Magnetic and electrical measurements have been made of the effect of impurity on the transitions to superconductivity in tin. Reproducible results were obtained only with well-annealed monocrystalline specimens. Solution of up to 6 % indium in pure tin decreases the electronic mean free path
l
from about 3 x 10<super>-3</super> to 3 x 10<super>- 6 </super>cm, and over this range magnetic measurements show that there is only a small depression of the transition temperature
T
c
and a small alteration in the critical field curve of
H
c
and
T
. Electrical measurements show that if
l / > l
c
, where
lc;
— 8 x 10<super>-6</super> cm, the resistance transitions are sharp and almost concurrent with the magnetic transitions. However, if
l<l
c
superconducting nucleation apparently occurs, since a state of partial superconductivity exists with zero resistance, but no exclusion of magnetic induction, in fields greater than
H
c
but less than
H'
c
, where it has been found that at any one temperature HJH'C — This relation describes in broad outline the dependence of
H'
c
on
l
and temperature, although the interpretation of the results is complicated by considerable broadening of the resistance transitions and the appearance of a sensitive non-linear dependence on the measuring current of the temperature of nucleation. These complicating effects may wholly or partly be due to inhomogeneities in indium concentration. The concept of a range of coherence g of the superconducting phase is used in formulating the thermodynamic conditions for the formation in a magnetic field of superconducting nuclei with cylindrical and spherical symmetry. It is shown that the main features of superconducting nucleation in homogeneous tin-indium alloys can be accounted for if g-2 A 0l where t= T/Tc T(i and A0 is the penetration depth at 0°K. The implication that g greatly exceeds
/
just below is supported by a consideration of the sharpness of resistance transition and the shape of the critical field curve near
T
c
. The formula for g resembles that given in Pippard’s phenomenological theory of superconductivity (1953).
Phenomenological theory of Superconductivity in the frameworks of QCD and
RCD
