EXPERIMENTAL STUDIES OF QUANTUM OSCILLATIONS IN THE TRANSVERSE MAGNETORESISTANCE OF SINGLE-CRYSTALLINE, ULTRAPURE MAGNESIUM: NON-OHMIC EFFECTS.

Abstract

We report here the observation of non-ohmic behavior in the dominant oscillatory component (the cigar component) of ρ('H) in ultrapure magnesium when the magnetic field, 'H, is parallel to [0001] and the current density, 'J, lies in the basal plane. In order to study this new phenomenon systematically, we had to overcome two experimental problems. The first was the design of an experimental probe which could reproducibly control, at low temperatures, the contact resistance at one of the points where current is injected into the sample without disturbing the sample's orientation with respect to 'H. Special micromanipulators, controlled by helium gas pressure, were designed into the probe for this purpose. The second problem was the construction of a detector which had the sensitivity to measure small signals from the magnesium samples in an effort to investigate the low current regime where the oscillations appear to satisfy Ohm's Law. A superconducting chopper amplifier was built which had the sensitivity to measure 10⁻¹¹ volt signals. We present evidence which directly relates this non-ohmic behavior to the long-range influence of a relatively large contact resistance at a point where current is injected into the sample. Data are presented which indicate that when this non-ohmic behavior is present, the corresponding oscillation amplitudes are proportional to the contact resistance. Measurements are also presented which show that the effects of this local current injection are so nonlocal that they extend over distances which are comparable to the dimensions of the sample.