SQUIDs and SQUID-microscopy

Transcription

1 1 SQUIDs and SQUID-microscopy Klaus Hasselbach

2 2 outline Basic principles of SQUIDs Applications of SQUIDs SQUID microscopy

3 3 Basic principles of SQUIDs Flux quantization in superconducting Ring DC and AC Josephson effect Realization of SQUIDs

4 Flux quantization in superconducting Ring 4 ψ = ρ 1/2 exp(iϕ(r )) = ψ = ρ1/2 exp(i pr / ) s s p = p kin + p pot = 2mv s 2 e A js = 2 e ρ vs s p = (m / e ρ s ) 2 e A Rigid phase: total phase change must be single valued Δϕ = ( p / ) dl = m e ρ s js d l 2 e A dl = 2πn φ ' = (m / 2ρ s e 2 ) js dl + φ = n(2π) / (2 e ) = nφ 0

5 5 Flux quantization in superconducting Ring φ ' = (m / 2ρ s e 2 ) js dl + φ = n(2π) / (2 e ) = nφ 0 =0 in bulk sc as J s = 0 in bulk φ ' = φ = nφ 0 Φ 0 = h / 2e = Tm 2 Flux(oid) quantization Deaver and Fairbanks (PRL 7, 43 (1961)) Doll and Naebauer (PRL 7, 51 (1961))

6 6 Experiment proof of flux quantization Torsional oscillator Doll and Näbauer (PRL 7, 51 (1961)) Torsional oscillator Deaver and Fairbanks (PRL 7, 43 (1961)) Magnetometry of tin coated copper wire

7 7 Josephson junction φ 1 φ 2 JR Kirtley Josephson tunneling- transfer of Cooper pairs rather than single electrons I s = I 0 sinϕ V = 2e dϕ dt B.D. Josephson (PRL 1, 251(1962)) I s -Cooper pair current across junction I 0 -Junction critical current φ -difference in pair phases across junction

8 8 Josephson Junctions Tunnel barrier SNS Junction Micro Bridge junction M. Tinkham

9 DC-SQUID 9 I 1,I 2 critical current of junctions L 1,L 2 - inductances of arms of SQUID loops I B I B = I 1 sin(ϕ 1 ) + I 2 sin(ϕ 2 ) 2πn = ϕ2 ϕ1+ 2π Φ 0 (Φ a + L 2 I 2 L 1 I 1 ) n = 0 Maximize I B for each value of Φ a φ 1,I 1,L 1 φ 2,I 2,L 2 X X I B Φ a /Φ 0

10 10 Design considerations JR Kirtley SQUID size ( inductance L) not too small I c sufficient big compared to thermal noise

11 Magnetic field scales 11 J. Clarke

12 Operation of SQUID 12 R. Kleiner

13 13 Noise in DC SQUID Nyquist noise of shunt resistor Spectral density of flux noise S φ (f ) 16k B TL 2 / R L=200 ph, R= 6 Ohm T=4.2 K S 1/2 φ (f ) φ 0 / Hz Noise energy ε(f ) = S φ (f ) / 2L JHz C. Tesche, J Clarke 1977

14 Flux noise in the SQUID 14 J. Clarke

15 15 Applications of SQUIDs Magnetometer Geophysics Medical research Non destructive testing Current amplifier How is the SQUID coupled to the world?

16 16 Electronics readout The SQUID is part of a feed-back loop, maintaining it at given Flux -> allows to measure important fields R Kleiner

17 Transformer gradiometer 17 J. Clarke Input coil

18 18 Noise thermometry Sφ(f,T) = 4k b TM 2 R(1+ (f / f c ) 2 ) J. Engert et al. PTB f c =R/2πL Pd resistor in the circuit of 1 mohm (green)

19 19 Measure the fluctuating currents in a piece of copper (thermal magnetic flux noise) -> better thermalization of electrons. Sφ(f,T) = 4k b TM 2 R(1+ (f / f c ) 2a ) b

20 20 Noise thermometry

21 21 Nano-SQUID Mailly PRL 1993 Wernsdorfer PRL 1996

22 SQUID Microscopy 22 Sensitivity and spatial resolution depend on size of pickup area and spacing to sample JR Kirtley

23 23 Stanford IBM J.R. Kirtley

24 24 Weizmann SQUID on Tip

25 25 Grenoble NanoSQUID microscope Former PhD students: C. Veauvy, V.O. Dolocan, D. Hykel Present PhD Z.S. Wang PostDoc D. Hazra Technical support: T. Crozes, T. Fournier, G. Garde, J. Minet, P. Carecchio, O. Exshaw, M. Grollier Collaborations: K. Schuster IRAM, J.R. Kirtley,..

