Atomic-Layer-Controlled Magnetic Orders in MnBi2Te4-Bi2Te3 Topological Heterostructures.

The natural van der Waals superlattice MnBi2Te4-(Bi2Te3)m provides an optimal platform to combine topology and magnetism in one system with minimal structural disorder. Here, we show that this system can harbor both ferromagnetic (FM) and antiferromagnetic (AFM) orders and that these magnetic orders can be controlled in two different ways by either varying the Mn-Mn distance while keeping the Bi2Te3/MnBi2Te4 ratio constant or vice versa. We achieve this by creating atomically engineered sandwich structures composed of Bi2Te3 and MnBi2Te4 layers. We show that the AFM order is exclusively determined by the Mn-Mn distance, whereas the FM order depends only on the overall Bi2Te3/MnBi2Te4 ratio regardless of the distance between the MnBi2Te4 layers. Our results shed light on the origins of the AFM and FM orders and provide insights into how to manipulate magnetic orders not only for the MnBi2Te4-Bi2Te3 system but also for other magneto-topological materials.

Design and Development of Stable Nanocrystalline High-Entropy Alloy: Coupling Self-Stabilization and Solute Grain Boundary Segregation Effects.

Grain growth is prevalent in nanocrystalline (NC) materials at low homologous temperatures. Solute element addition is used to offset excess energy that drives coarsening at grain boundaries (GBs), albeit mostly for simple binary alloys. This thermodynamic approach is considered complicated in multi-component alloy systems due to complex pairwise interactions among alloying elements. Guided by empirical and GB-segregation enthalpy considerations for binary-alloy systems, a novel alloy design strategy, the "pseudo-binary thermodynamic" approach, for stabilizing NC-high entropy alloys (HEAs) and other multi-component-alloy variants is proposed. Using Al25Co25Cr25Fe25 as a model-HEA to validate this approach, Zr, Sc, and Hf, are identified as the preferred solutes that would segregate to HEA-GBs to stabilize it against growth. Using Zr, NC-Al25Co25Cr25Fe25 HEAs with minor additions of Zr are synthesized, followed by annealing up to 1123 K. Using advanced characterization techniques- in situ X-ray diffraction (XRD), scanning/transmission electron microscopy (S/TEM), and atom probe tomography, nanograin stability due to coupling self-stabilization and solute-GB segregation effects is reported in HEAs up to substantially high temperatures. The self-stabilization effect originates from the preferential GB-segregation of constituent HEA-elements that stabilizes NC-Al25Co25Cr25Fe25 up to 0.5Tm (Tm-melting temperature). Meanwhile, solute-GB segregation originates from Zr segregation to NC-Al25Co25Cr25Fe25 GBs; this results in further stabilization of the phase and grain-size (≈14 nm) up to ≈0.58 and ≈0.64Tm, respectively.

Secondary-electron imaging of bulk crystalline specimens in an aberration corrected STEM.

Atomic-scale electron microscopy traditionally probes thin specimens, with thickness below 100 nm, and its feasibility for bulk samples has not been documented. In this study, we explore the practicality of scanning transmission electron microscope (STEM) imaging with secondary electrons (SE), using a silicon-wedge specimen having a maximum thickness of 18 µm. We find that the atomic structure is present in the entire thickness range of the SE images although the background intensity increases moderately with thickness. The consistent intensity of secondary electron (SE) images at atomic positions and the modest increase in background intensity observed in silicon suggest a limited contribution from SEs generated by backscattered electrons, a conclusion supported by our multislice calculations. We conclude that achieving atomic resolution in SE imaging for bulk specimens is indeed attainable using aberration-corrected STEM and an aberration-corrected scanning electron microscope (SEM) may have the capacity for atomic-level resolution, holding great promise for future strides in materials research.

Probing Oxidation-Driven Amorphized Surfaces in a Ta(110) Film for Superconducting Qubit.

Recent advances in superconducting qubit technology have led to significant progress in quantum computing, but the challenge of achieving a long coherence time remains. Despite the excellent lifetime performance that tantalum (Ta) based qubits have demonstrated to date, the majority of superconducting qubit systems, including Ta-based qubits, are generally believed to have uncontrolled surface oxidation as the primary source of the two-level system loss in two-dimensional transmon qubits. Therefore, atomic-scale insight into the surface oxidation process is needed to make progress toward a practical quantum processor. In this study, the surface oxidation mechanism of native Ta films and its potential impact on the lifetime of superconducting qubits were investigated using advanced scanning transmission electron microscopy (STEM) techniques combined with density functional theory calculations. The results suggest an atomistic model of the oxidized Ta(110) surface, showing that oxygen atoms tend to penetrate the Ta surface and accumulate between the two outermost Ta atomic planes; oxygen accumulation at the level exceeding a 1:1 O/Ta ratio drives disordering and, eventually, the formation of an amorphous Ta2O5 phase. In addition, we discuss how the formation of a noninsulating ordered TaO1-δ (δ < 0.1) suboxide layer could further contribute to the losses of superconducting qubits. Subsurface oxidation leads to charge redistribution and electric polarization, potentially causing quasiparticle loss and decreased current-carrying capacity, thus affecting superconducting qubit coherence. The findings enhance the comprehension of the realistic factors that might influence the performance of superconducting qubits, thus providing valuable guidance for the development of future quantum computing hardware.

