Crystalline Order Research Articles

Flat Band and η-Pairing States in a One-Dimensional Moiré Hubbard Model

A Moiré system is formed when two periodic structures have a slightly mismatched period, resulting in unusual strongly correlated states in the presence of particle-particle interactions. The periodic structures can arise from the intrinsic crystalline order and periodic external field. We investigate a one-dimensional Hubbard model with periodic on-site potential of period n 0, which is commensurate to the lattice constant. For large n 0, the exact solution demonstrates that there is a midgap flat band with zero energy in the absence of Hubbard interaction. Each Moiré unit cell contributes two zero energy levels to the flat band. In the presence of Hubbard interaction, the midgap physics is demonstrated to be well described by a uniform Hubbard chain in which the effective hopping and on-site interaction strength can be controlled by the amplitude and period of the external field. Numerical simulations are performed to demonstrate the correlated behaviors in the finite-sized Moiré Hubbard system, including the existence of an η-pairing state and bound pair oscillation. This finding provides a method to enhance the correlated effect by a spatially periodic external field.

Non‐Oxidative Mechanism in Oxygen‐Based Magneto‐Ionics

AbstractThe Ta/CoFeB/Pt/MgO/HfO2 system is investigated, whose magnetic anisotropy can be controlled through magneto‐ionic gating, using both ionic liquid and solid state gating, via a non‐oxidative mechanism combining reversible and irreversible gating effects. Analysis of X‐ray absorption spectroscopy at the Co and Fe edges reveals no indications of oxidation after gating, while a reversible change at the oxygen K edge suggests the involvement of oxygen species in the magneto‐ionic process. In addition, X‐ray diffraction measurements reveal that gating can irreversibly increase the crystalline volume of MgO, through an increase in the MgO/Mg(OH)2 ratio. This is in line with measurements in solid state devices showing that in a series of 150 gating cycles a reversible effect combines with a progressive increase in the strength of the perpendicular magnetic anisotropy contribution that saturates after extensive cycling. Consequently, the observed gate‐induced changes in magnetic anisotropy can be attributed to the combined effects of Mg(OH)2 dehydration into MgO (irreversible) and most likely a gentle reordering of oxygen species at the CoFeB interface (reversible) leading to a non‐oxidative magneto‐ionic mechanism. This study provides valuable insights into the underlying mechanisms governing the complex magneto‐ionic phenomena, including the coexistence of both reversible and irreversible effects, and a pathway to voltage‐control of crystalline order in spintronics materials.

Backbone and Side Group Interchain Correlations Govern Wide-Angle X-ray Scattering of Poly(3-hexylthiophene).

Identifying the origin of scattering from polymer materials is crucial to infer structural features that can relate to functional properties. Here, we use our recently developed virtual-site coarse graining to accelerate atomistic simulations and show how various molecular features govern wide-angle X-ray scattering from a conjugated polymer, poly(3-hexylthiophene) (P3HT). The efficient molecular dynamics simulations can represent the structure and capture the emergence of crystalline order from amorphous melts upon cooling while retaining atomistic details of chain configurations. The scattering extracted from simulations shows good agreement with wide-angle X-ray scattering experiments. Amorphous P3HT exhibits broad scattering peaks: a high-q peak from interchain side-group correlations and a low-q peak from interchain backbone-backbone correlations. During amorphous to crystalline phase transitions, the distance between backbones along the side-group direction increases because of lack of interdigitation in the crystalline phase. Scattering from π-π stacking emerges only after crystallization takes place. Intrachain correlations contribute negligibly to the scattering from the amorphous and crystalline phases.

Effect of boron concentration on local structure and spontaneous polarization in AlBN thin films

The discovery of ferroelectricity in polar wurtzite-based ternary materials, such as Al1−xBxN, has attracted attention due to their compatibility with complementary metal–oxide–semiconductor processes and potential use in integrated non-volatile memory devices. However, the origin of ferroelectricity and the fundamental control of the polarization switching in these materials are still under intensive investigation but appear to be related to local disorder induced from the alloying. In this work, we report the effect of boron alloying on the local structure of Al1−xBxN films deposited by magnetron sputtering. Our results reveal a diminished crystalline order as a function of boron concentration, accompanied by a reduction in the spontaneous polarization. The film disorder is primarily associated with the dissimilar bond lengths between Al–N and B–N and the formation of threading dislocations induced by B incorporation in the structure.

Divergence among Local Structure, Dynamics, and Nucleation Outcome in Heterogeneous Nucleation of Close-Packed Crystals.

