134 research outputs found
Column ratio mapping: a processing technique for atomic resolution high angle annular dark field(HAADF) images
An image processing technique is presented for atomic resolution high-angle annular dark-field (HAADF) images that have been acquired using scanning transmission electron microscopy (STEM). This technique is termed column ratio mapping and involves the automated process of measuring atomic column intensity ratios in high-resolution HAADF images. This technique was developed to provide a fuller analysis of HAADF images than the usual method of drawing single intensity line profiles across a few areas of interest. For instance, column ratio mapping reveals the compositional distribution across the whole HAADF image and allows a statistical analysis and an estimation of errors. This has proven to be a very valuable technique as it can provide a more detailed assessment of the sharpness of interfacial structures from HAADF images. The technique of column ratio mapping is described in terms of a [1 1 0]-oriented zinc-blende structured AlAs/GaAs superlattice using the 1 Å-scale resolution capability of the aberration-corrected SuperSTEM 1 instrument
Experimental evaluation of interfaces using atomic-resolution high angle annular dark field (HAADF) imaging
Aberration-corrected highangleannulardarkfield (HAADF) imaging in scanning transmission electron microscopy (STEM) can now be performed at atomic-resolution. This is an important tool for the characterisation of the latest semiconductor devices that require individual layers to be grown to an accuracy of a few atomic layers. However, the actual quantification of interfacial sharpness at the atomic-scale can be a complicated matter. For instance, it is not clear how the use of the total, atomic column or background HAADF signals can affect the measured sharpness or individual layer widths. Moreover, a reliable and consistent method of measurement is necessary. To highlight these issues, two types of AlAs/GaAs interfaces were studied in-depth by atomic-resolutionHAADFimaging. A method of analysis was developed in order to map the various HAADF signals across an image and to reliably determine interfacial sharpness. The results demonstrated that the level of perceived interfacial sharpness can vary significantly with specimen thickness and the choice of HAADF signal. Individual layer widths were also shown to have some dependence on the choice of HAADF signal. Hence, it is crucial to have an awareness of which part of the HAADF signal is chosen for analysis along with possible specimen thickness effects for future HAADF studies performed at the scale of a few atomic layers
Deterministic approach to microscopic three-phase traffic theory
Two different deterministic microscopic traffic flow models, which are in the
context of the Kerner's there-phase traffic theory, are introduced. In an
acceleration time delay model (ATD-model), different time delays in driver
acceleration associated with driver behaviour in various local driving
situations are explicitly incorporated into the model. Vehicle acceleration
depends on local traffic situation, i.e., whether a driver is within the free
flow, or synchronized flow, or else wide moving jam traffic phase. In a speed
adaptation model (SA-model), vehicle speed adaptation occurs in synchronized
flow depending on driving conditions. It is found that the ATD- and SA-models
show spatiotemporal congested traffic patterns that are adequate with empirical
results. In the ATD- and SA-models, the onset of congestion in free flow at a
freeway bottleneck is associated with a first-order phase transition from free
flow to synchronized flow; moving jams emerge spontaneously in synchronized
flow only. Differences between the ATD- and SA-models are studied. A comparison
of the ATD- and SA-models with stochastic models in the context of three phase
traffic theory is made. A critical discussion of earlier traffic flow theories
and models based on the fundamental diagram approach is presented.Comment: 40 pages, 14 figure
Criterion for traffic phases in single vehicle data and empirical test of a microscopic three-phase traffic theory
A microscopic criterion for distinguishing synchronized flow and wide moving
jam phases in single vehicle data measured at a single freeway location is
presented. Empirical local congested traffic states in single vehicle data
measured on different days are classified into synchronized flow states and
states consisting of synchronized flow and wide moving jam(s). Then empirical
microscopic characteristics for these different local congested traffic states
are studied. Using these characteristics and empirical spatiotemporal
macroscopic traffic phenomena, an empirical test of a microscopic three-phase
traffic flow theory is performed. Simulations show that the microscopic
criterion and macroscopic spatiotemporal objective criteria lead to the same
identification of the synchronized flow and wide moving jam phases in congested
traffic. It is found that microscopic three-phase traffic models can explain
both microscopic and macroscopic empirical congested pattern features. It is
obtained that microscopic distributions for vehicle speed difference as well as
fundamental diagrams and speed correlation functions can depend on the spatial
co-ordinate considerably. It turns out that microscopic optimal velocity (OV)
functions and time headway distributions are not necessarily qualitatively
different, even if local congested traffic states are qualitatively different.
