Center Of Brillouin Zone Research Articles

Electronic, Mechanical, and Infrared Properties of BiOX (X = Cl, Br, I) Monolayers

BiOX (X = Cl, Br, I) monolayers are photocatalytic materials having many potential applications, but their basic properties are still not systematically studied. Herein, first‐principles calculations are performed to study their electronic, mechanical, and infrared properties. Band structures calculated by the Heyd–Scuseria–Ernzerhof (HSE06) functional with and without spin orbital coupling uncover that they are indirect bandgap materials. Partial densities of states of these materials are computed and compared to reveal the reason for the differences among their band structures. Studies imply that the BiO bonds are chiefly covalent while the interaction between the Bi and the X atoms is mainly ionic. Elastic constants and ideal in‐plane stresses of these materials are computed to explore their mechanical properties. Performed factor group analyses indicate that there are nine Raman‐active and six infrared‐active modes in their Brillouin zone centers. Their infrared spectra are simulated and the reason for the disappearance of some infrared peaks is given. Comparisons on the above properties are also made for these BiOX monolayers.

Multiple flatbands and localized states in photonic super-Kagome lattices.

We demonstrate multiple flatbands and compact localized states (CLSs) in a photonic super-Kagome lattice (SKL) that exhibits coexistence of singular and nonsingular flatbands within its unique band structure. Specifically, we find that the upper two flatbands of an SKL are singular-characterized by singularities due to band touching with their neighboring dispersive bands at the Brillouin zone center. Conversely, the lower three degenerate flatbands are nonsingular and remain spectrally isolated from other dispersive bands. The existence of such two distinct types of flatbands is experimentally demonstrated by observing stable evolution of the CLSs with various geometrical shapes in a laser-written SKL. We also discuss the classification of the flatbands in momentum space, using band-touching singularities of the Bloch wave functions. Furthermore, we validate this classification in real space based on unit cell occupancy of the CLSs in a single SKL plaquette. These results may provide insights for the study of flatband transport, dynamics, and nontrivial topological phenomena in other relevant systems.

First-principles study of the structural, mechanical, dynamical, and transport properties of Cs2NaInX6[X = Br,I] for thermoelectric applications

Thermoelectricity generation techniques utilized for conversion of waste heat into electric power are currently gaining prominence. It is necessary that for improved efficiency, the materials used for such applications should have a high thermoelectric figure of merit (ZT) and appropriate stability. In this study, the properties of Cs2NaInX6 double perovskites (DPs) were examined using a first-principles approach to ascertain their suitability as thermoelectric materials. The findings revealed that the materials are mechanically stable according to the Born stability criteria for cubic crystals. They were also observed to be anisotropic given isotropic values of 0.702 and 0.662 for Cs2NaInBr6 and Cs2NaInI6 DPs respectively. Phonon calculations indicate soft modes at the Brillouin zone boundary and center (Γ) for Cs2NaInI6 DP, which suggests ferroelectric and anti-ferroelectric distortions. Similarly for Cs2NaInBr6 DP, the soft modes were exclusively found at the BZ center (Γ), suggesting ferroelectric distortion. The transport coefficient properties calculated are used for the determination of the ZT values. The calculated values of ZT at ambient temperature are of the order 1.0 for all the DPs which is high enough for thermoelectric applications suggesting that they may be potential thermoelectric materials

Study of the electronic, optical, elastic and infrared properties of trigonal Mg3As2

Trigonal Mg3As2 is a narrow band gap semiconductor, which makes it possible for many technical applications. However, its fundamental properties, such as electronic, optical, elastic and infrared properties, are still not well studied. Here we systematically studied its these properties with the help of first principles calculations. Band structure is obtained and the origins of the bands are revealed by the calculated partial density of states. Its bonding property is analyzed based on the computed electron density difference and Bader charges. Optical dielectric function and the related quantities are derived and discussed. Its mechanical stability and anisotropy is confirmed by the calculated elastic constants. The phonon vibrational modes in its Brillouin zone center are assigned and infrared emission spectrum is simulated. The reason for the disappearance of one infrared peak in the simulated infrared spectrum is revealed. The brittleness, hardness and melting temperature of this crystal are also predicted. The present study can enrich our understanding of trigonal Mg3As2 and facilitate its future applications.

