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1. Spontaneous Supercrystal Formation During a Strain‐Engineered Metal–Insulator Transition

   https://doi.org/10.1002/adma.202403873  

   Gorobtsov, Oleg Yu; Miao, Ludi; Shao, Ziming; Tan, Yueze; Schnitzer, Noah; Goodge, Berit Hansen; Ruf, Jacob; Weinstock, Daniel; Cherukara, Mathew; Holt, Martin Victor; et al (August 2024, Advanced Materials)

   Abstract Mott metal–insulator transitions possess electronic, magnetic, and structural degrees of freedom promising next‐generation energy‐efficient electronics. A previously unknown, hierarchically ordered, and anisotropic supercrystal state is reported and its intrinsic formation characterized in‐situ during a Mott transition in a Ca₂RuO₄thin film. Machine learning‐assisted X‐ray nanodiffraction together with cryogenic electron microscopy reveal multi‐scale periodic domain formation at and below the film transition temperature (T_Film ≈ 200–250 K) and a separate anisotropic spatial structure at and aboveT_Film. Local resistivity measurements imply an intrinsic coupling of the supercrystal orientation to the material's anisotropic conductivity. These findings add a new degree of complexity to the physical understanding of Mott transitions, opening opportunities for designing materials with tunable electronic properties. 

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2. Local structure elucidation of tungsten-substituted vanadium dioxide (V$$_{1-x}$$W$$_x$$O$$_2$$)

   https://doi.org/10.1038/s41598-022-18575-0  

   Wilson, Catrina E.; Gibson, Amanda E.; Cuillier, Paul M.; Li, Cheng-Han; Crosby, Patrice H.; Trigg, Edward B.; Najmr, Stan; Murray, Christopher B.; Jinschek, Joerg R.; Doan-Nguyen, Vicky (December 2022, Scientific Reports)

   Abstract Initially, vanadium dioxide seems to be an ideal first-order phase transition case study due to its deceptively simple structure and composition, but upon closer inspection there are nuances to the driving mechanism of the metal-insulator transition (MIT) that are still unexplained. In this study, a local structure analysis across a bulk powder tungsten-substitution series is utilized to tease out the nuances of this first-order phase transition. A comparison of the average structure to the local structure using synchrotron x-ray diffraction and total scattering pair-distribution function methods, respectively, is discussed as well as comparison to bright field transmission electron microscopy imaging through a similar temperature-series as the local structure characterization. Extended x-ray absorption fine structure fitting of thin film data across the substitution-series is also presented and compared to bulk. Machine learning technique, non-negative matrix factorization, is applied to analyze the total scattering data. The bulk MIT is probed through magnetic susceptibility as well as differential scanning calorimetry. The findings indicate the local transition temperature ( $$T_c$$ T c ) is less than the average $$T_c$$ T c supporting the Peierls-Mott MIT mechanism, and demonstrate that in bulk powder and thin-films, increasing tungsten-substitution instigates local V-oxidation through the phase pathway VO $$_2\, \rightarrow$$ 2 → V $$_6$$ 6 O $$_{13} \, \rightarrow$$ 13 → V $$_2$$ 2 O $$_5$$ 5 . 

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3. X-ray nano-imaging of defects in thin film catalysts via cluster analysis

   https://doi.org/10.1063/5.0125268  

   Luo, Aileen; Gorobtsov, Oleg Yu.; Nelson, Jocienne N.; Kuo, Ding-Yuan; Zhou, Tao; Shao, Ziming; Bouck, Ryan; Cherukara, Mathew J.; Holt, Martin V.; Shen, Kyle M.; et al (October 2022, Applied Physics Letters)

   Functional properties of transition-metal oxides strongly depend on crystallographic defects; crystallographic lattice deviations can affect ionic diffusion and adsorbate binding energies. Scanning x-ray nanodiffraction enables imaging of local structural distortions across an extended spatial region of thin samples. Yet, localized lattice distortions remain challenging to detect and localize using nanodiffraction, due to their weak diffuse scattering. Here, we apply an unsupervised machine learning clustering algorithm to isolate the low-intensity diffuse scattering in as-grown and alkaline-treated thin epitaxially strained SrIrO 3 films. We pinpoint the defect locations, find additional strain variation in the morphology of electrochemically cycled SrIrO 3 , and interpret the defect type by analyzing the diffraction profile through clustering. Our findings demonstrate the use of a machine learning clustering algorithm for identifying and characterizing hard-to-find crystallographic defects in thin films of electrocatalysts and highlight the potential to study electrochemical reactions at defect sites in operando experiments. 

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4. Anisotropic strain relaxation-induced directional ultrafast carrier dynamics in RuO ₂ films

   https://doi.org/10.1126/sciadv.adw7125  

   Jeong, Seung Gyo; Choi, In Hyeok; Lee, Seungjun; Oh, Jin Young; Nair, Sreejith; Lee, Jae Hyuck; Kim, Changyoung; Seo, Ambrose; Choi, Woo Seok; Low, Tony; et al (June 2025, Science Advances)

   Ultrafast light-matter interactions inspire potential functionalities in picosecond optoelectronic applications. However, achieving directional carrier dynamics in metals remains challenging due to strong carrier scattering within a multiband environment, typically expected for isotropic carrier relaxation. In this study, we demonstrate epitaxial RuO₂/TiO₂(110) heterostructures grown by hybrid molecular beam epitaxy to engineer polarization selectivity of ultrafast light-matter interactions via anisotropic strain engineering. Combining spectroscopic ellipsometry, x-ray absorption spectroscopy, and optical pump-probe spectroscopy, we revealed the strong anisotropic transient optoelectronic response at an excitation energy of 1.58 eV in strain-engineered RuO₂/TiO₂(110) heterostructures along both in-plane [001] and [1 $\overline{1}$ 0] crystallographic directions. Theoretical analysis identifies strain-induced modifications in band nesting as the underlying mechanism for enhanced anisotropic carrier relaxation observed at this excitation energy. These findings establish epitaxial strain engineering as a powerful tool for tuning anisotropic optoelectronic responses with near-infrared excitations in metallic systems, paving the way for next-generation polarization-sensitive ultrafast optoelectronic devices. 

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5. Room-temperature valence transition in a strain-tuned perovskite oxide

   https://doi.org/10.1038/s41467-022-35024-8  

   Chaturvedi, Vipul; Ghosh, Supriya; Gautreau, Dominique; Postiglione, William M.; Dewey, John E.; Quarterman, Patrick; Balakrishnan, Purnima P.; Kirby, Brian J.; Zhou, Hua; Cheng, Huikai; et al (December 2022, Nature Communications)

   Abstract Cobalt oxides have long been understood to display intriguing phenomena known as spin-state crossovers, where the cobalt ion spin changes vs. temperature, pressure, etc. A very different situation was recently uncovered in praseodymium-containing cobalt oxides, where a first-order coupled spin-state/structural/metal-insulator transition occurs, driven by a remarkable praseodymium valence transition. Such valence transitions, particularly when triggering spin-state and metal-insulator transitions, offer highly appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr 1- x Ca x CoO 3 ). Here, we show that in thin films of the complex perovskite (Pr 1- y Y y ) 1- x Ca x CoO 3-δ , heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291 K, i.e., around room temperature. The technological implications of this result are accompanied by fundamental prospects, as complete strain control of the electronic ground state is demonstrated, from ferromagnetic metal under tension to nonmagnetic insulator under compression, thereby exposing a potential novel quantum critical point. 

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