Heterogeneous crystal nucleation is the dominant mechanism of crystallization in most systems, yet its underlying physics remains an enigma. While emergent interfacial crystalline order precedes heterogeneous nucleation, its importance in the nucleation mechanism is unclear. Here, we use path sampling simulations of two model systems to demonstrate that crystalline order in its traditional sense is not predictive of the outcome of the heterogeneous nucleation of close-packed crystals. Consequently, structure-based collective variables (CVs) that reliably describe homogeneous nucleation can be poor descriptors of heterogeneous nucleation. This divergence between structure and nucleation outcome is accompanied by an intriguing dynamical anomaly, wherein low-coordinated crystalline particles outpace their liquid-like counterparts. We use committor analysis, high-throughput screening, and machine learning to devise CV optimization strategies and present suitable structural heuristics within the metastable fluid for CV prescreening. Employing such optimized CVs is pivotal for properly characterizing the mechanism of heterogeneous nucleation in metallic and colloidal systems.

Helix-coil and solvation-induced reentrant nematic-smectic A transitions in a polypeptide solution

ABSTRACT A simple model is introduced to describe re-entrant isotropic-nematic and nematic-smectic A phase transitions mediated helix-coil transition and solvation within individual liquid crystalline semiflexible polymers in a solution. We consider hydrogen bonding between the polymer and solvent molecules (solvation) as an intermolecular interaction and helix-coil transformations of the polymer as an intramolecular interaction. The helix part is rigid and can form nematic and smectic A phases. On the other hand, the solvation prevents liquid crystalline ordering of the polymer chains. At high temperatures, a polymer is in a coiled state without solvation. With decreasing temperatures, solvation and helix-coil transitions compete in the individual semiflexible polymers, and the re-entrant phase transitions occur. We also discuss the chain conformation by calculating Kuhn length.

Effect of different proportions of glycerol and D-mannitol as plasticizer on the properties of extruded corn starch.

In this study, thermoplastic starch (TPS) was prepared by melt extrusion process, in which glycerol and/or D-mannitol were used as plasticizers, and the effect of different glycerol/D-mannitol ratios (4:0, 3:1, 2:2, 1:3, and 0:4) on the physicochemical properties of the extruded starch samples was investigated. The short-range molecular order, crystallization, gelatinization, thermal stability, and thermal properties of the TPS samples were analyzed through attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray diffraction (XRD), rapid visco analysis (RVA), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The results showed that the crystallinity and short-range molecular order of the TPS samples increased with increasing glycerol content. Conversely, the water absorption index (WAI) and water solubility index (WSI) of the TPS samples decreased with increasing glycerol content. In addition, the TPS samples with higher glycerol content exhibited higher gel and thermal stabilities. This study provides a theoretical basis for starch extrusion and plasticization in the preparation of TPS-based materials with specific properties.

Exploring the magnetic and magnetocaloric behavior of nanocrystalline melt-spun R2Fe17 (R = Pr, Nd) ribbons

Single-phase nanocrystalline R2Fe17 (R = Pr and Nd) ribbons with rhombohedral Th2Zn17-type crystalline structure (space group R3̅m) have been fabricated by melt-spinning technique. The microstructure of the polycrystalline ribbons is composed of quasi-spherical grains with an average size below 100 nm. Transmission electron microscopy reveals that these grains are formed by agglomeration of smaller nanocrystalline entities around 15 nm in diameter, separated by disordered boundaries where the long-range crystalline order is lost. Two different ferro-to-paramagnetic phase transitions are observed, one of them coincides with that of the parent bulk alloy (290 and 326 K for R = Pr and Nd, respectively), and the other one can be ascribed to the disordered boundaries (323 and 350 K for R = Pr and Nd, respectively). For R= Pr, this fact gives rise to a significant broadening (c.a. 120 K under a magnetic field change of 2 T) of the full-width at the half-maximum of the magnetic entropy change curve, |∆SM(T)| (that adopts a "table-like" shape), resulting in a considerable increase of the refrigerant capacity.

Facile synthesis of β-Ga2O3 based high-performance electronic devices via direct oxidation of solution-processed transition metal dichalcogenides

