Superfluidity ofHe4confined in microscopic bubbles in copper

Abstract

We have created extremely small and independent Bose systems by confining "^He in bubbles of radius of less than 110 A in Cu foils. The size distribution and density of the bubbles were adjusted by annealing the samples and were determined by transmission electron microscopy. At low temperatures the confined helium becomes first solid, then liquid, and eventually superfluid as the mean bubble size is increased by annealing. The modification of superfluidity in this new confining geometry by size effects has been investigated by low-temperature calorimetry.PACS numbers: 64.70.Dv, 65.20.+w, 67.40.Kh, 67.70.+ n Because of its purity and homogeneity, liquid helium is a unique system for studies of finite-size effects. The superfluid transition of helium has been investigation in various confining geometries, e.g., helium films on planar substrates, and helium in Vycor glass, Nuclepore filters, or fine powders. Observed modifications of superfluidity were in qualitative agreement with expectations of the mean-field theory and were also compared to finite-size scaling arguments. Experimental data could not, however, always be used to test theoretical predictions because of insufficient knowledge and of complexity of the confining system. In the case of porous materials an additional disadvantage is the connectivity of the pores, which may reduce the effectiveness of the confinement.Following a different approach we have confined high-pressure helium gas in nonconnected cavities (bubbles) in Cu. At low temperatures the confined helium liquifies. Here we report our results on lowtemperature calorimetry, particularly the effects of pore size on the superfluid transition in bubbles of average radius between 25 and 108 A. Low-temperature calorimetrie evidence for superfluidity of helium in voids in irradiated solids has previously been reported by Keesom and co-workers,^ Aslanian and Weil,^ and Gmelin^; indications for size effects can be inferred from the data in Ref. 2.This new confining geometry is well defined and is topologically simple as we show by transmission electron microscopy (TEM). It creates an ensemble of extremely small Bose systems completely isolated from each other and consisting of about 10^ helium atoms each. Our work presents, in addition, experimental thermodynamic support for the properties of helium in small bubbles in a metal.Helium is practically insoluble in metals.^ Therefore, when helium is injected or produced in a metal it precipitates into microscopic bubbles whose size can be increased by annealing of the specimens. Significant reemission of helium from bulk metallic specimens charged with low helium concentrations is observed only in the neighborhood of the melting temperature of the metal. The gas pressure in the bubbles is in the kilobar range so that its density can be very high.'^^^ According to the equation of state,^ liquid helium is expected to exist in Cu in bubbles of radius r > 40 A at low temperatures.We have introduced helium into 99.999%-pure Cu disks of 7-mm diam x 100-/xm th...

Localized Bose–Einstein Condensation of 4He Confined in Nanoporous Media

“…These features are commonly observed in a number of confined 4 He systems, such as 4 He in Vycor, [24][25][26]28,29) in slab geometries, 27) and even in 4 He nanodroplets formed in metal. 30) We believe that all of these heat capacity peaks are manifestations of the LBEC states. In particular, a heat capacity peak without macroscopic superfluidity has been observed in liquid 4 He nanodroplets formed in metal foils.…”

“…In particular, a heat capacity peak without macroscopic superfluidity has been observed in liquid 4 He nanodroplets formed in metal foils. 30) In this case the liquid droplets of several nanometers in diameter are separate by the host metal the droplets never exhibit superfluidity in the macroscopic sense. 4 He in nanoporous Gelsil is similar to this droplet system.…”

Superfluid effects in rotating helium clusters

“…Recently the specific heat of helium bubbles confined in Cu foils has been measured [1] in the region of the 2 transition. Recently the specific heat of helium bubbles confined in Cu foils has been measured [1] in the region of the 2 transition.…”

Superfluidity of He4 confined in nanoporous media

“…Heat capacity peak without macroscopic superfluidity has been observed in liquid 4 He droplets formed in metal foils 14 . In this case each droplet that is several nanometers in diameter is perfectly independent, and the droplets never exhibit superfluidity in macroscopic sense.…”