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State-of-the-art
Superconducting Quantum Interference Devices
-SQUIDs-
for electrical and magnetic measurements
at low and very low temperatures
Thomas Schurig
Seminaire Daniel Dautreppe , Grenoble, Nov 21 -25 201 1
Thomas SchurigDietmar Drung, Jörn Beyer, Alexander Kirste, Rainer Körber, Margret
Peters, Cornelia Aßmann, Sylke Bechstein, Frank Ruede, Marco Schmidt, Jan Storm,
Hans Koch, Lutz Trahms, Martin Burghoff, Allard Schnabel,
Physikalisch-Technische Bundesanstalt Abbestr. 2-12, 10587 Berlin, Germany
Berlin
Braunschweig
The Physikalisch-Technische Bundesanstalt
National Metrology Instituteof Germany
1500 staff members (400 in Berlin)
Braunschweig Berlin-Charlottenburg
BraunschweigBerlin-Charlottenburg
Berlin
Motivation
Introducing Superconducting Quantum Interference Devices (SQUIDs) as a tool for sensitve electrical and
Yesterday: talk by Frank Hekkingabout the physics of Josephson junctions and dc SQUIDs
Devices (SQUIDs) as a tool for sensitve electrical and magnetic measurements.
Description of SQUID devices presently (commerciall y) available. Talk will concentrate on low critical te mperature superconductor (LTS) dc SQUIDs.
3
SQUID basics
SQUID current sensor basics
State-of-the-art SQUID current sensors
Application examples
OutlineOutline
Application examples
Handling of SQUIDs
Auxiliary components
4
Most sensitive detector of magnetic flux (changes)
SQUIDs are very versatile and can be used for sensi tive magnetic and electrical measurements
Operating at cryogenic temperature (77 K down to th e mK range)
OutlineWhat is a SQUID?
SQUID function based onFlux quantization in a superconducting ringJosephson effect
Low-noise read-out electronics usually operating at room temperature is required
5
Magnetic flux Φ
The dc SQUID
Superconducting ring
Bias current Ib
Josephson junction
Physicist: “A dc SQUID is a superconducting ring interrupted by two Josephson junctions ...”
Electronics engineer: “A dc SQUID is a low-noise, nonlinear flux-to-voltage converter”
How does a dc SQUID work?
Ib½Ib ½Ib
Iscr
CJ Rs V
Φ
n Φ0
L
I-V and V-ΦΦΦΦ characteristics
How does a dc SQUID work?
V Ib
(n+½) Φ0
I0 – Critical current of the JJ
∆V – Voltage swing
W – Working point
Transfer function
VΦ = δV/δΦshould be maximum
I0 2I0 I
n Φ0
Small-signal SQUID readout
Main problems:
∆V W
Φ
V Φ0
δV δΦ
Φlin
Small change in applied flux δΦresults in
small change in SQUID voltage δV
How does a dc SQUID work?
Amplifies the weak SQUID voltage without adding noi seLinearizes transfer function to provide sufficient dy namic range
Main tasks of a SQUID electronics:
Very small voltage across the SQUID: ∆∆∆∆V ≈≈≈≈ 10...50 µV Transfer coefficient VΦΦΦΦ = δδδδV/δΦδΦδΦδΦ depends on SQUID working point Very small linear flux range: ΦΦΦΦlin << ΦΦΦΦ0
Main problems:
Example: Magnetometer with 1 nT/Φ0 → Human heart signal ≈ 0.05 Φ0
Power line interference ≈ 300 Φ0
T ≤ 5K
Flux Locked Loop (FLL) with direct SQUID read-out
Rf
dc SQUID
V
Amplifier
VbΦf
Integrator
⌠⌠⌠⌠⌡⌡⌡⌡
How does a dc SQUID work?
FLL linearizes the V-Φ characteristics andenables a high dynamic range (100 Φ0)
V∞ Φ
Low-noise preamplifier required
Noise and energy resolution
Thermally induced white noise
Power spectral density of flux noise : SΦ ≈ SV / VΦ2
with SV(f) power spectral density of voltage noise
SΦ ≈ 16kBTL2/R (keep L low)SΦ ≈ 16kBTL /R (keep L low)
Noise energy : ε = SΦ/2L
1/f noise
sources: moving flux vortices,
critical current fluctuations in the junctions (HTS SQUIDs)
See e.g. Clarke, „SQUID fundamentals“ in SQUID Sensors, Fundamentals, Fabrication andApplications, NATO ASI vol.329 (1996)
10µ
/H
z1/2
C5XL1R-C509-O8, 2007-01-15T = 4.2 K
SQUID
Noise and energy resolution
Flux noise of a PTB SQUID current sensor
0.1 1 10 100 1000 10000 100000100n
1µ
SΦ
1/2 /
µΦ
0/H
z
f / Hz
Noise and energy resolution
Example (John Clarke)
L = 200 pH, R = 6 Ω, T = 4.2 K
Spectral flux noise density: √SΦ ≈ 1.2 µΦ0/√Hz
Noise energy : ε ≈ 1.5 x 10-32 J/Hz ≈ 150 ħ
1 x 10-32 J ≈ 10-13 eV is the energy to raise 1 electron through 1 mm in the earth‘s gravitational field, or 10-14 x the ground state energy of onehydrogen atom (John Clarke)
Outline
How to make a SQUID?
