Center Of Brillouin Zone Research Articles

Double Dirac point in a photonic graphene

The band structure of the photonic graphene composed of an array of dielectric cylinders in air with honeycomb lattice is computed in this work. The results reveal that there is a double Dirac point at the Brillouin zone center with a frequency of 0.870 61c/a in our proposed photonic graphene structure. By using average eigen-field calculation, we demonstrate that such a photonic graphene structure possesses both zero effective permittivity and effective permeability, which implied that this structure can serve as zero-index medium. Furthermore, applications including focusing, multidirectional emission and super-coupling are numerically demonstrated with the proposed structure at the double Dirac point frequency, which further conformed the zero-index properties of the photonic graphene structure.

Experimental Determination of the Ionization Energies of MoSe2, WS2, and MoS2 on SiO2 Using Photoemission Electron Microscopy.

The values of the ionization energies of transition metal dichalcogenides (TMDs) are needed to assess their potential usefulness in semiconductor heterojunctions for high-performance optoelectronics. Here, we report on the systematic determination of ionization energies for three prototypical TMD monolayers (MoSe2, WS2, and MoS2) on SiO2 using photoemission electron microscopy with deep ultraviolet illumination. The ionization energy displays a progressive decrease from MoS2, to WS2, to MoSe2, in agreement with predictions of density functional theory calculations. Combined with the measured energy positions of the valence band edge at the Brillouin zone center, we deduce that, in the absence of interlayer coupling, a vertical heterojunction comprising any of the three TMD monolayers would form a staggered (type-II) band alignment. This band alignment could give rise to long-lived interlayer excitons that are potentially useful for valleytronics or efficient electron-hole separation in photovoltaics.

Surface optical phonon modes in hexagonal shaped Al0.97Ga0.03N nanostructures

Raman modes of plasma assisted molecular beam epitaxy grown, c-plane oriented and hexagonal shaped cylindrical Al0.97Ga0.03N nanostructures are reported. Apart from the group theoretically allowed optical phonons at zone center (wave vector, q = 0), two additional phonon modes around 763 and 846 cm−1 are observed in between the reported values of longitudinal optical (LO q=0) and transverse optical (TO q=0) phonon modes. The observed phonon mode frequencies are matched with the simulated dispersion curve for the surface optical (SO) phonon modes and subsequently assigned as SO(A 1) and SO(E 1) with A 1 and E 1 symmetries, respectively. A change in the dielectric constant of the surrounding medium using CCl4 is demonstrated to show a significant red shift of the additional modes with respect to those in air, and they show good agreement with the calculated SO phonon frequencies. The origin of SO phonon modes are attributed to the relaxation of Raman selection rules away from the Brillouin zone centre because of the presence of periodic surface modulations of sharp edges at the corners of well-faceted Al0.97Ga0.03N hexagonal crystallites in the sample. An appropriate quantification is also made in this regard to find out the Fourier component of the surface potential responsible for activating the SO mode.

New group-V elemental bilayers: A tunable structure model with four-, six-, and eight-atom rings

Two-dimensional (2D) group V elemental materials have attracted widespread attention due to their nonzero band gap while displaying high electron mobility. Using first-principles calculations, we propose a series of new elemental bilayers with group V elements (Bi, Sb, As). Our study reveals the dynamical stability of 4, 6, and 8-atom ring structures, demonstrating their possible coexistence in such bilayer systems. The proposed structures for Sb and As are large-gap semiconductors that are potentially interesting for applications in future nanodevices. The Bi structures have nontrivial topological properties with a large and direct nontrivial band gap. The nontrivial gap is shown to arise from a band inversion at the Brillouin zone center due to the strong intrinsic spin-orbit coupling (SOC) in Bi atoms. Moreover, we demonstrate the possibility to tune the properties of these materials by enhancing the ratio of 6-atom rings to 4 and 8-atom rings, which results in wider nontrivial band gaps and lower formation energies.

