Superlattice Model Research Articles

Deciphering Absolute O 2p Position as a Practical Descriptor for Strain‐Dependent Perovskite Nickelate Electrocatalysts

AbstractDeveloping effective descriptors for perovskite oxygen evolution reaction (OER) electrocatalysts is crucial for advancing clean energy technologies, which are often constrained by incomplete theoretical calculations. Herein, using step‐wise strained (−3–3%) NdNiO3 as a model, the density functional theory (DFT) is employed, considering both bulk superlattice model (B‐model) and surface slab models (S‐model) to garner a set of electronic descriptors. The S‐model‐derived metal─oxygen orbital hybridization is found to agree with the X‐ray absorption spectroscopy (XAS) results, which, however, are disentangled from the observed OER activity trend. Further detailed analysis discovers that the alignment of the calculated O 2p band position with the OER redox potential on an absolute scale can serve as a hitherto unexplored descriptor for comprehending the activity of lattice oxygen and its bonding strength to OH*. Combined DFT and XAS data reveals that excessive strain hinders surface oxygen exchange, slows down the rate‐determining step of OH* adsorption, and impairs the durability of the electrocatalyst by altering the random electron occupancy in frontier orbitals. The work sheds light on the rational design of cost‐effective, high‐performance perovskite‐based electrocatalysts.

Influence of interposed graphene sheets on mechanical and electronic properties of Al/graphene superlattice

Graphene-reinforced aluminum matrix composites have drawn remarkable attention in several fields of high-tech industries, but the understanding of their material properties remains unclear. This work reports a first-principles study of interface binding nature, mechanical strength, and electronic properties of aluminum/graphene (Al/G) composites using superlattice models as varying graphene content. Our calculations reveal the weak binding between Al and graphene layers with no new chemical bonding at the interface and the gradual decrease in binding strength as increasing graphene content. While demonstrating the enhancement of mechanical strength by interposing graphene layers, the critical value of graphene content for keeping ductility is determined to be 14.7%. Atom-projected band structures and local density of states are analyzed to get an insight into electronic conductance of superlattices.

Wavelength-selective thermal emission metasurfaces based on synthetic dimensional topological Weyl points

Blackbody emission such as the emission from incandescent sources usually possesses a broadband emission spectrum covering the whole infrared wavelength range. Most of emission energy goes into the unwanted infrared range and consequently causes low emission efficiency. Recently, metasurfaces with two-dimensional subwavelength artificial nanostructures have been widely studied due to their flexibility in modulating optical properties, thus providing an ideal platform for controlling thermal emission. The use of synthetic dimension methods in metasurfaces has opened up new avenues for fine-tuning thermal emission, especially highlighting the physical properties beyond traditional three-dimensional systems and rich topological physics. Although it is theoretically possible to explore physical phenomena through complete three-dimensional structures, such structures are difficult to construct in practice. In contrast, studying one-dimensional system or two-dimensional system is more feasible and efficient. The synthetic dimension approach introduces the possibility of manipulating intrinsic degrees of freedom in photon systems by introducing structural or physical parameters. In this work, we propose utilizing synthetic dimension methods to achieve wavelength-selective thermal emission. Firstly, we construct synthetic Weyl point in a superlattice model and validate it theoretically. Subsequently, experimental characterization of synthetic Weyl cones is conducted by using angle-resolved thermal emission spectroscopy (ARTES). The experimental results demonstrate that we can achieve reasonable wavelength-selective thermal emission while suppressing emission at other wavelengths as much as possible. This is essential for practical infrared applications such as thermalphotovoltaics and thermal management devices.

VO2/SnO2 superlattice enables metal-insulator alternating and Li migration barrier modulating

The VO2/SnO2 heterojunctions with adjustable electrical properties are considered to be of potential applications in micro-electronics. In this paper, the VO2/SnO2 superlattice model was constructed by stacking rutile VO2 and SnO2 along the c-axis direction, and first-principles calculations were conducted to reveal the electrical properties and Li migration properties of the VO2/SnO2 superlattices. Results show that the metal and semiconductor appear alternately in the VO2/SnO2 superlattice with the increasing number of VO2 layers. When the number of VO2 layers is odd (1, 3 or 5), the VO2/SnO2 superlattice presents a metallic state, the electronic conductivity increases and the Li migration energy barrier decrease with the increasing number of VO2 layers. When the number of VO2 layers is even (2, 4 or 6), the VO2/SnO2 superlattice displays a semiconducting state, and the electronic conductivity and Li migration energy show slight changes. The high electronic conductivity and controllable Li migration barrier in the VO2/SnO2 superlattices provide a reference for the application of VO2/SnO2 in Li-ion-based devices.