Three-dimensional nanoscale metal, metal oxide, and semiconductor frameworks through DNA-programmable assembly and templating.

Controlling the three-dimensional (3D) nanoarchitecture of inorganic materials is imperative for enabling their novel mechanical, optical, and electronic properties. Here, by exploiting DNA-programmable assembly, we establish a general approach for realizing designed 3D ordered inorganic frameworks. Through inorganic templating of DNA frameworks by liquid- and vapor-phase infiltrations, we demonstrate successful nanofabrication of diverse classes of inorganic frameworks from metal, metal oxide and semiconductor materials, as well as their combinations, including zinc, aluminum, copper, molybdenum, tungsten, indium, tin, and platinum, and composites such as aluminum-doped zinc oxide, indium tin oxide, and platinum/aluminum-doped zinc oxide. The open 3D frameworks have features on the order of nanometers with architecture prescribed by the DNA frames and self-assembled lattice. Structural and spectroscopic studies reveal the composition and organization of diverse inorganic frameworks, as well as the optoelectronic properties of selected materials. The work paves the road toward establishing a 3D nanoscale lithography.

Assembling Vertical Block Copolymer Nanopores via Solvent Vapor Annealing on Homopolymer-Functionalized Substrates.

Utilizing the self-assembly of block copolymers with large Flory-Huggins interaction parameters (χ) for nanofabrication is a formidable challenge due to the attendant large surface energy differences between the blocks. This work reports a robust protocol for the fabrication of thin films with highly ordered cylindrical nanopore arrays via the self-assembly of an asymmetric poly(styrene-block-4-vinylpyridine) (PS-b-P4VP) diblock copolymer blended with a P4VP homopolymer. The desired vertical domain orientation is achieved at the air-polymer interface by controlled solvent vapor annealing (SVA) using acetone, a solvent with weak selectivity for PS over P4VP, and at the substrate interface by functionalization using a hydroxy-terminated poly(2-vinylpyridine) (P2VP-OH) homopolymer brush. In contrast, the vertical cylinder orientation is unstable during acetone SVA on substrates functionalized using hydroxy-terminated poly(methyl methacrylate) (PMMA-OH). Although PMMA exhibits more balanced interfacial energies between PS and P4VP than P2VP in the dry state, it is also swollen more selectively by acetone. We hypothesize that the nearly balanced solvent swelling of the three polymers (P2VP, P4VP, and PS) stabilizes the vertical cylinder orientation, while unbalanced swelling (PMMA > P4VP and PS) does not. We further characterize pore formation by addition of a P4VP homopolymer and its postassembly extraction using ethanol, revealing a narrow window of pore size tunability. Notably, minimal differences in nanopore morphologies are observed for P4VP volume fractions as high as 0.1, regardless of the P4VP molar mass. However, further increasing the P4VP volume fraction results in domain reorientation or macrophase separation when its molar mass is less than or greater than the P4VP block molar mass, respectively. Using a P4VP homopolymer that is nearly equal in length to the P4VP block enables the fabrication of well-ordered arrays of vertical, through-film nanopores with high aspect ratios (>10), small periods (<23 nm), and diameters less than 10 nm.

Geometry and Surface Area Optimization in Iron Oxide Nanoparticles for Enhanced Magnetic Properties.

Iron oxide nanoparticles (IONPs) are recognized for their potential in biomedical applications due to their distinctive physicochemical properties. This study investigates the synthesis of IONPs with various geometric morphologies-cubic, star-like, truncated icosahedron, and spherical-via thermal decomposition to enhance their utility in magnetic resonance imaging (MRI) and targeted drug delivery. X-ray diffraction analysis verified the Fe3O4 phase in all nanoparticles, illustrating the synthesis's efficacy. Particle morphologies were well-defined, with sizes ranging from 10 to 150 nm, as determined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Magnetic evaluations using a vibrating sample magnetometer (VSM-PPMs) demonstrated their superparamagnetic behavior, with larger particles exhibiting greater saturation magnetization. Notably, truncated icosahedron and cubic IONPs showed superior transverse relaxation rates, with r2 values of 56.77 s1 mM1 and 42.67 s1 mM1, respectively. These results highlight the potential of customizing IONP geometries to optimize their magnetic properties and increase surface area available for functionalization, thereby improving their efficacy for biomedical applications.

