In the summaries below, the numbers in brackets refer to the group publication list (attached below). Our results have been presented also in numerous contributed and invited talks, seminars, and colloquia. A number of Invited Talks were given by our students and postdocs.

Critical Phenomena near the Superfluid Transition in liquid Helium

We continued our very-high resolution measurements of properties near the lambda-point in He-4 under unusual conditions. This work included studies of finite-size effects [176], of the effect of impurities on the transition [158,159, 171], of the propagation of nonlinear second sound near T-lambda [154, 163] and of the transition under non-equilibrium conditions [168, 188,191]. During the course of this work significant additional new technology [151,194] was developed which made our high-resolution measurements possible. Our results have stimulated theoretical work by a number of groups in the US, Japan, and Germany.

Our work in this area is funded by NASA. The finite-size measurements, under the name Boundary Effects on th eSuperfluid Transition (BEST) are expected to lead to a flight experiment on the International Space Station in 2007. Meanwhile, complementary ground-based measurements will be made in our well-equipped laboratory on Earth.

The next two paragraphs give a slightly more detailed explanation of the science involved in the above two projects.

One of the important issues in condensed-matter physics is the nature of the interface between solids and fluids. The quantitative study of finite-size effects near critical points can shed light on this issue. This is so because the correlation length becomes large and thus boundary effects involve relatively large portions of a (small) sample. The ultra-high temperature resolution and stability at the nano-Kelvin level which was developed by us [151,194] enabled us to measure quantitatively the finite-size effect on the thermal conductivity of helium near T_,( in long cylindrical tubes of diameter 2 micron [176]. These are the first, and so far only, measurements of finite-size effects on transport properties near a critical point.

The statistical dynamics of many-particle systems out of equilibrium is an important field of physics which is far from being well developed and well understood. Theoretical as well as experimental work is usually limited to the regime near equilibrium where linear response theory applies. In laboratory systems it often is difficult to achieve sufficiently extreme conditions for linear response theory to break down. However, such conditions can be achieved more readily in the vicinity of critical points where typical relaxation times become large. The superfluid transition of He-4 is a system which is particularly suitable for the study of these phenomena. We measured the effect of a heat current on this transition. In this work we discovered a new dissipative phase between the superfluid and the normal liquid. We measured the temperature range (as a function of the heat current) over which it exists, as well as its thermal conductivity [168,188,191].

Recent Publications

188. New Dissipative Region Below the Superfluid Transition of ⁴He in a Heat Current , F.-C. Liu and G. Ahlers, Phys. Rev. Lett. 76 , 1300 (1996).

191. Thermal Resistance below the Superfluid Transition of ⁴He in a Heat Current , F.-C. Liu and G. Ahlers, Czechoslovak J. Phys. 46 -Suppl, 189 (1996).

192. ⁴He Very Near T_lambda Heated From Above , G. Ahlers and F.-C. Liu, Czechoslovak J. Phys. 46 -Suppl, 187 (1996).

193. The Thermal Boundary Resistance in Superfluid ⁴He Near T_lambda , H. Baddar, H. Fu, F.-C. Liu, and G. Ahlers, Czechoslovak J. Phys. 46 -Suppl, 191 (1996).

194. A High-Resolution Susceptibility Thermometer for use Near 2 K , H. Baddar, H. Fu, M. Larson, N. Mulders, and G. Ahlers, Czechoslovak J. Phys. 46 -Suppl, 2859 (1996).

196. ⁴He Very Near T_lambda Heated From Above , G. Ahlers and F.-C. Liu, J. Low Temp. Phys. 105 , 255 (1996).

203. Opportunities for Microgravity Research along the Lambda Line of Liquid Helium , G. Ahlers, Proceedings of the St. Petersburg Symposium on Physical Sciences in Microgravity (1997), in press.

204. The Boundary Resistance Between Superfluid ⁴He Near T and a Solid Surface , H. Fu, H. Baddar, K. Kuehn, and G. Ahlers, Low Temp. Phys. 24 , 69 (1998).

205. A High-Resolution Thermometer for the Range 1.6 to 5 K , H. Fu, H. Baddar, K. Kuehn, M. Larson, N. Mulders, A. Schegolev, and G. Ahlers, J. Low Temp. Phys. 111 , 49 (1998).

213. Opportunities for High-Resolution Measurements and Microgravity Research near the Superfluid Transition of Liquid ⁴He , G. Ahlers, J. Low Temp. Phys. 113 , 835 (1998).

215. Finite-Size Effects on the Thermal Conductivity of ⁴He near T_lambda , G. Ahlers, Proceedings of the 1998 NASA/JPL Microgravity Fundamental Physics Workshop , Oxnard, CA, June 23-25, 1998.

218. Finite-Size and Gravity Effects on the Thermal Conductivity of ⁴He Near the Lambda Line , G. Ahlers, J. Low Temp. Phys. 59, 2975 (1998).

221. Mutual Friction very close to the Superfluid Transition, H. Baddar, G. Ahlers, K. Kuehn, and H.-Y Fu, in Proceedings of the 1999 NASA/JPL Microgravity Fundamental Physics Workshop, April 29 to May 1, 1999.

223. Boundary Effects near the Superfluid Transition (BEST), an Experiment Proposed for the ISS , G. Ahlers, P. Finley, E. Genio, K. Kuehn, F. Liu, Y. Liu, S. Mehta, and D. Murphy, in CP504, Space Technology and Applications International Forum - 2000, edited by M.S. El-Genk (American Institute of Physics, 2000), p. 662.

224. Thermal Resistance of ⁴He Below but Very Near the Superfluid Transition , H. Baddar, G. Ahlers, K. Kuehn, and H. Fu, J. Low Temp. Phys. 119, 1 (2000).

229. Fundamental Physics in Space, C.W.F. Everitt, G. Ahlers, M. Jacob, S. Vitale, and N. Bigelow, Advances in Space Research, in press.

230. Singularity in the boundary resistance between superfluid $^4$He and a solid surface, K. Kuehn, S. Mehta, H. Fu, E. Genio, D. Murphy, F. Liu, Y. Liu, and G. Ahlers, Phys. Rev. Lett. 88, 096702 (2002).

237. Finite-size scaling and universality of the thermal resistivity of liquid $^4$He near $T_\lambda$, D. Murphy, E. Genio, G. Ahlers, F.C. Liu, and Y. Liu, Phys. Rev. Lett. {\bf 90}, 025301 (2003).

239. Scaling and universality of the thermal conductivity of liquid $^4$He near the superfluid transition and in restricted geometries, E. Genio, D. Murphy, G. Ahlers, F.-C. Liu, and Y.-M. Liu, Advances in Space Research, in press.