1. PULSE-TUBE PRECOOLED AND HYPERFINE-FIELD-ENHANCED NUCLEAR REFRIGERATION WITH NOISE THERMOMETRY Aya Shibahara, Microkelvin Workshop 2013 JRA1 Task 2 JRA4 Task 3a

2. 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

3. InK: Implementing the new Kelvin  To resolve the long-standing discrepancy between the PLTS 2000 measurements through low uncertainty primary thermometry.

4. Praseodymium Nickel-5: PrNi 5  Hyperfine enhanced paramagnet  Contains rare earth ion Pr 3+, 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 B int at the Pr nucleus, which is enhanced compared to the externally applied field B.  K = B int /B = 11.2  B enhanced by (1+K), l n enhanced by (1+K) 2

5. Entropy curves

6. Comparison of PrNi 5 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 PrNi 5 Copper  Spontaneous ferromagnetic nuclear ordering T = 400 µK  Internal field 65 mT  Spontaneous antiferromagnetic nuclear ordering T = 50 nK  Internal field 0.36 mT

7. The Oxford Instruments Triton 200 Commercial cryogen free dilution fridge Decoupled pulse tube cooler first and second stages from the refrigerator plates

8. 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

9. The Nuclear stage  128 g of PrNi 5 (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

10. Current sensing Noise thermometer Noise sensor: 0.24 m Ω 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

11. Current sensing Noise thermometer  C636 G24 XXL SQUID from PTB  Input coil L i = 1.8 µH

12. Noise spectra at various temperatures

13. Here we present the performance of the PrNi 5 nuclear stage on the Triton 200 Results

14. 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

15. 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

16. Heat capacity of PrNi 5 Kubota et al. Phys. Rev. Lett. 45, 22 (1980)

17. 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

18. Fast PtW noise thermometer T N = 130 µK

19. Precision of Fast noise thermometer  R = 1.29 Ω  Dilution fridge  1% precision in 100 ms

20. Conclusions  A bolt-on PrNi 5 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

21. Thank you to all our collaborators and funding bodies! And thank you for listening! arXiv:1307.7049

22. Heat leak measurements