Colloque

The Galvanomagnetic effects in metals under extreme conditions

D. K. C. MacDonald

Membre a labase

D. K. C. MacDonald

Résumé du colloque

In a metal of conductivity σ the electric current density, J, is related to the electric field, E, through J=σE. In the case of a cubic metal (such as the alkalis or group IB metals) G is a scalar and the current everywhere follows the lines of electric field. When a magnetic field is applied, the Lorentz force qv x H/c acts on the charge-carriers q (usually electrons) tending to displace the lines of current flow. The initial displacement of current flow generally gives rise to an electric field tending to oppose further displacement; this electric field is the Hall field, EH, whose magnitude is of the order (ℓ/R) E where ℓ is the electron mean free path and R the radius of curvature of a free conduction electron. At the same time a change of resistance, the magneto-resistance, occurs which is also a function of (ℓ/R). Now at room temperature in sodium, for example, ℓ/R is only of order 10-2 in magnetic fields as high as 20,000 gauss and these effects are small; in particular, it is generally assumed (cf. e.g. Smythe (1939), p. 227) that one may neglect entirely any distortion of the current flow pattern by the magnetic field. In liquid helium, however, (≈ 4.2°K) we can achieve values of ℓ/R in very pure sodium in excess of 50, and the galvanomagnetic effects then become dominant and very striking. Recent experiments (cf. also MacDonald 1956a, 1956b) and the new problems presented thereby will be reviewed.