LT 21 Proceedings of the 21s1 Intcrnalional Conference on Low Temperature Physics Prague, August 8-14, 1996

Part $1 - Quantum Fluids and Solids: Liquid Helium

T h e E f f e c t o f R o u g h S u r f a c e s o n t h e S t a b i l i t y o f S u p e r c o o l e d a H e - A

Matthew O'Keefe, Barry Barker, D.D. Osheroff

Physics Department, Stanford University, Varian building, Stanford, CA 94305-4060

We have studied the first order A-B phase transition in superfluid SIIe in 'rough' surface morphologies. Surfaces which present a small radius of curvature to the supercooled A-phase decrease the barrier to B- phase nucleation. Preliminary data indicate that the lifetime for the supercooled A-phase at T ~ 0.56Tr is substantially decreased when ionizing radation acts in concert with specific surface roughnesses. ~,Ve have observed for the first time B-phase nucleation coincident with mechanical vibrations in tubes containing specific surface roughness. Also, we show that in the absence of a strong magnetic field the A-B interface can be prevented from entering a smooth-walled tube by a membrane filter of sub-micron pore diameter. *

1. I N T R O D U C T I O N

The first order A-B phase transistion in super- fluid SHe can attain the deepest degree of supercool- ing of all know phase transitions [5]. The high en- ergy of the A-B interface [2],~AB, coupled with the rather low difference in bulk free energies, AF, leads to an enormous critical volume for homogeneous nu- cleation which makes this mechanism for nucleation, practically speaking, impossible.

In 1984 Leggett [1] proposed an alternate, 'exotic' nucleation mechanism, involving cosmic rays, which he called the baked-Alaska model. Cosmic ray muons can cause a shower of secondary electrons in the 3He which deposit several hundred eV as they stop in the liquid. The locally heated 3He can then re-cool directly into the more stable B-phase and possibly lead to nucleation.

Schiffer et al [3] investigated this hypothesis ex- perimentally in a smooth walled sample cell which al- lowed extremely deep supercooling of the metastable A-phase. The researchers simulated the effect of cos- mic ray muons by shining a 6~ 7-ray source at the supercooled A-phase and found nucleation behavior consistent with the baked-Alaska process. In addi- tion they found, at low magnetic fields, evidence for a parallel nucleation mechanism. These last observa- tions are the motivation for the present experiment.

2. S A M P L E CELL We have designed a sample cell which allows us to

test the importance of rough surfaces on tile nucle- ation problem while simultaneously monitoring the

baked-Alaska effect in a smooth-walled environment. The sample cell consists of six fused silica tubes ( lmm I.D., 2mm O.D.) contained in a polycarbon- ate and brass cell body. The open end of five of the tubes passes through a NdFeB permanent magnet which provides a high magnetic field to exclude the B-phase. The sixth tube, which is half the length of the others did not pass through the magnet. All tubes were cleaned in a class 100 clean-room and capped with a 0.1pm pore diameter polycarbonate filter membrane.

Three of these fused silica tubes, including the short one, were prepared in the manner described by Schiffer [4] to provide microscopically smooth c o n -

tact surfaces for the Site. The remaining three tubes each contained characterized rough surfaces which would presumably affect the nucleation mechanism in different ways.

One of the three remaining tubes contains a transversely oriented thin (0.012" dia.) copper wire with SiC grit impregnated in the insulation. The two remaining tubes contain thin silicon wafers: one with etched micropits, the other without. Since the edge roughness of these wafer slices cannot be ignored, and indeed may provide more total surface rough- ness than the SiC grit, we included the blank wafer to help distinguish between the effect of micropits from that of the wafer's rough edges.

* Th i s work was s u p p o r t e d by the Nat iona l Science Foun-

da t i on u n d e r G r a n t No. D M 8 - 9 4 0 9 5 9 0 .

