Tunable surface electron gas and effect of phonons in Sr2CuO3 : A first-principles study

While the conducting ${\mathrm{CuO}}_{2}$ planes in cuprate superconductors have been widely recognized as a crucial component in producing high superconducting ${T}_{\text{c}}$, recent experimental and theoretical studies on ${\mathrm{Ba}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{3+\ensuremath{\delta}}$ have also drawn much attention to the importance of Cu-O chains in one-dimensional (1D) cuprates. To better understand the cuprates containing Cu-O chains, here we have studied the electronic, magnetic, and phonon properties of ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$ bulk and films based on the spin-polarized density functional theory calculations. We first reproduced the typical Mott insulator feature of the cuprate parent compound for bulk ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$, and then built a ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$ thin film with Cu-O chains exposed on the surface to directly investigate their characteristics. Different from the insulating bulk phase, the ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$ surface shows interesting metallic properties. Further electronic structure calculations reveal the existence of spin-polarized electron gas between surface Sr atoms that strongly depends on the interchain coupling of Cu spins. Moreover, the phonon modes that involve the vibrations of in-chain and out-of-chain O atoms can induce strong charge and spin fluctuations in the surface layer of ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$ film, which suggests significant multiple degree-of-freedom couplings that may be important for the superconductivity in 1D cuprates. Our work provides a comprehensive viewpoint of the properties of Cu-O chains in ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$, facilitating a complete understanding of 1D cuprate superconductors.