SUPERCONDUCTORS  |   TOPICS


INTRODUCTION

Superconductors are materials which below a critical temperature have a resistance of zero.  They can carry currents that will not decay and are very useful in the development of super computers.  They fall into two categories, type 1 and type 2.  Which one they belong to is dependent upon the effect that a magnetic field has upon them.
Type 1 SUPERCONDUCTORS 
Are perfect diamagnets below their critical temperature, that is they repel any magnetic flux that attempts to enter.  This means that a strong enough magnet will induce a current in the conductor. This will produce an  opposing  magnetic field and the    magnet will float above the superconductor.  This is called the Meissner- Ochsenfeld effect.  Above the critical temperature, or the critical field strength,then superconductivity fails, and so the conductor's opposing field collapses and the magnet drops.
The Meissner-Ochenfeld Effect
Type 2 SUPERCONDUCTORS

The original superconductors like mercury all fall into the category of type 1 conductors.  As research continued a new group emerged.  These had two critical field strengths for a given temperature.  Below the lower one, these materials, (mostly alloys, with the exception of niobium and vanadium) act exactly like type 1 superconductors, and above the higher field strength act like normal materials at low temperatures.

But between these field strengths the superconductors have a unique properties that type 1 conductors don't.  They have a resistance of zero (the definition of a superconductor), but do allow a certain amount of flux penetration (a magnetic field exists inside the conductor).  This means that they are no longer a perfect diamagnet. 

What is believed to happen (as no complete theory of superconductors exist) is the superconductor is split into many layers, some superconducting, some not.  The normal conductors each produce 1 quantum of flux (magnetic flux is also quantized on these scales, like energy).  As the field strength increases, the number of cores of normal material increases.  Eventually the material become completely dominated by the cores of normal material and the superconductivity is eliminated.  This causes superconductivity to stop.
BCS THEORY

Where as no complete theory on superconductors as yet to be put forward, BCS theory does help establish a mechanism for superconuctors.  It was first proposed in the fifties, but took seven years to complete all the needed experiments.  It was known that the quantum of magnetic flux was inversely proportional to twice the charge on an electron.  It was also known that heavier isotopes of elements became superconducting at lower temperatures than their lighter version.  The BCS theory explained these observations by stated the the current in a superconductor is made up of electron pairs.  These interact by one pull on the nucleus, and hence pulling on the other electron.  Since electron nucleus attractions greater the electron electron repulsion, the pair of electrons would move though a lattice of atoms.  Only electrons of equal and opposite momentum are allowed to pair like this.  Therefore there total momentum of any pair is zero.  Using the equation lp = h.  If the momentum of the electrons is zero, then the wavelength they have is infinite.  It is impossible to interact with an infinitely long wave, and therefore there is no resistance.
This theory also showed that a metal which vibrates easily will have a high critical temperature.  This is because the electron pair can move more freely though the lattice.  This explained why the rigid structures of gold and silver, which makes them good conductors at room temperature, aren't very good superconductors, and why the higher resistance metals like tin and lead were very good superconductors.
HIGH TEMPERATURE SUPERCONDUCTORS
Up till around 1986, all known superconductors had critical temperatures of below 23 Kelvin.  These could only be cooled by helium, which in a liquid form is expensive and dangerous.  But in 1987, a research laboratory in Zurich started experiments into using metal oxides as superconductors.  The oxide used would be copper, to act as the in-between for the electron pairs (see above).  It was found that this had a critical temperature of 30 Kelvin. 
The research focused on trying to find a room temperature superconductor.  As time went on the critical temperature rose.  In a year and a half the critical    temperature had now reached over 77 Kelvin.  This  was a major breakthrough.  Superconductivity could now be achieved by the use of liquid nitrogen.  This is cheaper and much safer then liquid helium.  Today, the highest critical temperatures are approaching 200 Kelvin, over two thirds of the way to achieving a superconducting material with a critical temperature that can be achieved without cooling.
Page contributed by Robert Lambert
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