NaCl has , and . Someone argues: "Between and the ions in NaCl are driven strongly…

PHYSICS · Subjective · Fermi University Physics · Condensed Matter, Materials & Soft Matter · Dielectric and optical properties of solids · Dielectric and optical properties of solids

Solid State Physics, Ch Collective and coupled excitations: plasmons, polaritons and polarons

Problem

NaCl has ωTO/2π=4.9 THz\omega_{TO}/2\pi = 4.9\,\mathrm{THz}, ωLO/2π=7.9 THz\omega_{LO}/2\pi = 7.9\,\mathrm{THz} and ε∞=2.34\varepsilon_\infty = 2.34. Someone argues: "Between ωTO\omega_{TO} and ωLO\omega_{LO} the ions in NaCl are driven strongly out of phase with the incident infrared field and absorb it very effectively. That is why the reflectivity of NaCl drops to almost zero across this band, and this near-total absorption is what is meant by the reststrahlen band." Find the error, and support the correction with a numerical check at the midpoint frequency ω=(ωTO+ωLO)/2\omega = (\omega_{TO}+\omega_{LO})/2, using the lossless ionic model ε(ω)=ε∞(ωLO2−ω2)/(ωTO2−ω2)\varepsilon(\omega) = \varepsilon_\infty(\omega_{LO}^2-\omega^2)/(\omega_{TO}^2-\omega^2).

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