Superconductivity Continued……�

Superconductors……

Superconductors are materials that conduct electricity with no resistance. This means that, unlike the more familiar conductors such as copper or steel, a superconductor can carry a current indefinitely without losing any energy. They also have several other very important properties, such as the fact that no magnetic field can exist within a superconductor.

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A superconductor is an element or metallic alloy which, when cooled to near absolute zero, dramatically lose all electrical resistance. In principle, superconductors can allow electrical current to flow without any energy loss

What is a superconductor?

Types I and II Superconductors

• There are thirty pure metals which exhibit zero resistivity at low temperatures and have the property of excluding magnetic fields from the interior of the superconductor (Meissner effect). They are called Type I superconductors. The superconductivity exists only below their critical temperatures and below a critical magnetic field strength. Type I superconductors are well described by the BCS theory (Bardeen, Cooper and Schrieffer) .

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Starting in 1930 with lead-bismuth alloys, a number of alloys were found which exhibited superconductivity; they are called Type II superconductors. They were found to have much higher critical fields and therefore could carry much higher current densities while remaining in the superconducting state.

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The time dependence of the magnetic field in a perfect conductor is described via the classical equation

is known as the penetration depth. This has the solution:

the magnetic field inside the perfect conductor obeys the condition

The Classification of Superconductors��

Difference between type 1 and type 2 superconductors

Difference between type 1 and type 2 superconductors

• Type 1. This type has a characteristic as shown in figure 1. The dependence of the critical magnetic field on the temperature can be seen clearly.

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• Type 2. Discovered in 1962 this has two critical magnetic fields associated with its behaviour, enabling it to have a composite N/S structure. (N stands for normal state, S for the superconducting state). See figure 1. Above the first critical value B_c1 the flux penetrates in a series of regular positioned filaments. The core of each filament is in the normal state, thus magnetic flux flows through in quanta given by equation 1. When the second critical field B_c2 is reached, the flux penetrates completely and the material reverts to its normal state.

List of Superconductors

The Benchmarks of Superconductivity

The Meissner Effect

Perfect Diamagnetism

• Magnetic Fields and Superconductors are not generally compatible

Super-

conductor

T>T_c

T<T_c

The Meissner Effect

• When describing superconductors, it is the magnetic properties that are important. The superconductor is not a perfect conductor, but a perfect diamagnetic material with zero electrical resistance. This is known as the Meissner effect.

The Meissner effect is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state

Applications of Superconductors

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1. Superconducting electronics applications

There are many thin film applications in ultra fast microelectronics or instrumentation

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2. Magnetically-Levitated Train (Maglev Train)

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3. MRI (Magnetic Resonance Image)

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4. Nuclear Magnetic Resonance (NMR)

The High-T_c Cuprate Superconductors�

• Layered structure – quasi-two-dimensional
• Anisotropic physical properties
• Ceramic materials (brittle, poor ductility, etc.)
• Oxygen content is critical for superconductivity

YBa₂Cu₃O_7-δ

Tl₂Ba₂CaCu₂O₈

Thanks