Center Of Brillouin Zone Research Articles

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.

FeTe1−xSex monolayer films: towards the realization of high-temperature connate topological superconductivity

We performed angle-resolved photoemission spectroscopy studies on a series of FeTe1−xSex monolayer films grown on SrTiO3. The superconductivity of the films is robust and rather insensitive to the variations of the band position and effective mass caused by the substitution of Se by Te. However, the band gap between the electron- and hole-like bands at the Brillouin zone center decreases towards band inversion and parity exchange, which drive the system to a nontrivial topological state predicted by theoretical calculations. Our results provide a clear experimental indication that the FeTe1−xSex monolayer materials are high-temperature connate topological superconductors in which band topology and superconductivity are integrated intrinsically.

Initial process of photoluminescence dynamics of self-trapped excitons in aβ−Ga2O3single crystal

We investigate the photoluminescence (PL) dynamics of self-trapped excitons (STEs) in a $\ensuremath{\beta}\ensuremath{-}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ single crystal from the viewpoint of the transition process from the free exciton to the STE. We succeed in measuring the PL rise time ($\ensuremath{\sim}24$ ps) at 8 K corresponding to the tunneling time through the barrier between the free exciton and STE states in the adiabatic potential. From the analysis of the PL rise time of the STE based on perturbation theory for the tunneling time considering exciton-phonon interactions, we obtain the following results. Acoustic phonons near the Brillouin zone center contribute to the tunneling process. This suggests that the wave function of the STE is still spatially extended at the final state in the tunneling process. Furthermore, we investigate temperature dependence of the PL rise time of the STE. It is found that the PL rise time decreases with increasing temperature. The PL rise times in the temperature range from 8 to 100 K can be quantitatively explained by an adiabatic theory for the tunneling process. Consequently, the self-trapping process is dominated by the tunneling process at low temperatures.

Thermal transport and anharmonic phonons in strained monolayer hexagonal boron nitride

Thermal transport and phonon-phonon coupling in monolayer hexagonal boron nitride (h-BN) under equibiaxial strains are investigated from first principles. Phonon spectra at elevated temperatures have been calculated from perturbation theory using the third-order anharmonic force constants. The stiffening of the out-of-plane transverse acoustic mode (ZA) near the Brillouin zone center and the increase of acoustic phonon lifetimes are found to contribute to the dramatic increase of thermal transport in strained h-BN. The transverse optical mode (TO) at the K point, which was predicted to lead to mechanical failure of h-BN, is found to shift to lower frequencies at elevated temperatures under equibiaxial strains. The longitudinal and transverse acoustic modes exhibit broad phonon spectra under large strains in sharp contrast to the ZA mode, indicating strong in-plane phonon-phonon coupling.

First principles study of the vibrational, dielectric and thermal properties of SrClF

SrClF is an important optical crystal and has many technological applications. In this work, vibrational, dielectric and thermal properties of SrClF were investigated by density functional perturbation calculation. The calculated Born effective charges are close to their nominal ionic charges, revealing the ionic characteristic of SrClF. Group theory analysis indicates that there are two E u and A 2u infrared modes at the Brillouin zone center of SrClF. The LO-TO splitting of these infrared modes were calculated and discussed and their vibrational modes were sketched. Static dielectric constants were studied, which show that SrClF has a larger ionic dielectric contribution than its electrons. Its birefringence was calculated and the infrared reflectance spectra were simulated, which can be used to explain the experimental findings. Based on the computed phonon dispersion curves, the lattice heat capacity, the Gruneisen parameter and the thermal expansion coefficient as functions of temperature were predicted.

