2,355 research outputs found
Status of Neutrino Masses and Mixing and Future Perspectives
Status of the problem of neutrino masses, mixing and oscillations is
discussed. Future perspectives are briefly considered.Comment: Report at the conference IRGAC 2006, Barcelona July 11-15 200
Compact and efficient power electronics with applications to battery management systems
This work investigates the use of advanced power electronics techniques for a variety of applications to both improve efficiency and decrease the size. The first area of research is on investigating limitations in high density switched-capacitor converters for voltage step-up applications. The results from this could be particularly useful for pulsed power applications. This work uses techniques such as resonance, advanced control, and interleaving in the Dickson converter to avoid common limitations in switched-capacitor circuits. Another area of research is on fast battery charging using active battery management system topologies. These topologies have been proven before but this work expands upon those by using modern power electronics techniques to minimize the size and maximize the efficiency. This is achieved by using high frequency, GaN switches, planar magnetics, and active core resetting in a forward converter. This system was developed to be isolated and bi-directional so multiple active battery management system topologies could be used from the same design
The low-temperature energy calibration system for the CUORE bolometer array
The CUORE experiment will search for neutrinoless double beta decay (0nDBD)
of 130Te using an array of 988 TeO_2 bolometers operated at 10 mK in the
Laboratori Nazionali del Gran Sasso (Italy). The detector is housed in a large
cryogen-free cryostat cooled by pulse tubes and a high-power dilution
refrigerator. The TeO_2 bolometers measure the event energies, and a precise
and reliable energy calibration is critical for the successful identification
of candidate 0nDBD and background events. The detector calibration system under
development is based on the insertion of 12 gamma-sources that are able to move
under their own weight through a set of guide tubes that route them from
deployment boxes on the 300K flange down into position in the detector region
inside the cryostat. The CUORE experiment poses stringent requirements on the
maximum heat load on the cryostat, material radiopurity, contamination risk and
the ability to fully retract the sources during normal data taking. Together
with the integration into a unique cryostat, this requires careful design and
unconventional solutions. We present the design, challenges, and expected
performance of this low-temperature energy calibration system.Comment: To be published in the proceedings of the 13th International Workshop
on Low Temperature Detectors (LTD), Stanford, CA, July 20-24, 200
Stripe phases in high-temperature superconductors
Stripe phases are predicted and observed to occur in a class of
strongly-correlated materials describable as doped antiferromagnets, of which
the copper-oxide superconductors are the most prominent representative. The
existence of stripe correlations necessitates the development of new principles
for describing charge transport, and especially superconductivity, in these
materials.Comment: 5 pp, 1 color eps fig., to appear as a Perspective in Proc. Natl.
Acad. Sci. US
Ultrafast holography and transient-absorption spectroscopy in charge-transfer polymers
Charge-transfer polymers are a new class of nonlinear optical materials which can be used for generating femtosecond holographic gratings. Using semiconducting polymers sensitized with varying concentrations of C{sub 60}, holographic gratings were recorded by individual ultrafast laser pulses; the diffraction efficiency and time decay of the gratings were measured using non-degenerate four-wave mixing. Using a figure of merit for dynamic data processing, the temporal diffraction efficiency, this new class of materials exhibits between two and 12 orders of magnitude higher response than previous reports. The charge transfer range at polymer/C{sub 60} interfaces was further studied using transient absorption spectroscopy. The fact that charge-transfer occurs in the picosecond-time scale in bilayer structures (thickness 200 {angstrom}) implies that diffusion of localized excitations to the interface is not the dominant mechanism; the charge transfer range is a significant fraction of the film thickness. From analysis of the excited state decay curves, we estimate the charge transfer range to be 80 {angstrom} and interpret that range as resulting from quantum delocalization of the photoexcitations
Concentration-dependent mobility in organic field-effect transistors probed by infrared spectromicroscopy of the charge density profile
We show that infrared imaging of the charge density profile in organic
field-effect transistors (FETs) can probe transport characteristics which are
difficult to access by conventional contact-based measurements. Specifically,
we carry out experiments and modeling of infrared spectromicroscopy of
poly(3-hexylthiophene) (P3HT) FETs in which charge injection is affected by a
relatively low resistance of the gate insulators. We conclude that the mobility
of P3HT has a power-law density dependence, which is consistent with the
activated transport in disorder-induced tails of the density of states.Comment: 3+ pages, 2 figure
Magnetic Structure of Nano-Graphite Moebius Ribbon
We consider the electronic and magnetic properties of nanographite ribbon
with zigzag edges under the periodic or Moebius boundary conditions. The zigzag
nano-graphite ribbons possess edge localized states at the Fermi level which
cause a ferrimagnetic spin polarization localized at the edge sites even in the
very weak Coulomb interaction. The imposition of the Moebius boundary condition
makes the system non-AB-bipartite lattice, and depress the spin polarization,
resulting in the formation of a magnetic domain wall. The width of the magnetic
domain depends on the Coulomb interaction and narrows with increasing U/t.Comment: 4 pages; 6 figures; published at J. Phys. Soc. Jpn. Vol. 72 No. 5 pp.
998-1001 (2003
Dimerization structures on the metallic and semiconducting fullerene tubules with half-filled electrons
Possible dimerization patterns and electronic structures in fullerene tubules
as the one-dimensional pi-conjugated systems are studied with the extended
Su-Schrieffer-Heeger model. We assume various lattice geometries, including
helical and nonhelical tubules. The model is solved for the half-filling case
of -electrons. (1) When the undimerized systems do not have a gap, the
Kekule structures prone to occur. The energy gap is of the order of the room
temperatures at most and metallic properties would be expected. (2) If the
undimerized systems have a large gap (about 1eV), the most stable structures
are the chain-like distortions where the direction of the arranged
trans-polyacetylene chains is along almost the tubular axis. The electronic
structures are ofsemiconductors due to the large gap.Comment: submitted to Phys. Rev. B, pages 15, figures 1
Topological Excitations of One-Dimensional Correlated Electron Systems
Properties of low-energy excitations in one-dimensional superconductors and
density-wave systems are examined by the bosonization technique. In addition to
the usual spin and charge quantum numbers, a new, independently measurable
attribute is introduced to describe elementary, low-energy excitations. It can
be defined as a number w which determines, in multiple of , how many times
the phase of the order parameter winds as an excitation is transposed from far
left to far right. The winding number is zero for electrons and holes with
conventional quantum numbers, but it acquires a nontrivial value w=1 for
neutral spin-1/2 excitations and for spinless excitations with a unit electron
charge. It may even be irrational, if the charge is irrational. Thus, these
excitations are topological, and they can be viewed as composite particles made
of spin or charge degrees of freedom and dressed by kinks in the order
parameter.Comment: 5 pages. And we are not only splitting point
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