Transcript Superconductivity - An overview of science and technology Prof Damian P. Hampshire

Superconductivity
- An overview of science and technology
Prof Damian P. Hampshire
Durham University, UK
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Structure of the Talk
I)
The fundamental building blocks
-
II)
The important materials
-
III)
(G-L) Ginzburg-Landau and (B-C-S) Bardeen-CooperSchrieffer theories
The Josephson effect
Critical current and pinning (zero resistance)
Classic LTS high field materials – NbTi and Nb3Sn
The high temperature superconductors
The pnictides (Superconductivity and magnetism)
Technology – MRI, LHC, ITER and beyond..
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There are two main theories in
superconductivity:
i) Ginzburg-Landau Theory – describes the
properties of superconductors in magnetic fields
ii) Microscopic BCS theory – describes why materials
are superconducting
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Ginzburg-Landau Theory
Ginzburg and Landau (G-L) postulated a Helmholtz energy
density for superconductors of the form:
where α and β are constants and ψ is the wavefunction. α
is of the form α’(T-TC) which changes sign at TC
High magnetic fields penetrate superconductors in units of
quantised flux (fluxons)!
A fluxon has quantised magnetic flux its structure is like a tornado
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The Mixed State in Nb
Vortex lattice in niobium – the triangular layout can clearly be
seen. (The normal regions are preferentially decorated by
ferromagnetic powder).
Reversible Magnetic Properties of
‘Perfect’ Superconductors
Below Hc, Type I superconductors are in the Meissner state: current flows in a thin layer
around the edge of the superconductor, and there is no magnetic flux in the bulk of the
superconductor. (Hc : Thermodynamic Critical Field.)
In Type II superconductors, between the lower critical field (Hc1), and the upper critical
field (Hc2), magnetic flux – fluxons - penetrates into the sample, giving a “mixed” state.
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Josephson dc. SQUID
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Josephson diffraction
The voltage across a biased SQUID as a function of field
BCS Theory
- the origin of superconductivity
Bardeen Cooper and Schrieffer derived two expressions
that describe the mechanism that causes
superconductivity,

