Anion ordering transition and Fermi surface electron-hole instabilities in the (TMTSF)₂ClO₄ and (TMTSF)₂NO₃ Bechgaard...

Area of Science:

• Condensed Matter Physics
• Materials Science

Background:

• Bechgaard salts, specifically (TMTSF)2X where X = ClO4 and NO3, exhibit complex electronic properties influenced by anion ordering (AO) transitions.
• Understanding the electron-hole Lindhard response function is crucial for characterizing the electronic instabilities and emergent phenomena in these quasi-1D materials.

Purpose of the Study:

• To perform first-principles calculations of the electron-hole Lindhard response function for (TMTSF)2ClO4 and (TMTSF)2NO3 salts.
• To analyze the temperature dependence and structural effects (relaxed vs. quenched) on the electron-hole response, particularly around anion ordering transitions.
• To correlate the calculated Lindhard response with experimental observations like SDW modulation wave vectors and understand its role in superconductivity.

Main Methods:

• First-principles calculations of the electron-hole Lindhard response function.
• Utilizing real triclinic low-temperature crystal structures for accurate modeling.
• Analyzing the evolution of the response with temperature for both relaxed and quenched salt configurations.

Main Results:

• Quenched (TMTSF)2X samples exhibit a low-temperature, curved, and tilted triangular continuum of 2kF response maxima, absent in relaxed samples.
• (TMTSF)2ClO4 in the AO state shows a more quasi-1D response, while relaxed (TMTSF)2NO3 displays a sharp maximum.
• The broad maxima in quenched samples arise from multiple Fermi surface nesting, indicating a wide range of electron-hole fluctuations, with varying Brillouin zone area percentages for different X anions.
• Calculated Lindhard response maxima align well with experimentally determined SDW modulation wave vectors.

Conclusions:

• The distinct electron-hole response in quenched vs. relaxed samples, driven by Fermi surface warping and nesting, influences the range of electronic fluctuations.
• The strong reduction of associated spin-density wave (SDW) fluctuations in (TMTSF)2NO3 may explain the absence of SDW-mediated superconductivity.
• Critical AO wave vectors occur at minima in the Lindhard response, suggesting they are not driven by electron-hole instabilities.
• First-principles calculations reveal significant 3D effects in the Lindhard response at low temperatures, challenging analytical models.