Plasmon-LO Phonon Coupled Modes in Nb-Doped Strontium Titanate

Abstract

We found previously two spectral peaks in a Nb-doped SrTiO ₃ sample (conduction-electron concentration n ∼ 12.0 × 10 ¹⁸ cm⁻³) and assigned them tentatively to “plasmon-LO phonon coupled modes”. In order to confirm this assignment, the 180° light scattering experiment is performed in the samples with different n values. The experimental results strongly support this assignment. The value of the highest limit for a plasma frequency is estimated in each sample at T ∼ 15 K. One of the coupled modes is found to have a property similar to a broad doublet reported in a pure SrTiO ₃ sample.

Keywords:

• Nb-doped SrTiO₃
• plasmon
• broad doublet
• LO phonon
• quantum paraelectrics
• light scattering

PACS numbers:

• 71.45.Gm
• 77.80.-e
• 77.84.Dy
• 78.35.+c

Acknowledgments

The authors express their appreciation to Mr. Takao Ohta for his polishing the samples so as to have the optical grade surfaces, and to Mr. Yasushi Hirata and Mr. Tatsuji Meike for their improvement of the sample cell. The present work has been supported in part by a Grant-in-Aid for Scientific Research (C) from the Ministry of Education, Science, Sports and Culture of Japan, No. 19540325.