Band Dispersion Research Articles

Conduction Band Replicas in a 2D Moiré Semiconductor Heterobilayer.

Stacking monolayer semiconductors creates moiré patterns, leading to correlated and topological electronic phenomena, but measurements of the electronic structure underpinning these phenomena are scarce. Here, we investigate the properties of the conduction band in moiré heterobilayers of WS2/WSe2 using submicrometer angle-resolved photoemission spectroscopy with electrostatic gating. We find that at all twist angles the conduction band edge is the K-point valley of the WS2, with a band gap of 1.58 ± 0.03 eV. From the resolved conduction band dispersion, we deduce an effective mass of 0.15 ± 0.02 me. Additionally, we observe replicas of the conduction band displaced by reciprocal lattice vectors of the moiré superlattice. We argue that the replicas result from the moiré potential modifying the conduction band states rather than final-state diffraction. Interestingly, the replicas display an intensity pattern with reduced 3-fold symmetry, which we show implicates the pseudo vector potential associated with in-plane strain in moiré band formation.

Isolated flat band in artificially designed Lieb lattice based on macrocycle supramolecular crystal

Isolated flat bands are known to host various strongly correlated phases due to the enhanced Coulomb interactions when the flat bands are gapped from dispersive bands. One way to achieve an isolated flat band is by breaking the on-site energy symmetry in a Lieb lattice. In this study, we demonstrate the design of such a Lieb lattice. The self-assembly of square-shaped macrocycle molecules on a Ag(111) surface forms a two-dimensional supramolecular crystal, comprising three types of nanopores with different sizes arranged in a Lieb lattice. The surface-state electrons of the Ag(111) substrate confined by these nanopores behave as quantum dots with specific energies depending on the pore size. Using scanning tunneling spectroscopy and plane-wave quantum simulation, we reveal that this artificial Lieb lattice exhibits an isolated flat band gapped at 0.16 eV from the nearest band. The supramolecular crystal is nearly defect-free and extends to sub-micrometer size, making it a practical platform for exploring the exotic properties of the isolated flat band.

Plasmon dispersion in bilayer cuprate superconductors

The essential building blocks of cuprate superconductors are two-dimensional CuO2 sheets interspersed with charge reservoir layers. In bilayer cuprates, two closely spaced CuO2 sheets are separated by a larger distance from the subsequent pair in the next unit cell. In contrast to single-layer cuprates, prior theoretical work on bilayer systems has predicted two distinct acoustic plasmon bands for a given out-of-plane momentum transfer. Here we report random phase approximation (RPA) calculations for bilayer systems which corroborate the existence of two distinct plasmon bands. We find that the intensity of the lower-energy band is negligibly small in most parts of the Brillouin zone, whereas the higher-energy band carries significant spectral weight. We also present resonant inelastic x-ray scattering (RIXS) experiments at the O K-edge on the bilayer cuprate Y0.85Ca0.15Ba2Cu3O7 (Ca-YBCO), which show only one dispersive plasmon branch, in agreement with the RPA calculations. In addition, the RPA results indicate that the dispersion of the higher-energy plasmon band in Ca-YBCO is not strictly acoustic but exhibits a substantial energy gap of approximately 250 meV at the two-dimensional Brillouin zone center. Published by the American Physical Society 2024

A neural network model for point defect microcavities of two-dimensional GaAs media background photonic crystals

