14,832 research outputs found
The Effect of Hydrostatic Weighting on the Vertical Temperature Structure of the Solar Corona
We investigate the effect of hydrostatic scale heights in
coronal loops on the determination of the vertical temperature structure
of the solar corona. Every method that determines an average temperature at a
particular line-of-sight from optically thin emission (e.g. in EUV or soft
X-ray wavelengths) of a mutli-temperature plasma, is subject to the emission
measure-weighted contributions from different temperatures. Because
most of the coronal structures (along open or closed field lines) are close to
hydrostatic equilibrium, the hydrostatic temperature scale height introduces a
height-dependent weighting function that causes a systematic bias in the
determination of the temperature structure as function of altitude .
The net effect is that the averaged temperature seems to increase with
altitude, , even if every coronal loop (of a multi-temperature
ensemble) is isothermal in itself. We simulate this effect with differential
emission measure distributions observed by {\sl SERTS} for an instrument with a
broadband temperature filter such as {\sl Yohkoh/SXT} and find that the
apparent temperature increase due to hydrostatic weighting is of order \Delta
T \approx T_0 \times h/r_{\sun}. We suggest that this effect largely explains
the systematic temperature increase in the upper corona reported in recent
studies (e.g. by Sturrock et al., Wheatland et al., or Priest et al.), rather
than being an intrinsic signature of a coronal heating mechanism.Comment: 7 pages, 3 figures. ApJ Letters, accepted 2000 April 6, in pres
Immense magnetic response of exciplex light emission due to correlated spin-charge dynamics
As carriers slowly move through a disordered energy landscape in organic
semiconductors, tiny spatial variations in spin dynamics relieve spin blocking
at transport bottlenecks or in the electron-hole recombination process that
produces light. Large room-temperature magnetic-field effects (MFE) ensue in
the conductivity and luminescence. Sources of variable spin dynamics generate
much larger MFE if their spatial structure is correlated on the nanoscale with
the energetic sites governing conductivity or luminescence such as in
co-evaporated organic blends within which the electron resides on one molecule
and the hole on the other (an exciplex). Here we show that exciplex
recombination in blends exhibiting thermally-activated delayed fluorescence
(TADF) produces MFE in excess of 60% at room temperature. In addition, effects
greater than 4000% can be achieved by tuning the device's current-voltage
response curve by device conditioning. These immense MFEs are both the largest
reported values for their device type at room temperature. Our theory traces
this MFE and its unusual temperature dependence to changes in spin mixing
between triplet exciplexes and light-emitting singlet exciplexes. In contrast,
spin mixing of excitons is energetically suppressed, and thus spin mixing
produces comparatively weaker MFE in materials emitting light from excitons by
affecting the precursor pairs. Demonstration of immense MFE in common organic
blends provides a flexible and inexpensive pathway towards magnetic
functionality and field sensitivity in current organic devices without
patterning the constituent materials on the nanoscale. Magnetic fields increase
the power efficiency of unconditioned devices by 30% at room temperature, also
showing that magnetic fields may increase the efficiency of the TADF process.Comment: 12 pages, PRX in pres
Liquid-liquid interfacial tension of electrolyte solutions
It is theoretically shown that the excess liquid-liquid interfacial tension
between two electrolyte solutions as a function of the ionic strength I behaves
asymptotically as O(- I^0.5) for small I and as O(+- I) for large I. The former
regime is dominated by the electrostatic potential due to an unequal
partitioning of ions between the two liquids whereas the latter regime is
related to a finite interfacial thickness. The crossover between the two
asymptotic regimes depends sensitively on material parameters suggesting that,
depending on the actual system under investigation, the experimentally
accessible range of ionic strengths can correspond to either the small or the
large ionic strength regime. In the limiting case of a liquid-gas surface where
ion partitioning is absent, the image chage interaction can dominate the
surface tension for small ionic strength I such that an Onsager-Samaras
limiting law O(- I ln(I)) is expected. The proposed picture is consistent with
more elaborate models and published measurements.Comment: Accepted for publication in Physical Review Letter
Effective chiral restoration in the rho'-meson in lattice QCD
In simulations with dynamical quarks it has been established that the ground
state rho in the infrared is a strong mixture of the two chiral representations
(0,1)+(1,0) and (1/2,1/2)_b. Its angular momentum content is approximately the