Gallium oxide (Ga2O3) is a promising wide bandgap semiconductor that is viewed as a contender for the next generation of high-power electronics due to its high theoretical breakdown electric field and large Baliga’s figure of merit. Here, we report a facile route of synthesizing β-Ga2O3 via direct oxidation conversion using solution-processed two-dimensional (2D) GaS semiconducting nanomaterial. Higher order of crystallinity in x-ray diffraction patterns and full surface coverage formation in scanning electron microscopy images after annealing were achieved. A direct and wide bandgap of 5 eV was calculated, and the synthesized β-Ga2O3 was fabricated as thin film transistors (TFT). The β-Ga2O3 TFT fabricated exhibits remarkable electron mobility (1.28 cm2 Vs−1) and a good current ratio (I on/I off) of 2.06 × 105. To further boost the electrical performance and solve the structural imperfections resulting from the exfoliation process of the 2D nanoflakes, we also introduced and doped graphene in β-Ga2O3 TFT devices, increasing the electrical device mobility by ∼8-fold and thereby promoting percolation pathways for the charge transport. We found that electron mobility and conductivity increase directly with the graphene doping concentration. From these results, it can be proved that the β-Ga2O3 networks have excellent carrier transport properties. The facile and convenient synthesis method successfully developed in this paper makes an outstanding contribution to applying 2D oxide materials in different and emerging optoelectronic applications.

Towards the epitaxial growth of Au thin films on MgO substrates for plasmonic applications

Surface Plasmon Polaritons (SPPs) in Au thin films are nowadays intensively exploited for sensing applications that leverage the strong optical field confinement at the metal/dielectric interface and the easy functionalization of the Au surface. Moreover, Au thin films represent one of the common starting points for the top-down nanofabrication of plasmonic nanostructures supporting localized resonances. In this framework, strategies for the growth of high-quality Au films on transparent substrates are crucial and not yet fully established. In this study, we exploit MgO(001) substrates for the growth of thin (about 45 nm) Au films, also including an additional buffer layer of Fe. We successfully demonstrate Au samples with reduced roughness and presenting Low-Energy Electron Diffraction (LEED) features, indicating a high degree of crystalline ordering. This is supported by the experimental evidence of an increased (by almost a factor of 3) propagation length compared to a reference Au sample grown on standard glass slides, which is however still significantly lower than the one expected from first principles.

Supramolecular gels - a panorama of low-molecular-weight gelators from ancient origins to next-generation technologies.

Supramolecular gels, self-assembled from low-molecular-weight gelators (LMWGs), have a long history and a bright future. This review provides an overview of these materials, from their use in lubrication and personal care in the ancient world, through to next-generation technologies. In academic terms, colloid scientists in the 19th and early 20th centuries first understood such gels as being physically assembled as a result of weak interactions, combining a solid-like network having a degree of crystalline order with a highly mobile liquid-like phase. During the 20th century, industrial scientists began using these materials in new applications in the polymer, oil and food industries. The advent of supramolecular chemistry in the late 20th century, with its focus on non-covalent interactions and controlled self-assembly, saw the horizons for these materials shifted significantly beyond their historic rheological applications, expanding their potential. The ability to tune the LMWG chemical structure, manipulate hierarchical assembly, develop multi-component systems, and introduce new types of responsive and interactive behaviour, has been transformative. Furthermore, the dynamics of these materials are increasingly understood, creating metastable gels and transiently-fueled systems. New approaches to shaping and patterning gels are providing a unique opportunity for more sophisticated uses. These supramolecular advances are increasingly underpinning and informing next-generation applications - from drug delivery and regenerative medicine to environmental remediation and sustainable energy. In summary, this article presents a panorama over the field of supramolecular gels, emphasising how both academic and industrial scientists are building on the past, and engaging new fundamental insights and innovative concepts to open up exciting horizons for their future use.

Encapsulation of crystalline and amorphous Sb2S3 within carbon and boron nitride nanotubes.

The recent rediscovery of 1D and quasi-1D (q-1D) van der Waals (vdW) crystals has laid foundation for the realization of emergent electronic, optical, and quantum-confined physical phenomena in both bulk and at the nanoscale. Of these, the highly anisotropic q-1D vdW crystal structure and the visible-light optical/optoelectronic properties of antimony trisulfide (Sb2S3) have led to its widespread consideration as a promising building block for photovoltaic and non-volatile phase change devices. However, while these applications will greatly benefit from well-defined and sub-nanometer-thick q-1D structures, little has been known about feasible synthetic routes that can access single covalent chains of Sb2S3. In this work, we explore how encapsulation in single or multi-walled carbon nanotubes (SWCNTs or MWCNTs) and visible-range transparent boron nitride nanotubes (BNNTs) influences the growth and phase of Sb2S3 nanostructures. We demonstrate that nanotubes with smaller diameters had a more pronounced effect in the crystallographic growth direction and orientation of Sb2S3 nanostructures, promoting the crystallization of the guest structures along the long-axis [010]-direction. As such, we were able to reliably access well-ordered few to single covalent chains of Sb2S3 when synthesized within defect-free SWCNTs with sub-2 nm inner diameters. Intriguingly, we found that the degree of crystalline order of Sb2S3 nanostructures was strongly influenced by the presence of defects and discontinuities along the Sb2S3-nanotube interface. We show that amorphous nanowire domains of Sb2S3 form around defect sites in larger, multi-walled nanotubes that manifest inner wall defects and discontinuities, suggesting a means to manipulate the crystallization dynamics of confined sub-10 nm-thick Sb2S3 nanostructures within nanotubes. Lastly, we show that ultranarrow amorphous Sb2S3 can impart functionality onto isolable BNNTs with photocurrent generation in the pA range which, alongside the dispersibility of the Sb2S3@BNNTs, could be leveraged to easily fabricate photoresistors only a few nm in width. Altogether, our results serve to solidify the understanding of how q-1D vdW pnictogen chalcogenides crystallize within confined synthetic platforms and are a step towards realizing functional materials from ensembles of encapsulated heterostructures.

