As. Alexandrov et Am. Bratkovsky, Massive spectral weight transfer and colossal magneto-optical effect in doped manganites, J APPL PHYS, 87(9), 2000, pp. 5016-5018
We calculate the value of the Frohlich electron-phonon interaction in manga
nites, cuprates, and some other charge-transfer insulators and show that th
is interaction (similar to 1 eV) is much stronger than any relevant magneti
c interaction. This suggests that carriers in those systems are small (bi)p
olarons at all temperatures and doping levels, in agreement with the oxygen
isotope effect and other data. As these materials are cooled below the Cur
ie temperature, the colossal magnetoresistance (CMR) is accompanied by a ma
ssive transfer of the spectral weight of the optical conductivity to lower
frequencies. As with the CMR itself, this change in the optical conductivit
y is explained by the dissociation of bipolarons into small polarons by the
exchange interaction with localized Mn spins during the transition. The co
rresponding current carrier density collapse in doped manganites leads to a
colossal change of the optical conductivity in an external magnetic field
at temperatures close to the ferromagnetic transition, in agreement with av
ailable experimental observations. (C) 2000 American Institute of Physics.
[S0021-8979(00)53808-0].