2,117 research outputs found
Nanoscale tunnel field effect transistor based on a complex oxide lateral heterostructure
We demonstrate a tunnel field effect transistor based on a lateral
heterostructure patterned from an electron gas.
Charge is injected by tunneling from the /
contacts and the current through a narrow channel of insulating
is controlled via an electrostatic side gate. Drain-source
I/V-curves have been measured at low and elevated temperatures. The transistor
shows strong electric-field and temperature-dependent behaviour with a steep
sub-threshold slope %of up to as small as and a
transconductance as high as . A fully
consistent transport model for the drain-source tunneling reproduces the
measured steep sub-threshold slope.Comment: 20 pages, 6 figures, Supplementary material: 4 pages, 2 figure
Optimization Approaches for the Traveling Salesman Problem with Drone
The fast and cost-efficient home delivery of goods ordered online is logistically challenging. Many companies are looking for new ways to cross the last-mile to their customers. One technology-enabled opportunity that recently has rec
Delay Management including Capacities of Stations
The question of delay management is whether trains should wait for delayed feeder
trains or should depart on time. Solutions to this problem strongly depend on the available
capacity of the railway infrastructure. While the limited capacity of the tracks has been
considered in delay management models, the limited capacity of the stations has been
neglected so far. In this paper, we develop a model for the delay management problem that
includes the stationsâ capacities. This model allows to reschedule the platform assignment
dynamically. Furthermore, we propose an iterative algorithm in which we first solve the
delay management model with a fixed platform assignment and then improve this platform
assignment in each step. We show that the latter problem can be solved in polynomial
time by presenting a totally unimodular IP formulation. Finally, we present an extension
of the model that balances the delay of the passengers on the one hand and the number of
changes in the platform assignment on the other. All models are evaluated on real-world
instances from Netherlands Railways
Delay Management with Re-Routing of Passengers
The question of delay management is whether trains should wait for a delayed feeder train
or should depart on time. In classical delay management models passengers always take
their originally planned route. In this paper, we propose a model where re-routing of
passengers is incorporated.
To describe the problem we represent it as an event-activity network similar to the one
used in classical delay management, with some additional events to incorporate origin
and destination of the passengers. We present an integer programming formulation of
this problem. Furthermore, we discuss the variant in which we assume fixed costs for
maintaining connections and we present a polynomial algorithm for the special case of
only one origin-destination pair. Finally, computational experiments based on real-world
data from Netherlands Railways show that significant improvements can be obtained by
taking the re-routing of passengers into account in the model
Spin injection and electric field effect in degenerate semiconductors
We analyze spin-transport in semiconductors in the regime characterized by
(intermediate to degenerate), where is the Fermi
temperature. Such a regime is of great importance since it includes the lightly
doped semiconductor structures used in most experiments; we demonstrate that,
at the same time, it corresponds to the regime in which carrier-carrier
interactions assume a relevant role. Starting from a general formulation of the
drift-diffusion equations, which includes many-body correlation effects, we
perform detailed calculations of the spin injection characteristics of various
heterostructures, and analyze the combined effects of carrier density
variation, applied electric field and Coulomb interaction. We show the
existence of a degenerate regime, peculiar to semiconductors, which strongly
differs, as spin-transport is concerned, from the degenerate regime of metals.Comment: Version accepted for publication in Phys. Rev.
A Rolling Horizon Heuristic with Optimality Guarantee for an On-Demand Vehicle Scheduling Problem
We consider a basic vehicle scheduling problem that arises in the context of travel demand models:
Given demanded vehicle trips, what is the minimal number of vehicles needed to fulfill the demand?
In this paper, we model the vehicle scheduling problem as a network flow problem. Since instances
arising in the context of travel demand models are often so big that the network flow model becomes
intractable, we propose using a rolling horizon heuristic to split huge problem instances into smaller
subproblems and solve them independently to optimality. By letting the horizons of the subproblems
overlap, it is possible to look ahead to the demand of the next subproblem. We prove that composing
the solutions of the subproblems yields an optimal solution to the whole problem if the overlap of
the horizons is sufficiently large. Our experiments show that this approach is not only suitable for
solving extremely large instances that are intractable as a whole, but it is also possible to decrease
the solution time for large instances compared to a comprehensive approach
First experiences with Personal Networks as an enabling platform for service providers
By developing demonstrators and performing small-scale user trials, we found various opportunities and pitfalls for deploying personal networks (PNs) on a commercial basis. The demonstrators were created using as many as possible legacy devices and proven technologies. They deal with applications in the health sector, home services, tourism, and the transportation sector. This paper describes the various architectures and our experiences with the end users and the technology. We conclude that context awareness, service discovery, and content management are very important in PNs and that a personal network provider role is necessary to realize these functions under the assumptions we made. The PNPay Travel demonstrator suggests that PN service platforms provide an opportunity to develop true trans-sector services
An iterative heuristic for passenger-centric train timetabling with integrated adaption times
In this paper we present a method to construct a periodic timetable from a tactical
planning perspective. We aim at constructing a timetable that is feasible with respect
to infrastructure constraints and minimizes average perceived passenger travel time. In
addition to in-train and transfer times, our notion of perceived passenger time includes
the adaption time (waiting time at the origin station). Adaption time minimization allows
us to avoid strict frequency regularity constraints and, at the same time, to ensure regular
connections between passengersâ origins and destinations. The combination of adaption
time minimization and infrastructure constraints satisfaction makes the problem very
challenging.
The described periodic timetabling problem can be modelled as an extension of a Peri-
odic Event Scheduling Problem (PESP) formulation, but requires huge computing times if
it is directly solved by a general-purpose solver for instances of realistic size. In this paper,
we propose a heuristic approach consisting of two phases that are executed iteratively.
First, we solve a mixed-integer linear program to determine an ideal timetable that mini-
mizes the average perceived passenger travel time but neglects infrastructure constraints.
Then, a Lagrangian-based heuristic makes the timetable feasible with respect to infras-
tructure constraints by modifying train departure and arrival times as little as possible.
The obtained feasible timetable is then evaluated to compute the resulting average per-
ceived passenger travel time, and a feedback is sent to the Lagrangian-based heuristic so as to possibly improve the obtained timetable from the passenger perspective, while
still respecting infrastructure constraints. We illustrate the proposed iterative heuristic
approach on real-life instances of Netherlands Railways and compare it to a benchmark
approach, showing that it finds a feasible timetable very close to the ideal one
Railway timetabling with integrated passenger distribution
Timetabling for railway services often aims at optimizing travel times for passengers. At the same
time, restricting assumptions on passenger behavior and passenger modeling are made. While research
has shown that passenger distribution on routes can be modeled with a discrete choice model, this has
not been considered in timetabling yet. We investigate how a passenger distribution can be integrated
into an optimization framework for timetabling and present two mixed-integer linear programs for
this problem. Both approaches design timetables and simultaneously find a corresponding passenger
distribution on available routes. One model uses a linear distribution model to estimate passenger
route choices, the other model uses an integrated simulation framework to approximate a passenger
distribution according to the logit model, a commonly used route choice model. We compare both
new approaches with three state-of-the-art timetabling methods and a heuristic approach on a set of
artificial instances and a partial network of Netherlands Railways (NS)
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