Electrically mediated self-assembly and manipulation of drops at an interface.

The fluid-fluid interface is a complex environment for a floating object where the statics and dynamics may be governed by capillarity, gravity, inertia, and other external body forces. Yet, the alignment of these forces in intricate ways may result in beautiful pattern formation and self-assembly of these objects, as in the case of crystalline order observed with bubble rafts or colloidal particles. While interfacial self-assembly has been explored widely, controlled manipulation of floating objects, e.g. drops, at the fluid-fluid interface still remains a challenge largely unexplored. In this work, we reveal the self-assembly and manipulation of water drops floating at an oil-air interface. We show that the assembly occurs due to electrostatic interactions between the drops and their environment. We highlight the role of the boundary surrounding the system by showing that even drops with a net zero electric charge can self-assemble under certain conditions. Using experiments and theory, we show that the depth of the oil bath plays an important role in setting the distance between the self-assembled drops. Furthermore, we demonstrate ways to manipulate the drops actively and passively at the interface.

Thermal evolution of a polymer-nanoparticle binary mixture.

We experimentally probe the microscopic variations in a model polymer-nanoparticle (NP) binary mixture (mixture of polybutadiene and clay nanoplatelets) across a thermal evolution path for which Tevolution > Tg(polymer). The evolution of the NP dispersion, NP crystallinity, polymer chain-NP interface, and nature of polymer chain-NP interaction are mapped for a spectrum of temperatures and NP concentrations constrained by experiments. Multiple pieces of evidence indicate that thermal evolution does not influence the nature of interparticle dispersion and is also independent of NP concentration in the binary mixture. However, the NP crystalline order significantly reduces across the thermal evolution path. Thermal evolution induces a transition of a sharp polymer chain-NP interface to a diffuse interfacial layer. In contrast, an already diffuse polymer-NP interface existing in the binary mixture due to particle crowding at high NP concentrations undergoes no significant change in its nature across the evolution path. At all particle concentrations, thermal evolution changes the dominant interaction from polymer chain-polymer chain to polymer chain-NP. These insights aid in explaining the molecular origins of unique and anomalous behaviors shown by polymer-nanoparticle binary mixtures while undergoing thermal evolution.

Amorphization of Gd2Zr2O7 induced by mechanical deformation in nanoindentation

Amorphization has become an important plastic deformation mechanism for covalently-bonded ceramics, which can disturb their crystalline order and affect their mechanical behaviors. Unlike the covalently-bonded ceramics, however, amorphization induced by mechanical deformation is rarely observed in ionically-bonded ceramics. In this work, we show direct experimental evidence that nano-scale amorphous zones in Gadolinium Zirconate ceramic are the ultimate stage of plasticity and relate to an accumulation of dislocations. In addition, the occurrence of amorphization also causes the unusual indentation size effect.

Designing the stripe-ordered cuprate phase diagram through uniaxial-stress.

The ability to efficiently control charge and spin in the cuprate high-temperature superconductors is crucial for fundamental research and underpins technological development. Here, we explore the tunability of magnetism, superconductivity, and crystal structure in the stripe phase of the cuprate La[Formula: see text]Ba[Formula: see text]CuO[Formula: see text], with [Formula: see text] = 0.115 and 0.135, by employing temperature-dependent (down to 400 mK) muon-spin rotation and AC susceptibility, as well as X-ray scattering experiments under compressive uniaxial stress in the CuO[Formula: see text] plane. A sixfold increase of the three-dimensional (3D) superconducting critical temperature [Formula: see text] and a full recovery of the 3D phase coherence is observed in both samples with the application of extremely low uniaxial stress of [Formula: see text]0.1 GPa. This finding demonstrates the removal of the well-known 1/8-anomaly of cuprates by uniaxial stress. On the other hand, the spin-stripe order temperature as well as the magnetic fraction at 400 mK show only a modest decrease under stress. Moreover, the onset temperatures of 3D superconductivity and spin-stripe order are very similar in the large stress regime. However, strain produces an inhomogeneous suppression of the spin-stripe order at elevated temperatures. Namely, a substantial decrease of the magnetic volume fraction and a full suppression of the low-temperature tetragonal structure is found under stress, which is a necessary condition for the development of the 3D superconducting phase with optimal [Formula: see text]. Our results evidence a remarkable cooperation between the long-range static spin-stripe order and the underlying crystalline order with the three-dimensional fully coherent superconductivity. Overall, these results suggest that the stripe- and the SC order may have a common physical mechanism.

