Superconductors
The Chemist’s Perspective
Randolph Miller
Superconductors
• Introduction
• History
• Common Types
– Ferropnictides
– Cuprates
– Organics
• Applications
What is a superconductor?
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http://supermanlogo.org/
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http://mikejory.blogspot.com/2010/12/superconductor.html
http://www.berkshirefinearts.com/05-07-2010_launching-125th-season-of-boston-pops.htm
What is a superconductor?
Normal metal
Pin(electrical)
Pout(electrical)
Pout(non-electrical)
Normal metal: Pout(electrical) < Pin(electrical)
Superconductor: Pout(electrical) = Pin(electrical)
Why do metals have resistance?
http://cnx.org/content/m22750/1.3/
•The crystal lattice has vibrations.
•These vibrations scatter the electrons.
•Higher temperature = more vibration = more scattering
How is a superconductor different?
http://webs.mn.catholic.edu.au/physics/emery/hsc_ideas_implementation.htm
•The first electron distorts the lattice
•The distortion attracts a second electron
•The lattice is returned to normal after a pair of electrons go by
http://en.wikipedia.org/wiki/John_Ambrose_Fleming
http://en.wikipedia.org/wiki/James_Dewar
Resistance goes down linearly with temperature.
James Dewar and John Fleming predicted that any pure metal would
have zero resistance at absolute zero, but Dewar later changed his mind.
http://en.wikipedia.org/wiki/Walther_Nernst
Walther Nernst stated that absolute zero is unattainable.
http://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvin
Lord Kelvin predicted that electrons would stop completely
at absolute zero, causing infinite resistance, but he also
believed that absolute zero was unattainable.
http://th.physik.uni-frankfurt.de/~jr/gif/phys/onnes.jpg
http://hoffman.physics.harvard.edu/materials/SCintro.php
•H. Kamerlingh Onnes was first to liquify helium and was using the extreme cold
to study metals.
•Temperatures below 123 K are called cryogenic temperatures.
•As the temperature of mercury went down, the resistance went down linearly
until 4.2 K.
•From 4.2 K lower, the resistance was 0.
•TC is the critical temperature, maximum temperature it’s superconducting.
•Onnes won the Nobel Prize in Physics in 1913.
Ferropnictides: LaFeAsO
http://jolisfukyu.tokai-sc.jaea.go.jp/fukyu/mirai-en/2009/12_3.html
http://www.natureasia.com/asia-materials/highlight.php?id=290
•Ferro means iron
•Pnictogen means from Nitrogen’s group (Group 15 in IUPAC notation)
•Ferropnictide layer is the superconducting layer
•Some of the O atoms are replaced with F
•In this example, As is the pnictogen
•FeAs layer is –
•LaO is +
Ferropnictide: overhead view
(Ren 9)
Re = rare earth metal: Sm, Nd, Pr, Ce, La (same as lanthanoids)
Pr stands for praseodymium, Ce is Cerium
Ferropnictides: comparing Re elements
(Ren 9)
These compounds have the same structure, but very different Tc.
Ferropnictides: bond angle
(Ishida 9)
•The vertical line is at 109.4°, the regular tetrahedral angle.
•The light green sphere is Fe, the 4 orange spheres are As.
Ferropnictide: LaOFeP
http://pubs.acs.org/doi/full/10.1021/ja063355c
•Alternating stack of layers
•Layered structure allows researchers to try different carrier densities
•Impurity doping in the LaO layer transfers carriers to the FeP layer
Doping
1. Chemical substitution dopes carriers into the system,
by introducing heterovalent ions
2. Chemical substitution deforms the crystal structures,
caused by ionic radius mismatch
•Substituting F for O does both
“Chemical substitution results in (i) the doping of carriers into the system, by
introducing heterovalent ions, and (ii) deformation of the crystal structures, caused
by the ionic radius mismatch of the guest elements. F- and K-substitution and Odeficiency are considered to play both roles, namely, to supply electron/hole
carriers and to suppress the crystal structural transition occurring in the parent
compounds.” (Miyaza 11)
•Band: mobile electronic state within a solid, electron
is free to move within the atomic lattice
•Hole: empty electronic state in a band, a traveling
vacancy in a band
Ferropnictide: SmFeAsO1-xFx
http://www.nanotech-now.com/news.cgi?story_id=31632
•Formula specifies some O atoms are replaced with F atoms
Doping: CeFeAsO1-xFx
(Lynn 9)
•SC stands for superconducting
•Like many ferropnitictides, has a minimum doping level to be superconducting.
Doping: (Ba1−xKx)Fe2As2
(Rotter 3)
Ferropnictide: SmFeAsO0.85F0.15
Only superconducting at low pressure.
(Yi 10)
Magnetic Field Dependence
(Karpinski 23)
TC is reduced by an external magnetic field.
Ferropnictide synthesis
LnAs can react with
moisture, making arsine!!!
http://en.wikipedia.org/wiki/File:Arsine-3D-vdW.png
http://iopscience.iop.org/1367-2630/11/4/045002/fulltext
•Explosion can result in contamination with arsenide compounds.
•This is the HP (high pressure) technique.
