Magnetic-field-induced far-infrared transmission in bismuth

Abstract

It has been demonstrated that high fields render the semimetal bismuth sufficiently transparent at far-infrared frequencies to study effects not previously seen in reflection experiments. The frequency interval 10–400 cm −1 was explored at fields up to 100 kG using Fourier transform spectroscopic techniques and 3 He-cooled bolometer detectors, and samples were cooled to liquid helium temperatures. Transmission was observed in all three principal crystal directions when the magnetic field was sufficiently large to drive one or more sets of electrons into the extreme quantum limit. The reduced absorptivity at these fields results from a large reduction in the relaxation rate. Efforts were concentrated on the geometry where the magnetic field and Poynting vectors were both parallel to the crystal bisectrix axis, and measurements at the highest fields indicated that the relaxation rate increases as the 1.4±0.1 power of the frequency and that ωτ ≈ 700 at 30 cm −1 . The temperature dependence of the relaxation rate continues to be quadratic at these fields. Three resonances were observed at the hole cyclotron resonance frequency ω c and at 2ω c and 3ω c , and they appear to arise from small departures of the hole Fermi surface from a perfect ellipsoid. Since these harmonics occur in the normal mode where the electric vector and the holes are rotating in opposite senses, they correspond to then=−1, −2, and −3 Fourier components of the motion in a labeling scheme where the cyclotron resonance itself is the much strongern=+1 component. Other resonances were seen, and one was identified as a magnetoplasma resonance.