Vorticity Fluctuations in Turbulent Counterflow of Superfluid Helium

A model of vorticity fluctuations in turbulent helium based on Vinen\u27s dynamical equation is developed. Its predictions are compared with measurements of ⟨δL2⟩ recently reported by Mantese, Bischoff, and Moss. The result is interpreted as supporting the validity of Vinen\u27s equation

Fragmentation Clusters Formed in Supercritical Expansions of \u3csup\u3e4\u3c/sup\u3eHe

We have measured the mass distribution of cluster ions formed from a supercritical expansion of helium gas. We find two distinct cluster groups which we identify as condensation and fragmentation clusters. The latter first appear when the expansion conditions approach the critical isentrope. The measurements also suggest that the neutral fragmentation cluster mass distribution is a universal function of the source entropy

Charged Droplets in Cryogenic \u3csup\u3e4\u3c/sup\u3eHe Vapor

We have measured the mobility of positive ions in 4He vapor for temperatures between 1.3 and 2.0 K and for saturation ratios between 0.1 and 1.0. We present a model which relates the size of a charged droplet to its mobility and find good quantitative agreement with our data when we calculate the size of the droplet which forms about the ion with classical macroscopic thermodynamic arguments. The radius thus obtained ranges from 7 to 9 Å

Vortex Velocity in Turbulent He II Counterflow

We have used the ion-vortex interaction to measure the drift velocity of the vorticity present in turbulent counterflow, and find that it moves in the direction of normal-fluid flow. The result is in direct conflict with a central assumption of the presently accepted model of the turbulent state

Capture of Neon Atoms by \u3csup\u3e4\u3c/sup\u3eHe Clusters

Neon atoms are captured by helium clusters in a crossed-beam experiment. The capture process depends strongly on the cluster beam source conditions. We identify a sharply defined region corresponding to expansions passing near the critical point for which the capture probability is anomalously large

A Dynamical Definition of Atomic Clusters

The assignment of an atom to a particular cluster of atoms is defined dynamically in terms of the power flow to the atom. The definition avoids some of the ambiguities present in structural definitions of cluster membership and enables a precise determination of the time at which evaporation and capture events occur in clusters. The definition is illustrated with molecular dynamics studies of evaporation events in Lennard‐Jones clusters, and comparisons are made with structural definitions

Structure of Charged Argon Clusters Formed in a Free Jet Expansion

We describe measurements of the mass spectrum of charged argon clusters generated in a low-temperature free jet expansion. It contains detailed intensity variations which can be understood in terms of a simplified competing-lattice model of charged-cluster structure

Theoretical Studies of the Energetics and Structures of Atomic Clusters

Comparative calculations of the binding energy and structure of relaxed closed-shell clusters of icosahedral and cuboctahedral point group symmetry are reported. The atoms are presumed to interact via either the Lennard-Jones or the Aziz-Chen (HFD-C) pair potential. The IC structure is found to be lower in total energy for less than 14 shells (10 179 atoms) in the Lennard-Jones case and for less than 13 shells (8217 atoms) in the HFD-C case. Detailed energetics are analzyed in order to elucidate the mechanism for the transition from icosahedral to cuboctahedral symmetry

High-Field Specific-Heat and Susceptibility Measurements: Relevance to the Spin-Peierls Phase Diagram and the Validity of a Soliton Picture

We discuss recent high-field specific-heat measurements on the spin-Peierls compound tetrathiafulvalinium bis-cis-(1,2-perfluoromethylethylene-1-2-dithiolato)–gold [TTF-BDT(Au)]. An ordering anomaly can clearly be detected which defines a high-field phase boundary, despite some extraneous structure in the data apparently reflecting experimental limitations. The high-field phase boundary is in good quantitative agreement with the theory of Cross and Fisher. Further, the specific-heat data are in good agreement with earlier ac susceptibility data, the reliability of which as indicators of a phase transition has been somewhat open to doubt because pronounced relaxation effects in the high-field regime result in striking differences between χac and χdc. Some observations are made concerning the extent of hysteresis associated with the phase boundary between dimerized and high-field phases. Finally, the extensive experimental data are employed in several tests to determine the applicability of recent soliton theories