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

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

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.

Analytical Estimate of the Critical Velocity for Vortex Pair Creation in Trapped Bose Condensates

We use a modified Thomas-Fermi approximation to estimate analytically the critical velocity for the formation of vortices in harmonically trapped BEC. We compare this analytical estimate to numerical calculations and to recent experiments on trapped alkali condensates.Comment: 12 page

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

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

Watching a superfluid untwist itself: Recurrence of Rabi oscillations in a Bose-Einstein condensate

The order parameter of a condensate with two internal states can continuously distort in such a way as to remove twists that have been imposed along its length. We observe this effect experimentally in the collapse and recurrence of Rabi oscillations in a magnetically trapped, two-component Bose-Einstein condensate of ^87Rb

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

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

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 $\Delta_\perp\approx$ 1.2meV and a singlet at $\Delta_\parallel\approx$ 2.5meV. With pressure we observe appreciable shifts in these levels, which move to $\Delta_\perp{(2.5GPa)}\approx$ 1.45 meV and $\Delta_\parallel(2.5GPa)\approx$ 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 $\Delta_H\approx$ 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.