Meissner effect |
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| Exhibiting diamagnetic properties to the total exclusion of all magnetic fields. (Named for Walter Meissner.) This is a classic hallmark of superconductivity and can actually be used to levitate a strong rare-earth magnet. Click here to see a movie of a magnet being levitated or here to see a graphic of a magnet's flux lines folding around a superconductor. | ||||
| When a material makes the
transition from the normal to superconducting state, it
actively excludes magnetic fields from its interior; this
is called the Meissner effect. This constraint to zero magnetic field inside a superconductor is distinct from the perfect diamagnetism which would arise from its zero electrical resistance. Zero resistance would imply that if you tried to magnetize a superconductor, current loops would be generated to exactly cancel the imposed field (Lenz's law). But if the material already had a steady magnetic field through it when it was cooled trough the superconducting transition, the magnetic field would be expected to remain. If there were no change in the applied magnetic field, there would be no generated voltage (Faraday's law) to drive currents, even in a perfect conductor. Hence the active exclusion of magnetic field must be considered to be an effect distinct from just zero resistance. A mixed state Meissner effect occurs with Type II materials. One of the theoretical explanations of the Meissner effect comes from the London equation. It shows that the magnetic field decays exponentially inside the superconductor over a distance of 20-40 nm. It is described in terms of a parameter called the London penetration depth. |
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| In Type II superconductors the magnetic field is not excluded completely, but is constrained in filaments within the material. These filaments are in the normal state, surrounded by supercurrents in what is called a vortex state. Such materials can be subjected to much higher external magnetic fields and remain superconducting. | ||||
| Perfect Diamagnet If a conductor already had a steady magnetic field through it and was then cooled through the transition to a zero resistance state, becoming a perfect diamagnet, the magnetic field would be expected to stay the same. |
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| A conductor will oppose any change in externally applied magnetic field. Circulating currents will be induced to oppose the buildup of magnetic field in the conductor (Lenz's law). In a solid material, this is called diamagnetism, and a perfect conductor would be a perfect diamagnet. That is, induced currents in it would meet no resistance, so they would persist in whatever magnitude necessary to perfectly cancel the external field change. A superconductor is a perfect diamagnet, but there is more than this involved in the Meissner effect. | ||||
| Superconductor Remarkably, the magnetic behavior of a superconductor is distinct from perfect diamagnetism. It will actively exclude any magnetic field present when it makes the phase change to the superconducting state. |
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| With a strong enough magnet, the repulsion due to the Meissner effect can be sufficient to support the weight of the magnet. Such a magnet will levitate above a superconductor. | ||||
| You can determine T0
by observing the Meissner effect. Above the critical
temperature a material will be in a normal state and will
no longer expel magnetic field. By measuring the
temperature at which the Meissner effect disappears as
the superconductor is warming up you can determine the T0 . The temperature of the superconductor will be determined with a thermocouple attached to the superconductor. At this stage only the thermocouple leads (blue and red) will be used. The thermocouple junction thermometer consists of two different metals connected at a point to form a junction. This kind of bi-metallic junction produces a small electrical voltage which varies with the temperature of the thermocouple junction. |
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| You will measure the resistance of the superconductor device as a function of temperature to determine T0. You will use the thermocouple to measure the temperature, as above, and use a four point probe to measure the resistance. | ||||
http://teacher.nsrl.rochester.edu/phy_labs/Superconductivity/Superconductivity.html |
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