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1. Making BaZrS ₃ Chalcogenide Perovskite Thin Films by Molecular Beam Epitaxy

   https://doi.org/10.1002/adfm.202105563  

   Sadeghi, Ida; Ye, Kevin; Xu, Michael; Li, Yifei; LeBeau, James_M; Jaramillo, Rafael (August 2021, Advanced Functional Materials)

   Abstract The making of BaZrS₃thin films by molecular beam epitaxy (MBE) is demonstrated. BaZrS₃forms in the orthorhombic distorted‐perovskite structure with corner‐sharing ZrS₆octahedra. The single‐step MBE process results in films smooth on the atomic scale, with near‐perfect BaZrS₃stoichiometry and an atomically sharp interface with the LaAlO₃substrate. The films grow epitaxially via two competing growth modes: buffered epitaxy, with a self‐assembled interface layer that relieves the epitaxial strain, and direct epitaxy, with rotated‐cube‐on‐cube growth that accommodates the large lattice constant mismatch between the oxide and the sulfide perovskites. This work sets the stage for developing chalcogenide perovskites as a family of semiconductor alloys with properties that can be tuned with strain and composition in high‐quality epitaxial thin films, as has been long‐established for other systems including Si‐Ge, III‐Vs, and II‐VIs. The methods demonstrated here also represent a revival of gas‐source chalcogenide MBE. 

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2. Impact of Mechanical Strain on Auger Recombination in InGaAs/InP

   https://doi.org/10.1364/CLEO_AT.2024.JTh2A.14  

   Simmons, Y Lange; Dickson, Killian; Ricks, Amberly F; Skipper, Alec M; Briggs, Andrew F; Muhowski, Aaron J; Bank, Seth R; Gopinath, Juliet T (January 2024, Optica Publishing Group)

   We characterized the impact of mechanically-applied biaxial strain on Auger recombination in InGaAs quantum wells using time-resolved photoluminescence. Our results support that Auger recombination is reduced by mechanical distortion introduced by strained-layer epitaxy. 

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3. Strain and strain gradient engineering in membranes of quantum materials

   https://doi.org/10.1063/5.0146553  

   Du, Dongxue; Hu, Jiamian; Kawasaki, Jason K. (April 2023, Applied Physics Letters)

   Strain is powerful for discovery and manipulation of new phases of matter; however, elastic strains accessible to epitaxial films and bulk crystals are typically limited to small ( < 2 %), uniform, and often discrete values. This Perspective highlights emerging directions for strain and strain gradient engineering in free-standing single-crystalline membranes of quantum materials. Membranes enable large ( ∼ 10 %), continuously tunable strains and strain gradients via bending and rippling. Moreover, strain gradients break inversion symmetry to activate polar distortions, ferroelectricity, chiral spin textures, superconductivity, and topological states. Recent advances in membrane synthesis by remote epitaxy and sacrificial etch layers enable extreme strains in transition metal oxides, intermetallics, and Heusler compounds, expanding beyond the natively van der Waals (vdW) materials like graphene. We highlight emerging opportunities and challenges for strain and strain gradient engineering in membranes of non-vdW materials. 

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4. Hybridization and deconfinement in colloidal quantum dot molecules

   https://doi.org/10.1063/5.0112443  

   Verbitsky, Lior; Jasrasaria, Dipti; Banin, Uri; Rabani, Eran (October 2022, The Journal of Chemical Physics)

   The structural, electronic, and optical properties of CdSe/CdS core–shell colloidal quantum dot molecules, a new class of coupled quantum dot dimers, are explored using atomistic approaches. Unlike the case of dimers grown by molecular beam epitaxy, simulated strain profile maps of free-standing colloidal dimers show negligible additional strain resulting from the attachment. The electronic properties of the relaxed dimers are described within a semiempirical pseudopotential model combined with the Bethe–Salpeter equation within the static screening approximation to account for electron–hole correlations. The interplay of strain, hybridization (tunneling splitting), quantum confinement, and electron–hole binding energies on the optical properties is analyzed and discussed. The effects of the dimensions of the neck connecting the two quantum dot building blocks, as well as the shell thickness, are studied. 

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5. Nanoscale decoupling of electronic nematicity and structural anisotropy in FeSe thin films

   https://doi.org/10.1038/s41467-020-20150-y  

   Ren, Zheng; Li, Hong; Zhao, He; Sharma, Shrinkhala; Wang, Ziqiang; Zeljkovic, Ilija (January 2021, Nature Communications)

   Abstract In a material prone to a nematic instability, anisotropic strain in principle provides a preferred symmetry-breaking direction for the electronic nematic state to follow. This is consistent with experimental observations, where electronic nematicity and structural anisotropy typically appear hand-in-hand. In this work, we discover that electronic nematicity can be locally decoupled from the underlying structural anisotropy in strain-engineered iron-selenide (FeSe) thin films. We use heteroepitaxial molecular beam epitaxy to grow FeSe with a nanoscale network of modulations that give rise to spatially varying strain. We map local anisotropic strain by analyzing scanning tunneling microscopy topographs, and visualize electronic nematic domains from concomitant spectroscopic maps. While the domains form so that the energy of nemato-elastic coupling is minimized, we observe distinct regions where electronic nematic ordering fails to flip direction, even though the underlying structural anisotropy is locally reversed. The findings point towards a nanometer-scale stiffness of the nematic order parameter. 

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