27,644 research outputs found
On the accuracy of conservation of adiabatic invariants in slow-fast systems
Let the adiabatic invariant of action variable in slow-fast Hamiltonian
system with two degrees of freedom have two limiting values along the
trajectories as time tends to infinity. The difference of two limits is
exponentially small in analytic systems. An iso-energetic reduction and
canonical transformations are applied to transform the slow-fast systems to
form of systems depending on slowly varying parameters in a complexified phase
space. On the basis of this method an estimate for the accuracy of conservation
of adiabatic invariant is given for such systems.Comment: 27 pages, 14 figure
Fulde-Ferrel-Larkin-Ovchinnikov Inhomogeneous Superconducting State and Phase Transitions Induced by Spin Accumulation in a Ferromagnet--Wave Superconductor-Ferromagnet Tunnel Junction
Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) inhomogeneous superconducting (SC)
state, first- and second-order phase transitions, and quantum criticality
induced by spin accumulation in a ferromagnet--wave
superconductor-ferromagnet tunnel junction are theoretically predicted. A
complex phase diagram in the temperature-bias voltage plane is determined. It
is found that the phase transitions from the homogeneous BCS state to the
inhomogeneous FFLO state, and from the FFLO state with the momentum 's azimuthal angle to that with , are of the first-order; while the transitions from all SC
states to the normal state at critical voltages are of the second-order. A
Lifshitz point, a bicritical point and a quantum critical point are identified.Comment: 5 pages, 5 figure
Analytical Results for Cold Asymmetrical Fermion Superfluids at the Mean-Field Level
We present the analytical results at the mean-field level for the
asymmetrical fermion system with attractive contact interaction at the zero
temperature. The results can be expressed in terms of linear combinations of
the elliptic integrals of the first and second kinds. In the limit of small gap
parameter, we discuss how the asymmetry in fermion species affects the phases
of the ground state. In the limit of large gap parameter, we show that two
candidate phases are competing for the system's ground state. The Sarma phase
containing a pure Fermi fluid and a mixed condensate is favored at large degree
of asymmetry. The separated phase consisting of a pure Fermi fluid and a boson
condensate supports the system at smaller degree of asymmetry. The two phases
are degenerate in the limit of infinite pairing gap.Comment: 23 pages, no figur
Detection of a new methanol maser line with ALMA
Aims. We aimed at investigating the structure and kinematics of the gaseous
disk and outflows around the massive YSO S255 NIRS3 in the S255IR-SMA1 dense
clump. Methods. Observations of the S255IR region were carried out with ALMA at
two epochs in the compact and extended configurations. Results. We
serendipitously detected a new, never predicted, bright maser line at about
349.1 GHz, which most probably represents the CHOH A transition. The emission covers most of the 6.7 GHz methanol maser emission
area of almost 1 in size and shows a velocity gradient in the
same sense as the disk rotation. No variability was found on the time interval
of several months. It is classified as Class II maser and probably originates
in a ring at a distance of several hundreds AU from the central star.Comment: 4 pages, 4 figures, accepted by Astronomy and Astrophysic
Atomistic origins of the phase transition mechanism in Ge2Sb2Te5
Combined static and molecular dynamics first-principles calculations are used
to identify a direct structural link between the metastable crystalline and
amorphous phases of Ge2Sb2Te5. We find that the phase transition is driven by
the displacement of Ge atoms along the rocksalt [111] direction from the
stable-octahedron to high-energy-unstable tetrahedron sites close to the
intrinsic vacancy regions, which give rise to the formation of local 4-fold
coordinated motifs. Our analyses suggest that the high figures of merit of
Ge2Sb2Te5 are achieved from the optimal combination of intrinsic vacancies
provided by Sb2Te3 and the instability of the tetrahedron sites provided by
GeTe
Comment on "Giant Plasticity of a Quantum Crystal"
In their Letter, Haziot et al. [Phys. Rev. Lett. 110 (2013) 035301] report a
novel phenomenon of giant plasticity for hcp Helium-4 quantum crystals. They
assert that Helium-4 exhibits mechanical properties not found in classical
plasticity theory. Specifically, they examine high-quality crystals as a
function of temperature and applied strain, where the shear modulus reaches a
plateau and dissipation becomes close to zero; both quantities are reported to
be independent of stress and strain, implying a reversible dissipation process
and quantum tunneling. In this Comment, we show that these signatures can be
explained with a classical model of thermally activated dislocation glide
without the need to invoke quantum tunneling or dissipationless motion.
