Electricity and Magnetism

19–128 Electricity and Magnetism

The [linear] temperature coefficient of resistivity, α, may be positive, as one might expect,

or negative, which is rather a surprise.

1

The behaviour at low temperatures is widely

divergent in these two cases.

ρ

T

ρ

res

α > 0

ρ

T

ρ

res

α < 0

FIGURE 19.2 Positive- and Negative-Alpha Materials

The precise shapes of the resistivity curves shown in Figure 19.2 are material-dependent

and dictated by the degree to which the free electrons scatter with the lattice, phonons,

and each other. In the α > 0 case, naive extrapolation [indicated by the dashed line] would

suggest that the resistivity declines to zero at some value of temperature. ...