Cuprates
(Jin 400)
•Cuprates have two alternating types of layers or blocks.
•Charge reservoir layer can be rock salt, perovskite, or fluorite substructure.
•The CuO2 plane is the “infinite layer.”
“The role of the charge reservoir block is to generate and inject charge carriers
into the [CuO2] plane.” (Jin 400)
Cuprate: Hg-1223
(Jin 404)
Cu-12(n-1) homologous series
(Jin 405)
Ca is the spacer layer, BaO is the interfacial layer
Cuprates: Doping
(Liu 24)
Sr2CuO2+dCl2-y
•Apical means axial (or not coplanar)
•CuO layer is superconducting
•Apical oxygen is connection between superconducting layer and charge reservoir
•Doping means substituting Cl- for O2-
Cuprates: Magnetic Fields
•SCCO stands for Sm2-xCexCuO4-d
•TC goes down with increasing magnetic fields
(Kawakami 017001-2)
Cuprate: YBCO
http://www.fhi-berlin.mpg.de/~hermann/Balsac/BalsacPictures/YBaCuO.gif
http://en.wikipedia.org/wiki/File:Ybco002.svg
•YBa2Cu3O7 was first superconducting cuprate discovered
•Cu4O4 layer is superconducting layer
•Cuprate means compound has Cu2-(cupric) anions
•Yttrium is the spacer layer.
•"The fundamental building block of the copper oxide superconductors is a Cu4O4 square
plaquette." Hinkov
Cuprate: BSCCO
http://commons.wikimedia.org/wiki/File:BSCCO-2212.gif
•BSCCO is pronounced bisco
•Bi2Sr2Ca2Cu3O10
•The CuO2 layer is the superconducting
plane
http://hoffman.physics.harvard.edu/materials/CuprateIntro.php
Cuprate synthesis
Common method:
1.
2.
3.
4.
5.
6.
7.
8.
•1.825g or 0.005M Y(NO3)3.5H2O
•2.614g or 0.010M Ba(NO3)2
•3.624g or 0.015M Cu(NO3)2.3H2O
Grind all three ingredients
Heat with a slow flow of oxygen at 350°C for an hour
Cook at 950°C for a few hours
Cool down
Grind into powder
Crush into pellets with 12 tons of force
Heat up to 950°C again with a slow flow of oxygen (sintering)
Cool at 50°C per hour past 690°C. (tetragonal-orthorhombic phase transition)
Sintering: making an object from powder by heating it below its
melting point until its particles adhere to each other.
Organics
BEDT-TTF
(bis(ethylenedithio)tetrathiafulvalene)
http://www.riken.go.jp/r-world/info/release/press/2008/080623/detail.html
http://www.lcsim.univ-rennes1.fr/thematiques/Ouahab/ouahab_index.htm
Organics: BEDT-TTF
•To get the molecules to stack up, they are usually put in a “charge transfer salt.”
•The BEDT-TTF donates an electron to the other molecule, becoming the donor or cation.
•The other molecule receives the electron and becomes the anion.
•This makes the layers bond, similar to ionic bonding.
“For example, the BEDT-TTF molecule is roughly flat, so that it can be packed
in a variety of arrangements in a solid, and it is surrounded by voluminous
molecular orbitals; to create electronic bands, it is merely necessary to stack
the BEDT-TTF molecules next to each other, so that the molecular orbitals
can overlap. Crudely one might say that this enables the electrons to transfer
from molecule to molecule.”
(Singleton and Mielke 3)
Organics: BEDT-TTF
•The BEDT-TTF molecules line up flatly
against each other while the I atoms line up
in planes above and below in the chargetransfer salt β-(BEDT-TTF)2I3.
•The β means the arrangement of
molecules.
(Singleton and Mielke 5)
Organics: BEDT-TTF
(Singleton and Mielke 4)
•The BEDT-TTF molecules line up flatly against each other in pairs while the
Cu(NCS)2 groups line up at the ends in Κ-(BEDT-TTF)2Cu(NCS)2.
•Each pair is called a dimer.
Organics: Lateral Interactions
(Misaki 2)
Lateral interactions in ladder like array of sulfur
atoms cause it to form 2-D conducting sheets.
Organics: Lateral Interactions
(Misaki 15)
•A schematic drawing of overlaps between the donor molecules in λ-(ET-PDT)4PF6(cn); bars and
broken lines denote the donor molecules projected along the long molecular axis and relatively
large intra- and interstack interactions, respectively.
•cn stands for 1-chloronaphthalene
Organics: Lateral Interactions
(Wang et al. 2270)
•Stereogram of packing structure of β-(ET)2I3
•Dashed lines show short intermolecular contacts
Organics: Lateral Interactions
(Wang et al. 2270)
•Stereogram of packing structure of α-(ET)2I3
•Dashed lines show short intermolecular contacts
(Schlueter 268)
•Packing diagram shows layers.
•Lines show S to S bonds shorter than Van der Waals radius of 3.60Å
•Molecule shown is β”-(ET)2SF5CH2CF2SO3
Organics: Other Donor Molecules
(Kobayashi and Cui 5267)
Donor molecules for organic superconductors
come in many sizes but not shapes:
They’re all flat!
