363 research outputs found
Quenching of phase coherence in quasi-one dimensional ring crystals
The comparison of the single-particle (SP) dynamics between the whisker and
ring NbSe crystals provides new insight into the phase transition
properties in quasi-one-dimensional charge density wave (CDW) systems.Comment: 9 pages, 4 figure
Shot Noise with Interaction Effects in Single Walled Carbon Nanotubes
We have measured shot noise in single walled carbon nanotubes (SWNT) with
good contacts at 4.2 K at low frequencies ( MHz). We find a strong
modulation of shot noise over the Fabry-Perot pattern; in terms of differential
Fano factor the variation ranges over 0.4 - 1.2. The shot noise variation, in
combination with differential conductance, is analyzed using two
(spin-degenerate) modes with different, energy-dependent transmission
coefficients. No power law dependence of shot noise, as expected for Luttinger
liquids, was found in our measurements.Comment: 5 pages, 4 figure
Diffusive Josephson junctions made out of multiwalled carbon nanotubes
We have investigated electrical transport in diffusive multiwalled carbon nanotubes (MWNT) contacted using superconducting leads made of Ti/Al/Ti sandwich structure. We measure proximity-induced supercurrents up to Icm = 1.3 nA and find tunability by the gate voltage due to variation of the Thouless energy via the diffusion constant that is controlled by scattering in the MWNT. The modeling of these results by long, diffusive SNS junctions, supplemented with phase diffusion effects is discussed: the agreement between theory and experiments is tested especially on the basis of the temperature dependence of the Josephson coupling energy. In order to prove conclusively that the diffusive model works for MWNT proximity junctions, we propose an improved measurement scheme that is based on the kinetic inductance of superconducting junctions.Peer reviewe
The Mixed State of Charge-Density-Wave in a Ring-Shaped Single Crystals
Charge-density-wave (CDW) phase transition in a ring-shaped crystals,
recently synthesized by Tanda et al. [Nature, 417, 397 (2002)], is studied
based on a mean-field-approximation of Ginzburg-Landau free energy. It is shown
that in a ring-shaped crystals CDW undergoes frustration due to the curvature
(bending) of the ring (geometrical frustration) and, thus, forms a mixed state
analogous to what a type-II superconductor forms under a magnetic field. We
discuss the nature of the phase transition in the ring-CDW in relation to
recent experiments.Comment: 6 pages, 4 figure
Drift-Kinetic Modeling of Particle Acceleration and Transport in Solar Flares
Based on the drift-kinetic theory, we develop a model for particle
acceleration and transport in solar flares. The model describes the evolution
of the particle distribution function by means of a numerical simulation of the
drift-kinetic Vlasov equation, which allows us to directly compare simulation
results with observations within an actual parameter range of the solar corona.
Using this model, we investigate the time evolution of the electron
distribution in a flaring region. The simulation identifies two dominant
mechanisms of electron acceleration. One is the betatron acceleration at the
top of closed loops, which enhances the electron velocity perpendicular to the
magnetic field line. The other is the inertia drift acceleration in open
magnetic field lines, which produces antisunward electrons. The resulting
velocity space distribution significantly deviates from an isotropic
distribution. The former acceleration can be a generation mechanism of
electrons that radiate loop-top nonthermal emissions, and the latter be of
escaping electrons from the Sun that should be observed by in-situ measurements
in interplanetary space and resulting radio bursts through plasma
instabilities.Comment: 32 Pages, 11 figures, accepted by Ap
Formation Process of a Light Bridge Revealed with the Hinode Solar Optical Telescope
The Solar Optical Telescope (SOT) aboard HINODE successfully and continuously
observed a formation process of a light bridge in a matured sunspot of the NOAA
active region 10923 for several days with high spatial resolution. During its
formation, many umbral dots were observed emerging from the leading edges of
penumbral filaments, and intruding into the umbra rapidly. The precursor of the
light bridge formation was also identified as the relatively slow inward motion
of the umbral dots which emerged not near the penumbra, but inside the umbra.
The spectro-polarimeter on SOT provided physical conditions in the photosphere
around the umbral dots and the light bridges. We found the light bridges and
the umbral dots had significantly weaker magnetic fields associated with
upflows relative to the core of the umbra, which implies that there was hot gas
with weak field strength penetrating from subphotosphere to near the visible
surface inside those structures. There needs to be a mechanism to drive the
inward motion of the hot gas along the light bridges. We suggest that the
emergence and the inward motion are triggered by a buoyant penumbral flux tube
as well as the subphotospheric flow crossing the sunspot.Comment: 8 pages, 6 figures, accepted in the PASJ Hinode special issu
Measurements of shot noise in single walled carbon nanotubes
We have measured shot noise in single walled carbon nanotubes (SWNT) at 4.2K over frequencies f = 600 – 850 MHz. Here we report results obtained on shot noise without DC bias by applying an AC modulation at ω0 and recording the noise variation at 2ω0: the Fano factor is obtained by extrapolating down to zero excitation amplitude. We also discuss the applicability of this method in samples which have strongly non‐linear IV characteristics like carbon nanotubes. The obtained results are compared with regular differential noise measurements where both DC and AC bias are employed.Peer reviewe
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