Chemistry Reference
In-Depth Information
in the circuit, namely the Josephson junction, giving an energy difference
E
=
2
eV
across the junction, so that
+
Et
θ
(
t
)
−
θ
(
t
)
=
θ
(8.70)
L
R
0
Substituting this into eq. (8.66a) we therefore deduce that a DC voltage,
V
,
across a Josephson junction leads to an AC current flow,
2
eVt
j
(
r
,
t
)
=
j
0
sin
(θ
+
)
(8.71)
0
with angular frequency
ω
=
2
eV
/
and frequency
ν
=
2
eV
/
h
. Measur-
ing
for a given applied voltage has allowed the determination of the
ratio of the fundamental constants
e
and
h
to 1 part in 10
7
, and is now
also used as a means of measuring and calibrating voltage standards. The
AC current also leads to the emission of electromagnetic radiation of fre-
quency
ν
. (Energy is dissipated across the junction through the emission
of a photon of energy
h
ν
2
eV
each time a Cooper pair crosses the junc-
tion.) Although the power radiated by a single junction is very low, of
order 10
−
10
W, Josephson junctions do nevertheless find some application
as microwave power sources: an applied voltage of order 10
−
4
V leads
to microwave emission about 50GHz (wavelength,
ν
=
, with the
emission wavelength also being tunable through variation of the applied
voltage.
λ
∼
6mm
)
8.14 High-temperature superconductivity
The lure of high-
T
c
superconductors is partly psychological. These materi-
als become virtually perfect conductors when plunged into liquid nitrogen
at 77 K and, before one's very eyes, become capable of levitating a magnet.
The discovery by Bednorz and Müller in late 1986 that the ceramic mate-
rial, lanthanum barium copper oxide, lost all electrical resistance when
cooled to 35 K gained them the Nobel prize in Physics within a year and
unleashed an unparalleled explosion of research activity. Within eighteen
months a wide range of further material combinations had been tested,
leading to the discovery of compounds such as Tl
2
Ba
2
Ca
2
Cu
3
O
10
with
a superconducting transition temperature as high as 125 K. Surprisingly,
all of the high-temperature superconductors discovered to date share a
common crystal structure: they all contain lightly doped copper-oxide
layers, with other metal atoms sitting between these layers. Extensive
research to find high-
T
c
superconductivity in other families of materials
has been singularly unsuccessful. The cuprate family of materials con-
tinues, therefore, to be of immense theoretical and experimental interest.
Even a decade and a half after its discovery, the mechanism underpinning
