Novel aspects of charge and lattice dynamics in the hole-doped manganite La_0.67Sr_0.33MnO₃

Abstract

Previous infrared studies on the hole-doped manganite La_0.67Sr_0.33MnO₃ (LSMO) have analysed its charge dynamics in terms of one type of charge carrier despite evidence of both electron and hole Fermi surfaces. Here, we investigate the charge dynamics of an LSMO film with infrared and optical spectroscopy in order to provide a complete picture of metallic conduction. In the ferromagnetic metallic phase, the low-frequency optical conductivity is best explained by a two-carrier model comprising electrons and holes. The number densities, effective masses and relaxation response of the delocalized electrons and holes are quantified. We discover that only one-third of the doped charges are coherent and contribute to the dc transport. Metallic LSMO cannot be classified as a bad metal at low temperatures because the mean free path of the coherent, mobile charge carriers exceeds the Ioffe–Regel–Mott limit. The incoherent spectral response of the doped charges manifests itself as a broad mid-infrared feature. We also report the first observation of splitting of an infrared-active phonon due to local Jahn–Teller distortion in the vicinity of the thermally driven transition to the nonmetallic, paramagnetic phase in LSMO. This demonstrates that infrared spectroscopy is capable of detecting the presence of local lattice distortions in correlated electron systems.

Keywords:

• manganite
• metal–insulator transition
• infrared
• optical spectroscopy

Acknowledgement

The authors acknowledge experimental assistance by Chris Jensen in the measurement of the dc resistivity of La_0.67Sr_0.33MnO₃.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the NSF DMR [grant number 1255156]; the Jeffress Memorial Trust; the NSF ECCS [grant number 1128586].