1,150 research outputs found
Liquid-induced damping of mechanical feedback effects in single electron tunneling through a suspended carbon nanotube
In single electron tunneling through clean, suspended carbon nanotube devices
at low temperature, distinct switching phenomena have regularly been observed.
These can be explained via strong interaction of single electron tunneling and
vibrational motion of the nanotube. We present measurements on a highly stable
nanotube device, subsequently recorded in the vacuum chamber of a dilution
refrigerator and immersed in the 3He/4He mixture of a second dilution
refrigerator. The switching phenomena are absent when the sample is kept in the
viscous liquid, additionally supporting the interpretation of dc-driven
vibration. Transport measurements in liquid helium can thus be used for finite
bias spectroscopy where otherwise the mechanical effects would dominate the
current.Comment: 4 pages, 3 figure
Negative frequency tuning of a carbon nanotube nano-electromechanical resonator
A suspended, doubly clamped single wall carbon nanotube is characterized as
driven nano-electromechanical resonator at cryogenic temperatures.
Electronically, the carbon nanotube displays small bandgap behaviour with
Coulomb blockade oscillations in electron conduction and transparent contacts
in hole conduction. We observe the driven mechanical resonance in dc-transport,
including multiple higher harmonic responses. The data shows a distinct
negative frequency tuning at finite applied gate voltage, enabling us to
electrostatically decrease the resonance frequency to 75% of its maximum value.
This is consistently explained via electrostatic softening of the mechanical
mode.Comment: 4 pages, 4 figures; submitted for the IWEPNM 2013 conference
proceeding
Quantum Metallicity on the High-Field Side of the Superconductor-Insulator Transition
We investigate ultrathin superconducting TiN films, which are very close to
the localization threshold. Perpendicular magnetic field drives the films from
the superconducting to an insulating state, with very high resistance. Further
increase of the magnetic field leads to an exponential decay of the resistance
towards a finite value. In the limit of low temperatures, the saturation value
can be very accurately extrapolated to the universal quantum resistance h/e^2.
Our analysis suggests that at high magnetic fields a new ground state, distinct
from the normal metallic state occurring above the superconducting transition
temperature, is formed. A comparison with other studies on different materials
indicates that the quantum metallic phase following the magnetic-field-induced
insulating phase is a generic property of systems close to the disorder-driven
superconductor-insulator transition.Comment: 4 pages, 4 figures, published versio
Aharonov-Bohm differential conductance modulation in defective metallic single-wall carbon nanotubes
Using a perturbative approach, the effects of the energy gap induced by the
Aharonov-Bohm (AB) flux on the transport properties of defective metallic
single-walled carbon nanotubes (MSWCNTs) are investigated. The electronic waves
scattered back and forth by a pair of impurities give rise to Fabry-Perot
oscillations which constitutes a coherent backscattering interference pattern
(CBSIP). It is shown that, the CBSIP is aperiodically modulated by applying a
magnetic field parallel to the nanotube axis. In fact, the AB-flux brings this
CBSIP under control by an additional phase shift. As a consequence, the extrema
as well as zeros of the CBSIP are located at the irrational fractions of the
quantity , where is the flux piercing the
nanotube cross section and is the magnetic quantum flux. Indeed,
the spacing between two adjacent extrema in the magneto-differential
conductance (MDC) profile is decreased with increasing the magnetic field. The
faster and higher and slower and shorter variations is then obtained by
metallic zigzag and armchair nanotubes, respectively. Such results propose that
defective metallic nanotubes could be used as magneto-conductance switching
devices based on the AB effect.Comment: 11 pages, 4 figure
Phase transition curves for mesoscopic superconducting samples
We compute the phase transition curves for mesoscopic superconductors.
Special emphasis is given to the limiting shape of the curve when the magnetic
flux is large. We derive an asymptotic formula for the ground state of the
Schr\"odinger equation in the presence of large applied flux. The expansion is
shown to be sensitive to the smoothness of the domain. The theoretical results
are compared to recent experiments.Comment: 8 pages, 1 figur
Altered DNA methylation in human placenta after (suspected) preterm labor
Aim: The aim of this study was to determine if alterations in DNA methylation in the human placenta would support suspected preterm labor as a pathologic insult associated with diminished placental health.
Methods: We evaluated placental DNA methylation at seven loci differentially methylated in placental pathologies using targeted bisulfite sequencing, in placentas associated with preterm labor (term birth after suspected preterm labor [n = 15] and preterm birth [n = 15]), and controls (n = 15).
Results: DNA methylation levels at the NCAM1 and PLAGL1 loci in placentas associated with preterm labor did differ significantly (p < 0.05) from controls.
Discussion: Specific alterations in methylation patterns indicative of an unfavourable placental environment are associated with preterm labor per se and not restricted to preterm birth
Mesoscopic Ferromagnet/Superconductor Junctions and the Proximity Effect
We have measured the electrical transport of submicron ferromagnets (Ni) in
contact with a mesoscopic superconductor (Al) for a range of interface
resistances. In the geometry measured, the interface and the ferromagnet are
measured separately. The ferromagnet itself shows no appreciable
superconducting proximity effect, but the ferromagnet/superconductor interface
exhibits strong temperature, field and current bias dependences. These effects
are dependent on the local magnetic field distribution near the interface
arising from the ferromagnet. We find that the temperature dependences may be
fit to a modified version of the Blonder-Tinkham-Klapwijk theory for
normal-superconductor transport.Comment: 4 eps fig
Altered gene expression in human placenta after suspected preterm labour
Introduction Suspected preterm labour occurs in around 9% of pregnancies. However, almost two-thirds of women admitted for threatened preterm labour ultimately deliver at term and are considered risk-free for fetal development. Methods We examined placental and umbilical cord blood samples from preterm or term deliveries after threatened preterm labour as well as term deliveries without threatened preterm labour. We quantitatively analysed the mRNA expression of inflammatory markers (IL6, IFN?, and TNFa) and modulators of angiogenesis (FGF2, PGF, VEGFA, VEGFB, and VEGFR1). Results A total of 132 deliveries were analysed. Preterm delivery and term delivery after suspected preterm labour groups showed similar increases in TNFa expression compared with the term delivery control group in umbilical cord blood samples. Placental samples from preterm and term deliveries after suspected preterm labour exhibited significantly increased expression of TNFa and IL6 and decreased expression of IFN?. Suspected preterm labour was also associated with altered expression of angiogenic factors, although not all differences reached statistical significance. Discussion We found gene expression patterns indicative of inflammation in human placentas after suspected preterm labour regardless of whether the deliveries occurred preterm or at term. Similarly, a trend towards altered expression of angiogeneic factors was not limited to preterm birth. These findings suggest that the biological mechanisms underlying threatened preterm labour affect pregnancies independently of gestational age at birth
Broken SU(4) symmetry in a Kondo-correlated carbon nanotube
Understanding the interplay between many-body phenomena and non-equilibrium
in systems with entangled spin and orbital degrees of freedom is a central
objective in nano-electronics. We demonstrate that the combination of Coulomb
interaction, spin-orbit coupling and valley mixing results in a particular
selection of the inelastic virtual processes contributing to the Kondo
resonance in carbon nanotubes at low temperatures. This effect is dictated by
conjugation properties of the underlying carbon nanotube spectrum at zero and
finite magnetic field. Our measurements on a clean carbon nanotube are
complemented by calculations based on a new approach to the non-equilibrium
Kondo problem which well reproduces the rich experimental observations in Kondo
transport.Comment: 8 pages, 6 figures; appendix of 14 pages, 7 figure
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