Continuous Line-Narrowed, Coherent Radiation from PumpedBi92Sb08Nuclear Electric-Quadrupole Levels

We report the observation of strong line narrowing of microwave radiation (870 MHz) emitted from a continuously operated, dc-pumped ${\mathrm{Bi}}_{92}$ ${\mathrm{Sb}}_{08}$ semiconducting alloy at 1.6 K. Bandwidths of less than 1 kHz are observed. The origin of the line narrowing is thought to be a maser effect involving the $\frac{3}{2}\to \frac{1}{2}$ nuclear electric-quadrupole transition of the ${\mathrm{Bi}}^{209}$ nucleus. A new dc pumping mechanism is proposed to explain the effect. In this effect the nuclei are pumped by electronic spin levels, saturated as in the Overhauser effect, except that saturation is achieved by carrier avalanche, instead of rf pumping.