than the precession time of nuclei in the electron
hyperfine field, it is found that three thermodynamic potentials (,
, ) characterize the polarized electron-nuclear spin
system. The values of these potentials are calculated assuming a sharp
transition from short to long correlation times, and the relaxation mechanisms
of these potentials are discussed. The relaxation of the nuclear spin potential
is simulated numerically showing that high nuclear polarization decreases
relaxation rate.Comment: RevTeX 4, 12 pages, 9 figure
Monte Carlo simulations of ordering in ferromagnetic-antiferromagnetic bilayers
Monte Carlo simulations have been used to study phase transitions on coupled
anisotropic ferro/antiferromagnetic (FM/AFM) films of classical Heisenberg
spins. We consider films of different thicknesses, with fully compensated
exchange across the FM/AFM interface. We find indications of a phase transition
on each film, occuring at different temperatures. It appears that both
transition temperatures depend on the film thickness.Comment: Revtex, 4 pages, 4 figure
Mucosal Immunity in Toxoplasma Gondii Infection
Toxoplasma gondii is an intracellular parasite that frequently infects a large spectrum of warm-blooded animals. This parasite induces abortion and establishes both chronic and silent infections, particularly in the brain. Parasite penetration into the host activates a strong anti-parasite immune response. In the present paper, we will discuss the interplay between innate and adaptive immunity that occurs within the infected intestine to clear the parasite and to maintain intestinal homeostasis despite the exacerbation of an inflammatory immune response
Mott Transition of MnO under Pressure: Comparison of Correlated Band Theories
The electronic structure, magnetic moment, and volume collapse of MnO under
pressure are obtained from four different correlated band theory methods; local
density approximation + Hubbard U (LDA+U), pseudopotential self-interaction
correction (pseudo-SIC), the hybrid functional (combined local exchange plus
Hartree-Fock exchange), and the local spin density SIC (SIC-LSD) method. Each
method treats correlation among the five Mn 3d orbitals (per spin), including
their hybridization with three O orbitals in the valence bands and their
changes with pressure. The focus is on comparison of the methods for rocksalt
MnO (neglecting the observed transition to the NiAs structure in the 90-100 GPa
range). Each method predicts a first-order volume collapse, but with variation
in the predicted volume and critical pressure. Accompanying the volume collapse
is a moment collapse, which for all methods is from high-spin to low-spin (5/2
to 1/2), not to nonmagnetic as the simplest scenario would have. The specific
manner in which the transition occurs varies considerably among the methods:
pseudo-SIC and SIC-LSD give insulator-to-metal, while LDA+U gives
insulator-to-insulator and the hybrid method gives an insulator-to-semimetal
transition. Projected densities of states above and below the transition are
presented for each of the methods and used to analyze the character of each
transition. In some cases the rhombohedral symmetry of the
antiferromagnetically ordered phase clearly influences the character of the
transition.Comment: 14 pages, 9 figures. A 7 institute collaboration, Updated versio
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