26 26 SQUID Flux penetrating the SQUID loop modulates the critical current

27 27 Hysteretic Nano-SQUID K. Hasselbach et al. Physica C 332(2000)

28 28 Critical current measurement Field range ± 100 G Magnetic sensitivity

29 29 SQUID tip first generation Aluminium 0.6 µm / 1.1 µm diameter To minimize the distance between SQUID and sample we cut the wafer T. Crozes (IN) D. Mailly (LPN) Y. Gamberini

30 30 SQUID tip second generation 50 µm Deep Si etching allows for better edge SQUID alignment T. Crozes (Neel), A. Barbier, K. Schuster (IRAM)

31 31 Squid Force Microscopy 2 mm Quartz tuning fork-> Distance Control Review of Scientific Instruments C. Veauvy et al

32 32 AFM Principle TF is a force sensor Its resonance frequency increases with decreasing tip sample distance

33 33 AFM Electronic DSP contains Two PI in series 10 khz bandwidth

34 Microscope 34

35 35 Vortices in a perforated Al film Flux quantization at each hole Vortex transitions: Fusion and localized SC T=0.4K C. Veauvy, et al. Phys. Rev. B 70, , (2004) T=1.18 K T=1.2 K

36 36 Magnetic anisotropy in the superconducting state of Sr 2 RuO 4 state Crossing vortex lattices: in-plane vortex pin crossing vortices In Sr 2 RuO 4 Dolocan, V.O. et al. Phys. Rev. B 74, , 2006 Dolocan, V.O. et al. Phys. Rev. Lett 95, 97004, 2005

37 37 Penetration depth and vortex stray field E. H. Brandt et al. PRB, 61,6370 A. M. Chang et al. Appl.Phys.Lett. 61(16),1974

38 38 Absolute value of penetration depth z=0.45µm λ=101nm ZS Wang 2011 λ=103nm T c =1.6K

39 39 Scientific projects Vortex state and penetration depth (doping) in Pnictide SC Domain structure in superconducting Ferromagnets Imaging noise sources in Qbit circuits.

40 40 Outlook Imaging Smaller SQUID better aligned 500 nm SQUID loop 1 µm alignment of etch 1 µm SQUID loop

41 41 Outlook Imaging At least 10 times higher sensitivity at 4.2 K Flux noise at 1 khz Nb + Fib deposited W L.Hao et al. NPL PTB APL (2008)

42 42 Acknowledgements PhD Danny Hykel feb 2011 D. Aoki SQUIDs D. Mailly, K. Schuster, T. Crozes J. Kirtley, C. Paulsen ESF NES

43 43 references O. V. Lounasma, Experimental Principles and Methods Below 1 K M. Tinkham, Introduction to Superconductivity Superconducting Quantum Interference Devices: State of the Art and Applications R. Kleiner, D. Koelle, F.Ludwig, AND J. Clarke PROCEEDINGS OF THE IEEE, VOL. 92, NO. 10, OCTOBER 2004, p Practical noise thermometers J. Engert, J. Beyer, D. Drung, A. Kirste, D. Heyer, A. Fleischmann, C. Enss and H.-J. Barthelmess, LT25, IOP Publishing Journal of Physics: Conference Series 150 (2009) Experimental Observation of persistent Currents in GaAs-AlGaAs Single Loop D. Mailly, C. Chapelier, A. Benoit PRL Nucleation of Magnetization Reversal in Individual Nanosized Nickel Wires W. Wernsdorfer, A. Doudin, D. Mailly, K. Hasselbach, A. Benoit, J. Meier, J-Ph Ansermet, B. BarbaraPRL Martin Huber, Nick Koshnick, et al., RSI 79, (2008) Koshnick et al., APL 93, (2008)