1 
  2 D exp  

 N 0 V 

1 
k BTc  1.14 D exp  

 N 0 V 
where Tc is the critical temperature, Δ is a constant
energy gap around the Fermi surface, N(0) is the
density of states and V is the strength of the coupling.
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Model for a polycrystalline superconductor
– with strong pinning
A collection of
truncated octahedra
G. J. Carty and Damian P. Hampshire - Phys. Rev. B. 77 (2008) 172501 also published in
Virtual journal of applications of Superconductivity 15th May 2008
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Critical current (Jc) measurements
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Electric Field, E (Vm )
V (nV)
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2
1
0
-1
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Current, I (A)
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100
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-1
Ec = 100 Vm
100
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20
2.0
90
T = 4.2 K
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Initial ( = 0 %)
After 1 strain cycle
to  = +0.455%
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12.5 T
13 T
13.5 T
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1.0
14 T
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14.5 T
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-1
Ec = 10 Vm
10
15 T
0
-2
0
10
20
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I (A)
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-10
0
0
1
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-2
Current Density, J (10 Am )
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77 K, zero field YBCO
4.2 K, variable B-field, Nb3Sn
Voltage, V (V)
110
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0
Fluxons do not move smoothly through a
polycrystalline superconductor
The motion of flux through the
system takes place predominantly
along the grain boundaries.
TDGL movie 0.430Hc2 Psi2
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Structure of the Talk
I)
The fundamental building blocks
-
II)
The important materials
-
III)
(G-L) Ginzburg-Landau and (B-C-S) Bardeen-CooperSchrieffer theories
The Josephson effect
Critical current and pinning (zero resistance)
Classic LTS high field materials – NbTi and Nb3Sn
The high temperature superconductors
The pnictides (Superconductivity and magnetism)
Technology – MRI, LHC, ITER and beyond..
NbTi multifilamentary wire
– the workhorse for fields up to ~ 10 Tesla
Alloy - NbTi
Tc ~ 9 K
BC2 ~ 14 T
Ductile
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Nb3Sn superconducting wires
- the workhorse for ITER
OST MJR Nb3Sn
EM-LMI ITER
Internal-tin Nb3Sn
Outokumpu Italy (OCSI)
ITER Internal tin Nb3Sn
Furukawa ITER
Bronze-route Nb3Sn
Intermetallic
compound Nb3Sn
Tc ~ 18 K
BC2 ~ 30 T
Brittle
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Superconducting
magnets: large strains
due to the differential
thermal contraction
during cool-down and
the Lorentz-forces
during high-field
operation.
1000
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Nb3Sn Wire
Magnetic Field: 8 T
Temperature: 4.2 K
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23 T
1
6
10
0.1
5
10
-1.5
-1.0
-0.5
0.0
Applied Strain (%)
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Critical Current (A)
The critical current
density (JC) depends on
the magnetic field, the
temperature and the
strain-state of the
superconductor.
Engineering Critical Current Density (Am-2)
Why is the effect of strain on JC
important ?
HTS – BiSrCaCuO (BiSCCO)
- Powder-in-tube fabrication
- Granularity is an issue
- d-wave
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HTS coated conductors
- Kilometre long single crystals
Configuration of SuperPower 2G HTS Wire™
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MgB2 - Brittle compound
Tc ~ 40 K, BC2 (//c) ~ 20 T
A nodeless BCS-type gap !
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Conductors in the USA
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Conductors in the USA
10000
YBCO B Tape Plane
YBCO B|| Tape Plane
Nb-Ti
Complied from
ASC'02 and
ICMC'03 papers
(J. Parrell OI-ST)
1000
JE (A/mm²)
SuperPower tape
used in record
breaking NHMFL
insert coil 2007
RRP Nb3Sn
427 filament strand with
Ag alloy outer sheath
tested at NHMFL
2212
YBCO Insert Tape (B|| Tape Plane)
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MgB2
Maximal JE for
entire LHC NbTi strand
production
(CERNT. Boutboul '07)
YBCO Insert Tape (B Tape Plane)
MgB2 19Fil 24% Fill (HyperTech)
Bronze
Nb3Sn
2212 OI-ST 28% Ceramic Filaments
NbTi LHC Production 38%SC (4.2 K)
Nb3Sn RRP Internal Sn (OI-ST)
4543 filament High Sn
Bronze-16wt.%Sn0.3wt%Ti (MiyazakiMT18-IEEE’04)
18+1 MgB2/Nb/Cu/Monel
Courtesy M. Tomsic, 2007
Nb3Sn High Sn Bronze Cu:Non-Cu 0.3
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Applied Field (T)
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HTS materials and exotic materials
A schematic of a high-Tc
phase diagram
Phase diagram for the
ferromagnet UGe2
The Pnictide Superconductors
– the iron age revisited
Iron Man : In cinemas now from Paramount Pictures and Marvel Entertainment
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The Pnictides
- the original discovery
Layered structure
Original material:
Tc 3-5 K 2006 LaOFeP
A big class of new materials
