Superconductor_ceramics.pdf

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1、SuperconductorCeramicsWhat'sasuperconductor?Superconductorshavetwooutstandingfeatures:1).Zeroelectricalresistivity.●Thismeansthatanelectricalcurrentinasuperconductingringcontinuesindefinitelyuntilaforceisappliedtoopposethecurrent.2).Themagneticfieldin

2、sideabulksampleiszero(theMeissnereffect).●Whenamagneticfieldisappliedcurrentflowsintheouterskinofthematerialleadingtoaninducedmagneticfieldthatexactlyopposestheappliedfield.●Thematerialisstronglydiamagneticasaresult.●IntheMeissnereffectexperiment,amag

3、netfloatsabovethesurfaceofthesuperconductorWhat'sasuperconductor?●Mostmaterialswillonlysuperconduct,atverylowtemperatures,nearabsolutezero.●Abovethecriticaltemperature,thematerialmayhaveconventionalmetallicconductivityormayevenbeaninsulator.●Asthetemp

4、eraturedropsbelowthecriticalpoint,T,resistivityrapidlydropstozeroandcurrentcancflowfreelywithoutanyresistance.What'sasuperconductor?●Linearreductioninresistivityastemperatureisdecreased:=(1+(T-T))oowhere:resistivityand:thelineartemperaturecoeffic

5、ientofresistivity.-23●Resistivity:~4x10cmforssuperconductor.-13●Resistivity:~1x10cmformnonsuperconductormetal.Hallmark1–ZeroResistance●MetallicRvsT○e-pscattering(latticeinteractions)athightemperature○ImpuritiesatlowtemperaturesRLattice(phonon)Elec

6、tricalinteractionsresistanceImpuremetalResidualRResistance0(impurities)PuremetalT/TemperatuD3reHallmark1–ZeroResistance●SuperconductingRvsTRSuperconductorR0TTemperatu“Transitioncretemperature”Hallmark1–ZeroResistanceMagnetic(dipole)●Hardtomeasure“zero

7、”fielddirectly●Cantrytolookataneffectofthezeroresistance●CurrentflowinginaSCringI○Notthoughtexperiment–standardconfigurationforhigh-fieldlaboratorymagnets(10-20T)CirculatingSuperconduct●Nonzeroresistancesupercurreorntchangingcurrentchangingmagneticf

8、ield●OnesuchmeasurementFromUstinov“Superconductivity”Lectures(WS2008-2009)Hallmark1–ZeroResistance●R=0onlyforDC●ACresponsearisesfromkineticinductanceofsuperconductingelectrons○Changingcurrentelectricfield●Model:perfectresistor(normalelectron