Imaginary phonon modes and phonon-mediated superconductivity in Y2C3

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.

6 pages, 9 figures and 1 table

References in corpus (5)

• Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
• Advanced capabilities for materials modelling with Quantum ESPRESSO
• Superconductivity in the elements, alloys and simple compounds
• Multigap superconductivity in sesquicarbides La${}_2$C${}_3$ and Y${}_2$C${}_3$
• Strong electron-phonon coupling in the rare-earth carbide superconductor La2C3

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