Spatiotemporal Changes in Atomic and Molecular Architecture of Mineralized Bone under Pathogenic Conditions

Mechanisms responsible for spatiotemporal changes in the atomic-molecular architecture of the human femur in intact and osteoarthritis-affected areas were studied using high-resolution X-ray diffraction and spectroscopic techniques. Comparison of the experimental data demonstrates strong deviations of core electron-binding energies, lattice constants of hydroxyapatite crystal cells, linear sizes of crystallites, and degrees of crystallinity for both intact and osteoarthritic areas. The quantitative values of these characteristics and their standard deviations in each area are measured and presented. A systematic analysis of the site-dependent deviations was carried out within the framework of the 3D superlattice model. It is argued that the main mechanism responsible for the deviations arises primarily as a result of carbonization and catalytic reactions at the mineral-cartilage interface. The impact of the mechanism is enhanced in the vicinities of the area of sclerosed bone, but not inside the area where mechanical loads are maximum. Restoration of the atomic-molecular architecture of mineralized bone in the sclerosis area is revealed. Statistical aspects of the spatiotemporal changes in mineralized bone under pathogenic conditions are discussed.

Effective medium model for graphene superlattices with electrostatic and magnetic vector potentials

In this article we develop an effective medium model to characterize the electron wave propagation in graphene based nanostructures with electrostatic and magnetic vector potentials imposed on their surface. We use a numerical algorithm to determine the effective medium parameters of the heterostructure and calculate the electronic band structure of the system. We apply our formalism to analyze superlattices with solely a magnetic potential and reveal that the response of the structure remains reciprocal and is characterized by a decrease in the velocity of the charge carriers. We also study the response of superlattices with both potentials superimposed on graphene and show that the response of the system becomes nonreciprocal with a dispersion characterized by a tilted Dirac cone. We demonstrate that it is possible to alternate between type-I, type-II, or type-III Dirac cones by properly tuning the amplitude of the potentials.

Optoelectronic properties of Ga1−xAlxN superlattice nanowires tuned by multilayer Al composition via first-principles

Considering that the graded structure could generate an electric field, the structure and optoelectronic properties of [Formula: see text][Formula: see text]N superlattice nanowires are considered via first-principles. The structural stability and optoelectronic properties of single-component and component-graded nanowires are discussed. For [Formula: see text][Formula: see text]N superlattice nanowires, the formation energy decreases with increasing Al composition, resulting in a structure that tends to be stable. The [Formula: see text][Formula: see text]N superlattice model is more stable and the bond length changes more dramatically in the superlattice structure with bigger component divergence. The bandgap [Formula: see text] increases with increasing Al composition. The direct bandgap of [Formula: see text][Formula: see text]N superlattice nanowires is also affected by the nanowire sublayers. The absorption coefficient tends to increase with the increase of Al composition in the nanowires. These studies can serve as the basis for the preparation of ideal materials for deep ultraviolet photocathodes and improve the optoelectronic properties of deep ultraviolet photocathodes.

Formation Mechanism and Bandgap Reduction of GaN-ZnO Solid Solution Thin Films Fabricated by Nanolamination of Atomic Layer Deposition.

Nanolamination of GaN and ZnO layers by atomic layer deposition (ALD) is employed to fabricate GaN-ZnO homogeneous solid solution thin films because it offers more precise control of the stoichiometry. By varying the ALD cycle ratios of GaN:ZnO from 5:10 to 10:5, the (GaN)1- x (ZnO)x films with 0.39≦x≦0.79 are obtained. The formation of solid solution is explained based on the atomic stacking and preferred orientation of the layers of GaN and ZnO. However, the growth rates of GaN and ZnO during the lamination process are different from those of pure GaN and ZnO films. It is found that GaN grows faster on ZnO, whereas ZnO grows slower on GaN. The density function theory (DFT) calculations are performed using a superlattice model for GaN and ZnO laminated layers fabricated by ALD to understand the difference of density of states (DOS) and evaluate the bandgaps for various atomic configurations in the solid solution films. The band positions are experimentally defined by ultraviolet photoelectron spectroscopy. Significant bandgap reduction of the solid solutions is observed, which can be explained by the DOS from the DFT calculations. Visible-light-driven photocatalytic hydrogen evolution is conducted to confirm the applicability of the solid solution films. This article is protected by copyright. All rights reserved.

