Sharp Heat-Capacity Signature at the Superfluid Transition of Helium Films in Porous Glasses

Abstract

We report the first observation of a sharp heat-capacity signature related to the superfluid transition in thin 4 He films adsorbed on porous Vycor and xerogel glasses. The transition temperatures of these films range from 0.14 to 1.1 K. This new feature is found in addition to the broad peak centered at a higher temperature reported in a number of earlier studies.PACS numbers: 67.40. Kh, 67.40.Hf, 67.40.Rp, The confinement of He in a multiply connected geometry such as porous glass has received renewed experimental 1,2 as well as theoretical interest. 3 " 6 Superfluid density (p s ) measurements 1,2 of 4 He films in porous Vycor glass (pore diameter -70 A) reveal a sharp transition at T c with an asymptotic power-law dependence similar to the bulk X transition, i.e.,This behavior was seen for films with T c > 70 mK up to full pores where T c = 1.955 K (Refs. 1 and 2). More recently, studies of 4 He in two other porous glasses have revealed different exponents: 0.89 for 10" 3 <r<10 _1 in xerogel 7 and 0.813 for 10 ~4 < / < 10~2 in aerogel. 2 The pores in xerogel glass have a diameter near 100 A and those in aerogel span a wide range in size. The power-law dependence in these systems suggests that the superfluid transition is sharp and indicative of genuine critical behavior. 2 Previous heat-capacity measurements of confined 4 He have revealed a broad peak with a maximum at a temperature T m , which is apparently higher than T c .* As the confining dimension is descreased, the height of this broad peak decreases, whereas the width and 7\ -T m increase (7\ is the bulk transition temperature). Some of these observations have been analyzed in terms of finitesize scaling. 9,10 No sharp signature at T c has been observed. Qualitatively similar behavior has been seen for 4 He films in Vycor glass by Brewer. 8 For partial filling of the pores they found broad peaks with T m =1.25 and 1.75 K having widths of -0.60 and 0.55 K (FWHM) at coverages which we calculate to be 32.1 and 40.9 ^mole/m 2 , respectively. From p s measurements, 7Vs for these films should be 0.4 and 1.1 K, respectively. 1 No signature at T c was found in either film. At full pores, a broad anomaly 8 J/mol K in height and 0.40 K in width was observed at 7V, =2.1 K. Again, no signature was observed at T c = 1.955 K. Similar results for filled pores were observed by Joseph and Gasparini. n To our knowledge, no sharp heat-capacity signatures have been observed for any thin helium films whether in Vycor or on a planar substrate. Broad heat-capacity peaks were observed by Yuyama and Watanabe 12 and Bretz 13 above 1.2 K in 4 He on graphite. These peaks appear to be

Onset of superfluidity inHe4films adsorbed on disordered substrates

“…A peak can only barely be resolved for the film with T c = 20.7 mK. This trend is consistent with that observed in earlier studies, 28,29 in which the magnitude of the heat capacity peak for 4 He films adsorbed in Vycor was shown to scale according to the hyperuniversality relation, 44…”

“…In the vicinity of the superfluid transition, there is a small peak in C/T due to critical fluctuations. 28,29 The size of the peak decreases with decreasing thickness. As can be seen in Fig.…”

“…Each of the data sets in the lower panel of the figure shows a sharp cusp, corresponding to a superfluid transition. 28 The superfluid transition temperatures determined from the locations of the cusps are shown as closed circles in Fig. 14.…”

Equilibrium phase transitions in a porous medium

“…Some experiments suggest that while the coexistence regions are narrowed over a range of temperatures below T c , the universality class of the transitions remains the same as in the bulk homogeneous systems. There is also some experimental evidence [3][4][5]7 for different critical exponents for phase-separation and liquid-vapor transitions in Vycor, aerogel, and other porous media. Questions of exponents are thus far from settled.…”

Numerical study of phase transitions inside the pores of aerogels

“…Number of experiments have been dedicated to study of new features of transitions in helium [1][2][3][4][5] or liquid crystals [6][7][8][9][10] when they are confined within the pores of an aerogel. Theoretical reconstruction of those phenomena has been attempted through various models [11][12][13][14][15][16].…”