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arXiv2013

Parameters Influencing the Optical Properties Of SnS Thin Films

Tin Sulphide (SnS) thin films have been recognized as a potential candidate for solar cells. Many fabrication techniques have been used to grow SnS thin films. The band-gap, ${\rm E_g}$ of SnS films as reported in literature, were found to vary from 1.2-2.5 eV depending on the film fabrication technique. The present work reports the structural, compositional, morphological and optical characterization of SnS thin films fabricated by thermal evaporation at room temperature. Results show that for the given fabrication technique/ condition, the band-gap functionally depends on the lattice parameter and grain size. The well-defined variation allows for tailoring SnS film as per requirement.

cond-mat.mtrl-sci↗

arXiv2013

The Influence of Ca and Y on the Microstructure and Corrosion Resistance of Vacuum Die Casting AZ91 Alloy

The influence of Ca and Y on the microstructure and corrosion resistance of vacuum die casting AZ91 alloy is investigated using optical microscope, electron scanning microscope, weight-loss test and electrochemical corrosion test. The results indicate that the microstructure of AZ91 alloy can be refined, amount of Mg17Al12 phases is reduced, making Mg17Al12 phases transform from banding to reticular, and stringer Al2Ca phases and block Al2Y phases are formed through adding both Ca and Y. The corrosion resistance of AZ91 magnesium alloy can be increased greatly by adding both Ca and Y. The corrosion rate of AZ91-1.5Ca-1.0Y alloy is dropped to 16.2% of that of AZ91 alloy immersed in 3.5% NaCl aqueous solution for 24 hours. The corrosion current density of AZ91-1.5Ca-1.0Y alloy is dropped by one order of magnitude.

cond-mat.mtrl-sci↗

arXiv2013

Potassium under pressure: electronic origin of complex structures

Recent high-pressure x-ray diffraction studies of alkali metals revealed unusual complex structures that follow the body-centered and face-centered cubic structures on compression. The structural sequence of potassium under compression to 1 Megabar is as follows: bcc - fcc - h-g (tI19*), hP4 - oP8 - tI4 - oC16. We consider configurations of Brillouin-Jones zones and the Fermi surface within a nearly-free-electron model in order to analyze the importance of these configurations for the crystal structure energy that contains two main contributions: electrostatic (Ewald) and electronic (band structure) energies. The latter can be lowered due to a formation of Brillouin zone planes close to the Fermi surface opening an energy gap at these planes. Under pressure, the band structure energy term becomes more important leading to a formation of complex low-symmetry structures. The stability of the post-fcc phases in K is attributed to the changes in the valence electron configuration implying the overlap of valence band with the upper core electrons. This effect offers an understanding of structural complexity of alkali metals under strong compression.

cond-mat.mtrl-sci↗

arXiv2017

Fabrication and Characterization of Fe100-xNix Nanoparticles in the Invar Region

In this work, Fe100-xNix nanoparticle systems in the invar region (x = 29, 32, and 37) were prepared by the method of chemical co-precipitation. X-Ray Diffraction (XRD) patterns confirmed the coexistence of both bcc and fcc phases for x = 29 and 32. However, only fcc phase was observed for x = 37. Scanning Electron Microscope (SEM) images indicated that the particle size is ~ 120-500 nm, and is almost independent of x. Mössbauer spectroscopy (MS) on the prepared nanoparticles indicated the development of a paramagnetic phase characteristic of the low-spin (antitaenite) fcc phase, in addition to the magnetic components characteristic of the bcc phase and the high-spin fcc phases. Since it is not possible to resolve the low- spin fcc phase from the high-spin fcc phase in XRD patterns, MS proves to be an effective tool for studying the prepared nanoparticle systems.

cond-mat.mtrl-sci↗

arXiv2017

Magnetization measurements on as prepared and annealed Fe3-xMnxSi alloys

The magnetic properties of the alloy system Fe3-xMnxSi have been studied by measuring magnetization for samples with x = 0, 0.1, 0.25, 0.5, and by thermal scanning techniques for samples with x = 0, 0.1. The results reveal that the system is ferromagnetic in this composition range. Zero field cooling and field cooling magnetization measurements indicate a similar magnetic ordering and magnetic anisotropy in all samples. The saturation magnetization for the annealed samples was higher than that for as prepared samples. This is attributed to the reduction of magnetic domain boundaries rather than to improving magnetic order as a result of annealing. Further, TC values determined from thermal DSC measurements are in good agreement with previously reported results based on magnetic measurements.