The reason for this is that important spatiotemporal features of congested
traffic patterns are it lost in these as well as in many other macroscopic and
microscopic traffic characteristics, which are widely used as the empirical
basis for a test of traffic flow models, specifically, cellular automata
traffic flow models.Comment: 27 pages, 16 figure
Interface atomic structure of epitaxial ErAs layers on (001) In0.53Ga0.47As and GaAs
High-angle annular dark-field (HAADF) imaging in scanning transmission electron microscopy was used to determine the atomic structure of interfaces between epitaxial ErAs layers with the cubic rock salt structure and In0.53Ga0.47As and GaAs, respectively. All layers were grown by molecular beam epitaxy. We show that the interfacial atomic arrangement corresponds to the so-called chain model, in which the zinc blende semiconductor is terminated with a Ga layer. Image analysis was used to quantify the expansion between the first ErAs plane and the terminating Ga plane. In the HAADF images, a high intensity transfer from the heavy Er columns into the background was observed in the ErAs layer, whereas the background in In0.53Ga0.47As was of much lower intensity
Cellular automata approach to three-phase traffic theory
The cellular automata (CA) approach to traffic modeling is extended to allow
for spatially homogeneous steady state solutions that cover a two dimensional
region in the flow-density plane. Hence these models fulfill a basic postulate
of a three-phase traffic theory proposed by Kerner. This is achieved by a
synchronization distance, within which a vehicle always tries to adjust its
speed to the one of the vehicle in front. In the CA models presented, the
modelling of the free and safe speeds, the slow-to-start rules as well as some
contributions to noise are based on the ideas of the Nagel-Schreckenberg type
modelling. It is shown that the proposed CA models can be very transparent and
still reproduce the two main types of congested patterns (the general pattern
and the synchronized flow pattern) as well as their dependence on the flows
near an on-ramp, in qualitative agreement with the recently developed continuum
version of the three-phase traffic theory [B. S. Kerner and S. L. Klenov. 2002.
J. Phys. A: Math. Gen. 35, L31]. These features are qualitatively different
than in previously considered CA traffic models. The probability of the
breakdown phenomenon (i.e., of the phase transition from free flow to
synchronized flow) as function of the flow rate to the on-ramp and of the flow
rate on the road upstream of the on-ramp is investigated. The capacity drops at
the on-ramp which occur due to the formation of different congested patterns
are calculated.Comment: 55 pages, 24 figure
Josephson -junctions based on structures with complex normal/ferromagnet bilayer
We demonstrate that Josephson devices with nontrivial phase difference in the ground state can be realized in structures composed
from longitudinally oriented normal metal (N) and ferromagnet (F) films in the
weak link region. Oscillatory coupling across F-layer makes the first harmonic
in the current-phase relation relatively small, while coupling across N-layer
provides negative sign of the second harmonic. To derive quantitative criteria
for a -junction, we have solved two-dimensional boundary-value problem
in the frame of Usadel equations for overlap and ramp geometries of S-NF-S
structures. Our numerical estimates show that -junctions can be
fabricated using up-to-date technology.Comment: 14 pages, 9 figure
Control of Spatial-Temporal Congested Traffic Patterns at Highway Bottlenecks
A microscopic theory of control of spatial-temporal congested traffic pattern
at freeway bottlenecks is presented. Based on empirical spatial-temporal
features of congested patterns at freeway bottlenecks which have recently been
found, different control strategies for prevention or reducing of the patterns
are simulated and compared. The studied control strategies include the on-ramp
metering with feedback and automatic cruise control (ACC) vehicles. A recent
microscopic traffic flow model within the author's three-phase traffic theory
is used for validation of spatial-temporal congested pattern control.Comment: 19 pages, 7 figure
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