Stripe phase in high-<i>T</i><sub>c</sub> superconductor FeSe/SrTiO<sub>3</sub>

The enhancement of superconductivity in one unit-cell FeSe grown on SrTiO₃ is an important discovery in high-temperature superconductivity. In this system, the crucial role of the SrTiO₃ substrate has been extensively studied. Its contribution mainly manifests in two aspects: charge transfer and interfacial electron-phonon coupling. However, study of the intrinsic properties of the FeSe thin film itself is still insufficient. In this article, we review the latest research progress of the mechanism of the enhancement of superconductivity in FeSe/SrTiO₃, covering the newly discovered stripe phase and its relationship with superconductivity. By using scanning tunneling microscope and molecular beam epitaxy growth method, we find that the electrons in FeSe thin film tend to form stripe patterns, and show a thickness-dependent evolution of short-range to long-range stripe phase. The stripe phase, a kind of electronic liquid crystal state (smectic), originates from the enhanced electronic correlation in FeSe thin film. Surface doping can weaken the electronic correlation and gradually suppress the stripe phase, which can induce superconductivity as well. More importantly, the remaining smectic fluctuation provides an additional enhancement to the superconductivity in FeSe film. Our results not only deepen the understanding of the interfacial superconductivity, but also reveal the intrinsic uniqueness of the FeSe films, which further refines the mechanism of superconductivity enhancement in FeSe/SrTiO₃.