arXiv subjects

Arkadiy Simonov

Publications and source records attributed to Arkadiy Simonov.

16 recordsLinked to original sources

arXiv2026

X-ray Thermal diffuse scattering from real-space displacement correlations

We introduce a method for calculating X-ray thermal diffuse scattering based on the three-dimensional difference pair distribution function (3D-dPDF). Within the harmonic approximation the method is exact, as it includes all orders of multi-phonon scattering. A single Fourier transform delivers the diffuse intensity over large volumes of reciprocal space. We tested the method against experimental diffuse scattering measured on a silicon single crystal. With nothing refined beyond a scale and a background, the calculation reproduces the measured intensity to an R2 residual below 5%. Because the method uses the same pair-correlation language as the established Yell program, thermal and static correlated disorder enter the analysis on equal footing.

cond-mat.mtrl-sci↗

arXiv2025

Symmetry breaking in Prussian Blue Analogues via growth--guided local ordering of hexacyanometallate vacancies

We report Growth--Guided Local Ordering, a novel mechanism of symmetry reduction in disordered crystals. This mechanism operates through the directional ordering of point defects during crystal growth, where defect correlations develop preferentially along the growth direction, resulting in reduced symmetry that persists in the final structure through the spatial distribution of defects. We demonstrate this phenomenon in Mn[Co]-Prussian Blue Analogues, disordered cyanide crystals containing numerous Co(CN)$_6$ vacancies. Single crystal diffuse scattering reveals pronounced anisotropy in vacancy distribution: strong correlations along [001] growth direction contrast with weak correlations perpendicular to it. This local ordering reduces the Laue symmetry to tetragonal $4/mmm$, evident in properties such as birefringence, while the average structure retains cubic $m\bar 3m$ symmetry. When growth proceeds along [111] direction, the same mechanism produces domains with trigonal symmetry. Because this mechanism relies on fundamental aspects of crystal growth rather than specific material properties, it offers a general strategy for symmetry control in disordered crystals. Crucially, it transforms the complex task of altering crystal symmetry into the more manageable challenge of controlling growth direction, achievable through various established techniques such as the use of surfactants during crystallization.

cond-mat.mtrl-sci↗

arXiv2024

Disentangling competing interactions in disordered materials using interaction space modelling

Understanding and manipulating the relationship between intentionally introduced disorder and material properties necessitates efficient characterization techniques. For example, single crystal diffuse scattering experiments provide insights into the driving forces behind local order phenomena. In this work, we present a time- and resource-efficient approach based on mean field theory, that quantifies local interaction energies but unlike other techniques does not require computationally expensive supercell models. The method is employed to quantify competing interactions in functionally disordered materials such as disordered rock salt cathode materials and Prussian blue analogs that share an underlying face-centred lattice.

cond-mat.mtrl-sci↗

arXiv2024

Magnetoelectric domain engineering from micrometer to {\AA}ngstr{\o}m scales

The functionality of magnetoelectric multiferroics depends on the formation, size, and coupling of their magnetic and electric domains. Knowing the parameters guiding these criteria is a key effort in the emerging field of magnetoelectric domain engineering. Here we show, using a combination of piezoresponse-force microscopy, non-linear optics, and x-ray scattering, that the correlation length setting the size of the ferroelectric domains in the multiferroic hexagonal manganites can be engineered from the micron range down to a few unit cells under the substitution of Mn$^{3+}$ ions with Al$^{3+}$ ions. The magnetoelectric coupling mechanism between the antiferromagnetic Mn$^{3+}$ order and the distortive-ferroelectric order remains intact even at substantial replacement of Mn$^{3+}$ by Al$^{3+}$. Hence, chemical substitution proves to be an effective tool for domain-size engineering in one of the most studied classes of multiferroics.

