Fluctuative Mechanism of Vortex Nucleation in the Flow of 4He^4He

We propose a mechanism of a vortex nucleation in a flow of a superfluid $^4He$. The mechanism is related to the creation by critical fluctuations of a "plasma" of half-vortex rings located near the wall. The "plasma" screens the attraction of the vortex to the wall and permits vortex nucleation. In the spirit of Williams-Shenoy theory we derive the scaling laws in the critical region and estimate the scaling relation and the critical exponent $p$ for critical velocity; we find $V_c\sim V_{0}(1-T/T_c)$, so that $p=1$. Various applications of the obtained results are discussed.Comment: 16 pages (Latex,Revtex), appears Physical Review Letters, v76, N13 (1996

Aharonov-Bohm Effect in Luttinger Liquid and Beyond

In systems with the spin-charge separation, the period of the Aharonov-Bohm (AB) oscillation becomes half of the flux quantum. This effect is at least related to the fact that for the creation of the holons (spinons) are needed two electrons. The effect is illustrated on the example of the Hubbard Hamiltonian with the aid of the bosonization including topological numbers and exists also in the Luttinger liquid on two chains. The relation to a fractional 1/N- AB effect, which can be associated with a modified Luttinger liquid, is discussed.Comment: 12 pages (Latex) in Technical Reports of ISSP , Ser.A, Oktober (1994) and in JETP Lett. v60, N9 (1994

Intrinsic and extrinsic vortex nucleation mechanisms in the flow

We propose very general vortex nucleation mechanisms analogous to a hydrodynamic instability and calculate associated critical velocity in agreement with experiments. The creation of vortices via extrinsic mechanism is driven by a formation of the surface vorticity sheet created by the flow, which reaches a critical size. Such a sheet screens an attraction of a half-vortex ring to the wall, the barrier for the vortex nucleation disappears and the vortex nucleation is started. In the intrinsic mechanism the creation of a big vortex ring, which transforms into the vortex, is driven by a fluctuative generation of small vortex ringsComment: Contribution paper to LT21 (to be published in Physica B

Phase diagram of the 2D 4^4He in the density-temperature plane

Thin $^4$He films adsorbed to weakly attractive substrates form nearly 2D layers. We describe the vortices in 2D superfluid $^4$He like quasiparticles. With the aid of a variational many-body calculation we estimate their inertial mass and describe their interactions with the $^4$He particles and other vortices. Third sound measurements revealed anomalous behavior below the BKT-phase transition temperature. We ascribe this to the sound mode traveling in the fluid of vortex-antivortex pairs. These pairs forms a crystal (or liquid crystal) when the film thickness increases, the third sound mode splits into two separate modes as seen in experiments. Our many-body calculation predicts the critical density, at which the phase transition into the vortex-antivortex state at zero temperature occurs. We also describe the phase diagram of thin $^4$He films.Comment: Contribution paper to LT21 (to be published in Physica B

Fast Quantum Computing with Buckyballs

We have found that encapsulated atoms in fullerene molecules, which carry a spin, can be used for fast quantum computing. We describe the scheme for performing quantum computations, going through the preparation of the qubit state and the realization of a two-qubit quantum gate. When we apply a static magnetic field to each encased spin, we find out the ideal design for the preparation of the quantum state. Therefore, adding to our system a time dependent magnetic field, we can perform a phase-gate. The operational time related to a $\pi-$phase gate is of the order of $ns$. This finding shows that, during the decoherence time, which is proportional to $\mu s$, we can perform many thousands of gate operations. In addition, the two-qubit state which arises after a $\pi-$gate is characterized by a high degree of entanglement. This opens a new avenue for the implementation of fast quantum computation.Comment: 9 pages, 4 figure

Switching and symmetry breaking behaviour of discrete breathers in Josephson ladders

We investigate the roto-breathers recently observed in experiments on Josephson ladders subjected to a uniform transverse bias current. We describe the switching mechanism in which the number of rotating junctions increases. In the region close to switching we find that frequency locking, period doubling, quasi-periodic behaviour and symmetry breaking all occur. This suggests that a chaotic dynamic occurs in the switching process. Close to switching the induced flux increases sharply and clearly plays an important role in the switching mechanism. We also find three critical frequencies which are independent of the dissipation constant $\alpha$, provided that $\alpha$ is not too large