assad bcs theory
TRANSCRIPT
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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