This paper aims to address the challenge of constructing, training, and optimizing a neural network model for the point defect microcavity of a two-dimensional GaAs media background photonic crystal. The objective is to achieve more accurate predictions and analyses of photon modes, thereby enhancing the performance of the microcavity. Due to the structural complexity and multi-parameter nature of point defect microcavities, traditional analysis methods often fail to capture their optical behavior adequately. Consequently, there is a need for a novel approach to establishing an optical model for point defect microcavities, enabling precise predictions of their optical properties and optimal design. In this study, the MPB computing tool developed by MIT, in conjunction with the programming language Scheme, MATLAB, and the Ubuntu virtual machine platform, was utilized for data acquisition. The neural network model was employed to process the input data of the photonic crystal point defect microcavity, leading to predictions and analyses. This interdisciplinary approach results in a high-precision model of the microcavity. The neural network model is capable of nonlinear mapping and prediction by learning the patterns and features of the data. Additionally, this combination holds the promise of enabling more efficient optical information processing and increased data transfer rates. The relationship between the energy band data and the structure of photonic crystals exhibits a high degree of nonlinearity. The neural network model proposed in this paper can infer the structure of photonic crystals based on the photon energy band (dispersion relation ω-K) of micro-cavity structures with point defects in different photonic crystals. This provides a valuable reference for more accurate predictions and analyses of photon modes in complex photonic crystal micro-cavities, ultimately leading to improved micro-cavity performance.

Upper-branch thermal Hall effect in quantum paramagnets

Inspired by the persistent thermal Hall effects at finite temperatures in various quantum paramagnets, we explore the origin of the thermal Hall effects from the perspective of the upper branch parts by invoking dispersive and twisted crystal field excitations. It is shown that the upper branches of the local energy levels could hybridize and form dispersive bands. The observation is that, upon time-reversal symmetry breaking by the magnetic fields, these upper branch bands could acquire a Berry curvature distribution and contribute to the thermal Hall effect even in the paramagnetic regime. As a proof of principle, we consider the setting on kagomé lattice with one ground-state singlet and an excited doublet, and show this is indeed possible. We expect this effect to be universal and that it has no strong connection with the underlying lattice. Although the thermal Hall signal can be contributed from other sources such as phonons and their scattering in the actual materials, we discuss the application to the relevant quantum materials. Published by the American Physical Society 2024

Fatigue life estimation of nickel-based single crystal superalloy with different inclined film cooling holes: Initial damage quantification and coupling of damage-fracture mechanics models

Quantitative assessment of the initial damage state of a structure and fatigue life prediction on this basis, especially when it is sensitive to manufacturing quality, is crucial for both engineering application and material science. Traditional solutions based on fracture or damage mechanics either require precise constitutive relationships and cumbersome physical mechanism models or simply ignore the initial damage state. This study developed a novel equivalent initial flaw size (EIFS) quantitative evaluation method for film cooling holes in Nickel-based single crystal superalloy turbine blades to address the strong correlation between the initial damage in and fatigue life of the material. The temperature and stress field differences at the edges of holes with different inclination angles were simulated using a solid-liquid-gas three-phase level-set model and considered to represent the same initial damage. Next, the coupled damage–fracture mechanics model was used to achieve equivalent crack insertion, crack propagation, and fatigue life prediction based on the EIFS. The results indicate that this method can provide a highly robust EIFS distribution interval and that the crack geometry correction factor and EIFS distribution range are weakly correlated with the loading conditions (with an error within 2 %). The EIFS-based fatigue life predictions demonstrated a notable degree of accuracy, with the majority of the predicted and experimental fatigue lives falling within a three-fold dispersion band and all predictions remaining confined within a five-fold dispersion band of the actual lifetime. These results represent a substantial advancement in accuracy compared to traditional damage mechanics predictions. Therefore, this study provides a powerful approach for evaluating the fatigue life of turbine blades considering the initial damage state and can be widely applied to guide drilling process optimization and blade fatigue analysis.

