Microkelvin investigations (µKI, [ myːkiː ])
Experiments on condensed matter well below 1 mK can be performed routinely in a carefully designed cryostat combining a powerful dilution refrigerator and an adiabatic nuclear demagnetization stage.
Systems still thermally active at such low temperatures include most nuclear spin ensembles, fermion fluids (pure and dilute He-3), and some conduction electron systems, which fail to develop superconductivity or magnetic ordering at higher temperatures.
µKI Group currently focus to two main lines of research: to the search of superfluidity of He-3 in dilute mixtures of the helium isotopes and to the study of helium crystals. These subjects were earlier studied by two separate groups: YKI and Interface, correspondingly.
The past achievements include the observations of spontaneous long range magnetic ordering of the nuclear spin systems in copper, silver, and lithium. Experiments at negative absolute temperatures have been performed in silver and rhodium. The lowest temperature ever achieved was recorded in 1999 - the spin temperature of nuclei in rhodium metal was reduced to about 0.000 000 000 1 K = 100 pK by a cascade nuclear demagnetization technique.
The search of superfluidity of He-3 in dilute mixtures of the helium isotopes
We develop a new method for cooling helium mixtures by a so-called adiabatic melting technique. The micro-Kelvin capacity of our cryostat will be used to cool pure He-3 into the superfluid state prior to letting it mix with pure He-4. The isotope separation in the experimental chamber at ultra low temperatures will be achieved by the solidification of He-4 due to applied pressure, which, once released, will lead to melting of the He-4 crystal, mixing of the isotopes, and consequent dilution cooling. The cooling ratio by dilution of the thermally isolated helium mixture increases dramatically as the initial temperature of pure He-3 is decreased below its superfluid transition point. This way, temperatures far below 0.1 mK, which is the practical limit for cooling helium mixtures by conventional techniques, should become achievable.
The study of helium crystals
Helium crystals present a beautiful model system in which general properties of crystalline surfaces such as the equilibrium crystal shape, surface phase transitions (roughening) and elementary mechanisms of the crystal growth can be studied over a wide temperature range, in principle down to absolute zero. In addition, helium crystals reveal several exceptional phenomena like crystallization waves which are the melting-freezing waves on the superfluid-solid interface and which are due to quantum properties of liquid and solid helium at low temperatures.
In the current project the shape and growth dynamics of He-3 and He-4 crystals has been studied at ultra low temperatures. For imaging the crystals we used a unique low-temperature Fabry-Pérot interferometer, which is built inside the nuclear demagnetization cryostat and which allows simultaneous observations of the global shape of the crystals as well as fine details of the liquid-solid interface. In past several original results were obtained, as the discovery of a multitude of different types of facets (smooth flat faces) on He-3 crystals and the accurate measurements on the growth dynamics of He-3 crystals along the melting curve down to 0.0005 K. In future the effect of a magnetic field on the properties of the liquid-solid interface of He-3 will be investigated.
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Reivinen, M., Salonen, E.-M., Todoshchenko, I., and Vaskelainen, V.P., "Equilibrium crystal shapes by virtual work", Journal of Low Temperature Physics, 170, 75-90 (2013). [DOI]
Tuoriniemi, J., Rysti, J., Salmela, A., and Manninen, M., "Mode analysis for a quartz tuning fork coupled to acoustic resonances of fluid in a cylindrical cavity", Journal of Physics: Conference Series, 400, 012077 (2012). [URL]
Rysti, J., Tuoriniemi, J., Salmela, A., and Sebedash, A., "Melting pressure of saturated helium mixture at temperatures between 10 mK and 0.5 K", Journal of Physics: Conference Series, 400, 012065 (2012). [URL]
Rysti, J., Tuoriniemi, J., Salmela, A., "Effective 3He interactions in dilute 3He-4He mixtures", Physical Review B, 85, 134529/1-8 (2012). [DOI]