cond-mat.mtrl-sci↗

arXiv2017

Solution processable and optically switchable 1D photonic structures

In this work, we report the first demonstration of a solution processable, optically switchable 1D photonic crystal by implementing phototunable doped metal oxide nanocrystals. The resulting device structure shows bi-photonic response with the photonic bandgap covering the visible spectral range and the plasmon resonance of the doped metal oxide the near infrared. By means of a facile photodoping process, we tuned the plasmonic response and switched effectively the optical properties of the photonic crystal, translating the effect from the near infrared to the visible. The ultrafast bandgap pumping induces a signal change in the region of the photonic stopband, with recovery times of several picoseconds, providing a step toward the ultrafast optical switching. Optical modeling uncovers the importance to understand largely the variations of the dielectric function of the photodoped material, and variations in the high frequency region of the Drude response are responsible for the strong switching in the visible after photodoping. Our device configuration offers unprecedented tunablility due to flexibility in device design, cover wavelength ranges from the visible to the near infrared. Our findings indicate a new protocol to modify the optical response of photonic devices by optical triggers only.

cond-mat.mtrl-sci↗

arXiv2017

First Principles Phase Diagram Calculation for the 2D TMD system $WS_{2}-WTe_{2}$

First principles phase diagram calculations, that included van der Waals interactions, were performed for the bulk transition metal dichalcogenide system $(1-X) \cdot WS_{2} - (X) \cdot WTe_{2}$. To obtain a converged phase diagram, a series of cluster expansion calculations were performed with increasing numbers of structure-energies, ($N_{Str}$) up to $N_{Str}=435$, used to fit the cluster expansion Hamiltonian. All calculated formation energies are positive and all ground-state analyses predict that formation energies for supercells with 16 or fewer anion sites are positive; but when $\approx 150 N_{Str} \leq 376$, false ordered ground-states are predicted. With $N_{Str} \geq 399$, only a miscibility gap is predicted, but one with dramatic asymmetry opposite to what one expects from size-effect considerations; i.e. the calculations predict more solubility on the small-ion S-rich side of the diagram and less on the large-ion Te-rich side. This occurs because S-rich low-energy metastable ordered configurations have lower energies than their Te-rich counterparts.

cond-mat.mtrl-sci↗

arXiv2018

Near-Perfect Absorption of Light by Plasmene Sheets

Near-perfect absorbers (NPAs) efficiently absorb visible light with a layered nanostructure that is thinner than the diffusion lengths of photogenerated charge carriers. We overcame existing limitations in fabricating their nanoparticulate surface by depositing \textit{plasmene}, a tightly-packed two-dimensional lattice of metal nanoparticles formed through self-assembly. The plasmene NPAs absorb up to 98\% of incident visible light, with modelling showing the improvement on existing NPAs arises from the structural ordering of the plasmene. We also demonstrate control of NPAs' absorption profile through the use of anisotropic building blocks in plasmene. These property enhancements may broaden the application of NPAs to structural colour, sensing and photocatalysis.

physics.app-ph↗

arXiv2018

Fingerprints of stacking order in graphene layers from ab initio second-order Raman spectra

We present an \textit{ab initio} study based on density-functional theory of first- and second-order Raman spectra of graphene-based materials with different stacking arrangements and numbers of layers. Going from monolayer and bilayer graphene to periodic graphitic structures, we investigate the behavior of the first-order G-band and of the second-order 2D-band excited by the same set of photon energies. The former turns out to be very similar in all considered graphene-based materials, while in the latter we find the signatures of individual structures. With a systematic analysis of the second-order Raman spectra at varying frenquency of the incident radiation, we monitor the Raman signal and identify the contributions from different phonon modes that are characteristic of each specific arrangement. Supported by good agreement with experimental findings and with previous theoretical studies based on alternative approaches, our results propose an effective tool to probe and analyze the fingerprints of graphene-based and other low-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2018

Nanoscale compositional evolution in complex oxide based resistive memories

Functional oxides based resistive memories are recognized as potential candidate for the next-generation high density data storage and neuromorphic applications. Fundamental understanding of the compositional changes in the functional oxides is required to tune the resistive switching characteristics for enhanced memory performance. Herein, we present the micro/nano-structural and compositional changes induced in a resistive oxide memory during resistive switching. Oxygen deficient amorphous chromium doped strontium titanate (Cr:$a$-SrTiO$_{3-x}$) based resistance change memories are fabricated in a Ti/Cr:$a$-SrTiO$_{3-x}$ heterostructure and subjected to different biasing conditions to set memory states. Transmission electron microscope based cross-sectional analyses of the memory devices in different memory states shows that the micro/nano-structural changes in amorphous complex oxide and associated redox processes define the resistive switching behavior. These experimental results provide insights and supporting material for Ref. [1].

