BBardeenardeenCCooperooper

SSchriefferchriefferTheory of Theory of

SuperconductivitySuperconductivity Presented toPresented to

Prof. Dr. Jameel Un Nabi Prof. Dr. Jameel Un Nabi

Presented ByPresented ByAssad Ur Rehman Assad Ur Rehman

ES-1717ES-1717Faculty Of Engineering SciencesFaculty Of Engineering Sciences

Ghulam Ishaq Khan Institute Of Engineering Ghulam Ishaq Khan Institute Of Engineering Sciences & TechnoogySciences & Technoogy

ContentsContents

Introduction.Introduction.

Theories.Theories.

Cooper Pairs.Cooper Pairs.

BCS Theory.BCS Theory.

Drawbacks.Drawbacks.

Summary.Summary.

IntroductionIntroduction

BCS successfully and completely explains the BCS successfully and completely explains the

microscopic origin of super conductivity.microscopic origin of super conductivity.

It is a mean filed theory.It is a mean filed theory.

Applicable to conventional super conductors only. Applicable to conventional super conductors only.

SuperconductivitySuperconductivity

Superconductivity is a phenomenon displayed by Superconductivity is a phenomenon displayed by certain conductors that demonstrate no resistance to certain conductors that demonstrate no resistance to the flow of an electric current. Superconductors also the flow of an electric current. Superconductors also exhibit strong diamagnetism; that is, they are repelled exhibit strong diamagnetism; that is, they are repelled by magnetic fields. Superconductivity is manifested by magnetic fields. Superconductivity is manifested only below a certain critical temperature only below a certain critical temperature TTc and a c and a critical magnetic field critical magnetic field HHc, which vary with the c, which vary with the material used. Before 1986, the highest Tc was 23.2 material used. Before 1986, the highest Tc was 23.2 K (-249.8°C, -417.6° F) in niobium-germanium K (-249.8°C, -417.6° F) in niobium-germanium compounds. compounds.

SuperconductivitySuperconductivity

Discovered by Discovered by Kamerlingh Onnes Kamerlingh Onnes in in

1911 during first low temperature 1911 during first low temperature

measurements to liquefy helium.measurements to liquefy helium.

Whilst measuring the resistivity of Whilst measuring the resistivity of

“ “pure” Hg he noticed that the pure” Hg he noticed that the

electrical resistance dropped to zeroelectrical resistance dropped to zero

at 4.2K.at 4.2K.

In 1912 he found that the resistive In 1912 he found that the resistive

state is restored in a magnetic fieldstate is restored in a magnetic field

or temperatures. or temperatures.

19131913

Type Type II Superconductors Superconductors

In type Type I superconductors transition from normal In type Type I superconductors transition from normal state to superconducting state occurs instantly i.e. at state to superconducting state occurs instantly i.e. at exactly it's critical/transition temperature Tc like pure exactly it's critical/transition temperature Tc like pure metals mercury, lead and aluminum. metals mercury, lead and aluminum.

Type Type IIII Superconductors Superconductors

In Type II superconductors  transition  from  a normal state to a superconducting state occurs "slowly" i.e. as temperature decreases from it's critical  temperature  superconducting properties  increase.  Examples  of  type  II superconductors  are  alloys  and  ceramic  like niobium-titanium and niobium-tin. 

Theories Theories Drude  Model  of  Metals  failed  to  explain  the Drude  Model  of  Metals  failed  to  explain  the 

superconductivity.superconductivity.

London  Equations  explained  superconductivity  with London  Equations  explained  superconductivity  with 

some limitations.some limitations.

Giznberg    Landaue  proposed  a  macroscopic  model Giznberg    Landaue  proposed  a  macroscopic  model 

based on thermodynamic parameters.based on thermodynamic parameters.

BCS TheoryBCS Theory

Bardeen-Cooper-Schrieffer theory of Bardeen-Cooper-Schrieffer theory of

superconductivity explains the origin of superconductivity explains the origin of

superconductivity as effect caused by the superconductivity as effect caused by the

condensation of condensation of Cooper-pairsCooper-pairs into a boson-like into a boson-like

state[2].state[2].

19721972

Cooper PairsCooper Pairs

In condensed matter physics, a In condensed matter physics, a Cooper pairCooper pair or or BCS BCS pairpair is a is a pairpair of electrons (or other fermions) of electrons (or other fermions) bound together at low temperatures in a certain bound together at low temperatures in a certain manner first described in 1956 by American physicist manner first described in 1956 by American physicist Leon Leon Cooper[3]Cooper[3]..

The physical idea is that, the first electron polarizes The physical idea is that, the first electron polarizes the medium by the medium by attracting positive ions &attracting positive ions &

These excess positive ions in turn attract the second These excess positive ions in turn attract the second electron (with opposite spin and momenta), giving an electron (with opposite spin and momenta), giving an effective attractive interaction between the electrons.effective attractive interaction between the electrons.

Cooper PairsCooper Pairs

Cooper PairsCooper Pairs

DrawbacksDrawbacks

Does not explain the superconductivity of Type II

superconductors.

Valid at low temperatures only.Valid at low temperatures only.

Does not predict which materials may be

superconductors.

SummarySummary

BCS successfully explains the microscopic origin of BCS successfully explains the microscopic origin of

superconductivity.superconductivity.

An attractive interaction between electrons will result An attractive interaction between electrons will result

in forming bound Cooper pairs.in forming bound Cooper pairs.

BCS can only explain superconductivity in type I BCS can only explain superconductivity in type I

superconductors only.superconductors only.

ReferencesReferences1.1.Dirk (2007) Freezing physics, Heike Kamerlingh Onnes and the quest for cold, Edita, Amsterdam.

2. Phys. Rev. 108, 1175 (1957)

3.3.Cooper, Leon N. (1956). "Bound electron pairs in a degenerate Fermi gas". Physical Review. 104 (4): 1189–1190.

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