Showing papers on "Brillouin zone published in 2014"
TL;DR: In this article, the authors show that a family of nonmagnetic materials including TaAs, TaP, NbAs and NbP are Weyl semimetal (WSM) without inversion center.
Abstract: Based on first principle calculations, we show that a family of nonmagnetic materials including TaAs, TaP, NbAs and NbP are Weyl semimetal (WSM) without inversion center. We find twelve pairs of Weyl points in the whole Brillouin zone (BZ) for each of them. In the absence of spin-orbit coupling (SOC), band inversions in mirror invariant planes lead to gapless nodal rings in the energy-momentum dispersion. The strong SOC in these materials then opens full gaps in the mirror planes, generating nonzero mirror Chern numbers and Weyl points off the mirror planes. The resulting surface state Fermi arc structures on both (001) and (100) surfaces are also obtained and show interesting shapes, pointing to fascinating playgrounds for future experimental studies.
1,266 citations
TL;DR: In this article, the relationship between surface scattering properties, the bulk band properties, and the formation of interface states is investigated, which can enable the design of systems with interface states in a rational manner.
Abstract: Surface impedance is an important concept in classical wave systems such as photonic crystals (PCs). For example, the condition of an interface state formation in the interfacial region of two different one-dimensional PCs is simply Z_SL +Z_SR=0, where Z_SL (Z_SR)is the surface impedance of the semi-infinite PC on the left- (right-) hand side of the interface. Here, we also show a rigorous relation between the surface impedance of a one-dimensional PC and its bulk properties through the geometrical (Zak) phases of the bulk bands, which can be used to determine the existence or non-existence of interface states at the interface of the two PCs in a particular band gap. Our results hold for any PCs with inversion symmetry, independent of the frequency of the gap and the symmetry point where the gap lies in the Brillouin Zone. Our results provide new insights on the relationship between surface scattering properties, the bulk band properties and the formation of interface states, which in turn can enable the design of systems with interface states in a rational manner.
296 citations
TL;DR: It is found that, as for other transition-metal dichalcogenides, Raman scattering provides a powerful diagnostic tool for studying layer thickness and also layer orientation in few-layer ReSe2.
Abstract: Rhenium diselenide (ReSe2) is a layered indirect gap semiconductor for which micromechanical cleavage can produce monolayers consisting of a plane of rhenium atoms with selenium atoms above and below. ReSe2 is unusual among the transition-metal dichalcogenides in having a low symmetry; it is triclinic, with four formula units per unit cell, and has the bulk space group P1. Experimental studies of Raman scattering in monolayer, few-layer, and bulk ReSe2 show a rich spectrum consisting of up to 16 of the 18 expected lines with good signal strength, pronounced in-plane anisotropy of the intensities, and no evidence of degradation of the sample during typical measurements. No changes in the frequencies of the Raman bands with layer thickness down to one monolayer are observed, but significant changes in relative intensity of the bands allow the determination of crystal orientation and of monolayer regions. Supporting theory includes calculations of the electronic band structure and Brillouin zone center phon...
295 citations
TL;DR: In this paper, the electronic structures of moir\'e superlattices arising in monolayer/bilayer graphene stacked on hexagonal boron nitride (hBN) in the presence and absence of magnetic field were theoretically investigated.
Abstract: We theoretically investigate the electronic structures of moir\'e superlattices arising in monolayer/bilayer graphene stacked on hexagonal boron nitride (hBN) in the presence and absence of magnetic field. We develop an effective continuum model from a microscopic tight-binding lattice Hamiltonian and calculate the electronic structures of graphene-hBN systems with different rotation angles. Using the effective model, we explain the characteristic band properties such as the gap opening at the corners of the superlattice Brillouin zone (mini-Dirac point). We also investigate the energy spectrum and quantum Hall effect of graphene-hBN systems in uniform magnetic field and demonstrate the evolution of the fractal spectrum as a function of the magnetic field. The spectrum generally splits in the valley degrees of freedom ($K$ and ${K}^{\ensuremath{'}}$) due to the lack of the inversion symmetry, and the valley splitting is more significant in bilayer graphene on hBN than in monolayer graphene on hBN because of the stronger inversion-symmetry breaking in bilayer.
184 citations
TL;DR: In this paper, the authors present an experimental study of spin-wave excitation and propagation in microstructured waveguides consisting of a 100nm thick yttrium iron garnet/platinum bilayer.
Abstract: We present an experimental study of spin-wave excitation and propagation in microstructured waveguides consisting of a 100 nm thick yttrium iron garnet/platinum (Pt) bilayer. The life time of the spin waves is found to be more than an order of magnitude higher than in comparably sized metallic structures, despite the fact that the Pt capping enhances the Gilbert damping. Utilizing microfocus Brillouin light scattering spectroscopy, we reveal the spin-wave mode structure for different excitation frequencies. An exponential spin-wave amplitude decay length of 31 μm is observed which is a significant step towards low damping, insulator based micro-magnonics.
177 citations
TL;DR: In this article, the optical conductivity of 2D honeycomb crystals is computed using independent-quasiparticle approximation of the complex dielectric function for optical interband transitions.
