Absence Of Long-range Order Research Articles

Elastic properties of Fe95-yNb2Mo2Cu1Siy-xBx (x=5-8, y=20-26) at % amorphous alloys

Amorphous materials exhibit complex atomic structures characterized by the absence of long-range order, in contrast to the well-defined periodicity of crystalline materials. This structural complexity is further pronounced in compositions such as FeMCuSiB (M = Nb, Mo, W and Ta), which incorporate elements with varying atomic radii and valencies. The Young's modulus of structures generated using traditional methods, such as melt-quenching and random packing, shows poor agreement with experimental data. In this study, we employ hybrid methods for generating the structures, which involve randomly packing elements without overlap, followed by thermalization at room temperature using Ab-initio Molecular Dynamics simulations. The Young's modulus evaluated from these structures aligns well with values measured using Dynamic Mechanical Analysis. Additionally, we utilize these structures to determine other elastic moduli including the bulk modulus and tetragonal shear modulus and find that the obtained values are consistent with the expected range for these compounds. We attribute the improved accuracy to a more representative approximation of the amorphous structure and the direct application of energy-strain relationships, rather than stress-strain relationships, for elastic moduli determination. Our methodology facilitates reliable predictions of the physical properties of amorphous materials and contributes to the design of FeMCuSiB (M = Nb, Mo, W and Ta) alloys with enhanced mechanical properties.

Griffiths phase like behaviour in CoCrVZ (Z = Al, Ga) Heusler alloys

We report here the studies of structural and magnetic properties of two equiatomic quaternary Heusler alloys CoCrVZ (Z = Al, Ga). Both the alloys crystallize in LiMgPdSn type crystal structure (space group #216) with B2 (CoCrVAl) and A2 (CoCrVGa) type anti-site disorder between elemental sites. Intended stoichiometry was confirmed from the compositional analysis. The temperature dependence of dc magnetic susceptibility and the low temperature moment values show a nearly compensating ferrimagnetic behaviour at low temperatures for both the alloys. The inverse of dc magnetic susceptibility starts to deviate from the Curie–Weiss law below 220 K and 164 K for the alloys containing Al and Ga respectively. The Arrott plots confirms the absence of long range order down to 2 K. Further analysis of the dc and ac susceptibility shows that the observed behaviour is due to the presence of Griffiths like phase in these alloys.

Construction of Ideal One-Dimensional Spin Chains by Topochemical Dehydration/Rehydration Route.

One-dimensional (1D) Heisenberg antiferromagnets are of great interest due to their intriguing quantum phenomena. However, the experimental realization of such systems with large spin S remains challenging because even weak interchain interactions induce long-range ordering. In this study, we present an ideal 1D S = 5/2 spin chain antiferromagnet achieved through a multistep topochemical route involving dehydration and rehydration. By desorbing three water molecules from (2,2'-bpy)FeF3(H2O)·2H2O (2,2'-bpy = 2,2'-bipyridyl) at 150 °C and then intercalating two water molecules at room temperature (giving (2,2'-bpy)FeF3·2H2O 1), the initially isolated FeF3ON2 octahedra combine to form corner-sharing FeF4N2 octahedral chains, which are effectively separated by organic and added water molecules. Mössbauer spectroscopy reveals significant dynamical fluctuations down to 2.7 K, despite the presence of strong intrachain interactions. Moreover, results from electron spin resonance (ESR) and heat capacity measurements indicate the absence of long-range order down to 0.5 K. This controlled topochemical dehydration/rehydration approach is further extended to (2,2'-bpy)CrF3·2H2O with S = 3/2 1D chains, thus opening the possibility of obtaining other low-dimensional spin lattices.

