arXiv subjects

D. Kwabena Bediako

Publications and source records attributed to D. Kwabena Bediako.

At least 19 recordsLinked to original sources

arXiv2026

Bragg Interferometry of Moiré Superlattices: From Geometric Phase Principles to Atomic Reconstruction

The emergence of moiré superlattices formed by twisting and stacking two-dimensional materials has created a need for characterization techniques capable of mapping sub-angstrom atomic reconstruction across micron-scale fields of view. This review surveys a suite of methodologies developed to extract geometric phases in electron microscopy and X-ray spectroscopy, with a primary focus on the interference of overlapping Bragg reflections in the dark field. We provide a comparative analysis of some established techniques, including geometric phase analysis, converged beam electron diffraction holography, and various ptychographic paradigms, culminating in a discussion of Bragg interferometry for measuring interlayer displacement fields and strain in moiré materials. We demonstrate the utility of Bragg interferometry through case studies of twisted bilayer and trilayer graphene as well as transition metal dichalcogenide moiré systems. Special attention is given to the unique advantages of this dark-field interferometric method for probing buried interfaces and encapsulated heterostructures, as well as the inherent challenges of interpreting incomplete phase information in the presence of dynamical scattering. By examining the physical principles underlying these approaches, this review highlights the conceptual similarities and practical trade-offs involved in high-resolution structural mapping of materials in which the interplay between structural relaxation and electronic behavior defines a scientific frontier at the nexus of modern condensed matter physics, nanomaterials engineering, and interfacial chemistry.

cond-mat.mtrl-sci↗

arXiv2026

Topochemically-engineered coexistence of charge and spin orders in intercalated endotaxial heterostructures

Correlated electron systems that host multiple electronic orders offer routes to multifunctional quantum materials, but strong competition between these orders often prevents their coexistence. Here we show that nanoscale, metastable intercalated heterostructures can stabilize a rare combination of long-range magnetism and a commensurate charge density wave (C-CDW) order in a single material. We synthesize a two-dimensional (2D) metastable crystal, T/H-Fe$_x$TaS2, which comprises an endotaxial polytype heterostructure of 1T-TaS$_2$ and H-TaS$_2$ with Fe intercalated in the van der Waals interfaces. In T/H-Fe$_x$TaS2, Fe intercalants provide localized spins that support ferromagnetism, while 1T layers host a robust commensurate charge density wave (C-CDW) that persists to room temperature. In these intercalated heterostructures, Fe content simultaneously tunes ordering of spin and charge degrees of freedom, positioning topochemically-prepared intercalated endotaxial heterostructures as a route to stabilize and control competing quantum phases in 2D materials.

cond-mat.str-el↗

arXiv2026

Enantiopurity-Controlled Magnetism in a Two-Dimensional Organic-Inorganic Material

Extended solids that combine unpaired electron spin and structural chirality can host unconventional magnetic behaviors with potential for electronic technologies. A versatile strategy for creating chiral solids is incorporation of chiral organic molecules into inorganic crystals. However, such hybrid organic-inorganic materials have so far been examined through the lens of absolute chirality, leaving enantiomeric excess (ee) underexplored as a tuning parameter. Here, we report two-dimensional (2D) intercalation compounds with controllable ee produced by cation exchange of MnPS$_3$ with chiral organic molecules. We show that these materials' magnetism is determined by intercalant ee rather than absolute chirality. Moreover, low-ee materials display thermally activated dynamic magnetism absent from enantiopure analogs. These ee-dependent magnetic behaviors are explained by local ordering of Mn vacancies, directed by correlated vacancy-intercalant electrostatics and confined molecular packing. Together, these results demonstrate a distinctive tuning strategy for molecule-material hybrids and establish design principles for 2D chiral and magnetically dynamic materials.

