Optical and acoustic plasmons in the layered material Sr₂RuO₄

Area of Science:

• Condensed matter physics
• Quantum materials research

Background:

• Strange metals exhibit puzzling linear temperature dependence of electrical resistivity.
• Holographic theories predict overdamped plasmons in strange metals due to electronic susceptibility.
• Previous electron energy-loss spectroscopy on cuprates and ruthenates supported these predictions.

Purpose of the Study:

• Investigate collective charge excitations in the layered metal Sr2RuO4 using transmission electron energy-loss spectroscopy.
• Explore plasmon behavior across a range of in-phase and out-of-phase oscillations.
• Compare experimental findings with theoretical predictions, particularly from holographic models.

Main Methods:

• Utilized transmission electron energy-loss spectroscopy (EELS) to study Sr2RuO4.
• Analyzed collective charge excitations, including optical and acoustic plasmons.
• Employed a model for Coulomb interaction in layered systems and compared with resonant inelastic X-ray scattering (RIXS) data.

Main Results:

• Observed well-defined optical and acoustic plasmons in Sr2RuO4, contradicting overdamped predictions.
• Optical plasmons showed quadratic dispersion, while acoustic plasmons exhibited linear dispersion.
• Demonstrated an energy-dependent effective mass, crucial for reconciling EELS and RIXS data.

Conclusions:

• The study challenges holographic theory predictions of overdamped plasmons in strange metals.
• Well-defined plasmons with distinct dispersions exist in Sr2RuO4.
• Correlation effects significantly influence acoustic plasmon velocity at low energies.