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

2. 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

3. 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

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

5. Temperature Conversion
Kelvin (K):
K = C
K = 5/9 F
Fahrenheit
Celsius
Kelvin
comments
212
100
373.15
water boils 32
273.15
water freezes
77.36
liquid nitrogen boils
4.2
liquid helium boils
absolute zero

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

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

8. Magnetic Fields Magnet has two poles: North and South
Like poles repel, unlike poles attract
Detect magnetic field
 iron filings
© John Wiley & Sons, Inc.

9. 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. =

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

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

12. 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

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

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

15. 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!!

16. 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)

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

18. RIVALRY SATURDAY!
GO BLUE!