Charge ordering fluctuation and optical pseudogap in La(1-x)Ca(x)MnO(3)

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid-State Chemistry

Background:

• La(1-x)Ca(x)MnO(3) exhibits complex phase transitions influenced by doping (x) and temperature (T).
• Charge ordering (CO) and pseudogap phenomena are key features in understanding its electronic properties.
• Previous studies have explored the interplay between these phenomena, but detailed optical investigations near x=0.5 were limited.

Purpose of the Study:

• To investigate the optical gap (2Δ) associated with charge ordering (CO) in La(1-x)Ca(x)MnO(3) across a range of doping levels (0.48 ≤ x ≤ 0.67).
• To examine the development of the optical pseudogap and its temperature dependence relative to the CO transition temperature (TCO).
• To elucidate the relationship between CO correlation strength and doping concentration, particularly near x = 0.5.

Main Methods:

• Utilized optical spectroscopy to probe the electronic structure and energy gaps.
• Systematically varied doping concentration (x) and temperature (T) to observe spectral changes.
• Analyzed the optical gap (2Δ) and pseudogap features as a function of x and T.

Main Results:

• Observed a surprisingly large ratio of 2Δ/kBTCO (up to 30) for x ≈ 0.5, indicating strong CO effects.
• Found that this ratio rapidly decreases with increasing x.
• Demonstrated a significant enhancement of the optical pseudogap near x = 0.5, with its onset temperature (T*) increasing systematically as x approaches 0.5.
• Observed non-BCS temperature dependence for 2Δ, with substantial magnitudes persisting far above TCO.

Conclusions:

• The results unequivocally indicate systematically enhanced charge ordering correlations as x approaches 0.5.
• Despite the decrease in the charge ordering transition temperature (TCO) with increasing x, the correlation strength remains robust near x = 0.5.
• The enhanced pseudogap and its T-dependence provide crucial insights into the complex electronic phase diagram of La(1-x)Ca(x)MnO(3).