Elastic, electronic, lattice dynamical properties and electron-phonon interaction in the ternary superconductors Sr(GaGe) and Sr(GaSi) at high pressure

We report the results of an ab−initio study of electronic, and detailed lattice dynamical properties of superconducting materials, Sr(GaGe) and Sr(GaSi) under high pressure. The phonon dispersion curves along the high-symmetry directions and phonon frequencies parameters at the Brillouin zone center are computed by using density functional perturbation theory while the elastic constants are calculated in metric-tensor formulation. The Vickers hardness belonging to the compounds is also evaluated clearly. Moreover the band structures, partial densities of states and Fermi surface topologies are discussed in detail. At the same time we advance in-depth understanding of the relationship between the properties determined and superconducting characteristic. The transition temperatures for the compounds have been detected. We find the values of electron-phonon coupling constant as 0.99 and 0.92 for Sr(GaSi) and Sr(GaGe), respectively.

Nontrivial topology of cubic alkali bismuthides

We report an ab initio study of the effect of pressure on vibrational and electronic properties of ${\mathrm{K}}_{3}\mathrm{Bi}$ and ${\mathrm{Rb}}_{3}\mathrm{Bi}$ in the cubic $Fm\overline{3}m$ structure. It is shown that the high-temperature cubic phase of ${\mathrm{K}}_{3}\mathrm{Bi}$ and ${\mathrm{Rb}}_{3}\mathrm{Bi}$ is dynamically unstable at $T=0$ but can be stabilized by pressure. The electronic spectra of alkali bismuthides are found to possess the bulk band touching at the Brillouin zone center and an inverted spin-orbit bulk band structure. Upon hydrostatic compression the compounds transform from the topologically nontrivial semimetal (${\mathrm{K}}_{3}\mathrm{Bi}$)/metal (${\mathrm{Rb}}_{3}\mathrm{Bi}$) into a trivial semiconductor (metal) with a conical Dirac-type dispersion of electronic bands at the point of the topological transition. In ${\mathrm{K}}_{3}\mathrm{Bi}$ the dynamical stabilization occurs before the system undergoes the topological phase transition.

Giant Magnetic Band Gap in the Rashba-Split Surface State of Vanadium-Doped BiTeI: A Combined Photoemission and Ab Initio Study

One of the most promising platforms for spintronics and topological quantum computation is the two-dimensional electron gas (2DEG) with strong spin-orbit interaction and out-of-plane ferromagnetism. In proximity to an s-wave superconductor, such 2DEG may be driven into a topologically non-trivial superconducting phase, predicted to support zero-energy Majorana fermion modes. Using angle-resolved photoemission spectroscopy and ab initio calculations, we study the 2DEG at the surface of the vanadium-doped polar semiconductor with a giant Rashba-type splitting, BiTeI. We show that the vanadium-induced magnetization in the 2DEG breaks time-reversal symmetry, lifting Kramers degeneracy of the Rashba-split surface state at the Brillouin zone center via formation of a huge gap of about 90 meV. As a result, the constant energy contour inside the gap consists of only one circle with spin-momentum locking. These findings reveal a great potential of the magnetically-doped semiconductors with a giant Rashba-type splitting for realization of novel states of matter.

Layer dependence of the electronic band alignment of few-layer MoS2 on SiO2 measured using photoemission electron microscopy

Tailoring band alignment layer-by-layer using heterojunctions of two-dimensional (2D) semiconductors is an attractive prospect for producing next-generation electronic and optoelectronic devices that are ultrathin, flexible, and efficient. The 2D layers of transition metal dichalcogenides (TMDs) in laboratory devices have already shown favorable characteristics for electronic and optoelectronic applications. Despite these strides, a systematic understanding of how band alignment evolves from monolayer to multilayer structures is still lacking in experimental studies, which hinders development of novel devices based on TMDs. Here, we report on the local band alignment of monolayer, bilayer, and trilayer $\mathrm{Mo}{\mathrm{S}}_{2}$ on a 285-nm-thick $\mathrm{Si}{\mathrm{O}}_{2}$ substrate using an approach to probe the occupied electronic states based on photoemission electron microscopy and deep-ultraviolet light. Local measurements of the vacuum level and the valence band edge at the Brillouin zone center show that the addition of layers to monolayer $\mathrm{Mo}{\mathrm{S}}_{2}$ increases the relative work function and pushes the valence band edge toward the vacuum level. We also deduced $n$-type doping of few-layer $\mathrm{Mo}{\mathrm{S}}_{2}$ and type-I band alignment across monolayer-to-bilayer and bilayer-to-trilayer lateral junctions. Conducted in isolation from environmental effects owing to the vacuum condition of the measurement and an insulating $\mathrm{Si}{\mathrm{O}}_{2}$ substrate, this study shows a metrology to uncover electronic properties intrinsic to $\mathrm{Mo}{\mathrm{S}}_{2}$ semiconducting layers and emerging 2D crystals alike.

