Superconductivity “Superconductivity is perhaps the most remarkable physical property in the Universe” David Pines
Discovered by Kamerlingh Onnes in 1911 during first low temperature measurements to liquefy helium Whilst measuring the resistivity of “pure” Hg he noticed that the electrical resistance dropped to zero at 4.2K In 1912 he found that the resistive state is restored in a magnetic field or at high transport currents 1913 Discovery of Superconductivity
George Bednorz Alex Muller The breakthrough -1986
Superconductor Types The Type 1 category of superconductors is mainly comprised of metals and metalloids that show some conductivity at room temperature. They require incredible cold to slow down molecular vibrations sufficiently to facilitate unimpeded electron flow in accordance with what is known as BCS theory. The Type 2 category of superconductors is comprised of metallic compounds and alloys. The recently-discovered superconducting "perovskites" (metal-oxide ceramics that normally have a ratio of 2 metal atoms to every 3 oxygen atoms) belong to this Type 2 group. They achieve higher Tc's than Type 1 superconductors by a mechanism that is still not completely understood. Conventional wisdom holds that it relates to the planar layering within the crystalline structure
Bardeen Cooper Schreiffer Theory BCS theory requires: (a) low temperatures - to minimise the number of random (thermal) phonons (ie those associated with electron-ion interactions must dominate) (b) a large density of electron states just below EF (the electrons associated with these states are those that are energetically suited to form pairs) (c) strong electron phonon coupling BCS theory is an effective, all encompassing microscopic theory of superconductivity from which all of the experimentally observed results emerge naturally
Electron-Lattice Interactions • Electron moving throughlattice exerts an attractiveforce on nearby + ions. • Causes a lattice deformation& local concentration of + charge.
THE BCS THEORY OF SUPERCONDUCTIVITY BCS Theory suggests that superconductors have zero electrical resistance below their critical temperatures because at such temperatures the electrons pass unimpeded through the crystal lattice and therefore lose no energy. The theory states that the supercurrent in a superconductor is carried by many millions of bound electron pairs, called Cooper pairs.
THE BCS THEORY According to the theory, as one negatively charged electron passes by positively charged ions in the lattice of the superconductor, the lattice distorts. This in turn causes phonons to be emitted which forms a trough of positive charges around the electron. Before the electron passes by and before the lattice springs back to its normal position, a second electron is drawn into the trough. It is through this process that two electrons, which should repel one another, link up. The forces exerted by the phonons overcome the electrons' natural repulsion. The electron pairs are coherent with one another as they pass through the conductor in unison. The electrons are screened by the phonons and are separated by some distance. When one of the electrons that make up a Cooper pair and passes close to an ion in the crystal lattice, the attraction between the negative electron and the positive ion cause a vibration to pass from ion to ion until the other electron of the pair absorbs the vibration. The net effect is that the electron has emitted a phonon and the other electron has absorbed the phonon. It is this exchange that keeps the Cooper pairs together. It is important to understand, however, that the pairs are constantly breaking and reforming. Because electrons are indistinguishable particles, it is easier to think of them as permanently paired.
The Meissner Effect The Meissner Effect So far everything we have discussed is equally true for a “perfect conductor” as well as a “superconductor” In 1933 Meissner and Oschenfeld made a discovery which distinguished between the two “A superconductor excludes all magnetic flux from its interior”
Meissner eﬀect A superconductor in magnetic ﬁeld Superconductor always expels the magnetic ﬂux
Superconductivity and magnetism are “natural enemies” When a superconducting material is cooled below its critical temperature in the presence of an applied magnetic field, it expels all magnetic flux from its interior. Supercurrents induced by the magnet flow through the superconductor and produce a magnetic field that exactly cancels out the magnet’s own field.
Principle of Maglev The principal of a Magnet train is that floats on a magnetic field and is propelled by a linear induction motor. They follow guidance tracks with magnets. These trains are often refered to as Magnetically Levitated trains which is abbreviated to MagLev. Although maglevs don't use steel wheel on steel rail usually associated with trains, the dictionary definition of a train is a long line of vehicles travelling in the same direction - it is a train. A maglev train floats about 10mm above the guidway on a magnetic field. It is propelled by the guidway itself rather than an onboard engine by changing magnetic fields. Once the train is pulled into the next section the magnetism switches so that the train is pulled on again. The Electro-magnets run the length of the guideway.
Maglev Train The train runs in a concrete guide way on sides of which there are three systems of copper coils. One system serves for the train levitation, another one for the train propulsion, and the third one for lateral stability in the guideway. The left figure demonstrates the principle of the train levitation. The superconducting coils on the cars produce high magnetic field of about 5 Tesla. At sufficiently high speed (above 130 km/h) this field induces magnetic field in the stable copper coils on the bed sides that is high enough to keep the train safely above the bottom. Below the critical speed the train is driven by a conventional electrical motor and runs on rubber wheels. Electric current passing through the copper coils on the ground produce alternating magnetic field that attracts the superconducting magnets of the train and propells the train forward.
Maglev Train The first MagLev train was developed in Japan in 1972 and Japan has been the leaders in levitated transport since. In 1990, the Yamanashi MagLev test line opened and has been operating ever since. The test line is an 18.4 km stretch of track that runs solely on the technology of superconductors. The MagLev trains are much safer, faster and environmentally friendly than their traditional counterparts. Japan is leading the way, continually investing more money into the further research of levitated vehicles. The MagLev trains that run on the Yamanashi test line have been clocked at speeds up to 581 km h-1.
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