Synthesis and Characterization of Carbon-Based Quantum Dots and Doped Derivatives for Improved Andrographolide's Hydrophilicity in Drug Delivery Platforms.

Carbon-based quantum dots (CBQDs), sulfur-doped carbon-based quantum dots (S-CBQDs), and nitrogen-doped carbon-based quantum dots (N-CBQDs) have strong potential for drug delivery platforms. They were conjugated with andrographolide, a well-known hydrophobic drug, to study the concomitant changes in hydrophilicity. The interactions between these nanomaterials and the drug were studied by characterizing the optical and structural properties of the nanoparticles before and after coupling with the drug. It was found that the interaction of the drug with these nanomaterials produced noticeable changes in their optical and structural properties. Moreover, the partition coefficient for the nanocomposites was determined by NMR. The results indicate that conjugating the drug with the nanoparticles significantly enhanced its affinity for the aqueous phase, from 2.632 to 0.1117, thereby opening the possibility of using this approach for developing an effective drug delivery platform for this hydrophobic drug.

Hierarchically porous and single Zn atom-embedded carbon molecular sieves for H2 separations.

Hierarchically porous materials containing sub-nm ultramicropores with molecular sieving abilities and microcavities with high gas diffusivity may realize energy-efficient membranes for gas separations. However, rationally designing and constructing such pores into large-area membranes enabling efficient H2 separations remains challenging. Here, we report the synthesis and utilization of hybrid carbon molecular sieve membranes with well-controlled nano- and micro-pores and single zinc atoms and clusters well-dispersed inside the nanopores via the carbonization of supramolecular mixed matrix materials containing amorphous and crystalline zeolitic imidazolate frameworks. Carbonization temperature is used to fine-tune pore sizes, achieving ultrahigh selectivity for H2/CO2 (130), H2/CH4 (2900), H2/N2 (880), and H2/C2H6 (7900) with stability against water vapor and physical aging during a continuous 120-h test.

Surpassing millisecond coherence in on chip superconducting quantum memories by optimizing materials and circuit design.

The performance of superconducting quantum circuits for quantum computing has advanced tremendously in recent decades; however, a comprehensive understanding of relaxation mechanisms does not yet exist. In this work, we utilize a multimode approach to characterizing energy losses in superconducting quantum circuits, with the goals of predicting device performance and improving coherence through materials, process, and circuit design optimization. Using this approach, we measure significant reductions in surface and bulk dielectric losses by employing a tantalum-based materials platform and annealed sapphire substrates. With this knowledge we predict the relaxation times of aluminum- and tantalum-based transmon qubits, and find that they are consistent with experimental results. We additionally optimize device geometry to maximize coherence within a coaxial tunnel architecture, and realize on-chip quantum memories with single-photon Ramsey times of 2.0 - 2.7 ms, limited by their energy relaxation times of 1.0 - 1.4 ms. These results demonstrate an advancement towards a more modular and compact coaxial circuit architecture for bosonic qubits with reproducibly high coherence.

Ultrathin Magnesium-Based Coating as an Efficient Oxygen Barrier for Superconducting Circuit Materials.

Scaling up superconducting quantum circuits based on transmon qubits necessitates substantial enhancements in qubit coherence time. Over recent years, tantalum (Ta) has emerged as a promising candidate for transmon qubits, surpassing conventional counterparts in terms of coherence time. However, amorphous surface Ta oxide layer may introduce dielectric loss, ultimately placing a limit on the coherence time. In this study, a novel approach for suppressing the formation of tantalum oxide using an ultrathin magnesium (Mg) capping layer is presented. Synchrotron-based X-ray photoelectron spectroscopy studies demonstrate that oxide is confined to an extremely thin region directly beneath the Mg/Ta interface. Additionally, it is demonstrated that the superconducting properties of thin Ta films are improved following the Mg capping, exhibiting sharper and higher-temperature transitions to superconductive and magnetically ordered states. Moreover, an atomic-scale mechanistic understanding of the role of the capping layer in protecting Ta from oxidation is established based on computational modeling. This work provides valuable insights into the formation mechanism and functionality of surface tantalum oxide, as well as a new materials design principle with the potential to reduce dielectric loss in superconducting quantum materials. Ultimately, the findings pave the way for the realization of large-scale, high-performance quantum computing systems.