Session A66: Structure, Growth, Stabilization and Function

A first-principles study on Ga stabilized δ-Pu alloys

The puzzling behavior of the 5f electrons makes plutonium metal one of most complex materials in the periodic table. It has six solid phases that go through five solid-state phase transformations in a relatively short temperature range, and these solid phases have interestingly crystal symmetries coupled with different mechanical properties. The room temperature low-symmetry monoclinic α-phase is brittle and the high-temperature high symmetry face-centered-cubic δ-phase is ductile. Essentially, ductility makes the δ-phase metallurgically easier to manipulate. Stabilizing the δ-phase to room temperature requires alloying with an element from Group IIIA (Al, Ga, In, and Tl) where the stability zones is dependent on alloying solute concentrations and temperature. For Ga stabilized δ-Pu, upon cooling the alloy undergoes a decomposition into other pure solid phases of Pu and Pu-Ga alloys according to the Russian phase diagram contrary to the US phase diagram which indicates metastability. In this work, we employ density functional theory to investigate the thermodynamics and electronic structures of Ga-δ-Pu alloys from 0 to 9at% of Ga to further understand the metastability of δ-Pu from a first-principles perspective.   

Effect of Entropy and Hydrogen Bonding on Water Configuration in Sub-Nanometric Active Confinements

Water in sub-nanometeric confinements demonstrates properties that are absent in its bulk counterpart. In this study, we have conducted reactive molecular dynamics (MD) simulations to study the role of water entropy and hydrogen bond (HB) strength on its structure when confined inside boron nitride nanotubes (BNNTs) of different diameters. The nitrogen atoms of the BNNT participate in such HB formation making the BNNT an active confinement. The ReaxFF force field parameters used here were shown to accurately capture water structure and predict a water-hexagonal-Boron-Nitride (water-hBN) contact angle of ~70° that agrees with the experimental results. Most previous works involving MD simulations of water confined in BNNT have underestimated the water-hBN contact angle (~40°) resulting in a significant overestimation of energetic contribution to the water free energy inside the BNNTs. As such, our study shows that water does not form a continuous single-file inside sub-nanometer BNNT at 1 atmospheric pressure and 300K temperature which is at odds with results from previous MD simulations: we associate such behavior to an intricate interplay of the entropic effects (associated with the entry of the water molecules into the BNNT) and the corresponding HB formation dynamics.   

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Quantum anomalous Hall effect (QAHE) in magnetic ion-doped topological insulators (TIs) has been actively researched with great attention by virtue of its fascinating phenomena of quantized Hall resistance at zero external magnetic field. Even though various approaches have been reported so far to increase the QAHE temperature, the observable temperature of the QAHE has been limited below 500 mK without the help of gating.  

In this presentation, we investigate the QAHE with tuning Cr and Bi doping concentrations in multilayer magnetic TIs. As a platform, we employed a trilayer Cr-doped (Bi, Sb)₂Te₃ structure with heavy Cr doping at outer (top and bottom) layers. To increase the QAHE temperature, we systematically tune Bi and Cr concentrations in inner and outer MTI layers and, in a sample with optimized doping, observe close to the fully quantized value at 2 K without the external electric field for gating. We believe that this finding might be helpful for further study in high-temperature QAHE.

 

    

Elemental Hydrogen Diffusion in fcc-bcc Grain Boundaries

Hydrogen-rich operating environments and processing conditions have led to a manufacturability problem of environmentally-assisted cracking due to hydrogen embrittlement. In the case of materials fabricated by additive manufacturing, the increased occurrence of dislocations and vacancies have been theorized to correspond to an increase susceptibility to hydrogen embrittlement. A component of mitigating hydrogen embrittlement is a fundamental understanding of the hydrogen-material interactions at the most impactful structures and defects. In the following work, two approaches have been used to calculate the probabilistic behavior of hydrogen near and at grain boundaries in face centered and body centered cubic structures of FeCrNi. The first approach employs high-throughput ab initio calculations coupled with determination of transition states via the embedding energy. The second utilizes a classical atomistic forcefield based on the embedded atom method but includes a novel short-range term that enables simulation of collision cascades. The resultant diffusion pathways and potential barriers can be used to guide engineering applications of the alloys.   

