Research
Overview
Research in
Gasparinilab focuses on
Finite-size
effects and
universality. We
confine
liquid helium in one or more
dimensions and measure its thermodynamic properties at temperatures near the
superfluid transition temperature. We are able to test finite-size
scaling theories and determine critical exponents for the "XY" universality class. We
have measured helium confined in one, two and all three dimensions (refered to
as 2D, 1D, and 0D systems respectively), and found that, although there is
strong evidence for correlation length scaling, it does not seem to apply
everywhere in the
critical region.
Confinements
Our
confinements are achieved between two bonded silicon oxide wafers. An oxide
layer is grown on 2 inch silicon wafers and then patterned to produce the
desired confining structures using photolithography. The two wafers are then
bonded to one another and filled with helium via a hole drilled through the
top wafer.
3D to 2D Crossover
Helium confined
to 2D films has been studied extensively. Gasparinilab has studied films
ranging from 48.3nm to 986.9nm. These data show strong evidence for
correlation length scaling above the bulk transition temperature. The success
of scaling continues slightly below the bulk transition temperature. However
as the temperature of the specific heat maximum in the confined system is
approached scaling breaks down, and data no longer collapses onto a univeral
locus. This suggests that scaling breaks down as fluctuations in the confined system become largest.
3D to 1D Crossover
Data from 1D
systems are similar to that from the planar systems. They show strong
evidence for correlation-length scaling above the bulk transistion temperature
and down towards temperatures near the specific heat maximum in the confined
systems. Again scaling appears to break down around the specific heat maximum.
However, there is no systematic deviation from the universal locus in the 1D
data. This suggests that here the physics causing the lack of scaling differs
from that involved in the 2D data, where the deviations from the locus are
systematic with confinement size.
3D to 0D Crossover
To date we have
measured two sets of data confined in all three spatial dimensions.
Measurements have been made using cells comprised of 1 micrometer cubes and 2
micrometer cubes. Unlike the 2D and 1D data, there is an overall failure of
scaling throughout the critical region. There are, however, anomalies in the
data, such as the magnitude of the specific heat maximums, that suggest
neighbouring confinements may actually be coupled through the shallow fill
channels which span the entire cell. This has inspired future measurements to
investigate this apparent coupling of relatively large regions of helium
across very restricted regions.
Last Updated:
Monday, 21-Apr-2008 11:33:06 EDT
by
MOK
