5,330 research outputs found
Deformation of generic submanifolds in a complex manifold
This paper shows that an arbitrary generic submanifold in a complex manifold
can be deformed into a 1-parameter family of generic submanifolds satisfying
strong nondegeneracy conditions. The proofs use a careful analysis of the jet
spaces of embeddings satisfying certain nondegeneracy properties, and also make
use of the Thom transversality theorem, as well as the stratification of
real-algebraic sets. Optimal results on the order of nondegeneracy are given.Comment: 24 page
A Discrete Version of the Inverse Scattering Problem and the J-matrix Method
The problem of the Hamiltonian matrix in the oscillator and Laguerre basis
construction from the S-matrix is treated in the context of the algebraic
analogue of the Marchenko method.Comment: 11 pages. The Laguerre basis case is adde
Quantitative Simulation of the Superconducting Proximity Effect
A numerical method is developed to calculate the transition temperature of
double or multi-layers consisting of films of super- and normal conductors. The
approach is based on a dynamic interpretation of Gorkov's linear gap equation
and is very flexible. The mean free path of the different metals, transmission
through the interface, ratio of specular reflection to diffusive scattering at
the surfaces, and fraction of diffusive scattering at the interface can be
included. Furthermore it is possible to vary the mean free path and the BCS
interaction NV in the vicinity of the interface. The numerical results show
that the normalized initial slope of an SN double layer is independent of
almost all film parameters except the ratio of the density of states. There are
only very few experimental investigations of this initial slope and they
consist of Pb/Nn double layers (Nn stands for a normal metal). Surprisingly the
coefficient of the initial slope in these experiments is of the order or less
than 2 while the (weak coupling) theory predicts a value of about 4.5. This
discrepancy has not been recognized in the past. The autor suggests that it is
due to strong coupling behavior of Pb in the double layers. The strong coupling
gap equation is evaluated in the thin film limit and yields the value of 1.6
for the coefficient. This agrees much better with the few experimental results
that are available.
PACS: 74.45.+r, 74.62.-c, 74.20.F
Experimental evidence of ageing and slow restoration of the weak-contact configuration in tilted 3D granular packings
Granular packings slowly driven towards their instability threshold are
studied using a digital imaging technique as well as a nonlinear acoustic
method. The former method allows us to study grain rearrangements on the
surface during the tilting and the latter enables to selectively probe the
modifications of the weak-contact fraction in the material bulk. Gradual ageing
of both the surface activity and the weak-contact reconfigurations is observed
as a result of repeated tilt cycles up to a given angle smaller than the angle
of avalanche. For an aged configuration reached after several consecutive tilt
cycles, abrupt resumption of the on-surface activity and of the weak-contact
rearrangements occurs when the packing is subsequently inclined beyond the
previous maximal tilting angle. This behavior is compared with literature
results from numerical simulations of inclined 2D packings. It is also found
that the aged weak-contact configurations exhibit spontaneous restoration
towards the initial state if the packing remains at rest for tens of minutes.
When the packing is titled forth and back between zero and near-critical
angles, instead of ageing, the weak-contact configuration exhibits "internal
weak-contact avalanches" in the vicinity of both the near-critical and zero
angles. By contrast, the stronger-contact skeleton remains stable
Inelastic Decay of Electrons in the Shockley-type Metal-Organic Interface States
We present a theoretical study of lifetimes of interface states (IS) on
metal-organic interfaces PTCDA/Ag(111), NTCDA/Ag(111), PFP/Ag(111), and
PTCDA/Ag(100), describing and explaining the recent experimental data. By means
of unfolding the band structure of one of the interfaces under study onto the
Ag(111) Brillouin zone we demonstrate, that the Brillouin zone folding upon
organic monolayer deposition plays a minor role in the phase space for electron
decay, and hence weakly affects the resulting lifetimes. The presence of the
unoccupied molecular states below the IS gives a small contribution to the IS
decay rate mostly determined by the change of the phase space of bulk states
upon the energy shift of the IS. The calculated lifetimes follow the
experimentally observed trends. In particular, we explain the trend of the
unusual increase of the IS lifetimes with rising temperature.Comment: 8 pages, 5 figure
High intermodulation gain in a micromechanical Duffing resonator
In this work we use a micromechanical resonator to experimentally study small
signal amplification near the onset of Duffing bistability. The device consists
of a PdAu beam serving as a micromechanical resonator excited by an adjacent
gate electrode. A large pump signal drives the resonator near the onset of
bistability, enabling amplification of small signals in a narrow bandwidth. To
first order, the amplification is inversely proportional to the frequency
difference between the pump and signal. We estimate the gain to be about 15dB
for our device
Shadow on the wall cast by an Abrikosov vortex
At the surface of a d-wave superconductor, a zero-energy peak in the
quasiparticle spectrum can be observed. This peak appears due to Andreev bound
states and is maximal if the nodal direction of the d-wave pairing potential is
perpendicular to the boundary. We examine the effect of a single Abrikosov
vortex in front of a reflecting boundary on the zero-energy density of states.
We can clearly see a splitting of the low-energy peak and therefore a
suppression of the zero-energy density of states in a shadow-like region
extending from the vortex to the boundary. This effect is stable for different
models of the single Abrikosov vortex, for different mean free paths and also
for different distances between the vortex center and the boundary. This
observation promises to have also a substantial influence on the differential
conductance and the tunneling characteristics for low excitation energies.Comment: 5 pages, 5 figure
Nonlinear resonance in a three-terminal carbon nanotube resonator
The RF-response of a three-terminal carbon nanotube resonator coupled to
RF-transmission lines is studied by means of perturbation theory and direct
numerical integration. We find three distinct oscillatory regimes, including
one regime capable of exhibiting very large hysteresis loops in the frequency
response. Considering a purely capacitive transduction, we derive a set of
algebraic equations which can be used to find the output power (S-parameters)
for a device connected to transmission lines with characteristic impedance
.Comment: 16 pages, 8 figure
Strong-Pinning Effects in Low-Temperature Creep: Charge-Density Waves in TaS_3
Nonlinear conduction in the quasi-one dimensional conductor o-TaS_3 has been
studied in the low-temperature region down to 30 mK. It was found that at
temperatures below a few Kelvins the current-voltage (I-V) characteristics
consist of several branches. The temperature evolution of the I-V curve
proceeds through sequential freezing-out of the branches. The origin of each
branch is attributed to a particular strong pinning impurity type.
Similar behavior is expected for other physical systems with collective
transport (spin-density waves, Wigner crystals, vortex lattices in type-II
superconductors etc.) in the presence of strong pinning centers.Comment: 11 pages, 3 ps figures, Revtex, To be published in Phys. Rev. Letters
(1997
Severe discrepancies between experiment and theory in the superconducting proximity effect
The superconducting proximity effect is investigated for SN double layers in
a regime where the resulting transition temperature T_{c} does not depend on
the mean free paths of the films and, within limits, not on the transparency of
the interface. This regime includes the thin film limit and the normalized
initial slope S_{sn}= (d_{s}/T_{s})|dT_{c}/dd_{n}|. The experimental results
for T_{c} are compared with a numerical simulation which was recently developed
in our group. The results for the SN double layers can be devided into three
groups: (i) When N = Cu, Ag, Au, Mg a disagreement between experiment and
theory by a factor of the order of three is observed, (ii) When N = Cd, Zn, Al
the disagreement between experiment and theory is reduced to a factor of about
1.5, (iii) When N = In, Sn a reasonably good agreement between experiment and
theory is observed
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