Density-functional Tight-binding Method for Simulation of Rare-earth Mono-pnictide (RE-V):III-V

Embedding rare-earth mono-pnictide (RE-V) nanoparticles into III-V matrices allows for semiconductor composites with a wide range of optical, electrical, and thermoelectric properties. The inclusion of nanoparticles in a III-V increases the number of interfaces leading to increased phonon scattering and decreased thermal conductivity, also providing enhanced electrical conduction through electron filtering mechanisms. The electronic structure of these hybrid quantum materials at device-relevant (nm) length scales dictates functionality, yet poses challenges for ab initio simulation which is limited to systems of few hundreds up to a thousand of atoms. A promising way to simulate the electronic structure of nanoscale materials would be to combine density functional theory with an approximate method like tight binding, such as in the DFTB method. Here we discuss our parameterization of semimetallic rare-earth pnictides, III-V semiconductors, and their realistic size composite nanostructures in DFTB, and discuss effects of quantum confinement in rare-earth films on III-V substrates and rare-earth nanoparticles embedded in III-V matrices. In particular, we discuss the effects of atomic arrangements at the interface and interface orientation of the film and nanoparticle systems.   

Unveiling the physical origin of the electro-plasticity of metals

It has been widely known that the plasticity of metals enhances under electric current. Such electro-plasticity has been utilized for various industrial purposes. Even until recently, Joule heating was mostly regarded as the origin of this phenomenon, but it turned out that the temperature raised in this phenomenon may not be high enough to explain the enhanced plasticity. In this work, we unveil the physical origin of such electro-plasticity using first-principle calculation. We investigate the effects of extra charges supplied by the current on phonon properties and shear deformation. We reveal that bulk phonon properties change little with a small amount of supplied charges. On the other hand, extra charges affect quite considerably the phonon properties of imperfect structures like a slab system or a system with grain boundaries resulting in significant phonon softening. Additionally, we discover that extra charges can reduce the energy barrier of shear motions in various metal slabs. Eventually, we suggest that electro-plasticity originates from local charge accumulation or deficiency occurring in imperfect regions.   

Individual and collective atomic transportation in deformation of solids at nanoscale

Solids response to stress through deformation. At nanoscale, how the deformation happens and whether the mechanisms can transisit for different length scales, temperatures, and material classes are of high interest for both materials science and nanofabrication techniques such as nanomolding.

In this presentation, we will talk about a transition in deformation mechanisms between individual and collective atomic transportation in solids revealed by thermomechanical nanomolding (TMNM). Specifically, ordered materials (e.g., crystallline metals and alloys) deform through dislocation-slip at high temperature and large size and interface diffusion at high temperature and small size, while disordered materials (e.g., BMGs) transit between viscous flow at large size and interface diffusion at fine scales at temperatures below glass transition. The dislocation-slip and the viscous flow mechanism induce collective atomic motion that maintain the chemical composition of feedstock materials. As a contrast, the interface diffusion mechanism induces atomic motion of each element individually and results in varied chemical composition from feedstock materials through different diffusivities among components. These findings may inspire new insights for research in nanomechanics and materials science and open new possibilities for future nanofabrication techniques.   

Photocurrents in Plasmonic Metals. Spectral and material dependence.

Significant electrical effects associated with plasmonic excitations present interest for various applications including plasmonic biosensors with compact electric detection. The origin of these effects is not fully clear as the magnitude of the photocurrents significantly exceeds the predictions of the theoretical momentum loss approach.  In the experiment, we study photoinduced voltages at plasmon resonance conditions as a function of the wavelength in various metals, including silver, gold, aluminum, copper, platinum, and permalloy.  The effects are found to be strongly different in different materials; they increase in relative magnitude upon decreasing wavelength toward the localized plasmon resonance.  Experimental data is used to develop a better understanding the nature of plasmon-induced electrical effects and to characterize them for possible applications.   

Formation of a single Al-Co-Ni quasicrystal upon collision of multiple grains

Quasicrystals are solids that have long-range orientational order but lack translational periodicity. These materials are of significant interest as they possess mechanical and transport properties typically not observed in periodically crystalline materials with similar compositions. However, experimental fabrication of defect-free quasicrystals poses a significant barrier to commercial applications. It is critical to understand how conventional strain (phonon strain) and phason strain, which is unique to quasicrystals, contribute to grain boundary formation and grain coalescence. We present a joint experimental-computational study where we elucidate the contributions of phasons to grain coalescence mechanisms in Al-Co-Ni quasicrystals, and investigate the effects of initial misorientation on the formation of grain boundaries. Our study reveals how the unique symmetries of quasicrystals govern grain coalescence, providing a novel handle that can be used to design defect-free fabrication of quasicrystalline materials.   

