Low-Energy Excitations in Quantum Spin Liquids Identified by Optical Spectroscopy

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Magnetism

Background:

• Organic spin liquids on triangular lattices exhibit complex electronic behaviors.
• Mott insulators are characterized by Hubbard band transitions.
• Quantum spin liquid (QSL) states represent exotic phases of matter with fractionalized excitations.

Purpose of the Study:

• To investigate the electrodynamic response of organic spin liquids.
• To identify and characterize low-energy excitations attributed to the QSL state.
• To explore the relationship between magnetic coupling and conductivity in these materials.

Main Methods:

• Electrodynamic response measurements across a wide energy range.
• Optical spectroscopy at low temperatures and frequencies.
• Analysis of conductivity and its dependence on frequency and magnetic coupling.

Main Results:

• Optical spectra are dominated by Hubbard band transitions.
• Distinct in-gap excitations were observed at low temperatures, attributed to the QSL state.
• Enhanced conductivity was found in β^{'}-EtMe_{3}Sb[Pd(dmit)_{2}]_{2} below 175 cm^{-1}, correlating with magnetic coupling (J≈250 K).
• Low-frequency excitations vanish faster than charge-carrier response, forming a dome-shaped band peaked at 100 cm^{-1}.

Conclusions:

• The observed in-gap excitations provide evidence for the quantum spin liquid state in organic materials.
• The enhanced conductivity is linked to the QSL state and magnetic interactions.
• Further investigation into the nature of these excitations (spinons, magnons, disorder) is warranted.