(> 2000 compounds)
Re-O-TM-Pn.
TM =
Re = La+
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Pn
Comparing HTS and pnictide structure
In both cases, the superconductivity is in metallic layers,
there is a charge reservoir and they are
antiferromagnetic in their undoped state.
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Tc of the iron-based system is quite high
Tc 3-5 K 2006 LaOFeP
Tc 26 K, LaOFFeAs. Jun. 2008
Tc 43 K with high pressure (4 GPa) LaOFeAs.
Feb. 2008
Possibly the 1st 40K-class LTS superconductor
Tc 55 K NdFeAsO1-d. April/May 2008.
(Also 111 phase and 122 phase)
Oxygen concentration is critical for
superconductivity
• For the NdFeAsO1-d with different O concentration
• A dome-shaped superconducting bubble has been found
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Page 1224
Point-contact spectroscopy
Tc ~ 42K
Sweep the V
I-V
dI/dV - V
A nodeless BCS-type gap !
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Does Superconductivity coexist or
compete with magnetism ?
This sharp drop about 150 K is due to a SDW – confirmed
using neutron diffraction - P. C. Dai Nature (2008)
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BC2 is high
Larbalestier et al measured the resistance of F doped
LaOFeAs at high fields up to 45 T. Nature 453 903
Two-gap model is
qualitatively
consistent with
their data.
H.H. Wen et al measured F doped NdOFeAs. Hc2 ~ 300 T in
the ab plane and ~60-70T in c axis. Arxive:cond-mat/0806.0532
High critical current
in polycrystalline pnictides !
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Structure of the Talk
I)
The fundamental building blocks
-
II)
The important materials
-
III)
(G-L) Ginzburg-Landau and (B-C-S) Bardeen-CooperSchrieffer theories
The Josephson effect
Critical current and pinning (zero resistance)
Classic LTS high field materials – NbTi and Nb3Sn
The high temperature superconductors
The pnictides (Superconductivity and magnetism)
Technology – MRI, LHC, ITER and beyond..
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Applications using Superconductors
MRI Body scanners
LHC
ITER
Transport
Power transmission
Public outreach
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MRI - $1B annual market
Large hadron collider – LHC ~ $ 6B
6000 superconducting magnets will accelerate proton beams in opposite
directions around a 27 km-long ring and smash them together at energies
bordering on 14 TeV.
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Some facts about the LHC
Protons are accelerated to 99.999999991% of the speed
of light
The LHC lets us glimpse the conditions 1/100th of a
billionth of a second after the Big Bang: a travel back in
time by 13.7 billion years
High energy collisions create particles that haven’t
existed in nature since the Big Bang
Find out what makes the Universe tick at
the most fundamental level
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ITER – Building a star on planet earth
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We need extreme conditions …
At 200 million ºC,
Matter becomes a plasma
Picture courtesy of the
SOHO/EIT collaboration
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ITER – A large transformer
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The fuel for ITER is from seawater
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16 Nb3Sn toroidal field coils - each coil is ~ 290 tonnes, has
1100 strands, ~ 0.8 mm diameter to form a conductor 820
m long.
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A burning plasma
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Fusion powers the Sun and stars
and has many potential attractions
• Essentially limitless fuel
• No green house gases
• Major accidents impossible
• No long-lived radioactive waste
• Could be a reality in 30 years
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Applications using Superconductors
Transport
In Jan 08, the Central Japan Railway Company (JR Central) announced that
it plans to construct the world's fastest train, a second-generation maglev
train that will run from Tokyo to central Japan.
Cost ~ 44.7 billion dollars
Completion in 2025
Speed ~ 500 kilometers per hour
Length ~ 290 kilometers
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Applications using Superconductors
Superconducting power transmission
- currently we waste ~ 20 % of our
energy just transporting it around
- potentially the next industrial
revolution
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Conclusions
The many uses for superconductivity means that many of
the technological tools required to exploit new materials
are in place.
The new materials discovered in the last 20 years were
found by relatively small determined groups.
Using world-class science to produce technology is
tough.
It requires first class scientists, time,
perserverance, creativity, luck and funding.
Superconductivity offers excellent science,
excellent technology, excellent training and
the possibility of saving the planet !!
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References + Acknowledgements
Acknowledgements: Xifeng Lu + colleagues in Beijing, Mark Raine, Georg
Weiglein (IPPP, Durham), Eric Hellstrom (ASC Florida), Chris Carpenter
(Culham) + many others …….
Bibliography/electronic version of all talks and publications are available at:
http://www.dur.ac.uk/superconductivity.durham/