Brillouin zone

About: Brillouin zone is a research topic. Over the lifetime, 13849 publications have been published within this topic receiving 383077 citations.

Intervalley scattering by acoustic phonons in two-dimensional MoS2 revealed by double-resonance Raman spectroscopy.

Abstract: Double-resonance Raman scattering is a sensitive probe to study the electron-phonon scattering pathways in crystals. For semiconducting two-dimensional transition-metal dichalcogenides, the double-resonance Raman process involves different valleys and phonons in the Brillouin zone, and it has not yet been fully understood. Here we present a multiple energy excitation Raman study in conjunction with density functional theory calculations that unveil the double-resonance Raman scattering process in monolayer and bulk MoS2. Results show that the frequency of some Raman features shifts when changing the excitation energy, and first-principle simulations confirm that such bands arise from distinct acoustic phonons, connecting different valley states. The double-resonance Raman process is affected by the indirect-to-direct bandgap transition, and a comparison of results in monolayer and bulk allows the assignment of each Raman feature near the M or K points of the Brillouin zone. Our work highlights the underlying physics of intervalley scattering of electrons by acoustic phonons, which is essential for valley depolarization in MoS2.

High-resolution angle-resolved photoemission study of the Fermi surface and the normal-state electronic structure of Bi2Sr2CaCu2O8

Abstract: High-resolution angle-resolved photoelectron spectroscopic measurements were made of the Fermi edge of a single crystal of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} at 90 K along several directions in the Brillouin zone. The resultant Fermi-level crossings are consistent with local-density band calculations, including a point calculated to be of Bi-O character. Additional measurements were made where bands crossed the Fermi level between 100 and 250 K, along with measurements on an adjacent Pt foil. The Fermi edges of both materials agree to within the noise. Below the Fermi level the spectra show correlation effects in the form of an increased effective mass, but the essence of the single-particle band structure is retained. The shape of the spectra can be explained by a lifetime-broadened photohole and secondary electrons. The effective inverse photohole lifetime is linear in energy.

Raman Spectrum of Cubic ZnS

Abstract: We present a detailed study of the first- and second-order Raman effect in cubic ZnS using both 4880 and 5145 \AA{} excitation. Our primary interest is to determine to what extent information on lattice dynamics can be extracted from Raman studies in a case where the crystal structure is relatively simple. Aside from interpreting the observed spectra and determining the energies of the phonon branches at the Brillouin-zone center and boundary, the study emphasizes two things. First, we examine experimentally the relation between first-order Raman intensities and the linear electro-optic effect. We find that the linear electro-optic constant derived from the absolute Raman intensities of the longitudinal- (LO) and transverse- (TO) optic modes agrees quite well with the constant obtained from direct measurement. This close agreement is particularly significant in the zinc-blende structure, since only one electro-optic constant and two optical modes are involved in the comparison. Also, it provides experimental evidence that the macroscopic, and not the local, electric field caused by the polar phonons should be used in calculations involving semiconducting crystals. Second, emphasis was also placed on comparing the observed selection rules or polarization properties of the second-order Raman effect with that predicted at the two most important or highest-symmetry critical points ($X$ and $L$) on the Brillouin-zone boundary. The agreement between observed and calculated selection rules is quite good, although in some cases, certain polarization characteristics which are allowed by symmetry are not detected. Also, some evidence of two-phonon scattering from other points in the Brillouin zone is found. The polarization properties of single-crystal Raman spectra are more conveniently discussed in terms of the irreducible representations of the polarizability, rather than depolarized spectra or depolarization ratios which apply more directly to liquid or polycrystalline samples. The observed and calculated selection rules for second-order Raman scattering are compared in detail, using these polarizability representations. The LO and TO phonon energies are 271 and 352 ${\mathrm{cm}}^{\ensuremath{-}1}$ at the zone center and 306 and 333 ${\mathrm{cm}}^{\ensuremath{-}1}$ at the zone boundary, and the TA and LA energies are 88 and 110 ${\mathrm{cm}}^{\ensuremath{-}1}$ at the boundary.

Topological phases and surface flat bands in superconductors without inversion symmetry

Abstract: We examine different topological phases in three-dimensional noncentrosymmetric superconductors with time-reversal symmetry by using three different types of topological invariants. Due to the bulk boundary correspondence, a nonzero value of any of these topological numbers indicates the appearance of zero-energy Andreev surface states. We find that some of these boundary modes in nodal superconducting phases are dispersionless, i.e., they form a topologically protected flat band. The region where the zero-energy flat band appears in the surface Brillouin zone is determined by the projection of the nodal lines in the bulk onto the surface. These dispersionless Andreev surface bound states have many observable consequences, in particular, a zero-bias conductance peak in tunneling measurements. We also find that in the gapless phase there appear Majorana surface modes at time-reversal invariant momenta which are protected by a ${\mathbb{Z}}_{2}$ topological invariant.

First-principles calculation of the electronic structure and EELS spectra at the graphene/Ni(111) interface

Abstract: A spin-polarized first-principles calculation of the atomic and electronic structure of the graphene/Ni(111) interface is presented. Different structural models have been considered, which differ in the positions of the carbon atoms with respect to the nickel topmost layer. The most probable structure, which has the lowest energy, has been determined. The distance between the floating carbon layer and the nickel surface is found smaller than the distance between graphene sheets in bulk graphite, in accordance with experimental measurements. The electronic structure of the graphene layer is strongly modified by interaction with the substrate and the magnetic moment of the surface nickel atoms is lowered in the presence of the graphene layer. Several interface states have been identified in different parts of the interface two-dimensional Brillouin zone. Their influence on the electron energy loss spectra has been evaluated.