superconducting quantum interference device is a mechanism used to measure extremely weak signals, such as subtle changes in the human body's electromagnetic energy field. Using a device called a Josephon junction, a SQUID can detect a change of energy as much as 100 billion times weaker than that of the electromagnetic energy which moves a compass needle. SQUIDs have been used for a variety of testing purposes which require extreme sensitivity, including engineering, medical, and geological equipment. Since SQUIDS measure changes in a magnetic field with very high sensitivity so that they do not have to come in contact with a system that they are testing.
A SQUID (superconducting quantum interference device) is a very sensitive magnetometer used to measure extremely weak magnetic fields based on superconducting loops containing Josephson junctions..SQUIDs are sensitive enough to measure fields as low as 5 aT(5Ã-10âˆ’18 T) within a few days of averaged measurements Their noise levels are as low as 3 fTÂ·Hz Â½. For comparison, a typical refrigerator magnet produces 0.01 tesla (10âˆ’2 T), and some processes in animals produce very small magnetic fields between 10âˆ’9 T and 10âˆ’6 T. Recently invented SERF atomic magnetometers are potentially more sensitive and do not require cryogenic refrigeration but are orders of magnitude larger in size (~1Â cm3) and must be operated in a near-zero magnetic field.
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Superconductors are the best known for their ability to conduct current without developing a corresponding voltage. Perhaps a more fundamental property is the pairing of conduction electrons. The property of the microscopic charges carriers leads to the macroscopic phenomenon associated with superconductivity. It also led to the invention of the most sensitive detectors of magnetic flux known, the superconducting quantum interference device, or SQUID. SQUIDs are used to measure extremely tiny magnetic fields; they are currently the most sensitive such devices known, with noise levels as low as 3 fT/sqrt(Hz) . Discovery of High Temperature superconductivity in 1986 by Karl Müller and Johannes Bednorz and YBCO in 1987 by Paul Chu and his students changed the prospective of SQUID usages and superconductors . The SQUID is, to the engineer, a magnetic flux to voltage transducer of unparalleled sensitivity. It arguably the most sensitive detector of any kind, with an equivalent energy sensitivity that approaches the quantum limit .The principle of operation, the methods of manufacture, and the application of SQUIDs are each discussed in turn.
A superconducting quantum interference device (SQUID) is a mechanism used to measure extremely weak signals, such as subtle changes in the human body's electromagnetic energy field. Using a device called a Josephson junction, a SQUID can detect a change of energy as much as 100 billion times weaker than the electromagnetic energy that moves a compass needle. A Josephson junction is made up of two superconductors, separated by an insulating layer so thin that electrons can pass through. A SQUID consists of tiny loops of superconductors employing Josephson junctions to achieve superposition each electron moves simultaneously in both directions. Because the current is moving in two opposite directions, the electrons have the ability to perform as qubits(that theoretically could be used to enable quantum computing). SQUIDs have been used for a variety of testing purposes that demand extreme sensitivity, including engineering, medical, and geological equipment. Because they measure changes in a magnetic field with such sensitivity, they do not have to come in contact with a system that they are testing. SQUIDs are usually made of either a lead alloy (with 10% gold or indium) and/or niobium, often consisting of the tunnel barrier sandwiched between a base electrode of niobium and the top electrode of lead alloy. A radio frequency (RF) SQUID is made up of one Josephson junction, which is mounted on a superconducting ring. An oscillating current is applied to an external circuit, whose voltage changes as an effect of the interaction between it and the ring. The magnetic flux is then measured. A direct current (DC) SQUID, which is much more sensitive, consists of two Josephson junctions employed in parallel so that electrons tunneling through the junctions demonstrate quantum interference, dependent upon the strength of the magnetic field within a loop. DC SQUIDs demonstrate resistance in response to even tiny variations in a magnetic field, which is the capacity that enables detection of such minute changes.
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Superconductivity is a phenomenon sometimes occurs in special types of materials, when they are kept below a certain temperature. In superconductors the resistanceless current is carried by pairs of electrons, known as Cooper Pairs which is the pair of electrons. Each electron has a quantized wavelength. With a Cooper pair each electrons wave couples with its opposite number over a large distances. This phenomenon is a result of the very low temperatures at which many materials will superconduct.