Electronic structure and effective mass of pristine and Cl-doped CsPbBr3

Organic–inorganic lead halide perovskites (LHPs) have attracted great interest owing to their outstanding optoelectronic properties. Typically, the underlying electronic structure would determinate the physical properties of materials. But as for now, limited studies have been done to reveal the underlying electronic structure of this material system, comparing to the huge amount of investigations on the material synthesis. The effective mass of the valance band is one of the most important physical parameters which plays a dominant role in charge transport and photovoltaic phenomena. In pristine CsPbBr3, the Fröhlich polarons associated with the Pb–Br stretching modes are proposed to be responsible for the effective mass renormalization. In this regard, it would be very interesting to explore the electronic structure in doped LHPs. Here, we report high-resolution angle-resolved photoemission spectroscopy (ARPES) studies on both pristine and Cl-doped CsPbBr3. The experimental band dispersions are extracted from ARPES spectra along both Γ¯ – M¯ – Γ¯ and X¯ – M¯ – X¯ high symmetry directions. DFT calculations are performed and directly compared with the ARPES data. Our results have revealed the band structure of Cl-doped CsPbBr3 for the first time, which have also unveiled the effective mass renormalization in the Cl-doped CsPbBr3 compound. Doping dependent measurements indicate that the chlorine doping could moderately tune the renormalization strength. These results will help understand the physical properties of LHPs as a function of doping.

Enhancing the emulsion properties and bioavailability of loaded astaxanthin by selecting the reaction sequence of ternary conjugate emulsifiers in nanoemulsions

This study investigated the effects of the conjugate reaction sequences of whey protein concentrate (WPC), epigallocatechin gallate (EGCG) and dextran (DEX) on the structure and emulsion properties of conjugates and the bioaccessibility of astaxanthin (AST). Two types of ternary covalent complexes were synthesised using WPC, EGCG and DEX, which were regarded as emulsifiers of AST nanoemulsions. Results indicated that the WPC-DEX-EGCG conjugate (referred to as ‘con’) exhibits a darker SDS-PAGE dispersion band and higher contents of α-helix (6%), β-angle (24%) and random coil (32%), resulting in a greater degree of unfolding structure and fluorescence quenching. These findings suggested WPC-DEX-EGCG con had the potential to exhibit better emulsification properties than WPC-EGCG-DEX con. AST encapsulation efficiency (76.22%) and bioavailability (31.89%) also demonstrated the superior performance of the WPC-DEX-EGCG con emulsifier in nanoemulsion delivery systems. These findings indicate that altering reaction sequences changes protein conformation, enhancing the emulsification properties and bioavailability of AST.

Nonlinear optical diode effect in a magnetic Weyl semimetal

Diode effects are of great interest for both fundamental physics and modern technologies. Electrical diode effects (nonreciprocal transport) have been observed in Weyl systems. Optical diode effects arising from the Weyl fermions have been theoretically considered but not probed experimentally. Here, we report the observation of a nonlinear optical diode effect (NODE) in the magnetic Weyl semimetal CeAlSi, where the magnetization introduces a pronounced directionality in the nonlinear optical second-harmonic generation (SHG). We demonstrate a six-fold change of the measured SHG intensity between opposite propagation directions over a bandwidth exceeding 250 meV. Supported by density-functional theory, we establish the linearly dispersive bands emerging from Weyl nodes as the origin of this broadband effect. We further demonstrate current-induced magnetization switching and thus electrical control of the NODE. Our results advance ongoing research to identify novel nonlinear optical/transport phenomena in magnetic topological materials and further opens new pathways for the unidirectional manipulation of light.

Layer Hall counterflow as a model probe of magic-angle twisted bilayer graphene

The recent constructions of flat moiré minibands in specifically twisted multilayer graphene and twisted transition metal dichalcogenides (TMDs) have facilitated the observation of strong correlations with convenient tunability. These correlations in flat bands result in band dispersion heavily influenced by carrier densities, leading to filling-dependent quasiparticle band renormalizations. Particularly, in magic-angle twisted bilayer graphene (MATBG), the band structure—including the quasiparticle energy and wave function—is crucial in understanding the correlated properties. Previous theoretical studies have demonstrated the presence of a time-reversal-even charge Hall counterflow in response to a direct current (DC) electric field in twisted bilayers as chiral structures. In this study, we show that such layer Hall counterflow can serve as a sensitive probe for MATBG model parameters, which are currently ambiguous as a result of unavoidable structural relaxation and twist-angle disorder. We present the layer Hall counterflow and the associated in-plane magnetization for three different MATBG continuum models, based on which many-body interacting models have been widely applied to study strong correlations in MATBG. At the single-particle level, our findings indicate notable differences in layer-projected Hall conductivity, both in magnitude and sign, between different MATBG continuum models. Furthermore, our self-consistent Hartree calculations, performed on each of these single-particle continuum models, reveal renormalized layer-projected Hall conductivity by the self-consistent Hartree field. Published by the American Physical Society 2024