Symmetry and order of the kinetic heterogeneity in Pd-Si amorphous alloys

In amorphous alloys, the atomic arrangement exhibits short-range order while lacking long-range order. Despite this absence of long-range order, the local atomic arrangements and interactions can still significantly influence the movement of atoms.The microstructural features and structural evolution mechanisms of amorphous materials are key areas of research, and the dynamics of amorphous alloys can provide insights into their formation processes and structural evolution. The cage effect refers to the phenomenon where atoms are trapped by their surrounding atoms, making it difficult for them to migrate or diffuse freely. This leads to slower diffusion rates and higher viscosities in these materials. Atomic concentration is one of the crucial factors that influence the structure and properties of amorphous materials. Changes in concentration can significantly alter the material’s structure. Adjusting the atomic concentration can lead to differences in the diffusion rates between elements in the amorphous alloys, resulting in a heterogeneous distribution of elements in different regions, which in turn affects the deformation characteristics of amorphous materials.This study aims to investigate the effect of Pd atomic concentration on the diffusion hindrance of Si atoms, as well as its impact on the local symmetry and order of the system. To achieve this objective, molecular dynamics simulations are employed to explore the relaxation process of atoms in Pd-Si amorphous alloys at different Pd atomic concentrations, and parameters related to atomic diffusion, displacement distribution, system symmetry, and order are analyzed. The results show that increasing the concentration of Pd atoms leads to a more pronounced hindrance in the diffusion of Si atoms, manifested by an increase in the abnormal peak values of the non-Gaussian parameters and a decrease in the standard deviation of the displacements. This indicates that a higher Pd atom concentration enhances the cage effect of Si atoms, thus restricting their diffusion. Additionally, the increase in Pd concentration promotes the transition from unsaturated to saturated bond types in the Pd-Si amorphous alloys, and also leads to a decrease in the system's configurational entropy. This consequently enhances the local symmetry and order of the Pd-Si amorphous alloys, resulting in Si atoms locate at the center of more closed, higher-symmetry, and more compact cluster structures, which strengthens their cage effect and local symmetry. This study investigates the impact of Pd atom concentration on the diffusion behavior and local environment of Si atoms, providing a new insight into the structural evolution of amorphous alloys.

Anion-polarisation-directed short-range-order in antiperovskite Li2FeSO.

Short-range ordering in cation-disordered cathodes can have a significant effect on their electrochemical properties. Here, we characterise the cation short-range order in the antiperovskite cathode material Li2FeSO, using density functional theory, Monte Carlo simulations, and synchrotron X-ray pair-distribution-function data. We predict partial short-range cation-ordering, characterised by favourable OLi4Fe2 oxygen coordination with a preference for polar cis-OLi4Fe2 over non-polar trans-OLi4Fe2 configurations. This preference for polar cation configurations produces long-range disorder, in agreement with experimental data. The predicted short-range-order preference contrasts with that for a simple point-charge model, which instead predicts preferential trans-OLi4Fe2 oxygen coordination and corresponding long-range crystallographic order. The absence of long-range order in Li2FeSO can therefore be attributed to the relative stability of cis-OLi4Fe2 and other non-OLi4Fe2 oxygen-coordination motifs. We show that this effect is associated with the polarisation of oxide and sulfide anions in polar coordination environments, which stabilises these polar short-range cation orderings. We propose that similar anion-polarisation-directed short-range-ordering may be present in other heterocationic materials that contain cations with different formal charges. Our analysis illustrates the limitations of using simple point-charge models to predict the structure of cation-disordered materials, where other factors, such as anion polarisation, may play a critical role in directing both short- and long-range structural correlations.