(schematic representation of the scattering of electrons as they pass through a vibrating lattice)
The material of a superconductor is designed to have very small vibrations, these vibrations are lessened even more by cooling the material to extremely low temperatures. With no vibrations there is no scattering of the electrons and this allows the material to superconduct. The origin of a Cooper pair is that as the electron passes through a crystal lattice at superconducting temperatures it negative charge pulls on the positive charge of the nuclei in the lattice through coulombic interactions producing a ripple. An electron traveling in the opposite direction is attracted by this ripple. This is the origin of the coupling in a Cooper pair.
(schematic representation of the copper pair coupling model)
This effect has two most important characteristics: absolutely zero electrical resistivity and exclusion of the interior magnetic field, latter one is known as Meissner effect. The electrical resistivity of a metallic conductor has a linear relation with temperature; it decreases gradually while the temperature is lowered. However, in ordinary conductors such as copper and silver, impurities and other defects impose a lower limit. On the contrary, the resistance of a superconductor drops dramatically to zero when the material is cooled below a temperature called "critical temperature" (figure 1). It has been found that an electric current, flowing in a loop of superconducting wire can persist indefinitely with no external power source. Superconductivity is a quantum mechanical phenomenon. It cannot be understood simply as the idealization of "perfect conductivity" in classical physics.
Since the discovery of superconductive materials in 1911 by Heike Kamerlingh Onnes and co-workers, their magnetic properties attracted considerable attention. A breakthrough came in 1933 when Meissner and Ochsenfeld showed that in magnetic fields below a certain threshold value the flux inside the superconductor was expelled. They concluded that this property defined a new thermodynamic state and is not a consequence of infinite conductivity.
Operation - Josephson effect
The Josephson effect occurs when an electric current (Cooper pairs) flows between two superconductors separated by a thin non super conducting layer through quantum tunnelling. Junction is called a Josephson junction.
Josephson junction can only support a certain maximum(critical) current in a superconducting state.
Operation - Superconducting loop
â-A SQUID consists of a loop of superconductor with one or more Josephson junctions, called
â- Inner diameter of loop ~ 100 mm..
â- Generally made from either an alloy of lead and
gold or indium, or pure niobium.
â- Ceramic superconductors such as yttrium-,barium, copper-oxide also possible, but difficult to manufacture.
Types of SQUIDs :
There are two main types of SQUID, DC and RF (or AC). RF SQUIDs have only one Josephson junction whereas DC SQUIDs have two or more junctions .This makes DC SQUIDs more difficult and expensive to produce, but DC SQUIDs are much more sensitive
1. DC SQUIDs
2. RF (or AC) SQUIDs
Operation - DC SQUID
â- Current made to flow around the loop through
both Josephson junctions.
â- Electrons tunnel through
the junctions, interfere.
â- Magnetic field through the loop causes a phase
electrons, affects current
through the loop.
Operation - RF SQUID
â- Also called AC SQUID
â- Only one Josephson junction.
â- Radio frequency oscillating current
â- Measure interactions between the
superconducting ring and an external resonant LC
- External inductor induces current in SQUID ring, and
when the Josephson junction enters the resistive state
it damps the LC circuit
Weak Links and Josephson Effects:
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Two superconductors which are coupled together in such a way that the critical current in the contact region is much lower than that of the individual constituents, set up a 'weak link'. Josephson in 1962 predicted that such a junction should be able to sustain a supercurrent without application of a voltage. He also mentioned that if such a junction was driven by an external current source to exceed its critical current, radiation of high frequency electromagnetic waves would appear.
Several configuration of a weak link are illustrated in figure 3. The method which is showed in (figure 3-c) is a recently developed method, in which the weak link is achieved by depositing a high-Tc film across a naturally occurring grain boundary in a substrate like SrTiO3. Such contacts are commonly referred to as grain boundary junctions or bicrystal junctions. Along the length of the grain boundary, it forces the superconductor to develop a chain of defects. Above the grain boundary, therefore, there is a weak link between the two parts of the superconductor film.
The weak link creates a junction which is called Josephson junction. An interesting fact about this junction relates to its critical current: All superconductors cease to be resistanceless as soon as the current carrying through them exceeds a maximum value called the critical current; but the critical current of a weak link is much smaller than a superconductor itself. Considering Ic as the critical current of a Josephson junction, the current passes through a weak link can be determined by:
Where are the phases of wave function. This equation is called the dc Josephson Effect.
(Typical SQUID, V-I characteristic)
The critical current of the SQUID is shown as a flat spot in the middle of the curve. In this region, there is current flowing with no voltage which is a supercurrent.
