Superconductivity Eton College Physics WJEC AS Level Resistivity

Superconductivity Eton College Physics WJEC AS Level

Resistivity ρ characterises the resistive behaviour of a material. l ρ is an intrinsic quantity but is not constant. l In most metals, ρ increases linearly with temperature: l

Resistivity v Temperature resistivity / Ω m temperature / ºC

Onnes - 1911 Heike Kamerlingh Onnes was studying the resistivity of solid mercury at very low temperatures. l Mercury behaved as expected down to below 5 K. l

Resistivity of Mercury resistivity / Ω m 4. 2 temperature / K

Resistivity of Mercury l The resistivity of mercury abruptly vanishes once its temperature falls below 4. 2 K. l Below 4. 2 K, mercury is said to be in a superconducting state; l It offers zero electrical resistance to any current flowing through it.

Implications of Superconductivity l In the superconducting state, electrons pair up. l These ‘Cooper pairs’ of electrons no longer interact with the lattice ions. l Since there are no collisions between electrons and lattice ions, there is no energy transfer from the electrons to the ions. l The lattice therefore offers no resistance to current flow.

Critical Temperature l 4. 2 K is known as the superconducting transition temperature or critical temperature, TC, of mercury. l At this temperature, mercury undergoes a phase-transition from a normal conducting state to a superconducting state.

Subsequent Discoveries 1913 – lead observed in superconducting state with TC = 7 K. l 1941 – niobium nitride (Nb. N) observed in superconducting state with TC = 16 K. l Most superconductors have critical temperatures of a few kelvin. l Until the 1980 s it was thought that superconductivity was not possible above 30 K. l

‘High’ Temperature Superconductors (HTSCs) l 1986 – TC = 35 K observed (Nobel Prize for Bednorz and Müller). l Shortly afterwards ‘YBCO’ was observed with TC = 92 K. l This is significant as nitrogen condenses at 77 K, enabling liquid-nitrogen cooling. l TC > 77 K is known as high temperature superconductivity.

Modern HTSCs l 2006 - Hg 12 Tl 3 Ba 30 Cu 45 O 125 has TC = 138 K; possibly > 160 K under pressure. l In recent months, claims of TC ≈ 200 K for some crystals have been made.

Uses of Superconductors l Superconductors enable very high currents to be sustained in circuits. l They are therefore very useful in the construction of extremely powerful electromagnets. l Superconducting electromagnets have many applications.

Particle Accelerators Superconducting steering magnets are used to deflect charged particles. l For example: the new LHC at CERN. l

MRI Scanners Intense magnetic fields are required to produce images in MRI scanners. l This is one of the most sophisticated diagnostic tools in modern medicine. l

Tokamaks A tokamak is a containment device for the storage of high-temperature plasma. l Such devices may ultimately make sustained, controlled nuclear fusion possible. l

The Future… It is hoped that future applications of superconducting materials will include: l Motors, l Generators, l Transformers, l Power storage devices, l Power transmission devices, l Magnetic levitation devices.

Related Physics… l Superconductivity is an example of lowtemperature bosonic behaviour. l Paired electrons are classified as bosons; alone they belong to a class known as fermions. l Other examples include superfluidity of helium-4 (henlium-3 atoms are fermions) and Bose-Einstein condensation.