What Puts the Super in Superconductors? Saturday Morning Physics November 22, 2003 Dr. Sa-Lin Cheng Bernstein

Conductors © John Wiley & Sons, Inc. Outer electrons of the atoms in conductors are loosely bound and free to move through the material FREE electrons Metals are conductors Energy is carried by charge from power plant to appliances

Resistance Life is tough for free electrons Resistance: Repulsion from other electrons Vibration of atoms Impurities Life is tough for free electrons, especially on hot days Energy is wasted http://regentsprep.org/Regents/physics/phys03/bresist/default.htm

Extreme Low Temperature Kelvin (1824-1907): electrons freeze and resistance increases Onnes (1853-1926): Resistance drops to zero

Temperature Conversion Kelvin (K): K = C + 273.15 K = 5/9 F + 255.37 Fahrenheit Celsius Kelvin comments 212 100 373.15 water boils 32 273.15 water freezes -300.42 -195.79 77.36 liquid nitrogen boils -452.11 -268.95 4.2 liquid helium boils -459.67 -273.15 absolute zero

Discovery of Superconductivity Heike Kamerlingh Onnes 1908 - liquefied helium (~4 K = - 452°F ) 1911- investigated low temperature resistance of mercury 1913 - Nobel Prize in physics

Conductors vs. Superconductors Normal conductors: r=r0 at T=0 Superconductors: r=0 at T<Tc (superconducting state) Tc = critical temperature

Magnetic Fields Magnet has two poles: North and South Like poles repel, unlike poles attract Detect magnetic field  iron filings © John Wiley & Sons, Inc. http://www.school-for-champions.com/science/magnetism.htm

Electromagnet Current flowing in a loop of wire creates a magnetic field Current loop can be imagined to be a phantom bar magnet © John Wiley & Sons, Inc. = http://www.windows.ucar.edu/spaceweather/info_mag_fields.html

Which side is north pole? © John Wiley & Sons, Inc. Right hand rule N

Lenz’s Law A conductor opposes any change in externally applied magnetic fields. N S N S Induced current = N S

Meissner Effect 1933 – Walther Meissner and Robert Ochsenfeld T<Tc: external magnetic field is perfectly expelled from the interior of a superconductor Strong external magnetic fields can destroy superconductivity http://www.jsf.or.jp/sln/aurora_e/step2.html

Demo Superconductor: YBa2Cu3O7 Tc ~ 90 K (5.90 mV) Voltmeter: measure voltage across superconductor V = I R

Superconducting State T<Tc H<Hc Normal State Superconducting State T Hc Tc H Hc = critical magnetic field

Type I Superconductors Perfect diamagnetism Strength of diamagnetism increases linearly as the applied field increases When the applied field = Hc Field uniformly crosses sample QUENCH!!

Examples of Type I Mostly elements Highest Tc ~ 22 K (Phosphorus) Mercury 4.153 Lead 7.193 Aluminum 1.196 Tin 3.722 Zinc 0.85 Titanium 0.39 Phosphorus 14~22 (pressure) Mostly elements Highest Tc ~ 22 K (Phosphorus) Highest Hc ~ 800 G (lead)

The “But”. . . Tc too low (highest: 22K) Hc too low (highest: 800 G) Little potential for applications Discouraged!!!

RIVALRY SATURDAY! GO BLUE!