Chemistry Reference
In-Depth Information
(a)
(b)
(i)
E
F
Energy
E
F
Energy
eV
eV
(ii)
Tunnelling
I
I
eV
=2
∆
(iii)
Figure 8.13
Comparison of I-V characteristics of (a) two normal metals, and (b) two
superconductors separated by a thin insulating layer. (a)(i) When
V
=
0,
the two metals share a common Fermi energy,
E
F
, and there is no net flow
of carriers. (a)(ii) When
V
=
0, the voltage is dropped mainly across the
resistive insulating layer, and carriers can tunnel from filled (shaded) states
in one metal to empty states in the second metal, leading (a)(iii) to a linear
increase in current,
I
, with applied voltage,
V
. (b)(i) For
V
=
0, the two
superconductors also share a common
E
F
. (b)(ii) For small applied voltage,
(
eV
<
2
), the voltage drop across the insulating layer is insufficient to
align filled (Cooper pair) states in one superconductor with empty (single
particle) states in the second, so that (b)(iii) the current only starts to
increase significantly when
eV
>
2
.
tunnel from the filled states in the first superconductor through to the
energy gap of the second. The tunnelling current is observed to switch
on sharply at an applied voltage,
V
such that
eV
, at which point
electrons can tunnel from the filled (Cooper pair) states in the first
superconductor through to the empty (single-particle) states in the
=
2