14
First SQUIDs were made of bulk material
Resistive SQUID for noise thermometry (PTB)with Nb point contact
15A. Hoffmann and B. Buchholz, J. Phys. E, 17, 1984, 1035
3” Si wafer with 183 SQUIDs
LTS SQUID Fabrication
Magnetometers fabricated using a Nb/AlOx/Nbthin-film technology
7.2 mm
Si/SiO2-substrate
Nb-base electrode
Nb2O5-insulation
LTS SQUID Fabrication
Layers
Josephson junction
shuntresistor
transformercoils
SiXNY-film
Au/Pd-resistive layer
Al2O3-barrier
Nb-counter electrode
17
Example of a LTS SQUID magnetometer
Multiloop SQUID magnetometer
To measure B, a fairly large sensing area is needed, beca use ΦΦΦΦ = ∫BdA
Chip-size: 7,2 mm x 7,2 mm
√SΦ = 2.6 µΦ0/√Hz @ 1kHz
√SB = 1.2 fT/√Hz @ 1 kHz
Noise @ T = 4.2 K
Drung type design
Dietmar Drung
L = 400 pH
Aktuelle Forschungsvorhaben -Beispiele
Heavily magneticallyshielded rooms
Fetal MCG
Outline
For a large number of experiments
we do not need magnetometers, but
sensitive SQUID current sensors.
20
R
V
Preamp
V
Integrator
⌠⌠⌠⌠⌡⌡⌡⌡
T ≤ 5 K
I
How to make a SQUID current sensors?
RfVbMf
V
Preamp Integrator
⌠⌠⌠⌠⌡⌡⌡⌡
T ≤ 5 K
I
Application example of SQUID current sensor
Magnetometer consisting ofwire wound pickup coiland SQUID current sensor
RfVbMf
I
Magnetometer made of SQUID current sensor
SQUID chip
Nb chip holderNb cylinder
SQUID chip
Nb chip holderNb cylinder
Simple SQUID current sensor
closed capsule
open capsuleopen capsule
1.5 mm
√SI ≤ 1 pA/Hz½,
εC ≤ 400 h
B/Φ ≈ 10 nT/Φ0
TOP: 4.2K
W9SI:
Drawbacks
• extremely sensitive to magnetic interferences
• carefull magnetic shielding required
• extremely sensitive to vibrations
Simple SQUID current sensors
• limited flexibility regarding input inductance
• operational temperature 1 K – 5 K
Not well suited for many applications , in particular for those in
novel cryogen-free cryostats
with mechanical precooling.
What SQUIDs are required?