BirchiteCd2Cu2(PO4)2SO4·5H2Oas a model antiferromagnetic spin-1/2 HeisenbergJ1−J2chain

$S=1/2$ Heisenberg ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ chain antiferromagnets have been investigated extensively due to their exotic magnetic states. Here, we report the magnetic behavior of birchite ${\mathrm{Cd}}_{2}{\mathrm{Cu}}_{2}{({\mathrm{PO}}_{4})}_{2}{\mathrm{SO}}_{4}\ifmmode\cdot\else\textperiodcentered\fi{}5{\mathrm{H}}_{2}\mathrm{O}$ and its effective spin model. Experimental studies by magnetic susceptibility, magnetization, heat capacity, and $\ensuremath{\mu}\mathrm{SR}$ measurements indicate the absence of long-range order down to 0.4 K. Theoretical studies reveal that birchite is a model compound for the ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ antiferromagnetic chain: the intrachain interactions ${J}_{1}$ and ${J}_{2}$ are antiferromagnetic and their magnitude is about 100 times larger than the interchain interactions. The magnitude of ${J}_{2}$ is two to three times larger than that of ${J}_{1}$, thus the spin gap is expected to be only a few percent of that of ${J}_{1}$. The temperature dependence of the specific heat shows a broad peak at about 1 K ($\ensuremath{\simeq}0.036{J}_{1}$), which suggests the presence of a spin gap.

Possible realization of three-dimensional quantum spin liquid behavior in HoVO4

The study of geometrically frustrated magnetic systems with unusual crystal field ground states offers a possibility of realizing the new aspects of physics of disordered systems. In this study, we report our results of structural, magnetic susceptibility, heat capacity measurements, along with density functional theory (DFT) calculations on HoVO4; a compound in which the presence of a distorted kind of HoO8 polyhedral leads to multiple magnetic interaction paths. The observed broad maximum below 10 K in the temperature response of DC susceptibility curves implies the presence of short-range correlations. AC susceptibility rules out the possibility of any kind of spin freezing. Temperature dependent heat capacity measurement at zero field indicate towards the absence of long-range ordering, along with the presence of a broad maximum centered around 14 K. The residual heat capacity exhibits a characteristic power-law (Tα ) behavior with the exponent α nearly equal to 2, which is analogous to that observed for other three-dimensional (3D) quantum spin liquid (QSL) systems. The DFT calculations signify the presence of dominant second and third nearest neighbor interactions, which in turn lead to magnetic frustration in our system. Our investigations suggest that HoVO4 can be a candidate for realizing a 3D QSL state.

Cooperative freezing of the L12 ordered domains at the critical cooling temperature of Ni3Fe alloy

It is well known that Ni3Fe transforms from a disordered solid solution to an ordered intermetallic with L12 superstructure when the alloy is cooled slowly. Here we elucidate the underlying cooperative phenomenon and the atomistic mechanism of this ordering process based on simulations using embedded atom potentials. As the simulated alloy is cooled from the disordered state to the critical cooling temperature (T c), Ni atoms with L12 order [denoted as Ni(L12 ⩾ 1) atoms] increase significantly along with Ni atoms having the least deviation from L12 local order (denoted as Ni([IP]3) atoms). The ordering (up to T c) occurs predominantly through random increase in Ni(L12 ⩾ 1) atoms throughout the system, as indicated by absence of long-range order. At T c, L12 ordered domains formed by Ni(L12 ⩾ 1) atoms ‘freeze’, i.e. these domains, collectively, achieve a threshold strength against thermal fluctuations. This is indicated by (i) dissipation of large-scale fluctuations of Ni(L12 ⩾ 1) atoms at T c and (ii) the growth of the L12 domains through propagation (at the expense of atoms with non-L12 local environment) as the alloy is cooled below T c. The stability threshold of the L12 ordered domains at T c is qualitatively consistent with (i) the critical slowing down, i.e. a significant increase in annealing time (to about 41 days) at 497 °C close to T c (∼500 °C) and (ii) sharp changes in bulk properties (due to loss of stability of the domains) when the alloy is heated across T c to about 550 °C. Further, the experimental long-range order parameter values as a function of reduced temperature are in reasonable agreement with the corresponding values of the simulated alloys. The contribution of Ni([IP]3) atoms to ordering in the actual alloy is potentially significant since such atoms together with nearest neighbours constitute about 75% of the total atoms in the simulated alloys at T c.

