SuperconductivitySuperconductivity Dr. Viswanath Tanikella

Conductors 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 © John Wiley & Sons, Inc.

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

Extreme Low Temperature Kelvin ( ): electrons freeze and resistance increases Onnes ( ): Resistance drops to zero

Temperature Conversion Kelvin (K): –K = C –K = 5/9 F Fahrenhei t CelsiusKelvincomments water boils water freezes liquid nitrogen boils liquid helium boils absolute zero

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

Conductors vs. Superconductors Normal conductors: = 0 at T=0 Superconductors: = 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.

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.

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

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

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

Superconductor: YBa2Cu3O7 Tc ~ 90 K (5.90 mV) Voltmeter: measure voltage across superconductor V = I R Demo 0 0

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

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

Examples of Type I Mostly elements Highest Tc ~ 22 K (Phosphorus) Highest Hc ~ 800 G (lead) ElementTc (K) Mercury4.153 Lead7.193 Aluminum1.196 Tin3.722 Zinc0.85 Titanium0.39 Phosphoru s 14~22 (pressure)

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