254
QUANTUM WELLS, WIRES, AND DOTS
. 6
5
.- b
n
E 102:
c
e
a,
c
L
9
F
0
c
(0
1
E
- As Cleaved, L, = 5040 pm
- pulsed
;
f '
/'
/ 0 /
l o r - -
-
- -
- - W = 5 9 p m
W = 2 9 p m
I
I
I
I
I
I
magnetic field B,,, can penetrate into a pure type I superconductor. Magnetic flux,
that is, B fields, are excluded from the bulk of a type I superconductor, and applied
magnetic fields that exceed the critical field B, cause the superconductor to revert to
being a normal metal. Superconductivity is present in a material when pairs of
electrons condense into a bound state called a Cooper pair with dimensions of the
order of the coherence length (. Thus Cooper pairs, the basic charge carriers of the
supercurrent, can be looked on as nanoparticles.
A type I1 superconductor has both a lower and an upper critical magnetic field,
B,, < B,,, and three regions of behavior in an applied magnetic field Bapp. For low
Table 9.6. Transition temperatures Tc, coherence lengths 5 ,
and penetration depths h of some typical superconductors
Cd
In
Pb
P b I n alloy
NbN alloy
V3Si
Nb3Ge
PbMO6Sg
K3C60
I
I
I
I1
I1
I1
I1
I1
I1
0.56
3.4
7.2
7.0
16
15
16
23
19
760
3 60
82
30
5
2
3
3
2.6
110
40
39
150
200
200
60
90
240
Source: Data are from C. P. Poole Jr, H. A. Farach, and R. J. Creswick,
Superconductivi@, Academic Press, San Diego, 1995, p. 271.