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 Introduction

 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.

Type I  Sudden loss of magnetisation  Exhibit Meissner Effect  One H C = 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 H C s – H C1 & H C2 (≈30 tesla)  Mixed state present  Hard superconductor  Eg.s – Nb-Sn, Nb-Ti

Superconducting ElementsT C (K) Sn (Tin)3.72 Hg (Mercury)4.15 Pb (Lead)7.19 Superconducting Compounds NbTi (Niobium Titanium)10 Nb 3 Sn (Niobium Tin)18.1

MEISSNER EFFECT When the superconducting material is placed in a magnetic field under the condition when T≤T C and H ≤ H C, the flux lines are excluded from the material. Material exhibits perfect diamagnetism or flux exclusion. BB T>TcT<Tc Levitation of a magnet by superconductor

Applications of Meissner Effect Standard test – proof for a superconductor Repulsion of external magnets - levitation Magnet Superconductor Yamanashi MLX01 MagLev train 430 km/h = mph

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. The Yamanashi MLX01MagLev Train

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.

SQUIDS

Types Two main types of SQUID: 1) RF SQUIDs have only one Josephson junction 2)DC SQUIDs have two or more junctions. Thereby,  more difficult and expensive to produce.  much more sensitive.  Study of earthquakes  Removing paramagnetic impurities  Detection of magnetic signals from brain, heart etc. uses

APPLICATION OF SUPER CONDUCTORS

 Large distance power transmission (ρ = 0)  Switching device (easy destruction of superconductivity)  Sensitive electrical equipment (small V variation  large constant current)  Memory / Storage element (persistent current)  Highly efficient small sized electrical generator and transformer Applications

Medical Applications NMR – Nuclear Magnetic Resonance – Scanning Brain wave activity – brain tumour, defective cells Separate damaged cells and healthy cells

 2001: Detroit, USA › Detroit Edison at the Frisbie Substation › three 400-foot HTS cables › 100 million watts of power

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