Tunnel Diode - Quantum Device Lab

Meissner Effect
- superconductors are perfectly diamagnetic
- in a type I superconductor field below the
critical field is expelled completely from the
material when cooled through T c , see figure
- in this Meissner effect screening currents
are induced in the superconductor to cancel the
externally applied field
- this effect distinguishes a superconductor from an ideal conductor
- type II superconductors below a first critical field Bc1 behave like type I superconductors,
above B c1 and below a second critical field B c2 magnetic flux can penetrate into the material
bringing it to a mixed superconducting/normal state
- B c2 critical fields can be high so that these
materials are interesting for generating
magnetic fields
material T c [K] B c2 [T]
Nb 3 Sn 18.0 24.5
phys4.19 Page 9
Cooper Pairs and Bardeen-Cooper-Schrieffer (BCS) theory
- in conventional superconductors electrons attract each other through deformations induced
in the crystal lattice
- materials with strong lattice vibrations are usually poor conductors at room temperature but
maybe superconductors at low temperatures
- a hint of this fact was first found when it was noted that the T c of different superconductors
depends on the isotope used, e.g. T c ( 199 Hg) = 4.161 and T c ( 204 Hg) = 4.126
- two electrons (Fermions) form a single Cooper pair (Boson) with the electrons being in a
singlet state with zero angular momentum
- the binding energy E g , also called the gap energy, is
typically on the order of 1 meV and can be measured using
microwave absorption
- at temperatures above 0 K some Cooper pairs are broken up
by thermal fluctuations, the remaining electrons interact
with the Cooper pairs effectively reducing the gap energy
(see figure)
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