3S1 partial wave which is consistent with the quark model. Effective chiral
restoration in an excited rho-meson would require that in the infrared this
meson couples predominantly to one of the two representations. The variational
method allows one to study the mixing of interpolators with different chiral
transformation properties in the non-perturbatively determined excited state at
different resolution scales. We present results for the first excited state of
the rho-meson using simulations with n_f=2 dynamical quarks. We point out, that
in the infrared a leading contribution to rho'= rho(1450) comes from
(1/2,1/2)_b, in contrast to the rho. Its approximate chiral partner would be a
h_1(1380) state. The rho' wave function contains a significant contribution of
the 3D1 wave which is not consistent with the quark model prediction.Comment: 4 pp, a few short remarks have been added, a reference updated. To
appear in PR
Condensate density and superfluid mass density of a dilute Bose gas near the condensation transition
We derive, through analysis of the structure of diagrammatic perturbation
theory, the scaling behavior of the condensate and superfluid mass density of a
dilute Bose gas just below the condensation transition. Sufficiently below the
critical temperature, , the system is governed by the mean field
(Bogoliubov) description of the particle excitations. Close to , however,
mean field breaks down and the system undergoes a second order phase
transition, rather than the first order transition predicted in Bogoliubov
theory. Both condensation and superfluidity occur at the same critical
temperature, and have similar scaling functions below , but
different finite size scaling at to leading order in the system size.
Through a simple self-consistent two loop calculation we derive the critical
exponent for the condensate fraction, .Comment: 4 page
Agrin isoforms and their role in synaptogenesis
Agrin is thought to mediate the motor neuron-induced aggregation of synaptic proteins on the surface of muscle fibers at neuromuscular junctions. Recent experiments provide direct evidence in support of this hypothesis, reveal the nature of agrin immunoreactivity at sites other than neuromuscular junctions, and have resulted in findings that are consistent with the possibility that agrin plays a role in synaptogenesis throughout the nervous system
ALLOREACTIVE T LYMPHOCYTES CULTURED FROM LIVER TRANSPLANT BIOPSIES: ASSOCIATIONS OF HLA SPECIFICITY WITH CLINICOPATHOLOGICAL FINDINGS.
Lymphocyte cultures grown from liver allograft biopsies were shown to exhibit alloreactivity towards donor cells as measured by primed lymphocyte testing (PLT). The PLT specificity was determined in assays using HLA typed panel cells and/or by inhibition testing with HLA specific monoclonal antibodies. Certain cultures exhibited PLT specificity towards class I HLA antigens of the donor, whereas others were specific for class II HLA antigens or recognized mixtures of class I and II antigens. These PLT specificity patterns were compared with clinical, histological and laboratory findings on the liver transplant patients at the time of the biopsy. Biopsies yielding class I specific PLT cells were taken generally during the earlier posttransplant period, whereas class II specific cells were grown from later biopsies. There was no significant correlation of the PLT specificity towards class I vs II antigens with the levels of total or direct bilirubin, serum glutamate oxaloacetic transaminase (SGOT), and serum glutamate pyruvate transaminase (SGPT), although a trend towards higher values was noted for biopsies presenting with a class II specific infiltrate. However, the levels of gamma glutamyl transpeptidase (GGTP) and alkaline phosphatase (AP) were significantly increased when biopsies yielded class II specific rather than class I specific PLT cells. Biopsy histology showed more damage to bile duct epithelium in association with class II PLT specificity whereas intense but often reversible infiltrates were found in biopsies yielding class I specific cells. The elevated GGTP and AP levels are probably related to the interaction of class II specific T cells with bile duct epithelium, which has been shown to express induced class II HLA antigens on their cell surface
Driven Spin Systems as Quantum Thermodynamic Machines: Fundamental Limits
We show that coupled two level systems like qubits studied in quantum
information can be used as a thermodynamic machine. At least three qubits or
spins are necessary and arranged in a chain. The system is interfaced between
two split baths and the working spin in the middle is externally driven. The
machine performs Carnot-type cycles and is able to work as heat pump or engine
depending on the temperature difference of the baths and the energy
differences in the spin system . It can be shown that the efficiency
is a function of and .Comment: 9 pages, 11 figures, accepted for publication in Phys. Rev.
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