REFRIGERATOR:
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.
SAMPLE
REGION: CALORIMETER
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.
ADIABATIC
NUCLEAR DEMAGNETIZATION:
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
~500 µK.
NMR
SAMPLE REGION:
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.
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| Cryo2 | |
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| Nuclear Stage |
Cryo2 has a PrNi5 nuclear demagnetization stage precooled by a model DRP-42
dilution fridge from SHE.
If you are wondering what this means, you have
not payed attention to our web-site closely enough. We know that life is
short and it may be a little too short for you to stare at our web-site
all day, but since you have made the effort to come this far, please take
some time and look at our web-site a little more carefully. It'll be a great
opportunity to learn about ultra low temperature physics. Now, if you have already looked at our web-site inside out and are still
going 'what does this mean?', you can contact Johannes Pollanen (j-pollanen@northwestern.edu) or Hyoungsoon Choi (choi@northwestern.edu)
and we will be happy to discuss low temperature physics and answer any questions you might have about the work we do.
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| Calorimeter | |
Anyway, back to Cryo2. Most recently Cryo2 had a calorimeter which we used to
measure heat capacity of liquid 3He in aerogel. Those that are interested
in what this means exactly may read or attempt to read
PRL Vol. 93 145301. For those
that are mildly interested in what this means, here is what it is. We liquify
3He, which exists as gas at room temperature, at around one Kelvin into
a silver container of roughly two mL. In that silver container, we have
a heater and a thermometer. Just to be fancy or to make it sound difficult,
we call this set up a calorimeter. Now we cool this silver cell down to
about one mK. You turn on the heater and measure the temperature.
Cryo2 is also equipped with a 3.5 Tesla magnet at 44.2 Ampere for nuclear
magnetic resonance (NMR) experiments. Johannes is setting up a NMR experiment to study the properties of superfluid 3He in aerogels that were grown right here in our lab. Speaking of NMR, our advisor, Prof. W.P.Halperin
also runs an NMR lab.
As of December, 2007, Choi has finished up with his experiments on heat capacity and the calorimeter has been removed. Johannes is currently working on assembling the NMR sample cell and will perform experiments on liquid 3He in aerogel that will hopefully give us some exciting results.
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