11,610 research outputs found
Quantum Fluctuations Driven Orientational Disordering: A Finite-Size Scaling Study
The orientational ordering transition is investigated in the quantum
generalization of the anisotropic-planar-rotor model in the low temperature
regime. The phase diagram of the model is first analyzed within the mean-field
approximation. This predicts at a phase transition from the ordered to
the disordered state when the strength of quantum fluctuations, characterized
by the rotational constant , exceeds a critical value . As a function of temperature, mean-field theory predicts a range of
values of where the system develops long-range order upon cooling, but
enters again into a disordered state at sufficiently low temperatures
(reentrance). The model is further studied by means of path integral Monte
Carlo simulations in combination with finite-size scaling techniques,
concentrating on the region of parameter space where reentrance is predicted to
occur. The phase diagram determined from the simulations does not seem to
exhibit reentrant behavior; at intermediate temperatures a pronounced increase
of short-range order is observed rather than a genuine long-range order.Comment: 27 pages, 8 figures, RevTe
Density-functional study of Cu atoms, monolayers, and coadsorbates on polar ZnO surfaces
The structure and electronic properties of single Cu atoms, copper monolayers
and thin copper films on the polar oxygen and zinc terminated surfaces of ZnO
are studied using periodic density-functional calculations. We find that the
binding energy of Cu atoms sensitively depends on how charge neutrality of the
polar surfaces is achieved. Bonding is very strong if the surfaces are
stabilized by an electronic mechanism which leads to partially filled surface
bands. As soon as the surface bands are filled (either by partial Cu coverage,
by coadsorbates, or by the formation of defects), the binding energy decreases
significantly. In this case, values very similar to those found for nonpolar
surfaces and for copper on finite ZnO clusters are obtained. Possible
implications of these observations concerning the growth mode of copper on
polar ZnO surfaces and their importance in catalysis are discussed.Comment: 6 pages with 2 postscript figures embedded. Uses REVTEX and epsf
macro
CHALLENGES IN FLYING QUADROTOR UNMANNED AERIAL VEHICLE FOR 3D INDOOR RECONSTRUCTION
Three-dimensional modelling plays a vital role in indoor 3D tracking, navigation, guidance and emergency evacuation. Reconstruction of indoor 3D models is still problematic, in part, because indoor spaces provide challenges less-documented than their outdoor counterparts. Challenges include obstacles curtailing image and point cloud capture, restricted accessibility and a wide array of indoor objects, each with unique semantics. Reconstruction of indoor environments can be achieved through a photogrammetric approach, e.g. by using image frames, aligned using recurring corresponding image points (CIP) to build coloured point clouds. Our experiments were conducted by flying a QUAV in three indoor environments and later reconstructing 3D models which were analysed under different conditions. Point clouds and meshes were created using Agisoft PhotoScan Professional. We concentrated on flight paths from two vantage points: 1) safety and security while flying indoors and 2) data collection needed for reconstruction of 3D models. We surmised that the main challenges in providing safe flight paths are related to the physical configuration of indoor environments, privacy issues, the presence of people and light conditions. We observed that the quality of recorded video used for 3D reconstruction has a high dependency on surface materials, wall textures and object types being reconstructed. Our results show that 3D indoor reconstruction predicated on video capture using a QUAV is indeed feasible, but close attention should be paid to flight paths and conditions ultimately influencing the quality of 3D models. Moreover, it should be decided in advance which objects need to be reconstructed, e.g. bare rooms or detailed furniture
Low frequency 1/f noise in doped manganite grain-boundary junctions
We have performed a systematic analysis of the low frequency 1/f-noise in
single grain boundary junctions in the colossal magnetoresistance material
La_{2/3}Ca_{1/3}MnO_{3-delta}. The grain boundary junctions were formed in
epitaxial La_{2/3}Ca_{1/3}MnO_{3-delta} films deposited on SrTiO_3 bicrystal
substrates and show a large tunneling magnetoresistance of up to 300% at 4.2 K
as well as ideal, rectangular shaped resistance versus applied magnetic field
curves. Below the Curie temperature T_C the measured 1/f noise is dominated by
the grain boundary. The dependence of the noise on bias current, temperature
and applied magnetic field gives clear evidence that the large amount of low