Direct modeling of near field thermal radiation in a metamaterial.

The study of near field thermal radiation is gaining renewed interest thanks in part to their great potential in energy harvesting applications. It is well known that plasmonic or polaritonic materials exhibit strongly enhanced fields near the surface, but it is not trivial to quantitatively predict their impact on thermal radiation intensity in the near field. In this paper, we present a case study for a metamaterial that supports a surface plasmon mode in the terahertz region and consequently exhibits strongly enhanced near field thermal radiation at the plasmon resonance frequency. We implemented a finite-difference time-domain method that thermally excites the metamaterial with randomly fluctuating dipoles according to the fluctuation-dissipation theorem. The calculated thermal radiation from the metamaterial was then compared with the case of optical excitation by the plane wave incident on the metamaterial surface. The optical excitation couples only to the mode that satisfies the momentum matching condition while thermal excitation is not bound by it. As a result, the near field thermal radiation exhibits substantial differences compared to the optically excited surface plasmon modes. Under thermal excitation, the near field intensity at 1 µm away from metal surface of the metamaterial reaches a maximum enhancement of 43 fold over the far field at the frequency of the Brillouin zone boundary mode while the near field intensity under optical excitation reaches a maximum enhancement of 24 fold at the frequency of the Brillouin zone center mode. In addition, the peak near field intensity under thermal excitation shows a 4-fold enhancement over blackbody radiation with linear polarization radiation in the far field. The ability to precisely predict the local field intensity under thermal excitation is critical to the development of advanced energy devices that take advantage of this near field enhancement and could lead to the development of new generation of novel energy technology.

Coherent Two-Dimensional Terahertz Magnetic Resonance Spectroscopy of Collective Spin Waves.

We report a demonstration of two-dimensional (2D) terahertz (THz) magnetic resonance spectroscopy using the magnetic fields of two time-delayed THz pulses. We apply the methodology to directly reveal the nonlinear responses of collective spin waves (magnons) in a canted antiferromagnetic crystal. The 2D THz spectra show all of the third-order nonlinear magnon signals including magnon spin echoes, and 2-quantum signals that reveal pairwise correlations between magnons at the Brillouin zone center. We also observe second-order nonlinear magnon signals showing resonance-enhanced second-harmonic and difference-frequency generation. Numerical simulations of the spin dynamics reproduce all of the spectral features in excellent agreement with the experimental 2D THz spectra.

Electronic structure of FeS

Here we report the electronic structure of FeS, a recently identified iron-based superconductor. Our high-resolution angle-resolved photoemission spectroscopy studies show two hole-like ($\alpha$ and $\beta$) and two electron-like ($\eta$ and $\delta$) Fermi pockets around the Brillouin zone center and corner, respectively, all of which exhibit moderate dispersion along $k_z$. However, a third hole-like band ($\gamma$) is not observed, which is expected around the zone center from band calculations and is common in iron-based superconductors. Since this band has the highest renormalization factor and is known to be the most vulnerable to defects, its absence in our data is likely due to defect scattering --- and yet superconductivity can exist without coherent quasiparticles in the $\gamma$ band. This may help resolve the current controversy on the superconducting gap structure of FeS. Moreover, by comparing the $\beta$ bandwidths of various iron chalcogenides, including FeS, FeSe$_{1-x}$S$_x$, FeSe, and FeSe$_{1-x}$ Te$_x$, we find that the $\beta$ bandwidth of FeS is the broadest. However, the band renormalization factor of FeS is still quite large, when compared with the band calculations, which indicates sizable electron correlations. This explains why the unconventional superconductivity can persist over such a broad range of isovalent substitution in FeSe$_{1-x}$Te$_{x}$ and FeSe$_{1-x}$S$_{x}$.

