35 research outputs found
Depinning transition in type-II superconductors
The surface impedance Z(f) of conventional isotropic materials has been
carefully measured for frequencies f ranging from 1 kHz to 3 MHz, allowing a
detailed investigation of the depinning transition. Our results exhibit the
irrelevance of classical ideas to the dynamics of vortex pinning. We propose a
new picture, where the linear ac response is entirely governed by disordered
boundary conditions of a rough surface, whereas in the bulk vortices respond
freely. The universal law for Z(f) thus predicted is in remarkable agreement
with experiment, and tentatively applies to microwave data in YBaCuO films.Comment: 4 pages, 4 figures, 14 reference
Separation of neutral and charge modes in one dimensional chiral edge channels
Coulomb interactions have a major role in one-dimensional electronic
transport. They modify the nature of the elementary excitations from Landau
quasiparticles in higher dimensions to collective excitations in one dimension.
Here we report the direct observation of the collective neutral and charge
modes of the two chiral co-propagating edge channels of opposite spins of the
quantum Hall effect at filling factor 2. Generating a charge density wave at
frequency f in the outer channel, we measure the current induced by
inter-channel Coulomb interaction in the inner channel after a 3-mm propagation
length. Varying the driving frequency from 0.7 to 11 GHz, we observe damped
oscillations in the induced current that result from the phase shift between
the fast charge and slow neutral eigenmodes. We measure the dispersion relation
and dissipation of the neutral mode from which we deduce quantitative
information on the interaction range and parameters.Comment: 23 pages, 6 figure
rf-studies of vortex dynamics in isotropic type-II superconductors
We have measured the surface impedance of thick superconductors in the mixed
state over a broad 2 kHz - 20 MHz frequency range. The depinning cross-over is
observed; but it is much broader than expected from classical theories of
pinning. A striking result is the existence of size effects which invalidate
the common interpretation of the low-frequency surface inductance in terms of a
single penetration depth. Instead, a two-mode description of vortex dynamics,
assuming free vortex flow in the bulk and surface pinning, accounts
quantitatively for the spectrum of the complex apparent penetration depth.Comment: 20 pages, 6 figures, 28 reference
Electron quantum optics : partitioning electrons one by one
We have realized a quantum optics like Hanbury Brown and Twiss (HBT)
experiment by partitioning, on an electronic beam-splitter, single elementary
electronic excitations produced one by one by an on-demand emitter. We show
that the measurement of the output currents correlations in the HBT geometry
provides a direct counting, at the single charge level, of the elementary
excitations (electron/hole pairs) generated by the emitter at each cycle. We
observe the antibunching of low energy excitations emitted by the source with
thermal excitations of the Fermi sea already present in the input leads of the
splitter, which suppresses their contribution to the partition noise. This
effect is used to probe the energy distribution of the emitted wave-packets.Comment: 5 pages, 4 figure
Why pinning by surface irregularities can explain the peak effect in transport properties and neutron diffraction results in NbSe2 and Bi-2212 crystals?
The existence of a peak effect in transport properties (a maximum of the
critical current as function of magnetic field) is a well-known but still
intriguing feature of type II superconductors such as NbSe2 and Bi-2212. Using
a model of pinning by surface irregularities in anisotropic superconductors, we
have developed a calculation of the critical current which allows estimating
quantitatively the critical current in both the high critical current phase and
in the low critical current phase. The only adjustable parameter of this model
is the angle of the vortices at the surface. The agreement between the
measurements and the model is really very impressive. In this framework, the
anomalous dynamical properties close to the peak effect is due to co-existence
of two different vortex states with different critical currents. Recent neutron
diffraction data in NbSe2 crystals in presence of transport current support
this point of view
Sex differences in intestinal carbohydrate metabolism promote food intake and sperm maturation
Physiology and metabolism are often sexually dimorphic, but the underlying mechanisms remain incompletely understood. Here, we use the intestine of Drosophila melanogaster to investigate how gut-derived signals contribute to sex differences in whole-body physiology. We find that carbohydrate handling is male-biased in a specific portion of the intestine. In contrast to known sexual dimorphisms in invertebrates, the sex differences in intestinal carbohydrate metabolism are extrinsically controlled by the adjacent male gonad, which activates JAK-STAT signalling in enterocytes within this intestinal portion. Sex reversal experiments establish roles for this malebiased intestinal metabolic state in controlling food intake and sperm production through gutderived citrate. Our work uncovers a male gonad-gut axis coupling diet and sperm production, and reveals that metabolic communication across organs is physiologically significant. The instructive role of citrate in inter-organ communication may be significant in more biological contexts than previously recognised
Observation of edge magnetoplasmon squeezing in a quantum Hall conductor
Squeezing of the quadratures of the electromagnetic field has been
extensively studied in optics and microwaves. However, previous works focused
on the generation of squeezed states in a low impedance () environment. We report here on the demonstration of the squeezing of
bosonic edge magnetoplasmon modes in a quantum Hall conductor whose
characteristic impedance is set by the quantum of resistance (), offering the possibility of an enhanced coupling to low-dimensional
quantum conductors. By applying a combination of dc and ac drives to a quantum
point contact, we demonstrate squeezing and observe a noise reduction 18\%
below the vacuum fluctuations. This level of squeezing can be improved by using
more complex conductors, such as ac driven quantum dots or mesoscopic
capacitors.Comment: 6+2 pages, 3+1 figure
Critical Velocity of Vortex Nucleation in Rotating Superfluid 3He-A
We have measured the critical velocity v_c at which 3He-A in a rotating
cylinder becomes unstable against the formation of quantized vortex lines with
continuous (singularity-free) core structure. We find that v_c is distributed
between a maximum and minimum limit, which we ascribe to a dependence on the
texture of the orbital angular momentum l(r) in the cylinder. Slow cool down
through T_c in rotation yields l(r) textures for which the measured v_c's are
in good agreement with the calculated instability of the expected l texture.Comment: 4 pages, 3 figure
Defect Formation in Quench-Cooled Superfluid Phase Transition
We use neutron absorption in rotating 3He-B to heat locally a 10
micrometer-size volume into normal phase. When the heated region cools back in
microseconds, vortex lines are formed. We record with NMR the number of lines
as a function of superflow velocity and compare to the Kibble-Zurek theory of
vortex-loop freeze-out from a random network of defects. The measurements
confirm the calculated loop-size distribution and show that also the superfluid
state itself forms as a patchwork of competing A and B phase blobs. This
explains the A to B transition in supercooled neutron-irradiated 3He-A.Comment: RevTex file, 4 pages, 3 figures, resubmitted to Phys. Rev. Let