Polaronic Quasiparticles in the Valence-Transition Compound TmSe_{1-x}Te_{x}

Area of Science:

• Condensed Matter Physics
• Materials Science
• Spectroscopy

Background:

• Mixed-valent compounds are predicted to host exotic quasiparticle states during valence transitions.
• Identifying these exotic states spectroscopically has been a significant challenge in condensed matter physics.

Purpose of the Study:

• To provide clear spectroscopic evidence for exotic quasiparticle states in mixed-valent compounds.
• To investigate the nature of quasiparticle excitations across a semimetal-insulator transition in TmSe_{1-x}Te_{x}.

Main Methods:

• Utilized synchrotron-based hard x-ray and extreme ultraviolet photoemission spectroscopy.
• Analyzed Tm 3d and 4f emissions in TmSe_{1-x}Te_{x} samples with varying Te concentrations.

Main Results:

• Observed a Te concentration-dependent semimetal-insulator transition coinciding with a valence transition.
• Spectroscopic results tracked this combined transition, revealing a noninteger valence in the insulating phase.
• Identified a novel quasiparticle excitation, a Holstein polaron, in the semimetallic phase.

Conclusions:

• The findings provide the first clear spectroscopic identification of exotic quasiparticle states in mixed-valent compounds.
• The novel Holstein polaron excitation observed extends beyond the predictions of the standard periodic Anderson model.
• This research offers new insights into the complex electronic behavior of mixed-valent materials near quantum critical points.