Atomic order evolution on the length scale in metallic glasses

Possessing the short-range order but lacking of the long-range order is ubiquitous in the atomic structures of metallic glasses. We compare the inherent structures of metallic glasses with different compositions simulated via the molecular dynamics to explore the order evolution at different length scales. The short-range order mostly contributes to the dominant clusters with higher local order metrics, which is not universal for all atoms. Only the Bergman type of medium-range order centered by icosahedral clusters is clearly identified, resulting in a much lower degree of medium-range order. With the further increase of length scale, locations of neighboring atoms are less correlated, including the icosahedral cluster network. The absence of long-range order is the result of the decreasing evolution of order at the length scale.

Is icosahedral short-range order presented in supercooled transition metals?

Supercooled transition metals are characterized by the absence of long-range order and the presence of a specific short-range order in the arrangement of atoms. So, the presence of shoulders and broadenings at the second maximum in the experimentally measured quantity—in the static structure factor S(k)—is usually interpreted as a manifestation of the icosahedral (ideal or distorted) short-range order (ISRO—Icosahedral Short-Range Order). Icosahedral short-range order is a structure with fivefold symmetry in the arrangement of atoms, which can lead to the possibility of achieving deep supercooling. In this work, we study the local structural features of equilibrium and supercooled nickel melts under various cooling protocols ( K s−1) in order to clarify the mechanism of formation of the icosahedral short-range order in pure transition metals. Comprehensive studies of the properties of nickel melts were carried out using experiments on x-ray diffraction, large-scale molecular dynamics (MD) simulations with subsequent structural and cluster analysis. A good agreement was found between the results of x-ray diffraction data and the MD simulations results for an equilibrium nickel melt. It was found that the nickel melt is characterized by a short-range order, formed by fragments of icosahedra and distorted icosahedral clusters. It was revealed that the ‘liquid-crystal’ phase transition in nickel is accompanied by the transformation of distorted icosahedral clusters into clusters with the fcc/hcp-symmetry. It is shown that, in contrast to the Voronoi tessellation method, the cluster analysis method based on the () rotational invariants does not allow sufficiently correct identification of the distorted icosahedral short-range order in metal melts.

Absence of long-range order in a three-dimensional stacked Ising antiferromagnet on kagome lattice

We study the possibility of long-range ordering (LRO) in a 3D system of vertically stacked layers of Ising antiferromagnet on a kagome lattice (SIAKL). Monte Carlo simulations are carried out for a varying interlayer coupling strength and a finite-size scaling analysis is performed for selected cases. Unlike in the related Ising system on a triangular lattice, in which even a finite number of the layers can stabilize LRO, or the Heisenberg system on the same kagome lattice, in which LRO emerged in 3D for a sufficient strength of the interlayer coupling, no LRO could be observed in the present model. This makes SIAKL a rare example of a 3D Ising paramagnet composed of frustrated layers coupled by unfrustrated interaction.

Hydrothermal Synthesis and a Composite Crystal Structure of Na6Cu7BiO4(PO4)4[Cl,(OH)]3 as a Candidate for Quantum Spin Liquid

A novel sodium bismuth oxo-cuprate phosphate chloride, Na6Cu7BiO4(PO4)4[Cl2.23(OH)0.77], containing square-kagomé layers of Cu2+ has been synthesized by hydrothermal techniques. The compound crystallizes in the tetragonal space group P4/nmm, a = 10.0176(4), c = 10.8545(6), Z = 2, V = 1089.3(1) Å3, R1 = 0.021, wR = 0.053, S = 1.32. Its composite crystal structure includes [O4Cu6Bi]7+ layers, which are formed by the clusters of oxygen-centered tetrahedra [OCu3Bi]. These positively charged two periodic fragments are intercalated in a negatively charged [CuNa6Cl3(PO4)4]7- matrix built by Na-centered polyhedra, PO4 tetrahedra, and CuO4Cl pyramids. The composite character of the crystal structure of Na6Cu7BiO4(PO4)4[Cl2.23(OH)0.77], as well as the way of its self-assembly, are discussed in close connection with the sulfohalite Na6ClF(SO4)2 salt. It is shown that the "host-guest" model of the formation of the tetragonal Na6Cu7BiO4(PO4)4[Cl2.23(OH)0.77] phase is due to the group-subgroup symmetry relation with the cubic crystal structure of mineral sulfohalite and is also supported by the crystallization condition in excess sodium chloride. The magnetic subsystem of Na6Cu7BiO4(PO4)4[Cl2.23(OH)0.77] is represented by a dense square-kagomé network of 2Cu1 and 4Cu2 ions, decorated with weakly bonded Cu3 ions. Measurements of magnetization and heat capacity indicate the absence of long-range order up to 2 K, which makes this compound a candidate for a highly demanded spin liquid.

