Imaginary phonon modes and phonon-mediated superconductivity in Y2C3

In high-throughput computation to search for new phonon-mediated superconductors, compounds with calculated imaginary frequency phonon modes are regarded as dynamically unstable and often discarded as promising candidates. Using density functional perturbation theory (DFPT) to investigate Y2C3 with an experimentally measured Tc ~ 18 K, we find imaginary optical phonon modes in its high symmetric body-centered cubic (BCC) structure. By following the imaginary phonon modes to distort the structure, we find the low-symmetry structures have lower electronic total energy than the BCC structure. Our electron-phonon coupling (EPC) calculations then reveal that the strong EPC contributions from these low-energy optical phonon modes are responsible for the observed sizable Tc. 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.