Pumping with symmetry

Re-configurable materials and meta-materials can jump between space symmetry classes during their deformations. Here, we introduce the concept of singular symmetry enhancement, which refers to an abrupt jump to a higher symmetry class accompanied by an un-avoidable reduction in the number of dispersion bands of the excitations of the material. Such phenomenon prompts closings of some of the spectral resonant gaps along singular manifolds in a parameter space. In this work, we demonstrate that these singular manifolds can carry topological charges. As a concrete example, we show that a deformation of an acoustic crystal that encircles a configuration of an array of cavity resonators results in an adiabatic cycle that carries a Chern number in the bulk and displays Thouless pumping at the edges. This points to a very general guiding principle for recognizing cyclic adiabatic processes with high potential for topological pumping in complex materials and meta-materials, which rests entirely on symmetry arguments.

Broken Screw Rotational Symmetry in the Near-Surface Electronic Structure of AB-Stacked Crystals.

We investigate the electronic structure of 2H-NbS_{2} and h-BN by angle-resolved photoemission spectroscopy (ARPES) and photoemission intensity calculations. Although in bulk form, these materials are expected to exhibit band degeneracy in the k_{z}=π/c plane due to screw rotation and time-reversal symmetries, we observe gapped band dispersion near the surface. We extract from first-principles calculations the near-surface electronic structure probed by ARPES and find that the calculated photoemission spectra from the near-surface region reproduce the gapped ARPES spectra. Our results show that the near-surface electronic structure can be qualitatively different from the bulk electronic structure due to partially broken nonsymmorphic symmetries.

PtTe2 photodetectors with negative photoconductivity under different wavelength laser irradiation

Platinum telluride (PtTe2), as a newly emerged type II Dirac semimetal, has attracted much attention. Due to its excellent linear dispersive band structure and ultra-high carrier mobility, PtTe2 show great potential in the new quantum phenomena, topological phase transitions, unconventional superconductivity and other photodetecting fields. In this paper, we report the negative photoelectric effect of photodetectors based on the type II Dirac semimetal PtTe2 film under the laser irradiation of 405 nm, 532 nm and 808 nm. Under the condition of applying 1 V voltage, the R of the PtTe2 device under 405 nm laser irradiation was 20 A/W and the D* was 6.9×109 Jones; the R of the PtTe2 device under 532 nm laser irradiation was 1.3 A/W and the D* was 4.1×108 Jones; the R of the PtTe2 device under 808 nm laser irradiation was 0.6 A/W and the D* was 2.1×108 Jones. Under normal atmospheric pressure and room temperature conditions, we observed a negative photoconductivity phenomenon for the PtTe2 device, where the change in photoconductivity is attributed to the light-induced desorption of O or H2O molecules from the surface of the material. In addition, a single-site scanning imaging system based on a PtTe2 photodetector was designed to test the detection capability of the device. The analysis of the photocurrent effect produced by the PtTe2 device further deepens the understanding of the photoconductivity mechanism in the field of photodetection.

