Some of the topics we are working on are:
- Non-linear second sound experiments
near the tricritical point of
3 He-4He mixtures
It turns out that D=3, i.e. the highest spatial
dimension that is experimentally accessible, is also the marginal
spatial dimension for tricritical behavior. As a result we have
closed form prediction for tricritical exponents and (logarithmic)
corrections to scaling. Unfortunately, just where the theory makes
its most accurate predictions there is a dearth of high precession
experimental data. Our experiments near the tricritical point
in helium mixtures aim at improving this situation. We use non-linear
second sound time-of flight techniques to obtain accurate values
for the superfluid density.
M. Mohazzab
et al., Second sound measurements near the
tricritical point in He-3-He-4 mixtures
M. Mohazzab and N. Mulders, Nonlinear second- and first-sound wave equations in 3 He-
4Hemixtures
- Vortex formation on quenching from the normal
into the superfluid state
Quite generally, we associate critical phenomena,
such as the superfluid transition in 4He with diverging
length and time scales. Trying to quench a system through a continuous
phase transition faster than the rate at which the order parameter
can adjust leads to the formation of topological defects such
as domain walls and vortex lines. We are working on experiments
in which liquid helium is quenched from the normal into the superfluid
state using focused ultrasound. The expectation is that this will
result in the formation of a dense vortex tangle, the strength
of which can be investigated by determining the absorption and
scattering of second sound.
W.H.
Zurek, Cosmological experiments in superfluid helium, Nature 317,
505 (1985).
- Phase transitions in helium in
porous media
Absorption of a fluid in a porous medium
can substantially alter the fluid’s behavior. When the ratio
of pore surface to pore volume is large, one observes significant
shifts in phase boundaries, sometimes to the extent that the topology
of the phase diagram is altered and new phases appear. Both 3He
and 4He, as well as their isotopic mixtures, in variety
of porous media, provide striking examples. The properties of
these superfluid-solid matrix composites can be studied using
low frequency acoustic resonance techniques as well as time-of-flight
ultrasound.
M.H.W. Chan, N. Mulders and J.D. Reppy, Helium in
Aerogel, The effects of disorder on phase transitions
- The interaction between helium
and the TLS in glasses
The low-temperature properties of amorphous
solids differ substantially from those of their crystalline counterparts.
There appears to be little difference in the behavior of porous
glasses such as Vycor and completely dense systems. However, when
helium is adsorbed in a porous glass, a well defined new relaxation
channel for the TLS appears, the origin of which is not clear.
We are investigating the relations between pore size and type
of adsorbent for various silica porous media.
Mulders N, Molz
E and Beamish J.R The
Effect of adsorption of 3He and 4He
on the low-temperature ultrasonic properties of porous Vycor
