710 research outputs found
The boiling suppression of liquid nitrogen
When He gas is injected from room temperature into boiling liquid N,
boiling is suppressed, leaving liquid surface flat like a mirror. Although the
qualitative explanation for this phenomenon is known [Minkoff G J \textit{et
al}. Nature 1957;180(4599):1413-4.], it has not been studied quantitatively and
comprehensively yet. In this report, we made careful simultaneous measurements
of temperature and weight variation of the liquid. The results clearly indicate
that the boiling suppression is caused by cooling of the liquid with "internal
evaporation" of N into the He bubbles.Comment: 14 pages, 7 figure
Determination of the mosaic angle distribution of Grafoil platelets using continuous-wave NMR spectra
We described details of a method to estimate with good accuracy the mosaic
angle distributions of microcrystallites (platelets) in exfoliated graphite
like Grafoil which is commonly used as an adsorption substrate for helium thin
films. The method is based on analysis of resonance field shifts in
continuous-wave (CW) NMR spectra of He ferromagnetic monolayers making
use of the large nuclear polarization of the adsorbate itself. The mosaic angle
distribution of a Grafoil substrate analyzed in this way can be well fitted to
a gaussian form with a deg spread. This distribution is quite
different from the previous estimation based on neutron scattering data which
showed an unrealistically large isotropic powder-like component.Comment: 6 pages, 5 figure
Dilution effect in correlated electron system with orbital degeneracy
Theory of dilution effect in orbital ordered system is presented. The
orbital model without spin degree of freedom and the spin-orbital coupled model
in a three-dimensional simple-cubic lattice are analyzed by the Monte-Carlo
simulation and the cluster expansion method. In the orbital model without
spin degree of freedom, reduction of the orbital ordering temperature due to
dilution is steeper than that in the dilute magnet. This is attributed to a
modification of the orbital wave-function around vacant sites. In the
spin-orbital coupled model, it is found that magnetic structure is changed from
the A-type antiferromagnetic order into the ferromagnetic one. Orbital
dependent exchange interaction and a sign change of this interaction around
vacant sites bring about this novel phenomena. Present results explain the
recent experiments in transition-metal compounds with orbital dilution.Comment: 12 pages, 16 figure
Dilution Effects in Two-dimensional Quantum Orbital System
We study dilution effects in a Mott insulating state with quantum orbital
degree of freedom, termed the two-dimensional orbital compass model. This is a
quantum and two-dimensional version of the orbital model where the interactions
along different bond directions cause frustration between different orbital
configurations. A long-range correlation of a kind of orbital at each row or
column, termed the directional order, is studied by means of the quantum
Monte-Carlo method. It is shown that decrease of the ordering temperature due
to dilution is much stronger than that in spin models. Quantum effect enhances
the effective dimensionality in the system and makes the directional order
robust against dilution. We discuss an essential mechanism of the dilute
orbital systems.Comment: 5pages, 4 figure
Laser control of magnonic topological phases in antiferromagnets
We study the laser control of magnon topological phases induced by the Aharonov-Casher effect in insulating antiferromagnets (AFs). Since the laser electric field can be considered as a time-periodic perturbation, we apply the Floquet theory and perform the inverse frequency expansion by focusing on the high frequency region. Using the obtained effective Floquet Hamiltonian, we study nonequilibrium magnon dynamics away from the adiabatic limit and its effect on topological phenomena. We show that a linearly polarized laser can generate helical edge magnon states and induce the magnonic spin Nernst effect, whereas a circularly polarized laser can generate chiral edge magnon states and induce the magnonic thermal Hall effect. In particular, in the latter, we find that the direction of the magnon chiral edge modes and the resulting thermal Hall effect can be controlled by the chirality of the circularly polarized laser through the change from the left-circular to the right-circular polarization. Our results thus provide a handle to control and design magnon topological properties in the insulating AF
A Local One-Zone Model of MHD Turbulence in Dwarf Nova Disks
The evolution of the magnetorotational instability (MRI) during the
