213 research outputs found
Symmetric Hubbard Systems with Superconducting Magnetic Response
In purely repulsive, -symmetric Hubbard clusters a correlation effect
produces an effective two-body attraction and pairing; the key ingredient is
the availability of W=0 pairs, that is, two-body solutions of appropriate
symmetry. We study the tunneling of bound pairs in rings of 5-site units
connected by weak intercell links; each unit has the topology of a CuO
cluster and a repulsive interaction is included on every site. Further, we test
the superconducting nature of the response of this model to a threading
magnetic field. We present a detailed numerical study of the two-unit ring
filled with 6 particles and the three-unit ring with 8 particles; in both cases
a lower filling yields normal behavior. In previous studies on 1d Hubbard
chains, level crossings were reported (half-integer or fractional Aharonov-Bohm
effect) which however cannot be due to superconducting pairs. In contrast, the
nontrivial basis of clusters carrying W=0 pairs leads to genuine
Superconducting Flux Quantization (SFQ). The data are understood in terms of a
cell-perturbation theory scheme which is very accurate for weak links. This
low-energy approach leads to an effective hard core boson Hamiltonian which
naturally describes itinerant pairs and SFQ in mesoscopic rings. For the
numerical calculations, we take advantage of a recently proposed exact
diagonalization technique which can be generally applied to many-fermion
problems and drastically reduces the size of the matrices to be handled.Comment: 12 pages, 11 figure
Waterfall Traffic Classification: A Quick Approach to Optimizing Cascade Classifiers
Heterogeneous wireless communication networks, like 4G LTE, transport diverse kinds of IP traffic: voice, video, Internet data, and more. In order to effectively manage such networks, administrators need adequate tools, of which traffic classification is the basis for visualizing, shaping, and filtering the broad streams of IP packets observed nowadays. In this paper, we describe a modular, cascading traffic classification systemâthe Waterfall architectureâand we extensively describe a novel technique for its optimizationâin terms of CPU time, number of errors, and percentage of unrecognized flows. We show how to significantly accelerate the process of exhaustive search for the best performing cascade. We employ five datasets of real Internet transmissions and seven traffic analysis methods to demonstrate that our proposal yields valid results and outperforms a greedy optimizer
Repulsion-Sustained Supercurrent and Flux Quantization in Rings of Symmetric Hubbard Clusters
We test the response to a threading magnetic field of rings of 5-site
-symmetric repulsive Hubbard clusters connected by weak intercell
links; each 5-site unit has the topology of a CuO cluster and a repulsive
interaction is included on every site. In a numerical study of the three-unit
ring with 8 particles, we take advantage of a novel exact-diagonalization
technique which can be generally applied to many-fermion problems. For O-O
hopping we find Superconducting Flux Quantization (SFQ), but for purely Cu-Cu
links bound pair propagation is hindered by symmetry. The results agree with
W=0 pairing theory.Comment: 4 pages, 2 figure
"Spin-Disentangled" Exact Diagonalization of Repulsive Hubbard Systems: Superconducting Pair Propagation
By a novel exact diagonalization technique we show that bound pairs propagate
between repulsive Hubbard clusters in a superconducting fashion. The size of
the matrices that must be handled depends on the number of fermion
configurations {\em per spin}, which is of the order of the square root of the
overall size of the Hilbert space. We use CuO units connected by weak O-O
links to model interplanar coupling and c-axis superconductivity in Cuprates.
The numerical evidence on CuO and CuO prompts a new
analytic scheme describing the propagation of bound pairs and also the
superconducting flux quantization in a 3-d geometry.Comment: 5 pages, 3 figure
Unsupervised machine learning and geometric morphometrics as tools for the identification of inter and intraspecific variations in the Anopheles Maculipennis complex
Geometric morphometric analysis was combined with two different unsupervised machine learning algorithms,
UMAP and HDBSCAN, to visualize morphological differences in wing shape among and within four Anopheles
sibling species (An. atroparvus, An. melanoon, An. maculipennis s.s. and An. daciae sp. inq.) of the Maculipennis
complex in Northern Italy. Specifically, we evaluated: (1) wing shape variation among and within species; (2) the
consistencies between groups of An. maculipennis s.s. and An. daciae sp. inq. identified based on COI sequences
and wing shape variability; and (3) the spatial and temporal distribution of different morphotypes. UMAP
detected at least 13 main patterns of variation in wing shape among the four analyzed species and mapped
intraspecific morphological variations. The relationship between the most abundant COI haplotypes of An. daciae
sp. inq. and shape ordination/variation was not significant. However, morphological variation within haplotypes
was reported. HDBSCAN also recognized different clusters of morphotypes within An. daciae sp. inq. (12) and An.
