259 research outputs found
Trapping electrons in electrostatic traps over the surface of helium
We have observed trapping of electrons in an electrostatic trap formed over
the surface of liquid helium-4. These electrons are detected by a Single
Electron Transistor located at the centre of the trap. We can trap any desired
number of electrons between 1 and . By repeatedly (
times) putting a single electron into the trap and lowering the electrostatic
barrier of the trap, we can measure the effective temperature of the electron
and the time of its thermalisation after heating up by incoherent radiation.Comment: Presented at QFS06 - Kyoto, to be published in J. Low Temp. Phys., 6
pages, 3 figure
Wigner islands with electrons over helium
We present here the first experimental study of Wigner islands formed by
electrons floating over helium. Electrons are trapped electrostatically in a
mesoscopic structure covered with a helium film, behaving as a quantum dot. By
removing electrons one by one, we are able to find the addition spectrum, i.e.
the energy required to add (or extract) one electron from the trap with
occupation number . Experimental addition spectra are compared with Monte
Carlo simulations for the actual trap geometry, confirming the ordered state of
electrons over helium in the island.Comment: 12 pages, 13 figures, draft pape
Low frequency acoustics in solid He at low temperature
The elastic properties of hcp He samples have been investigated using low
frequency (20 Hz to 20 kHz) high sensitivity sound transducers. In agreement
with the findings of other workers, most samples studied grew very
significantly stiffer at low temperature; Poisson's ratio was observed to
increase from 0.28 below 20 mK to at 0.7 K. The span of the
variation of varies from sample to sample according to their thermal and
mechanical history. Crystals carefully grown at the melting curve show a
different behavior, the change in taking place at lower and being
more abrupt
Search for supersolidity in 4He in low-frequency sound experiments
We present results of the search for supersolid 4He using low-frequency,
low-level mechanical excitation of a solid sample grown and cooled at fixed
volume. We have observed low frequency non-linear resonances that constitute
anomalous features. These features, which appear below about 0.8 K, are absent
in 3He. The frequency, the amplitude at which the nonlinearity sets in, and the
upper temperature limit of existence of these resonances depend markedly on the
sample history.Comment: Submitted to the Quantum Fluids and Solids Conf. Aug. 2006 Kyot
Detection of the Rotation of the Earth with a Superfluid Gyrometer
The effect of the rotation of the Earth, Ω⊕, on a superfluid resonator equipped with a 4.0 cm exp 2 rotation pickup loop and with a microaperture is reported. The velocity circulation induced in the loop by the rotation is detected by phase-slippage techniques. The magnitude of Ω⊕ is measured to better than 1%, and the north direction to ±0.5° for a 10 h observation time. This experiment is the superfluid counterpart of interferometric measurements based on the Sagnac effect.Peer reviewe
Evidence for Single-Vortex Pinning and Unpinning Events in Superfluid 4He
We have observed two critical velocity levels for phase slips in superfluid 4He at temperatures between 14 and 44 mK, due to microparticles formed in the cell at low temperature. We interpret these observations as evidence for pinning and unpinning events of single vortices of nanometric size. From a study of the lifetime and unpinning velocities of the pinned vortices, we are led to conclude that unpinning takes place by quantum tunneling.Peer reviewe
Statistical Learning for Resting-State fMRI: Successes and Challenges
International audienceIn the absence of external stimuli, fluctuations in cerebral activity can be used to reveal intrinsic structures. Well-conditioned probabilistic models of this so-called resting-state activity are needed to support neuroscientific hypotheses. Exploring two specific descriptions of resting-state fMRI, namely spatial analysis and connectivity graphs, we discuss the progress brought by statistical learning techniques, but also the neuroscientific picture that they paint, and possible modeling pitfalls
Thermodynamic inequalities in superfluid
We investigate general thermodynamic stability conditions for the superfluid.
This analysis is performed in an extended space of thermodynamic variables
containing (along with the usual thermodynamic coordinates such as pressure and
temperature) superfluid velocity and momentum density. The stability conditions
lead to thermodynamic inequalities which replace the Landau superfluidity
criterion at finite temperatures.Comment: 7 pages, 1 figur
Giant Relaxation Oscillations in a Very Strongly Hysteretic SQUID ring-Tank Circuit System
In this paper we show that the radio frequency (rf) dynamical characteristics
of a very strongly hysteretic SQUID ring, coupled to an rf tank circuit
resonator, display relaxation oscillations. We demonstrate that the the overall
form of these characteristics, together with the relaxation oscillations, can
be modelled accurately by solving the quasi-classical non-linear equations of
motion for the system. We suggest that in these very strongly hysteretic
regimes SQUID ring-resonator systems may find application in novel logic and
memory devices.Comment: 7 pages, 5 figures. Uploaded as implementing a policy of arXiving old
paper
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