Abstract: We compute the optical conductivity of 2D honeycomb crystals beyond the usual Dirac-cone approximation. The calculations are mainly based on the independent-quasiparticle approximation of the complex dielectric function for optical interband transitions. The full band structures are taken into account. In the case of silicene, the influence of excitonic effects is also studied. Special care is taken to derive converged spectra with respect to the number of k points in the Brillouin zone and the number of bands. In this way both the real and imaginary parts of the optical conductivity are correctly described for small and large frequencies. The results are applied to predict the optical properties reflection, transmission and absorption in a wide range of photon energies. They are discussed in the light of the available experimental data.
168 citations
TL;DR: In a subwavelength-diameter optical fibre, the first experimental observation of Brillouin light scattering from surface acoustic waves is reported, which opens new opportunities for various sensing applications, not only in microwave photonics and nonlinear plasmonics.
Abstract: In optical fibres, stimulated Brillouin scattering is a fundamental interaction where light generates bulk elastic waves and it is backward scattered by them. Here, Beugnot et al. demonstrate the generation of backward-propagating surface acoustic wave Brillouin scattering in subwavelength-diameter optical fibres.
157 citations
TL;DR: This work demonstrates how an electronic structure characteristic of the isolated monolayer can be created at the surface of a bulk MoS2 crystal by intercalating potassium in the interlayer van der Waals gap, expanding its size while simultaneously doping electrons into the conduction band.
Abstract: Several transition-metal dichalcogenides exhibit a striking crossover from indirect to direct band gap semiconductors as they are thinned down to a single monolayer. Here, we demonstrate how an electronic structure characteristic of the isolated monolayer can be created at the surface of a bulk MoS2 crystal. This is achieved by intercalating potassium in the interlayer van der Waals gap, expanding its size while simultaneously doping electrons into the conduction band. Our angle-resolved photoemission measurements reveal resulting electron pockets centered at the K and K' points of the Brillouin zone, providing the first momentum-resolved measurements of how the conduction band dispersions evolve to yield an approximately direct band gap of ∼1.8 eV in quasi-freestanding monolayer MoS2. As well as validating previous theoretical proposals, this establishes a novel methodology for manipulating electronic structure in transition-metal dichalcogenides, opening a new route for the generation of large-area quasi-freestanding monolayers for future fundamental study and use in practical applications.
151 citations
TL;DR: In this paper, the optical emission spectrum of a resonantly excited molybdenum disulfide (MoS2) was studied at liquid helium temperature, and 20 peaks in the energy range spanning up to 1400 cm−1 from the laser line, which are related to multiphonon resonant Raman scattering processes, were observed.
Abstract: Optical emission spectrum of a resonantly (λ = 632.8 nm) excited molybdenum disulfide (MoS2) is studied at liquid helium temperature. More than 20 peaks in the energy range spanning up to 1400 cm−1 from the laser line, which are related to multiphonon resonant Raman scattering processes, are observed. The attribution of the observed lines involving basic lattice vibrational modes of MoS2 and both the longitudinal (LA(M)) and the transverse (TA(M) and/or ZA(M)) acoustic phonons from the vicinity of the high-symmetry M point of the MoS2 Brillouin zone is proposed.
133 citations
TL;DR: It is shown that confined electronic states at oxide surfaces can be endowed with novel, non-trivial properties that are both theoretically challenging to anticipate and promising for technological applications.
Abstract: Two-dimensional electron gases (2DEGs) forming at the interfaces of transition metal oxides exhibit a range of properties, including tunable insulator-superconductor-metal transitions, large magnetoresistance, coexisting ferromagnetism and superconductivity, and a spin splitting of a few meV (refs 10, 11). Strontium titanate (SrTiO3), the cornerstone of such oxide-based electronics, is a transparent, non-magnetic, wide-bandgap insulator in the bulk, and has recently been found to host a surface 2DEG (refs 12-15). The most strongly confined carriers within this 2DEG comprise two subbands, separated by an energy gap of 90 meV and forming concentric circular Fermi surfaces. Using spin- and angle-resolved photoemission spectroscopy (SARPES), we show that the electron spins in these subbands have opposite chiralities. Although the Rashba effect might be expected to give rise to such spin textures, the giant splitting of almost 100 meV at the Fermi level is far larger than anticipated. Moreover, in contrast to a simple Rashba system, the spin-polarized subbands are non-degenerate at the Brillouin zone centre. This degeneracy can be lifted by time-reversal symmetry breaking, implying the possible existence of magnetic order. These results show that confined electronic states at oxide surfaces can be endowed with novel, non-trivial properties that are both theoretically challenging to anticipate and promising for technological applications.
131 citations
TL;DR: In this article, the authors trace the evolution of the band structure as a function of the number of layers, starting from a monolayer which has a direct gap, to the bulk material, which has an indirect gap.
