Universiteit Leiden

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Tjerk Oosterkamp Lab

Setup: Low Temperatures

A lot of our research is performed at temperatures close to absolute zero, among other reasons because this increases the sensitivity of the measurements. 

In the case of nano MRI experiments, we are trying to break the sensitivity record of the technique: lower temperatures reduce the thermal motion of the cantilever, which makes it sensitive to smaller forces and therefore a smaller number of spins. Our ultimate goal is measuring one single nuclear spin!

We reach temperatures of around 15 mK in our 3 cryostats. If you have ever been in our lab, you might have heard the squeaky noise from the pulse tube coolers. They cool the setup to 4 K by compression and expansion of helium 4 gas. From 4 K down to 15 mK we use the cooling principle of dilution refrigeration: we mix the two helium isotopes helium 3 and helium 4. This mixing process requires energy and if you force the 2 isotopes to mix, you can cool the environment.

We spend a lot of time working on our cryostats, therefore we have given them names: Olaf and Marshmallow are the snowmen from the movie Frozen, the Yeti is a mysterious snow creature supposedly living in the Himalaya.

The Yeti cryostat can even go to temperatures below 1 mK, using a nuclear demagnetization stage. This works by aligning nuclear spins of PrNi5 in a strong external magnetic field. The entropy of the spins depends on the ratio between magnetic field and temperature. If you reduce the magnetic field in an adiabatic process, the temperature of the PrNi5 spins has to go down as well in order to keep the entropy constant.

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