cond-mat.mtrl-sci↗

arXiv2024

Giant Modulation of Refractive Index from Picoscale Atomic Displacements

Structural disorder has been shown to enhance and modulate magnetic, electrical, dipolar, electrochemical, and mechanical properties of materials. However, the possibility of obtaining novel optical and optoelectronic properties from structural disorder remains an open question. Here, we show unambiguous evidence of disorder in the form of anisotropic, picoscale atomic displacements modulating the refractive index tensor and resulting in the giant optical anisotropy observed in BaTiS$_3$, a quasi-one-dimensional hexagonal chalcogenide. Single crystal X-ray diffraction studies reveal the presence of antipolar displacements of Ti atoms within adjacent TiS$_6$ chains along the c-axis, and three-fold degenerate Ti displacements in the a-b plane. $^{47/49}$Ti solid-state NMR provides additional evidence for those Ti displacements in the form of a three-horned NMR lineshape resulting from a low symmetry local environment around Ti atoms. We used scanning transmission electron microscopy to directly observe the globally disordered Ti a-b plane displacements and find them to be ordered locally over a few unit cells. First-principles calculations show that the Ti a-b plane displacements selectively reduce the refractive index along the ab-plane, while having minimal impact on the refractive index along the chain direction, thus resulting in a giant enhancement in the optical anisotropy. By showing a strong connection between structural disorder with picoscale displacements and the optical response in BaTiS$_3$, this study opens a pathway for designing optical materials with high refractive index and functionalities such as large optical anisotropy and nonlinearity.

cond-mat.mtrl-sci↗

arXiv2023

Origin of correlated diffuse scattering in the hexagonal manganites

We use a combination of first-principles density functional calculations and spin-dynamics simulations to explain the unusual diffuse inelastic neutron scattering in the hexagonal multiferroic yttrium manganite, YMnO$_3$. Using symmetry considerations, we construct a model spin Hamiltonian with parameters derived from our density functional calculations and show that it captures the measured behavior. We then show that the observed directionality in the structured diffuse scattering in momentum space is a hallmark of the triangular geometry, and that its persistence across a wide range of temperatures, both above and below the Néel temperature, T$_\text{N}$, is a result of the strong magnetic frustration. We predict that this diffuse scattering exists in a yet-to-be-observed modulated continuum of energies, that its associated spin excitations have distinct in-plane and out-of-plane character and that the frustration influences the magnetism below the Néel temperature. Finally, we show that visualizing the magnetic order in terms of composite trimer magnetoelectric monopoles and toroidal moments, rather than individual spins, provides insight into the real space fluctuations, revealing clusters of emerging order in the paramagnetic state, as well as collective short-range excitations in the ordered antiferromagnetic phase. Our understanding of this directional diffuse scattering in such a wide temperature range, both below and above T$_\text{N}$, provides new insight into the magnetic phase transitions in frustrated systems.

cond-mat.mtrl-sci↗

arXiv2023

Effect of the depolarizing field on the domain structure of an improper ferroelectric

We show that, contrary to common belief, the depolarizing electric field generated by bound charges at thin-film surfaces can have a substantial impact on the domain structure of an improper ferroelectric with topological defects. In hexagonal-manganite thin films, we observe in phase-field simulations that through the action of the depolarizing field, (i) the average magnitude of the polarization decreases, (ii) the local magnitude of the polarization decreases with increasing distance from the domain walls, and (iii) there is a significant alteration of the domain-size distribution and average domain size, which is visualized with the pair-correlation function. We conclude that, in general, it is not appropriate to ignore the effects of the depolarizing field for thin film ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2021

Striping of orbital-order with charge-disorder in optimally doped manganites

The phase diagrams of LaMnO$_3$ perovskites have been intensely studied due to the colossal magnetoresistance (CMR) exhibited by compositions around the $\frac{3}{8}^{th}$ doping level. However, phase segregation between ferromagnetic (FM) metallic and antiferromagnetic (AFM) insulating states, which itself is believed to be responsible for the colossal change in resistance under applied magnetic field, has prevented an atomistic-level understanding of the orbital ordered (OO) state at this doping level. Here, through the detailed crystallographic analysis of the phase diagram of a prototype system (AMn$_3^{A'}$Mn$_4^B$O$_{12}$), we show that the superposition of two distinct lattice modes gives rise to a striping of OO Jahn-Teller active Mn$^{3+}$ and charge disordered (CD) Mn$^{3.5+}$ layers in a 1:3 ratio. This superposition only gives a cancellation of the Jahn-Teller-like displacements at the critical doping level. This striping of CD Mn$^{3.5+}$ with Mn$^{3+}$ provides a natural mechanism though which long range OO can melt, giving way to a conducting state.

cond-mat.str-el↗

arXiv2019

Designing Disorder into Crystalline Materials

Crystals are a state of matter characterised by periodic order. Yet crystalline materials can harbour disorder in many guises, such as non-repeating variations in composition, atom displacements, bonding arrangements, molecular orientations, conformations, charge states, orbital occupancies, or magnetic structure. Disorder can sometimes be random, but more usually it is correlated. Frontier research into disordered crystals now seeks to control and exploit the unusual patterns that persist within these correlated disordered states in order to access functional responses inaccessible to conventional crystals. In this review we survey the core design principles at the disposal of materials chemists that allow targeted control over correlated disorder. We show how these principles---often informed by long-studied statistical mechanical models---can be applied across an unexpectedly broad range of materials, including organics, supramolecular assemblies, oxide ceramics, and metal--organic frameworks. We conclude with a forward-looking discussion of the exciting link to function in responsive media, thermoelectrics, topological phases, and information storage.

