Department of Applied Physics

Microkelvin investigations (µKI)

In the µKI group, we study systems thermally active still in microkelvin range of temperatures. This applies to 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.
muKi cryostat

Different subsystems of matter may have very different temperatures at the same time, though they may be spatially inseparable. Fascinating examples of this are the assemblies of nuclear spins and conduction electrons in metals. 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.

Group leader

Dr. Juha Tuoriniemi

Research

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.

World record in low temperatures

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.

Eleven different types of facets observed on the surface of helium-3 crystals.

Facilities

Latest publications

A graphene resonator as an ultrasound detector for generalized Love waves in a polymer film with two level states

Antti Laitinen, Jukka-Pekka Kaikkonen, Thanniyil S. Abhilash, Igor Todoshchenko, Juuso Manninen, Vladislav Zavyalov, Alexander Savin, Andreas Isacsson, Pertti J. Hakonen 2019 Journal of Physics D - Applied Physics

Effects of 4He Film on Quartz Tuning Forks in 3He at Ultra-low Temperatures

T. S. Riekki, J. Rysti, J. T. Mäkinen, A. P. Sebedash, V. B. Eltsov, J. T. Tuoriniemi 2019 Journal of Low Temperature Physics

Thermal Conductivity of Superfluid 3 He-B in a Tubular Channel Down to 0.1 Tc at the 4 He Crystallization Pressure

T. S. Riekki, J. T. Tuoriniemi, A. P. Sebedash 2019 Journal of Low Temperature Physics

Thermodynamics of adiabatic melting of solid He 4 in liquid He 3

T. S. Riekki, A. P. Sebedash, J. T. Tuoriniemi 2019 Physical Review B

Finite-size effects in thermodynamics

I. Todoshchenko 2018 Physical Review B

An Enhanced Facet Determination Scheme in 3D

M. Reivinen, Eero-Matti Salonen, I. Todoshchenko, V. P. Vaskelainen 2017 Journal of Low Temperature Physics

Adiabatic Melting Experiment on Helium Mixtures

A. Sebedash, S. Boldarev, T. Riekki, J. Tuoriniemi 2017 Journal of Low Temperature Physics

Quartz tuning fork as a probe of surface oscillations

I. Todoshchenko, A. Savin, M. Haataja, J. P. Kaikkonen, P. J. Hakonen 2017 Applied Physics Letters

Surface Waves on the Superfluids 3He and 4He

Matti Manninen, Antti Ranni, J. Rysti, I. A. Todoshchenko, J. T. Tuoriniemi 2016 Journal of Low Temperature Physics
More information on our research in the Research database.
Research database

Research group members

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