Visualizing near-coexistence of massless Dirac electrons and ultra-massive saddle point electrons

Strong singularities in the electronic density of states amplify correlation effects and play a key role in determining the ordering instabilities in various materials. Recently high order van Hove singularities (VHSs) with diverging power-law scaling have been classified in single-band electron models. We show that the 110 surface of Bismuth exhibits high order VHS with an usually high density of states divergence \sim (E)^{-0.7}∼(E)−0.7. Detailed mapping of the surface band structure using scanning tunneling microscopy and spectroscopy combined with first-principles calculations show that this singularity occurs in close proximity to Dirac bands located at the center of the surface Brillouin zone. The enhanced power-law divergence is shown to originate from the anisotropic flattening of the Dirac band just above the Dirac node. Such near-coexistence of massless Dirac electrons and ultra-massive saddle points enables to study the interplay of high order VHS and Dirac fermions.

Ab-initio simulations of coherent phonon-induced pumping of carriers in zirconium pentatelluride

Laser-driven coherent phonons can act as modulated strain fields and modify the adiabatic ground state topology of quantum materials. Here we use time-dependent first-principles and effective model calculations to simulate the effect of the coherent phonon induced by strong terahertz electric field on electronic carriers in the topological insulator ZrTe5. We show that a coherent A1g Raman mode modulation can effectively pump carriers across the band gap, even though the phonon energy is about an order of magnitude smaller than the equilibrium band gap. We reveal the microscopic mechanism of this effect which occurs via Landau–Zener-Stückelberg tunneling of Bloch electrons in a narrow region in the Brillouin zone center where the transient energy gap closes when the system switches from strong to weak topological insulator. The quantum dynamics simulation results are in excellent agreement with recent pump-probe experiments in ZrTe5 at low temperature.

DFT study of electronic and optical properties of Cu2ZnSiS4 chalcogenide compound and experimental investigation of improved PEC response of α-MoO3/Cu2ZnGeS4 nanocomposite

The full potential linearized augmented plane wave method (FP-LAPW) is used to compute the optoelectronic properties of Cu2ZnSiS4 by employing the modified Becke-Johnson (mBJ) as the exchange correlation potential. The band structures, total and partial densities of states and optical properties including refractive indices are calculated. Moreover, we have fabricated the MoO3/Cu2ZnGeS4 nanocomposite and their structural, morphological, and photoelectrochemical performance is then experimentally investigated. According to the calculations, Si-p states contribute significantly to the formation of the conduction bands minima (CBM), whereas Cu-d states make up the valence bands maxima (VBM). A direct energy band gap exists in Cu2ZnSiS4 near the Brillouin Zone's centre (Γ-point) (BZ). We have observed the energy gap of 2.112 eV in mBJ which is significantly lower as compared to the experimental value of ∼3.07 eV. Using optical constants and electroreflectance plots, we have depicted the anisotropic behavior of Cu2ZnSiS4 and discovered that the ΔR/R in mBJ is 0.097 eV. The hydrothermal formation of Cu2ZnGeS4 compound with wide band gap photo catalyst (α-MoO3) as thin film photoelectrodes onto ITO substrates increased photocurrent density values by about 400 times when compared to pristine α-MoO3 photoelectrodes. The MoO3/Cu2ZnGeS4 nanocomposite exhibits greater photocurrent density values and better charge transfer because of their lower charge transfer resistance, enhanced charge carrier concentration, and decreased depletion layer width.

Bound states in the continuum in anisotropic photonic crystal slabs

We investigate the bound states in the continuum (BICs) in photonic crystal slabs composed of alternating anisotropic and isotropic dielectric materials. According to the orientation of optical axis plane, three different configurations are proposed for analyzing various types of BICs, associated with extremely large quality factors and vanishing spectral linewidths. In particular, symmetry-protected (SP) BICs exist at the Brillouin zone center for zero rotation angle of the optical axis, which exhibit antisymmetric field patterns that are decoupled from the symmetric radiating fields. Accidental BICs and Friedrich-Wintgen (FW) BICs also occur at the Brillouin zone center for particular rotation angles of the optical axis. The former emerge on isolated bands with quasi-symmetric or quasi-antisymmetric field patterns, while the latter appear near the avoided crossing between two dispersion bands. At off the Brillouin zone center, SP BICs do not exist while accidental BICs and FW BICs appear at particular optical axis rotation angles, with similar features but somewhat more asymmetric field patterns than those at the Brillouin zone center.

