703 research outputs found
Composite fermions from the algebraic point of view
Composite fermion wavefuctions have been used to describe electrons in a
strong magnetic field. We show that the polynomial part of these wavefunctions
can be obtained by applying a normal ordered product of suitably defined
annihilation and creation operators to an even power of the Vandermonde
determinant, which can been considered as a kind of a non-trivial Fermi sea. In
the case of the harmonic interaction we solve the system exactly in the lowest
Landau level. The solution makes explicit the boson-fermion correspondence
proposed recently.Comment: 11 pages 1 figur
The driving mechanisms of particle precipitation during the moderate geomagnetic storm of 7 January 2005
International audienceThe arrival of an interplanetary coronal mass ejection (ICME) triggered a sudden storm commencement (SSC) at ~09:22 UT on the 7 January 2005. The ICME followed a quiet period in the solar wind and interplanetary magnetic field (IMF). We present global scale observations of energetic electron precipitation during the moderate geomagnetic storm driven by the ICME. Energetic electron precipitation is inferred from increases in cosmic noise absorption (CNA) recorded by stations in the Global Riometer Array (GLORIA). No evidence of CNA was observed during the first four hours of passage of the ICME or following the sudden commencement (SC) of the storm. This is consistent with the findings of Osepian and Kirkwood (2004) that SCs will only trigger precipitation during periods of geomagnetic activity or when the magnetic perturbation in the magnetosphere is substantial. CNA was only observed following enhanced coupling between the IMF and the magnetosphere, resulting from southward oriented IMF. Precipitation was observed due to substorm activity, as a result of the initial injection and particles drifting from the injection region. During the recovery phase of the storm, when substorm activity diminished, precipitation due to density driven increases in the solar wind dynamic pressure (Pdyn) were identified. A number of increases in Pdyn were shown to drive sudden impulses (SIs) in the geomagnetic field. While many of these SIs appear coincident with CNA, SIs without CNA were also observed. During this period, the threshold of geomagnetic activity required for SC driven precipitation was exceeded. This implies that solar wind density driven SIs occurring during storm recovery can drive a different response in particle precipitation to typical SCs
Advanced surface characterization of silver nanocluster segregation in Ag-TiCN bioactive coatings by RBS, GDOES and ARXPS
Surface modification by means of wear protective and antibacterial coatings represents, nowadays, a crucial challenge in the biomaterials field in order to enhance the lifetime of bio-devices. It is possible to tailor the properties of the material by using an appropriate combination of high wear resistance (e.g., nitride or carbide coatings) and biocide agents (e.g., noble metals as silver) to fulfill its final application. This behavior is controlled at last by the outmost surface of the coating. Therefore, the analytical characterization of these new materials requires high-resolution analytical techniques able to provide information about surface and depth composition down to the nanometric level. Among these techniques are Rutherford backscattering spectrometry (RBS), glow discharge optical emission spectroscopy (GDOES), and angle resolved X-ray photoelectron spectroscopy (ARXPS). In this work, we present a comparative RBSâGDOESâARXPS study of the surface characterization of AgâTiCN coatings with Ag/Ti atomic ratios varying from 0 to 1.49, deposited at room temperature and 200 °C. RBS analysis allowed a precise quantification of the silver content along the coating with a non-uniform Ag depth distribution for the samples with higher Ag content. GDOES surface profiling revealed that the samples with higher Ag content as well as the samples deposited at 200 °C showed an ultrathin (1â10 nm) Ag-rich layer on the coating surface followed by a silver depletion zone (20â30 nm), being the thickness of both layers enhanced with Ag content and deposition temperature. ARXPS analysis confirmed these observations after applying general algorithm involving regularization in addition to singular value decomposition techniques to obtain the concentration depth profiles. Finally, ARXPS measurements were used to provide further information on the surface morphology of the samples obtaining an excellent agreement with SEM observations when a growth model of silver islands with a height dâ=â1.5 nm and coverage Ξâ=â0.20 was applied to the sample with Ag/Tiâ=â1.49 and deposited at room temperature.This work was financially supported by the Spanish Ministry of Science and Innovation (projects FUNCOAT CSD2008-00023 and RyC2007-0026). This research is sponsored by FEDER funds through the program COMPETE "Programa Operacional Factores de Competitividade" and by national funds through FCT "Fundacao para a Ciencia e a Tecnologia", in the framework of the Strategic Projects PEST-C/FIS/UI607/2011, and PEST-C/EME/UI0285/2011 and under the project PTDC/CTM/102853/2008. The authors would like to acknowledge I. Caretti and R. Velasco for the fruitful discussions and the proofreading of the manuscript
New type of ensemble of quasi-periodic, long-lasting VLF emissions at the auroral zone
A new type of the series of quasi-periodic (QP) very low
frequency (VLF) emissions in frequency range of 1â5 kHz, and not associated