(Energy Technology Division Graduate Student Award sponsored by Bio-Logic) Understanding Charge Transport for Current and Future Electrochemical Energy Storage Technologies

Advances in electrochemical energy storage are key for the integration of intermittent wind and solar energy sources into the grid, widespread adoption of electric vehicles, and development of new safe and reliable consumer devices. For grid, transportation, and electronic energy storage markets, each application has distinct needs. Therefore, developing battery technology for emerging and wide-spread applications demands greater understanding of the complex chemical and electrochemical processes that occur within rechargeable batteries. Electron transfer and ion transport are the foundation of battery function where the capacity, loaded voltage and current define the practical energy and power delivery of the system. During operation, repetitive addition and removal of electrons and ions can lead to structural change within the electrode and interfacial formation that can dramatically impact electrochemical outcomes. Thus, structural, chemical, and electrochemical characterization during operation is critically important for next generation electrochemical energy storage technologies. The research efforts highlighted in this talk use multiple advanced characterization techniques, including operando methods, to probe charge transport properties, structural changes, and interfacial evolution in multiple rechargeable battery systems. Specifically, spatiotemporal x-ray fluorescence mapping was used to quantify elemental composition near the electrode and in the electrolyte of operando cells. Isothermal microcalorimetry was used to determine heat evolution due to parasitic or decomposition reactions operando. X-ray absorption spectroscopy enabled structure and oxidation state elucidation in the absence of long-range order. X-ray diffraction was employed to track phase changes of distinct electroactive materials. Using advanced characterization, these research efforts provide insight into electron transfer and ion transport properties and their impact on application relevant outcomes (resistance, cycle life, delivered capacity) of electrochemical energy storage technologies.

Probing the interplay between lattice dynamics and short-range magnetic correlations in CuGeO3 with femtosecond RIXS

Investigations of magnetically ordered phases on the femtosecond timescale have provided significant insights into the influence of charge and lattice degrees of freedom on the magnetic sub-system. However, short-range magnetic correlations occurring in the absence of long-range order, for example in spin-frustrated systems, are inaccessible to many ultrafast techniques. Here, we show how time-resolved resonant inelastic X-ray scattering (trRIXS) is capable of probing such short-ranged magnetic dynamics in a charge-transfer insulator through the detection of a Zhang–Rice singlet exciton. Utilizing trRIXS measurements at the O K-edge, and in combination with model calculations, we probe the short-range spin correlations in the frustrated spin chain material CuGeO3 following photo-excitation, revealing a strong coupling between the local lattice and spin sub-systems.

