Why Do We Need Superconductors?

And How Are The Made?

Developed from the talks of: Dr. Athena S. Sefat

AndDr. 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 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 outEnergy Crisis Solved

.

Compact, powerful motors/generators!

Efficient/condensed underground transmission lines!

Why superconductors?

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The Facts• Copper wire can carry 5

Amps/mm2 • Superconductors conduct

electricity with significantly less resistance, but they must be cooled for the effect.

• NbTi can carry 2500 Amps/mm2

• A “low temperature” superconductor because it must be cooled to 4.2 Kelvin using liquid Helium (very expensive)

• YBCO can carry 10,000 Amps/mm2

• A “high temperature” superconductor because it must be cooled to 92 Kelvin using liquid Nitrogen (relatively inexpensive to produce)

Other PossibilitiesFrom

Superconductors

https://www.youtube.com/watch?v=Z4XEQVnIFmQ

- Element Properties are important: reactivity, toxicity, general properties

metal radius(pm) Tmelt(°C) Tboil (°C) D20°C(g/cm3)

Creating a Superconductor

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Ca 197 842 1494 1.55Sr 215 769 1382 2.63Ba 222 729 1805 3.59

Eutectic pts

Congruent reaction

Peritectic reaction

- Phase Diagrams Help to Target the Proper Elements

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** Tetragonal structures may be crucial

** ThCr2Si2-type structure may be important

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. . .

One can explore other Fe-based compounds with this structure:

- The Secret Appears to be in the Crystalline Structure

Fe-based superconductorCuprates

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 (Al2O3) zirconia (ZrO2)

1720 3452 ° F 177019002000

20722700

Tmax (°C) Tmelting (°C)

Alkali & alkaline-earth metals Al, Ga

Ta, steel Al2O3, MgO, BeO

MgCu, Ag,

Au Fe, Co, Ni Zn, Cd,

Hg InRare-earth metals

Bi, SnSb

MgO, Ta, graphite or steel graphite, MgO,

Al2O3, TaAl2O3, ZrO2

Al2O3

Al2O3, TaTa, Mo,

W, BeOAl2O3, SiO2, graphite

SiO2, graphite

Elements Container & tube choices

- Know how to confine reactions!

magnesia (MgO) tantalum

24001400

28523017

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borosilicate glass (Pyrex) 515 820gold 1013 1064silica (quartz) 1200 1853 3370 ° F

1111RFeAsO

TC = 56 K

111, 11AFeAs, FeSeTC ≈ 33, 15 K

Li or Fe

A or BR

O

Sr

T O

42622Sr4T2O6Fe2As2

TC ≈ 37 K

122

BFe2As2, AFe2Se2

TC ≈ 38 K, 45 K

La-214 La2CuO4

TC = 40 KHg-1223

HgBa2Ca2Cu3O9+δ

TC = 133 K

Ba

Y

Ba

Ca

Tl

Tl-2223Tl2Ba2Ca2Cu3O10

TC = 125 K

Y-123YBa2Cu3O7-δ

TC = 92 K

Ca

BaHg

La

FeCu

Superconducting Materials have Complex Structures

Sefat, et al. MRS Bulletin 36 (2011), 614.

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Normal Conductors

Superconductors

LaAs + x/3 Co3O4 + (3-4x)/9 Fe2O3 + (3-x)/9 Fe LaFe1-xCoxAsO

e.g. 1220 °C (2230 °F)!

Example:

Weigh accurately

Seal into a silica tubeHeat strongly

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fc

zfc

0

T (K)

χ (c

m3 /m

ol)

0T (K)

ρ (

mO

hm c

m)

A research workflow!

wires

mm to cm

Our Superconductivity Research

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(a) Decision on a crystal structure

(b) Synthesis

(c) Characterization

The Science of Low Temperature

Superconductors

Low Temperature Superconductivity

Tc

At Normal TemperaturesElectrons in Conductors move independentlyResistance within the conductor is due to the scattering.

Superconductivity is enabled by the Cooper

dance

Cooper Pair Flash Mob

Superconductivity = Cooper dance

Tc

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 away4. 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

Bardeen

Cooper

Schrieffer

140

100

60

20

LiquidHe

Tem

pera

ture

[K]

1920 1960 19801940 2000

Nb3Ge

MgB2 2001

Nb3SuNbN

Nb

HgPb

NbCV3Si

Low temperature BCS

BCS Theory

Bednorzand Müller

TIBaCaCuO 1988

HgBaCaCuO 1993

HgTlBaCuO 1995

BiSrCaCuO 1988

La2-xBaxCuO4 1986

YBa2Cu3O7 1987

High temperaturenon-BCS

Liquid N2

?

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-Tc

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 crossoverPlasmon

s

Spin liquids

Interlayer Coulomb

Interlayer tunneling

van Hove singularities

Marginal Fermi liquid

Anyon superconductivity

Phil Anderso

n Alexei Abrikosov

Bob Schrieffe

r

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.