Extremely sensitive, robust and easy to handle SQUID current sensorsrequired for (quantum) measurements at low and ultra-low temperatures
- low input inductance (nH) sensors: read-out of cryogenic radiation detectors TES, HEB, single-photon detectors
- high input inductance (µH) sensors: electromagnetic measurements electromagnetic measurements
metrology, Cryogenic Current Comperators (CCC)Nondestructive evaluation (NDE)susceptometryNMR, lf-NMR novel biomedical diagnostic toolsmagnetic nanoparticles
Sensitive magnetometers/gradiometers and nano-magnetometers - noise measurements - single magnetic moment detection
large polarizingfields (>>1 mT)are used
State-of-the-art SQUID current sensors
Goal
• robust against magnetic interferences
• operation down to ultra-low temperature (ca. 10 mK)
Solution
→ gradiometric sensor design, small line width
→ appropriate resistance materials and Josephson junctions
Towards robust low-noise SQUID current sensors
• high flexibility
• high bandwidth and high slew rate
• easy to handle
→ appropriate resistance materials and Josephson junctions
→ variety of sensor designs : single- and double stage SQUIDs, arrays
→ appropriate read-out techniques, e.g. OCF (on chip current feedback)
→ auxiliary components, computer controlled, user-friendly software
BI
Gradiometric sensor design
Parallel gradiometer: superconducting loop: →trapped flux
magnetometer parallel gradiometer
serial gradiometer
Robust low-noise SQUID current sensors
Serial gradiometer: →release of trapped flux with bias current
→ use serial gradiometers
magnetic microscopy of Nb stripes cooled down in ambient field
Bcool =85µT Bcool =5,3µTG. Stan, et. al.; Phys. Rev. Lett. 92, 097003 (2004)
Expulsion of vortices in the Nb thin-film patterns
→ Wmax ≤ 5µm
for cooling in Earth field
SQUID array
VOUT∫
RFB
I
LIN,SQA
- low input inductance
- high dynamic range
- not sensitive to magnetic
interference
- single-SQUID-like behavior
Josephson junctions
Cooling fins
SQUID loop
Single-turn input coil
SQUID-to-SQUID connection
50 µm
Gradiometer SQUID
SQUID array
3 mm3
mm
• 2 independent 16 SQUID-arrays per chip
• operable at mK-temperatures
• no shielding required
• integrated resistors for biasing radiation detectors
(e.g. TES) on chip
rf Filters
-F
+F
-V
+V+IN
-INRFeedback Coil
Bias Resistor
Shunted 16-SQUID Array-INR
+R+R
-F
+F
-V
+V+IN
-INRFeedback Coil
Bias Resistor
Shunted 16-SQUID Array-INR
+R+R
1
2
(e.g. TES) on chip
√SI <5 pA/√Hz @ 0.1 KLIN <3nHεC <57h @0,1 KPDiss∼1nW (per channel)
Technological limitation
SQUID current sensors with high input inductance
Problem :for high input inductance 10..100 turn input coil needed I
Currently Nb/AlOx/Nb thin-film technology with 2.5 µm photolithographic patterning
but
with 2.5 µm linewidth only 1 turn possible!
Solution:input flux transformer I
Current sensor with single SQUID
RB
SSASQ1
VOUT
LFB
∫I
LIN,SQ1
SSASQ1LIN,SSA
RFB
- high input inductance
- high sensitivity
- not sensitive to magnetic interference
2-stage SQUID current sensors
RB
SSASQ1
VOUT
LFB
∫I
LIN,SQ1
SSASQ1LIN,SSA
RFB
- minimum coupled energy sensitivity
- integrated 2stage SQUID current sensor
- not sensitive to magnetic interference
- single-SQUID-like behavior
2-stage SQUID current sensors
• front-end: single-SQUID, read out with SQUID array
• operable at mK-temperatures
• no shielding required
• V/Φ- characteristics like single-SQUID
• different sizes with LIN from 25 nH to 1.8 µH
3 mm
3 m
m
√SI < 0,05 pA/√Hz @ 0.1KLIN = 1,05µHεC < 8h @ 0.1KPDiss ∼2 nW
• different sizes with LIN from 25 nH to 1.8 µH
• adjustable input current limiter +FIN
-FIN
Z
-F
+F
+I
-V
+V
-IFX
+FX
-FQ
+FQ
+IN
+Q
-Q
-INShuntedSQUIDwith APF
Intermediate Loop
Feedback Transformer
BiasResistor
rf Filters
Shunted 16-SQUIDArray Amplifier
Unshunted16-SQUID ArrayCurrent Limiter
Example for robust and user-friendly FLL electronics : Directly coupled FLL electronics XXF (developed by PTB and Magnicon)
16 cm
670 surface-mount components on 13 ×××× 4.3 cm2 FLL board
Up to three channels in one FLL unit
Low noise 0.33 nV/√√√√Hz and 2.6 pA/√√√√Hz
High bandwidth 20 MHz (FLL) or 50 MHz (open-loop)
Built-in current sources for TES readout and two-stage SQUIDs
Fast < 1 µs external reset
Flexible and user-friendly → fully computer controlled
Novel SQUID current sensors below 1 K
• 4.2 K → <320 mK: white noise falls , but 1/f noise rises
• For this example: mK noise higher than 4.2 K noise above ≈≈≈≈40 Hz
• 1/f noise can become significant even in the kHz range
• Currently under investigation (other supercond. material)
Bandwidth limitation due to cable delay in the FLL1 m → 10 MHz
Wide bandwidth SQUID current sensors
SQUID+FLL at low temperature (4.2K)
• Very high FLL bandwidth ≈≈≈≈ 350 MHz
• Very high slew rate ≈≈≈≈ 80 ΦΦΦΦ0/µµµµs
• BUT: Power dissipation ≈≈≈≈ 10 mW
How to get faster?
