T. A. Knuuttila

A Versatile Nuclear Demagnetization Cryostat for Ultralow Temperature Research

A new cascade nuclear demagnetization cryostat has been designed and constructed. Our aim was to build a versatile, modular cryostat, with a large experimental space providing an excellent platform for various types of ultralow temperature measurements. A powerful dilution refrigerator, combined with a massive copper nuclear cooling stage, enables us to reach lattice temperatures below 100μK continuously for more than two months. The cryostat is equipped with a second magnet for operating a double-stage nuclear demagnetization setup. Details of the design and performance are presented.

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.

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.

Effect of heat treatment of pure and carbon-polluted rhodium samples on the low-temperature resistivity

We present a systematic investigation of conditions for heat treatment of Rh with the aim of increasing the residual resistivity ratio (RRR). The maximal value of RRR for a 25 μm thick foil was found to be 1050 and the optimal treatment conditions were high temperatures, above 1400°C, and a low pressure of pure oxygen, around 1 μbar. Another batch of foils, containing less magnetic impurities, showed an RRR of only 600. A 0.4 mm thick single crystal was heat treated to an RRR value of 740. Our findings are discussed in the light of a model with magnetic and non-magnetic impurities in Rh, where the latter is found to have an important contribution for this unusual metal. Especially carbon impurities were found to be quite detrimental for the resistivity, and the recovery of the RRR after a carbon contamination is extremely slow in subsequent heat treatments.

High Temperature Expansion and Exact Diagonalization of the Rhodium Nuclear Spin System Compared to Experimental Results

The nuclear spin system of Rh metal has recently been investigated experimentally. No nuclear magnetic ordering was observed in these measurements, in spite of the very low nuclear entropy achieved. To study this system theoretically, we have applied the method of exact diagonalization to a 16 spin I=1/2 fcc cluster with both Ruderman-Kittel and dipolar interactions. In this work, we compare the results of this method with the experimental data and the high temperature expansion results in the paramagnetic state. The high temperature expansions agree with the experimental data down to 1 nK, while the exact diagonalisation results are less accurate.

A new ultralow temperature cryostat in Helsinki

A new cascade nuclear demagnetization cryostat has been designed and constructed. Our aim has been to build a versatile, modular cryostat with a large experimental space providing an excellent platform for various types of ultra-low temperature measurements. A powerful dilution refrigerator, combined with massive nuclear cooling stages, will enable us to reach the low temperatures necessary in studies of nuclear magnetic ordering. First experiments include nuclear magnetism in Rh and search for superfluidity of3He in dilute mixtures. Details of the design and results of the performance tests will be discussed.