The investigation of adsorption and dissociation of H2O on Li2O (111) by ab initio theory

The adsorption and dissociation mechanism of H2O molecule on the Li2O (111) surface have been systematically studied by using the density functional theory calculations. The parallel and vertical configurations of H2O at six different symmetry adsorption sites on the Li2O (111) surface are considered. In our calculations, it is suggested that H2O can dissociate on the perfect Li2O surface, of which the corresponding adsorption energy is 1.118eV. And the adsorption energy decrease to be 0.241eV when oxygen atom of H2O bonds to lithium atom of the slab. The final configurations are sensitive to the initial molecular orientation. By Bader charge analysis, the charge transfer from slab to adsorbed H2O/OH can be found due to the downward shift of lowest-unoccupied molecular orbital. We also analyze the vibrational frequencies at the Brillouin Zone centre for H2O molecule adsorbed on the stoichiometric surface. Due to the slightly different structure parameters, the calculated values of the vibrational frequencies of hydroxyl group range from 3824 to 3767cm−1. Our results agree well with experimental results performed in FT-IR spectrum, which showed that an absorption peak of OH group appeared at 3677cm−1 at room temperature.

Electronic structure and optical properties of β-RbSm(MoO4)2 from spin polarization calculations: DFT+U

We explored the influence of the inclusion of spin polarization on the energy band gap of β-RbSm(MoO4)2. Calculations explored that the appearance of Sm-4f states at the conduction band minimum (CBM) of the spin-up case causes a significant influence on ground state properties of β-RbSm(MoO4)2. The total and partial densities of states confirm the existence of Sm-4f states in the CBM of the spin-up case. The partial densities of states exhibit a strong hybridization between some states which may lead to the formation of covalent bonds. The valence band maximum (VBM) and the CBM are located at the center of first Brillouin zone (Γ), resulting in a direct band gap for both of majority spin (↑) and minority spin (↓). The values of the band gap are 3.01 eV (↑) and 3.78 eV (↓). The all-electron full potential linear augmented plane wave (FPLAPW+lo) method within the generalized gradient approximation plus the Hubbard Hamiltonian (GGA+U) were used. We have applied U on 4f orbital of Sm atoms and 4d orbital of Mo atoms. We have taken a careful look at the electronic charge density distribution to visualize the charge transfer and the chemical bonding characters. The calculated bond lengths show very good agreements with the measured ones. The optical properties were calculated to seek a deep insight into the electronic structure. It has been found that below λ = 450 nm β-RbSm(MoO4)2 exhibits a positive uniaxial anisotropy, while at λ = 450 nm and above, the crystal exhibits a negative uniaxial anisotropy.

Raman and infrared characterization of gadolinium-doped manganese sulfide

ABSTRACTGadolinium manganese sulfide solid solutions are investigated by infrared and Raman spectroscopy. Longitudinal optical–transverse optical splitting of the manganese–sulfur bond vibration is observed in the vibrational spectra. The Raman spectra contain modes that are prohibited in the crystal structure of the alpha phase of manganese sulfide, which are associated with activation of the phonons from both the Brillouin zone center and its X and L points. The concentration dependence of transverse optical and longitudinal optical modes’ frequencies is calculated within the frame of the modified random-element-isodisplacement model, being in good agreement with the experimental results. Both theory and experiment show that the solid solution under study exhibits a “one-mode” behavior.

Resonance Raman Scattering in TlGaSe2 Crystals

The resonance Raman scattering for geometries Y(YX)Z and Y(ZX)Z at temperature 10 K and infrared reflection spectra in E∥a and E∥b polarizations at 300 K were investigated. The number of Aa (Ba) and Au (Bu) symmetry vibrational modes observed experimentally and calculated theoretically agree better in this case than when TlGa2Se4 crystals belong to D2h symmetry group. The emission of resonance Raman scattering and excitonic levels luminescence spectra overlap. The lines in resonance Raman spectra were identified as a combination of optical phonons in Brillouin zone center.