Dielectric and conductivity properties of liquid crystal MBBA doped with single-walled carbon nanotubes

The effect of single-walled carbon nanotubes (SWCNTs) on the dielectric and conductivity properties of nematic liquid crystal 4-methoxybenzylidene-40-butylaniline (MBBA) has been studied. It was shown that the additive of SWCNTs with concentration of 0.3% leads to an increase in the order parameter of MBBA. Observation under the polarization microscope has shown that the presence of SWCNTs in MBBA increases the clearing temperature from 46.3°C to 47.4°C. This change can be explained by the improvement of the liquid crystalline orientation order. As a result, the transition to a complete disordering of the molecules (transition to an isotropic state) requires more heat energy, correspondingly, more temperature. The dielectric and conductivity measurements have showed that the longitudinal component of the dielectric permittivity decreases while the transverse component increases as well as dielectric anisotropy. The percolation effect promotes to the dominance of hopping electron conductivity over ionic conductivity, leading to an increase in electric conductivity.

Thin film crystallization of oligoethylene glycol-benzothieno benzothiophene: Physical vapor deposition versus spin coating

The benzothieno-benzothiophene derivatives are among the best-performing organic semiconductor materials. Their triumphant application in electronic devices is correlated to their capability to form well-defined thin film structures featuring crystalline order at the substrate–organic interface. Thin films of the molecule 2,7-bis(2-(2-methoxy ethoxy)ethoxy) benzo[b]-benzo[4,5]thieno[2,3-d]thiophene (OEG-BTBT) are prepared by spin coating and physical vapor deposition using different experimental conditions. Atomically flat silicon oxide surfaces with defined surface energies are used as substrates. The crystalline morphology is studied for the monolayer regime up to thick films by X-ray reflectivity, grazing incidence X-ray diffraction and atomic force microscopy. Depending on the preparation process, substantial variations are found in the structure of the first monolayers. The highly flexible oligoethylene glycol side chains allow different monolayer formation processes. Also, the subsequent crystal growth strongly depends on the nucleation in the monolayer regime. Here, we observe two distinct nucleation scenarios, one directly at the substrate surface and one at the edges of sub-monolayer terraces. As a consequence, the mosaicity of the crystallites in thick films is strongly affected by the growth type. This work illustrates that different thin film preparation processes can result in different nucleation mechanisms for subsequent crystallization. Consequently, variations in the crystallographic order in device relevant films are observed.

Nonlocal Potentials and Crystalline Order in One and Two Dimensions

We revisit the seminal 1968 proof of the absence of crystalline order in two dimensions and analyze the importance played in the quantum theorem by the assumption of local pair potentials. We relax the assumption of local potentials and consider instead nonlocal pair potentials. We show that the 1/k2 -singularity that occurs in the Bogoliubov inequality, which leads to no crystalline order in two dimensions for local potentials and nonzero temperatures, does not occur for nonlocal potentials. Accordingly, crystalline order in one and two dimensions cannot be ruled out for nonlocal pair potentials at finite temperatures.

Effect of thickness on the electrical properties of PEDOT:PSS/Tween 80 films

The lower material and manufacturing costs of conductive polymers, particularly PEDOT:PSS, compared to indium tin oxide have led to significant research into their use in optoelectric devices. In this study, improvements of the electrical properties of PEDOT:PSS and PEDOT:PSS/Tween 80 via the production of multilayered films were investigated. A single layer of pristine PEDOT:PSS was found to give a sheet resistance of 1639 Ω□–1. The application of an additional three layers reduced this value to 29 Ω□-1, corresponding to an increase in conductivity from 2.6 to 18.3 Scm–1. A similar trend was also found with formulations containing Tween 80. X-ray diffraction and Raman spectroscopy showed that the additional layers increased the crystalline order and induced a slight benzoid to quinoid shift. Surface profiling showed progressive increases in surface roughness with each additional layer of pristine PEDOT:PSS; however, this was mitigated by the presence of Tween 80 in the formulations.