The intrinsic microwave absorption coefficient P of alkali halides is well explained by lifetime-broadened two-phonon difference processes, in contrast to energy-conserving twoand three-phonon processes as previously thought. A simple closed-form expression for p with no adjustable parameters gives excellent agreement with the magnitude, frequency dependence, and temperature dependence of P in LiF, NaC1, KBr, and KI. As tu increases from to & to, /3 to co=to, +yz93 the frequency and temperature dependence of p at 293 K Here co, =20 cm ' for NaCl is the frequency difference between the transverse-acoustical and transverse-optical phonon modes at the Brillouin-zone edge along the [111]direction, and y is the combined-phonon inverse lifetime at 293 K. The temperature dependence of the phonon frequencies and other parameters must be included in order to explain the temperature dependence of the experimental results. The present lifetime-broadened results reduce to previous energy-conservation results {apart from smoothing, which explains the lack of sharp structure in p, but otherwise has little effect in general) and agree with experimental results in the region co & co, +y. Previous energy-conserving results disagree with the T and co dependence and magnitude of the experimental results in the region co & co, .
The intrinsic microwave absorption coefficient β of alkali halides is well explained by lifetime-broadened two-phonon difference processes, in contrast to energy-conserving two- and three-phonon processes as previously thought. A simple closed-form expression for β with no adjustable parameters gives excellent agreement with the magnitude, frequency dependence, and temperature dependence of β in NaCℓ, KBr, and KI. As ω increases from ω<ωc/3 to ω=ωc+γ293, the frequency and temperature dependence of β at 293 K (or 1000 K) change from the low-frequency limit β∼Tℓω2 to β∼Thω2+, where 2.0 ≤ ℓ ≤ 2.6 (or 2.2 ≤ ℓ ≤ 4.0) and 1.1 ≤ h ≤ 1.2 (or 0.57 ≤ h ≤ 1.6) for the three materials. Here ωC = 20 cm−1 for NaCl is the frequency difference between the transverse acoustical and transverse-optical phonon modes at the Brillouin zone edge and γ is the combined phonon inverse lifetime at 293 K. The temperature dependence of the phonon frequencies and other parameters must be included in order to explain the temperature dependence of the experimental results. The present lifetime-broadened results agree with previous energy-conservation results (apart from smoothing) and with experimental results in the region ω>ωc+γ. Previous energy-conserving results disagree with the T and ω dependence and magnitude of the experimental results in the region ω<ωc.
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