Low Temperature Physics Group

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.