Showing papers by "Chi-Cheng Lee published in 2021"

Hidden competing phase revealed by first-principles calculations of phonon instability in the nearly optimally doped cuprate La 1.875 Sr 0.125 CuO 4

Abstract: The representative cuprate ${\mathrm{La}}_{2\ensuremath{-}x}{M}_{x}{\mathrm{CuO}}_{4}$ with $M=\mathrm{Sr}$ and $x=1/8$ is studied via first-principles calculations in the high-temperature tetragonal (HTT), low-temperature orthorhombic (LTO), and low-temperature less-orthorhombic (LTLO) structures. By suppressing the magnetism and superconductivity, the LTLO phase, which has rarely been observed in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$, is found to be the ground state where the structural phase transitions $\mathrm{HTT}\ensuremath{\rightarrow}\mathrm{LTO}\ensuremath{\rightarrow}\mathrm{LTLO}$ can be understood via phonon instability. Although the La-O composition is identified to be responsible for the phonon softening, the superconducting ${\mathrm{CuO}}_{2}$ layer is dynamically stable. The LTLO phase, which can exhibit an $\ensuremath{\sim}20\text{\ensuremath{-}}\mathrm{meV}$ splitting in the density of states, is proposed to have an intimate relationship with the observed pseudogap and the charge-density wave giving the stripe. We argue that at low temperatures, the superconducting LTO ${\mathrm{La}}_{1.875}{\mathrm{Sr}}_{0.125}{\mathrm{CuO}}_{4}$ competes with the phonon-preferred LTLO phase by spontaneously forming the Cooper pairs, resulting in suppressing the stripe. Therefore, the revealed LTLO phase is indispensable for understanding ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$.