We employ reactive molecular-beam epitaxy to synthesize the metastable perovskite SrIrO3 and utilize in situ angle-resolved photoemission to reveal its electronic structure as an exotic narrowband semimetal. We discover remarkably narrow bands which originate from a confluence of strong spin-orbit interactions, dimensionality, and both in-and out-of-plane IrO6 octahedral rotations. The partial occupation of numerous bands with strongly mixed orbital characters signals the breakdown of the single-band Mott picture that characterizes its insulating two-dimensional counterpart, Sr2IrO4, illustrating the power of structure-property relations for manipulating the subtle balance between spin-orbit interactions and electron-electron interactions. PACS numbers: Keywords:The combination of strong spin-orbit interactions (SOIs) with electron-electron correlations has recently been predicted to realize a variety of novel quantum states of matter, including topological Mott insulators [1, 2], quantum spin Hall, quantum anomalous Hall, and axion insulators [3-6], Weyl semimetals [7], and even high temperature superconductors [8]. Although typically viewed to be disparate properties, the recent discovery that SOI can sufficiently enhance the effective role of electron correlations to stabilize a Mott-like insulating state in the quasi-two-dimensional 5d transition metal oxide Sr 2 IrO 4 [9, 10] has opened a new frontier for exploring the rare interplay between these 2 degrees of freedom. Its three-dimensional perovskite analogue, SrIrO 3 , has been theoretically proposed as a key building block for engineering topological phases at interfaces and in superlattices [3,11,12]. In bulk, SrIrO 3 is believed to lie in close proximity to a metal-insulator transition [13,14], yet little is known of its electronic structure to date.Here we reveal the momentum-resolved electronic structure of SrIrO 3 using a combination of reactive oxide molecular-beam epitaxy (MBE) and in situ angleresolved photoemission spectroscopy (ARPES). Our measurements uncover an exotic semimetallic ground state, hosting an unusual coexistence of heavy holelike and light electronlike bands. Contrary to conventional expectations that increased coordination leads to broader bands in higher-dimensional materials [13], we find that the bandwidths of SrIrO 3 are, instead, narrower than its insulating two-dimensional counterpart. By combining first-principles calculations with spectroscopic measurements, we uncover the surprising interplay of spinorbit interactions, dimensionality, and octahedral rotations which drives the narrow-band, semimetallic state in SrIrO 3 . Our results indicate that subtle changes in the structure and rotation angles should drive substantial changes in the electronic structure and physical properties of SrIrO 3 . This highlights the important structure-property relationships in correlated quantum materials [15,16], much like in ferroelectrics and multiferroics [17]. This Letter also underscores that simple toy models which neglect these str...

Epitaxial SrRuO3 and CaRuO3 films were grown under an excess flux of elemental ruthenium in an adsorption-controlled regime by molecular-beam epitaxy (MBE), where the excess volatile RuOx (x = 2 or 3) desorbs from the growth front leaving behind a single-phase film. By growing in this regime, we were able to achieve SrRuO3 and CaRuO3 films with residual resistivity ratios (ρ300 K/ρ4 K) of 76 and 75, respectively. A combined phase stability diagram based on the thermodynamics of MBE (TOMBE) growth, termed a TOMBE diagram, is employed to provide improved guidance for the growth of complex materials by MBE.

Superconductivity is among the most fascinating and well-studied quantum states of matter. Despite over 100 years of research, a detailed understanding of how features of the normal-state electronic structure determine superconducting properties has remained elusive. For instance, the ability to deterministically enhance the superconducting transition temperature by design, rather than by serendipity, has been a long sought-after goal in condensed matter physics and materials science, but achieving this objective may require new tools, techniques and approaches. Here, we report the transmutation of a normal metal into a superconductor through the application of epitaxial strain. We demonstrate that synthesizing RuO2 thin films on (110)-oriented TiO2 substrates enhances the density of states near the Fermi level, which stabilizes superconductivity under strain, and suggests that a promising strategy to create new transition-metal superconductors is to apply judiciously chosen anisotropic strains that redistribute carriers within the low-energy manifold of d orbitals.

We report the growth of superconducting Sr2RuO4 thin films by molecular-beam epitaxy on (110) NdGaO3 substrates with transition temperatures of up to 1.8 K. We calculate and experimentally validate a thermodynamic growth window for the adsorption-controlled growth of superconducting Sr2RuO4 epitaxial thin films. The growth window for achieving superconducting Sr2RuO4 thin films is narrow in growth temperature, oxidant pressure, and ruthenium-to-strontium flux ratio.

The electronic correlation strength is a basic quantity that characterizes the physical properties of materials such as transition metal oxides. Determining correlation strengths requires both precise definitions and a careful comparison between experiment and theory. In this paper, we define the correlation strength via the magnitude of the electron self-energy near the Fermi level. For the case of LaNiO 3 , we obtain both the experimental and theoretical mass enhancements m /m by considering high resolution angle-resolved photoemission spectroscopy (ARPES) measurements and density functional + dynamical mean field theory (DFT + DMFT) calculations. We use valence-band photoemission data to constrain the free parameters in the theory and demonstrate a quantitative agreement between the experiment and theory when both the realistic crystal structure and strong electronic correlations are taken into account. In addition, by considering DFT + DMFT calculations on epitaxially strained LaNiO 3 , we find a strain-induced evolution of m /m in qualitative agreement with trends derived from optics experiments. These results provide a benchmark for the accuracy of the DFT + DMFT theoretical approach, and can serve as a test case when considering other complex materials. By establishing the level of accuracy of the theory, this work also will enable better quantitative predictions when engineering new emergent properties in nickelate heterostructures.

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In mixed-valent Kondo lattice systems, such as YbAl3, interactions between localized and delocalized electrons can lead to fluctuations between two different valence configurations with changing temperature or pressure. The impact of this change on the momentum-space electronic structure is essential for understanding their emergent properties, but has remained enigmatic. Here, by employing a combination of molecular beam epitaxy and in situ angle-resolved photoemission spectroscopy we show that valence fluctuations can lead to dramatic changes in the Fermi surface topology, even resulting in a Lifshitz transition. As the temperature is lowered, a small electron pocket in YbAl3 becomes completely unoccupied while the low-energy ytterbium (Yb) 4f states become increasingly itinerant, acquiring additional spectral weight, longer lifetimes, and well-defined dispersions. Our work presents a unified picture of how local valence fluctuations connect to momentum-space concepts such as band filling and Fermi surface topology in mixed valence systems.

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