Imaginary phonon modes and phonon-mediated superconductivity in Y2C3

TL;DR

This study reveals that imaginary phonon modes in Y2C3 lead to stable structures with strong electron-phonon coupling, explaining its superconductivity, and challenges the exclusion of dynamically unstable compounds in superconductor searches.

Contribution

It demonstrates that compounds with calculated dynamical instability can host superconductivity and explores the phase stability of Y2C3's structures using ab initio methods.

Findings

Imaginary phonon modes are stabilized after lattice distortion.

Strong electron-phonon coupling in the distorted structure explains the observed T_c.

I-43d structures with C dimers are stable under certain conditions.

Abstract

For Y$_2$C$_3$ with a superconducting critical temperature (T$_c$) $\sim$18 K, zone-center imaginary optical phonon modes have been found for the high-symmetry $I$-$43d$ 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 arise to the observed sizable T$_c$. Our work shows that compounds with the calculated dynamical instability should not be simply excluded in high-throughput search for new phonon-mediated superconductors. Moreover, we have studied the phase stability of the $I$-$43d$ 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 $ab$ $initio$ molecular dynamics near and out of the $I$-$43d$ structure. Our results show that the $I$-$43d$ 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 $I$-$43d$-distorted structures with C dimer wobbling motion at finite temperature still gives an overall $I$-$43d$ structure.