Imaginary phonon modes and phonon-mediated superconductivity in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi mathvariant="normal">Y</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">C</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>

Abstract

For ${\mathrm{Y}}_{2}{\mathrm{C}}_{3}$ with a superconducting critical temperature (${T}_{c}$) $\ensuremath{\sim}18\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, zone-center imaginary optical phonon modes have been found for the high-symmetry $I\text{\ensuremath{-}}43d$ structure due to C dimer wobbling motion and electronic instability from a flat band near Fermi energy. After lattice distortion to the more stable lowest-symmetry $P1$ structure, these stabilized low-energy phonon modes with a mixed C and Y character carry a strong electron-phonon coupling to give rise to the observed sizable ${T}_{c}$. Our work shows that compounds with the calculated dynamical instability should not be simply excluded in the high-throughput search for new phonon-mediated superconductors. Moreover, we have studied the phase stability of the $I\text{\ensuremath{-}}43d$ structure by calculating the enthalpy of different structural motifs of binary compounds containing group-IV elements at the 2:3 composition and also exploring the energy landscapes via $ab\phantom{\rule{4pt}{0ex}}initio$ molecular dynamics near and out of the $I\text{\ensuremath{-}}43d$ structure. Our results show that the $I\text{\ensuremath{-}}43d$-type structures with C dimers are preferred in the low to medium pressure range. Because of the wobbling motion of the C dimers, there are many local energy minimums with degenerated energies. Thus, the ensemble average of many $I\text{\ensuremath{-}}43d$-distorted structures with C dimer wobbling motion at finite temperature still gives an overall $I\text{\ensuremath{-}}43d$ structure.

Topics & Concepts