transition from outburst to quiescence in a dwarf nova disk is investigated
using three-dimensional MHD simulations. The shearing box approximation is
adopted for the analysis, so that the efficiency of angular momentum transport
is studied in a small local patch of the disk: this is usually referred as to a
one-zone model. To take account of the low ionization fraction of the disk, the
induction equation includes both ohmic dissipation and the Hall effect. We
induce a transition from outburst to quiescence by an instantaneous decrease of
the temperature. The evolution of the MRI during the transition is found to be
very sensitive to the temperature of the quiescent disk. As long as the
temperature is higher than a critical value of about 2000 K, MHD turbulence and
angular momentum transport is sustained by the MRI. However, MHD turbulence
dies away within an orbital time if the temperature falls below this critical
value. In this case, the stress drops off by more than 2 orders of magnitude,
and is dominated by the Reynolds stress associated with the remnant motions
from the outburst. The critical temperature depends slightly on the distance
from the central star and the local density of the disk.Comment: 20 pages, 2 tables, 6 figures, accepted for publication in Ap
Numerical study of t2g orbital system with ferromagnetic polarization
Finite temperature orbital state in a ferromagnetic Mott insulator with
triply-degenerate orbital is investigated numerically. We employ the
quantum Monte Carlo simulation with the loop algorithm. Indications for
conventional staggered-type orbital order are not remarkable down to the lowest
temperature to which the present simulation can get access. Physical parameters
monitoring the off-diagonal orbital order, which is characterized by a linear
combination of the orbital-wave functions with equal
weights, are not conspicuous. It is found that a orbital gap-like behavior
appears in the uniform orbital susceptibility. This is supported by a threshold
behavior in the staggered correlation function in a calculation with the
additional Ising-type interaction. Some rigorous remarks for the long-range
orbital order are also presented.Comment: 5 pages, 3 figure
Josephson junction in cobalt-doped BaFe2As2 epitaxial thin films on (La, Sr)(Al, Ta)O3 bicrystal substrates
Josephson junctions were fabricated in epitaxial films of cobalt-doped
BaFe2As2 on [001]-tilt (La,Sr)(Al,Ta)O3 bicrystal substrates. 10m-wide
microbridges spanning a 30-degrees-tilted bicrystal grain boundary (BGB bridge)
exhibited resistively-shunted-junction (RSJ)-like current-voltage
characteristics up to 17 K, and the critical current was suppressed remarkably
by a magnetic field. Microbridges without a BGB did not show the RSJ-like
behavior, and their critical current densities were 20 times larger than those
of BGB bridges, confirming BGB bridges display a Josephson effect originating
from weakly-linked BGB
The Effect of the Hall Term on the Nonlinear Evolution of the Magnetorotational Instability: I. Local Axisymmetric Simulations
The effect of the Hall term on the evolution of the magnetorotational
instability (MRI) in weakly ionized accretion disks is investigated using local
axisymmetric simulations. First, we show that the Hall term has important
effects on the MRI when the temperature and density in the disk is below a few
thousand K and between 10^13 and 10^18 cm^{-3} respectively. Such conditions
can occur in the quiescent phase of dwarf nova disks, or in the inner part
(inside 10 - 100 AU) of protoplanetary disks. When the Hall term is important,
the properties of the MRI are dependent on the direction of the magnetic field
with respect to the angular velocity vector \Omega. If the disk is threaded by
a uniform vertical field oriented in the same sense as \Omega, the axisymmetric
evolution of the MRI is an exponentially growing two-channel flow without
saturation. When the field is oppositely directed to \Omega, however, small
scale fluctuations prevent the nonlinear growth of the channel flow and the MRI
evolves into MHD turbulence. These results are anticipated from the
characteristics of the linear dispersion relation. In axisymmetry on a field
with zero-net flux, the evolution of the MRI is independent of the size of the
Hall term relative to the inductive term. The evolution in this case is
determined mostly by the effect of ohmic dissipation.Comment: 31 pages, 3 tables, 12 figures, accepted for publication in ApJ,
postscript version also available from
http://www.astro.umd.edu/~sano/publications
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