Organics: BEDT-TTF
•Salts of BEDT-TTF
•Note that the I3 salt has a
structural phase transition at
about 0.6 kbar.)
•“Decreasing the unit cell size,
either by using a shorter anion
or by increasing the pressure,
reduces TC”
•Should be (ET)2AuI2
(Singleton and Mielke 6)
κ-(BEDT-TTF)2Cu[N(CN)2]Cl
(Singleton and Mielke 24)
SC only above roughly 200 bars
Organics: Doping, T, and P
(Kobayashi and Cui 5274)
•Molecule is λ-(BETS)2GaBrxCl4-x.
•TC goes down with increasing pressure.
•TC is affected by Br content, ideal at x=0.8
•Above x=0.8, not a SC at ambient pressure.
Organic: Synthesis
(Kobayashi and Cui 5270)
•Steps are at ambient pressure.
•Most steps are ambient temperature.
•One step at low temperature
Organic: Synthesis
(Takimiya 1123)
1.
2.
3.
4.
5.
6.
a) BuLi, Se, CSe2 , THF
b) NCS(CH2)2CO2Me
c) 1,3-diselenole-2-selone, P(OMe)3, C6H6
d) CsOH-H2O
e) ClCH2I
f) NaI, 2-butanone
•Several steps have <100% yield
Levitation
(Saito 3)
•A magnet can levitate, above, below, or to the side of a superconductor
Application: Maglevs
http://www.n-sharyo.co.jp/business/tetsudo_e/pages/maglev.htm
•Maglevs are magnetically levitated trains
•Shown is a MLX01 maglev test train capable of achieving 361 mph, the current record
Application: Maglevs
http://www.dvorak.org/blog/2007/06/01/superconducting-mystery-solved/
Shown is a maglev vehicle at the end of a track. Notice the
electronmagnets visible underneath each side of the track.
Application: SQUID
SQUID is Superconducting Quantum Interference Device
http://www.learner.org/courses/physics/unit/text.html?unit=8&secNum=5
•SQUIDs are based on the principle that superconductors block magnetic fields
•Extremely sensitive detector of magnetic fields
Application: MEG
•MEG stands for magnetoencephalography
•Many SQUIDs (122 in example shown) are used
to measure brain activity
http://www.lanl.gov/quarterly/q_spring03/meg_helmet_measurements.shtml
http://www.hbci.com/~wenonah/hudson/index.html
Application: MEG
It takes 50,000 neurons firing
to make a detectable signal.
http://en.wikipedia.org/wiki/Magnetoencephalography
http://www.kumc.edu/hoglund/posters/brooks.html
http://www.unitn.it/en/cimec/10906/magnetoencephalography-lab
Application: MRI
MRI stands for Magnetic Resonance Imaging
MRI is the biggest market for superconductors
http://www.fightmesotheliomacancer.com/mri.html
http://www.conectus.org/technology.html
Application: particle acceleration
http://indiavisions.wordpress.com/2008/09/10/lhc-tunnel-the-path-of-proton-beam/
The LHC has a tube 27km in circumference, with superconducting magnets the
whole way to speed up charged particles to relativistic speeds.
1200 tonnes of NbTi superconducting cable at 1.9 K to make up to 8.3 T field
Application: Detectors
http://cdsweb.cern.ch/record/910381
•ATLAS detector in the LHC has 8 magnets, each with 100 tonnes of superconductor
•Measures energy and momentum of charged particles
•Stores 1.6 GJ
Application: Gravity Probe B
http://www.resonancepub.com/gravity.htm
•These are parts from Gravity Probe B.
•Gravity Probe B studied gravity from Earth orbit.
•Blue sphere is coated with a superconductor.
•The blue sphere rotates and acts as a gyroscope.
•SQUID detectors monitor the blue gyroscope.
Application: Motors
http://www.superconductorweek.com/free-content-photo-gallery
http://www.amsc.com/products/motorsgenerators/shipPropulsion.html
•Electric motors with superconductors are more energy efficient,
lighter, smaller, and a quieter.
•Shown is the first 36.5MW electric motor made from high temperature
superconductor (HTS) and the equivalent made with copper.
•That's 49,000 horsepower.
•It might be used for propulsion of Navy ships.
•It would make Navy ships more fuel efficient and free up valuable space.
Application: Power Transmission
http://www.amsc.com/products/htswire/LIPAHTSCableProjectBrief.html
http://www.superconductorweek.com/free-content-photo-gallery
•Shown are superconducting YBCO cables compared with the copper cables they replace.
•The superconducting cables carry 150 times as much electricity as same sized copper.
•They carry up to 574 MW.
•This is Holbrook, Long Island.
•It’s been operating since April 22, 2008.
Application: Plasma containment
http://blogs.knoxnews.com/munger/2010/01/fields_for_fusion.html
http://homepages.spa.umn.edu/~llrw/a1001/tokamak.html
•Hot plasma has to be contained by a magnetic field.
•Shown is a Tokamak type fusion reactor.
Superconductors
•Chemistry
•Physics
•Quantum Mechanics
Questions?
Randolph Miller