Czechoslovak Journal of Physics, Voi. 46 (1996), Suppl. S 1 163

3. R E S U L T S

Data were taken at 33.6 bar and 28.5 naT. Tile baked-Alaska scenario was tested in a temperature range of .5inK - 1.4mK (0.2-0.56 To), both with and without tile exposure to tile 6~ 7-ray source. Fig- ure 1 shows tile accumulated data.

IC16

0.4 0.6 0.8 1.0 1.2 1.4 1.6

Temp (mK)

FIG. 1. Metastable A-phase lifetimes plotted with the expected lifetimes (solid lines) based on the baked-Alaska scenario. Hollow symbols are for irradiated nucleations.

All of the empty, smooth tubes follow the baked- Alaska scenario quite well over a broad range of tem- peratures. In fact since the tubes arc phenomonolog- ically similar we have averaged the lifetimes of these three tubes together and recover an excellent fit to the baked-Alaska model.

Also, it is clear from the figure that the presence of rough surfaces severely attenuates the metastable A-phase lifetimes at high temperatures. Since the perturbed tube with the least amount of surface roughness, the SiC grit tube, obeys the baked-Alaska model as well as the empty tubes we can conclude that a small amount of roughness does not drasti- cally alter the nucleation behavior. This statement is consistent with the requirement that, in the ab- sence of thermal gradients, ionizing radiation nmst act in concert with a surface defect to enhance nu- cleation probability.

The fact that the short empty tube, when scaled to account for the difference in intercepted radiation flux, exhibited supercooled lifetimes as long as the full length tubes, even without the strong magnetic valve field, indicates that a smooth porous membrane can also be used to pin the A-B interface, protecting supercooled A-phase in the interior from external B- phase nucleations. It is noted that the presumably rough surface of the membrane filter does not affect nucleation.

The 'rough' tubes (with the exception of the SiC

tube during slow cooling) regularly nucleated while cooling at temperatures above 1.3mK, a temperature range where the metastable A-phase lifetime derived from the baked-Alaska model is tens of days.

On the occasion that equilibrium was reached at T~.I.2mK for the tubes with Si wafers, we could observe B-phase nucleation immediately coincident with mechanical vibrations. Specifically, we obtained nucleation in either or both of these tubes within a fraction of a second of gently rapping the cryostat with our knuckles.

Finally, the data of Fig.2 indicate that the B- phase nucleation temperature, in the presence of rough surfaces, is strongly dependent on the cool- ing rate. Therefore we propose the existence of an alternate nucleation mechanism which depends on superfluid flow and is effective over a broad temper- ature range. These high temperature nucleations are consistent with the observations of Hakonen et al [6] who found nucleations to occur only during cooling.

# 0

2.0

1.8.

1.6-

1.4-

1.2-

1.0-

0.8

. . . . . . . . i

blank wafer I pilled wafer

• ~ . ~

~ 1 7 6

~ 1 7 6

~

�9 • - ~ B o

I . . -''~~ �9 ~ 1 7 6 1 7 6 �9

~

. . . . . . . . i . . . . . . . . | . . . . . . . . i . . . . . . .

10' 102 103 104

Cooling Rate (nK/see)

FIG. 2. B*phase Nucleation temperature while cooling.

R E F E R E N C E S [1] A. J. Leggett, Phys. Rev. Lett. 53, 1096 (1984). [2] D. D. Osheroff and M. C. Cross, Phys. Rev.

Lett. 38,905 (1977). [3] P. Schiffer, M. T. O'Keefe, M. D. Hildreth, Hi-

roshi Fukuyama, and D. D. Osheroff, Phys. Rev. Lett. 69, 120 (1992).

[4] P. Schiffer, Ph.D. Thesis, Stanford University, 1993 (unpublished).

[5] G. W. Swift and D. S. Buchanan, in Proceed- ings of the 18th International Conference on Low Temperature Physics, Japanese Journal of Applied Physics 26-3, 1828 (1987).

[6] P. 5. llakonen, O. T. Ikkala, S. T. Islander, O. V. Lounasmaa, and G. E. Volovik, 3. Low Temp. Plays. 53,425 (1983).

164 Czech. J. Phys. 46 (1996), Suppl. $1