Center for Functional Nanomaterials Seminar

Abstract The spatial and energy resolution of electron microscopy today are defined by two new techniques: multislice electron ptychography (MEP) and monochromated electron energy loss spectroscopy (EELS). In this talk, I will introduce both methods and demonstrate how they bring new insight into the structure and properties of two superconducting systems—La3Ni2O7 and FeSe/SrTiO3. Bilayer nickelates have recently demonstrated high Tc superconductivity under hydrostatic pressure. Compressive strained La3Ni2O7 thin films can superconduct without external pressure, but with lower Tc. With much recent efforts on tuning the superconducting properties by the oxygen stoichiometry, we find that fluorine (F) incorporation is detrimental to the superconductivity of La3Ni2O7. I will present the discovery of La3Ni2O7F2, a new variant of bilayer nickelates that formed due to unintentional fluorination. This new phase features F intercalation in the rock salt layers, leading to over hole-doping and a distinct Ni-O bond length. The resulting Fermi surface is more two-dimensional and lacks the dz2 band, which may explain the absence of superconductivity. The second part of the talk will focus on a monochromated EELS study of the FeSe/SrTiO3 interface [1], where superconductivity is enhanced by electron-phonon coupling. We aim to understand the phonon modes at the interface by resolving the atomic scale vibrational snapshots. Using a momentum selective EELS detection geometry, we acquire atomic scale images with vibrationally scattered electrons in the 10-100 meV energy range, which carry information about the polarization direction. Combining experimental results with theoretical calculations, we identify that the out-of-plane stretching vibrations of apical oxygens in SrTiO3 contribute strongly to coupling with electrons, and that the coupling strength is related to the interfacial structure and interlayer spacing. Reference: [1] H. Yang et al., Phonon modes and electron–phonon coupling at the FeSe/SrTiO3 interface, Nature 635, 332 (2024).