Hard and soft x-ray standing-wave photoelectron spectroscopy and angle-resolved photoemission spectroscopy study of LaNiO3/SrTiO 3 superlattice and its interfaces.

摘要

Abstract Many classes of materials that exhibit interesting characteristics in the modulation of the electronic and magnetic properties when they are made of more than one compound, often arranged in multilayers and superlattices. In such cases, the electronic, electric, and magnetic properties of the multilayer, as well as their densities-of-states, are vastly different from the properties of the constituent materials, with the most important features often located at the interfaces. Specifically, perovskite nickelates are examples of materials that lie at the heart of correlated electron physics. Prior studies have been done on superlattices that contain multilayers of two perovskites. Specifically, it has been shown that LaNiO3 (LNO) undergoes a Mott metal-insulator transition when sandwiched between the layers of SrTiO3 (STO). However, even with prior theoretical simulations and experimental studies, no conclusion has been reached so far as to the exact reason for such a transition.;To further the investigation of these ideas, we are undertaking a detailed study of the electronic structure of a LaNiO3/SrTiO3 superlattice with 10 repeats of [4 unit-cell LNO/3 unit-cell STO] bilayer grown on an (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate. To provide a complete characterization of this superlattice, it is crucial to characterize the core levels of the atoms at the interface, as well as to measure the depth-dependent density of states and the element-specific magnetization through the interface.;The standing-wave photoemission technique provides a unique capability for characterizing the LNO/STO interfaces by depth-resolving the electronic structure of the superlattice, particularly in its momentum-resolving form of standing-wave angle-resolved photoemission using soft x-rays in the ca. 1 keV regime. The main advantages of SW-XPS are its non-destructiveness, large effective attenuation length, and the enhanced depth resolution for buried interfaces via standing-wave excitation, which makes use the interference of the incoming and outgoing x-rays setting up a standing wave inside and outside the multilayer. This standing wave can be subsequently varied in depth by either changing the incidence angle or the photon energy. Within the multilayer, a strong reflection at the first-order Bragg condition amplifies the standing wave. Being able to tune the standing wave by varying incidence angle, thus generating a rocking curve, or by varying the photon energy, permits a local enhancement or suppression of the regions directly at the interfaces between two materials or in the central regions of the two materials.;In this study, we are employing x-ray photoemission spectroscopy specifically, the soft x-ray standing-wave-excited angle-resolved photoemission, together with soft- and hard- x-ray photoemission measurements of core levels, with data collected at three different beamlines (BLs 7.0.1 and 9.3.1 at the Advanced Light Source at LBNL in Berkeley, CA and BL15XU at SPring-8 in Japan). More precisely, we are demonstrating several aspects of using the depth selectivity of soft x-ray standing-wave photoemission (SW-XPS), and standing-wave angle-resolved photoemission (SWARPES), for studying the electronic structure of core levels and the valence band in buried layers and interfaces, including momentum-resolved electronic structure. This study is coupled with complementary characterization using more depth sensitive hard x-ray excitation (several keV), and with optical modeling.;We are confirming our experimental findings for the LNO/STO multilayer by comparison with the theoretical calculations of band structures and densities of states. We further show that the behavior of the Ni 3d eg and t2g states near the Fermi level, as well as those at the bottom of the valence bands, is very similar to recently published results for an La0.7Sr0.3MnO 3/SrTiO3 superlattice that was studied using the same technique, which further validates this approach and our conclusions. As a specific point, we make direct comparison of the electronic structure of the two superlattices studied with the same technique and highlight the similarities between the eg and t2g states of the transition metal Mn and Ni components. We also discuss differences between the two systems, which we attribute to the greater difficulty of growing LNO/STO, which can lead to rougher, less well-defined interfaces. The fact that SWARPES can, even for this case, still provide interface-specific momentum-resolved information is encouraging for its future applications to other multilayer systems.