Magnetoresistance of Thallium in Large Magnetic Fields

We have measured the transverse magnetoresistance of a c-axis crystal of pure thallium at liquid-helium temperatures and in magnetic fields up to 150 kG (at the National Magnet Laboratory). At 1.2^∘{}K the resistance has saturated at this field for all directions in the basal plane. The field dependence of the resistance is correctly simulated by a simple model of orbits which become coupled by magnetic breakdown. By fitting the experimental results with the predictions of the model, we are able to estimate values of the breakdown field H₀, the largest value encountered being 220 kG. The model also correctly predicts the change of resistance with mean free path between 4.2 and 1.2^∘{}K.