cond-mat.mtrl-sci↗

arXiv2025

Electronic origin of reorganization energy in interfacial electron transfer

Electron transfer (ET) reactions underpin energy conversion and chemical transformations in both biological and abiological systems. The efficiency of any ET process relies on achieving a desired ET rate within an optimal driving force range. Marcus theory provides a microscopic framework for understanding the activation free energy, and thus the rate, of ET in terms of a key parameter: the reorganization energy. For electrified solid-liquid interfaces, it has long been conventionally understood that only factors in the electrolyte phase are responsible for determining the reorganization energy and the electronic density of states (DOS) of the electrode serves only to dictate the number of thermally accessible channels for ET. Here we show instead that the electrode DOS plays a central role in governing the reorganization energy, far outweighing its conventionally assumed role. Using atomically layered heterostructures, we tune the DOS of graphene and measure outer-sphere ET kinetics. We find the ensuing variation in ET rate arises from strong modulation in a reorganization energy associated with image potential localization in the electrode. This work redefines the traditional paradigm of heterogeneous ET kinetics, revealing a deeper role of the electrode electronic structure in interfacial reactivity.

cond-mat.mtrl-sci↗

arXiv2025

Local interface effects modulate global charge order and optical properties of 1T-TaS$_2$/1H-WSe$_2$ heterostructures

1T-TaS$_2$ is a layered charge density wave (CDW) crystal exhibiting sharp phase transitions and associated resistance changes. These resistance steps could be exploited for information storage, underscoring the importance of controlling and tuning CDW states. Given the importance of out-of-plane interactions in 1T-TaS$_2$, modulating interlayer interactions by heterostructuring is a promising method for tailoring CDW phase transitions. In this work, we investigate the optical and electronic properties of heterostructures comprising 1T-TaS$_2$ and monolayer 1H-WSe$_2$. By systematically varying the thickness of 1T-TaS$_2$ and its azimuthal alignment with 1H-WSe$_2$, we find that intrinsic moiré strain and interfacial charge transfer introduce CDW disorder in 1T-TaS$_2$ and modify the CDW ordering temperature. Furthermore, our studies reveal that the interlayer alignment impacts the exciton dynamics in 1H-WSe$_2$, indicating that heterostructuring can concurrently tailor the electronic phases in 1T-TaS$_2$ and the optical properties of 1H-WSe$_2$. This work presents a promising approach for engineering optoelectronic behavior of heterostructures that integrate CDW materials and semiconductors.

cond-mat.mtrl-sci↗

arXiv2025

Unconventional superlattice ordering in intercalated transition metal dichalcogenide V$_{1/3}$NbS$_2$

The interplay between symmetry and topology in magnetic materials makes it possible to engineer exotic phases and technologically useful properties. A key requirement for these pursuits is achieving control over local crystallographic and magnetic structure, usually through sample morphology (such as synthesis of bulk crystals versus thin-films) and application of magnetic or electric fields. Here we show that V$_{1/3}$NbS$_2$ can be crystallized in two ordered superlattices, distinguished by the periodicity of out-of-plane magnetic intercalants. Whereas one of these structures is metallic and displays the hallmarks of altermagnetism, the other superlattice, which has not been isolated before in this family of intercalation compounds, is a semimetallic noncollinear antiferromagnet that may enable access to topologically nontrivial properties. This observation of an unconventional superlattice structure establishes a powerful route for tailoring the tremendous array of magnetic and electronic behaviors hosted in related materials.

cond-mat.mtrl-sci↗

arXiv2024

Anomalous Hall effect from inter-superlattice scattering in a noncollinear antiferromagnet

Superlattice formation dictates the physical properties of many materials, including the nature of the ground state in magnetic materials. Chemical composition is commonly considered to be the primary determinant of superlattice identity, especially in intercalation compounds. Here, we find that, contrary to this conventional wisdom, kinetic control of superlattice growth leads to the coexistence of disparate domains within a compositionally "perfect" single crystal. We demonstrate that Cr$_{1/4}$TaS$_2$ is a bulk noncollinear antiferromagnet in which scattering between bulk and minority superlattice domains engenders complex magnetotransport below the Néel temperature, including an anomalous Hall effect. We characterize the magnetic phases in different domains, image their nanoscale morphology, and propose a mechanism for nucleation and growth. These results provide a blueprint for the deliberate engineering of macroscopic transport responses via microscopic patterning of magnetic exchange interactions in superlattice domains.

cond-mat.mtrl-sci↗

arXiv2024

Modeling the Superlattice Phase Diagram of Transition Metal Intercalation in Bilayer 2H-TaS$_2$

