108 research outputs found
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
Diversification and hybridization in firm knowledge bases in nanotechnologies
The paper investigates the linkages between the characteristics of
technologies and the structure of a firms' knowledge base. Nanotechnologies
have been defined as converging technologies that operate at the nanoscale, and
which require integration to fulfill their economic promises. Based on a
worldwide database of nanofirms, the paper analyses the degree of convergence
and the convergence mechanisms within firms. It argues that the degree of
convergence in a firm's nano-knowledge base is relatively independent from the
size of the firm's nano-knowledge base. However, while firms with small
nano-knowledge bases tend to exploit convergence in each of their
patents/publications, firms with large nano-knowledge bases tend to separate
their nano-R&D activities in the different established fields and achieve
diversity through the juxtaposition of the output of these independent
activitie
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
Josephson effects in dilute Bose-Einstein condensates
We propose an experiment that would demonstrate the ``dc'' and ``ac''
Josephson effects in two weakly linked Bose-Einstein condensates. We consider a
time-dependent barrier, moving adiabatically across the trapping potential. The
phase dynamics are governed by a ``driven-pendulum'' equation, as in
current-driven superconducting Josephson junctions. At a critical velocity of
the barrier (proportional to the critical tunneling current), there is a sharp
transition between the ``dc'' and ``ac'' regimes. The signature is a sudden
jump of a large fraction of the relative condensate population. Analytical
predictions are compared with a full numerical solution of the time dependent
Gross-Pitaevskii equation, in an experimentally realistic situation.Comment: 4 pages, 1 figur
Tuning the spin Hamiltonian of NENP by external pressure: a neutron scattering study
We report an inelastic neutron scattering study of antiferromagnetic spin
dynamics in the Haldane chain compound Ni(C2H8N2)2NO2ClO4 (NENP) under external
hydrostatic pressure P = 2.5 GPa. At ambient pressure, the magnetic excitations
in NENP are dominated by a long-lived triplet mode with a gap which is split by
orthorhombic crystalline anisotropy into a lower doublet centered at
1.2meV and a singlet at 2.5meV.
With pressure we observe appreciable shifts in these levels, which move to
1.45 meV and
2.2meV. The dispersion of these modes in the crystalline c-direction
perpendicular to the chain was measured here for the first time, and can be
accounted for by an interchain exchange J'_c approximately 3e-4*J which changes
only slightly with pressure. Since the average gap value 1.64
meV remains almost unchanged with P, we conclude that in NENP the application
of external pressure does not affect the intrachain coupling J appreciably, but
does produce a significant decrease of the single-ion anisotropy constant from
D/J = 0.16(2) at ambient pressure to D/J = 0.09(7) at P = 2.5 GPa.Comment: LaTeX file nenp_p.tex, 10 pages, 1 table, 5 figures. Submitted to
Phys. Rev.
Chemical-potential standard for atomic Bose-Einstein condensates
When subject to an external time periodic perturbation of frequency , a
Josephson-coupled two-state Bose-Einstein condensate responds with a constant
chemical potential difference , where is Planck's constant
and is an integer. We propose an experimental procedure to produce
ac-driven atomic Josephson devices that may be used to define a standard of
chemical potential. We investigate how to circumvent some of the specific
problems derived from the present lack of advanced atom circuit technology. We
include the effect of dissipation due to quasiparticles, which is essential to
help the system relax towards the exact Shapiro resonance, and set limits to
the range of values which the various physical quantities must have in order to
achieve a stable and accurate chemical potential difference between the
macroscopic condensates.Comment: 13 pages, 4 figure
Josephson Effect between Condensates with Different Internal Structures
A general formula for Josephson current in a wide class of hybrid junctions
between different internal structures is derived on the basis of the Andreev
picture. The formula extends existing formulae and also enables us to analyze
novel B-phase/A-phase/B-phase (BAB) junctions in superfluid helium three
systems, which are accessible to experiments. It is predicted that BAB
junctions will exhibit two types of current-phase relations associated with
different internal symmetries. A ``pseudo-magnetic interface effect'' inherent
in the system is also revealed.Comment: 4 pages, 2 figure
Looking back at superfluid helium
A few years after the discovery of Bose Einstein condensation in several
gases, it is interesting to look back at some properties of superfluid helium.
After a short historical review, I comment shortly on boiling and evaporation,
then on the role of rotons and vortices in the existence of a critical velocity
in superfluid helium. I finally discuss the existence of a condensate in a
liquid with strong interactions, and the pressure variation of its superfluid
transition temperature.Comment: Conference "Bose Einstein Condensation", Institut henri Poincare,
Paris, 29 march 200
Discovery of the Acoustic Faraday Effect in Superfluid 3He-B
We report the discovery of the acoustic Faraday effect in superfluid 3He-B.
The observation of this effect provides the first direct evidence for
propagating transverse acoustic waves in liquid 3He, a mode first predicted by
Landau in 1957. The Faraday rotation is large and observable because of
spontaneously broken spin-orbit symmetry in 3He-B. We compare the experimental
observations with a simulation of the transverse acoustic impedance that
includes the field-induced circular birefringence of transverse waves.Comment: 4 pages in RevTex plus 3 postscript figures; new version includes:
minor corrections to the text and an updated of list of reference
- …