Abstract
The project aims to investigate and control the physical properties of relativistic Mott insulators, a class of transition metal oxides (specifically iridates) characterized by strong spin-orbit coupling and electronic correlation. Focusing on the Ruddlesden-Popper Srₙ₊₁IrₙO₃ₙ₊₁ iridate series, the goal is to explore and synthesize previously inaccessible intermediate phases (n > 2) using artificial superlattices. The project combines experimental and theoretical approaches to understand the interplay of spin, charge, orbital, and lattice degrees of freedom. It also aims to achieve electrical control of transport and magnetic properties through strain engineering and ferroelectric interfaces, with potential applications in oxide spintronics. Additionally, the project seeks to establish a sustainable and international research team with expertise across condensed matter physics. Epitaxial Growth of Ruddlesden-Popper (RP) Iridate Thin Films and Superlattices -Successful fabrication of high-quality RP iridate thin films and superlattices with precise stoichiometry and atomic-layer control. -Computational prediction of stable structures for large-n RP iridates using DFT+U+SOC simulations. -Generation of simulated STEM images and structural data to guide synthesis. -Optimization of growth conditions (via PLD), confirmed through XPS, XRD, LEED/RHEED, and STEM characterizations. 2. Understanding Physical Properties through Experiment and Theory -Detailed experimental characterization of transport (e.g., magnetoresistance) and magnetic properties using MOKE, VSM, XMCD, and XRMS. -In-depth study of electronic and magnetic interactions using XAS, ARPES, and RIXS to reveal orbital character, excitations, and coupling mechanisms. -Theoretical modeling of ground states and excitations using DFT+U+SOC, GW+BSE, and DMFT, enabling direct comparison with experimental data. -Implementation of a new MOKE simulation module for theoretical-experimental benchmarking. 3. Design of Iridate/Ferroelectric Superlattices with Magnetoelectric Properties -Fabrication of novel superlattices combining RP iridates with ferroelectrics (e.g., PZT), aiming to realize magnetoelectric phases. -Identification of interfacial effects such as strain-induced distortions and their impact on magnetic and transport behavior. -Theoretical guidance to tailor combinations for desired functionalities relevant to oxide spintronics applications. Broader Outcomes: -Publications in high-impact, open-access journals -Presentations at international/national conferences -Hosting an international workshop to foster collaboration and future directions
Results achieved
: Significant progress has been achieved in establishing the experimental basis for the artificial synthesis of RP iridates. High-quality SrIrO₃ thin films have been stabilized over a broad range of growth parameters, and their structural properties and growth rate have been quantitatively determined. Systematic investigations of SrO deposition and alternating SrIrO₃/SrO growth cycles enabled the exploration of the parameter space relevant to RP phase formation. The experiments identified the main technological constraint: the optimal growth-temperature windows for SrIrO₃ and SrO do not overlap. This provides a clear understanding of the limiting growth conditions and defines the strategy for further optimization. The Hubbard interaction parameters U and J were investigated for SrIrO₃ and placed in the broader context of a large perovskite dataset reported by Si, Liu, and Franchini in Physical Review Materials 9, 015001 (2025). An automated protocol was developed to calculate quasiparticle energies at the GW level in transition-metal oxides, enabling systematic comparison between theory and spectroscopy, as demonstrated by Varrassi, Franchini, and co-workers in npj Computational Materials 11, 351 (2025). The successful stabilization of SrIrO₃, the identification of the key constraints in SrO/SrIrO₃ sequential growth, and the development of reliable computational protocols provide a coherent platform for pursuing higher-order RP iridates. These advances move the project toward its long-term goal: controlling dimensionality, electronic correlations, spin–orbit coupling, and magnetic interactions in engineered relativistic oxide superlattices. The PoliMi and UniSa units carried out the experimental synthesis using pulsed laser deposition at the NFFA-Ts facilities. Two polycrystalline targets, SrIrO₃ and SrO, were employed to explore sequential growth of RP iridate phases. The deposition process was supported by in situ and ex situ characterization, including RHEED, XPS, EDS, SEM, XRR, and XRD. SrTiO₃ and DyScO₃ were selected as substrates because their lattice parameters are compatible with those of the targeted RP phases. The synthesis campaign consisted of two 10-day sessions in July and October 2024. The initial plan was to optimize the growth parameters for SrIrO₃ and SrO separately during the first session,and then refine the deposition conditions for RP superlattices during