Recently, we proposed a dislocation glide model in solid Helium-4 containing
the dissipation contribution in the presence of other dislocations with
qualitatively similar behavior [Zhou et al., Philos. Mag. Lett. 92 (2012) 608].Comment: 1 page, 1 figure, comment; minor revision
The Fano resonance for Anderson impurity systems
We present a general theory for the Fano resonance in Anderson impurity
systems. It is shown that the broadening of the impurity level leads to an
additional and important contribution to the Fano resonance around the Fermi
surface, especially in the mixed valence regime. This contribution results from
the interference between the Kondo resonance and the broadened impurity level.
Being applied to the scanning tunnelling microscopic experiments, we find that
our theory gives a consistent and quantitative account for the Fano resonance
lineshapes for both Co and Ti impurities on Au or Ag surfaces. The Ti systems
are found to be in the mixed valence regime.Comment: 4 pages, 5 figures, published versio
Infrared Emission by Dust Around lambda Bootis Stars: Debris Disks or Thermally Emitting Nebulae?
We present a model that describes stellar infrared excesses due to heating of
the interstellar (IS) dust by a hot star passing through a diffuse IS cloud.
This model is applied to six lambda Bootis stars with infrared excesses.
Plausible values for the IS medium (ISM) density and relative velocity between
the cloud and the star yield fits to the excess emission. This result is
consistent with the diffusion/accretion hypothesis that lambda Bootis stars (A-
to F-type stars with large underabundances of Fe-peak elements) owe their
characteristics to interactions with the ISM. This proposal invokes radiation
pressure from the star to repel the IS dust and excavate a paraboloidal dust
cavity in the IS cloud, while the metal-poor gas is accreted onto the stellar
photosphere. However, the measurements of the infrared excesses can also be fit
by planetary debris disk models. A more detailed consideration of the
conditions to produce lambda Bootis characteristics indicates that the majority
of infrared-excess stars within the Local Bubble probably have debris disks.
Nevertheless, more distant stars may often have excesses due to heating of
interstellar material such as in our model.Comment: 10 pages, 5 figures, 4 tables, accepted by ApJ, emulateap
Superconductivity in Ca-doped graphene
Graphene, a zero-gap semimetal, can be transformed into a metallic,
semiconducting or insulating state by either physical or chemical modification.
Superconductivity is conspicuously missing among these states despite
considerable experimental efforts as well as many theoretical proposals. Here,
we report superconductivity in calcium-decorated graphene achieved by
intercalation of graphene laminates that consist of well separated and
electronically decoupled graphene crystals. In contrast to intercalated
graphite, we find that Ca is the only dopant that induces superconductivity in
graphene laminates above 1.8 K among intercalants used in our experiments such
as potassium, caesium and lithium. Ca-decorated graphene becomes
superconducting at ~ 6 K and the transition temperature is found to be strongly
dependent on the confinement of the Ca layer and the induced charge carrier
concentration. In addition to the first evidence for superconducting graphene,
our work shows a possibility of inducing and studying superconductivity in
other 2D materials using their laminates
Genome-wide profiling of chromosome interactions in Plasmodium falciparum characterizes nuclear architecture and reconfigurations associated with antigenic variation.
Spatial relationships within the eukaryotic nucleus are essential for proper nuclear function. In Plasmodium falciparum, the repositioning of chromosomes has been implicated in the regulation of the expression of genes responsible for antigenic variation, and the formation of a single, peri-nuclear nucleolus results in the clustering of rDNA. Nevertheless, the precise spatial relationships between chromosomes remain poorly understood, because, until recently, techniques with sufficient resolution have been lacking. Here we have used chromosome conformation capture and second-generation sequencing to study changes in chromosome folding and spatial positioning that occur during switches in var gene expression. We have generated maps of chromosomal spatial affinities within the P. falciparum nucleus at 25 Kb resolution, revealing a structured nucleolus, an absence of chromosome territories, and confirming previously identified clustering of heterochromatin foci. We show that switches in var gene expression do not appear to involve interaction with a distant enhancer, but do result in local changes at the active locus. These maps reveal the folding properties of malaria chromosomes, validate known physical associations, and characterize the global landscape of spatial interactions. Collectively, our data provide critical information for a better understanding of gene expression regulation and antigenic variation in malaria parasites
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