PULSE-TUBE PRECOOLED AND HYPERFINE-FIELD-ENHANCED NUCLEAR REFRIGERATION WITH NOISE THERMOMETRY

Aya Shibahara, Microkelvin Workshop 2013

JRA1 Task 2JRA4 Task 3a

Nanoscience community for fast turnaround µK measurements.

Oxford Instruments for industrial/commercial reasons

Us for testing noise thermometry

Staying below 1mK on a cryogen-free fridge

InK: Implementing the new Kelvin To resolve the long-standing discrepancy

between the PLTS 2000 measurements through low uncertainty primary thermometry.

Praseodymium Nickel-5: PrNi5 Hyperfine enhanced paramagnet Contains rare earth ion Pr3+, spin 5/2 At low temperatures, the 4f electrons have an

electronic singlet non-magnetic ground state. External magnetic field mixes higher non-singlet states

into the ground state, inducing an electronic magnetic moment.

This generates a hyperfine field Bint at the Pr nucleus, which is enhanced compared to the externally applied field B.

K = Bint/B = 11.2

B enhanced by (1+K), ln enhanced by (1+K)2

Entropy curves

Comparison of PrNi5 with Copper

Large entropy reduction possible

Less eddy-current heating

Less than 1 mol required

Only a maximum of 20% entropy reduction possible

High conductivity 10-100 mol used

PrNi5 Copper

Spontaneous ferromagnetic nuclear ordering T = 400 µK

Internal field 65 mT

Spontaneous antiferromagnetic nuclear ordering T = 50 nK

Internal field 0.36 mT

The Oxford Instruments Triton 200

Commercial cryogen free dilution fridge

Decoupled pulse tube cooler first and second stages from the refrigerator plates

Brass MC shield and Still shield

Dry 8 T magnet mounted at the second pulse tube stage 3K plate

Schematic of shields and magnet on Triton 200

The Nuclear stage

128 g of PrNi5 (0.3 mol) Nine rods, 6 mm Ø × 50 mm long 99.99% Cadmium solder to 1 mm Ø Cu

wires One wire per rod to upper plate Eight wires per rod to lower plate From Jeevak Parpia, Cornell Aluminium heat switch supplied by

Richard Haley, Lancaster. Noise thermometer heat sunk to lower

plate with 37 × 0.7 mm Ø annealed Cu wires

Current sensing Noise thermometer

Noise sensor:

0.24 mΩ copper foil resistor

Heat sinking

ground

Annealed copper

holder

Heat sinking copper washer

Heat sinking

Nb foil

Single calibration at 4.2 K Simple to install Compact

Nb screw

terminals

Current sensing Noise thermometer

C636 G24 XXL SQUID from PTB Input coil Li = 1.8 µH

Noise spectra at various temperatures

Here we present the performance of the PrNi5 nuclear stage on the Triton 200

Results

Typical demag procedure

Pump on vacuum can overnight With magnet, base temperature of DU reached in

48 hours Precool field limited to 6.2 T (90 A)

Due to stray field at aluminium heat switch But high enough field for significant entropy reduction

Precool duration 24 h for 20 mK Typically 40-45 h weekend precool for 19 mK Corresponds to 80% entropy reduction

Demagnetisation from 6.2 T to 0 T in 6 hours In steps from 90 A to 0 A with rates from 0.5 A/min to 0.03

A/min

How long does it stay cold?

Base temp of 600 µK reached in zero field

20 nW heat leak: 16 hours below 1 mK

5 nW heat leak: over 24 hours below 1 mK

Increased hold time for 210 mT

Heat capacity of PrNi5

Kubota et al. Phys. Rev. Lett. 45, 22 (1980)

Thermometer performance on a wet nuclear demag fridge

Traditional copper nuclear stage on a wet system Noise thermometer of the same design, base T = 190 µK

Fast PtW noise thermometer

TN = 130 µK

Precision of Fast noise thermometer

R = 1.29 Ω Dilution fridge 1% precision in

100 ms

Conclusions

A bolt-on PrNi5 nuclear demag stage for a pulse-tube pre-cooled system was shown to cool to 600 µK and remain below 1 mK for over 24 hours, with a heat leak of 5 nW.

The use of a current sensing dc SQUID noise thermometer allowed the direct measurement of these sub-mK temperatures.

A precool in a field of ~6.2 T and a starting temperature of ~20 mK with a 6 hour demag is feasible.

Compact and easy to use Cryogen-free sub-mK platforms seem to be a

realistic prospect, dramatically improving the accessibility of ultra-low temperatures

Thank you to all our collaborators and funding bodies!

And thank you for listening!arXiv:1307.7049

Heat leak measurements