Chemistry Reference
In-Depth Information
the Fermi energy are free to move through the metal, they also have
a diamagnetic susceptibility associated with this motion. Landau calcu-
lated that for a
free-electron
metal (whose density of states is given by
eq. (6.41)), the diamagnetic susceptibility,
1
3
P
,so
that the net susceptibility for free electrons is then positive and is equal to
2
3
χ
L
, is given by
χ
=−
χ
L
P
. Band structure and related effects modify this result, but nevertheless
it is still found that the susceptibility is temperature independent and is,
for many metals, of comparable magnitude to that predicted by the Pauli
model.
χ
6.9 Floating frogs
It is fascinating to see objects floating without material support or suspen-
sion. This became a familiar sight in the 1980s when pellets of the then
new high-temperature superconductors were levitated above permanent
magnets, and vice versa. We have emphasised throughout this chapter
how the magnetic response of most materials is very weak. It is, therefore,
surprising to find that ordinary diamagnetic objects can also be levitated
in achievable magnetic fields. Figure 6.8 illustrates a frog floating above
a 16 T magnet.
Figure 6.8
A small frog floating above a high field superconducting magnet, due to
diamagnetic repulsion. (Photograph courtesy of A. Geim, University of
Manchester.)