Van der Waals hosts intercalated with transition metal (TM) ions exhibit a range of magnetic properties strongly influenced by the structural order of the intercalants. However, predictive computational models for the intercalant ordering phase diagram are lacking, complicating experimental pursuits to target key structural phases. Here we use Density Functional Theory (DFT) to construct a pairwise lattice model and Monte Carlo to determine its associated thermodynamic phase diagram. To circumvent the complexities of modeling magnetic effects, we use the diamagnetic ions Zn$^{2+}$ and Sc$^{3+}$ as computationally accessible proxies for divalent and trivalent species of interest (Fe$^{2+}$ and Cr$^{3+}$), which provide insights into the high-temperature thermodynamic phase diagram well above the paramagnetic transition temperature. We find that electrostatic coupling between intercalants is almost entirely screened, so the pairwise lattice model represents a coarse-grained charge density reorganization about the intercalated sites. The resulting phase diagram reveals that the entropically-favored $\sqrt{3} \times \sqrt{3}$ ordering and coexisting locally ordered $\sqrt{3} \times \sqrt{3}$ and $2 \times 2$ domains persist across a range of temperatures and intercalation densities. This occurs even at quarter filling of interstitial sites (corresponding to bulk stoichiometries of M$_{0.25}$TaS$_2$; M = intercalant ion) where a preference for long-range $2 \times 2$ order is typically assumed.

cond-mat.mtrl-sci↗

arXiv2024

Considerations for extracting moiré-level strain from dark field intensities in transmission electron microscopy

Bragg interferometry (BI) is an imaging technique based on four-dimensional scanning electron microscopy (4D-STEM) wherein the intensities of select overlapping Bragg disks are fit or more qualitatively analyzed in the context of simple trigonometric equations to determine local stacking order. In 4D-STEM based approaches, the collection of full diffraction patterns at each real-space position of the scanning probe allows the use of precise virtual apertures much smaller and more variable in shape than those used in conventional dark field imaging, such that even buried interfaces marginally twisted from other layers can be targeted. A coarse-grained form of dark field ptychography, BI uses simple physically derived fitting functions to extract the average structure within the illumination region and is therefore viable over large fields of view. BI has shown a particular advantage for selectively investigating the interlayer stacking and associated moiré reconstruction of bilayer interfaces within complex multi-layered structures. This has enabled investigation of reconstruction and substrate effects in bilayers through encapsulating hexagonal boron nitride and of select bilayer interfaces within trilayer stacks. However, the technique can be improved to provide a greater spatial resolution and probe a wider range of twisted structures, for which current limitations on acquisition parameters can lead to large illumination regions and the computationally involved post-processing can fail. Here we analyze these limitations and the computational processing in greater depth, presenting a few methods for improvement over previous works, discussing potential areas for further expansion, and illustrating the current capabilities of this approach for extracting moiré-scale strain.

cond-mat.mtrl-sci↗

arXiv2024

Tailored topotactic chemistry unlocks heterostructures of magnetic intercalation compounds

The construction of thin film heterostructures has been a widely successful archetype for fabricating materials with emergent physical properties. This strategy is of particular importance for the design of multilayer magnetic architectures in which direct interfacial spin--spin interactions between magnetic phases in dissimilar layers lead to emergent and controllable magnetic behavior. However, crystallographic incommensurability and atomic-scale interfacial disorder can severely limit the types of materials amenable to this strategy, as well as the performance of these systems. Here, we demonstrate a method for synthesizing heterostructures comprising magnetic intercalation compounds of transition metal dichalcogenides (TMDs), through directed topotactic reaction of the TMD with a metal oxide. The mechanism of the intercalation reaction enables thermally initiated intercalation of the TMD from lithographically patterned oxide films, giving access to a new family of multi-component magnetic architectures through the combination of deterministic van der Waals assembly and directed intercalation chemistry.

cond-mat.mtrl-sci↗

arXiv2024

Local atomic stacking and symmetry in twisted graphene trilayers

Moiré superlattices formed from twisting trilayers of graphene are an ideal model for studying electronic correlation, and offer several advantages over bilayer analogues, including more robust and tunable superconductivity and a wide range of twist angles associated with flat band formation. Atomic reconstruction, which strongly impacts the electronic structure of twisted graphene structures, has been suggested to play a major role in the relative versatility of superconductivity in trilayers. Here, we exploit an inteferometric 4D-STEM approach to image a wide range of trilayer graphene structures. Our results unveil a considerably different model for moiré lattice relaxation in trilayers than that proposed from previous measurements, informing a thorough understanding of how reconstruction modulates the atomic stacking symmetries crucial for establishing superconductivity and other correlated phases in twisted graphene trilayers.

