Superconductivity By Shruti Sheladia Garrett M Leavitt Stephanie

Superconductivity By: Shruti Sheladia, Garrett M Leavitt, Stephanie Schroeder, Christopher Dunn, Kathleen Brackney Levitation of a magnet above a high temperature superconductor illustrating the Meissner Effect.

What is Superconductivity? �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)

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

Discovery & Theory of Superconductivity �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.

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 Temperature �At 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 Effect �When 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 Field �The 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 / Current �The 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 Effect �Lead 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.

Current Contributions � 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)

Future Contributions � Integrated circuits in computers � Would not lose power like semiconductor based circuits � Currently the cost of superconducting computers is too high. � Higher temperature superconductors may reduce this cost � Magnetic Levitation of Trains (Maglev) � Electromagnetic system; EMS (attractive maglev) � Unstable equilibrium � Electrodynamics system; EDS (repulsive maglev) � More stable � Requires expensive superconducting magnets

Future Contributions cont. � Electrical generators and motors � Decrease power losses � Lighter superconducting magnets could replace heavy iron cores to create larger generators • Superconducting transmission lines � Energy saved due to no resistive loss � Scientific research � Particle accelerators � Higher temperature magnets lower costs since liquid nitrogen costs less than liquid helium � Ceramic superconductors produce larger magnetic fields