4,733 research outputs found
On Some Positivity Properties of the Interquark Potential in QCD
We prove that the Fourier transform of the exponential e^{-\b V(R)} of the
{\bf static} interquark potential in QCD is positive. It has been shown by
Eliott Lieb some time ago that this property allows in the same limit of static
spin independent potential proving certain mass relation between baryons with
different quark flavors.Comment: 6 pages, latex with one postscript figur
Hierarchic Superposition Revisited
Many applications of automated deduction require reasoning in first-order
logic modulo background theories, in particular some form of integer
arithmetic. A major unsolved research challenge is to design theorem provers
that are "reasonably complete" even in the presence of free function symbols
ranging into a background theory sort. The hierarchic superposition calculus of
Bachmair, Ganzinger, and Waldmann already supports such symbols, but, as we
demonstrate, not optimally. This paper aims to rectify the situation by
introducing a novel form of clause abstraction, a core component in the
hierarchic superposition calculus for transforming clauses into a form needed
for internal operation. We argue for the benefits of the resulting calculus and
provide two new completeness results: one for the fragment where all
background-sorted terms are ground and another one for a special case of linear
(integer or rational) arithmetic as a background theory
Carbon nanotube quantum dots on hexagonal boron nitride
We report the fabrication details and low-temperature characteristics of the
first carbon nanotube (CNT) quantum dots on flakes of hexagonal boron nitride
(hBN) as substrate. We demonstrate that CNTs can be grown on hBN by standard
chemical vapor deposition and that standard scanning electron microscopy
imaging and lithography can be employed to fabricate nanoelectronic structures
when using optimized parameters. This proof of concept paves the way to more
complex devices on hBN, with more predictable and reproducible characteristics
and electronic stability.Comment: 4 pages, 4 figure
A second eigenvalue bound for the Dirichlet Schroedinger operator
Let be the th eigenvalue of the Schr\"odinger
operator with Dirichlet boundary conditions on a bounded domain and with the positive potential . Following the spirit of the
Payne-P\'olya-Weinberger conjecture and under some convexity assumptions on the
spherically rearranged potential , we prove that . Here denotes the ball, centered at the
origin, that satisfies the condition .
Further we prove under the same convexity assumptions on a spherically
symmetric potential , that decreases
when the radius of the ball increases.
We conclude with several results about the first two eigenvalues of the
Laplace operator with respect to a measure of Gaussian or inverted Gaussian
density
Large time dynamics and aging of a polymer chain in a random potential
We study the out-of-equilibrium large time dynamics of a gaussian polymer
chain in a quenched random potential. The dynamics studied is a simple Langevin
dynamics commonly referred to as the Rouse model. The equations for the
two-time correlation and response function are derived within the gaussian
variational approximation. In order to implement this approximation faithfully,
we employ the supersymmetric representation of the Martin-Siggia-Rose dynamical
action. For a short ranged correlated random potential the equations are solved
analytically in the limit of large times using certain assumptions concerning
the asymptotic behavior. Two possible dynamical behaviors are identified
depending upon the time separation- a stationary regime and an aging regime. In
the stationary regime time translation invariance holds and so is the
fluctuation dissipation theorem. The aging regime which occurs for large time
separations of the two-time correlation functions is characterized by history
dependence and the breakdown of certain equilibrium relations. The large time
limit of the equations yields equations among the order parameters that are
similar to the equations obtained in the statics using replicas. In particular
the aging solution corresponds to the broken replica solution. But there is a
difference in one equation that leads to important consequences for the
solution. The stationary regime corresponds to the motion of the polymer inside
a local minimum of the random potential, whereas in the aging regime the
polymer hops between different minima. As a byproduct we also solve exactly the
dynamics of a chain in a random potential with quadratic correlations.Comment: 21 pages, RevTeX
Low-temperature and high magnetic field dynamic scanning capacitance microscope
We demonstrate a dynamic scanning capacitance microscope (DSCM) that operates
at large bandwidths, cryogenic temperatures and high magnetic fields. The setup
is based on a non-contact atomic force microscope (AFM) with a quartz tuning
fork sensor with non-optical excitation and read-out for topography, force and
dissipation measurements. The metallic AFM tip forms part of an rf resonator
with a transmission characteristics modulated by the sample properties and the
tip-sample capacitance. The tip motion gives rise to a modulation of the
capacitance at the frequency of the AFM sensor and its harmonics, which can be
