Ü. Parts

Manipulation of Ag nanoparticles utilizing noncontact atomic force microscopy

We have developed a scheme to manipulate metallic aerosol particles on silicon dioxide substrates using an atomic force microscope. The method utilizes the noncontact mode both for locating and moving nanoparticles of size 10–100 nm. The main advantage of our technique is the possibility of “seeing” the moving particle in real time. Our method avoids well sticking problems that typically hamper the manipulation in the contact mode.

“Superconductor-Insulator Transition” in a Single Josephson Junction

VI-curves of resistively shunted single Josephson junctions with different capacitances and tunneling resistances are found to display a crossover between two types of VI-curves: one without and another with a resistance bump (negative second derivative) at zero-bias. The crossover corresponds to the dissipative phase transition (superconductor-insulator transition) at which macroscopic quantum tunneling delocalizes the Josephson phase and destroys superconductivity. Our measured phase diagram does not agree with the diagram predicted by the original theory, but does coincide with a theory that takes into account the accuracy of voltage measurements and thermal fluctuations.

Intrinsic and extrinsic mechanisms of vortex formation in superfluid3He-B

We report on the first comprehensive measurements of critical superflow velocities in3He-B which allow different mechanisms of vortex formation to be identified. As a function of temperatureT and pressureP, we measure the critical angular velocity Ωc(T, P) at which vortices start to form in slowly accelerating rotation in a cylindrical container filled with3He-B. Owing to the long coherence length ξ(T, P)∼10–100 nm, either trapped remanent vorticity or intrinsic nucleation may dominate vortex formation, depending on the roughness of the container wall and the presence of loaded traps.NMR measurement with a resolution of one single vortex line allows us to distinguish between different processes: (1) Three extrinsic mechanisms of vortex formation have been observed. One of them is the vortex mill, a continuous periodic source which is activated in a rough-walled container well below the limit for intrinsic nucleation. (2) In a closed smooth-walled container intrinsic nucleation is the only mechanism available, with a critical velocity vc(T, P)=Ωc(T, P), whereR is the radius of the container. We findvc(T, P) to be related to the calculated intrinsic stability limitvch(T, P) of homogeneous superflow. The existence of this connection in the form of a scaling law implies that nucleation takes place at an instability, rather than by thermal activation or quantum tunneling which become impossible because of an inaccessibly high energy barrier.

Critical Velocity of Vortex Nucleation in Rotating Superfluid {sup {bold 3}}He-A

We have measured the critical velocity {upsilon}{sub c} at which {sup 3}He{minus}A in a rotating cylinder becomes unstable against the formation of quantized vortex lines with continuous (singularity-free) core structure. We find that {upsilon}{sub c} is distributed between a maximum and a minimum limit, which we ascribe to a dependence on the texture of the orbital angular momentun axis {bold {cflx l}}({bold r}) in the cylinder. Slow cooldown through T{sub c} in rotation yields {bold {cflx l}}({bold r}) textures for which the measured {upsilon}{sub c} {close_quote}s are in good agreement with the calculated instability of the expected {bold {cflx l}} texture. {copyright} {ital 1997} {ital The American Physical Society}

Gap anisotropy, vortex cluster, and nucleation of vortices in rotating 3He-B

Abstract In rotating 3He-B the critical flow velocity for the nucleation of singular vortex lines is high and allows the existence of different rotating states: vortex-free counterflow, the equilibrium vortex state, and metastable states with a well-defined vortex cluster containing less than the equilibrium number of vortices. These states can be distinguished and characterized using CW NMR techniques. We derive an estimate for the magnetic field induced anistropy of the superfluid energy gap from an analysis of the spatial distribution of the order parameter texture in the vortex-free state. When vortices are introduced, their number in the vortex cluster can be counted using a simple experimental procedure. This allows us to measure the critical rotation velocity for the nucleation of vortices, which proves to correspond to the maximum flow velocity in the cylindrical NMR cell.

NMR signatures of topological objects in rotating superfluid3He-A

NMR spectrometry can be used to identify different topological objects in the order parameter field of rotating superfluid3He-A. We list their signatures in the cw NMR absorption line shape. Quantized vortex lines, domain walls, and their combination, the vortex sheet, all induce satellite peaks with specific intensities and frequency shifts in the NMR spectrum. Examples of spectra are presented to allow a comparison and to distinguish between different objects.

A–B phase transition in rotating superfluid 3He

Abstract Quantized vortices, when formed in rotating 3 He, can effectively be studied by NMR. Vortices of different structure have been identified, depending on the superfluid phase and the external conditions. Here we discuss what happens when the first-order phase boundary, which separates the A and B phases, traverses through a rotating container which initially contains an equilibrium vortex array. The measurements imply that a vortex layer is formed in front of a slowly propagating interface and part of the newly formed phase behind the interface is left devoid of vortices.

Measurements on the vortex sheet in rotating superfluid 3He-A

When a cylindrical container filled with superfluid 3HeA is rotated around its symmetry axis, several different configurations of quantized vorticity are possible: which of them will be preferred depends on the specifics how the rotating state is formed. The most unusual is the vortex sheet, a domain wall in the order parameter texture into which vortex lines are confined. This metastable structure has the lowest critical velocity of formation if a domain wall with the appropriate orientation is already present in the container. In this case the vortex sheet becomes the preferred rotating state which provides the solid-body rotation of the superfluid component on an averaged scale. Its presence can be identified from the cw NMR spectrum which samples the order parameter texture. Here the experimental properties of the vortex sheet are reviewed, as deduced from NMR measurements.

High sensitivity NMR measurements at low temperature and frequency

Abstract The sensitivity of continuous-wave NMR can be increased with a high- Q resonance tank circuit by using superconducting wire and a GaAs FET preamplifier operating at liquid helium temperatures. We discuss impedance matching to obtain an optimal signal-to-noise ratio for input circuits operating at ≈500 kHz, with Q ≈10 3 –10 4 at an impedance level of 1 Mω and a noise voltage of 1.3nV/√Hz. High rf compensation of 1:10 5 is achieved with orthogonally oriented transmitter coils and an astatically wound pick-up coil. The performance is evaluated on the basis of resolution that we achieved in the measurement of the NMR line in normal liquid 3 He; this is better than 1 part in 10 3 of the peak amplitude, while in the integrated intensity the standard deviation from the mean is better than 0.3%.

Coexistence of single and double quantum vortex lines

We discuss the configurations in which singly and doubly quantized vortex lines coexist in a rotating superfluid. General principles of energy minimization lead to the conclusion that in equilibrium the two vortex species segregate within a cylindrical vortex cluster in two coaxial domains where the singly quantized lines are in the outer annular region. This is confirmed with simulation calculations on discrete vortex lines. Experimentally the coexistence can be studied in rotating superfluid