Session R46: Complex Oxide Interfaces & Heterostructures -- Strong correlations

Magnetism and topology in SrRuO3- and SrVO3-based heterostructures: from DFT via DMFT to DΓA

Oxide heterostructures based on SrRuO₃ are promising candidates for thin film ferromagnetism, but experiments showed time and again that ferromagnetism breaks down below a critical thickness. By means of density functional theory (DFT) plus dynamical mean field theory (DMFT), we show [1] that growing SrRuO3 in the (111) direction instead yields a half-metallic ferromagnetic state with an ordered magnetic moment of 2 μB/Ru and survives the ultimate dimensional confinement down to a bilayer -- even at elevated temperatures of 500 K. This ferromagnetic state has been confirmed by experiment and also hosts Haldane’s quantum anomalous Hall state, without any external magnetic field or magnetic impurities [1]. Similar physics with an even larger gap is also observed in SrRhO3 (111) but not in LaNiO3 (111) [2] bilayers.

For SrRuO₃ grown in the (001)-direction, DFT+DMFT instead only yields ferromagnetism if the Ru is doped away from 4 d-electrons. This is possible by applying a gate voltage or, as we will show, by growing SrRuO₃ on an appropriate substrate that induces a self-doping between different SrRuO₃ layers.

For SrVO₃-based heterostructures, on the other hand, the metallic bulk behavior turns insulating below a thickness of 3 SrVO₃ layers, which has been proposed to be utilized as a Mott transistor [3]. Using DFT+DMFT and beyond that the dynamical vertex approximation (DΓA) [4], we here show that this insulating state is actually antiferromagnetic at low temperatures and can be switched to a ferromagnetic metallic state through a gate voltage, actually making it a Mott magnetotransistor. Here, strong non-local correlations further induce a Lifshitz transition at low temperatures.

[1] L. Si et al., PRL 119, 026402 (2017).
[2] O. Janson and KH, PRB 98, 115118 (2018).
[3] Z. Zhong et al., PRL 114, 246401 (2015).
[4] G. Rohringer et al., RMP 90, 025003 (2018).   

Designing new metastable nickelate thin films isostructural to the cuprates

Despite over thirty years of intense experimental and theoretical effort, there remain outstanding questions on the origin of superconductivity in the cuprates. One strategy for gaining insight into this phase has been to probe materials which are isoelectronic to the cuprates and contain many of the same characteristics of the known high-TC superconductors. Recent work on bulk crystals of layered square planar nickelates, R₄Ni₃O₈ (R = La, Pr) has re-ignited interest in nickelates for this purpose [1]. Here, we use atomically-precise synthesis to construct a full series of R_n+1Ni_nO_3n+1 (n = 1 – 5) thin films which we then reduce to form R_n+1Ni_nO_2n+2. We aim to develop a systematic understanding of these different nickelate thin films and to correlate the electronic behavior with the local crystal structure and nickel valence state.

[1] Junjie Zhang, A. S. Botana, J. W. Freeland2 , D. Phelan , Hong Zheng , V. Pardo, M. R. Norman and J. F. Mitchell, “Large orbital polarization in a metallic square-planar nickelate,” Nature Phys 13, 864-870 (2017)   

BoSS: Boson Slave Solver software for correlated electrons

We describe the recent release of the new open source software "BoSS" or Boson Slave Solver (bitbucket.org/yalebosscode/boss), written in MATLAB, which solves extended Hubbard models using the slave boson approach. The primary intended application is to study the electronic structure of transition metal oxides which are often affected significantly by strong and localized electron-electron interactions. Examples include renormalization of the mass and spectral weight of low energy quasiparticles and the formation of Hubbard bands: these dynamical effects can not be described by band theory approaches (e.g., DFT, DFT+U or hybrid methods). Slave-boson approaches represent computationally efficient electronic structure methods that can explicitly include such effects. We present an overview of our recent slave boson methodology [1,2], show applications to some transition metal oxides where we can compare to experimental and dynamical mean field theory (DMFT) results.
[1] Georgescu and Ismail-Beigi, PRB 92, 235117 (2015)
[2] Georgescu and Ismail-Beigi, PRB 96, 165135 (2017)   

Study of infinite layer CaCuO2/SrTiO3 interfaces

Thirty years after its discovery, superconductivity in cuprates still remains a lively field of research since the microscopic mechanisms leading to high T_c are not yet fully identified and understood. Recent work on the infinite layer CaCuO₂ [1,2] has shown that superconductivity can occur at the interface with SrTiO₃. The conduction at the interface has been related to a chemical doping mechanism with O atoms inserting into the apical position above Cu.
We present results on the study of CaCuO₂/SrTiO₃ bilayer structure. We investigate the influence of different growth parameters on the film quality and electrical properties explored through transport measurements.

