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11
Superconductivity and Superconductivity and
Quantum CoherenceQuantum CoherenceGGL Lent Term 2010
credits to: Christoph Bergemann,
David Khmelnitskii, John Waldram, …
• 12 Lectures: Tues & Thrs 11-12am Mott Seminar Room
• 3 Supervisions, each with one examples sheet
• This is a developing course – feedback is welcome!
Complete versions on course web site:www-qm.phy.cam.ac.uk/teaching.php
22
Literature:Literature:
JF Annett: Superconductivity, Superfluids and Condensates
JR Waldram: Superconductivity of Metals and Cuprates
AJ Leggett: Quantum Liquids – Bose Condensation & Cooper Pairing in Condensed-Matter Systems
R Feynman: Lectures on Physics Volume III
A Altland & B Simons: Condensed Matter Field Theory
CJ Pethick & H Smith: Bose-Einstein Condensationin Dilute Gases
M Tinkham: Introduction to Superconductivity
VV Schmidt: The Physics of Superconductors
GE Volovik: The Universe in a Helium Droplet
33
Outline:Outline:
• Ginzburg-Landau theory of the Super-
conducting State (4 lectures)
• Applications of Superconductivity (1)
• Bose-Einstein Condensates (1)
• Superfluidity in 4He (1)
• Quantum Coherence and Bardeen-
Cooper-Schrieffer (BCS) Theory (3)
• Superfluidity in 3He and Unconventional
Superconductivity in Advanced Materials (2)
PhenomenologicalGL Theory
Microscopic BCS Theory
New Developments
44
Lecture 1:Lecture 1:
• Historical overview
• Macroscopic manifestation of superconductivity: ρ, χ, C/T
• Meissner effect and levitation
• Type-I and type-II superconductivity
• Superconductivity as an ordered state – Landau theory as a precursor to the Ginzburg-Landau theory
• Literature: Waldram ch. 4 (or equivalent chapters in Annett, Leggett, Schmidt, or Tinkham)
55
Timeline:Timeline:
Unconventional superconductors, Unconventional superconductors, includingincluding
high temperature superconductorshigh temperature superconductors
1970s1970s--nownow
Josephson effect and SQUIDsJosephson effect and SQUIDs1962/641962/64
Superfluidity in Superfluidity in 33HeHe19711971
GinzburgGinzburg--Landau theory of superconductivityLandau theory of superconductivity
AbrikosovAbrikosov vorticesvortices
19501950
1952/571952/57
Prediction of BosePrediction of Bose--Einstein condensation (BEC)Einstein condensation (BEC)19251925
Superfluidity in Superfluidity in 44HeHe
MeissnerMeissner effecteffect
1927/381927/38
19331933
BEC and BCS in atomic gasesBEC and BCS in atomic gases1990s1990s--nownow
BCS theory of superconductivityBCS theory of superconductivity19571957
Superconductivity in mercurySuperconductivity in mercury19111911
Liquefaction of Liquefaction of 44HeHe19081908
?
Kamerlingh Onnes
66
Examples of SuperconductorsExamples of Superconductors
24.5 K
39 K
0.5 K
1.5 K
0.5 K to
164 K
92 K
35K
10 K
9.3 K
4.1 K
of ferromagnetism – p-waveUGe2
Tc above boiling point of nitrogenYBa2Cu3O7-δ
highest Tc to dateHgBa2Ca2Cu3O8+δ
superconducting magnets to ~ 9 TNbTi
superconducting magnets to ~ 20 T
high-Tc s-wave superconductivity
superconductivity on border of antiferromagnetism
Nb3Sn
MgB2
CeCu2Si2
superconductivity on the borderSr2RuO4
high-Tc d-wave superconductivityLa2-xBaxCuO4
highest Tc amongst the elementsNb
first superconductorHg
}carbon based compounds, iron arsenides, ….