Особенности измерения температурных зависимостей электрического сопротивления углеродных материалов, полученных термолизом смесей фенолфталеина с меламином

Carbon materials with high nitrogen content show great potential for modern electronics. To obtain such materials, it is convenient to use melamine as a source of nitrogen, since during its thermal decomposition a significant amount of nitrogen-containing groups is formed, which can be incorporated into the structure of the carbon material. The study of the temperature de-pendence of resistance for the obtained materials has made it possible to establish the effect of nitrogen atoms on the band structure of the samples. The article describes a method for obtaining single-phase nitrogen-containing solid solutions, based on amorphous carbon, by thermolysis of molten mixtures of melamine with phenolphthalein under heating to 500 °C. This method has made it possible to achieve the nitrogen content of 21 wt. %. The results of the study of new materials by the X-ray phase analysis are presented, confirming the chaotic structure of solid solutions and the absence of long-range order in the arrangement of carbon or nitrogen atoms. The morphology of the obtained samples with various concentrations of phenolphthalein varies from loosely bound powders to solid foam. The complexity of measuring the temperature dependences of resistivity is associated with the difficulty of preparing standard rectangular samples. To measure the temperature dependences of resistance, the samples were ground to a powder of the given granulometric composition, and a vacuum measuring cell was constructed, as well as measurement technique was developed in the range of 30–300 °C. The paper describes the experimental setup and sample preparation, and also presents the results of measurements of samples of solid solutions of nitrogen in amorphous carbon with a high nitrogen content. A monotonical decrease in the band gap of solid solutions from 1.07 to 0.61 eV with an increase in the nitrogen content in them has been shown.

GLAS FORMATION IN QUASI-TERNARY SYSTEMS AІ2S–ВIVS2–P2S5 (АI – Cu, Ag; ВIV – Ge, Sn)

The boundaries of the glass formation areas of quasiternary systems AІ2S–ВIVS2–P2S5 (АI – Cu, Ag; ВIV – Ge, Sn) have been established by the method of X-ray phase analysis. The compound GеS2 can exist in a glass state, so that germanium disulphide acts as a glass former in quasi-triple systems.The glass semiconductor alloys have been synthesized by the single-temperature method from the elementary substances of copper (99.99 wt.%), silver (99.99 wt.%), germanium (99.9999 wt.%), tin (99.999 wt.%), red phosphorus (99.998 wt.%) and sulfur (purity 99.997 wt.%). Maximum synthesis temperature was 1173 K, followed by quenching of the ampoules in a saturated solution of sodium chloride with crushed ice. Glasses with a high content of phosphorus are easily hydrolyzed under the influence of moisture of the air.Diffractograms have been recorded on a diffractometer DRON 4-13, CuKα-radiation, range of angles 2θ=10÷50°, stepped scan 0,05º, exposure time 2 seconds. Amorphousness of the obtained alloys has been controlled visually by the fracture characteristic of the glass and with the help of X-ray diffraction studies. «Gallo» has been observed on all diffractograms of glass samples, presence of which indicates the absence of long-range order in the alloy structure.There are continuous bands of glass formation on the sides GeS2–P2S5 in the studied germanium-containing systems АI2S–GеS2–P2S5. The obtained glasses have been transparent, yellow-red color with the fracture characteristic of the glass. The main factor is the tendency of GeS2 and P2S5 to the glass formation. There has been found that all samples have polycrystalline nature in the system Cu2S–GеS2. Only GeS2 and Ag2GeS3 have been obtained in the vitreous state from the cross section Ag2S–GeS2.The maximum content of Cu2S and Ag2S, which are a part of the glass in systems Сu(Ag)2S–GеS2–P2S5, is 10 and 70 mol.% accordingly.The areas of glass formation have been much smaller compared to similar germanium-containing in the state-containing systems of the glassformation area, which is associated with the strengthening of the metal component of the chemical bond by changing GeS2 to SnS2.We observe only two areas of glass formation in quasi-triple systems Сu(Ag)2S–SnS2–P2S5 which are on the side SnS2–P2S5: one in the area 5–15 mol.% P2S5, including approximately 5 mol.% Cu2S та Ag2S, the other – 35–65 mol.% P2S5, the maximum content of Cu2S and Ag2S is 13 and 4 mol.%.

