Chemistry Reference
In-Depth Information
but were not on their own sufficient to confirm d-wave symmetry, as they
could also have been consistent with a modified s-like state.
The key feature of the d-wave order parameter is that its phase varies
with direction, being of opposite sign along the
x
- and
y
-axes in fig. 8.18.
The first tests for d-wave symmetry, probing the angular dependence of
the energy gap, were insensitive to this phase variation, and so their results
were suggestive but not conclusive. We sawearlier in Sections 8.11 and 8.12
that themagnetic flux through a closed loop depends on the total change in
phase around the loop, and equals
n
0
for a conventional superconductor,
where
2
e
. The magnetic flux linking a closed loop turns out to be
a very useful probe of the order parameter symmetry, and has provided
the clearest evidence so far for unconventional d-wave symmetry. Consider
a superconducting circuit formed by linking an s-wave superconducting
element with a d-wave element, as shown in fig. 8.19. The superconduct-
ing order parameter must vary continuously round this circuit. With zero
=
h
/
0
s-wave superconductor
Magnetic flux
Interface a
Interface b
d
x
2
-
y
2
high
T
c
superconductor
Figure 8.19
Geometry for a superconducting circuit with two weak ( Josephson) links
between a d
x
2
−
y
2
high-
T
c
superconductor and a conventional s-wave
superconductor. With zero external field and no current flow, the super-
conducting phase,
θ
, is constant in the s-wave superconductor (illustrated
here as
θ
=
0), but changes by
π
between the a and b faces of the high-
T
c
superconductor. The phase discontinuity of
π
indicated at the b interface
is inconsistent with the general assumption that phase changes continu-
ously. For continuous variation of phase, we therefore require a current
flow, and conclude that the magnetic flux linking such a loop must equal
(
n
+
1
2
)
0
). (from James Annett,
Contemporary Physics
36
433 (1995) ©
Taylor & Francis.)
