Electron Localization Induced by Disordered Anions in an Organic Conductor

Area of Science:

• Materials Science
• Condensed Matter Physics
• Organic Electronics

Background:

• Organic conductors and superconductors are crucial in materials science.
• Polymorphism in organic salts can significantly alter electronic properties.
• The bis(ethylenedithio)tetrathiafulvalene (ET) molecule is a common building block for such materials.

Purpose of the Study:

• To characterize a new polymorph of an organic conductor, κ″-(ET)₂Cu[N(CN)₂]Br (κ″-Br).
• To elucidate the electronic state and properties of κ″-Br.
• To compare the electronic behavior of κ″-Br with its superconducting counterpart, κ-(ET)₂Cu[N(CN)₂]Br (κ-Br).

Main Methods:

• X-ray crystallography to determine crystal structure.
• Band structure calculations.
• Transport, magnetic, and optical measurements (including electron spin resonance and Raman spectroscopy).

Main Results:

• κ″-Br crystallizes in a monoclinic structure with disordered polymeric anion chains, unlike the orthorhombic κ-Br with ordered chains.
• Band calculations and optical spectra of κ″-Br are comparable to κ-Br.
• κ″-Br exhibits semiconducting behavior, with no magnetic or charge order detected.

Conclusions:

• The semiconducting behavior of κ″-Br is attributed to Anderson localization, likely caused by the disordered anion layers.
• Structural differences, specifically anion chain disorder, are key to the distinct electronic properties compared to κ-Br.
• This study highlights the sensitivity of organic conductor properties to subtle structural variations.