The generation of microwave phonons for studying the spin-lattice interaction∗

Abstract

We have produced microwave phonons and studied their interaction with paramagnetic impurity centers in quartz. Phonon packets were produced by the piezoelectric effect at the end surface of a quartz rod placed in the electric field of a re-entrant cavity. They propagated down the rod, were reflected from the opposite end, and were detected at the generating surface by the inverse effect. The amplitude envelope of the echoes consisted of a series of maxima and minima, or beats, superposed on an exponential decay. From the decay, the average lifetime of the coherent phonons was found to be 10 −5 sec. The t = 0 intercept of the envelope measured the square of the fraction of the microwave power converted into acoustic power, which was found to be smaller by a factor of 10 −3 than the computed value of 0.7 × 10 −6. This discrepancy, together with the departure from an exponential decay, is due, principally, to interference effects arising from a slight nonparallelism of the end surfaces. The absorption of phonons by spins produced in the natural quartz by gamma irradiation from a Co 60 source was detected as a partial saturation of the spin electron paramagnetic resonance. The partial saturation, S/ S 0, was found to be 0.75 at 1.7°K, 0.75 at 4.2°K, and 0.92 at 20°K,the error in S/ S 0 falling between 2 and 5 per cent. These values for S/ S 0 were independent of the difference between the microwave and phonon frequencies within 20 Mc/s, and at a given temperature were independent of the microwave power. From microwave saturation measurements, the order of magnitude of the spin-lattice relaxation time T 1 was found to be greater than 3 sec at 1.7°K, 0.3 sec at 4.2°K, 3 × 10 −4 sec at 35°K, and 3 × 10 −6 sec at 77°K. The spin-resonance absorption, which was measured as a function of microwave power, saturated in such a manner as to indicate partial inhomogeneous broadening, and cross relaxation took place between the closely spaced, narrow (approximately l G wide) lines of the spectrum. These two effects prevented quantitative determination of the spinphonon transition probability.