frequency noise is caused by localized sites with fluctuating magnetic moments
in a heavily disordered grain boundary region. At 4.2 K additional temporally
unstable Lorentzian components show up in the noise spectra that are most
likely caused by fluctuating clusters of interacting magnetic moments. Noise
due to fluctuating domains in the junction electrodes is found to play no
significant role.Comment: 9 pages, 7 figure
Proving Craig and Lyndon Interpolation Using Labelled Sequent Calculi
We have recently presented a general method of proving the fundamental
logical properties of Craig and Lyndon Interpolation (IPs) by induction on
derivations in a wide class of internal sequent calculi, including sequents,
hypersequents, and nested sequents. Here we adapt the method to a more general
external formalism of labelled sequents and provide sufficient criteria on the
Kripke-frame characterization of a logic that guarantee the IPs. In particular,
we show that classes of frames definable by quantifier-free Horn formulas
correspond to logics with the IPs. These criteria capture the modal cube and
the infinite family of transitive Geach logics
Voltage and temperature dependence of the grain boundary tunneling magnetoresistance in manganites
We have performed a systematic analysis of the voltage and temperature
dependence of the tunneling magnetoresistance (TMR) of grain boundaries (GB) in
the manganites. We find a strong decrease of the TMR with increasing voltage
and temperature. The decrease of the TMR with increasing voltage scales with an
increase of the inelastic tunneling current due to multi-step inelastic
tunneling via localized defect states in the tunneling barrier. This behavior
can be described within a three-current model for magnetic tunnel junctions
that extends the two-current Julliere model by adding an inelastic,
spin-independent tunneling contribution. Our analysis gives strong evidence
that the observed drastic decrease of the GB-TMR in manganites is caused by an
imperfect tunneling barrier.Comment: to be published in Europhys. Lett., 8 pages, 4 figures (included
Precision high voltage divider for the KATRIN experiment
The Karlsruhe Tritium Neutrino Experiment (KATRIN) aims to determine the
absolute mass of the electron antineutrino from a precise measurement of the
tritium beta-spectrum near its endpoint at 18.6 keV with a sensitivity of 0.2
eV. KATRIN uses an electrostatic retardation spectrometer of MAC-E filter type
for which it is crucial to monitor high voltages of up to 35 kV with a
precision and long-term stability at the ppm level. Since devices capable of
this precision are not commercially available, a new high voltage divider for
direct voltages of up to 35 kV has been designed, following the new concept of
the standard divider for direct voltages of up to 100 kV developed at the
Physikalisch-Technische Bundesanstalt (PTB). The electrical and mechanical
design of the divider, the screening procedure for the selection of the
precision resistors, and the results of the investigation and calibration at
PTB are reported here. During the latter, uncertainties at the low ppm level
have been deduced for the new divider, thus qualifying it for the precision
measurements of the KATRIN experiment.Comment: 22 pages, 12 figure
Faster Exponential-Time Approximation Algorithms Using Approximate Monotone Local Search
We generalize the monotone local search approach of Fomin, Gaspers,Lokshtanov and Saurabh [J.ACM 2019], by establishing a connection betweenparameterized approximation and exponential-time approximation algorithms formonotone subset minimization problems. In a monotone subset minimizationproblem the input implicitly describes a non-empty set family over a universeof size which is closed under taking supersets. The task is to find aminimum cardinality set in this family. Broadly speaking, we use approximatemonotone local search to show that a parameterized -approximationalgorithm that runs in time, where is the solutionsize, can be used to derive an -approximation randomized algorithm thatruns in time, where is the unique value in such that and is the Kullback-Leibler divergence. This running timematches that of Fomin et al. for , and is strictly better when, for any . Furthermore, we also show that this result can bederandomized at the expense of a sub-exponential multiplicative factor in therunning time. We demonstrate the potential of approximate monotone local search by derivingnew and faster exponential approximation algorithms for Vertex Cover,-Hitting Set, Directed Feedback Vertex Set, Directed Subset Feedback VertexSet, Directed Odd Cycle Transversal and Undirected Multicut. For instance, weget a -approximation algorithm for Vertex Cover with running time , improving upon the previously best known -approximationrunning in time by Bourgeois et al. [DAM 2011].<br
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