cond-mat.mes-hall↗

arXiv2023

Atomic engineering of interfacial polarization switching in van der Waals multilayers

In conventional ferroelectric materials, polarization is an intrinsic property limited by bulk crystallographic structure and symmetry. Recently, it has been demonstrated that polar order can also be accessed using inherently non-polar van der Waals materials through layer-by-layer assembly into heterostructures, wherein interfacial interactions can generate spontaneous, switchable polarization. Here, we show that introducing interlayer rotations in multilayer vdW heterostructures modulates both the spatial ordering and switching dynamics of polar domains, engendering unique tunability that is unparalleled in conventional bulk ferroelectrics or polar bilayers. Using operando transmission electron microscopy we show how changing the relative rotations of three WSe2 layers produces structural polytypes with distinct arrangements of polar domains, leading to either a global or localized switching response. Introducing uniaxial strain generates structural anisotropy that yields a range of switching behaviors, coercivities, and even tunable biased responses. We also provide evidence of physical coupling between the two interfaces of the trilayer, a key consideration for controlling switching dynamics in polar multilayer structures more broadly.

cond-mat.mtrl-sci↗

arXiv2023

Near room-temperature intrinsic exchange bias in an Fe intercalated ZrSe2 spin glass

Some magnetic systems display a shift in the center of their magnetic hysteresis loop away from zero field, a phenomenon termed exchange bias. Despite the extensive use of the exchange bias effect, particularly in magnetic multilayers, for the design of spin-based memory/electronics devices, a comprehensive mechanistic understanding of this effect remains a longstanding problem. Recent work has shown that disorder-induced spin frustration might play a key role in exchange bias, suggesting new materials design approaches for spin-based electronic devices that harness this effect. Here, we design a spin glass with strong spin frustration induced by magnetic disorder by exploiting the distinctive structure of Fe intercalated ZrSe2, where Fe(II) centers are shown to occupy both octahedral and tetrahedral interstitial sites and to distribute between ZrSe2 layers without long-range structural order. Notably, we observe behavior consistent with a magnetically frustrated, and multi-degenerate ground state in these Fe0.17ZrSe2 single crystals, which persists above room temperature. Moreover, this magnetic frustration leads to a robust and tunable exchange bias up to 250 K. These results not only offer important insights into the effects of magnetic disorder and frustration in magnetic materials generally, but also highlight as design strategy the idea that a large exchange bias can arise from an inhomogeneous microscopic environment without discernible long-range magnetic order. In addition, these results show that intercalated TMDs like Fe0.17ZrSe2 hold potential for spintronics technologies that can achieve room temperature applications.

cond-mat.mtrl-sci↗

arXiv2023

Modulating the electrochemical intercalation of graphene interfaces with $α$-RuCl$_3$ as a solid-state electron acceptor

Intercalation reactions modify the charge density in van der Waals (vdW) materials through coupled electronic-ionic charge accumulation, and are susceptible to modulation by interlayer hybridization in vdW heterostructures. Here, we demonstrate that charge transfer between graphene and $α$-RuCl$_3$, which dopes the graphene positively, greatly favors the intercalation of lithium ions into graphene-based vdW heterostructures. We systematically tune this effect on Li$^+$ ion intercalation, modulating the intercalation potential, by using varying thicknesses of hexagonal boron nitride (hBN) as spacer layers between graphene and $α$-RuCl$_3$. Confocal Raman spectroscopy and electronic transport measurements are used to monitor electrochemical intercalation and density functional theory computations help quantify charge transfer to both $α$-RuCl$_3$ and graphene upon Li intercalation. This work demonstrates a versatile approach for systematically modulating the electrochemical intercalation behavior of two-dimensional layers akin to electron donating/withdrawing substituent effects used to tune molecular redox potentials.

cond-mat.mtrl-sci↗

arXiv2023

Comparative Electronic Structures of the Chiral Helimagnets Cr1/3NbS2 and Cr1/3TaS2

