1,152 research outputs found
Emission Noise and High Frequency Cut-Off of the Kondo Effect in a Quantum Dot
By coupling on chip a carbon nanotube to a quantum noise detector, a
superconductor-insulator-superconductor junction, via a resonant circuit, we
measure the emission noise of a carbon nanotube quantum dot in the Kondo
regime. The signature of the Kondo effect in the current noise is measured for
different ratios of the Kondo temperature over the measured frequency and for
different asymmetries of the coupling to the contacts, and compared to finite
frequency quantum noise calculations. Our results point towards the existence
of a high frequency cut-off of the electronic emission noise associated with
the Kondo resonance. This cut-off frequency is of the order of a few times the
Kondo temperature when the electronic system is close to equilibrium, which is
the case for a strongly asymmetric coupling. On the other hand, this cut-off is
shifted to lower frequency in a symmetric coupling situation, where the bias
voltage drives the Kondo state out-of-equilibrium. We then attribute the low
frequency cut-off to voltage induced spin relaxation.Comment: 5 pages, 3 figures and appendi
0- quantum transition in a carbon nanotube Josephson junction: universal phase dependence and orbital degeneracy
We investigate experimentally the supercurrent in a clean carbon nanotube
quantum dot, close to orbital degeneracy, connected to superconducting leads in
a regime of strong competition between local electronic correlations and
superconducting proximity effect. For an odd occupancy of the dot and
intermediate coupling to the reservoir, the Kondo effect can develop in the
normal state and screen the local magnetic moment of the dot. This leads to
singlet-doublet transitions that strongly affect the Josephson effect in a
single-level quantum dot: the sign of the supercurrent changes from positive to
negative (0 to -junction). In the regime of strongest competition between
the Kondo effect and proximity effect, meaning that the Kondo temperature
equals the superconducting gap, the magnetic state of the dot undergoes a first
order quantum transition induced by the superconducting phase difference across
the junction. This is revealed experimentally by anharmonic current-phase
relations. In addition, the very specific electronic configuration of clean
carbon nanotubes, with two nearly orbitally degenerated states, leads to
different physics depending whether only one or both quasi-degenerate upper
levels of the dots participate to transport, which is determined by their
occupancy and relative widths. When the transport of Cooper pairs takes place
through only one of these levels, we find that the phase diagram of the
phase-dependent 0- transition is a universal characteristic of a
discontinuous level-crossing quantum transition at zero temperature. In the
case were two levels participate to transport, the nanotube Josephson current
exhibits a continuous 0- transition, independent of the superconducting
phase, revealing a different physical mechanism of the transition.Comment: 14 pages, 12 figure
Manipulating the magnetic state of a carbon nanotube Josephson junction using the superconducting phase
The magnetic state of a quantum dot attached to superconducting leads is
experimentally shown to be controlled by the superconducting phase difference
across the dot. This is done by probing the relation between the Josephson
current and the superconducting phase difference of a carbon nanotube junction
whose Kondo energy and superconducting gap are of comparable size. It exhibits
distinctively anharmonic behavior, revealing a phase mediated singlet to
doublet transition. We obtain an excellent quantitative agreement with
numerically exact quantum Monte Carlo calculations. This provides strong
support that we indeed observed the finite temperature signatures of the phase
controlled zero temperature level-crossing transition originating from strong
local electronic correlations.Comment: 5 pages, 4 figures + supp. material
Prompt Photon Identification in the ALICE Experiment: The Isolation Cut Method
Submitted for publication in NIMThe ALICE experiment at LHC will detect and identify prompt photons and light neutral mesons with the PHOS and EMCal detectors. Charged particles will be detected and identified by the central tracking system. In this paper, a method to identify prompt photons and to separate them from the background of hadrons and decay photons in PHOS with the help of isolation cuts is presented
Emission patterns of neutral pions in 40 A MeV Ta+Au reactions
Differential cross sections of neutral pions emitted in 181Ta + 197Au
collisions at a beam energy of 39.5A MeV have been measured with the photon
spectrometer TAPS. The kinetic energy and transverse momentum spectra of
neutral pions cannot be properly described in the framework of the thermal
model, nor when the reabsorption of pions is accounted for in a
phenomenological model. However, high energy and high momentum tails of the
pion spectra can be well fitted through thermal distributions with unexpectedly
soft temperature parameters below 10 MeV.Comment: 16 pages (double-spaced), 5 figures; corrections after referee's
comments and suggestion
Thermal bremsstrahlung probing the thermodynamical state of multifragmenting systems
Inclusive and exclusive hard-photon (E 30 MeV) production in five
different heavy-ion reactions (Ar+Au, Ag, Ni,
C at 60{\it A} MeV and Xe+Sn at 50{\it A} MeV) has been
studied coupling the TAPS photon spectrometer with several charged-particle
multidetectors covering more than 80% of 4. The measured spectra, slope
parameters and source velocities as well as their target-dependence, confirm
the existence of thermal bremsstrahlung emission from secondary nucleon-nucleon
collisions that accounts for roughly 20% of the total hard-photon yield. The
thermal slopes are a direct measure of the temperature of the excited nuclear
systems produced during the reaction.Comment: 4 pages, 3 figures, Proceedings CRIS 2000, 3rd Catania Relativistic
Ion Studies, "Phase Transitions in Strong Interactions: Status and
Perspectives", Acicastello, Italy, May 22-26, 2000 (to be published in Nuc.
Phys. A
Does shade improve light interception efficiency? A comparison among seedlings from shade-tolerant and -intolerant temperate deciduous tree species
• Here, we tested two hypotheses: shading increases light interception efficiency (LIE) of broadleaved tree seedlings, and shade-tolerant species exhibit larger LIEs than do shade-intolerant ones. The impact of seedling size was taken into account to detect potential size-independent effects on LIE. LIE was defined as the ratio of mean light intercepted by leaves to light intercepted by a horizontal surface of equal area. • Seedlings from five species differing in shade tolerance (Acer saccharum, Betula alleghaniensis, A. pseudoplatanus, B. pendula, Fagus sylvatica) were grown under neutral shading nets providing 36, 16 and 4% of external irradiance. Seedlings (1- and 2-year-old) were three-dimensionally digitized, allowing calculation of LIE. • Shading induced dramatic reduction in total leaf area, which was lowest in shade-tolerant species in all irradiance regimes. Irradiance reduced LIE through increasing leaf overlap with increasing leaf area. There was very little evidence of significant size-independent plasticity of LIE. • No relationship was found between the known shade tolerance of species and LIE at equivalent size and irradiance
Identification of photon-tagged jets in the ALICE experiment
The ALICE experiment at LHC will detect and identify prompt photons and light
neutral-mesons with the PHOS detector and the additional EMCal electromagnetic
calorimeter. Charged particles will be detected and identified by the central
tracking system. In this article, the possibility of studying the interaction
of jets with the nuclear medium, using prompt photons as a tool to tag jets, is
investigated by simulations. New methods to identify prompt photon-jet events
and to distinguish them from the jet-jet background are presented.Comment: NIM A: accepted manuscrip
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