SuperconductivityBy: Shruti Sheladia, Garrett M Leavitt, Stephanie Schroeder, Christopher Dunn, Kathleen Brackney

Levitation of a magnet above a high temperature superconductor illustrating the

Meissner Effect.

A physical state of matter that occurs at low temperatures with two key characteristics:

1. Zero Resistivity.

2. Expulsion of Magnetic Flux from within the material. (Meissner Effect)

What is Superconductivity?

Zero Resistivity in mercury at ~1 K first observed by Dutch physicist Heike Kamerlingh Onnes in Leiden in 1911 while he was studying the properties of matter at very low temperatures.

Discovery of Superconductivity

• Onnes was awarded the Nobel Prize in Physics in 1913 for his research

In Onnes’s original experiment, the resistivity of mercury abruptly disappeared at ~ 4.2 K

The Meissner Effect was discovered by W. Meissner and R. Ochsenfeld in 1933.

First successful theory proposed by Bardeen, Cooper, and Schrieffer in 1957.

Discovery & Theory of Superconductivity

BCS Theory

1. Electrons form pairs called Cooper Pairs

2. Electrons move in resonance with lattice vibrations.

Left to right: John Bardeen, Leon Cooper, J. Robert Schrieffer

Cooper Pairs

Critical TemperatureAt some critical temperature Tc, resistivity

abruptly drops to zero. Electrons flow freely through the lattice structure of the material.

• Resistivity of superconducting tin drops to zero at a critical temperature, whereas the normal conductor platinum does not.

Meissner EffectWhen a superconductor is placed within an exterior

applied magnetic field, it expels all magnetic flux from within the superconductor.

• Screening currents along the surface of the superconducting material cancel the magnetic field within the material.

• Magnetic flux is conserved; a larger applied field results in larger screening currents.

Critical FieldThe Meissner effect only works within a range. If the

applied field is too large, magnetic flux does penetrate the superconductor, and superconductivity is lost.

• The critical field, Bc, is temperature dependent and varies from material to material.

Bc → 0 as T → Tc

Critical Field / CurrentThe critical field varies with temperature by:

Similarly, there is a critical current above which zero resistivity is lost, which limits the environment in which certain superconductors can be used.

Isotope EffectLead to the successful BCS theory.

Critical temperature is dependent upon the mass of the atoms in the lattice:

Critical temperature is slightly higher for lighter isotopes.

The first practical application of superconductivity was developed in 1954 by Dudley Allen Buck. It was the invention of

cryotron switch- 2 superconductors with different values of magnetic

fields are combined to produce a fast, simple, switch for computer elements.

Scientific Research Superconducting magnets used to confine plasma Large particle accelerators

Brittle ceramic magnetic coils

Medical Application Magnetic Resonance Imaging (MRI)

Current Contributions

Integrated circuits in computersWould not lose power like semiconductor based

circuits Currently the cost of superconducting computers is

too high.Higher temperature superconductors may reduce this

costMagnetic Levitation of Trains (Maglev)

Electromagnetic system; EMS (attractive maglev)Unstable equilibrium

Electrodynamics system; EDS (repulsive maglev)More stableRequires expensive superconducting magnets

Future Contributions

Electrical generators and motorsDecrease power lossesLighter superconducting magnets could replace heavy

iron cores to create larger generators

• Superconducting transmission linesEnergy saved due to no resistive loss

Scientific researchParticle accelerators

Higher temperature magnets lower costs since liquid nitrogen costs less than liquid helium

Ceramic superconductors produce larger magnetic fields

Future Contributions cont.