Robust recognition and exploratory analysis of crystal structures using Bayesian deep learning

Atomic-resolution studies are  routinely being performed in modern materials-science experiments. Artificial-intelligence tools are promising candidates to leverage this valuable - yet underutilized - data in unprecedented, automatic fashion  to discover hidden patterns and eventually novel physics. Here, we introduce ARISE (Nat. Commun. 2021, DOI: 10.1038/s41467-021-26511-5), a crystal-structure-identification method based on Bayesian deep learning.  As a major step forward, ARISE is robust to structural noise and can treat more than 100 crystal structures, a number that can be extended on demand. While being trained on ideal structures only, ARISE correctly characterizes strongly perturbed single- and polycrystalline systems, from both synthetic and experimental sources. The probabilistic nature of the Bayesian-deep-learning model yields principled uncertainty estimates, which are found to be correlated with crystalline order of metallic nanoparticles in electron-tomography experiments. Application of unsupervised learning to the internal neural-network representations reveals grain boundaries and (unapparent) structural regions sharing interpretable geometrical properties. This work enables the hitherto hindered analysis of noisy atomic structural data from computations or experiments.   

Highly luminescent monolayer films with rare earth emitters in plasmonic environment

Amphiphilic complexes of Re³⁺ TTA where Re is a rare earth ion (Eu, Nd, Er) are developed for controlled deposition of monolayer and multilayer films on plasmonic surfaces, which provide an opportunity for controlling the emission rate and radiation patterns of electric and magnetic emitters. We show that EuTTA monolayer films are not fully quenched when deposited directly onto metal on contrary to the theoretical predictions. Efficiency of surface plasmon polariton excitation by both electric and magnetic dipoles is enhanced in comparison with thicker films.  The effects are discussed theoretically considering decoupling of plasmonic modes due to the surface geometry.   

An experimental investigation of bulk electronic structure of high-order approximants and quasicrystal

Approximants, the crystalline analog of quasicrystals, are assumed to represent the quasicrystal phase in the electronic structure calculations and have been very successful to predict the existence of the pseudogap at the Fermi level of the related quasicrystal which is generally considered to stabilize it via the Hume-Rothery mechanism[1]. However, experimental endeavor to explore the bulk electronic structures of approximants, especially of high-order ones is very limited. In this present work, using hard x-ray photoelectron spectroscopy we have investigated the bulk electronic structures of two high-order quaternary approximants of F-type icosahedral (i)-Al-Pd-TM quasicrystal: Al-Pd-Cr-Fe and Al-Pd-Mo-Fe, having similar electron to atom (e/a) ratio as i-Al-Pd-Mn quasicrystal[2]. Both the approximants show a well-formed pseudogap at the Fermi level. Moreover, the pseudogap turns out to be deeper in the approximants compared to i-Al-Pd-Mn, and this is supported by specific heat data. Modifications in the line shape of Al 2s core-level main peak as well as the plasmon loss peaks provide evidence for enhanced hybridization of Al sp and transition metal d states in the approximants than i-Al-Pd-Mn, which could be one of the possible reasons for their larger pseudogap. The absence of magnetic exchange splitting in the Fe 2p core-level spectra establishes the nonmagnetic nature of the approximants.

 1. J. Hafner et al., Phys. Rev. Lett. 68, 2321 (1992) 2. Krajci et al., Phys. Rev. B. 51, 24 (1995) 3. S. Sarkar et al., Phys. Rev. Res. 3, 013151 (2021)   

Scanning tunneling microscopy of thick quasiperiodic Sn layer

Quasicrystals (QCs) have an aperiodic long-range order with forbidden rotational symmetries but lack translational symmetry. These are very attractive due to their remarkable physical properties such as low surface energy, low thermal and electrical conductivity. Inorganic QCs exist mostly as ternary systems. However, elemental quasicrystal in bulk form has not been discovered so far, which would be important for understanding the physical properties of QCs independent of their chemical complexity. Although tremendous efforts in this direction have been made to grow elemental adlayers on quasicrystalline substrates but only 1-2 pseudomorphic monolayers of a few elemental metals have been reported to show quasiperiodicity since last three decades [1].  In the present work, we have established the thickest (4 nm) quasiperiodic Sn layer reported to date using scanning tunneling microscopy (STM) and other related techniques including density functional theory calculations [2]. The unique motifs made up of P-tiles and hexagonal tile such as a crown, wheel and triplet are different from the motifs observed on the substrate indicating different quasiperiodic arrangements of Sn. Also, the low-energy electron diffraction patterns are distinct from the substrate and support the STM results. Interestingly, the thicker Sn layer indicates 3-dimensional quasiperiodic growth. We propose Sn grows as a novel form of clathrate quasicrystal from our density functional theory calculations. The motifs observed by STM corroborate with the clathrate model. 

 

References:

[1] K. J. Franke et al., Phys. Rev. Lett. 89, 156104 (2002); A. K. Shukla et al., Phys. Rev. B 79, 134206 (2009); H. R. Sharma et al., Nat Commun 4, 2715 (2013).

[2] V. K. Singh et al., Phys. Rev. Research 2, 013023 (2020).