Since the critical current is dependent on the flux, the I-V characteristic will change with the flux as well, which means the voltage will also depend on and oscillate with the flux and have a minimum value for a integer numbers of flux quantum and maximum for half an integers .The high sensitivity of SQUID voltage to the applied magnetic field caused this device be extremely used as flux to voltage converter or in better words, as a magnetic flux sensor.
(Typical voltage created by applied flux in a SQUID)
Superconducting quantum interference device (SQUID)
A superconducting quantum interference device (SQUID) uses the properties of electron-pair wave coherence and Josephson Junctions to detect very small magnetic fields. The central element of a SQUID is a ring of superconducting material with one or more weak links called Josephesons Junctions. An example is shown in the below. With weak-links at points Wand X whose critical current, Ic, is much less than the critical current of the main ring. This produces a very low current density making the momentum of the electron-pairs small. The wavelength of the electron-pairs is thus very long leading to little difference in phase between any parts of the ring.
(superconducting quantum interference device (SQUID) as a simple magnetometer)
If a magnetic field, B is applied perpendicular to the plane of the ring (Figure 9), a phase difference is produced in the electron-pair wave along the path XYW and WZX. One of the features of a superconducting loop is that the magnetic flux (ø), passing through it which is the product of the magnetic field and the area of the loop and is quantized in units of , where h is Planck's constant, 2e is the charge of the Cooper pair of electrons, and has a value of 2 Ã- tesla . If there are no obstacles in the loop, then the superconducting current will compensate for the presence of an arbitrary magnetic field so that the total flux through the loop (due to the external field plus the field generated by the current) is a multiple of .
(schematic representation of a SQUID placed in a magnetic field)
Josephson predicted that a superconducting current can be sustained in the loop, even if its path is interrupted by an insulating barrier or a normal metal. The SQUID has two such barriers or 'Josephson junctions'. Both junctions introduce the same phase difference when the magnetic flux through the loop is 0, and so on, which results in constructive interference, and they introduce opposite phase difference when the flux is and so on, which leads to destructive interference. This interference causes the critical current density, which is the maximum current that the device can carry without dissipation, to vary. The critical current is so sensitive to the magnetic flux through the superconducting loop that even tiny magnetic moments can be measured. The critical current is usually obtained by measuring the voltage drop across the junction as a function of the total current through the device. Commercial SQUIDs transform the modulation in the critical current to a voltage modulation, which is much easier to measure. An applied magnetic field produces a phase change around a ring, which in this case is equal
where øa is the flux produced in the ring by the applied magnetic field. The magnitude of the critical measuring current is dependent upon the critical current of the weak-links and the limit of the phase change around the ring being an integral multiple of 2. For the whole ring to be superconducting the following condition must be met
where Î± and Î² are the phase changes produced by currents across the weak-links and 2 øa/øo is the phase change due to the applied magnetic field. When the measuring current is applied Î± and Î² are no longer equal, although their sum must remain constant. The phase changes can be written as
whereis related to the measuring current I. Using the relation between current and phase from the above Eqn. and rearranging to eliminate i we obtain an expression for I,
As sincannot be greater than unity we can obtain the critical measuring current, Ic from the above
which gives a periodic dependence on the magnitude of the magnetic field, with a maximum when this field is an integer number of fluxons and a minimum at half integer values as shown in the below figure.
(critical measuring current Ic as a function of applied magnetic field)
The Korean Superconductivity Group within KRISS has carried biomagnetic technology a step further with the development of a double-relaxation oscillation SQUID (Superconducting QUantum Interference Device) for use in Magnetoencephalography. SQUID's are capable of sensing a change in a magnetic field over a billion times weaker than the force that moves the needle on a compass (compass: 5e-5T, SQUID: e-14T.). With this technology, the body can be probed to certain depths without the need for the strong magnetic fields associated with MRI's.
Magnetic-levitation is an application where superconductors perform extremely well. Transport vehicles such as trains can be made to "float" on strong superconducting magnets, virtually eliminating friction between the train and its tracks. Not only would conventional electromagnets waste much of the electrical energy as heat, they would have to be physically much larger than superconducting magnets. A landmark for the commercial use of MAGLEV technology occurred in 1990 when it gained the status of a nationally-funded project in Japan. The Minister of Transport authorized construction of the Yamanashi Maglev Test Line which opened on April 3, 1997. In December 2003, the MLX01 test vehicle (shown above) attained an incredible speed of 361 mph (581 kph).
APPLICATIONS OF SQUID'S BY Ya.S.GREENBERG