maculipennis s.s. (4). All morphotypes shared a similar pattern of variation in the subcostal vein, in the anal vein
and in the radio-medial cross-vein of the wing. On the contrary, the marginal part of the wings remained unchanged
in all clusters of both species. Any spatial-temporal significant difference was observed in the frequency
of the identified morphotypes. Our study demonstrated that machine learning algorithms are a useful tool
combined with geometric morphometrics and suggest to deepen the analysis of inter and intra specific shape
variability to evaluate evolutionary constrains related to wing functionality
Can a single low-intensity premature stimulus induce ventricular arrhythmias in the normal heart?
Previously, we observed that a single low-intensity premature ventricular stimulation could occasionally induce spontaneous ectopic beats in normal rat hearts. Possible hypothesis for the arrhythmia is that a premature beat can encounter a zone of conduction block to initiate reentry. However, enhanced dispersion of repolarization, a necessary condition for initiation of reentry, is unlikely to be present in normal myocardium. Thus, the main objective of the present study was to perform detailed pace mapping measurements in normal ventricular myocardium with a view to identify pacing sites and critical coupling intervals which could induce spontaneous ectopic beats and to characterize the reentrant circuits
The role of the partner atom and resonant excitation energy in ICD in rare gas dimers
We show experimental evidence for Interatomic Coulombic Decay (ICD) in mixed rare gas dimers following resonant Auger decay. A velocity map imaging apparatus together with a cooled supersonic beam containing Ar2, ArNe and ArKr dimers was used to record electron VMI images in coincidence with two mass selected ions following excitation on five resonances converging to the Ar+ 2pâ11/2 and 2pâ13/2 thresholds using the synchrotron radiation. The results show that the kinetic energy distribution of the ICD electrons observed in coincidence with the ions from Coulomb explosion of the dimers depends on the partner ion and resonant photon energy
Pairing in Cu-O Models: Clues of Joint Electron-Phonon and Electron-Electron Interactions
We discuss a many-electron Hamiltonian with Hubbard-like repulsive
interaction and linear coupling to the phonon branches, having the Cu-O plane
of the superconducting cuprates as a paradigm. A canonical transformation
extracts an effective two-body problem from the many-body theory. As a
prototype system we study the \cu cluster, which yields electronic pairing in
the Hubbard model; moreover, a standard treatment of the Jahn-Teller effect
predicts distortions that destroy electronic pairing. Remarkably, calculations
that keep all the electronic spectrum into account show that vibrations are
likely to be synergic with electronic pairing, if the coupling to
half-breathing modes predominates, as experiments suggest.Comment: 4 pages, 3 figures, accepted by Phys. Rev.
Three-Dimensional Shapes of Spinning Helium Nanodroplets
A significant fraction of superfluid helium nanodroplets produced in a
free-jet expansion have been observed to gain high angular momentum resulting
in large centrifugal deformation. We measured single-shot diffraction patterns
of individual rotating helium nanodroplets up to large scattering angles using
intense extreme ultraviolet light pulses from the FERMI free-electron laser.
Distinct asymmetric features in the wide-angle diffraction patterns enable the
unique and systematic identification of the three-dimensional droplet shapes.
The analysis of a large dataset allows us to follow the evolution from
axisymmetric oblate to triaxial prolate and two-lobed droplets. We find that
the shapes of spinning superfluid helium droplets exhibit the same stages as
classical rotating droplets while the previously reported metastable, oblate
shapes of quantum droplets are not observed. Our three-dimensional analysis
represents a valuable landmark for clarifying the interrelation between
morphology and superfluidity on the nanometer scale
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