  We observe the appearance of a phonon near the lock-in temperature in
orthorhombic REMnO3 (RE: Lu and Ho) and anomalous phonon hardening in
orthorhombic LuMnO3. The anomalous phonon occurs at the onset of spontaneous
polarization. No such changes were found in incommensurate orthorhombic DyMnO3.
These observations directly reveal different electric polarization mechanisms
in the E-type and IC-type REMnO3.