[1] Di Castro, D. et al. Occurrence of a high-temperature superconducting phase in (CaCuO₂)_n/(SrTiO₃)_m superlattices. Phys. Rev. B 86, 134524 (2012).
[2] Di Castro, D. et al. High-Tc Superconductivity at the Interface between the CaCuO₂ and SrTiO₃ Insulating Oxides. Phys. Rev. Lett. 115, 147001 (2015).

  

Effects of epitaxial strain and oxygen underdoping on superconductivity in manganite/cuprate thin-film heterostructures

To distinguish the role of magnetism from other factors that affect superconductivity in c-axis La_2/3Ca_1/3MnO₃/YBa₂Cu₃O_7-δ (LCMO/YBCO) heterostructures, we grow and characterize various perovskite/YBCO/perovskite trilayers, using ferromagnetic LCMO, paramagnetic LaNiO₃ (LNO), and orthorhombic PrBa₂Cu₃O_7-δ (PBCO) as the sandwiching layers. LCMO and LNO trilayers show similarly large superconducting T_c reductions with decreasing YBCO layer thickness, whereas this T_c reduction is not seen in the PBCO trilayers. Our results indicate that epitaxial strain has a stronger effect on the T_c of LCMO/YBCO heterostructures than any long-range magnetic proximity effect present. [1] Using this system, we also study how the T_c reduction varies with oxygen underdoping by deoxygenating the cuprate layer. We discuss our results in the wider context of multiple competing orders, in particular the recent observation by resonant x-ray scattering of robust charge-density-wave order in LCMO/YBCO multilayers. [2]
[1] H. Zhang et al., arXiv:1710.10668v1 [cond-mat.supr-con]
[2] A. Frano et. al., Nat. Materials 15:831 (2016)   

Multi-band, two-dimensional superconductivity above the Pauli limit in Sr1-xNdxTiO3

We present evidence of two-dimensional, multi- band superconductivity in Sr_1-xNd_xTiO₃ thin films from the temperature dependence of the resistive upper critical field. The out-of-plane upper critical field shows pronounced positive curvature, which extends substantially below the critical temperature, and further exhibits a change in curvature at low temperatures. Quite unusually, the out-of-plane upper critical field exceeds the Pauli paramagnetic limit, by almost a factor of two. We attribute this unusual observation to the enhancement of critical field due to two-band superconductivity and the presence of strong spin-orbit coupling. We propose a model for the critical field of a two-band superconductor that includes paramagnetic breaking of Copper pairs and spin-orbital coupling in the dirty limit and find good agreement of this model with our data. Our results suggest that in SrTiO₃ intra-band superconducting coupling dominates over inter-band coupling.   

Prospects for creating complex oxide quantum electronic heterostructures via solid phase epitaxy: PrAlO3/SrTiO3 model system

Various oxide interfaces with composition RAlO₃/SrTiO₃ (R = La, Pr, Nd) can produce a two-dimensional electron gas (2DEG). Crystallization from amorphous precursors, termed solid phase epitaxy (SPE), is an approach for synthesizing these interfaces that complements vapor-phase epitaxy techniques. Atomic layer deposition (ALD) is a promising route for forming the amorphous films because it allows oxide interfaces to be created in non-planar geometries. SPE can be used with patterned substrates to grow epitaxial thin films in complex geometries, which may enable the creation of 2DEG beyond planar geometries.
The creation of PrAlO₃ thin films via SPE has required developing new ALD procedures, understanding crystallization kinetics, and probing the microstructure and interface structures of the crystallized thin films. Nearly stoichiometric amorphous PrAlO₃ thin films were grown via ALD at 300 °C using tris(isopropylcyclopentadienyl)praseodymium, AlMe₃, and water with a growth rate of 2.0 Å/cycle on TiO₂-terminated STO (001) substrates. The as-deposited amorphous PrAlO₃ crystallized via SPE to form an epitaxial layer on STO upon annealing at 800 °C for 3h. Our work provides new opportunities to form polar/nonpolar oxide interfaces, and the accompanying 2DEG in novel geometries.   

Suppression of Metal-Insulator Transition in VO2 by Interfacial Oxygen Migration

Oxygen defects are essential building blocks for designing functional oxides with remarkable properties, ranging from electrical and ionic conductivity to magnetism and ferroelectricity. Oxygen defects therefore can profoundly alter crystal and electronic structures and enables emergent phenomena. In this work, we achieved tunable metal-insulator transition (MIT) in oxide heterostructures by inducing interfacial oxygen vacancy migration. We chose VO_2-x as a model system due to its near room temperature metal-insulator transition temperature. We found that depositing a TiO₂ capping layer on an epitaxial VO2 thin film can effectively suppress the MIT in VO₂. We systematically studied the TiO₂/VO₂ heterostructures by structural and transport measurements, resonant inelastic x-ray scattering, X-ray photoelectron spectroscopy, and first principles calculations and found that that oxygen vacancy migration from TiO₂ to VO₂ is responsible for the suppression of MIT. Our findings underscore the importance of the interfacial oxygen “diode” effect in determining electronic structure and functionality, and provide new pathways of designing oxide heterostructures for novel ionotronics and computing devices.   

