“““Kings of Cool” Who are these people? SUPERCONDUCTORS

An Introduction

by

Prof George Walmsley Normal conductor eg copper

• Current, I. • Voltage drop, V. • Resistance, R = ? • Ans: V/I = R eg 2 Volts/1 Amp = 2 Ohms

I Copper I

V Normal conductor eg copper

• Source of resistance: • Electron collides with lattice ion to produce heat (). Copper lattice Lower Temperature

• What happens when we cool a metal?

• Ans 1: The electrons slow down and current is reduced maybe to zero. R→∞

• Ans 2: The lattice stops vibrating and resistance disappears. R=0 How do we cool things?

• Commonly used liquid refrigerants:

Element Boiling Pt Oxygen 90K Nitrogen 77K Hydrogen 20K Helium 4.2K Thomas Andrews, Chemist

• 9 Dec 1813 – 26 Nov 1885 • John (Flax spinner, Comber) [ggfather] • Michael (Linen, Ardoyne) [gfather] • Thomas (Linen merchant) [father] • Studied under James Thomson, RBAI • 1828 Univ of Glasgow, Thos Thomson • 1830 Paris, Dumas • 1830-34 Trinity College Dublin • 1835 MD U of Edinburgh • 1835-45 Prof of Chemistry RBAI • 1845 Vice-President, Queen’s College • 1847 Prof of Chemistry, Queen’s College

• 1869 Bakerian Lecture on CO 2 • 1871 Visit by Dr Janssen of Leiden

• Photo: Paris 1875 Andrews’ Isotherms

• Note critical temperature NORMAL CONDUCTOR: Electrical properties

Normal metal eg copper Resistance and (resistivity, ρ) >0 As temperature falls ρ falls smoothly too:

ρ

0 100 200 273.15 Temperature/K SUPERCONDUCTOR: Electrical properties

Superconductor eg mercury, lead Resistivity ( ρ) >0 like normal metal down to critical temperature, Tc At Tc resistivity drops abruptly to zero ( ρ =0). Dramatic effect . First seen by Kamerlingh Onnes in Leiden on 8 April 1911 SUPERCONDUCTORS: Electrical resistivity of metals at room temperature

Superconductors Non-superconductors • Al 2.74 X 10 -8 Ω-m • Cu 1.70 X 10 -8 Ω-m • In 8.75 • Ag 1.61 • Hg 95.9 • Au 2.20 • Pb 21.0 • Na 4.75 • Nb 14.5 • K 7.19

Question: • Source: C Kittel Introduction to Do you see any trend or tendency? State Physics (5ed) p 170 SUPERCONDUCTORS: Cooper pairs

Electrons pair off like hydrogen atoms in a molecule (H 2). Electrons pairs don’t scatter off anything (Cooper pairs).

If you search the internet you may find dynamic illustrations. (Those shown below are a different understanding of the problem.) Superconductor: Cooper (electron) pair SUPERCONDUCTORS: Magnetic properties

• Normal metals: only weakly (para)magnetic (Fig a)

• Superconductors (below Tc): expel magnetic flux (except from a surface layer in which persistent screening currents flow). () This is known as the MEISSNER Effect (Fig b)(1933)

Above Tc they are like normal metals.

SUPERCONDUCTORS: London Bros

• Heinz and Fritz London, 1953 Cambridge, photo: K. Mendelssohn

• Normal metal: J = E / ρ • Superconductor: J = - A / Λ SUPERCONDUCTORS: More magnetic properties

Type I Examples: simple metals eg mercury, tin, lead, aluminium, indium

Behaviour: perfect diamagnets up to critical field, H c.

Lev Shubnikov 1936

Type II Examples: alloys eg solder and HTc’s (see below)

Behaviour: perfect diamagnets up to lower critical field, H c1 . For H c1

• Alex Mueller and (IBM Zurich) 1986

found an oxide with T c around 35K and later Paul Chu (Houston) achieved 90K in a related material, YBa 2Cu 3O7-δ. Record 135K. SUPERCONDUCTORS: Theory I

• 1934 Thermal properties: Gorter and Casimir • 1935 Electrical properties: Fritz and Heinz London • 1950 Magnetic properties (Types I/II): Ginzburg and Landau

• 1957 Bardeen, Cooper and Schrieffer (BCS) gave comprehensive microscopic theory of conventional superconductors involving lattice vibrations.

Nobel Prize Day 1972

• ? Theory of high temperature superconductors: maybe you? • ? Find room temperature superconductors: maybe you too? SUPERCONDUCTORS: Theory II

Brian Josephson

Showed theoretically that Cooper pairs can tunnel between two superconductors in weak contact.

At the time he was a student. Thomas Andrews, Shipbuilder

• 7 Feb 1873 – 15 Apr 1912 • Born Ardara House, Comber • Parents Thos ANDREWS and Eliza PIRRIE • Brother John Miller ANDREWS, PM NI • Brother James ANDREWS, L C Justice NI • 1884 – 1889 RBAI • 1889 -1912 H & W SUPERCONDUCTORS: Nobel Prizes

1913 Matter at low temperature 1972 , Leon N. Cooper, J. Robert Schrieffer Theory of superconductivity 1973 (,) , Brian D. Josephson Tunneling in superconductors 1987 Georg Bednorz, Alex K. Müller High-temperature superconductivity 2003 Alexei A. Abrikosov, Vitaly L. Ginzburg(, Anthony J. Leggett) Pioneering contributions to the theory of superconductors and superfluids . SUPERCONDUCTORS : Websites

• Try Google: Superconductor • http://www.superconductors.org • http://www.en.wikipedia.org/wiki/Superconductivity • http://www.ornl.gov/info/reports/m/ornlm3063r1/contents.html (A teacher’s guide, 1994) • http://teachers.web.cern.ch/teachers/archiv/HST2001/accelerato rs/superconductivity/superconductivity.htm • http://www.newworldencyclopedia.org/entry/Maglev_train#Shan ghai_Maglev_Train • http://www.en.wikipedia.org/wiki/SQUID • http://hyperphysics.phy-astr.gsu.edu/hbase/solids/squid.html