A new algorithm for investigating strongly correlated systems using Hubbard model

In this work, a new algorithm introduces for calculating the average Green function of an interacting system that obeys the Hubbard model. This algorithm is applied to investigate the effects of onsite repulsion on the band structure of a square lattice in both the single-site approximations such as dynamical mean field theory (DMFT) and multi-site approximations such as effective medium supercell approximation (EMSCA). The advantages of our algorithm in comparison to the Hirsch-Fye algorithm and also the Blankenbecler, Scalapino, and Sugar (BSS) algorithm are the elimination of instabilities resulting from the Metropolis algorithm in the accepting and rejecting configurations, stability at low temperatures, the elimination of systematic errors resulting from the update of the Green's function in the quantum Monte Carlo process, and considering different probabilities for each possible configuration. Finally, by using our algorithm, it is possible to calculate the interacting three-dimensional system's band structure and the density of states that obey the Hubbard model. We have applied our algorithm to an interactive two-dimensional square lattice. As a result, phase transition boundaries can be easily recognized through calculated bands for different repulsions. Our results show that critical Coulomb repulsion values for Mott transition are uc = 9.05t and uc = 2.4t for DMFT and BEMSCA respectively. This means that DMFT significantly overestimates band splitting due to electrons' Coulomb repulsion. We found by starting at low repulsions and then increasing electrons' Coulomb repulsion, a partially flatted valence band around the center of the first Brillouin zone appears, but disappears at high repulsions.

Observation of a Folded Dirac Cone in Heavily Doped Graphene.

Superlattice potentials imposed on graphene can alter its Dirac states, enabling the realization of various quantum phases. We report the experimental observation of a replica Dirac cone at the Brillouin zone center induced by a superlattice in heavily doped graphene with Gd intercalation using angle-resolved photoemission spectroscopy (ARPES). The replica Dirac cone arises from the (√3× √3)R30° superlattice formed by the intervalley coupling of two nonequivalent valleys (e.g., the Kekulé-like distortion phase), accompanied by a bandgap opening. According to the findings, the replica Dirac band in Gd-intercalated graphene disappears beyond a critical temperature of 30 K and can be suppressed by potassium adsorption. The modulation of the replica Dirac band is primarily attributable to the residual frozen gas, which can act as a source of intervalley scattering at temperatures below 30 K. Our results highlight the persistence of the hidden Kekulé-like phase within the heavily doped graphene, enriching our current understanding of its replica Dirac Fermions.

Observation of flat band, Dirac nodal lines and topological surface states in Kagome superconductor CsTi3Bi5

Kagome lattices of various transition metals are versatile platforms for achieving anomalous Hall effects, unconventional charge-density wave orders and quantum spin liquid phenomena due to the strong correlations, spin-orbit coupling and/or magnetic interactions involved in such a lattice. Here, we use laser-based angle-resolved photoemission spectroscopy in combination with density functional theory calculations to investigate the electronic structure of the newly discovered kagome superconductor CsTi3Bi5, which is isostructural to the AV3Sb5 (A = K, Rb or Cs) kagome superconductor family and possesses a two-dimensional kagome network of titanium. We directly observe a striking flat band derived from the local destructive interference of Bloch wave functions within the kagome lattice. In agreement with calculations, we identify type-II and type-III Dirac nodal lines and their momentum distribution in CsTi3Bi5 from the measured electronic structures. In addition, around the Brillouin zone centre, {{mathbb{Z}}}_{2} nontrivial topological surface states are also observed due to band inversion mediated by strong spin-orbit coupling.

Laser‐induced Fano interference subsumed by electron–phonon coupling in orthorhombic KNbO3nano‐bricks: An ab initio vibrational and Raman spectroscopic investigation

AbstractHydrothermally prepared potassium niobate (KNbO3) nanoparticles are characterized meticulously to explore various crystallographic, microstructural, morphological, optical, compositional, and lattice‐vibrational properties. Laser power‐dependent Raman spectroscopy is employed to study the Fano effect in asymmetrically broadened optic (LO/TO) phonon modes in the orthorhombic KNbO3system, considering interference with the interband electronic continuum. The phonon softening, alterations in charge‐phonon coupling constant, linewidth and lifetime of phonon modes, and so on are explained in the light of theories formulated by Fano, Allen, and Hui, accompanying factor group analysis of the Raman‐active modes. Under local heating via focused laser irradiation, highly sensitive Raman spectra introspect perturbations in the generic vibrations at the first Brillouin zone center and moderations in the constructive/destructive interference conditions of resonant Fano scattering. Density functional theory (DFT)‐based calculations on phonon dispersion, density of phonon states, and relevant thermodynamic attributes decipher the theoretical background. Amid photoexcited electron plasma, the charge‐phonon coupling for distinct modes is strengthened considerably against strong excitation intensity without any unwanted anharmonicity, extensive lattice thermal expansion, phonon decay, or microscopic phase transformation. Finally, the direction of asymmetry, particle–particle, and particle–quasiparticle interactions are illustrated using Feynman diagrams for the associated and phonon modes.

