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SQUIDs

Compiled by

Akshay.Mukund

VII sem E&C

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Superconducting QUantum Interference Devices

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Abrief history about SQUIDs

The first biomagnetic signal, themagnetocardiogram (MCG), was detected in

1963 with an induction coil magnetometer(Baule and McFee, 1963).

A remarkable increase in the sensitivity ofbiomagnetic measurements was obtainedwith the introduction of SQUID, working atthe temperature of liquid helium (-269 C)(Cohen, 1972)

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Superconductivity

Phenomenon occurring in certain materials at verylow temperatures, characterized by exactly zeroelectrical resistance and the exclusion of the interior

magnetic field

Type I

Type II

Figure 1

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Cooper Pair

Electrons that are bound togetherat lowtemperatures in a certain manner

An electron is repelled from other electrons due to

their similar charge, but it also attracts the positiveions that make up the rigid lattice of the metal. This attraction can distort the positively charged

ion lattice in such a way as to attract otherelectrons (the electron-phonon interaction)

At long distances this attraction between electronsdue to the displaced ions can overcome the electrons'repulsion due to their negative charge, and causethem to pair-up.

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The energy of the pairing interaction is quite weak, and thermal energy

can easily break the pairs up.

So only at low temperatures are a significant number of the electrons

in a metal in Cooper pairs.Figure 2

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Schematic representation of sxattering of electrons as they pass through vibrating

lattice

Figure 3

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Figure 4

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Meissner effect

Activeexclusion of magnetic fieldfrom amaterial in its superconducting state

Figure 5

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If a small magnet is brought near asuperconductor, it will be repelled because inducedsupercurrent will produce mirror images of eachpole.

If a small permanent magnet is placed above asuperconductor, it can be levitated by this repulsiveforce.

Figure 6

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Josephson effect

Two superconductors separated by a thininsulating layer can experience tunneling of Cooperpairs of electrons through the junction. Under these

conditions, a current will flow through the junctionin the absence of an applied voltage

Magnetic flux quantum

Inverse of this , is the Josephson Constant

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SQUID

The superconducting quantum interferencedevice (SQUID) consists of twosuperconductors separated by thin

insulating layers to form two parallelJosephson junctions. If a constant biasing current is maintained

in the SQUID device, the measured voltageoscillates with the changes in phase at the twojunctions, which depends upon the change inthe magnetic flux.

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Figure 7

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Types of SQUIDS

dc SQUID

rf ( radio frequency ) SQUID

Figure 8b rf SQUIDFigure 8a dc SQUID

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Figure 9

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dc SQUID

Operates with a dc bias current, consists of twoJosephson junctions incorporated into a

superconducting loop. The maximum dcsupercurrent, known as the

critical current, and the current-voltage (I-V)characteristic of the SQUID oscillate when themagnetic field applied to the device is changed.

The oscillations are periodic in the magnetic flux

threading the loop with a period of one fluxquantum, 0 = h/2e 2.07 1015 weber Thus, when the SQUID is biased with a constant

current, the voltage is periodic in the flux.

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Working of DCsquid An external magnetic field H is applied to the loop.

A Josephson junction is incorporated into each of the twoarms of the dc SQUID. The Josephson junctions limit themaximum supercurrent Ic that can flow across the ring to amaximum value given by the sum of the critical currents ofthe two junctions.

The magnetic flux enclosed inside the SQUID ringmodulates Ic periodically,

This modulation, caused by an interference of thesuperconducting wave functions in the two SQUID arms,forms the basis of the working principle of the dc SQUID.

The SQUID is biased with a current slightly above themaximum value of Ic and the dc voltage V across thejunctions is read out directly as a function of externalmagnetic field. When Ic is maximum, V is minimum andvice versa. The dc SQUID thus directly acts as a flux-to-

voltage transducer.

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rf SQUID

Consists of a single Josephson junction incorporatedinto a superconducting loop and operates with an rfbias.

T

he SQUID is coupled to the inductor of an LC-resonant circuit excited at its resonant frequency,typically 30 MHz.

The characteristics of rf voltage across the tank

circuit versus the rf current depends on appliedflux.

With proper adjustment of the rf current, theamplitude of the rf voltage across the tank circuit

oscillates as a function of applied flux.

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Figure 10

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BiomedicalApplications of SQUID

Magnetocardiography

Magnetoencephalography

MagnetogastrographyLiver Susceptometry

Magnetic Relaxation Immunoassay

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MagnetoCardioGraphy

Magnetocardiography is the measurement andanalysis of the magnetic component of theelectro-magnetic field of the human heart.

In comparison to the electrical signals measuredby ECG, the magnetic signal is not disturbed onboundaries of tissues with different electricalproperties.

The MCG measurement is done inside amagnetically shielded room (MSR).

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The system has a 64-channel magnetic sensorsinside the dewar, and each magnetic sensor probeincorporates a first-order gradiometer.

T

his area covers the entire heart of an adult. The dewar containing SQUID sensors is placed asclose as possible to the anterior chest withouttouching it.

Each sensor is set to its exact position by using alaser pointer.

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Detection of myocardial ischemia by magneto cardiogram using

64-channel SQUID system

M. Sato, Department of Thoracic Surgery, Fujita HealthUniversity School of Medicine, Japan

Aim -- Visualize MI by measuring MCG using a 64channel

SQUID system, and compare the effectiveness of this

approach to the resting standard 12-lead ECG

Figure 11

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Mean of QRS and STsegment done in both IHD

and normal subjects

Figure 12

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Figure 13

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Figure 14

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Liver Susceptometry

The use of SQUIDs for the assessment of ironoverload was first suggested by Wikswo

The content of paramagnetic iron in the liver is

determined by measuring the response of Liver toan applied magnetic field.

Second-order gradiometers coupled to a helium-cooled SQUID for measuring the variation of themagnetic flux

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Figure15

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Magnetogastrography/Magnetoenterography

Measuring spontaneous activity of smooth muscleof stomach by measuring the biomagnetic field

Non invasive and no radiation unlike barium mealstudy

Magnetic Marker MonitoringMagnetic capsule swallowed by patient and motion studied

by magnetic tracing.

Used in studying the erosion or dissolution oforal drugsstudy of the function and efficacy of the peristaltic

activity of the gastrointestinal organs

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MagnetoEncephaloGraphy

Localizing and characterizing the electricalactivity of the central nervous system bymeasuring the associated magnetic field from the

Brain The neuronal current flow generates an associated

magnetic field of Magnitude only a few hundredfemtoTesla

Present-day MEG dewars are helmet-shaped andcontain as many as 300 sensors, covering most ofthe head

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Figure 16

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In contrast to electric fields, magnetic fields are lessdistorted by the resistive properties of the skull and

scalp, which result in a better spatial resolution ofthe MEG.MEG selectively measures the activity in the sulci,

whereas scalp EEG measures activity both in the

sulci and at the top of the cortical gyriMEG is therefore more sensitive to superficialcortical activity, which should be useful for thestudy of neocortical epilepsy.

Finally, MEG is reference-free which is in contrastto scalp EEG, where an active reference can lead toserious difficulties in the interpretation of the data

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References

The SQUID Handbook Vol I John Clarke The SQUID Handbook Vol II John Clarke http://hyperphysics.phy-astr.gsu.edu

http://citeseerx.ist.psu.edu

I hereby conclude my session.

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ANY QUERIES???

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