Page 410 - Electrical Properties of Materials

392 Superconductivity

very, very small. Obviously, what we need is not a single Josephson junction
but an array of Josephson junctions. That may indeed be the solution, but
then one has the problem of how to construct the array and, when we get
it, how to synchronize the oscillations from the many elements. However,
as it happens, we don’t need to worry about the way we construct the array.
Nature has very kindly provided not only the elements but the whole array.
Intrinsic Josephson junctions form naturally between the superconducting
CuO 2 layers in cuprates such as BSCCO (to be discussed in Section 14.9),
with bismuth oxide and strontium oxide layers acting as the Josephson-type
tunnel barriers. A device of 0.1 mm thickness contains about 70 000 such
junctions. Put them in a cavity and hope for the best. Experiments so far
have yielded power in the tens of nanowatts region. If a few microwatts
were available in the THz range, where oscillators hardly exist, that might
very well turn out to be a practical proposition.
3. A direct transition may be caused between the Josephson characteristics
and the ‘normal’ tunnelling characteristics by the application of a small
magnetic field (Fig. 14.19).
4. When two Josephson junctions are connected in parallel [Fig. (14.20)] the
maximum supercurrent that can flow across them is a periodic function of
the magnetic flux,
I J is a constant depending on the
junction parameters, is the en-

closed magnetic flux, and 0 is π
cos . (14.74)
the so-called flux quantum equal I max =2I J 0
to h/2e =2 × 10 –15 Wb.
I
Fig. 14.19
The current as a function of voltage
for a junction which may display both
‘normal’ and Josephson tunnelling.
I 0 is the current flowing without any I 0
accompanying voltage. The
application of a small magnetic field
causes a transition between the
Josephson and ‘normal’ tunnelling
characteristics. Once this extra
magnetic field is removed, the voltage 2Δ V
returns to zero. e

I
B

Fig. 14.20
Two Josephson junctions in parallel
Superconducting path
connected by a superconducting path.