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Superconductors The Chemist’s Perspective Randolph Miller
Superconductors Introduction History Common Types – Ferropnictides – Cuprates – Organics Applications
What is a superconductor? +=
What is a superconductor? P in(electrical) P out(electrical) Normal metal P out(non-electrical) Normal metal: P out(electrical) < P in(electrical) Superconductor: P out(electrical) = P in(electrical)
Why do metals have resistance? The crystal lattice has vibrations. These vibrations scatter the electrons. Higher temperature = more vibration = more scattering
How is a superconductor different? 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
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.
Walther Nernst stated that absolute zero is unattainable.
Lord Kelvin predicted that electrons would stop completely at absolute zero, causing infinite resistance, but he also believed that absolute zero was unattainable.
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. T C is the critical temperature, maximum temperature it’s superconducting. Onnes won the Nobel Prize in Physics in
Ferropnictides: LaFeAsO 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 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 “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) 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 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: SmFeAsO 1-x F x Formula specifies some O atoms are replaced with F atoms
Doping: CeFeAsO 1-x F x (Lynn 9) SC stands for superconducting Like many ferropnitictides, has a minimum doping level to be superconducting.
Doping: (Ba 1−x K x )Fe 2 As 2 (Rotter 3)
Ferropnictide: SmFeAsO 0.85 F 0.15 (Yi 10) Only superconducting at low pressure.
Magnetic Field Dependence (Karpinski 23) T C is reduced by an external magnetic field.
Ferropnictide synthesis Explosion can result in contamination with arsenide compounds. This is the HP (high pressure) technique. LnAs can react with moisture, making arsine!!!
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 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 O 2- (Liu 24) Sr 2 CuO 2+d Cl 2-y
Cuprates: Magnetic Fields SCCO stands for Sm 2-x Ce x CuO 4-d T C goes down with increasing magnetic fields (Kawakami )
Cuprate: YBCO YBa 2 Cu 3 O 7 was first superconducting cuprate discovered Cu 4 O 4 layer is superconducting layer Cuprate means compound has Cu 2- (cupric) anions Yttrium is the spacer layer. "The fundamental building block of the copper oxide superconductors is a Cu 4 O 4 square plaquette." Hinkov
Cuprate: BSCCO BSCCO is pronounced bisco Bi 2 Sr 2 Ca 2 Cu 3 O 10 The CuO 2 layer is the superconducting plane
Cuprate synthesis 1.825g or 0.005M Y(NO 3 ) 3.5H 2 O 2.614g or 0.010M Ba(NO 3 ) g or 0.015M Cu(NO 3 ) 2.3H 2 O Common method: 1.Grind all three ingredients 2.Heat with a slow flow of oxygen at 350°C for an hour 3.Cook at 950°C for a few hours 4.Cool down 5.Grind into powder 6.Crush into pellets with 12 tons of force 7.Heat up to 950°C again with a slow flow of oxygen (sintering) 8.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 “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) 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.
Organics: BEDT-TTF (Singleton and Mielke 5) 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) 2 I 3. The β means the arrangement of molecules.
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) 2 Cu(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) 4 PF 6 (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) 2 I 3 Dashed lines show short intermolecular contacts
Organics: Lateral Interactions (Wang et al. 2270) Stereogram of packing structure of α-(ET) 2 I 3 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) 2 SF 5 CH 2 CF 2 SO 3
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 (Singleton and Mielke 6) Salts of BEDT-TTF Note that the I 3 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 T C ” Should be (ET) 2 AuI 2
κ-(BEDT-TTF) 2 Cu[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) 2 GaBr x Cl 4-x. T C goes down with increasing pressure. T C 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.a) BuLi, Se, CSe 2, THF 2.b) NCS(CH 2 ) 2 CO 2 Me 3.c) 1,3-diselenole-2-selone, P(OMe) 3, C 6 H 6 4.d) CsOH-H 2 O 5.e) ClCH 2 I 6.f) NaI, 2-butanone Several steps have <100% yield
Levitation A magnet can levitate, above, below, or to the side of a superconductor (Saito 3)
Application: Maglevs Maglevs are magnetically levitated trains Shown is a MLX01 maglev test train capable of achieving 361 mph, the current record
Application: Maglevs 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 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
Application: MEG It takes 50,000 neurons firing to make a detectable signal.
Application: MRI MRI stands for Magnetic Resonance Imaging MRI is the biggest market for superconductors
Application: particle acceleration The LHC has a tube 27km in circumference, with superconducting magnets the whole way to speed up charged particles to relativistic speeds tonnes of NbTi superconducting cable at 1.9 K to make up to 8.3 T field
Application: Detectors 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 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 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 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,
Application: Plasma containment Hot plasma has to be contained by a magnetic field. Shown is a Tokamak type fusion reactor.
Superconductors Chemistry Physics Quantum Mechanics Randolph Miller Questions?