Magnetic materials with noncollinear spin textures are promising for spintronic applications. To realize practical devices, control over the length and energy scales of such spin textures is imperative. The chiral helimagnets Cr1/3NbS2 and Cr1/3TaS2 exhibit analogous magnetic phase diagrams with different real-space periodicities and field dependence, positioning them as model systems for studying the relative strengths of the microscopic mechanisms giving rise to exotic spin textures. Here, we carry out a comparative study of the electronic structures of Cr1/3NbS2 and Cr1/3TaS2 using angle-resolved photoemission spectroscopy and density functional theory. We show that bands in Cr1/3TaS2 are more dispersive than their counterparts in Cr1/3NbS2 and connect this result to bonding and orbital overlap in these materials. We also unambiguously distinguish exchange splitting from surface termination effects by studying the dependence of their photoemission spectra on polarization, temperature, and beam size. We find strong evidence that hybridization between intercalant and host lattice electronic states mediates the magnetic exchange interactions in these materials, suggesting that band engineering is a route toward tuning their spin textures. Overall, these results underscore how the modular nature of intercalated transition metal dichalcogenides translates variation in composition and electronic structure to complex magnetism.

cond-mat.mtrl-sci↗

arXiv2023

Consequences and control of multi-scale (dis)order in chiral magnetic textures

Transition metal-intercalated transition metal dichalcogenides (TMDs) are promising platforms for next-generation spintronic devices based on their wide range of electronic and magnetic phases, which can be tuned by varying the host lattice or the identity of the intercalant, along with its stoichiometry and spatial order. Some of these compounds host a chiral magnetic phase in which the helical winding of magnetic moments propagates along a high-symmetry crystalline axis. Previous studies have demonstrated that variation in intercalant concentrations can have a dramatic impact on the formation of chiral domains and ensemble magnetic properties. However, a systematic and comprehensive study of how atomic-scale order and disorder impacts collective magnetic behavior are so far lacking. Here, we leverage a combination of imaging modes in the (scanning) transmission electron microscope (S/TEM) to directly probe (dis)order across multiple length scales and show how subtle changes in the atomic lattice can be leveraged to tune the mesoscale spin textures and bulk magnetic response, with direct implications for the fundamental understanding and technological implementation of such compounds.

cond-mat.mtrl-sci↗

arXiv2023

Rotational and Dilational Reconstruction in Transition Metal Dichalcogenide Moiré Bilayers

Lattice reconstruction and corresponding strain accumulation play a key role in defining the electronic structure of two-dimensional moiré superlattices, including those of transition metal dichalcogenides (TMDs). Imaging of TMD moirés has so far provided a qualitative understanding of this relaxation process in terms of interlayer stacking energy, while models of the underlying deformation mechanisms have relied on simulations. Here, we use interferometric four-dimensional scanning transmission electron microscopy to quantitatively map the mechanical deformations through which reconstruction occurs in small-angle twisted bilayer MoS2 and WSe2/MoS2 heterobilayers. We provide direct evidence that local rotations govern relaxation for twisted homobilayers, while local dilations are prominent in heterobilayers possessing a sufficiently large lattice mismatch. Encapsulation of the moiré layers in hBN further localizes and enhances these in-plane reconstruction pathways, suppressing out-of-plane corrugation. We also find that extrinsic uniaxial heterostrain, which introduces a lattice constant difference in twisted homobilayers, leads to accumulation and redistribution of reconstruction strain, demonstrating another route to modify the moiré potential.

cond-mat.mtrl-sci↗

arXiv2023

Anomalous interfacial electron transfer kinetics in twisted trilayer graphene caused by layer-specific localization

Interfacial electron-transfer (ET) reactions underpin the interconversion of electrical and chemical energy. Pioneering experiments showed that the ET rate depends on the Fermi Dirac distribution of the electronic density of states (DOS) of the electrode, formalized in the Marcus Hush Chidsey (MHC) model. Here, by controlling interlayer twists in well-defined trilayergraphene moires, we show that ET rates are strikingly dependent on electronic localization in each atomic layer, and not the overall DOS. The large degree of tunability inherent to moire electrodes leads to local ET kinetics that range over three orders of magnitude across different constructions of only three atomic layers, even exceeding rates at bulk metals. Our results demonstrate that beyond the ensemble DOS, electronic localization is critical in facilitating interfacial ET, with implications for understanding the origin of high interfacial reactivity typically exhibited by defects at electrode electrolyte interfaces.

cond-mat.mes-hall↗