cond-mat.mtrl-sci↗

arXiv2018

Bi monocrystal formation on InAs(111)A and B substrates

The growth of Bi films deposited on both A and B faces of InAs(111) has been investigated by low-energy electron diffraction, scanning tunneling microscopy, and photoelectron spectroscopy using synchrotron radiation. The changes upon Bi deposition of the In 4d and Bi 5d5/2 photoelectron signals allow to get a comprehensive picture of the Bi/InAs(1 1 1) interface. From the initial stages the Bi growth on the A face (In-terminated InAs) is epitaxial, contrary to that on the B face (As- terminated InAs) that proceeds via the formation of islands. Angle-resolved photoelectron spectra show that the electronic structure of a $\approx 10$~BL deposit on the A face is identical to that of bulk Bi, while more than $\approx 30$ BL are needed for the B face. Both bulk and surface states are well accounted for by fully relativistic ab initio spin-resolved photoemission calculations.

cond-mat.mtrl-sci↗

arXiv2018

Role of thermal expansion heterogeneity in the cryogenic rejuvenation of metallic glasses

Cryogenic rejuvenation in metallic glasses reported in Ketov et al 's experiment (Nature(2015)524,200) has attracted much attention, both in experiments and numerical studies. The atomic mechanism of rejuvenation has been conjectured to be related to the heterogeneity of the glassy state, but the quantitative evidence is still elusive. Here we use molecular dynamics simulations of a model metallic glass to investigate the heterogeneity in the local thermal expansion. We then combine the resulting spatial distribution of thermal expansion with a continuum mechanics calculation to infer the internal stresses caused by a thermal cycle. Comparing the internal stress with the local yield stress, we prove that the heterogeneity in thermomechanical response has the potential to trigger local shear transformations, and therefore to induce rejuvenation during a cryogenic thermal cycling.

cond-mat.dis-nn↗

arXiv2018

Comment on "Evidence for absence of metallic surface states in BiO2-terminated BaBiO3 thin films"

In a recent work by Ji Seop Oh et al., BaBiO3(001) thin films were grown on SrTiO3 by Pulsed Laser Deposition. It was argued that the films are BiO2-terminated from the modelling of angle-resolved photoemission spectroscopy experiments. The authors claim, in opposition to previous theoretical predictions, that there are no metallic surface states on their films. In this short comment we question that the authors have enough evidence to make such a claim, as we consider that the large mismatch between SrTiO3 and BaBiO3 and the lack of control of their fabrication process with reflection high energy electron difraction could unlikely give high quality films with a single BiO2- termination, which is one of the requisites for the stabilization of these surface metallic states.

cond-mat.mtrl-sci↗

arXiv2021

Structural complexity in Prussian blue analogues

We survey the most important kinds of structural complexity in Prussian blue analogues, their implications for materials function, and how they might be controlled through judicious choice of composition. We focus on six particular aspects: octahedral tilts, A-site `slides', Jahn--Teller distortions, A-site species and occupancy, hexacyanometallate vacancies, and framework hydration. The promising K-ion cathode material K$_x$Mn[Fe(CN)$_6$]$_y$ serves as a recurrent example that illustrates many of these different types of complexity. Our article concludes with a discussion of how the interplay of various distortion mechanisms might be exploited to optimise the performance of this and other related systems, so as to aid in the design of next-generation PBA materials.

cond-mat.mtrl-sci↗

arXiv2022

Computational Understandings of the Cation Configuration Dependent Redox Activities and Oxygen Dimerizations in Li$_{1.22}$Ni$_{0.22}$Mn$_{0.56}$O$_2$ Cathode

