A dilution refrigerator is a device that can achieve and maintain
temperatures near 7 mK.
It can be divided into four main sections: 4He pot, still,
heat exchangers and mixing chamber. A mixture of 4He
and 3He gas is condensed into the fridge at the 4He
pot, which is cooled to 1 K by evaporative cooling of 4He.
The liquid helium mixture is pumped at the still which further cools
the mixture to around 0.3 K, by evaporative cooling of 3He.
Below 0.8 K the mixture of 4He and 3He phase
separates into two phases: one is pure 3He and the other
is 4He with a small quantity of 3He, the so-called
dilute phase. The boundary between these two phases sits in the
mixing chamber. As the still is pumped, differences in vapor pressure
between the two isotopes leads to 3He being primarily
removed from the dilute phase in the still. It is then energetically
favorable for 3He in the pure side to move across the
phase boundary to replenish the dilute side. This movement of 3He
from the concentrated phase into the dilute phase is analogous to
evaporation and has associated with it a latent heat. The 3He
that is pumped off at the still is returned to the pure side of
the mixing chamber by liquification at the 4He pot and
is precooled through a series of heat exchangers in order to continue
the cooling process.
Thermodynamic properties of superfluid 3He, such as specific
heat, are measured in this experimental sample region. Specific
heat measurement requires low and controlled heat capacity for materials
other than the substance of interest, 3He in this case.
Therefore, a superconducting cadmium heat switch is used to isolate
the sample cell from the nulcear stage for the measurements. Also,
low heat leak into 3He is crucial, and disconnecting
the sample cell from the nuclear stage enables us to reduce the
heat leak into 3He to as low as 80 pW. Once the sample
is cooled to sub-mK, a standard adiabatic heat pulse technique is
used to measure the heat capacity.
In the nuclear stage a paramagnetic material, either Hitachi copper
or praseodymium-nickel-5, is placed in an external magnetic field
of ~8 T. With the field in place the nuclear spins in the paramagnetic
material align parallel to the field. The field is then slowly (adiabatically)
lowered to zero and the system of nuclear spins disorder from the
low entropy (ordered) state into a configuration of higher entropy.
This proccess absorbs heat and cools the 3He down to
Using nuclear magnetic resonance (NMR) the spin state and spin dynamics
of superfluid phases of 3He are studied in this sample
region. This is, in fact, how the superfluid phases of 3He
were discovered and identified at Cornell University in 1972. This
discovery led to the Nobel Prize in Physics in 1996. Here at Northwestern
we have studied the magnetization of superfluid 3He-B
and NMR of superfluid 3He in aerogel.
Gap energy diagrams for various superfluid phases.