Berezinskii-Kosterlitz-Thouless phases in ultrathin PbTiO3/SrTiO3 superlattices

We study the emergence of Berezinskii-Kosterlitz-Thouless (BKT) phases in ${({\mathrm{PbTiO}}_{3})}_{3}/{({\mathrm{SrTiO}}_{3})}_{3}$ superlattices by means of second-principles simulations. Between a tensile epitaxial strain of $\ensuremath{\epsilon}=0.25--1\phantom{\rule{0.16em}{0ex}}%$, the local dipole moments within the superlattices are confined to the film-plane, and thus the polarization can be effectively considered as two-dimensional. The analysis of the decay of the dipole-dipole correlation with the distance, together with the study of the density of defects and its distribution as a function of temperature, supports the existence of a BKT phase in a range of temperature mediating the ordered ferroelectric (stable at low $T$), and the disordered paraelectric phase that appears beyond a critical temperature ${T}_{\mathrm{BKT}}$. This BKT phase is characterized by quasi-long-range order (whose signature is a power-law decay of the correlations with the distance), and the emergence of tightly bound vortex-antivortex pairs whose density is determined by a thermal activation process. The proposed ${\mathrm{PbTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ superlattice model and the imposed mechanical boundary conditions are both experimentally feasible, making it susceptible for an experimental observation of these new topological phases in ferroelectric materials.

Active and conductive layer stacked superlattices for highly selective CO2 electroreduction

Metal oxides are archetypal CO2 reduction reaction electrocatalysts, yet inevitable self-reduction will enhance competitive hydrogen evolution and lower the CO2 electroreduction selectivity. Herein, we propose a tangible superlattice model of alternating metal oxides and selenide sublayers in which electrons are rapidly exported through the conductive metal selenide layer to protect the active oxide layer from self-reduction. Taking BiCuSeO superlattices as a proof-of-concept, a comprehensive characterization reveals that the active [Bi2O2]2+ sublayers retain oxidation states rather than their self-reduced Bi metal during CO2 electroreduction because of the rapid electron transfer through the conductive [Cu2Se2]2- sublayer. Theoretical calculations uncover the high activity over [Bi2O2]2+ sublayers due to the overlaps between the Bi p orbitals and O p orbitals in the OCHO* intermediate, thus achieving over 90% formate selectivity in a wide potential range from −0.4 to −1.1 V. This work broadens the studying and improving of the CO2 electroreduction properties of metal oxide systems.

Moiré-induced band-gap opening in one-dimensional superlattices of carbon nanotubes on hexagonal boron nitride

Recently, two-dimensional (2D) moir\'e superlattices have been extensively studied, and many interesting physical phenomena have been observed. However, their one-dimensional (1D) counterpart---1D moir\'e superlattices---have been rarely explored yet. Here, we performed theoretical calculations of low-energy bands of 1D moir\'e superlattices of single-walled carbon nanotubes (CNTs) on hexagonal boron nitride (hBN) using a newly developed low-energy effective continuum model for 1D superlattices. We observed moir\'e-induced opening of small band gaps ranging from a few meV to a few tens of meV, which depends sensitively on the CNT chirality. The observed band-gap opening can be well understood by considering the coupling of electronic states between CNTs and the hBN using the effective continuum model. The results have been confirmed by the density functional theory calculations.

