The measurement of the Hall effect in single crystals of iodine and in an iodine complex by means of an AC technique

Abstract

An apparatus for the measurement of the Hall effect in materials of low mobility and high resistivity is described. An ac technique is used: the sample is rotated in a static magnetic field at 20 cps, an electric field is applied at 13 1/3 cps, and the Hall signal, preamplified by field-effect transistors mounted on the rotating shaft near the sample, is synchronously detected at 33 1/3 cps. Hall mobilities as small as 0.2 cm²/v-sec in samples of resistance up to 10⁹ ohms can be measured with negligible detector loading. A discussion is also given of the fundamental limitations of Hall effect measuring equipment. Typical Hall mobilities in iodine complexed 1:1 with poly-n-vinyl carbazole are reported. Extensive measurements of the Hall mobility in single crystals of iodine are presented. The Hall mobility in the ac crystallographic plane of iodine was found to vary rapidly with temperature, exhibiting a maximum of 460 cm²/v-sec at 1°C corresponding to a negative Hall coefficient, a Hall zero at 23°C, and a constant mobility of 4 cm²/v-sec from 33°C to 40° C corresponding to a positive Hall coefficient. Several interpretations are discussed and a model is proposed to explain the discrepancy between the Hall and drift mobilities on the basis of conduction in a narrow valence band. It is suggested that the data illustrate the transition from conduction characterized by a band model to conduction characterized by a hopping process.