Hallock, Robert
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Distinguished Professor, Physics Department
Last Name
Hallock
First Name
Robert
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Physics
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Introduction
Robert Hallock is a Distinguished Professor of Physics at the University of Massachusetts in Amherst. His research is in the general area of experimental Condensed Matter Physics, with an emphasis on low temperature phenomena, particularly liquid helium, liquid helium films, and solid helium. Specific research directions and other areas of interest, including teaching, communication information and a vitae, can be accessed by calling forward the individual highlighted items below.
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Publication Open Access Observation of mass transport through solid 4He(2009-01) Ray, M; Hallock, RBy use of an experimental design that provides for superfluid helium in contact with bulk hcp 4He off the melting curve, we have observed the dc transport of mass through a cell filled with solid 4He in the hcp region of the phase diagram. Flow, which shows characteristics of a superflow, is seen to be independent of the method used to grow the solid but depends on pressure and temperature. The temperature dependence suggests the possibility of hysteresis.Publication Open Access Mass flux and solid growth in solid 4He: 60 mK - 700 mK(2011-01) Ray, M; Hallock, RWe use the thermo-mechanical effect to create a chemical potential difference between two liquid reservoirs connected to each other through Vycor rods in series with solid hcp 4He to confirm that a DC flux of atoms takes place below 600 mK, but find that the flux falls abruptly in the vicinity of 80 mK. It is impossible to add density to a solid freshly made at 60 mK and samples freshly made at 60 mK do not allow mass flux, even when raised in temperature to 200 mK. Solids created above 300 mK and cooled to 60 mK accept added density and demonstrate finite mass flux.Publication Open Access Observation of Unusual Mass Transport in Solid hcp 4He(2008-01) Ray, M; Hallock, RSolid 4He has been created off the melting curve by growth at a nearly constant mass via the “blocked capillary” technique and growth from the 4He superfluid at constant temperature. The experimental apparatus allows injection of 4He atoms from superfluid directly into the solid. Evidence for the superfluidlike transport of mass through a sample cell filled with hcp solid 4He off the melting curve is found. This mass flux depends on temperature and pressure.Publication Open Access Growth of solid hcp 4He off the melting curve(2010-01) Ray, M; Hallock, RWe report studies of the growth of solid hcp 4He at pressures higher than the bulk freezing pressure using a cell design that allows us to inject atoms into the solid. Near the melting curve during injection we observe random events during which the pressure recorded in the cell drops abruptly. These events are accompanied by transient increases in the temperature of the cell. We discuss these transients and conclude that they represent the solidification of meta-stable liquid regions and the associated relief of strain in the local solid. We also observe that further from the melting curve the transients are no longer recorded, but that we can continue to add atoms to the solid, increasing its density at fixed volume. We document these changes in density with respect to changes in the chemical potential as a function of temperature and discuss these in the context of recent theoretical work.Publication Open Access Observation of thermomechanical equilibration in the presence of a solid 4He conduit(2010-01) Ray, M; Hallock, RWe observe a thermomechanical effect when a chemical-potential difference is created by a temperature difference imposed between two liquid reservoirs connected to each other through Vycor rods in series with solid hcp 4He. By creating a temperature difference, ΔT, between the two reservoirs, we induce a rate-limited growth of a pressure difference between the two reservoirs, ΔP. In equilibrium ΔP vs ΔT is in quantitative agreement with the thermomechanical effect in superfluid helium. These observations confirm that below ∼600 mK a flux-limited flow exists through the solid helium.Publication Metadata only Session C1: APS, AAPT, and SPS Poster Session - Temperature Dependence of Helium Diffusion Through Common Epoxies(2011-11-18) Lovinger, Dylan; Hallock, RobertHelium gas at room temperature is known to diffuse through the epoxies commonly used in various low temperature apparatus, which can complicate leak detection. The helium flux typically decreases with decreasing temperature. We have measured the flux of helium that passes though thin sections of as-cast clear Stycast 1266, Stycast 2850FT (black) and TRA-BOND 2151 (blue) epoxies as a function of temperature in the range 130K < T < 300K. We analyze the data to create normalized (to constant sample thickness and pressure differential) data for comparison. We report the preliminary temperature-dependent fluxes we have measured, which show significant differences among the epoxies studied.