11,561 research outputs found
Chaotic motion of space charge wavefronts in semiconductors under time-independent voltage bias
A standard drift-diffusion model of space charge wave propagation in
semiconductors has been studied numerically and analytically under dc voltage
bias. For sufficiently long samples, appropriate contact resistivity and
applied voltage - such that the sample is biased in a regime of negative
differential resistance - we find chaos in the propagation of nonlinear fronts
(charge monopoles of alternating sign) of electric field. The chaos is always
low-dimensional, but has a complex spatial structure; this behavior can be
interpreted using a finite dimensional asymptotic model in which the front
(charge monopole) positions and the electrical current are the only dynamical
variables.Comment: 12 pages, 8 figure
Magnetic phase diagram of a frustrated ferrimagnetic ladder: Relation to the one-dimensional boson Hubbard model
We study the magnetic phase diagram of two coupled mixed-spin
Heisenberg chains as a function of the frustration parameter related to
diagonal exchange couplings. The analysis is performed by using spin-wave
series and exact numerical diagonalization techniques. The obtained phase
diagram--containing the Luttinger liquid phase, the plateau phase with a
magnetization per rung , and the fully polarized phase--is closely
related to the generic phase diagram of the one-dimensional boson
Hubbard model.Comment: 4 pages, 2 figure
Chaos in resonant-tunneling superlattices
Spatio-temporal chaos is predicted to occur in n-doped semiconductor
superlattices with sequential resonant tunneling as their main charge transport
mechanism. Under dc voltage bias, undamped time-dependent oscillations of the
current (due to the motion and recycling of electric field domain walls) have
been observed in recent experiments. Chaos is the result of forcing this
natural oscillation by means of an appropriate external microwave signal.Comment: 3 pages, LaTex, RevTex, 3 uuencoded figures (1.2M) are available upon
request from [email protected], to appear in Phys.Rev.
Evidence of environmental strains on charge injection in silole based organic light emitting diodes
Using d. functional theory (DFT) computations, the authors demonstrated a
substantial skeletal relaxation when the structure of
2,5-bis-[4-anthracene-9-yl-phenyl]-1,1-dimethyl-3,4-diphenyl-silole (BAS) is
optimized in the gas-phase comparing with the mol. structure detd. from
monocrystal x-ray diffraction. The origin of such a relaxation is explained by
a strong environmental strains induced by the presence of anthracene entities.
Also, the estn. of the frontier orbital levels showed that this structural
relaxation affects mainly the LUMO that is lowered of 190 meV in the gas phase.
To check if these theor. findings would be confirmed for thin films of BAS, the
authors turned to UV photoemission spectroscopy and/or inverse photoemission
spectroscopy and electrooptical measurements. The study of the c.d. or voltage
and luminance or voltage characteristics of an ITO/PEDOT/BAS/Au device clearly
demonstrated a very unusual temp.-dependent behavior. Using a thermally
assisted tunnel transfer model, this behavior likely originated from the
variation of the electronic affinity of the silole deriv. with the temp. The
thermal agitation relaxes the mol. strains in thin films as it is shown when
passing from the cryst. to the gas phase. The relaxation of the intramol. thus
induces an increase of the electronic affinity and, as a consequence, the more
efficient electron injection in org. light-emitting diodes
A modified technique of orthotopic transplant of the kidney in rabbits
In this study kidneys were harvested from bred-for-research cats weighing 4 to 5 kg. General principles of donor bilateral nephrectomy en bloc with aorta, vena cava, renal vessels, and ureters were followed. After the harvest the grafts were placed in lactated Ringer slush. A cuff was prepared on the renal vein over a 10 French plastic tube. The aorta was divided and left in connection with the renal artery at each side. Twenty female checkered Flemish giant rabbits weighing 4.0-6.0 kg served as recipients. After premedication with 40 mg/kg of ketamine, anesthesia was maintained with repeated doses (every 10-15 min) of a 0.1-mL mixture of 5 parts ketamine and 1 part acepromazine diluted 50% in a normal saline. Arterial pressure, CVP, blood gases, and temperature were monitored. Through a limited midline incision a native left nephrectomy was performed. The venous anastomosis was performed with a cuff technique without clamping the vena cava (which causes severe hemodynamic instability); the anastomotic time was 2-3 min. The arterial anastomosis was performed with an end-to-side aorta-to-aorta anastomosis; the anastomotic time was 5 to 7 min. There were no episodes of venous or arterial thrombosis. The donor procedure took approximately 40 min, and the backtable preparation of the graft an additional 45 to 60 min. Preparation of the recipient for the anastomosis took 15 min and the anastomotic time (warm ischemia) was 13 +/- 5 min. In this model suitable for xenograft research the duration of the surgery in the recipient has been greatly reduced because of (1) the previous backtable preparation of the graft, and (2) the cuff technique used for venous anastomosis. The present anesthesia regimen and careful hemodynamic monitoring were also important in the success of this model
