Engineering topics

Zhu, Bonan

Publications and source records attributed to Zhu, Bonan.

5 records

DOE OSTI2023

Emergent and robust ferromagnetic-insulating state in highly strained ferroelastic LaCoO 3 thin films

Transition metal oxides are promising candidates for the next generation of spintronic devices due to their fascinating properties that can be effectively engineered by strain, defects, and microstructure. An excellent example can be found in ferroelastic LaCoO 3 with paramagnetism in bulk. In contrast, unexpected ferromagnetism is observed in tensile-strained LaCoO 3 films, however, its origin remains controversial. Here we simultaneously reveal the formation of ordered oxygen vacancies and previously unreported long-range suppression of CoO 6 octahedral rotations throughout LaCoO 3 films. Supported by density functional theory calculations, we find that the strong modification of Co 3d-O 2p hybridization associated with the increase of both Co-O-Co bond angle and Co-O bond length weakens the crystal-field splitting and facilitates an ordered high-spin state of Co ions, inducing an emergent ferromagnetic-insulating state. Our work provides unique insights into underlying mechanisms driving the ferromagnetic-insulating state in tensile-strained ferroelastic LaCoO 3 films while suggesting potential applications toward low-power spintronic devices.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Manipulating the metal-to-insulator transition and magnetic properties in manganite thin films via epitaxial strain

Strain engineering of epitaxial transition metal oxide heterostructures offers an intriguing opportunity to control electronic structures by modifying the interplay between spin, charge, orbital, and lattice degrees of freedom. Here, we demonstrate that the electronic structure, magnetic and transport properties of La 0.9 Ba 0.1 MnO 3 thin films can be effectively controlled by epitaxial strain. Spectroscopic studies and first-principles calculations reveal that the orbital occupancy in Mn eg orbitals can be switched from the d 3z 2 -r 2 orbital to the d x 2 -y 2 orbital by varying the strain from compressive to tensile. Further, the change of orbital occupancy associated with Mn 3d-O 2p hybridization leads to dramatic modulation of the magnetic and electronic properties of strained La 0.9 Ba 0.1 MnO 3 thin films. Under moderate tensile strain, an emergent ferromagnetic insulating state with an enhanced ferromagnetic Curie temperature of 215 K is achieved. These findings not only deepen our understanding of electronic structures, magnetic and transport properties in the La 0.9 Ba 0.1 MnO 3 system, but also demonstrate the use of epitaxial strain as an effective knob to tune the electronic structures and related physical properties for potential spintronic device applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Common workflows for computing material properties using different quantum engines

The prediction of material properties based on density-functional theory has become routinely common, thanks, in part, to the steady increase in the number and robustness of available simulation packages. This plurality of codes and methods is both a boon and a burden. While providing great opportunities for cross-verification, these packages adopt different methods, algorithms, and paradigms, making it challenging to choose, master, and efficiently use them. We demonstrate how developing common interfaces for workflows that automatically compute material properties greatly simplifies interoperability and cross-verification. We introduce design rules for reusable, code-agnostic, workflow interfaces to compute well-defined material properties, which we implement for eleven quantum engines and use to compute various material properties. Each implementation encodes carefully selected simulation parameters and workflow logic, making the implementer’s expertise of the quantum engine directly available to non-experts. All workflows are made available as open-source and full reproducibility of the workflows is guaranteed through the use of the AiiDA infrastructure.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Crystal Structure of Colloidally Prepared Metastable Ag 2 Se Nanocrystals

Structural polymorphism is known for many bulk materials; however, on the nanoscale metastable polymorphs tend to form more readily than in the bulk, and with more structural variety. One such metastable polymorph observed for colloidal Ag 2 Se nanocrystals has traditionally been referred to as the “tetragonal” phase. While there are reports on the chemistry and properties of this metastable polymorph, its crystal structure, and therefore electronic structure, has yet to be determined. We report that an anti-PbCl 2 -like structure type (space group P2 1 /n) more accurately describes the powder X-ray diffraction and X-ray total scattering patterns of colloidal Ag 2 Se nanocrystals prepared by several different methods. Density functional theory (DFT) calculations indicate that this anti-PbCl 2 -like Ag 2 Se polymorph is a dynamically stable, narrow-band-gap semiconductor. Here, the anti-PbCl 2 -like structure of Ag 2 Se is a low-lying metastable polymorph at 5–25 meV/atom above the ground state, depending on the exchange-correlation functional used.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2021

On the Crystal Structure of Colloidally Prepared Metastable Ag2Se Nanocrystals

Structural polymorphism is known for many bulk materials; however, on the nanoscale metastable polymorphs tend to form more readily than in the bulk, and with more structural variety. One such metastable polymorph observed for colloidal Ag2Se nanocrystals has traditionally been referred to as the “tetragonal” phase of Ag2Se. While there are reports on the chemistry and properties of this metastable polymorph, its crystal structure, and therefore electronic structure, has yet to be determined. We report that an anti-PbCl2-like structure type (space group P21/n) accurately describes the powder X-ray diffraction and X-ray total scattering patterns of colloidal Ag2Se nanocrystals prepared by several different methods. Density functional theory (DFT) calculations indicate that the anti-PbCl2-like Ag2Se polymorph is a dynamically stable, narrow-band gap semiconductor. DFT results reveal a dense theoretical Ag2Se phase space with many low-energy polymorphs, which helps explain the large number of polymorphs reported in the literature. Analysis and calculation data are stored in the zip archive. The `ag2se-calcs.aiida.` contains the provenance of the calculations and can be imported into an AiiDA database instance. The antiPbCl2like_Ag2Se_laboratory.cif file is the Reitveld refined Ag2Se structure starting from the PbCl2 structure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