cond-mat.mtrl-sci↗

arXiv2019

Hidden Diversity of Vacancy Networks in Prussian Blue Analogues

Prussian blue analogues (PBAs) are a broad and important family of microporous inorganic solids, famous for their gas storage, metal-ion immobilisation, proton conduction, and stimuli-dependent magnetic, electronic and optical properties. The family also includes the widely-used double-metal cyanide (DMC) catalysts and the topical hexacyanoferrate/hexacyanomanganate (HCF/HCM) battery materials. Central to the various physical properties of PBAs is the ability to transport mass reversibly, a process made possible by structural vacancies. Normally presumed random, vacancy arrangements are actually crucially important because they control the connectivity of the micropore network, and hence diffusivity and adsorption profiles. The long-standing obstacle to characterising PBA vacancy networks has always been the relative inaccessibility of single-crystal samples. Here we report the growth of single crystals of a range of PBAs. By measuring and interpreting their X-ray diffuse scattering patterns, we identify for the first time a striking diversity of non-random vacancy arrangements that is hidden from conventional crystallographic analysis of powder samples. Moreover, we show that this unexpected phase complexity can be understood in terms of a remarkably simple microscopic model based on local rules of electroneutrality and centrosymmetry. The hidden phase boundaries that emerge demarcate vacancy-network polymorphs with profoundly different micropore characteristics. Our results establish a clear foundation for correlated defect engineering in PBAs as a means of controlling storage capacity, anisotropy, and transport efficiency.

cond-mat.mtrl-sci↗

arXiv2019

Room temperature spin-ice physics in cadmium cyanide

Spin-ices are frustrated magnets that support a particularly rich variety of emergent physics. Typically, it is the interplay of magnetic dipole interactions, spin anisotropy, and geometric frustration on the pyrochlore lattice that drives spin-ice formation. The relevant physics occurs at temperatures commensurate with the magnetic interaction strength, which for most systems is 1--5\,K. This low energy scale poses severe challenges for experimental studies of spin-ices and the practical exploitation of their unusual properties. Here, we show that non-magnetic cadmium cyanide (Cd(CN)$_2$) exhibits analogous behaviour to magnetic spin-ices, but does so on a temperature scale that is nearly two orders of magnitude greater. The electric dipole moments of cyanide ions in Cd(CN)$_2$ assume the role of magnetic pseudospins, with the difference in energy scale reflecting the increased strength of electric \emph{vs} magnetic dipolar interactions. As a result, spin-ice physics influences the structural behaviour of Cd(CN)$_2$ even at room temperature.

cond-mat.mtrl-sci↗

arXiv2018

Improper Ferroelectric Polarisation in a Perovskite driven by Inter-site Charge Transfer and Ordering

It is of great interest to design and make materials in which ferroelectric polarisation is coupled to other order parameters such as lattice, magnetic and electronic instabilities. Such materials will be invaluable in next-generation data storage devices. Recently, remarkable progress has been made in understanding improper ferroelectric coupling mechanisms that arise from lattice and magnetic instabilities. However, although theoretically predicted, a compact lattice coupling between electronic and ferroelectric (polar) instabilities has yet to be realised. Here we report detailed crystallographic studies of a novel perovskite Hg$^{\textbf{A}}$Mn$^{\textbf{A'}}_{3}$Mn$^{\textbf{B}}_{4}$O$_{12}$ that is found to exhibit a polar ground state on account of such couplings that arise from charge and orbital ordering on both the A' and B-sites, which are themselves driven by a highly unusual Mn$^{A'}$-Mn$^B$ inter-site charge transfer. The inherent coupling of polar, charge, orbital and hence magnetic degrees of freedom, make this a system of great fundamental interest, and demonstrating ferroelectric switching in this and a host of recently reported hybrid improper ferroelectrics remains a substantial challenge.

