1,096 research outputs found

### Tetrahydrobiopterin analogues with NO-dependent pulmonary vasodilator properties

Reduced NO levels due to the deficiency of tetrahydrobiopterin (BH4) contribute to impaired vasodilation in pulmonary hypertension Due to the chemically unstable nature of BH4 it was hypothesised that oxidatively stable analogues of BR, would be able to support NO synthesis to improve Endothelial dysfunction in pulmonary hypertension Two analogues of BH4 namely 6-hydroxymethyl pterin (HMP) and 6-acetyl 7 7-dimethyl 7 8-dihydropterin (ADDP) were evaluated for vasodilator activity on precontracted rat pulmonary artery rings ADDP was administered to pulmonary hypertensive rats followed by measurement of pulmonary vascular resistance in perfused lungs and eNOS expression by immunohistochemistry ADDP and HMP caused significant relaxation in vitro in rat pulmonary arteries depleted of BH4 with a maximum relaxation at 0 3 mu M (both P<005) Vasodilator activity of ADDP and HMP was completely abolished following preincubation with the NO synthase inhibitor L-NAME ADDP and HMP did not alter relaxation induced by carbachol or spermine NONOate BH4 Itself did not produce relaxation In rats receiving ADDP 141 mg/kg/day pulmonary vasodilation induced by calcium ionophore A23187 was augmented and eNOS immunoreactivity was increased In conclusion ADDP and HMP are two analogues of BH4 which can act as oxidatively stable alternatives to BH4 in causing NO-mediated vasorelaxation Chronic treatment with ADDP resulted in Improvement of NO-mediated pulmonary artery dilation and enhanced expression of eNOS in the pulmonary vascular endothelium Chemically stable analogue, of BH4 may be able to limit endothelial dysfunction in the pulmonary vasculatur

### Trajectory generation for road vehicle obstacle avoidance using convex optimization

This paper presents a method for trajectory generation using convex optimization to find a feasible, obstacle-free path for a road vehicle. Consideration of vehicle rotation is shown to be necessary if the trajectory is to avoid obstacles specified in a fixed Earth axis system. The paper establishes that, despite the presence of significant non-linearities, it is possible to articulate the obstacle avoidance problem in a tractable convex form using multiple optimization passes. Finally, it is shown by simulation that an optimal trajectory that accounts for the vehicle’s changing velocity throughout the manoeuvre is superior to a previous analytical method that assumes constant speed

### Transport of Surface States in the Bulk Quantum Hall Effect

The two-dimensional surface of a coupled multilayer integer quantum Hall
system consists of an anisotropic chiral metal. This unusual metal is
characterized by ballistic motion transverse and diffusive motion parallel
(\hat{z}) to the magnetic field. Employing a network model, we calculate
numerically the phase coherent two-terminal z-axis conductance and its
mesoscopic fluctuations. Quasi-1d localization effects are evident in the limit
of many layers. We consider the role of inelastic de-phasing effects in
modifying the transport of the chiral surface sheath, discussing their
importance in the recent experiments of Druist et al.Comment: 9 pages LaTex, 9 postscript figures included using eps

### Mode-Locking in Quantum-Hall-Effect Point Contacts

We study the effect of an ac drive on the current-voltage (I-V)
characteristics of a tunnel junction between two fractional Quantum Hall fluids
at filling $\nu ^{-1}$ an odd integer. Within the chiral Luttinger liquid model
of edge states, the point contact dynamics is described by a driven damped
quantum mechanical pendulum. In a semi-classical limit which ignores electron
tunnelling, this model exhibits mode-locking, which corresponds to current
plateaus in the I-V curve at integer multiples of $I= e\omega /2\pi$, with
$\omega$ the ac drive angular frequency. By analyzing the full quantum model at
non-zero $\nu$ using perturbative and exact methods, we study the effect of
quantum fluctuation on the mode-locked plateaus. For $\nu=1$ quantum
fluctuations smear completely the plateaus, leaving no trace of the ac drive.
For $\nu \ge 1/2$ smeared plateaus remain in the I-V curve, but are not
centered at the currents $I=n e \omega /2\pi$. For $\nu < 1/2$ rounded plateaus
centered around the quantized current values are found. The possibility of
using mode locking in FQHE point contacts as a current-to-frequency standard is
discussed.Comment: 12 pages, 8 figures, minor change

### Signature of Quantum Hall Effect Skyrmions in Tunneling: A Theoretical Study

We present a theoretical study of the $I-V$ tunneling characteristic between
two parallel two-dimensional electron gases in a perpendicular magnetic field
when both are near filling factor $\nu=1$. Finite-size calculations of the
single-layer spectral functions in the spherical geometry and analytical
expressions for the disk geometry in the thermodynamic limit show that the
current in the presence of skyrmions reflects in a direct way their underlying
structure. It is also shown that fingerprints of the electron-electron
interaction pseudopotentials are present in such a current.Comment: 4 pages, 1 figur