• BUT: Power dissipation ≈≈≈≈ 10 mW3
mm
3 mm
SQUID with on chip current feedback (OCF)
• Semicond amplifier replaced by SQUID array
• very high small signal bandwidth >200 MHz
• large signal bandwidth 16 MHz
• current noise 7.4 pA/√Hz (L in < 5 nH)
• very high slew rate >50Φ0/µs
• Power dissipation ≈≈≈≈ 100 nW
Magnetometers and susceptometers
Integrated miniature multiloop SQUIDs
Sensor noise
< 4 fT/√√√√Hz size M (2.8 mm)
< 10 fT/√√√√Hz size S (1.7 mm)
2.8 mm
1.7 mm
39
Integrated susceptometers with 30 µm and 60 µm pick-up loop size for charcterization of small particles etc.
Large variety of devices available
40
Sources of SQUID devices:IPHT Jena, PTB Berlin, VTT Helsinki, Magnicon Hamburg, Supracon Jena US: Starcryoelectronics, Tristan
Outline
How to operate and handle SQUIDs?
41
Measurements
Characterization of SQUIDs
SQUID FLL-electronics Computer Oscilloscope
Spectrum analyzer
RF shielding
LHe can
ESD chair
42
Measurements
Characterization of SQUIDs
LHe can
Thermal super-
insulation
Take care of rf- and magnetic shielding!
Shielding
Inner vessel
insulation
Probe stick Cryoperm Pb
43
20 mm
Measurements
Characterization of SQUIDs
SQUID chip
PCB chipcarrier
44
20 mm
Bonding wires
MeasurementsHow to handle SQUIDs
Avoid electrostatic discharges (ESD)!(In particular if the SQUID input coil is connected!)
Always use ESD shoes and an ESD chair.
Use ESD bracelets and connect yourself with the mat and the soldering iron.
Connect the ground of your dip stick with the experimental setup 45
Measurements
Damages caused by ESD
PTB SQUID suszeptometer
Anna Repolles, Javier SeseUniversity Zaragoza
Repair with FIB
Application examples of SQUID current sensors
47
• TES Xray microcalorimeters at NASA/GSFC
• SSA chip directly mounted on Cu block close to TESs
• 6keV pulse → ∆ΦSQ = 1.3 Φ0
Read-out of TES X-ray detectors
8x8 Pixel Array
SSA Chips
GSFC TES setup
SSA noise
TES noise (baseline ∆E 1.63+/-0.02 eV )
102
103
104
105
10
100
√SI (pA/√Hz)
f (Hz)
LWL-Eingänge am mK-Kühler
TES detector-F
+F
-V
+V
INR
+R
-F
+F
+I
+FX
-
+V
-F
+F
-V
+V
INR
+R
-F
+F
+I
+FX
-
+V
-INR1
+IN1
-INR
+IN
-INR1
+IN1
-INR
+IN
-INR1
+IN1
-INR
+IN
-INR1
+IN1
-IN
+IN
SQUID sensor
TES detector-F
+F
-V
+V
INR
+R
-F
+F
+I
+FX
-
+V
-F
+F
-V
+V
INR
+R
-F
+F
+I
+FX
-VIFX
+V
-INR1
+IN1
-INR
+IN
-INR1
+IN1
-INR
+IN
-INR1
+IN1
-INR
+IN
-INR1
+IN1
-IN
+IN
SQUID sensor
Read-out of TES IR photon counter
49
LWL-gekoppeltes Detektormodul in DR200
Operation in cryogen free 3He/4He system
TES from NIST
cooperation with German Space Research Organisation and University Karlsruhe
4 µm x 4 µm
Read-out of superconducting single photon detectors
SQUID-SensorDetektor FLL-Elektronik
RBias
RD
+IN
+V
-V
-INI+I
ID
×25 ∫
Rf
Vout
Mf
Mf
+-
IBias
Preamp Integrator
hν
NbN, 88nm linewidth, 5,5 nm thicknessSQUID Chip
Detektor Chip
SQUID NMR and low-field SQUID NMR
• Ultralow field down to 4 Hz (93 nT)
• Small room-temperature sample of volume 0.14 ml
• Broadband input coupling scheme and direct readout
• 2stage SQUID with εεεεc = 50 h →→→→ S/N ~ 5 in single shot
300K300K300K300K
NMR signals (FID) from oil/water mixtures in a 4:1 mass ratio at 300 K (black) and 275 K (grey).
