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We report on the synthesis and electronic properties of epitaxial perovskite ${\mathrm{La}}_{1\text{\ensuremath{-}}x}{\mathrm{Sr}}_{x}\mathrm{Rh}{\mathrm{O}}_{3}$ thin films. Thin films with a Sr content ranging from $x=0$ to 0.5 have been grown using molecular beam epitaxy. Transport and x-ray photoemission spectroscopy data reveal an insulator-metal-insulator transition, accompanied by a $p$- to $n$-type carrier change observed in Hall measurements. Combined with theoretical calculations, we find that the addition of Sr does not directly dope carriers into the conduction band, but rather induces localized Rh $4d$ states within the $\mathrm{La}\mathrm{Rh}{\mathrm{O}}_{3}$ band gap. The bandwidth of the impurity band increases with Sr content, eventually causing the valence band (VB) and the localized Rh $4d$ band to overlap, which explains the first insulator-to-metal transition occurring at $x=0.35$. For Sr content $xg0.4$, possible cation ordering results in an increase of the gap between the VB and the Rh $4d$ band, leading to the second metal-to-insulator transition. We map out the electronic phase diagram of the Sr-doped $\mathrm{La}\mathrm{Rh}{\mathrm{O}}_{3}$ system and suggest strategies to engineer the electronic states in rhodate systems via delocalizing the ${\mathrm{Rh}}^{3+}$ states.

Experiments reveal interfacial atomic-scale structure that may be critical for the interface enhanced high Tc in FeSe/SrTiO3., Remarkable enhancement of the superconducting transition temperature (Tc) has been observed for monolayer (ML) FeSe films grown on SrTiO3 substrates. The atomic-scale structure of the FeSe/SrTiO3 interface is an important determinant of both the magnetic and interfacial electron-phonon interactions and is a key ingredient to understanding its high-Tc superconductivity. We resolve the atomic-scale structure of the FeSe/SrTiO3 interface through a complementary analysis of scanning transmission electron microscopy and in situ surface x-ray diffraction. We find that the interface is more strongly bonded for a particular registration, which leads to a coherently strained ML. We also determine structural parameters, such as the distance between ML FeSe and the oxide, Se─Fe─Se bond angles, layer-resolved distances between Fe─Se, and registry of the FeSe lattice relative to the oxide. This picoscale structure determination provides an explicit structural framework and constraint for theoretical approaches addressing the high-Tc mechanism in FeSe/SrTiO3.

Synthesizing monolayers and heterostructures is an enabling approach to extract new physical phenomena from bulk materials. Among the structures amenable to this approach is stanene, which is a monolayer of tin, similar to graphene, and has been predicted to host one-dimensional topological states at its edges. Stanene can be tuned by decorating with different adatoms, which makes it a promising platform on which to engineer topological devices. Here, we deposit Sn on Bi2Te3 and characterize the growth using anomalous synchrotron x-ray scattering and x-ray photoelectron spectroscopy (XPS). X-ray diffraction data reveal the formation of epitaxial Sn-based structures, along with penetration of Sn into the Bi2Te3, with Sn intercalating between the upper 10 Bi2Te3 quintuple layers. Additionally, XPS data show deposited Sn reacting to form SnTe and Bi at the Bi2Te3 surface. The calculated heat of reaction for Sn and Bi2Te3 is consistent with an exothermic reaction to SnTe and Bi. Using thermodynamic calculations as a guide, we identify several candidate substrates that can stabilize the stanene phase.

We report on a surface-induced, insulating, electrically dead layer in ultrathin conducting La-doped SrTiO3 thin films. Systematic studies on electrical properties as a function of film thickness and La-doping levels reveal that the insulating layer has a constant thickness and traps a constant amount of electron density regardless of La-doping levels. Growing an additional capping layer on top of the La-doped SrTiO3 surface counteracts the reduced conductivity, indicating a strong relationship between the insulating layer and the surface structure. Our results emphasize the importance of surface state studies for functional oxides in the thin film limit and provide a guiding principle for the fabrication of La-doped SrTiO3-based oxide nanoscale devices.

The alkaline earth stannate BaSnO3 is a semiconductor with high carrier mobility at room-temperature when doped with La3+. When a thin epitaxial layer of LaInO3 is grown on lightly doped BaSnO3, a polar discontinuity between the orthorhombic, polar LaInO3 and the cubic, nonpolar Ba0.998La0.002SnO3 leads to an electronic reconstruction, where LaInO3 remotely dopes Ba0.998La0.002SnO3, creating carriers for high mobility devices. We determine aspects of the crystalline structure of the LaInO3/BaSnO3 film that affect the polar discontinuity using synchrotron x-ray diffraction. Specifically, we examine the role of oxygen octahedral rotations and anti-parallel cation displacements in influencing the polarization of the LaInO3/BaSnO3 interface. These structural distortions are characterized by measuring half-order Bragg peaks of thin film LaInO3/BaSnO3/SrTiO3 heterostructures grown by pulsed laser deposition. We find that for films as thin as 3 unit cells, epitaxial LaInO3 has 2 distinct domains, one with the same Glazer tilt pattern as that of bulk LaInO3, a+b−b−, and a second rotated 90° in-plane from the first and having tilt pattern b−a+b−. Additionally, we observe a sudden and large increase in cation displacements along the [011] and [101] directions across the LaInO3/BaSnO3 interface due to the strength of the octahedral rotations.

We determine the surface reconstruction of $\mathrm{SrTi}{\mathrm{O}}_{3}$ used to achieve superconducting FeSe films in experiments, which is different from the $1\ifmmode\times\else\texttimes\fi{}1\mathrm{Ti}{\mathrm{O}}_{2}$-terminated $\mathrm{SrTi}{\mathrm{O}}_{3}$ assumed by most previous theoretical studies. In particular, we identify the existence of a double $\mathrm{Ti}{\mathrm{O}}_{2}$ layer at the FeSe/$\mathrm{SrTi}{\mathrm{O}}_{3}$ interface that plays two important roles. First, it facilitates the epitaxial growth of FeSe. Second, ab initio calculations reveal a strong tendency for electrons to transfer from an oxygen deficient $\mathrm{SrTi}{\mathrm{O}}_{3}$ surface to FeSe when the double $\mathrm{Ti}{\mathrm{O}}_{2}$ layer is present. The double layer helps to remove the hole pocket in the FeSe at the \ensuremath{\Gamma} point of the Brillouin zone and leads to a band structure characteristic of superconducting samples. The characterization of the interface structure presented here is a key step towards the resolution of many open questions about this superconductor.