5,090 research outputs found
Optical imaging of resonant electrical carrier injection into individual quantum dots
We image the micro-electroluminescence (EL) spectra of self-assembled InAs
quantum dots (QDs) embedded in the intrinsic region of a GaAs p-i-n diode and
demonstrate optical detection of resonant carrier injection into a single QD.
Resonant tunneling of electrons and holes into the QDs at bias voltages below
the flat-band condition leads to sharp EL lines characteristic of individual
QDs, accompanied by a spatial fragmentation of the surface EL emission into
small and discrete light- emitting areas, each with its own spectral
fingerprint and Stark shift. We explain this behavior in terms of Coulomb
interaction effects and the selective excitation of a small number of QDs
within the ensemble due to preferential resonant tunneling paths for carriers.Comment: 4 page
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
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
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 Tableaux Calculus for Reducing Proof Size
A tableau calculus is proposed, based on a compressed representation of
clauses, where literals sharing a similar shape may be merged. The inferences
applied on these literals are fused when possible, which reduces the size of
the proof. It is shown that the obtained proof procedure is sound,
refutationally complete and allows to reduce the size of the tableau by an
exponential factor. The approach is compatible with all usual refinements of
tableaux.Comment: Technical Repor
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
Extension and reconstruction theorems for the Urysohn universal metric space
We prove some extension theorems involving uniformly continuous maps of the
universal Urysohn space. We also prove reconstruction theorems for certain
groups of autohomeomorphisms of this space and of its open subsets.Comment: Final and shortened version, 25 pages, to appear in Czechoslovak
Math.
Scaling of polymers in aligned rods
We study the behavior of self avoiding polymers in a background of vertically
aligned rods that are either frozen into random positions or free to move
horizontally. We find that in both cases the polymer chains are highly
elongated, with vertical and horizontal size exponents that differ by a factor
of 3. Though these results are different than previous predictions, our results
are confirmed by detailed computer simulations.Comment: 4 pages, 4 figure
Intelligent Self-Repairable Web Wrappers
The amount of information available on the Web grows at an incredible high rate. Systems and procedures devised to extract these data from Web sources already exist, and different approaches and techniques have been investigated during the last years. On the one hand, reliable solutions should provide robust algorithms of Web data mining which could automatically face possible malfunctioning or failures. On the other, in literature there is a lack of solutions about the maintenance of these systems. Procedures that extract Web data may be strictly interconnected with the structure of the data source itself; thus, malfunctioning or acquisition of corrupted data could be caused, for example, by structural modifications of data sources brought by their owners. Nowadays, verification of data integrity and maintenance are mostly manually managed, in order to ensure that these systems work correctly and reliably. In this paper we propose a novel approach to create procedures able to extract data from Web sources -- the so called Web wrappers -- which can face possible malfunctioning caused by modifications of the structure of the data source, and can automatically repair themselves.\u
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