Weak Binding Substrates.
Most recent update: summer 1998
One of the research directions involves the behavior of helium on weak binding
substrates. Our initial work in this direction  involved a glass substrate which
held a third sound driver and several detectors (Figure 1). Third sound was observed to
propagate normally across a clean glass portion of the substrate, but for low values
of the chemical potential, no propagation was seen across a portion of the substrate
which had been covered by cesium in an in situ evaporation. Third sound was
observed to reflect from the edge of the cesium region (Figure 2). As the chemical potential
was increased (by adding helium to the experimental cell) a critical chemical
potential was reached above which thrid sound was observed to propagate across the
cesium (Figure 3). At this same chemical potential value, the reflected amplitude reversed
sign indicating a change in the boundary condition at the edge of the cesium. These
observations are interpreted as evidence for non-wetting of 4He to cesium, and the
first observation of a prewetting transition.
Figure 1. Here is shown a third sound substrate which is made
from a glass slide onto which superconducting bolometric detectors, (A, D), and
a silver thin film driver of third sound, (Z), have been placed. The region between
the Z and A has been coated with Cesium in situ. The slide is housed
in an experimental chamber in which the pressure of helium gas (and therefore the
chemical potential and the helium film thickness on the bare glass) can be
Figure 2. For values of the chemical potential such that the helium film thickness on the
bare glass substrate is 10.7 atomic layers or less, we observed that no third sound signal
reaches detector A. This is seen in the lower part of the figure where we show the signal
on detector A due to a pulse generated at Z. We see a normal third sound pulse at detector
D for the the direct propagation path Z-D, and we observe a pulse which has been reflected
from the edge of the Cesium, Z-Cs-D. These measurements were at T = 1.38 K. The conclusion
is that for this range of chemical potential values, the wetting of helium to Cesium is
anomalous, with the thickness not adequate for superfluidity to be present. This is consistent
with the non-wetting of helium to Cesium. Here the thickness on glass is 10.7 layers.
Figure 3. When the chemical potential is increased, so as to form a helium film on glass
of thickness 11.5 atomic layers, the behavior changes dramatically. There is observed the
abrupt onset of third sound arrival at detector A, and an inversion of the pulse amplitude
reflected from the edge of the Cesium. This behavior is hysteretic showing that the wetting
film is metastable. This abrupt jump to a film thick enough to propagate third sound on
the Cesium is the observation of a pre-wetting transition. Such had been predicted in the
mid-1970's but never seen in nature. For these data shown here the thickness of the helium
film on glass is 11.5 layers. Similar behavior to that shown here is observed for larger
values of the chemical potential.
Experiments with 3He-4He mixtures using quartz oscillator techniques  were
carried out and showed that the addition of 3He to the system resulted in reentrant
wetting as had been predicted earlier by Petersen and Saam . The lower wetting
temperature was established as a function of 3He concentration.
Work underway at present is designed to explore the nature of the 4He film
on Rubidium and questions asociated with the apparent very thin film superfluidity
which is claimed to be present on Rubidium below its wetting temperature. The effect
of Oxygen on the wetting temperature is being explored. Finally, we have also constructed a cesium boundary "corral" to confine
4He to a restricted regions of space and have studied the properties of third sound in
such a confined region [4,5]
Other work is directed at a study of the behavior of 4He on hydrogen surfaces. In
this direction we have recently confirmed the presence of sub-monolayer superfluidity
for 4He on hydrogen and determined the binding energy of 4He on two layers of
hydrogen on a gold substrate. For the case of 3He - 4He mixture films, we find the
presence of a second mass decoupling feature in addition to the Kosterlitz-Thouless
transion, likey caused by a rearrangement of the 3He in the film.
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