6,268 research outputs found
Metal-insulator transition in YHx: scaling of the sub-THz conductivity
The established scaling laws of the conductivity with temperature and doping
are strong indications for the quantum nature of the metal-insulator transition
in YH. Here we report the first results on the frequency scaling of the
conductivity. Samples were brought from the insulating to the metallic phase by
carrier doping via illumination. In the metallic phase, the sub-terahertz
conductivity coincides with the dc data. These results do not agree with the
simplest picture of a quantum-phase transition.Comment: 4 pages, accepted to PR
Ageing effects around the glass and melting transitions in poly(dimethylsiloxane) visualized by resistance measurements
The process of ageing in rubbers requires monitoring over long periods (days
to years). To do so in non-conducting rubbers, small amounts of carbon-black
particles were dispersed in a fractal network through the rubber matrix, to
make the rubber conducting without modifying its properties. Continuous
monitoring of the resistance reveals the structural changes around the glass
and melting transitions and especially details about the hysteresis and ageing
processes. We illustrate the method for the semicrystalline polymer
poly(dimethylsiloxane) (PDMS).Comment: 4 pages, 4 figure
Magnetic dipolar ordering and relaxation in the high-spin molecular cluster compound Mn6
Few examples of magnetic systems displaying a transition to pure dipolar
magnetic order are known to date, and single-molecule magnets can provide an
interesting example. The molecular cluster spins and thus their dipolar
interaction energy can be quite high, leading to reasonably accessible ordering
temperatures, provided the crystal field anisotropy is sufficiently small. This
condition can be met for molecular clusters of sufficiently high symmetry, as
for the Mn6 compound studied here. Magnetic specific heat and susceptibility
experiments show a transition to ferromagnetic dipolar order at T_{c} = 0.16 K.
Classical Monte-Carlo calculations indeed predict ferromagnetic ordering and
account for the correct value of T_{c}. In high magnetic fields we detected the
contribution of the ^{55}Mn nuclei to the specific heat, and the characteristic
timescale of nuclear relaxation. This was compared with results obtained
directly from pulse-NMR experiments. The data are in good mutual agreement and
can be well described by the theory for magnetic relaxation in highly polarized
paramagnetic crystals and for dynamic nuclear polarization, which we
extensively review. The experiments provide an interesting comparison with the
recently investigated nuclear spin dynamics in the anisotropic single molecule
magnet Mn12-ac.Comment: 19 pages, 11 eps figures. Contains extensive discussions on dipolar
ordering, specific heat and nuclear relaxation in molecular magnet
Testbeam and Laboratory Characterization of CMS 3D Pixel Sensors
The pixel detector is the innermost tracking device in CMS, reconstructing
interaction vertices and charged particle trajectories. The sensors located in
the innermost layers of the pixel detector must be upgraded for the ten-fold
increase in luminosity expected with the High- Luminosity LHC (HL-LHC) phase.
As a possible replacement for planar sensors, 3D silicon technology is under
consideration due to its good performance after high radiation fluence. In this
paper, we report on pre- and post- irradiation measurements for CMS 3D pixel
sensors with different electrode configurations. The effects of irradiation on
electrical properties, charge collection efficiency, and position resolution of
3D sensors are discussed. Measurements of various test structures for
monitoring the fabrication process and studying the bulk and surface
properties, such as MOS capacitors, planar and gate-controlled diodes are also
presented.Comment: 14 page
Direct link between Coulomb blockade and shot noise in a quantum coherent structure
We analyze the current-voltage characteristic of a quantum conduction channel
coupled to an electromagnetic environment of arbitrary frequency-dependent
impedance. In the weak blockade regime the correction to the ohmic behavior is
directly related to the channel current fluctuations vanishing at perfect
transmission in the same way as shot noise. This relation can be generalized to
describe the environmental Coulomb blockade in a generic mesoscopic conductor
coupled to an external impedance, as the response of the latter to the current
fluctuations in the former.Comment: 12 pages, 2 figures, submitted to Phys. Rev. Let
Localized and Delocalized Charge Transport in Single-Wall Carbon-Nanotube Mats
We measured the complex dielectric constant in mats of single-wall
carbon-nanotubes between 2.7 K and 300 K up to 0.5 THz. The data are well
understood in a Drude approach with a negligible temperature dependence of the
plasma frequency (omega_p) and scattering time (tau) with an additional
contribution of localized charges. The dielectric properties resemble those of
the best ''metallic'' polypyrroles and polyanilines. The absence of metallic
islands makes the mats a relevant piece in the puzzle of the interpretation of
tau and omega_p in these polymers.Comment: 4 pages including 4 figure
Measurement of the conductance of a hydrogen molecule
Recent years have shown steady progress in research towards molecular
electronics [1,2], where molecules have been investigated as switches [3-5],
diodes [6], and electronic mixers [7]. In much of the previous work a Scanning
Tunnelling Microscope was employed to address an individual molecule. As this
arrangement does not provide long-term stability, more recently
metal-molecule-metal links have been made using break junction devices [8-10].
However, it has been difficult to establish unambiguously that a single
molecule forms the contact [11]. Here, we show that a single H2 molecule can
form a stable bridge between Pt electrodes. In contrast to results for other
organic molecules, the bridge has a nearly perfect conductance of one quantum
unit, carried by a single channel. The H2-bridge provides a simple test system
and a fundamental step towards understanding transport properties of
single-molecule devices.Comment: 6 pages, 4 figure
Shot noise suppression in multimode ballistic Fermi conductors
We have derived a general formula describing current noise in multimode
ballistic channels connecting source and drain electrodes with Fermi electron
gas. In particular (at ), the expression describes the
nonequilibrium ''shot'' noise, which may be suppressed by both Fermi
correlations and space charge screening. The general formula has been applied
to an approximate model of a 2D nanoscale, ballistic MOSFET. At large negative
gate voltages, when the density of electrons in the channel is small, shot
noise spectral density approaches the Schottky value , where
is the average current. However, at positive gate voltages, when the
maximum potential energy in the channel is below the Fermi level of the
electron source, the noise can be at least an order of magnitude smaller than
the Schottky value, mostly due to Fermi effects.Comment: 4 page
Dopant-induced crossover from 1D to 3D charge transport in conjugated polymers
The interplay between inter- and intra-chain charge transport in bulk
polythiophene in the hopping regime has been clarified by studying the
conductivity as a function of frequency (up to 3 THz), temperature and doping
level. We present a model which quantitatively explains the observed crossover
from quasi-one-dimensional transport to three-dimensional hopping conduction
with increasing doping level. At high frequencies the conductivity is dominated
by charge transport on one-dimensional conducting chains.Comment: 4 pages, 2 figure
Shot noise in ferromagnetic single electron tunneling devices
Frequency dependent current noise in ferromagnetic double junctions with
Coulomb blockade is studied theoretically in the limit of sequential tunneling.
Two different relaxation processes are found in the correlations between spin
polarized tunneling currents; low frequency spin fluctuations and high
frequency charge fluctuations. Spin accumulation in strongly asymmetric
junctions is shown to lead to a negative differential resistance. We also show
that large spin noise activated in the range of negative differential
resistance gives rise to a significant enhancement of the current noise.Comment: 8 pages, 13 eps-figures include
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