Electronic depth profiles with atomic layer resolution from resonant soft x-ray reflectivity

Abstract

The analysis of x-ray reflectivity data from artificial heterostructures usually relies on the homogeneity of optical properties of the constituent materials. However, when the x-ray energy is tuned to the absorption edge of a particular resonant site, this assumption may no longer be appropriate. For samples realizing lattice planes with and without resonant sites, the corresponding regions containing the sites at resonance will have optical properties very different from regions without those sites. In this situation, models assuming homogeneous optical properties throughout the material can fail to describe the reflectivity adequately. As we show here, resonant soft x-ray reflectivity is sensitive to these variations, even though the wavelength is typically large as compared to the atomic distances over which the optical properties vary. We have therefore developed a scheme for analyzing resonant soft x-ray reflectivity data, which takes the atomic structure of a material into account by 'slicing' it into atomic planes with characteristic optical properties. Using LaSrMnO 4 as an example, we discuss both the theoretical and experimental implications of this approach. Our analysis not only allows to determine important structural information such as interface terminations and stacking of atomic layers, but also enables to extract depth-resolved spectroscopic information with atomic resolution, thus enhancing the capability of the technique to study emergent phenomena at surfaces and interfaces.

Site-selective spectroscopy with depth resolution using resonant x-ray reflectometry

“…25 . In this approach the YBCO unit cell has been subdivided into thinner slabs (slices) corresponding to its atomic planes.…”

“…The optical constants of each atomic slice have been defined according to the stoichiometry of the corresponding lattice plane. As described in our previous work 25 , when using the slice approach to reflectivity, a good way to model the transition from one layered material into another is using an envelope error function of the form . With μ  = ( z i  −  z 0 ), where z i is an integer number defining the location of an atomic layer of the material close to the interface, z 0 is an integer number defining the layer position where the interface is located and σ defines the width of the transition, i.e., the interface roughness.…”

“…The optical constants of the materials under study were constructed as described in refs 24,25,30 . First the measured XAS spectra are normalized to tabulated scattering factors 39 .…”

“…In this regard, RXR provides a unique experimental tool to study such effects. It is a state of the art technique that combines elastic scattering (reflectometry) with spectroscopy (X-ray absorption), hence allowing to acquire element specific information on electronic properties with monolayer depth resolution 24,25 . In the case of 3 d -TMOs, information on the relevant 3 d states is obtained by performing RXR measurements at the L 2,3 edges (i.e., 2 p  → 3 d ) of the TM ion.…”

“…Here we apply RXR to this complex situation and therefore go one step further towards monolayer resolution and site selectivity. To this end, based on our previous work 25 , we have developed an analysis scheme of RXR where the atomic structure of the material is taken into account by “slicing” it into atomic planes with characteristic anisotropic scattering properties. Using RXR at the Cu L 2,3 edges, the stacking sequence and atomic termination of a YBCO thin film is determined.…”