Superconductivity
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Superconductivity

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Presentation on superconductivity by Phelim Bradley for Lab module in University College Cork

Presentation on superconductivity by Phelim Bradley for Lab module in University College Cork

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  • BCS = BardeenCooper Schrieffer
  • 3 years after his 1st liquefaction of helium which was essential to cool mercury down to extremely low temperatures.Mercury was the 1st metal investigated as it was easily purified by distillation (although discovered later that purification was not essential)!Discovered when investigating resistance at low temperatures. !Resistance was discovered to drop suddenly at T=4.2K!The transition was extremely sharp, occurred over a range of 10^-3KThis discovery sparked interest and other substances were investigatedElement with highest niobium (Nb)!60 years later in 1972 was found to have a = 23KRemained the record for 14 years and researchers were mislead by theoretical justification into believing that significantly higher temperatures could not be found.!Breakthrough in 1986 with a t_c of 35, greater than the previous theoretical limit.!Limit now is, however is hydroscopic and therefore unstable. Most commonly used are ceramics with T_c of 100K.!R = 0 known to an accuracy of , so far no one has succeeded in measuring a finite resistance to small currents in a superconducting state. (i.e. any currents induced by a magnetic field do not dissipate called persistent currents). As the resistance is zero the current flows indefinitely and can be measured by investigating the magnetic field created by the circuit.It has been found that there is no reduction in current over the time period that anyone has had the patience to measure it (the record is over 2  years!). This proved that the resistance was indeed zero.
  • Even with a current flowing
  • !!E.G tin T_C=3.71K with 80Gauss applied T_C =3.1KAt 250Gauss T_c = 1K!ObeysThis magnetic field influence hinted that superconductivity was a completely new phenomenon and not just an extension of previous theories.
  • !What was actually observed was that not only was the B field consent, it was actually 0.!The Meissner effect is an independent phenomena consistent with but not a consequence of a perfect conductor.!First conclusive proof that superconductivity was not simply a perfect conductor and that an entirely new phenomenon was in play.! Ki=-1!As a result the current must flow on the surface only.
  • This is a short video of the messner effect in play. The superconductor repels the magnetic field of the magnet and the force is strong enough to overcome the gravitational force.The magnet also resists any movement
  • The next breakthrough in attempting to form an adequate theory came when two brothers, Fritz and Heinz London, made a useful connection between quantum mechanics and superconductivity. !to endThis ‘giant atom’ could be produced by having all of the electrons in the body correlated in such a way that the entire specimen could be described by a single wave function.
  • The London equations predict that the magnetic field will decay at the surface of the superconductor.!where ms, ns and es are the mass, number density and charge of the supercurrent. !This characteristic length, λ, is known as the London penetration depth. The number density of superconducting electrons is dependent on temperature and means that penetration depth is as well. the penetration depth rises asymptotically as the temperature approaches Tc. Thus the field penetrates further and further as the temperature approaches Tc and can penetrate so completely above Tc.
  • After talking about electron attracting each other::electrons repel each other due to the Coulombic interaction of the similar charge but for electrons to become coherent there must be some type of attraction between them.a paired state of electrons have a lower energy than the Fermi energy, which implies that the pair is bound.
  • The BCS theory is based on cooper pairs.Electrons act as Fermions which obey Pauli's exclusion principle and only 2 electrons spin up and spin down can occupy each energy level.Cooper pairs however, act as bosons with integer spin and obey Bose Einstein statistics.An easy way to see this difference is by considering how electrons and cooper pairs would behave in an electric field.If the thermal energy of the electron is less is less than the band gap, than the electron can travel in cooper pairs and the material exhibits zero resistivity.. This is the origin of the critical temperature.is less than the band gap, the material exhibits zero resistivity.The electron pairs have a slightly lower energy and leave an energy gap above them on the order of .001 eV which inhibits the kind of collision interactions which lead to ordinary resistivity. For temperatures such that the thermal energy is less than the band gap, the material exhibits zero resistivity.
  • Magnetic Levitation or MagLev for short is one of the most interesting applications of superconductivity. It is a system of transportation that suspends, guides and propels vehicles, predominantly trains, using magnetic levitation from a very large number of magnets for lift and propulsion.!Since the trains will be floating it provides low friction,!High Speeds!And since the electrical resistance is 0, very efficient.!!!To explain Maglev imagine a track as shown with NSN orientation!This creates a magnetic field like this!The addition of a superconductor changes the field as shown due to the Messiner Effect as you can see the magnet appears trapped within the field!The continuation of the field creates a magnetic “tunnel” ! which the magnet is forced to go through.!If the magnets are attached to the bottom of a train and the train is propelled the train can float along the line created by the “tunnel” with little resistance.
  • In this presentation I have shown an introduction to the theory behind superconductivity and a demonstration of one of the many applications of superconductivity magnetic levitation.
  • Except for the elements vanadium, technetium and niobium, the Type 2 category of superconductors is comprised of metallic compounds and alloys.They achieve higher Tc's than Type 1 superconductors by a mechanism that is still not completely understood.Since a Type 2 will allow some penetration by an external magnetic field into its surface, this creates some rather novel mesoscopic phenomena like superconducting “stripes” and “flux-lattice vortices”

Transcript

  • 1. Super Conductivity
    Zero Resistance at Low Temperatures
  • 2. Contents
    History
    Magnetic Properties of Superconductors
    Meissner Effect
    London Equations
    Cooper Pairs (BCS Theory)
    MagLev
  • 3. History
    • Discovered in 1911 by H. Kamerlingh Onnes.
    • 4. Discovered while investigating if resistance went to 0 Ω when T = 0K
    • 5. Zero resistance occurred for Mecuryat T = 4.2K
    • 6. The temperature where R=0 Ω occurs is called the critical temperature.
    • 7. After 60 years the highest was with = 23K
    • 8. Breakthrough in 1986 with discovery of with =35K
    • 9. Record at the moment held by (Tl4Ba)Ba2Ca2Cu7O13+ with a = 254K(-19C)
  • Conditions For Infinite Conductivity (R=0Ω)
    • No P-D between two Points (Constant Potential)
    • 10. E=0 everywhere (No EMF)
    • 11. B cannot change in Time
  • Magnetic Properties of Superconductors
    When a magnetic field is applied to a superconductor at the temperature must be lowered to maintain superconductive state.
    Every has an associated critical magnetic field .
  • 12. Messnier Effect
    Observed:
    Exclusion of a B field occurs, not a consequence of infinite conductivity
    Conclusive proof that superconductivity is a completely new phenomenon.
    Superconductors display perfect diamagnetism.
    Current flows only on surface.
  • 13. Messnier Effect
  • 14. London Conjecture
  • 15. London Equations
  • 16. Cu, Ag and Au, although excellent conductors do not rank among the superconducting elements. Why?
    Answer: Only one electron in outer shell.
    In Superconductive States electrons are bound together in Cooper Pairs
    This due to the electron-phonon interaction.
    Cooper Pairs
  • 17. BCS Theory
    The BCS theory predicts a bandgap of
  • 18. Maglev (magnetic levitation) is an application of superconductivity.
    Low friction.
    High Speeds
    Energy efficient.
    Maglev
  • 19. Summary
    History
    Magnetic Properties of Superconductors
    Meissner Effect
    London Equations
    Cooper Pairs (BCS Theory)
    MagLev
  • 20. Type 1 and Type 2 Superconductors