Tailoring the low-energy physics of RuO2 by epitaxial strain

Rutile ruthenium dioxide (RuO₂) was long presumed to be a paramagnetic metal with weak electronic correlations, until recent measurements of antiferromagnetism in bulk single crystals by Berlijn et al. Motivated by this discovery to better understand its electronic structure, we synthesized RuO₂ thin films by molecular-beam epitaxy on rutile TiO₂ substrates and characterized these films using in situ angle-resolved photoemission spectroscopy (ARPES). Comparing our ARPES data with density-functional calculations, we find that: (1.) electron-phonon coupling accounts for most of the modest quasiparticle mass renormalizations observed in RuO₂, and (2.) a sizable crystal field splitting in the rutile structure lifts the threefold degeneracy of the t_2g manifold spanning the Fermi level (E_F), causing strong departures from the prototypical Hund’s metal behavior observed in perovskite-based ruthenates, despite having the same electron count of 4d⁴. Guided by this understanding of the effective low-energy physics, we explain how epitaxial strain modifies the orbital occupations in RuO₂ films grown on different orientations of TiO₂ substrates, and discuss how concomitant changes to the density of states near E_F feed back into the instability of strained RuO₂ towards superconductivity.   

Physical properties of LaFeO3 and Pr2-xCexCuO4 superstructures

Charge transfer in electron-doped Sm₂CuO₄ (SCO) and LaFeO₃ (LFO) superstructures (SS) grown by RF sputtering was predicted based on their band structure. STEM-EELS measurements revealed the presence of extra electrons at the interface between SCO and LFO[1]. However, the growth conditions left open questions regarding the actual doping of the SCO layers. We have grown SS of electron-doped Pr_2-xCe_xCuO₄ (PCCO) and LaFeO₃ by pulsed-laser deposition with various thicknesses and annealing conditions. X-ray diffraction confirms the coherent growth of PCCO/LFO SS. Superconductivity is confirmed for selected samples by magnetic susceptibility and resistivity measurements, while the Hall Effect of low resistivity samples was carried out for all temperatures. A strong dependence of carrier density and the critical temperature as a function of annealing conditions and thickness of the SS subcomponents is observed. Our results indicate that charge transfer might not be the only source of charge carriers in these structures as annealing conditions are playing a central role in our PLD-grown superstructures.
[1] F.Y. Bruno et al. , Adv. Mat. 25, 1468 (2013).   

X-ray imaging of ionically-controlled nanoscale phase separation in cobaltites

Controlling ionic distributions and stoichiometry provides a unique way for manipulating functional properties in perovskite oxides (ABO₃). This talk will focus on Gd/La_0.67Sr_0.33CoO₃ (LSCO) heterostructures due to high oxygen ion conductivity, as well as the coupled magnetic and electronic properties of LSCO, which are strongly dependent on the oxygen stoichiometry. This enables the ionic control of the functional properties of LSCO thin films through the presence of oxygen getter layers such as Gd. We utilize x-ray nanodiffraction to directly image the nanoscale morphology of LSCO thin films as they are progressively transformed from the equilibrium perovskite phase to the metastable brownmillerite (ABO_2.5) phase with increasing Gd thickness. Our studies show the coexistence of perovskite and brownmillerite phases with a critical oxygen vacancy concentration threshold which leads to the formation of extended brownmillerite filaments. In addition to lateral phase separation, we observed phase separation along the thin film thickness due to pinning of perovskite or brownmillerite by substrate/LSCO or Gd/LSCO interface respectively. Our measurements highlight local structural effects and provide insights into nanoscale strain behavior in Gd/LSCO heterostructures   

Lifshitz transition and emergence of Van Hove singularity at the complex oxide heterostructure

Electronic correlations strongly depend on the topology of a given electronic structure. A Van Hove singularity (VHS), where the curvature of the electronic bands has opposite sign in two orthogonal directions, is a critical point for a change in electronic topology (Lifshitz transition). It has been considered to be the key to inducing many new unconventional states, especially when it is close to the Fermi level. By using angle-resolved photoemission spectroscopy, we will show the direct evidence of Lifshitz transition and an emergence of VHS in the oxide heterostructures with the different quantum well structures, revealing a new pathway to tune VHS and the corresponding electronic topology at a complex oxide heterointerface.