Multiscale modeling of ultrafast melting phenomena

Ultraviolet Nanosecond Laser Annealing (LA) is a powerful tool for both fundamental investigations of ultrafast, nonequilibrium phase-change phenomena and technological applications (e.g., the processing of 3D sequentially integrated nano-electronic devices) where strongly confined heating and melting is desirable. Optimizing the LA process along with the experimental design is challenging, especially when involving complex 3D-nanostructured systems with various shapes and phases. To this purpose, it is essential to model critical nanoscale physical LA-induced phenomena, such as shape changes or formation and evolution of point and extended defects. To date, LA simulators are based on continuum models, which cannot fully capture the microscopic kinetics of a solid–liquid interface. In this work a fully atomistic LA simulation methodology is presented, based on the parallel coupling of a continuum, finite elements, μm-scale electromagnetic-thermal solver with a super-lattice Kinetic Monte Carlo atomistic model for melting. Benchmarks against phase-field models and experimental data validate the approach. LA of a Si(001) surface is studied varying laser fluence and pulse shape, assuming both homogeneous and inhomogeneous nucleation, revealing how liquid Si nuclei generate, deform and coalesce during irradiation. The proposed methodology is applicable to any system where the atom kinetics is determined by a strongly space- and time-dependent field, such as temperature or strain.

Super-Klein tunneling and electron-beam collimation in the honeycomb superlattice

The honeycomb superlattice model from the nearly commensurate charge-density-wave phase of $1\mathrm{T}\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ has been shown to host multiple robust flat bands that depends on the number $(m)$ of extra atoms residing on each edge of the hexagon [Lee et al., Phys. Rev. Lett. 124, 137002 (2020)]. In this work we numerically study the transport phenomenon of the single-valley Dirac electrons around the band center $E=0$ where a flat band crosses. A super-Klein tunneling (SKT) phenomenon is found for $m=1$ that the Dirac electrons can tunnel through a rectangle barrier with a unity transmission irrespective of the incident angle of electrons. For the case of $m=3$, a super electron-beam collimation instead of the SKT phenomenon is identified that the Dirac electrons are allowed to tunnel through the barrier only within nearly zero incident angle, and such perpendicular transmission probability is optimal in the low-energy regime but suppressed in the high-energy regime.

Double Electron-Electron Resonance of Spin-Labeled Cholestane in Model Membranes: Evidence for Substructures inside the Lipid Rafts.

Plasma membranes are assumed to be highly compartmentalized, which is believed to be important for the membrane protein functionality. The liquid ordered-disordered phase segregation in the membranes results in nanoscale liquid-ordered assemblies-lipid rafts. Double electron-electron resonance spectroscopy (DEER, also known as PELDOR) is sensitive to spin-spin dipolar interactions between spin labels at the nanoscale range of distances. Here, DEER is applied to spin-labeled cholestane, 3β-doxyl-5α-cholestane (DChl), diluted in bilayers composed of an equimolar mixture of dioleoyl-glycero-phosphocholine (DOPC) and dipalmitoyl-glycero-phosphocholine (DPPC) phospholipids, with cholesterol (Chol) added. The DEER data allowed us to detect clustering of the DChl molecules. Their lateral distribution in the clusters in the absence of Chol was found to be random, while in the presence of Chol it became quasi-regular. DEER time traces are fairly well simulated within a simple square superlattice model. For the 20 mol % Chol content, for which at physiological temperatures, the lipid rafts are formed, the found superlattice parameter was 3.7 nm. Assuming that lipid rafts are captioned upon shock freezing at the temperature of investigation (80 K), the found regularity of DChl lateral distribution was interpreted by raft substructuring, with the DChl molecules embedded between the substructures.

Spontaneous Valley Spirals in Magnetically Encapsulated Twisted Bilayer Graphene.

Van der Waals heterostructures provide a rich platform for emergent physics due to their tunable hybridization of layers, orbitals, and spin. Here, we find that twisted bilayer graphene stacked between antialigned ferromagnetic insulators can feature flat electronic bands due to the interplay between twist, exchange proximity, and spin-orbit coupling. These flat bands are nearly degenerate in valley only and are effectively described by a triangular superlattice model. At half filling, we find that interactions induce spontaneous valley correlations that favor spiral order and derive a low-energy valley-Heisenberg model with symmetric and antisymmetric exchange couplings. We also show how electric interlayer bias broadens the bands and tunes these couplings. Our results put forward magnetic van der Waals heterostructures as a platform to explore valley-correlated states.

The Physics of Interlayer Exciton Delocalization in Ruddlesden-Popper Lead Halide Perovskites.

