Superconductivityo Zero resistivityo Meissner effecto Magnetic effectso Type I & II
Introduction Zero electrical resistance ◦ Superconductors carry current without energy loss Perfect diamagnetism ◦ Superconductors float (levitate) above magnetic fields
History of superconductors 1911: Onnes finds that at 4.2K the resistance of mercury suddenly drops to zero. He called this effect superconductivity and the temperature at which this occurs, critical temperature Tc 1933: Walter Meissner and Robert Ochsenfeld discover that a superconducting material repels a magnetic field (Meissner effect)
History contd. 1957: First widely-accepted theory by John Bardeen, Leon Cooper, and John Schrieffer (BCS theory) 1962: Brian D. Josephson predicts that electrical current would flow between two superconducting materials - even when they are separated by a non-superconductor or insulator. “Josephson effect”. 1986: Alex Müller and Georg Bednorz created the first superconducting cuprate: La2-xBaxCuO4 (Tc =30 K). Got Nobel in 1987. “High Tc superconductivity” 1987: Discovery of YBa2Cu3O6+ (YBCO) a material that superconducts at temperatures above the temperature of liquid nitrogen - a commonly available coolant The current world record Tc of 138 K is held by Hg0.8Tl0.2Ba2Ca2Cu3O8.33
Perfect diamagnet & superconductorPerfect diamagnet SuperconductorIf a conductor already had a Remarkably, the magneticsteady magnetic field through it behavior of a superconductor isand was then cooled through the distinct from perfecttransition to a zero resistance diamagnetism. It will activelystate, becoming a perfect exclude any magnetic fielddiamagnet, the magnetic field present when it makes the phasewould be expected to stay the change to the superconductingsame. state. Two mutually independent properties defining SC are = 0 and B =
Effect of magnetization Superconductivity can be destroyed also by an external magnetic field Hc which is also called the critical one Phase diagra m 2 T HC H C (T 0) 1 2 TC
TypesThere are two types of superconductors, Type I and TypeII, according to their behaviour in a magnetic field Type Isuperconducting state normal stateThis transition isabrupt Type I superconductors are pure metals and alloys
Type II superconducting normal state is gradual
Types I & II comparison The Type II superconductors have much higher critical magnetic fields than Type I, but for most of that field range they are mixtures of normal and superconducting.
BCS TheoryBCS Theory (1957) deals with the behaviour of electrons insuperconducting materials at very low temperaturesLow temperatures minimize the vibrational energy of individualatoms in the crystal lattice An electron moving freely through the material encounters less impedance due to vibrational distortions of the lattice at low temperatures The Coulomb attraction between the passing electron and the positive ion distorts the crystal structure
The region of increased positive charge density propagatesthrough the crystal as a quantized sound wave called a phonon 2 1 - -The passing electron has emitted a phononA second electron experiences a Coulomb attraction from theincreased region of positive charge density created by the firstelectron
BCS Theory contd.Electrons are said to pair into Cooper pairs through interaction withthe crystal lattice (indicated by isotope effect where TC is different fordifferent isotopes)Cooper pairs are formed by two electrons, which overcome theirCoulomb repulsion and experience an attraction through phononexchangesCooper Electron Pairs act like single particles (BOSONS)The electrons in a Cooper Pair possess antiparallel spin, resulting ina total spin of zero for the pairSince the Cooper Pair has zero spin, the pair is not required to obeythe Pauli exclusion principleBosons are particles which have integer spin and their energydistribution is described by Bose-Einstein statistics
Condensation: At low temperatures, bosons collect into the sameenergy stateCooper pairs condense into a highly ordered ground stateThe pairs retain this ordered structure while moving through thecrystal latticeEach pair becomes locked into its position with others pairs, and asa result no random scattering of electron pairs may occurZero resistivity may be defined as the absence of electronscattering; hence, the superconductor now demonstrates zeroresistivityThe binding energy of a Cooper pair at absolute zero is about 3KTCAs the temp rises the binding energy is reduced and goes to zerowhen T=Tc. Above T=Tc a Cooper pair is not bound.
Findings of BCS theory The binding energy of Cooper pair gives arise to energy gap of the order of 10-3 eV Eg(T = 0) = 3.53 kTC
Applications • DiamagnetismThe wide applicability of • Zero resistancesuperconductors is due to • Higher currentMedical Industry MRI Exploits the high magnetic fields expelled by superconducting wires for medical applicationsSince the superconducting coils are capable of producing very stable,large magnetic field strengths, they generate high quality images.
Transportation IndustrySuperconductor coils create strong magnetic fields that producethe effect of levitation by repulsionMaglev trains hover above a magnetic field without anycontact with the tracksAs a result, high speeds of up to 500 miles per hour arepossible with only a small consumption of energy
Electric power industryHigh temperature superconductors (HTS) can be used in theproduction of more cost effective motors and generatorsHTS power cables can carry Superconducting cyclotrontwo to ten times more power (MSU)in equally or smaller sizedcables
References A. Beiser – “Concepts of Modern Physics”, 6 Ed., Tata McGraw-Hill (New Delhi, 2003) Charles Kittel – “Introduction to Solid State Physics”, 7 Ed., John Wiley and Sons (New York, 1996) www.wikipedia.org http://hyperphysics.phy- astr.gsu.edu/hbase/hframe.html