Juha Martikainen

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.

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.

Anomalous Spin-Lattice Relaxation in Dilute RhFe at Positive and Negative Nanokelvin Spin Temperatures

We have investigated at ultralow temperatures the spin-lattice relaxation in rhodium samples with about 10 p.p.m. of iron impurities. A strong decrease in the Korringa constant κ, dependent on the sign of the nuclear spin temperature T, was observed below 1 mT. Our data on the spin-lattice relaxation time τ1, measured at the electronic temperature Te = 120 μK, fit the equation 1/τ1 - 1/(κ0/Te) = 1/τimp1 (B, T), where the impurity term τimp1 (B, T) has a contribution proportional to the inverse nuclear spin temperature in small fields.

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.

SQUID Amplifier System for Vibrating Wire Resonators

AbstractWe describe a simple coupling scheme of superconducting vibrating wire resonators to a SQUID amplifier for achieving supreme signal to noise characteristics. Our construction resulted in a gain factor near 108φ0/V at 1kHz with an amplifier noise level of about 20 μφ0/

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

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A new ultralow temperature cryostat in Helsinki

. Extremely small movements of the wires in very small magnetic fields can thus be detected.

Anomalous spin-lattice relaxation in dilute RhFe at positive and negative nanokelvin temperatures

We present our vibrating wire resonator data on dilute mixtures. Measurements have been performed for two saturated 3He concentrations and pressures (x=6.5%, p=0 bar and x=9.5%, p=10.2 bar). A DC SQUID readout of the resonator gave a good signal to noise ratio even at low magnetic fields. The data suggests that our experiment has cooled dilute mixtures to the lower temperature than in the earlier experiments has been reached.