Pressure dependence of transverse acoustic phonon energy in ferropericlase across the spin transition

We investigated transverse acoustic (TA) phonons in iron-bearing magnesium oxide (ferropericlase) up to 56 GPa using inelastic x-ray scattering (IXS). The results show that the energy of the TA phonon far from the Brillouin zone center suddenly increases with increasing pressure above the spin transition pressure of ferropericlase. Ab initio calculations revealed that the TA phonon energy far from the Brillouin zone center is higher in the low-spin state than in the high spin state; that the TA phonon energy depend weakly on pressure; and that the energy gap between the TA and the lowest-energy-optic phonons is much narrower in the low-spin state than in the high-spin state. This allows us to conclude that the anomalous behavior of the TA mode in the present experiments is the result of gap narrowing due to the spin transition and explains contradictory results in previous experimental studies.

Variable-temperature inelastic light scattering spectroscopy of nickel oxide: Disentangling phonons and magnons

We report the results of an investigation of the temperature dependence of the magnon and phonon frequencies in NiO. A combination of Brillouin-Mandelstam and Raman spectroscopies allowed us to elucidate the evolution of the phonon and magnon spectral signatures from the Brillouin zone center (GHz range) to the second-order peaks from the zone boundary (THz range). The temperature-dependent behavior of the magnon and phonon bands in the NiO spectrum indicates the presence of antiferromagnetic (AF) order fluctuation or a persistent AF state at temperatures substantially above the Néel temperature (TN=523 K). Tuning the intensity of the excitation laser provides a method for disentangling the features of magnons from acoustic phonons in AF materials without the application of a magnetic field. Our results are useful for the interpretation of the inelastic-light scattering spectrum of NiO and add to the knowledge of its magnon properties important for THz spintronic devices.

Electronic, elastic, lattice dynamic and thermal conductivity properties of Na3OBr via first principles

Na3OBr is expected to be a promising superionic conductor for use as solid state electrolyte in large scale energy storage systems, but its lattice dynamic and thermal conductivity properties have not been well studied till now. In this work, we performed a detailed study on these properties as well as its electronic and elastic properties using first principles method. The results indicate that Na3OBr is a direct band gap crystal. It is mechanically stable but elastically anisotropic. Its phonons at the Brillouin zone center were classified using group theory analysis and its Born effective charges, LO–TO splitting, and dielectric constants were calculated and discussed. Its phonon dispersion curves were obtained and their origins were revealed. Based on the phonon dispersion curves, its thermal conductivity as a function of temperature was predicted, which give a value of 7.30 Wm−1 K−1 at 300 K. Further study reveals that, for Na3OBr, the simple Slack's model can give almost the same thermal conductivity curve as that obtained from the phonon dispersion curves.

Isotropic Cooper pairs with emergent sign changes in a single-layer iron superconductor

We model a single layer of heavily electron-doped FeSe by spin-1/2 moments over a square lattice of iron atoms that include the 3d xz and 3d yz orbitals, at strong on-site Coulomb repulsion. Above half filling, we find emergent hole bands below the Fermi level at the center of the one-iron Brillouin zone in a half metal state characterized by hidden magnetic order and by electron-type Fermi surface pockets at wavenumbers that double the unit cell along the principal axes. "Replicas" of the emergent hole bands exist at lower energy in the two-iron Brillouin zone. Exact calculations with two mobile electrons find evidence for isotropic Cooper pairs that alternate in sign between the electron bands and the emergent hole bands.

Superconductivity in type-II Weyl semimetals

We study superconductivity in a Weyl semimetal with a tilted dispersion around two Weyl points of opposite chirality. In the absence of any tilt, the state with zero momentum pairing between two Fermi sheets enclosing each Weyl point has four point nodes in the superconducting gap function. Moreover, the surface of the superconductor hosts Fermi arc states and Majorana flat bands. We show that a quantum phase transition occurs at a critical value of the tilt, at which two gap nodes disappear by merging at the center of the first Brillouin zone, or by escaping at its edges, depending on the direction of the tilt. The region in the momentum space that the Majorana flat band occupies is found to increase as the tilt parameter is made larger. Additionally, the superconducting critical temperature and electronic specific heat can be enhanced in the vicinity of the quantum phase transition due to the singularity in the electronic density of states.