Evolution of structural and electronic properties standardized description in rhenium disulfide at the bulk-monolayer transition

The structural and electronic properties of ReS2 different forms — three-dimensional bulk and two-dimensional monolayer — were studied within density functional theory and pseudopotentials. A method for standardizing the description of bulk unit cells and "artificial" slab unit cells for DFT research has been proposed. The preference of this method for studying zone dispersion has been shown. The influence of the vacuum layer thickness on specified special high-symmetry points is discussed. Electron band dispersion in both classical 3D Brillouin zones and transition to 2D Brillouin zones in the proposed two-dimensional approach using the Niggli form of the unit cell was compared. The proposed two-dimensional approach is preferable for low-symmetry layered crystals such as ReS2. It was established that the bulk ReS2 is a direct gap semiconductor (band gap of 1.20 eV), with the direct transition lying in the X point of the first Brillouin zone, and it is in good agreement with published experimental data. The reduction in material dimension from bulk to monolayer was conducted with an increasing band gap up to 1.45 eV, with a moving direct transition towards the Brillouin zone center. The monolayer of ReS2 is a direct-gap semiconductor in a wide range of temperatures, excluding only a narrow range at low temperatures, where it comes as a quasi-direct gap semiconductor. The transition, situated directly in the Γ-point, lies 3.3 meV below the first direct transition located near this point. The electronic density of states of ReS2 in the bulk and monolayer cases of ReS2 were analyzed. The molecular orbitals were built for both types of ReS2 structures as well as the electron difference density maps. For all types of ReS2 structures, an analysis of populations according to Mulliken and Voronoi was carried out. All calculated data is discussed in the context of weak quantum confinement in the 2D case.

Stable lead free double perovskites Cs2XAu(Br/I)6 (X = Y, Sc) as an emerging aspirant for solar cells and thermoelectric applications

In present article, the electronic, optical, mechanical, and thermoelectric characteristics of Cs2XAu(Br/I)6, where X = Y, Sc are addressed comprehensively. The Born stability criteria ensure the mechanical stability, whereas the structural, thermodynamic, and lattice stabilities are calculated from tolerance factor, formation energy, and phonons dispersion band structures. The ductility, hardness, lattice thermal conductivity, melting, and Debye temperatures are discussed in terms of elastic constants. The direct band gaps 1.90/1.35 eV Cs2ScAu(Br/I)6, and 2.23/1.74 eV for Cs2YAu(Br/I)6 fall the absorption of light energy in visible to near ultraviolet regions. The Cs2ScAuI6 has ideal band gap (1.35 eV) for solar cells because broad absorption band in visible region. Finally, thermoelectric behavior has been studied by Seebeck coefficient, electrical conductivity, power factor, and thermal conductivity in the temperature range 200 K–600 K. The highest figure of merit (1.0 at 300 K) for Cs2YAuBr6 increases demand for thermoelectric generators.

Layer-polarized ferromagnetism in rhombohedral multilayer graphene

Flat-band systems with strongly correlated electrons can exhibit a variety of phenomena, such as correlated insulating and topological states, unconventional superconductivity, and ferromagnetism. Rhombohedral multilayer graphene has recently emerged as a promising platform for investigating exotic quantum states due to its hosting of topologically protected surface flat bands at low energy, which have a layer-dependent energy dispersion. However, the complex relationship between the surface flat bands and the highly dispersive high-energy bands makes it difficult to study correlated surface states. In this study, we introduce moiré superlattices as a method to isolate the surface flat bands of rhombohedral multilayer graphene. The observed pronounced screening effects in the moiré potential-modulated rhombohedral multilayer graphene indicate that the two surface states are electronically decoupled. The flat bands that are isolated promote correlated surface states in areas that are distant from the charge neutrality points. Notably, we observe tunable layer-polarized ferromagnetism, which is evidenced by a hysteretic anomalous Hall effect. This is achieved by polarizing the surface states with finite displacement fields.

Unconventional anomalous Hall effect in Zintl thermoelectric Eu[formula omitted]ZnSb[formula omitted

The outstanding performance of thermoelectric materials is intimately related to complicated electronic band structures that may comprise topological linear dispersion bands. In this work, the exotic magnetism and magneto-transport properties of thermoelectric Eu2ZnSb2 single crystal are investigated. Temperature- and field-dependent magnetization reveal an antiferromagnetic (AFM) ground state and highly localized magnetic moments of Eu2+ ions. The longitudinal resistivity [ρxx(T)] and magnetoresistivity (MR) unveil the coexistence of underlying ferromagnetic (FM) and AFM fluctuations. Furthermore, an unconventional anomalous Hall effect (UAHE) for both above and below the AFM phase transition temperature (TN) is observed, which is suggested to be caused by the vacancy-induced Weyl bands and strong magnetic fluctuations. Our work is benefit for understanding the exotic band structures and thermoelectric properties in Eu2ZnSb2.