Nanoarhitectonics of Inorganic-Organic Silica-Benzil Composites: Synthesis, Nanocrystal Morphology and Micro-Raman Analysis.

The synthesis of nanosized organic benzil (C6H5CO)2 crystals within the mesoporous SiO2 host matrix was investigated via X-ray diffraction, transmission electron microscopy, Raman spectroscopy, and ab initio lattice dynamics analysis. Combining these methods, we have proved that the main structural properties of benzil nanocrystals embedded into SiO2 host membranes with pore diameters of 6.0, 7.8, 9.4, and 13.0 nm are preserved compared to a bulk benzil crystal. Space confinement has an insignificant impact on the lattice vibrational properties of benzil crystals implanted into the host matrices. The ab initio lattice dynamics calculation of the phonon spectrum in the Brillouin zone center shows the mechanical and dynamical stability of benzil lattice, revealing very low optical frequency of 11 cm-1 at point Γ.

Momentum matching and band-alignment type in van der Waals heterostructures: Interfacial effects and materials screening

Momentum-matched type II van der Waals heterostructures (vdWHs) have been designed by assembling layered two-dimensional semiconductors (2DSs) with special band-structure combinations----that is, the valence band edge at the \ensuremath{\Gamma} point (the Brillouin-zone center) for one 2DS and the conduction-band edge at the \ensuremath{\Gamma} point for the other [Ubrig et al., Nat. Mater. 19, 299 (2020)]. However, the band offset sizes, band-alignment types, and whether momentum matched or not, all are affected by the interfacial effects between the component 2DSs, such as the quasichemical-bonding (QB) interaction between layers and the electrical dipole moment formed around the vdW interface. Here, based on density-functional theory calculations, first we probe the interfacial effects (including different QBs for valence and conduction bands, interface dipole, and the synergistic effects of these two aspects) on band-edge evolution in energy and valley (location in the Brillouin zone) and the resulting changes in band alignment and momentum matching for a typical vdWH of monolayer InSe and bilayer $\mathrm{W}{\mathrm{S}}_{2}$, in which the band edges of subsystems satisfy the special band-structure combination for a momentum-matched type II vdWH. Then, based on the conclusions of the studied interfacial effects, we propose a practical screening method for robust momentum-matched type II vdWHs. This practical screening method can also be applied to other band-alignment types. Our current study opens a way for practical screening and designing of vdWHs with robust momentum matching and band-alignment type.

Electronic structure studies of RbLn2Fe4As4O2 (Ln = Sm, Tb, Dy and Ho) compounds

We report a comprehensive density functional theory based first principles studies on newly discovered iron-based RbLn2Fe4As4O2 (Ln = Sm, Tb, Dy, Ho) superconducting compounds. A dominant Fe-d state, along with small arsenic state contribution is found in the low energy regime of all the four compounds, that constitutes conducting Fe-As layer. Orbital selective evolution of electronic structure is evident in hybrid RbLn2Fe4As4O2 compounds under the effect of chemical pressures induced by various lanthanide substitutions, and moderate on-site electron correlation. The characteristic electronic structure with multi-orbital derived multi-band nature undergoes an orbital-selective evolution. In all the four compounds, an orbital selective Lifshitz transition as well as an orbital-selective bandwidth renormalization is predicted due to on-site electron correlation. The calculated Fermi surfaces show evidence of bilayer splitting due to the interlayer inter-orbital interaction between the two Fe-As layers in a bilayer block. The splitting is found to be minimum along the Brillouin zone centre to the corner and is maximum along the zone centre to (π,0) direction. The largest significant effect due to spin-orbit coupling is the splitting of dxz and dyz derived bands above the Fermi level. The size of spin-orbit coupling is found to be consistent with earlier theoretical calculations on 122 materials. Splitting increases with substitutions of lanthanides in decreasing order of radii. Spin-orbit coupling influences the size of the Fermi radii and thus likely to influence nesting conditions and superconductivity as well. Predictions on possible pairing symmetry based on orbital characters of the Fermi surfaces together with qualitative estimation of nesting conditions for Fermi pockets are provided. The calculated large Fe-As hopping amplitudes based on 32-band Wannier function tight-binding Hamiltonian indicates that electrons mainly hop through As atoms.

