Energy Loss from Reconnection with a Vortex Mesh

Experiments in superfluid 4He show that at low temperatures, energy dissipation from moving vortices is many orders of magnitude larger than expected from mutual friction. Here we investigate other mechanisms for energy loss by a computational study of a vortex that moves through and reconnects with a mesh of small vortices pinned to the container wall. We find that such reconnections enhance energy loss from the moving vortex by a factor of up to 100 beyond that with no mesh. The enhancement occurs through two different mechanisms, both involving the Kelvin oscillations generated along the vortex by the reconnections. At relatively high temperatures the Kelvin waves increase the vortex motion, leading to more energy loss through mutual friction. As the temperature decreases, the vortex oscillations generate additional reconnection events between the moving vortex and the wall, which decrease the energy of the moving vortex by transfering portions of its length to the pinned mesh on the wall.Comment: 9 pages, 10 figure

Local contribution of a quantum condensate to the vacuum energy density

We evaluate the local contribution g_[mu nu]L of coherent matter with lagrangian density L to the vacuum energy density. Focusing on the case of superconductors obeying the Ginzburg-Landau equation, we express the relativistic invariant density L in terms of low-energy quantities containing the pairs density. We discuss under which physical conditions the sign of the local contribution of the collective wave function to the vacuum energy density is positive or negative. Effects of this kind can play an important role in bringing about local changes in the amplitude of gravitational vacuum fluctuations - a phenomenon reminiscent of the Casimir effect in QED.Comment: LaTeX, 8 pages. Final journal versio

The high-lying 6^6Li levels at excitation energy around 21 MeV

The $^3$H+$^3$He cluster structure in $^6$Li was investigated by the $^3$H($\alpha$,$^3$H $^3$He)n kinematically complete experiment at the incident energy $E_\alpha$ = 67.2 MeV. We have observed two resonances at $E_x^*$ = 21.30 and 21.90 MeV which are consistent with the $^3$He($^3$H, $\gamma$)$^6$Li analysis in the Ajzenberg-Selove compilation. Our data are compared with the previous experimental data and the RGM and CSRGM calculations.Comment: 12 pages, 6 figures. Accepted for publication in J. Phys. Soc. Jp

Dissipative Transport of a Bose-Einstein Condensate

We investigate the effects of impurities, either correlated disorder or a single Gaussian defect, on the collective dipole motion of a Bose-Einstein condensate of $^7$Li in an optical trap. We find that this motion is damped at a rate dependent on the impurity strength, condensate center-of-mass velocity, and interatomic interactions. Damping in the Thomas-Fermi regime depends universally on the disordered potential strength scaled to the condensate chemical potential and the condensate velocity scaled to the peak speed of sound. The damping rate is comparatively small in the weakly interacting regime, and the damping in this case is accompanied by strong condensate fragmentation. \textit{In situ} and time-of-flight images of the atomic cloud provide evidence that this fragmentation is driven by dark soliton formation.Comment: 14 pages, 20 figure

Polarization states of polydomain epitaxial Pb(Zr1-xTix)O3 thin films and their dielectric properties

Ferroelectric and dielectric properties of polydomain (twinned) single-crystal Pb(Zr1-xTix)O3 thin films are described with the aid of a nonlinear thermodynamic theory, which has been developed recently for epitaxial ferroelectric films with dense laminar domain structures. For Pb(Zr1-xTix)O3 (PZT) films with compositions x = 0.9, 0.8, 0.7, 0.6, 0.5, and 0.4, the "misfit strain-temperature" phase diagrams are calculated and compared with each other. It is found that the equilibrium diagrams of PZT films with x > 0.7 are similar to the diagram of PbTiO3 films. They consist of only four different stability ranges, which correspond to the paraelectric phase, single-domain tetragonal ferroelectric phase, and two pseudo-tetragonal domain patterns. In contrast, at x = 0.4, 0.5, and 0.6, the equilibrium diagram displays a rich variety of stable polarization states, involving at least one monoclinic polydomain state. Using the developed phase diagrams, the mean out-of-plane polarization of a poled PZT film is calculated as a function of the misfit strain and composition. Theoretical results are compared with the measured remanent polarizations of PZT films grown on SrTiO3. Dependence of the out-of-plane dielectric response of PZT films on the misfit strain in the heterostructure is also reported.Comment: 23 pages, 4 figure

Direct measurement of quantum phase gradients in superfluid 4He flow

We report a new kind of experiment in which we generate a known superfluid velocity in a straight tube and directly determine the phase difference across the tube's ends using a superfluid matter wave interferometer. By so doing, we quantitatively verify the relation between the superfluid velocity and the phase gradient of the condensate macroscopic wave function. Within the systematic error of the measurement (~10%) we find v_s=(hbar/m_4)*(grad phi)

Beyond the Landau Criterion for Superfluidity

According to the Landau criterion for superfluidity, a Bose-Einstein condensate flowing with a group velocity smaller than the sound velocity is energetically stable to the presence of perturbing potentials. We found that this is strictly correct only for vanishingly small perturbations. The superfluid critical velocity strongly depends on the strength and shape of the defect. We quantitatively study, both numerically and with an approximate analytical model, the dynamical response of a one-dimensional condensate flowing against an istantaneously raised spatially periodic defect. We found that the critical velocity $v_c$ decreases by incresing the strength of the defect $V_0$, up to to a critical value of the defect intensity where the critical velocity vanishes

The two-fluid model with superfluid entropy

The two-fluid model of liquid helium is generalized to the case that the superfluid fraction has a small entropy content. We present theoretical arguments in favour of such a small superfluid entropy. In the generalized two-fluid model various sound modes of He$\;$II are investigated. In a superleak carrying a persistent current the superfluid entropy leads to a new sound mode which we call sixth sound. The relation between the sixth sound and the superfluid entropy is discussed in detail.Comment: 22 pages, latex, published in Nuovo Cimento 16 D (1994) 37