recorded simultaneously with the AFM data. We use an intuitive model to
describe and analyze the resonator transmission and show that for most
experimental conditions it is proportional to the complex tip-sample
conductance, which depends on both the tip-sample capacitance and the sample
resistivity. We demonstrate the performance of the DSCM on metal disks buried
under a polymer layer and we discuss images recorded on a two-dimensional
electron gas in the quantum Hall effect regime, i.e. at cryogenic temperatures
and high magnetic fields, where we directly image the formation of compressible
stripes at the physical edge of the sample
The Vampire and the FOOL
This paper presents new features recently implemented in the theorem prover
Vampire, namely support for first-order logic with a first class boolean sort
(FOOL) and polymorphic arrays. In addition to having a first class boolean
sort, FOOL also contains if-then-else and let-in expressions. We argue that
presented extensions facilitate reasoning-based program analysis, both by
increasing the expressivity of first-order reasoners and by gains in
efficiency
Real-space imaging of quantum Hall effect edge strips
We use dynamic scanning capacitance microscopy (DSCM) to image compressible
and incompressible strips at the edge of a Hall bar in a two-dimensional
electron gas (2DEG) in the quantum Hall effect (QHE) regime. This method gives
access to the complex local conductance, Gts, between a sharp metallic tip
scanned across the sample surface and ground, comprising the complex sample
conductance. Near integer filling factors we observe a bright stripe along the
sample edge in the imaginary part of Gts. The simultaneously recorded real part
exhibits a sharp peak at the boundary between the sample interior and the
stripe observed in the imaginary part. The features are periodic in the inverse
magnetic field and consistent with compressible and incompressible strips
forming at the sample edge. For currents larger than the critical current of
the QHE break-down the stripes vanish sharply and a homogeneous signal is
recovered, similar to zero magnetic field. Our experiments directly illustrate
the formation and a variety of properties of the conceptually important QHE
edge states at the physical edge of a 2DEG.Comment: 7 page
A 23 GHz Survey of GRB Error Boxes
The Haystack 37-meter telescope was used in a pilot project in May 1995 to
observe GRB error boxes at 23~GHz. Seven BATSE error boxes and two IPN arcs
were scanned by driving the beam of the telescope rapidly across their area.
For the BATSE error boxes, the radio observations took place two to eighteen
days after the BATSE detection, and several boxes were observed more than once.
Total power data were recorded continuously as the telescope was driven at a
rate of 0.2~degrees/second, yielding Nyquist sampling of the beam with an
integration time of 50~milliseconds, corresponding to a theoretical rms
sensitivity of 0.5~Jy. Under conditions of good weather, this sensitivity was
achieved. In a preliminary analysis of the data we detect only two sources,
3C273 and 0552+398, both catalogued sources that are known to be variable at
23~GHz. Neither had a flux density that was unusally high or low at the time of
our observations.Comment: 5 pages, 1 postscript figure. To appear in Proceedings of the Third
Huntsville Symposium on Gamma-Ray Bursts (eds. C. Kouveliotou, M. S. Briggs,
and G. J. Fishman
Cyanoacrylic- and (1-cyanovinyl)phosphonic acid anchoring ligands for application in copper-based dye-sensitized solar cells
The syntheses and characterization of four new anchoring ligands (2–5) for copper(I) bis(diimine) dyes in dye-sensitized solar cells (DSCs) are reported. Ligands 2 and 3 contain a 6,6′-dimethyl-2,2′-bipyridine copper-binding unit, while 4 and 5 contain a 2,9-dimethyl-1,10-phenanthroline unit; 2 and 4 contain cyanoacrylic acid anchoring units, and 3 and 5 possess (1-cyanovinyl)phosphonic acid anchors. The performance of DSCs sensitized by [Cu(Lanchor)(Lancillary)]+ in which Lanchor is 2–5 and Lancillary is either 6,6′-dimethyl-2,2′-bipyridine (6) or 6-trifluoromethyl-2,2′-bipyridine (7) are compared with those of DSCs containing the dyes [Cu(1)(6)]+ or [Cu(1)(7)]+ where anchoring ligand 1 is the previously reported and well-performing ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid). Among dyes incorporating 2–5, the best performing dye contained anchor 3 (6,6′-Me2bpy/(1-cyanovinyl)phosphonic acid combination). The better performances of dyes containing the bpy-based 2 and 3 compared to the phen-based 4 and 5 are rationalized largely in terms of the greater flexibility of the bpy vs. phen unit, allowing dyes containing 2 and 3 to adopt a conformation that leads to better surface coverage on mesoporous TiO2. Replacing 1 by 3 leads to a small gain in the short-circuit current density (JSC), but dyes with anchor 1 (in place of 3) have enhanced open-circuit voltage (VOC). The results of electrochemical impedance spectroscopy (EIS) support the trends found from the J–V measurements. The EIS data for DSCs with dyes containing anchors 3 or 1 are compared; the latter has a higher recombination resistance and chemical capacitance although the former exhibits a lower transport resistance
- …