Plasmon-magnon interactions in two-dimensional honeycomb magnets

Two-dimensional honeycomb ferromagnets offer the unprecedented opportunity to study interactions between collective modes that in standard bulk ferromagnets do not cross paths. Indeed, they harbor an optical spin-wave branch, i.e. a spin wave which disperses weakly near the Brillouin zone center. When doped with free carriers, they also host the typical gapless plasmonic mode of 2D itinerant electron/hole systems. When the plasmon branch meets the optical spin-wave branch, energy and momentum matching occurs, paving the way for interactions between the charge and spin sector. In this Letter we present a microscopic theory of such plasmon-magnon interactions, which is based on a double random phase approximation. We comment on the possibility to unveil this physics in recently isolated 2D honeycomb magnets such as ${\rm Cr}_2{\rm Ge}_2{\rm Te}_6$.

Phonon transition across an isotopic interface

Isotopic mixtures result in distinct properties of materials such as thermal conductivity and nuclear process. However, the knowledge of isotopic interface remains largely unexplored mainly due to the challenges in atomic-scale isotopic identification. Here, using electron energy-loss spectroscopy in a scanning transmission electron microscope, we reveal momentum-transfer-dependent phonon behavior at the h−10BN/h-11BN isotope heterostructure with sub-unit-cell resolution. We find the phonons’ energy changes gradually across the interface, featuring a wide transition regime. Phonons near the Brillouin zone center have a transition regime of ~3.34 nm, whereas phonons at the Brillouin zone boundary have a transition regime of ~1.66 nm. We propose that the isotope-induced charge effect at the interface accounts for the distinct delocalization behavior. Moreover, the variation of phonon energy between atom layers near the interface depends on both of momentum transfer and mass change. This study provides new insights into the isotopic effects in natural materials.

Acoustic circular dichroism in a three-dimensional chiral metamaterial

Circular dichroism (CD) is an intriguing chiroptical phenomenon associated with the interaction of chiral structures with circularly polarized lights. Although the CD effect has been extensively studied in optics, it has not yet been demonstrated in acoustic systems. Here, we demonstrate the acoustic CD effect in a three-dimensional chiral metamaterial supporting circularly polarized transverse sound. We find that the effect is negligible in the lossy metamaterial possessing ${C}_{4}$ rotational symmetry but can be strongly enhanced in the ${C}_{2}$-symmetric system with inhomogeneous loss. The phenomena can be understood based on the properties of the metamaterial's complex band structure and the quality factors of its eigenmodes. We show that the enhanced CD in the ${C}_{2}$-symmetric system is attributed to the polarization band gaps and the non-Hermitian exceptional points appearing near the Brillouin-zone center and boundaries. The results contribute to the understanding of chiral sound-matter interactions and can find applications in acoustic sensing of chiral structures and sound manipulations based on vector properties.

In search of Pca21 phase ferroelectrics

In recent years, hafnia-based ferroelectrics have attracted enormous attention due to their capability of maintaining ferroelectricity below 10 nm thickness and excellent compatibility with microelectronics flow lines. However, the physical origin of their ferroelectricity is still not fully clear, although it is commonly attributed to a polar Pca21 orthorhombic phase. The high-temperature paraelectric phases (the tetragonal phase or the cubic phase) do not possess a soft mode at the Brillouin zone center, thus the ferroelectric distortion has to be explained in terms of trilinear coupling among three phonon modes in the tetragonal phase. It is necessary to explore new materials with possible ferroelectricity due to the polar Pca21 phase, which in turn should be very helpful in evaluating the microscopic theory for ferroelectric hafnia. In this work, based on the idea of the Materials Genome Engineering, a series of hafnia-like ferroelectrics have been found, exemplified by LaSeCl, LaSeBr, LuOF and YOF, which possess adequate spontaneous polarization values and also relatively favorable free energies for the polar phase. Their common features and individual differences are discussed in detail. In particular, a promising potential ferroelectric material, Pca21 phase LuOF, is predicted and recommended for further experimental synthesis and investigation.

Two-dimensional Dirac fermions in a mass superlattice

We study two-dimensional (2D) Dirac fermions in the presence of a periodic mass term alternating between positive and negative values along one direction. This scenario could be realized for a graphene monolayer or for the surface states of topological insulators. The low-energy physics is governed by chiral Jackiw-Rebbi modes propagating along zero-mass lines, with the energy dispersion of the Bloch states given by an anisotropic Dirac cone. By means of the transfer matrix approach, we obtain exact results for a piecewise constant mass superlattice. On top of Bloch states, two different classes of boundary and/or interface modes can exist in a finite-size geometry or in a nonuniform electrostatic potential, respectively. We compute the dispersion relation for both types of boundary and interface modes, which originate either from states close to the superlattice Brillouin zone (BZ) center or, via a Lifshitz transition, from states near the BZ boundary. In the presence of a potential step, we predict that the interface modes, the Bloch wave functions, and the electrical conductance will sensitively depend on the step position relative to the mass superlattice.