### Anomalous tunneling conductances of a spin singlet \nu=2/3 edge states: Interplay of Zeeman splitting and Long Range Coulomb Interaction

The point contact tunneling conductance between edges of the spin singlet
$\nu=2/3,\hat{K}=(3/3/0)$ quantum Hall states is studied both in the
quasiparticle tunneling picture and in the electron tunneling picture. Due to
the interplay of Zeeman splitting and the long range Coulomb interaction
between edges of opposite chirality novel spin excitations emerge, and their
effect is characterized by anomalous exponents of the charge and spin tunneling
conductances in various temperature ranges. Depending on the kinds of
scatterings at the point contact and the tunneling mechanism the anomalous
interaction in spin sector may enhance or suppress the tunneling conductances.
The effects of novel spin excitation are also relevant to the recent NMR
experiments on quantum Hall edges.Comment: Revtex File, 7 pages: To be published in Physical Reviews

### Magnetotunneling as a Probe of Luttinger-Liquid Behavior

A novel method for detecting Luttinger-liquid behavior is proposed. The idea
is to measure the tunneling conductance between a quantum wire and a parallel
two-dimensional electron system as a function of both the potential difference
between them, $V$, and an in-plane magnetic field, $B$. We show that the
two-parameter dependence on $B$ and $V$ allows for a determination of the
characteristic dependence on wave vector $q$ and frequency $\omega$ of the {\it
spectral function}, $A_{\rm LL}(q,\omega)$, of the quantum wire. In particular,
the separation of spin and charge in the Luttinger liquid should manifest
itself as singularities in the $I$-$V$-characteristic. The experimental
feasibility of the proposal is discussed.Comment: Accepted for publication in Phys. Rev. Let

### Nonuniversal behavior of scattering between fractional quantum Hall edges

Among the predicted properties of fractional quantum Hall states are
fractionally charged quasiparticles and conducting edge-states described as
chiral Luttinger liquids. In a system with a narrow constriction, tunneling of
quasi-particles between states at different edges can lead to resistance and to
shot noise. The ratio of the shot noise to the backscattered current, in the
weak scattering regime, measures the fractional charge of the quasi-particle,
which has been confirmed in several experiments. However, the non-linearity of
the resistance predicted by the chiral Luttinger liquid theory was apparently
not observed in some of these cases. As a possible explanation for these
discrepancies, we consider a model where a smooth edge profile leads to
formation of additional edge states. Coupling between the current carrying edge
mode and the additional phonon like mode can lead to {\it nonuniversal}
exponents in the current-voltage characteristic, while preserving the ratio
between shot noise and the back-scattered current, for weak backscattering. For
special values of the coupling, one may obtain a linear I-V behavior.Comment: 10 pages, 3 figure

### A unified framework for the Kondo problem and for an impurity in a Luttinger liquid

We develop a unified theoretical framework for the anisotropic Kondo model
and the boundary sine-Gordon model. They are both boundary integrable quantum
field theories with a quantum-group spin at the boundary which takes values,
respectively, in standard or cyclic representations of the quantum group
$SU(2)_q$. This unification is powerful, and allows us to find new results for
both models. For the anisotropic Kondo problem, we find exact expressions (in
the presence of a magnetic field) for all the coefficients in the
``Anderson-Yuval'' perturbative expansion. Our expressions hold initially in
the very anisotropic regime, but we show how to continue them beyond the
Toulouse point all the way to the isotropic point using an analog of
dimensional regularization. For the boundary sine-Gordon model, which describes
an impurity in a Luttinger liquid, we find the non-equilibrium conductance for
all values of the Luttinger coupling.Comment: 36 pages (22 in double-page format), 7 figures in uuencoded file,
uses harvmac and epsf macro

### Anomalous Exponent of the Spin Correlation Function of a Quantum Hall Edge

The charge and spin correlation functions of partially spin-polarized edge
electrons of a quantum Hall bar are studied using effective Hamiltonian and
bosonization techniques. In the presence of the Coulomb interaction between the
edges with opposite chirality we find a different crossover behavior in spin
and charge correlation functions. The crossover of the spin correlation
function in the Coulomb dominated regime is characterized by an anomalous
exponent, which originates from the finite value of the effective interaction
for the spin degree of freedom in the long wavelength limit. The anomalous
exponent may be determined by measuring nuclear spin relaxation rates in a
narrow quantum Hall bar or in a quantum wire in strong magnetic fields.Comment: 4 pages, Revtex file, no figures. To appear in Physical Revews B,
Rapid communication

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