2 samples of distilled water
304 SQUID-Sensor
52
BPolarization 250 µT
3 seconds
BDetection 1.5 µT
Trahms, Burghoff, Hartwig (PTB)
53
He-probe stick for nanoSQUID measurement
Read-out of nanoSQUIDs
Read-out of nanoSQUIDs
IRB
SQUID-Array
Nano
SQUID
IBIAS
Exc..
coilWP
Read-outelectronics
Feedback
ΦSig
Collaboration with L. Hao, J. Gallop, O. Kazakova,
• NanoSQUID read-out with SQUID array current sensor• Working point adjustment with auxiliary coil • Nanoparticle excitation with exitation coil
T = 4.2K
T = 4.2…12 K
Exc..coil
T = 300K
Feedback
Characterization of nanoSQUID
10
T = 6.8 K, I = 60 µAT = 6.8 K, I = 60 µµµµA
7.6 mT
A
Collaboration with Gallop, Kazakova, Hao
SQUID loop ≈ 200 nm
Read-out of nanoSQUIDs
10-1
100
101
102
103
104
105
0.1
1
√SΦ (
µΦ0/√
Hz)
f (Hz)
0.2 0.2 0.2 0.2 µΦµΦµΦµΦ0000////√√√√HzHzHzHz
18.8
µA
NanoSQUIDwith magnetic nanoparticle
Micro- and NanoSQUIDsRead-out of nanoSQUIDs
Bare SQUID With particle
SQUID noise thermometers
MIN
LIn
RN
T
Integrated current sensing noise thermometer
MFFT
<U2> = 4kBTR∆fNyquist Formel
CSNT
Magnetic field fluctuation thermometer
3 mm
2 mm
Chip with SQUID current sensor and noise resistor
Cu, 5N8
3x3 mm2 SQUID gradiometer chip
LIn
SQUID current sensor
SQUID noise thermometers have to be calibrated at one
reference temperature only.
T = TRef.SΦ(fp, TRef)SΦ(fp,TMeas)
with fp plateau frequency
SQUID noise thermometers
101
102
103
10-10
10-9
10-8
SΦ (
Φ0
2/ Hz)
f (Hz)
103
104
105
10-11
10-10
10-9
10-8
SΦ (
Φ02 /H
z)
f (Hz)
T2000 = 698 mK
T2000 = 10 mK
T2000 = 676 mK
T2000 = 10 mK
CSNT MFFT
MFFT mounted in cryogen-free 3He/4He- dilution refrigerator
SQUID noise thermometers
Geophysical sounding
61
Air borne SQUID system with highly balanced SQUID gradiometersR. Stolz, et al., “Magnetic full-tensor SQUID gradiometer system for geophysical applications, The Leading Edge 25;.178-180 (2006)
Supracon AG, Wildenbruchstr. 15, 07745 Jena Germany, www.supracon.com
IPHT Jena
Nondestructive evaluation of materials
62
Michael Mück, ez SQUID Mess- und Analysesysteme, Herborner Str. 9,
Commercial LT SQUID NDE system (eddy current testing) for defect inspection of materials
Fetal magnetocardiography
Biomagnetism
Mother: heart breathing304 channel system child
B/fT
Brain response
SQUID Magnetoencephalography
Biomagnetism
Loudspeaker for acousticstimulation
Stimulus
Goal: Combine MEG with lf-MRI
SQUID MRI system
1. step: lf MRI imaging of the brain
Goal: Combine MEG with lf-MRI
Modified Modified NeuroMagNeuroMag 122 for MEG and ULF122 for MEG and ULF--MRIMRI•• 11stst step: 16 clusters (MRI channels), 64 magnetometersstep: 16 clusters (MRI channels), 64 magnetometers•• 22ndnd step: step: 61 clusters (61 clusters (MRI channels)MRI channels), 244 magnetometers, 244 magnetometers•• 1515--24 reference channels24 reference channels
LNLN22 cooled Bcooled Bpp coils designed for up to 0.2 Tcoils designed for up to 0.2 T
Whole-head system at LANL for MEG & ULF-MRI
67Slide 67
P. Magnelind, EUCAS 2011,
Idea:
Monitor brain activity of a personwith room temperature SQUIDs
while he is doing strange things…
Than transmit the recorded data
VisionVision
Than transmit the recorded datato the brain of another person…
SummaryVariety of SQUIDs are available for practical use a t very low temperatures→→→→ low noise, high dynamic performance→→→→ easy to use and robust →→→→ input inductance range 1 nH ... 2 µµµµH→→→→ noise thermometers available for temperature range 10 mK … 6 K→→→→ operation with commercialised electronics (see www.m agnicon.com)
If you are interested in using PTB SQUIDsyou are welcome to discuss your application with us