Understanding the lattice oxygen dimerization is quite essential for the optimal design for the Li-rich Mn-based cathode materials. In this work, based on the density functional theory (DFT) calculations, a Ni-honeycomb Li-Ni-Mn cation configuration for Li$_{1.22}$Ni$_{0.22}$Mn$_{0.56}$O$_2$ cathode was carefully proposed and examined, which can coexist with the well-known Li-honeycomb structure in the experimentally synthesized Li$_{1.2}$Ni$_{0.2}$Mn$_{0.6}$O$_2$ samples. Li-Ni-Mn cation configurations have significant impacts on oxygen redox activities and oxygen dimerizations in the delithiated Li$_x$Ni$_{0.22}$Mn$_{0.56}$O$_2$. There is no necessary consistency between the high lattice oxygen redox activity and easy oxygen dimerization, such as the Li-honeycomb structures showing higher redox activities and higher activation energy barriers to prohibit oxygen dimerizations than Ni-honeycomb structures. Avoiding the Ni-honeycomb structures with more favorable lattice oxygen dimerization and making full use of the Li-honeycomb structures with better redox activities is important to optimally design the high-performance Li-rich Mn-based cathode materials.

cond-mat.mtrl-sci↗

arXiv2022

Learning Local Equivariant Representations for Large-Scale Atomistic Dynamics

A simultaneously accurate and computationally efficient parametrization of the energy and atomic forces of molecules and materials is a long-standing goal in the natural sciences. In pursuit of this goal, neural message passing has lead to a paradigm shift by describing many-body correlations of atoms through iteratively passing messages along an atomistic graph. This propagation of information, however, makes parallel computation difficult and limits the length scales that can be studied. Strictly local descriptor-based methods, on the other hand, can scale to large systems but do not currently match the high accuracy observed with message passing approaches. This work introduces Allegro, a strictly local equivariant deep learning interatomic potential that simultaneously exhibits excellent accuracy and scalability of parallel computation. Allegro learns many-body functions of atomic coordinates using a series of tensor products of learned equivariant representations, but without relying on message passing. Allegro obtains improvements over state-of-the-art methods on the QM9 and revised MD-17 data sets. A single tensor product layer is shown to outperform existing deep message passing neural networks and transformers on the QM9 benchmark. Furthermore, Allegro displays remarkable generalization to out-of-distribution data. Molecular dynamics simulations based on Allegro recover structural and kinetic properties of an amorphous phosphate electrolyte in excellent agreement with first principles calculations. Finally, we demonstrate the parallel scaling of Allegro with a dynamics simulation of 100 million atoms.

physics.comp-ph↗

arXiv2022

Expanding the Materials Search Space for Multivalent Cathodes

Multivalent batteries are an energy storage technology with the potential to surpass lithium-ion batteries, however their performance has been limited by the low voltages and poor solid-state ionic mobility of available cathodes. A computational screening approach to identify high-performance multivalent intercalation cathodes among materials that do not contain the working ion of interest has been developed which greatly expands the search space that can be considered for materials discovery. This approach has been applied to magnesium cathodes as a proof of concept and four resulting candidate materials (NASICON V$_2$(PO$_4$)$_3$, birnessite NaMn$_4$O$_8$, tavorite MnPO$_4$F, and spinel MnO$_2$) are discussed in further detail. In examining the ion migration environment and associated Mg$^{2+}$ migration energy in these materials, local energy maxima are found to correspond with pathway positions where Mg$^{2+}$ passes through a plane of anion atoms. While previous works have established the influence of local coordination on multivalent ion mobility, these results suggest that considering both the type of local bonding environment as well as available free volume for the mobile ion along its migration pathway can be significant for improving solid-state mobility.

cond-mat.mtrl-sci↗

arXiv2022

Stabilization and application of asymmetric Néel skyrmions in hybrid nanostructures

Increasing amounts of information force the continuous improvement of information storage and processing technologies, further device miniaturization, and their efficiency increase. Magnetic skyrmions, topological quasiparticles, and the smallest stable magnetic textures possess intriguing properties and potential for data storage applications. Hybrid nanostructures with elements of different magnetization orientations can offer additional advantages for developing skyrmion-based spintronic and magnonic devices. We show that an Néel-type skyrmion confined within a nanodot placed on top of a ferromagnetic stripe produces a unique and compelling platform for exploring mutual coupling between magnetization textures. The skyrmion induces an imprint upon the stripe, which, in turn, asymmetrically squeezes the skyrmion in the dot, increasing their size and the range of skyrmion stability for small values of DMI, as well as introducing skyrmion bi-stability. At the end, we present a proof-of-concept technique for unconstrained transport of a skyrmion along a racetrack based on proposed hybrid systems. Our results demonstrate a hybrid structure that is promising for applications in magnonics and spintronics.

cond-mat.mes-hall↗