cond-mat.str-el↗

arXiv2018

Extreme cooperative swelling in topologically disordered fibre entanglements

Entangled states are ubiquitous amongst fibrous materials, whether naturally occurring (keratin, collagen, DNA) or synthetic (nanotube assemblies, elastane). A key mechanical characteristic of these systems is their ability to reorganise in response to external stimuli, as implicated in e.g. hydration-induced swelling of keratin fibrils in human skin. During swelling, the curvature of individual fibres changes to give a cooperative and reversible structural reorganisation that opens up a pore network. The phenomenon is known to be highly dependent on topology, even if the nature of this dependence is not well understood: certain ordered entanglements (`weavings') can swell to many times their original volume while others are entirely incapable of swelling at all. Given this sensitivity to topology, it is puzzling how the disordered entanglements of many real materials manage to support cooperative dilation mechanisms. Here we use a combination of geometric and lattice-dynamical modelling to study the effect of disorder on swelling behaviour. The model system we devise spans a continuum of disordered topologies and is bounded by ordered states whose swelling behaviour is already known to be either vanishingly small or extreme. We find that while topological disorder often quenches swelling behaviour, certain disordered states possess a surprisingly large swelling capacity. Crucially, we show that the extreme swelling response previously observed only for certain specific weavings can be matched---and even superseded---by that of disordered entanglements. Our results establish a counterintuitive link between topological disorder and mechanical flexibility that has implications not only for polymer science but also for our broader understanding of collective phenomena in disordered systems.

cond-mat.soft↗

arXiv2017

Hybrid Local-Order Mechanism for Inversion Symmetry Breaking

Using classical Monte Carlo simulations, we study a simple statistical mechanical model of relevance to the emergence of polarisation from local displacements on the square and cubic lattices. Our model contains two key ingredients: a Kitaev-like orientation-dependent interaction between nearest neighbours, and a steric term that acts between next-nearest neighbours. Taken by themselves, each of these two ingredients is incapable of driving long-range symmetry breaking, despite the presence of a broad feature in the corresponding heat capacity functions. Instead each component results in a "hidden" transition on cooling to a manifold of degenerate states, the two manifolds are different in the sense that they reflect distinct types of local order. Remarkably, their intersection---\emph{i.e.} the ground state when both interaction terms are included in the Hamiltonian---supports a spontaneous polarisation. In this way, our study demonstrates how local ordering mechanisms might be combined to break global inversion symmetry in a manner conceptually similar to that operating in the "hybrid" improper ferroelectrics. We discuss the relevance of our analysis to the emergence of spontaneous polarisation in well-studied ferroelectrics such as BaTiO$_3$ and KNbO$_3$.

cond-mat.mtrl-sci↗

arXiv2017

Correlated local dipoles in PbTe

We present a combined single-crystal x-ray diffuse scattering and ab-initio molecular dynamics study of lead telluride, PbTe. Well-known for its thermoelectric and narrow-gap semiconducting properties, PbTe recently achieved further notoriety following the report of an unusual off-centering of the lead atoms, accompanied by a local symmetry breaking, on heating. This observation, which was named emphanisis, ignited considerable controversy regarding the details of the underlying local structure and the appropriate interpretation of the total scattering experiments. In this study, we demonstrate close agreement between our diffuse scattering measurements and our calculations, which allows us to analyze features such as higher-order correlations that are accessible in the simulations but not experimentally. This allowed us to discover an unusual correlated local dipole formation extending over several unit cells with an associated local reduction of the cubic symmetry in both our x-ray diffuse scattering measurements and our molecular dynamics simulations. Importantly, when averaged spatially or temporally, the most probable positions for the ions are at the centers of their coordination polyhedra. Our results therefore clarify the nature of the local symmetry breaking, and reveal the source of the earlier controversy regarding the existence or absence of off-centering. Finally, we provide an interpretation of the behavior in terms of coupled soft optical and acoustic modes, which is linked also to the high thermoelectric performance of PbTe.

cond-mat.mtrl-sci↗

arXiv2016

Phonon broadening from supercell lattice dynamics: random and correlated disorder

We demonstrate how supercell implementations of conventional lattice dynamical calculations can be used to determine the extent and nature of disorder-induced broadening in the phonon dispersion spectrum of disordered crystalline materials. The approach taken relies on band unfolding, and is first benchmarked against virtual crystal approximation phonon calculations. The different effects of mass and interaction disorder on the phonon broadening are then presented, focussing on the example of a simple cubic binary alloy. For the mass disorder example, the effect of introducing correlated disorder is also explored by varying the fraction of homoatomic and heteroatomic neighbours. Systematic progression in the degree of phonon broadening, on the one hand, and the form of the phonon dispersion curves from primitive to face-centered cubic type, on the other hand, is observed as homoatomic neighbours are disfavoured. The implications for rationalising selection rule violations in disordered materials and for using inelastic neutron scattering measurements as a means of characterising disorder are discussed.

cond-mat.dis-nn↗