Imaginary phonon modes and phonon-mediated superconductivity in Y2C3

Abstract

For Y2C3 with a superconducting critical temperature (Tc) ∼18K, 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 rise to 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. 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 abinitio 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.

Source-reported materials — not catalogue approval

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