Two-dimensional (2D) lead halide Ruddlesden-Popper perovskites (RPP) have recently emerged as a prospective material system for optoelectronic applications. Their self-assembled multi quantum-well structure gives rise to the novel interwell energy funnelling phenomenon, which is of broad interests for photovoltaics, light-emission applications, and emerging technologies (e.g., spintronics). Herein, we develop a realistic finite quantum-well superlattice model that corroborates the hypothesis of exciton delocalization across different quantum-wells in RPP. Such delocalization leads to a sub-50 fs coherent energy transfer between adjacent wells, with the efficiency depending on the RPP phase matching and the organic large cation barrier lengths. Our approach provides a coherent and comprehensive account for both steady-state and transient dynamical experimental results in RPPs. Importantly, these findings pave the way for a deeper understanding of these systems, as a cornerstone crucial for establishing material design rules to realize efficient RPP-based devices.

Model phonon spectra of Cu7SiS5I and Ag7SiS5I crystals

In the concept of superspatial symmetry the crystal structure of Cu7SiS5I and Ag7SiS5I superionic conductors has been analyzed. To calculate the phonon spectra, the model of FCC superlattice (8а, 8а, 0; 8а, 0, 8а; 0, 8а, 8а) in the metric of protocrystal (а, а, 0; а, 0, а; 0, а, а) has been developed. For the developed model, the general (3+3)-dimensional basis, the array of modulation vectors and mass modulation functions have been presented. The model calculations of phonon spectra dispersion for Cu7SiS5I and Ag7SiS5I crystals in schemes with various partial occupation of crystallographic orbits by Cu(Ag) atoms have been performed. The dispersion dependences of phonon spectra for Cu7SiS5I and Ag7SiS5I crystals in the high-symmetric directions of Brillouin zone have been presented. The genesis of phonon branches of vibrational spectra has been analyzed.

Higher-order entanglement and many-body invariants for higher-order topological phases

We discuss how strongly interacting higher-order symmetry protected topological (HOSPT) phases can be characterized from the entanglement perspective: First, we introduce a topological many-body invariant which reveals the non-commutative algebra between flux operator and $C_n$ rotations. We argue that this invariant denotes the angular momentum carried by the instanton which is closely related to the discrete Wen-Zee response and fractional corner charge. Second, we define a new entanglement property, dubbed `higher-order entanglement', to scrutinize and differentiate various higher-order topological phases from a hierarchical sequence of the entanglement structure. We support our claims by numerically studying a super-lattice Bose-Hubbard model that exhibits different HOSPT phases.

Fractional corner charges in a two-dimensional superlattice Bose-Hubbard model

We study a two dimensional super-lattice Bose-Hubbard model with alternating hoppings in the limit of strong on-site interactions. We evaluate the phase diagram of the model around half-filling using the density matrix renormalization group method and find two gapped phases separated by a gapless superfluid region. We demonstrate that the gapped states realize two distinct higher order symmetry protected topological phases, which are protected by a combination of charge conservation and $C_4$ lattice symmetry. The phases are distinguished in terms of a quantized fractional corner charge and a many-body topological invariant that is robust against arbitrary, symmetry preserving edge manipulations. We support our claims by numerically studying the full counting statistics of the corner charge, finding a sharp distribution peaked around the quantized values. These results are experimentally observable in ultracold atomic settings using state of the art quantum gas microscopy.

Hierarchy-induced X-ray linear dichroism in cortical bone

The high-resolution X-ray absorption spectra of rat tibia cortex have been measured for two different orientations of the bone long axis in respect to the electric field vector of linear polarized synchrotron radiation. Our measurements performed near the Ca 2p and O 1s absorption edges have revealed the substantial element-specific orientation dependence of the X-ray absorption coefficient. Strong changes in intensity of core-to-valence transitions are detected near the Ca L2,3 absorption edges, whereas minor changes occur near the O K absorption edge. The specific behavior is attributed to the emergent properties of X-ray linear dichroism in cortex. This optical phenomenon is assigned with relationship between the chemical Ca–O bonds in bone and hierarchical organization of the skeleton. The hierarchy-induced X-ray linear dichroism is discussed in the framework of the 3D superlattice model and Wolff paradigm. Prospects for using this phenomenon for medical diagnosis are also being discussed.