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Superconductors The Chemist’s Perspective

Superconductors The Chemist’s Perspective . Randolph Miller. Superconductors. Introduction History Common Types Ferropnictides Cuprates Organics Applications. What is a superconductor?. +. =. http://supermanlogo.org/. http://mikejory.blogspot.com/2010/12/superconductor.html.

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Superconductors The Chemist’s Perspective

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  1. SuperconductorsThe Chemist’s Perspective Randolph Miller

  2. Superconductors • Introduction • History • Common Types • Ferropnictides • Cuprates • Organics • Applications

  3. What is a superconductor? + = http://supermanlogo.org/ http://mikejory.blogspot.com/2010/12/superconductor.html http://www.berkshirefinearts.com/05-07-2010_launching-125th-season-of-boston-pops.htm

  4. What is a superconductor? Normal metal Pin(electrical) Pout(electrical) Pout(non-electrical) Normal metal: Pout(electrical) < Pin(electrical) Superconductor: Pout(electrical) = Pin(electrical)

  5. 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

  6. 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

  7. 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.

  8. http://en.wikipedia.org/wiki/Walther_Nernst Walther Nernst stated that absolute zero is unattainable.

  9. http://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvinhttp://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.

  10. http://th.physik.uni-frankfurt.de/~jr/gif/phys/onnes.jpg http://hoffman.physics.harvard.edu/materials/SCintro.php • H. KamerlinghOnnes 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.

  11. 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 +

  12. Ferropnictide: overhead view (Ren 9) Re = rare earth metal: Sm, Nd, Pr, Ce, La (same as lanthanoids) Pr stands for praseodymium, Ce is Cerium

  13. Ferropnictides: comparing Re elements (Ren 9) These compounds have the same structure, but very different Tc.

  14. 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.

  15. 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

  16. Doping • Chemical substitution dopes carriers into the system, by introducing heterovalent ions • 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 O-deficiency 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

  17. Ferropnictide: SmFeAsO1-xFx http://www.nanotech-now.com/news.cgi?story_id=31632 • Formula specifies some O atoms are replaced with F atoms

  18. Doping: CeFeAsO1-xFx (Lynn 9) • SC stands for superconducting • Like many ferropnitictides, has a minimum doping level to be superconducting.

  19. Doping: (Ba1−xKx)Fe2As2 (Rotter 3)

  20. Ferropnictide: SmFeAsO0.85F0.15 Only superconducting at low pressure. (Yi 10)

  21. Magnetic Field Dependence (Karpinski 23) TC is reduced by an external magnetic field.

  22. 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.

  23. 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)

  24. Cuprate: Hg-1223 (Jin 404)

  25. Cu-12(n-1) homologous series (Jin 405) Ca is the spacer layer, BaO is the interfacial layer

  26. 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-

  27. Cuprates: Magnetic Fields (Kawakami 017001-2) • SCCO stands for Sm2-xCexCuO4-d • TC goes down with increasing magnetic fields

  28. 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 Cu4O4square plaquette." Hinkov

  29. http://commons.wikimedia.org/wiki/File:BSCCO-2212.gif Cuprate: BSCCO • BSCCO is pronounced bisco • Bi2Sr2Ca2Cu3O10 • The CuO2 layer is the superconducting plane http://hoffman.physics.harvard.edu/materials/CuprateIntro.php

  30. Cuprate synthesis Common method: • 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.

  31. 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

  32. 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)

  33. 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 charge-transfer salt β-(BEDT-TTF)2I3. • The β means the arrangement of molecules. (Singleton and Mielke 5)

  34. 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.

  35. Organics: Lateral Interactions (Misaki 2) Lateral interactions in ladder like array of sulfur atoms cause it to form 2-D conducting sheets.

  36. 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

  37. Organics: Lateral Interactions (Wang et al. 2270) • Stereogram of packing structure of β-(ET)2I3 • Dashed lines show short intermolecular contacts

  38. Organics: Lateral Interactions (Wang et al. 2270) • Stereogram of packing structure of α-(ET)2I3 • Dashed lines show short intermolecular contacts

  39. (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

  40. Organics: Other Donor Molecules Donor molecules for organic superconductors come in many sizes but not shapes: They’re all flat! (Kobayashi and Cui 5267)

  41. 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)

  42. κ-(BEDT-TTF)2Cu[N(CN)2]Cl (Singleton and Mielke 24) SC only above roughly 200 bars

  43. 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.

  44. Organic: Synthesis (Kobayashi and Cui 5270) • Steps are at ambient pressure. • Most steps are ambient temperature. • One step at low temperature

  45. Organic: Synthesis (Takimiya 1123) 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

  46. Levitation (Saito 3) • A magnet can levitate, above, below, or to the side of a superconductor

  47. 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

  48. 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 electronmagnetsvisible underneath each side of the track.

  49. 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

  50. Application: MEG • MEG stands for magnetoencephalography • Many SQUIDs (122 in example shown) are used to measure brain activity http://www.hbci.com/~wenonah/hudson/index.html http://www.lanl.gov/quarterly/q_spring03/meg_helmet_measurements.shtml

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