102 research outputs found
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
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
Quantum whistling in superfluid 4He
Fundamental considerations predict that macroscopic quantum systems such as
superfluids and the electrons in superconductors will exhibit oscillatory
motion when pushed through a small constriction. Here we report the observation
of these oscillations between two reservoirs of superfluid 4He partitioned by
an array of nanometer-sized apertures. They obey the Josephson frequency
equation and are coherent amongst all the apertures. This discovery at the
relatively high temperature of 2K (2000 times higher than related phenomena in
3He) may pave the way for a new class of practical rotation sensors of
unprecedented precision.Comment: 6 pages, 3 figures, to be published in Natur
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
Pinhole calculations of the Josephson effect in 3He-B
We study theoretically the dc Josephson effect between two volumes of
superfluid 3He-B. We first discuss how the calculation of the current-phase
relationships is divided into a mesoscopic and a macroscopic problem. We then
analyze mass and spin currents and the symmetry of weak links. In quantitative
calculations the weak link is assumed to be a pinhole, whose size is small in
comparison to the coherence length. We derive a quasiclassical expression for
the coupling energy of a pinhole, allowing also for scattering in the hole.
Using a selfconsistent order parameter near a wall, we calculate the
current-phase relationships in several cases. In the isotextural case, the
current-phase relations are plotted assuming a constant spin-orbit texture. In
the opposite anisotextural case the texture changes as a function of the phase
difference. For that we have to consider the stiffness of the macroscopic
texture, and we also calculate some surface interaction parameters. We analyze
the experiments by Marchenkov et al. We find that the observed pi states and
bistability hardly can be explained with the isotextural pinhole model, but a
good quantitative agreement is achieved with the anisotextural model.Comment: 20 pages, 21 figures, revtex
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
Transition from phase slips to the Josephson effect in a superfluid 4He weak link
The rich dynamics of flow between two weakly coupled macroscopic quantum
reservoirs has led to a range of important technologies. Practical development
has so far been limited to superconducting systems, for which the basic
building block is the so-called superconducting Josephson weak link. With the
recent observation of quantum oscillations in superfluid 4He near 2K, we can
now envision analogous practical superfluid helium devices. The characteristic
function which determines the dynamics of such systems is the current-phase
relation Is(phi), which gives the relationship between the superfluid current
Is flowing through a weak link and the quantum phase difference phi across it.
Here we report the measurement of the current-phase relation of a superfluid
4He weak link formed by an array of nano-apertures separating two reservoirs of
superfluid 4He. As we vary the coupling strength between the two reservoirs, we
observe a transition from a strongly coupled regime in which Is(phi) is linear
and flow is limited by 2pi phase slips, to a weak coupling regime where Is(phi)
becomes the sinusoidal signature of a Josephson weak link.Comment: 12 pages, 4 figure
Geometric Laws of Vortex Quantum Tunneling
In the semiclassical domain the exponent of vortex quantum tunneling is
dominated by a volume which is associated with the path the vortex line traces
out during its escape from the metastable well. We explicitly show the
influence of geometrical quantities on this volume by describing point vortex
motion in the presence of an ellipse. It is argued that for the semiclassical
description to hold the introduction of an additional geometric constraint, the
distance of closest approach, is required. This constraint implies that the
semiclassical description of vortex nucleation by tunneling at a boundary is in
general not possible. Geometry dependence of the tunneling volume provides a
means to verify experimental observation of vortex quantum tunneling in the
superfluid Helium II.Comment: 4 pages, 2 figures, revised version to appear in Phys. Rev.
Field induced long-range-ordering in an S=1 quasi-one-dimensional Heisenberg antiferromagnet
We have measured the heat capacity and magnetization of the spin one
one-dimensional Heisenberg antiferromagnet NDMAP and constructed a magnetic
field versus temperature phase diagram. We found a field induced long-range
magnetic ordering. We have been successful in explaining the phase diagram
theoretically.Comment: 6 pages, 18 figure
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