Superconductivity

[seminar class]

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Superconducting Materials
Introduction
WHAT IS SUPERCONDUCTIVITY ?
For some materials, the resistivity vanishes at some low temperature: they become superconducting
Superconductivity is the ability of certain materials to conduct electrical current with no resistance. Thus, superconductors can carry large amounts of current with little or no loss of energy.
 Superconductivity was discovered in 1911 by Heike Kammerlingh Onnes
 Applications of superconductors
 Engineering field
Medical field
BCS Theory of Superconductivity
 The properties of type I superconductors were modeled by the efforts of John Bardeen, Leon Cooper, and Robert Schrieffer in what is commonly called the BCS theory.
 A key conceptual element in this theory is the pairing of electrons close to the Fermi level into Cooper pairs through interaction with the crystal lattice.
This pairing results from a slight attraction between the electrons related to lattice vibrations; the coupling to the lattice is called a phonon interaction
Types of Superconductors
Type I
 Sudden loss of magnetisation
 Exhibit Meissner Effect
 One HC = 0.1 tesla
 No mixed state
 Soft superconductor
 Eg.s – Pb, Sn, Hg
Type II
 Gradual loss of magnetisation
 Does not exhibit complete Meissner Effect
 Two HCs – HC1 & HC2 (≈30 tesla)
 Mixed state present
 Hard superconductor
 Eg.s – Nb-Sn, Nb-Ti
Occurrence of Superconductivity
APPLICATIONS:
Superconducting
Magnetic Levitation
The track are walls with a continuous series of vertical coils of wire mounted inside. The wire in these coils is not a superconductor.
As the train passes each coil, the motion of the superconducting magnet on the train induces a current in these coils, making them electromagnets.
The electromagnets on the train and outside produce forces that levitate the train and keep it centered above the track. In addition, a wave of electric current sweeps down these outside coils and propels the train forward
JOSEPHSON EFFECT
 A Josephson junction is made up of two superconductors, separated by a nonsuperconducting layer so thin that electrons can cross through the insulating barrier.
 The flow of current between the superconductors in the absence of an applied voltage is called a Josephson current,
 the movement of electrons across the barrier is known as Josephson tunneling.
 Two or more junctions joined by superconducting paths form what is called a Josephson interferometer.