Ideal strength, phonon stability and thermodynamics of the CrB-type CaX IV (X IV = Si, Ge, and Sn) binary compounds

Abstract We reported a theoretical investigation of the ideal strengths, dynamical and thermodynamic properties of the CrB-type CaXIV (XIV = Si, Ge, and Sn) alkaline-earth metal compounds by employing density functional theory (DFT) and density functional perturbation theory (DFPT). The calculated ideal tensile strengths have the sequence of CaSi > CaGe > CaSn for these intermetallic compounds along the [100], [010], and [001] high-symmetry lines. The present results mean the tensile and shear abilities of CaSi are best, followed by CaGe and CaSn calcium-based compounds. Based on the linear response method within DFPT, the full phonon dispersion curves and the phonon densities of state of the CaXIV (XIV = Si, Ge, and Sn) intermetallic compounds are investigated systematically. Furthermore, the Raman-active and Infrared-active phonon modes at Г point of Brillouin Zone (BZ) center are assigned combining with the point group theory. Our calculated results indicate that three calcium-based compounds are all dynamically stable at ambient conditions. Additionally, within the calculated vibration phonon densities of states, the thermo-physical properties of three calcium-based compounds such as the vibration internal energy, Helmholtz free energy, the lattice entropy and the lattice heat capacity have been also obtained. These investigations serve for new references for further synthesis of various calcium-based alkaline-earth metal compounds.

Enhanced superconductivity accompanying a Lifshitz transition in electron-doped FeSe monolayer

The origin of enhanced superconductivity over 50 K in the recently discovered FeSe monolayer films grown on SrTiO3 (STO), as compared to 8 K in bulk FeSe, is intensely debated. As with the ferrochalcogenides AxFe2−ySe2 and potassium-doped FeSe, which also have a relatively high-superconducting critical temperature (Tc), the Fermi surface (FS) of the FeSe/STO monolayer films is free of hole-like FS, suggesting that a Lifshitz transition by which these hole FSs vanish may help increasing Tc. However, the fundamental reasons explaining this increase of Tc remain unclear. Here we report a 15 K jump of Tc accompanying a second Lifshitz transition characterized by the emergence of an electron pocket at the Brillouin zone centre, which is triggered by high-electron doping following in situ deposition of potassium on FeSe/STO monolayer films. Our results suggest that the pairing interactions are orbital dependent in generating enhanced superconductivity in FeSe.

Observation of acoustic Dirac-like cone and double zero refractive index

Zero index materials where sound propagates without phase variation, holds a great potential for wavefront and dispersion engineering. Recently explored electromagnetic double zero index metamaterials consist of periodic scatterers whose refractive index is significantly larger than that of the surrounding medium. This requirement is fundamentally challenging for airborne acoustics because the sound speed (inversely proportional to the refractive index) in air is among the slowest. Here, we report the first experimental realization of an impedance matched acoustic double zero refractive index metamaterial induced by a Dirac-like cone at the Brillouin zone centre. This is achieved in a two-dimensional waveguide with periodically varying air channel that modulates the effective phase velocity of a high-order waveguide mode. Using such a zero-index medium, we demonstrated acoustic wave collimation emitted from a point source. For the first time, we experimentally confirm the existence of the Dirac-like cone at the Brillouin zone centre.

Thermal conductance of graphene/hexagonal boron nitride heterostructures

The lattice-based scattering boundary method is applied to compute the phonon mode-resolved transmission coefficients and thermal conductances of in-plane heterostructures built from graphene and hexagonal boron nitride (hBN). The thermal conductance of all structures is dominated by acoustic phonon modes near the Brillouin zone center that have high group velocity, population, and transmission coefficient. Out-of-plane modes make their most significant contributions at low frequencies, whereas in-plane modes contribute across the frequency spectrum. Finite-length superlattice junctions between graphene and hBN leads have a lower thermal conductance than comparable junctions between two graphene leads due to lack of transmission in the hBN phonon bandgap. The thermal conductances of bilayer systems differ by less than 10% from their single-layer counterparts on a per area basis, in contrast to the strong thermal conductivity reduction when moving from single- to multi-layer graphene.