Helium Droplets and Films on Alkali Metal Substrates

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					               Helium Droplets and Films on Alkali Metal Substrates
                          P. Taborek, E. Van Cleve and J. C. Burton

                               University of California, Irvine

Alkali metals have unusually weak adsorption potentials, and the phases and superfluid
behavior of helium on these substrates are very different from conventional strong binding
substrates. After briefly reviewing the status of experimental work on cesium and
rubidium, I will discuss recent results on lithium and sodium. The lighter alkalis have higher
binding energies and also higher melting temperatures. We have developed a pulsed laser
deposition system which allows us to form atomically clean substrates of these materials at
low temperature onto the surface of quartz crystal microbalances (QCM)[1]. Lithium is a
particularly interesting case because it is strong enough that there is no prewetting
transition, so the coverage can be continuously tuned, but not strong enough to cause the
formation of solid layers[2]. The measured value of the binding energy of 4He to lithium is
approximately -13K. As we vary the temperature and coverage on lithium in the
temperature range 0.75K-1.5K, we find a line of KT transitions with the conventional
features of a nearly discontinuous jump in the frequency and a peak in the dissipation. The
size of the frequency jumps indicate that the lithium surfaces have a low tortuosity (χ≈0.2) ,
which is much lower than conventional mylar or graphite torsional oscillators. Below 0.75K,
however, the conventional KT features abruptly disappear. Other features in the isotherms
are consistent with a 2D liquid-vapor coexistence region. At temperatures below 0.75K on
lithium, the KT transition is preempted by phase separation which leads to patches of
relatively high density liquid which is “born “ superfluid . These patches do not couple to the
oscillator, and do not lead to signatures in the dissipation.

1.     Van Cleve, E., P. Taborek, and J.E. Rutledge, Helium adsorption on lithium substrates.
       Journal of Low Temperature Physics, 2008. 150(1-2): p. 1-11.
2.     Boninsegni, M. and L. Szybisz, Structure and energetics of helium films on alkali
       substrates. Physical Review B, 2004. 70(2).