1,335 research outputs found
A Trace Finite Element Method for Vector-Laplacians on Surfaces
We consider a vector-Laplace problem posed on a 2D surface embedded in a 3D
domain, which results from the modeling of surface fluids based on exterior
Cartesian differential operators. The main topic of this paper is the
development and analysis of a finite element method for the discretization of
this surface partial differential equation. We apply the trace finite element
technique, in which finite element spaces on a background shape-regular
tetrahedral mesh that is surface-independent are used for discretization. In
order to satisfy the constraint that the solution vector field is tangential to
the surface we introduce a Lagrange multiplier. We show well-posedness of the
resulting saddle point formulation. A discrete variant of this formulation is
introduced which contains suitable stabilization terms and is based on trace
finite element spaces. For this method we derive optimal discretization error
bounds. Furthermore algebraic properties of the resulting discrete saddle point
problem are studied. In particular an optimal Schur complement preconditioner
is proposed. Results of a numerical experiment are included
Re-establishing communication in teams of mobile robots
As communication is important for cooperation, teams of mobile robots need a way to re-establish a wireless connection if they get separated. We develop a method for mobile robots to maintain a belief of each other's positions using locally available information. They can use their belief to plan paths with high probabilities of reconnection. This approach also works for subteams cooperatively searching for a robot or group of robots that they would like to reconnect with. The problem is formulated as a constrained optimization problem which is solved using a branch-and-bound approach. We present simulation results showing the effectiveness of this strategy at reconnecting teams of up to five robots and compare the results to two other strategies
Local current distribution at large quantum dots (QDs): a self-consistent screening model
We report the implementation of the self-consistent Thomas-Fermi screening
theory, together with the local Ohm's law to a quantum dot system in order to
obtain local current distribution within the dot and at the leads. We consider
a large dot (size
nm) defined by split gates, and coupled to the leads. Numerical
calculations show that the non-dissipative current is confined to the
incompressible strips. Due to the non-linear screening properties of the 2DES
at low temperatures, this distribution is highly sensitive to external magnetic
field. Our findings support the phenomenological models provided by the
experimental studies so far, where the formation of the (direct) edge channels
dominate the transport.Comment: 6 Pages, 2 Figure
On Secure Implementation of an IHE XUA-Based Protocol for Authenticating Healthcare Professionals
The importance of the Electronic Health Record (EHR) has been addressed in recent years by governments and institutions.Many large scale projects have been funded with the aim to allow healthcare professionals to consult patients data. Properties such as confidentiality, authentication and authorization are the key for the success for these projects. The Integrating the Healthcare Enterprise (IHE) initiative promotes the coordinated use of established standards for authenticated and secure EHR exchanges among clinics and hospitals. In particular, the IHE integration profile named XUA permits to attest user identities by relying on SAML assertions, i.e. XML documents containing authentication statements. In this paper, we provide a formal model for the secure issuance of such an assertion. We first specify the scenario using the process calculus COWS and then analyse it using the model checker CMC. Our analysis reveals a potential flaw in the XUA profile when using a SAML assertion in an unprotected network. We then suggest a solution for this flaw, and model check and implement this solution to show that it is secure and feasible
Time-resolved observation of spin-charge deconfinement in fermionic Hubbard chains
Elementary particles such as the electron carry several quantum numbers, for
example, charge and spin. However, in an ensemble of strongly interacting
particles, the emerging degrees of freedom can fundamentally differ from those
of the individual constituents. Paradigmatic examples of this phenomenon are
one-dimensional systems described by independent quasiparticles carrying either
spin (spinon) or charge (holon). Here we report on the dynamical deconfinement
of spin and charge excitations in real space following the removal of a
particle in Fermi-Hubbard chains of ultracold atoms. Using space- and
time-resolved quantum gas microscopy, we track the evolution of the excitations
through their signatures in spin and charge correlations. By evaluating
multi-point correlators, we quantify the spatial separation of the excitations
in the context of fractionalization into single spinons and holons at finite
temperatures
A subradiant optical mirror formed by a single structured atomic layer
Efficient and versatile interfaces for the interaction of light with matter
are an essential cornerstone for quantum science. A fundamentally new avenue of
controlling light-matter interactions has been recently proposed based on the
rich interplay of photon-mediated dipole-dipole interactions in structured
subwavelength arrays of quantum emitters. Here we report on the direct
observation of the cooperative subradiant response of a two-dimensional (2d)
square array of atoms in an optical lattice. We observe a spectral narrowing of
the collective atomic response well below the quantum-limited decay of
individual atoms into free space. Through spatially resolved spectroscopic
measurements, we show that the array acts as an efficient mirror formed by only
a single monolayer of a few hundred atoms. By tuning the atom density in the
array and by changing the ordering of the particles, we are able to control the
cooperative response of the array and elucidate the interplay of spatial order
and dipolar interactions for the collective properties of the ensemble. Bloch
oscillations of the atoms out of the array enable us to dynamically control the
reflectivity of the atomic mirror. Our work demonstrates efficient optical
metamaterial engineering based on structured ensembles of atoms and paves the
way towards the controlled many-body physics with light and novel light-matter
interfaces at the single quantum level.Comment: 8 pages, 5 figures + 12 pages Supplementary Infomatio
Density-functional theory investigation of oxygen adsorption at Pd(11N)(N=3,5,7) vicinal surfaces
We present a density-functional theory study addressing the on-surface
adsorption of oxygen at the Pd(11N) (N =3,5,7) vicinal surfaces, which exhibit
(111) steps and (100) terraces of increasing width. We find the binding to be
predominantly governed by the local coordination at the adsorption site. This
leads to very similar bonding properties at the threefold step sites of all
three vicinal surfaces, while the binding at the central fourfold hollow site
in the four atomic row terrace of Pd(117) is already very little disturbed by
the presence of the neighboring steps.Comment: 9 pages including 4 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Floquet Prethermalization in a Bose-Hubbard System
Periodic driving has emerged as a powerful tool in the quest to engineer new
and exotic quantum phases. While driven many-body systems are generically
expected to absorb energy indefinitely and reach an infinite-temperature state,
the rate of heating can be exponentially suppressed when the drive frequency is
large compared to the local energy scales of the system -- leading to
long-lived 'prethermal' regimes. In this work, we experimentally study a
bosonic cloud of ultracold atoms in a driven optical lattice and identify such
a prethermal regime in the Bose-Hubbard model. By measuring the energy
absorption of the cloud as the driving frequency is increased, we observe an
exponential-in-frequency reduction of the heating rate persisting over more
than 2 orders of magnitude. The tunability of the lattice potentials allows us
to explore one- and two-dimensional systems in a range of different interacting
regimes. Alongside the exponential decrease, the dependence of the heating rate
on the frequency displays features characteristic of the phase diagram of the
Bose-Hubbard model, whose understanding is additionally supported by numerical
simulations in one dimension. Our results show experimental evidence of the
phenomenon of Floquet prethermalization, and provide insight into the
characterization of heating for driven bosonic systems
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