Why Do We Need Superconductors? And How Are The Made? Developed from the talks of: Dr. Athena S. Sefat And Dr. Thomas Maier

This photo shows the Meissner effect, the expulsion of a magnetic field from a superconductor in super conducting state. Image courtesy Argonne National LaboratoryArgonne National Laboratory

Why are we interested in Superconductors? Power plants must increase their current to high voltages when transmitting it across country: to overcome energy lost due to resistance. Resistance is to electrons moving down a wire as rocks are in a stream. Imagine if we could find a way to remove resistance; Energy in would equal Energy out Energy Crisis Solved

. Compact, powerful motors/generators! Efficient/condensed underground transmission lines! Why superconductors? 4 Managed by UT-Battelle for the U.S. Department of Energy The Facts Copper wire can carry 5 Amps/mm 2 Superconductors conduct electricity with significantly less resistance, but they must be cooled for the effect. NbTi can carry 2500 Amps/mm 2 A “low temperature” superconductor because it must be cooled to 4.2 Kelvin using liquid Helium (very expensive) YBCO can carry 10,000 Amps/mm 2 A “high temperature” superconductor because it must be cooled to 92 Kelvin using liquid Nitrogen (relatively inexpensive to produce)

Other Possibilities From Superconductors

- Element Properties are important: reactivity, toxicity, general properties metal radius(pm) T melt ( ° C)T boil ( ° C)D 20 ° C (g/cm 3 ) Creating a Superconductor 6 Managed by UT-Battelle for the U.S. Department of Energy Ca Sr Ba

Eutectic pts Congruent reaction Peritectic reaction - Phase Diagrams Help to Target the Proper Elements 7 Managed by UT-Battelle for the U.S. Department of Energy

** Tetragonal structures may be crucial ** ThCr 2 Si 2 -type structure may be important 8 Managed by UT-Battelle for the U.S. Department of Energy One can explore other Fe-based compounds with this structure: - The Secret Appears to be in the Crystalline Structure

Fe-based superconductor Cuprates B. R. Pamplin, Crystal Growth, Pergamonpress (1980). 9 Managed by UT-Battelle for the U.S. Department of Energy - Preparation Methods are Critical

platinum alumina (Al 2 O 3 ) zirconia (ZrO 2 ) ° F T max ( ° C) T melting ( ° C) Alkali & alkaline-earth metals Al, Ga Ta, steel Al 2 O 3, MgO, BeO Mg Cu, Ag, Au Fe, Co, Ni Zn, Cd, Hg In Rare-earth metals Bi, Sn Sb MgO, Ta, graphite or steel graphite, MgO, Al 2 O 3, Ta Al 2 O 3, ZrO 2 Al 2 O 3 Al 2 O 3, Ta Ta, Mo, W, BeO Al 2 O 3, SiO 2, graphite SiO 2, graphite Elements Container & tube choices - Know how to confine reactions! magnesia (MgO) tantalum Managed by UT-Battelle for the U.S. Department of Energy borosilicate glass (Pyrex) gold silica (quartz) ° F

1111 RFeAsO T C = 56 K 111, 11 AFeAs, FeSe T C ≈ 33, 15 K Li or Fe A or BA or B R O Sr T O Sr 4 T 2 O 6 Fe 2 As 2 T C ≈ 37 K 122 BFe 2 As 2, AFe 2 Se 2 T C ≈ 38 K, 45 K La-214 La 2 CuO 4 T C = 40 K Hg-1223 HgBa 2 Ca 2 Cu 3 O 9+ δ T C = 133 K Ba Y Ca Tl Tl-2223 Tl 2 Ba 2 Ca 2 Cu 3 O 10 T C = 125 K Y-123 YBa 2 Cu 3 O 7- δ T C = 92 K Ca Ba Hg La Fe Cu Superconducting Materials have Complex Structures Sefat, et al. MRS Bulletin 36 (2011), Managed by UT-Battelle for the U.S. Department of Energy Normal Conductors

LaAs + x/3 Co 3 O 4 + (3-4x)/9 Fe 2 O 3 + (3-x)/9 FeLaFe 1-x Co x AsO e.g ° C (2230 ° F)! Example: Weigh accurately Seal into a silica tube Heat strongly 12 Managed by UT-Battelle for the U.S. Department of Energy

fc zfc 0 T (K)T (K) χ ( cm 3 /mol) 0 T (K)T (K) ρ (m O h m c m ) A research workflow ! wires mm to cm Our Superconductivity Research 13 Managed by UT-Battelle for the U.S. Department of Energy (a) Decision on a crystal structure (b) Synthesis (c) Characterization

The Science of Low Temperature Superconductors

Low Temperature Superconductivity TcTc At Normal Temperatures Electrons in Conductors move independently Resistance within the conductor is due to the scattering.

Superconductivity is enabled by the Cooper dance Cooper Pair Flash Mob Cooper Pair Flash Mob

Superconductivity = Cooper dance TcTc Electrons form “Cooper” pairs. Cooper pairs synchronize electrons eliminating the scattering affect and removing resistance.

But negative charges repel! e-e- So how can electron pairs form?

+ e- Everything you wanted to know about pair formation … in low-temperature superconductors ++ + e e- e- 1.A lattice structure is formed as a part of the creation of the superconducting material. 2.The first electron deforms the lattice of metal ions (ions shift their position due to Coulomb interaction) 3.First electron moves away 4.Second electron is attracted by lattice deformation and moves for former position of first electron

Duplicating a Nobel Prize Winning Experiment

Low-temperature (conventional) superconductivity is a solved problem ▻ We know that ion vibrations cause the electrons to pair High-temperature superconductivity is an unsolved problem ▻ We know that ion vibrations play no role in superconductivity ▻ We don’t know (agree) what causes the electrons to pair BardeenCooperSchrieffer Liquid He Temperature [K] Nb 3 Ge MgB Nb 3 Su NbN Nb Hg Pb NbC V 3 Si Low temperature BCS BCS Theory Bednorz and Müller TIBaCaCuO 1988 HgBaCaCuO 1993 HgTlBaCuO 1995 BiSrCaCuO 1988 La 2-x Ba x CuO YBa 2 Cu 3 O High temperature non-BCS Liquid N 2 ?

SOLVED The Science of Low Temperature Superconductors is a SOLVED Problem High Temperature Superconductors are still Mysterious.

Complexity in high-temperature superconducting cuprates Low-temperature superconductors behave like normal metals above the transition (to superconductor) temperature High-temperature superconductors display very strange behavior in their normal state ▻ Stripes ▻ Charge density waves ▻ Spin density waves ▻ Inhomogeneities ▻ Nematic behavior ▻ … Many theories have been proposed, most of them are refuted by experiments “If one looks hard enough, one can find in the cuprates something that is reminiscent of almost any interesting phenomenon in solid state physics.” (Kivelson & Yao, Nature Mat. ’08)

(Incomplete) list of theories for high-T c Resonating valence bonds Spin fluctuations Stripes Small q phonons Anisotropic phonons Bipolarons Excitons Kinetic energy d-density wave Orbital currents Flux phases Charge fluctuations Spin bags Gossamer superconductivity SO(5) BCS/BEC crossover Plasmons Spin liquids Interlayer Coulomb Interlayer tunneling van Hove singularities Marginal Fermi liquid Anyon superconductivity Phil Anderson Alexei Abrikosov Bob Schrieffer Tony Leggett Bob Laughlin Karl Müller

If we can solve Room Temperature Superconductors We can generate power from Solar Energy in the Deserts and use it anywhere. We can trim our energy budget while increasing our thirst for energy. Energy generation methods that are not viable become viable.