Absence of Spontaneous Spin Symmetry Breaking in 1D and 2D Quantum Ferromagnetic Systems with Bilinear and Biquadratic Exchange Interactions

Some measurements have shown that the second-order exchange interaction is non-negligible in ferromagnetic compounds whose microscopic interactions are described by means of half-odd integer quantum spins. In these spin systems the ground state is either ferromagnetic or antiferromagnetic when the bilinear exchange interaction is dominant. Instead, in ferromagnetic systems characterized by bilinear and biquadratic exchange interactions of comparable magnitude, the energy minimum occurs when spins are in a canting ground-state. To this aim, a one-dimensional (1D) quantum spin chain and a two-dimensional (2D) lattice of quantum spins subjected to periodic boundary conditions are modeled via the generalized quantum Heisenberg Hamiltonian containing, in addition to the isotropic and short-range bilinear exchange interaction of the Heisenberg type, a second-order interaction, the isotropic and short-range biquadratic exchange interaction between nearest-neighbors quantum spins. For these 1D and 2D quantum systems a generalization of the Mermin–Wagner–Hohenberg theorem (also known as Mermin–Wagner–Berezinksii or Coleman theorem) is given. It is demonstrated, by means of quantum statistical arguments, based on Bogoliubov’s inequality, that, at any finite temperature, (1) there is absence of long-range order and that (2) the law governing the vanishing of the order parameter is the same as in the bilinear case for both 1D and 2D quantum ferromagnetic systems. The physical implications of the absence of a spontaneous spin symmetry breaking in 1D spin chains and 2D spin lattices modeled via a generalized quantum Heisenberg Hamiltonian are discussed.

Local structural investigations of Fe-doped TiO2 amorphous thin films

The present study deals with the local structure of amorphous TiO2 and Fe-doped TiO2 thin films deposited on Si substrate using magnetron sputtering. The absence of long range order is confirmed by grazing incidence x-ray diffraction and the short range order is studied using x-ray absorption near edge structure (XANES) measurements at Ti, Fe and O K-edges and Fe and Ti L3-edge. The XANES provides understanding about the electronic transition induced from the short range ordering and dependence of electronic hybridization of the central Ti-p orbitals with the Ti-d orbitals. The XANES measurements also show that maximum Fe doping concentration allowed to replace Ti is 2 at.% and after that separate phase starts appearing, which is consistent with simulation by ab-initio multiple scattering theory based code FEFF9. Ellipsometry measurements show gradual decrease in band-gap energy upto 2 at.% Fe doping from pristine TiO2, however a drastic decrease in band-gap energy is observed at 4 at.% or higher doping concentration due to formation of other phases.

Generating Multiscale Amorphous Molecular Structures Using Deep Learning: A Study in 2D.

Amorphous molecular assemblies appear in a vast array of systems: from living cells to chemical plants and from everyday items to new devices. The absence of long-range order in amorphous materials implies that precise knowledge of their underlying structures throughout is needed to rationalize and control their properties at the mesoscale. Standard computational simulations suffer from exponentially unfavorable scaling of the required compute with system size. We present a method based on deep learning that leverages the finite range of structural correlations for an autoregressive generation of disordered molecular aggregates up to arbitrary size from small-scale computational or experimental samples. We benchmark performance on self-assembled nanoparticle aggregates and proceed to simulate monolayer amorphous carbon with atomistic resolution. This method bridges the gap between the nanoscale and mesoscale simulations of amorphous molecular systems.

Absence of long-range order in a general spin-S kagome lattice Ising antiferromagnet

The possibility of the emergence of some kind of long-range ordering (LRO) due to the increase of multiplicity of the local degrees of freedom (spin value S) is studied in an Ising antiferromagnet on a kagome lattice (IAKL) by Monte Carlo simulation. In particular, the critical exponent of the spin correlation function, obtained from a finite-size scaling analysis, is evaluated for various values of S, including S=∞, with the goal to determine whether there exists some threshold value of the spin SC above which the system would show true or quasi-LRO, similar to a related model on a triangular lattice (IATL). It is found that, unlike in the IATL case, the IAKL model remains disordered for any spin value and any finite temperature.