the second. Unexpected delays in target delivery and technical issues complicated the workflow. Background noise in the RHEED signal limited real-time monitoring of crystal structure and layer thickness, while SEM-EDS was unavailable during the October session. XRR and XRD were used to estimate thickness and assess phase formation, while XPS provided complementary information on chemical composition. A systematic exploration of laser fluence, substrate temperature, and oxygen pressure showed that SrIrO₃ thin films can be stabilized over a broad range of conditions, provided that the substrate temperature exceeds approximately 650 °C. XRD yielded an out-of-plane lattice parameter of about 4.04 Å, while XRR enabled a reliable estimate of the growth rate, corresponding to approximately 10–20 laser shots per SrIrO₃ unit cell, depending on the deposition conditions. Attempts to stabilize single-phase SrO under the same conditions used for SrIrO₃ were unsuccessful. At lower temperatures, XRD indicated weak SrO-related features, possibly associated with a (111)-oriented phase, but weak XRR oscillations made the deposition rate less reliable. The available data suggest an approximate growth rate of 14 laser shots per SrO unit cell, or about 7 shots per atomic layer. Without reliable RHEED monitoring, precise control over SrO deposition remained difficult. Using the estimated growth rates of SrIrO₃ and SrO, the experimental activity was extended to Sr₂IrO₄-like n = 1 RP structures through alternating SrIrO₃/SrO cycles. Several conditions were tested by keeping the SrIrO₃ dose fixed and varying the number of SrO shots. Increasing the SrO dose modified the resulting structure, confirming that the alternating protocol affects film formation. The characteristic diffraction peaks expected for ideal Sr₂IrO₄ were not observed, indicating that the target n = 1 RP structure was not fully stabilized. Chemical analysis was attempted by XPS, with samples transferred directly from the PLD chamber to the XPS setup through a vacuum tunnel to reduce contamination. However, the extracted elemental ratios were not fully reliable. Similar inconsistencies were observed in reference measurements on Nb-doped SrTiO₃, where the expected Sr stoichiometry was not recovered. This limited the quantitative interpretation of the sample composition and highlighted the need to refine the chemical characterization protocol. The experimental work established a solid foundation while identifying the main technological bottlenecks. SrIrO₃ films were successfully grown and characterized, whereas SrO growth remained less reproducible. The central limitation is the apparent mismatch between the optimal temperature windows: SrIrO₃ requires relatively high temperatures, above about 650 °C, while SrO appears to be favored at lower temperatures. This non-overlap complicates direct sequential growth of SrIrO₃/SrO superlattices and will be the key issue to address in the next phase. The theoretical activity established reliable first-principles methodologies for iridates and related transition-metal oxides. Hubbard U and J parameters for SrIrO₃ were computed using the constrained random-phase approximation, yielding U = 3.36 eV and J = 0.52 eV. These values provide robust input for future DFT+U+SOC modelling of experimentally realized RP structures. The analysis was also extended to a broad class of ABO₃ perovskites, clarifying the evolution of electronic interaction parameters across 3d, 4d, and 5d compounds. A second development was the implementation of an automated G₀W₀ workflow within the AiiDA framework using the PAW method. The workflow estimates errors in quasiparticle energies arising from basis-set truncation and norm violations in ultrasoft PAW potentials, reducing the need for demanding multidimensional convergence studies. The protocol was validated against experimental data and state-of-the-art GW calculations, and its scalability was demonstrated through a database of quasiparticle energies for more than 320 bulk materials, including iridates. The research activities have optimized the use of existing infrastructures and laboratory equipment, limiting unnecessary resource consumption. Open Access principles will be ensured through open-access publications and the deposit of research outputs in institutional and international repositories. Gender equality and equal opportunities have been supported through transparent, merit-based recruitment procedures. Generational inclusion has been promoted through the involvement of early-career postdoctoral fellows, who contributed to experimental and data-analysis activities. Training and mentoring actions have also supported skill development and knowledge transfer between senior and junior researchers.Project details
Unibo Team Leader: Cesare Franchini
Unibo involved Department/s:
Dipartimento di Fisica e Astronomia "Augusto Righi"
Coordinator:
Politecnico di MILANO(Italy)
Total Unibo Contribution: Euro (EUR) 65.199,00
Project Duration in months: 24
Start Date:
28/09/2023
End Date:
27/09/2025