High photovoltaic performance of lead-free Cs2AgInCl6-xBrx perovskite solar cell using DFT and SCAPS-1D simulations

The search of the lead-free highly efficient photovoltaic material is a key challenge for next generation solar cells. The structural, electronic, and optical properties of lead-free double perovskite Cs2AgInCl6-xBrx (x= 0,1,2,3,4,5 and 6) were investigated using density functional theory and its photovoltaic performance by using one-dimensional solar cell capacitance simulator SCAPS-1D. By increasing Br content in Cs2AgInCl6, the lattice parameter increases and the electronic band gap decreases without affecting the direct nature of band gap at Г- point. By mixing Br in Cs2AgInCl6 also provides an effective way to tune the energy band gap. In addition, increasing the Br content in mixed halide composition Cs2AgInCl6-xBrx (x= 0,1,2,3,4,5 and 6) reproduces more dispersive conduction band minima and valence band maxima result in lower effective masses of both charge carriers and consequently higher bipolar carrier mobility. The optical properties of Cs2AgInCl6-xBrx, such as absorption coefficient, reflectivity, dielectric constants, refractive index, and extinction coefficient were investigated with different Br concentrations. Among all compositions, the Cs2AgInClBr5 exhibits better optical properties and behaves as poor reflector and good absorber in the visible region. The photovoltaic performance of different absorber layers of Cs2AgInCl6-xBrx compositions in the perovskites solar cell module with configuration FTO/ETL/Cs2AgInCl6-xBrx /HTL/Au is analysed by using the SCAPS-1D. It is also observed that Cs2AgInClBr5 has high bipolar carrier mobility, relatively higher stability and direct band gap value of 1.75 eV with the PCE of 21.95% in the visible region indicate the suitability of the Cs2AgInClBr5 as a promising lead-free photovoltaic material.

High harmonic generation in monolayer indium nitride

In our work, we theoretically investigate high harmonic generation (HHG) in monolayer hexagonal indium nitride (h-InN) based on the semiconductor Bloch equation under strong laser fields. Compared with h-BN, there is no multiplateau in h-InN. This is because the intraband mechanism dominates the total HHG, and the harmonic generated by the intraband current is about three orders of magnitude higher than that generated by the interband polarization. We find that the higher order part of the HHG is mainly supplied by the interband current, which can be analyzed on the basis of the transition dipole moments between the energy bands. In addition, we found that the HHG of h-InN is sensitive to the external strains due to the modified band dispersion in the electronic structures. This study provides a useful reference for understanding the microscopic mechanism of laser-solid interaction.

Tuning of the flat band and its impact on superconductivity in Mo5Si3−xPx

The superconductivity in systems containing dispersionless (flat) bands is seemingly paradoxical, as traditional Bardeen-Cooper-Schrieffer theory requires an infinite enhancement of the carrier masses. However, the combination of flat and steep (dispersive) bands within the multiple band scenario might boost superconducting responses, potentially explaining high-temperature superconductivity in cuprates and metal hydrides. Here, we report on the magnetic penetration depths, the upper critical field, and the specific heat measurements, together with the first-principles calculations for the Mo5Si3−xPx superconducting family. The band structure features a flat band that gradually approaches the Fermi level as a function of phosphorus doping x, reaching the Fermi level at x ≃ 1.3. This leads to an abrupt change in nearly all superconducting quantities. The superfluid density data placed on the ’Uemura plot‘ results in two separated branches, thus indicating that the emergence of a flat band enhances correlations between conducting electrons.