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Phonon anomalies predict superconducting T(c) for AlB2-type structures.

Jose A Alarco1, Peter C Talbot, Ian D R Mackinnon

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The Kohn anomaly accurately predicts the superconducting transition temperature (Tc) for AlB2-type structures. This ab initio study validates the Kohn anomaly

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Area of Science:

  • Condensed Matter Physics
  • Materials Science
  • Computational Materials Science

Background:

  • The AlB2 crystal structure is a common motif in materials exhibiting superconductivity.
  • Predicting the superconducting transition temperature (Tc) of materials remains a significant challenge in condensed matter physics.
  • The Kohn anomaly, a feature in phonon dispersions, has been hypothesized to relate to superconductivity.

Purpose of the Study:

  • To investigate the predictive power of the Kohn anomaly for the superconducting transition temperature (Tc) in Mg1-xAlxB2.
  • To explore the relationship between electronic structure, phonon anomalies, and superconductivity in AlB2-type materials.
  • To predict Tc for novel compositions and structures within the AlB2 family.

Main Methods:

  • Utilizing ab initio density functional theory (DFT) to compute phonon dispersions.
  • Analyzing phonon spectra for Kohn anomalies in Mg1-xAlxB2 across various compositions (x).
  • Correlating the magnitude and characteristics of Kohn anomalies with experimentally observed Tc values.

Main Results:

  • A direct correlation was established between the magnitude of the Kohn anomaly and the experimental Tc for Mg1-xAlxB2.
  • Key features of the observed phonon anomalies were found to be linked to the electronic band structure.
  • The Kohn anomaly was successfully used to predict Tc for other known AlB2-type structures and new compositions.

Conclusions:

  • The Kohn anomaly serves as a reliable predictor for Tc in ordered AlB2-type superconductors.
  • Calculations predict a Tc of 63.6 ± 6.6 K for Mg0.5Ba0.5B2, with other Mg1-xBaxB2 compositions also showing potential superconductivity.
  • The end-member BaB2 is predicted to exhibit a high Tc, potentially requiring ~16 GPa pressure for structural stabilization.