Juha Tuoriniemi

Superconductivity in lithium below 0.4 millikelvin at ambient pressure.

Elements in the alkali metal series are regarded as unlikely superconductors because of their monovalent character. A superconducting transition temperature as high as 20 K, recently found in compressed lithium (the lightest alkali element), probably arises from pressure-induced changes in the conduction-electron band structure. Superconductivity at ambient pressure in lithium has hitherto remained unresolved, both theoretically and experimentally. Here we demonstrate that lithium is a superconductor at ambient pressure with a transition temperature of 0.4 mK. As lithium has a particularly simple conduction electron system, it represents an important case for any attempts to classify superconductors and transition temperatures, especially to determine if any non-magnetic configuration can exclude superconductivity down to zero temperature. Furthermore, the combination of extremely weak superconductivity and relatively strong nuclear magnetism in lithium would clearly lead to mutual competition between these two ordering phenomena under suitably prepared conditions.

Towards Superfluidity of 3He Diluted by 4He

Search for the superfluid state of dilute 3He dissolved to 4He is one of the major remaining problems of low temperature physics. We describe our two experiments designed to pursue the lowest achieved temperature in such mixtures essentially below the values reported before.

Polarized Nuclei in Normal and Superconducting Rhodium

We performed SQUID-NMR measurements on a rhodium single crystal at ultra-low nuclear-spin temperatures. With initial polarizations up to p=0.95, the antiferromagnetic tendency was clear, but surprisingly no indication of actual nuclear magnetic ordering was obtained. The lowest nuclear temperatures achieved were below 100 pK, whereas the lowest directly measured temperature was 280 pK. Double-spin-flip and evidence for triple-spin-flip resonance lines were detected, yielding direct information of the interactions between the nuclear spins. The superconducting transition of rhodium was observed with the critical values, Tc=210 μK and Bc(0)=3.4 μT. For the first time, measurements with substantially correlated nuclei were performed in the superconducting state, where the effect of the coherent electron system on the spin-lattice relaxation rate was studied. The spin-lattice relaxation time was longer in the superconducting state at all temperatures and displayed a strong dependence on nuclear susceptibility.

Nuclear cooling and spin properties of rhodium down to picokelvin temperatures

Abstract The lowest temperatures ever measured have been produced by cascade adiabatic nuclear demagnetization of copper and rhodium. Ultimately, spontaneous antiferromagnetic ordering of rhodium nuclei is anticipated. High-sensitivity SQUID-NMR measurements at high nuclear polarization in low magnetic fields have been used to study the mutual interactions between the nuclear spins of rhodium in the metal.

Microsphere viscometers for low temperature applications

We describe two types of viscometer utilizing metallic microspheres. These spheres are set into stationary translational motion or into translational oscillations. The motion of these charged microparticles is induced and detected via an electric field. The observed damping of the motion yields information on the dissipation mechanisms and the viscosity of the medium investigated. The advantages of our devices for low temperature use are a simple geometry, high resolution, low heat input into the sample and the small sample volume needed. The two viscometers are complementary to each other in the sense that one is applicable at high viscosities while the other is sensitive at low viscosities.

Melting Pressure Thermometry for Dilute 3He‐4He Mixtures

At very low temperatures, the melting pressure of a dilute 3He‐4He mixture depends quadratically on temperature. This can be used for thermometry much in a similar fashion that has become customary for pure 3He. We have measured the difference between the melting curves of pure 4He and a mixture with about 7% 3He down to 1 mK. At the T = 0 limit, such mixture solidifies at a pressure 34.1 kPa higher than pure 4He. The melting curve followed pm(T) = p0 + 1.1 Pa (T/mK)2 with good precision at least up to 50 mK. The noise level of our system corresponded to 6 mPa variation in pressure.

Thermal contact to lithium metal

Abstract Experiments at ultra-low temperatures require very low thermal contact resistances, a serious issue even for metallic specimens below 1 mK . Customary practices include pressed contacts or welding by some means, e.g. diffusion welding. When dissimilar metals are joined, one must avoid excess formation of an alloy, usually a poor thermal conductor. The most firm contact with possibly deep alloying does not always have the best thermal conductivity. We have studied this problem when pressing lithium metal to contact with copper, silver and gold. The results are surprisingly different—good contacts could be produced only between Li and Cu, not with Li and Ag or Au. This is obviously due to easy alloying of Ag and Au with Li even at room temperature. This information is essential for proper materials choice in our planned experiment on superconductivity and nuclear magnetism on lithium metal.

Vibrating Wire Measurements in 3He-4He Mixtures in the Ballistic Quasiparticle Regime

We present our first high resolution vibrating wire resonator data on dilute 3He-4He mixtures at ultra low temperatures. Measurements were performed at saturated vapor pressure with 3He concentrations below the phase separation limit. The behavior of the damping at very low temperatures does not follow the modified slip correction analysis previously applied to extend the validity of the hydrodynamic treatment.

Quasiparticle damping of surface waves in superfluid He-3 and He-4

Oscillations on free surfaces of superfluids at the inviscid limit are damped by quasiparticle scattering. We study this effect in both superfluid

Quartz Tuning Forks and Acoustic Phenomena: Application to Superfluid Helium

^{3}\mathrm{He}