Topological phase transitions caused by a simple rotational operation in two-dimensional acoustic crystals

We design a two-dimensional acoustic crystal (AC) to obtain topologically protected edge states for sound waves. The AC is composed of a triangular array of a complex unit cell consisting of two identical triangle-shaped steel rods arranged in air. The steel rods are placed on the vertices of the hexagonal unit cell so that the whole lattice possesses the C6v symmetry. We show that by simply rotating all triangular rods around their respective centers by 180 degrees, a topological phase transition can be achieved, and more importantly, such a transition is accomplished with no need of changing the fill ratios or changing the positions of the rods. Interestingly, the achieved topologically nontrivial band gap has a very large frequency width, which is really beneficial to future applications. The topological properties of the AC are rooted in the spatial symmetries of the eigenstates. It is well known that there are two doubly-degenerate eigenstates at the point for a C6v point group, and they are usually called the p and d states in electronic system. By utilizing the spatial symmetries of the p and d states in the AC, we can construct the pseudo-time reversal symmetry which renders the Kramers doubling in this classical system. We find pseudospin states in the interface between topologically trivial and nontrivial ACs, where anticlockwise (clockwise) rotational behaviors of time-averaged Poynting vectors correspond to the pseudospin-up (pseudospin-down) orientations of the edge states, respectively. These phenomena are very similar to the real spin states of quantum spin Hall effect in electronic systems. We also develop an effective Hamiltonian for the associated bands to characterize the topological properties of the AC around the Brillouin zone center by the kp perturbation method. We calculate the spin Chern numbers of the ACs, and reveal the inherent link between the band inversion and the topological phase transition. With full-wave simulations, we demonstrate the one-way propagation of sound waves along the interface between topologically distinct ACs, and demonstrate the robustness of the edge states against different types of defects including bends, cavity and disorder. Our design provides a new way to realize acoustic topological effects in a wide frequency range spanning from infrasound to ultrasound. Potential applications and acoustic devices based on our design are expected, so that people can manipulate and transport sound waves in a more efficient way.

Electronic structure of buried LaNiO3 layers in (111)-oriented LaNiO3/LaMnO3 superlattices probed by soft x-ray ARPES

Taking advantage of the large electron escape depth of soft x-ray angle resolved photoemission spectroscopy, we report electronic structure measurements of (111)-oriented [LaNiO3/LaMnO3] superlattices and LaNiO3 epitaxial films. For thin films, we observe a 3D Fermi surface with an electron pocket at the Brillouin zone center and hole pockets at the zone vertices. Superlattices with thick nickelate layers present a similar electronic structure. However, as the thickness of the LaNiO3 is reduced, the superlattices become insulating. These heterostructures do not show a marked redistribution of spectral weight in momentum space but exhibit a pseudogap of ≈50 meV.

Controlling graphene plasmons with a zero-index metasurface.

Graphene plasmons, owing to their diverse applications including electro-optical modulation, optical sensing, spectral photometry and tunable lighting at the nanoscale, have recently attracted much attention. One key challenge in advancing this field is to precisely control the propagation of graphene plasmons. Here, we propose an on-chip integrated platform to engineer the wave front of the graphene plasmons through a metasurface with a refractive index of zero. We demonstrate that a well-designed graphene/photonic-crystal metasurface can possess conical plasmonic dispersion at the Brillouin zone center with a triply degenerate state at the Dirac frequency, giving rise to the zero-effective-index of graphene plasmons. Plane-wave-emission and focusing effects of the graphene plasmons are achieved by tailoring such a zero-index metasurface. In addition to the tunable Dirac point frequency enabled by the electrical tuning of the graphene Fermi level, our highly integrated system also provides stable performance even when defects exist. This actively controllable on-chip platform can potentially be useful for integrated photonic circuits and devices.

Semi-Dirac semimetal in silicene oxide.

A semi-Dirac semimetal is a material that exhibits linear band dispersion in one direction and quadratic band dispersion in the orthogonal direction and, therefore, hosts massless and massive fermions at the same point in the momentum space. While a number of interesting physical properties have been predicted in semi-Dirac semimetals, it has been rare to realize such materials in condensed matter. Based on the fact that some honeycomb materials are easily oxidized or chemically absorb other atoms, here, we theoretically propose an approach of modifying their band structures by covalent addition of group-VI elements and strain engineering. We predict a silicene oxide with the chemical formula of Si2O to be a candidate semi-Dirac semimetal. Our approach is backed by the analysis and understanding of the effect of p-orbital frustration on the band structure of graphene-like materials.