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

Buckyball Quantum Computer: Realization of a Quantum Gate

We have studied a system composed by two endohedral fullerene molecules. We have found that this system can be used as good candidate for the realization of Quantum Gates Each of these molecules encapsules an atom carrying a spin,therefore they interact through the spin dipole interaction. We show that a phase gate can be realized if we apply on each encased spin static and time dependent magnetic field. We have evaluated the operational time of a $\pi$-phase gate, which is of the order of ns. We made a comparison between the theoretical estimation of the gate time and the experimental decoherence time for each spin. The comparison shows that the spin relaxation time is much larger than the $\pi$-gate operational time. Therefore, this indicates that, during the decoherence time, it is possible to perform some thousands of quantum computational operations. Moreover, through the study of concurrence, we get very good results for the entanglement degree of the two-qubit system. This finding opens a new avenue for the realization of Quantum Computers.Comment: 13 pages, 5 figures. Submitted to Physical Review