1 Introduction to SQUIDs and their applications ESAS Summer School 17.6.2011 Jari Penttilä, Espoo, Finland
2 Outline Flux quantization and Josephson junction Theoretical DC SQUID Practical DC SQUID Fabrication of SQUID devices Applications in magnetometry Summary
3 Flux quantization Superconductor characterized by macroscopic wave function Ψ(r) = (n s (r)) 1/2 exp(iθ(r)) Phase difference when moving around a closed path must be multiple of 2π
4 Josephson junction - Weak link between two superconductors - Cooper pairs tunnel across the junction - Physically small area of reduced critical current DC Josephson effect: I = I c sin(φ) AC Josephson effect: dφ/dt = V / Φ 0 Typical Josephson frequency 1 100 GHz
5 Josephson junction Resistively and capacitively shunted junction model (RCSJ)
6 Theoretical DC SQUID Equations:
7 Theoretical DC SQUID 2D washboard: - Current bias tilts the trough along the voltage axis - Flux bias shifts the bumps sideways Ryhänen et al 1989
8 Winter analog: Theoretical DC SQUID + =
9 Flux-to-voltage: Flux noise: Energy resolution: Shunt resistance: Theoretical DC SQUID Maximize critical current density - loop inductance decreases -> coupling - shunt resistance decreases -> matching - compensate with smaller junction -> lithography
SQUID nomograph 10
11 Practical DC SQUID Typically large pickup coil needs matching to SQUID via flux transformer Ls ~ 10 ph Lp ~ 10 nh Typically small gain of SQUID needs local feedback
12 Typical characteristics Courtesy of J. Luomahaara, VTT
d [nm] 13 Fabrication of SQUID devices Josephson junction is formed in Nb/Al/AlOx/Nb trilayer process where Al layer is oxidized and Nb counterelectrode is deposited in vacuo Superconductors: Nb, NbN, Al, Pb Normal metals: Mo (>1K), TiW, Pd, Cu 2000 1500 1000 500 0 Ins3 Ins2 Ins1 Nb4 CF Al/AlO x Nb3 Nb Nb2 Res 2 O 5 Res Nb1 0 50 x [ m] 100 150
14 Fabrication of SQUID devices Typical linewidth 2-3 um (contact lithography) < 1 um with projection lithography Damping of unwanted microstrip resonances
15 Fabrication of SQUID devices Several SQUID foundries available: Hypres, Starcryo, IPHT, PTB, VTT,... Multiproject or contract manufacturing Variations mk or 4.2K operation (normal state metal) layer structure and number of Nb layers critical current densities chip size
16 Readout electronics Due to APF/NC, readout electronics can be placed at room temperature SQUID output is linearized using flux-locked loop technique zeroing flux at SQUID or at flux transformer due to high gain of integrator
17 Applications in magnetometry Magnetoencephalography (MEG) measures ~1 pt fields arising from neuronal activity Elekta TRIUX: 306 independent SQUID channels in helmet shape Immersed into liquid helium in glass fibre dewar Inside magnetically shielded room ms / mm - resolution Presurgical mapping, locating epileptic areas Stroke recovery, autism, Alzheimer
Origin of the Magnetic Field Antti Ahonen Elekta Oy MEG-Fields are small, 100 ft => SQUID-Sensors Magnetic Shielding
MEG EEG MEG and EEG Distributions radial tilted radial tilted Courtesy of Prof. Matti Hämäläinen, Harvard
Silicon Chip Sensor Array Field distribution sampled by pickup loops at 510 distinct locations Pick-up loops configured into 306 channels using triple sensor elements comprising: Two planar gradiometers One magnetometer
21 Applications in magnetometry Ultra low-field MRI Measurement fields in ut regime (instead of T for conventional MRI) + open and silent, T1- contrast, allows metals - poorer SNR Combined MEG and MRI Vesanen et al 2011
22 Challenges in magnetometry MEG: Shielding against external fields -> MSR, gradiometer, software MEG: lower 1/f noise -> materials ULF-MRI: eddy current transients & magnetization when metals present nearby -> compensation coils ULF-MRI: flux trapping in SQUIDs -> narrow lines, local shielding, current limiter (Q-spoiler) Lower total noise when using room temperature amplifiers -> array SQUIDs to increase output impedance for better matching
23 Summary SQUIDs are ultra-sensitive quantum-limited sensors of magnetic field Fabrication in standard cleanroom processes Room temperature readout possible in low frequency applications Applications both existing (MEG, MCG, geophysics) and arising (ULF-MRI)