with geomagnetic pulsations, has been discovered at auroral latitudes
(<I>L</I> = 5.3) during the Finnish VLF campaign (held in December 2011). At least
five unusually spectacular events, each with a duration of several hours,
have been observed during the night under conditions of quiet geomagnetic
activity (Kp = 0â1), although QPs usually occur during the daytime. Contrary
to the QP emissions typically occurring during the day, the spectral
structure of these QP events represented an extended, complicated sequence
of repeated discrete rising VLF signals. Their duration was about 2â3 min
each, with the repetition periods ranging from ~1 min to
~10 min. Two such nighttime non-typical events are reported
in this paper. The fine structure of the separated QP elements may represent
a mixture of the different frequency band signals, which seem to have
independent origins. It was found that the periodic signals with lower
frequency appear to trigger the strong dispersive upper frequency
signals. The temporal dynamics of the spectral
structure of the QPs studied were significantly controlled by some
disturbances in the solar wind and interplanetary magnetic field (IMF). This finding is very important for
future theoretical investigations because the generation mechanism of this
new type of QP emissions is not yet understood
Magnetism in one-dimensional quantum dot arrays
We employ the density functional Kohn-Sham method in the local spin-density
approximation to study the electronic structure and magnetism of quasi
one-dimensional periodic arrays of few-electron quantum dots. At small values
of the lattice constant, the single dots overlap, forming a non-magnetic
quantum wire with nearly homogenous density. As the confinement perpendicular
to the wire is increased, i.e. as the wire is squeezed to become more
one-dimensional, it undergoes a spin-Peierls transition. Magnetism sets in as
the quantum dots are placed further apart. It is determined by the electronic
shell filling of the individual quantum dots. At larger values of the lattice
constant, the band structure for odd numbers of electrons per dot indicates
that the array could support spin-polarized transport and therefore act as a
spin filter.Comment: 11 pages, 6 figure
Ag-Ti(C,N)-based coatings for biomedical applications : influence of silver content on the structural properties
AgâTiCN coatings were deposited by dc reactive magnetron sputtering and their structural and
morphological properties were evaluated. Compositional analysis showed the existence of
AgâTiCN coatings with different Ag/Ti atomic ratios (ranging from 0 to 1.49). The structural
and morphological properties are well correlated with the evolution of Ag/Ti atomic ratio. For
the samples with low Ag/Ti atomic ratio (below 0.20) the coatings crystallize in a B1-NaCl
crystal structure typical of TiC0.3N0.7. The increase in Ag/Ti atomic ratio promoted the
formation of Ag crystalline phases as well as amorphous CNx phases detected in both x-ray
photoelectron spectroscopy and Raman spectroscopy analysis. Simultaneously to the
formation of Ag crystalline phases and amorphous carbon-based phases, a decrease in
TiC0.3N0.7 grain size was observed as well as the densification of coatings.Spanish Ministry of Science and InnovationFundação para a CiĂȘncia e Tecnologia (FCT)CRUP InstitutionMCIN
Close-Packing of Clusters: Application to Al_100
The lowest energy configurations of close-packed clusters up to N=110 atoms
with stacking faults are studied using the Monte Carlo method with Metropolis
algorithm. Two types of contact interactions, a pair-potential and a many-atom
interaction, are used. Enhanced stability is shown for N=12, 26, 38, 50, 59,
61, 68, 75, 79, 86, 100 and 102, of which only the sizes 38, 75, 79, 86, and
102 are pure FCC clusters, the others having stacking faults. A connection
between the model potential and density functional calculations is studied in
the case of Al_100. The density functional calculations are consistent with the
experimental fact that there exist epitaxially grown FCC clusters starting from
relatively small cluster sizes. Calculations also show that several other
close-packed motifs existwith comparable total energies.Comment: 9 pages, 7 figure
Magnetic phases of one-dimensional lattices with 2 to 4 fermions per site
We study the spectral and magnetic properties of one-dimensional lattices
filled with 2 to 4 fermions (with spin 1/2) per lattice site. We use a
generalized Hubbard model that takes account all interactions on a lattice
site, and solve the many-particle problem by exact diagonalization. We find an
intriguing magnetic phase diagram which includes ferromagnetism, spin-one
Heisenberg antiferromagnetism, and orbital antiferromagnetism.Comment: 8 pages, 6 figure
Resonant tunneling through a macroscopic charge state in a superconducting SET transistor
We predict theoretically and observe in experiment that the differential
conductance of a superconducting SET transistor exhibits a peak which is a
complete analogue in a macroscopic system of a standard resonant tunneling peak
associated with tunneling through a single quantum state. In particular, in a
symmetric transistor, the peak height is universal and equal to . Away from the resonance we clearly observe the co-tunneling current
which in contrast to the normal-metal transistor varies linearly with the bias
voltage.Comment: 11 pages, 3 figures, Fig. 1 available upon request from the first
autho
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