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Wed. Oct 19
Phy107 Lecture 30
From Last Time…
•
Solids are large numbers of atoms arranged in a regular
crystal structure.
•
Each atom has electron quantum states, but
interactions shift the energies.
•
End result is each type atomic electron state (e.g. 1
s
)
corresponds to a broadened ‘band’ of energy levels in
a solid.
•
Band filling determines electrical properties
–
Partially full bands = metal
–
Bands completely full or empty = insulator / seminconductor
•
Substitutional doping of a semiconductor leads to a
material useful in electronic devices.
Wed. Oct 19
Phy107 Lecture 30
Superconductivity
•
Superconductors are materials
that have exactly zero
electrical resistance.
•
But this only occurs at
temperatures below a
critical temperature, T
c
•
In most cases this
temperature is far below
room temperature.
Hg,
mercury
Wed. Oct 19
Phy107 Lecture 30
Brief History of Superconductivity
Discovery in
1911
Theoretical
explanation
in 1957
Hightemperature
superconductivity
1986
Wed. Oct 19
Phy107 Lecture 30
Persistent currents
•
How zero is zero?
• EXACTLY!
•
Can set up a persistent current in
a ring.
•
The magnitude of the current
measured by the magnetic field
generated.
•
No current decay detected over
many years!
Persistent
supercurrent
Flux
lines
Wed. Oct 19
Phy107 Lecture 30
Critical current
•
If the current is too big,
superconductivity is
destroyed.
•
Maximum current for
zero resistance is called
the ‘critical’ current.
•
For larger currents, the
voltage is no longer zero,
and power is dissipated.
Current
Voltage
Critical
current
Wed. Oct 19
Phy107 Lecture 30
Superconducting elements
•
Many elements are in fact superconducting
•
But the critical temperatures are quite low.
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Wed. Oct 19
Phy107 Lecture 30
Elemental Critical Temperatures
Wed. Oct 19
Phy107 Lecture 30
Temperature Scales
•
All these are near 450˚F
•
This is because they are near 0˚ K
(Kelvin)
•
Kelvin is an absolute temperature scale
•
0 K is the coldest temperature possible
–
This is 459.67 ˚F
•
This is because temperature describes
the average internal kinetic energy of
the system.
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 Spring '08
 n/a
 Conductivity, Superconductivity, magnetic resonance, Magnetic levitation, Wed. Oct, Phy107 Lecture

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