Charged magnons and magneto-elastic polarons in the mid-infrared spectrum of YBa Cu O

The optical conductivity of undoped and low-doped YBa Cu O is studied in detail in the mid-infrared range. Full 2 3 6 16 18 Ž . Ž . isotope substitution of O with O is used to a study the optical phonons in the far-infrared and b identify the rich structure emerging in the mid-infrared from multiphonon and bimagnon-plus-phonon absorption. Substitution of Zn in the CuO planes allows direct observation of bimagnon excitations. Very light chemical doping with oxygen introduces 2 localized charge carriers in the form of magneto-elastic polarons, i.e., holes which are strongly coupled both to the vibrational and magnetic degrees of freedom. q 1999 Elsevier Science B.V. All rights reserved.