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Reversible writing of high-mobility and high-carrier-density doping patterns in two-dimensional van der Waals heterostructures
A key feature of two-dimensional materials is that the sign and concentration of their carriers can be externally controlled with techniques such as electrostatic gating. However, conventional electrostatic gating has limitations, including a maximum carrier density set by the dielectric breakdown, and ionic liquid gating and direct chemical doping also suffer from drawbacks. Here, we show that an electron-beam-induced doping technique can be used to reversibly write high-resolution doping patterns in hexagonal boron nitride-encapsulated graphene and molybdenum disulfide (MoS2) van der Waals heterostructures. The doped MoS2 device exhibits an order of magnitude decrease of subthreshold swing compared with the device before doping, whereas the doped graphene devices demonstrate a previously inaccessible regime of high carrier concentration and high mobility, even at room temperature. We also show that the approach can be used to write high-quality p–n junctions and nanoscale doping patterns, illustrating that the technique can create nanoscale circuitry in van der Waals heterostructures
Radiative Shock-Induced Collapse of Intergalactic Clouds
Accumulating observational evidence for a number of radio galaxies suggests
an association between their jets and regions of active star formation. The
standard picture is that shocks generated by the jet propagate through an
inhomogeneous medium and trigger the collapse of overdense clouds, which then
become active star-forming regions. In this contribution, we report on recent
hydrodynamic simulations of radiative shock-cloud interactions using two
different cooling models: an equilibrium cooling-curve model assuming solar
metallicities and a non-equilibrium chemistry model appropriate for primordial
gas clouds. We consider a range of initial cloud densities and shock speeds in
order to quantify the role of cooling in the evolution. Our results indicate
that for moderate cloud densities (>1 cm^{-3}) and shock Mach numbers (<20),
cooling processes can be highly efficient and result in more than 50% of the
initial cloud mass cooling to below 100 K. We also use our results to estimate
the final H_2 mass fraction for the simulations that use the non-equilibrium
chemistry package. This is an important measurement, since H_2 is the dominant
coolant for a primordial gas cloud. We find peak H_2 mass fractions of >0.01
and total H_2 mass fractions of >10^{-5} for the cloud gas. Finally, we compare
our results with the observations of jet-induced star formation in
``Minkowski's Object.'' We conclude that its morphology, star formation rate (~
0.3M_solar/yr) and stellar mass (~ 1.2 x 10^7 M_solar) can be explained by the
interaction of a 90,000 km/s jet with an ensemble of moderately dense (~ 10
cm^{-3}), warm (10^4 K) intergalactic clouds in the vicinity of its associated
radio galaxy at the center of the galaxy cluster.Comment: 30 pages, 7 figures, submitted to Astrophysical Journa
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Direct Autocrine Action of Insulin on β-Cells: Does It Make Physiological Sense?
In recent years there has been a growing interest in the possibility of a direct autocrine effect of insulin on the pancreatic β-cell. Indeed, there have been numerous intriguing articles and several eloquent reviews written on the subject (1–3); however, the concept is still controversial. Although many in vitro experiments, a few transgenic mouse studies, and some human investigations would be supportive of the notion, there exist different insights, other studies, and circumstantial evidence that question the concept. Therefore, the idea of autocrine action of insulin remains a conundrum. Here we outline a series of thoughts, insights, and alternative interpretations of the available experimental evidence. We ask, how convincing are these, and what are the confusing issues? We agree that there is a clear contribution of certain downstream elements in the insulin signaling pathway for β-cell function and survival, but the question of whether insulin itself is actually the physiologically relevant ligand that triggers this signal transduction remains unsettled
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