77
Superconducting elements:Superconducting elements:
www.webelements.com-examples sheet
88
Basic experimental facts:Basic experimental facts:• The resistivity of a superconductor drops to zero below some transition temperature Tc
• Immediate corollary: can’t change the magnetic field inside a superconductor
B = 0 B
Switch on external B:
zero field cooled
0 since ,0 curl curl ==−≡−=∂
∂ρρΕ J
t
B
99
What if we cool a superconductor in a magnetic field and then switch the field off – do we get something like a permanent magnet?
field cooled
BExperimentally, this does not work – even when field cooled, the superconductor expels the field!
B
field cooled
This is known as the Meissner effect. Superconductivity arises through a thermodynamic phase transition (state depends only on final conditions, e.g., Tand B).
1010
The Meissner effect leads to the stunning levitation effects that underlie many of the proposed technological applications of superconductivity (see examples sheet).
The superconducting state is destroyed above a critical field Hc
Ideal magnetisation curve…
Hc
…and so-called type-II superconductivity(which we’ll discuss later)
Hc1 Hc2H
M
NB: These curves apply for a magnetic field along a long rod.
B
1111
• exponential low-Tbehaviour indicative ofenergy gap(explained by BCS)
• power-law behaviour at low-T in unconventional superconductors(to be discussed later)
• matching areas means entropy is continuous at Tc consistent with second order phase transition
The electronic specific heat around the superconducting transition temperature Tc:
exponential in simple
superconductors
1212
From the form of C/T we find that the entropy S vs. temperature has the following form:
T
S
TcThe superconducting state has lower entropy than the normal state and is therefore the more ordered state. A general theory based on just a few reasonable assumptions about the order parameter is remarkably powerful. It describes not just BCS superconductors but also the high-Tc superconductors, superfluids, and Bose-Einstein condensates. This is known as Ginzburg-Landau theory.
normal state
superconducting state
1313
Landau Theory:Landau Theory:
For a second order phase transition, the order parameter vanishes continuously at Tc. In the Landau theory one assumes that sufficiently close to Tc the free energy density relative to the normal state can be expanded in a Taylor series in the order parameter, ψ
This assumes that the order parameter is real and that the free energy density is an even function of the order parameter.
Where is the free energy minimum?
)0(2
42)( >+= βψβ
αψψf
1414
Free energy curves:
Picture credits: A. J. Schofield
α > 0 α < 0
ψ ψψ0−ψ0
The phase transition takes place at α(Tc) = 0. Thus, a power series expansion of α(T) around Tc may be expected to have the following leading form:
This is enough to describe a second order phase transition, complete with specific heat jump (examples sheet).
f f
0) ( )( >−= aTTa cα
1515
This is appropriate for, e.g., ferrromagnetism where ψ in the uniform magnetization along a given axis. In the Ginzburg-Landau (GL) theory, however, ψ is assumed to be complex rather than real as is the case for a macroscopic wave function. We will see in a later lecture how a complex order parameter arises naturally from a microscopic theory. The assumptions in the GL theory are:
• ψ can be complex-valued
• ψ can vary in space – but this carries an energy penaltyproportional to
• ψ couples to the electromagnetic field in the same way as an ordinary wavefunction (Feynman, ch. 21)
Here, A is the magnetic vector potential and q is the relevant charge, which experimentally turns out to be q = –2e.
4422 , ψψψψ →→
2ψ∇
h/iqA−∇→∇
1616
This provides the first clue that superconductivity has got something to do with electron pairs. The idea of electron pairing is central to the microscopic theory.
A final part in the free energy that must not be forgotten is the relevant magnetic field energy density BM
2/2µ0, where BM=B-BE is due to currents in the superconductor and BE is due to external sources. (Note that when the material is introduced the total field energy density changes from BE
2/2µ0 to B2/2µ0, but
BMBE/µ0 is taken up by the external sources (Waldram, Ch.6)).
So finally we arrive at the Ginzburg-Landau free energy density:
We have written the free energy so that the gradient term involve an effective mass m = 2me , which is consistent withq = –2e. This represents an effective field theory unifying matter field ψ & gauge field A (recall B=curlA) in the static limit.
2242)(
2
1)2(
2
1
2E
oBBeAi
m−++∇−++=
µψψ
βψα hf