Abstract: Exploring two-dimensional layered materials, such as molybdenum disulfide $({\mathrm{MoS}}_{2})$, for (opto)electronic applications requires detailed knowledge of their electronic band structures. Using first-principles calculations we trace the evolution of the band structure as a function of the number of layers, starting from a monolayer, which has a direct gap, to the bulk material, which has an indirect gap. We find that, with respect to the vacuum level, the valence-band maximum (VBM) increases rapidly with the number of layers, while the conduction-band minimum (CBM) remains almost constant. For two or more layers the VBM always occurs at $\ensuremath{\Gamma}$ and the CBM occurs at K. These findings are analyzed in terms of the orbital composition of the valence- and conduction-band edges at the various high-symmetry points in the Brillouin zone.
TL;DR: A simple scheme for tomography of band-insulating states in one- and two-dimensional optical lattices with two sublattice states that maps out the Berry curvature in the entire Brillouin zone and extracts topological invariants such as the Chern number is proposed.
Abstract: We propose a simple scheme for tomography of band-insulating states in one- and two-dimensional optical lattices with two sublattice states. In particular, the scheme maps out the Berry curvature in the entire Brillouin zone and extracts topological invariants such as the Chern number. The measurement relies on observing---via time-of-flight imaging---the time evolution of the momentum distribution following a sudden quench in the band structure. We consider two examples of experimental relevance: the Harper model with $\ensuremath{\pi}$ flux and the Haldane model on a honeycomb lattice. Moreover, we illustrate the performance of the scheme in the presence of a parabolic trap, noise, and finite measurement resolution.
Book•
01 Jan 2014
TL;DR: In this paper, the Hartree-Fock equation and Kohn-Sham equation were derived from first principles and the Schrodinger equation was used to model the properties of materials.
Abstract: 1. Computational materials modelling from first principles 2. Many-body Schrodinger equation 3. Density-functional theory 4. Equilibrium structures of materials: fundamentals 5. Equilobrium structures of materials: calculation vs. experiment 6. Elastic properties of materials 7. Vibrations of molecules and solids 8. Phonons, vibrational spectroscopy, and thermodynamics 9. Band structures and photoelectron spectroscopy 10. Dielectric function and optical spectra 11. Density-functional theory and magnetic materials Appendix A: Derivation of the Hartree-Fock equations Appendix B: Derivation of the Kohn-Sham equations Appendix C: Numerical solution of the Kohn-Sham equations Appendix D: Reciprocal lattice and Brillouin zone Appendix E: Pseudopotentials
TL;DR: The specification and improved understanding of scattering parameters using the SPB model are important and instructive for further optimization of the thermoelectric performance of n-type Mg2Si0.3Sn0.7.
Abstract: The well-known single parabolic band (SPB) model has been useful in providing insights into the understanding of transport properties of numerous thermoelectric materials. However, the conduction and valence bands of real semiconductors are rarely truly parabolic which limits the predictive power of the SPB model. The coincidence of the band edges of two parabolic bands, a situation arising in Mg2Si1−xSnx solid solutions when x ∼ 0.7, naturally makes the SPB approximation applicable to evaluate all transport parameters. We demonstrate this in the case of Bi-doped Mg2Si0.3Sn0.7 where the minima of the two conduction bands at the X-point of the Brillouin zone coincide. The combination of a large density-of-states effective mass m* ∼ 2.6 me arising from the enhanced valley degeneracy Nv, high mobility μd due to low deformation potential Ed (8.77–9.43 eV), and ultra-low alloy scattering parameter Ea (0.32–0.39 eV) leads to an outstanding power factor, PFmax ∝ (m*)3/2μd, of up to 4.7 mW m−1 K−2 at around 600 K. The specification and improved understanding of scattering parameters using the SPB model are important and instructive for further optimization of the thermoelectric performance of n-type Mg2Si0.3Sn0.7.
TL;DR: In this paper, the effects of spin-lattice coupling on the magnon spectrum of thin ferromagnetic films consisting of the magnetic insulator yttrium iron garnet were investigated.
Abstract: We calculate the effects of the spin-lattice coupling on the magnon spectrum of thin ferromagnetic films consisting of the magnetic insulator yttrium iron garnet. The magnon-phonon hybridization generates a characteristic minimum in the spin dynamic structure factor which quantitatively agrees with recent Brillouin light scattering experiments. We also show that at room temperature the phonon contribution to the magnon damping exhibits a rather complicated momentum dependence: In the exchange regime the magnon damping is dominated by Cherenkov type scattering processes, while in the long-wavelength dipolar regime these processes are subdominant and the magnon damping is two orders of magnitude smaller. We supplement our calculations by actual measurements of the magnon relaxation in the dipolar regime. Our theory provides a simple explanation of a recent experiment probing the different temperatures of the magnon and phonon gases in yttrium iron garnet.
TL;DR: In this article, the effect of interlayer coupling on the valence band properties of a variety of few-layer, two-dimensional materials has been investigated using ab initio calculations for GaS, GaSe, InS, InSe, for Bi$2}$Se$3}$, for monolayer Bi, and for bilayer graphene as a function of vertical field.
Abstract: The valence band of a variety of few-layer, two-dimensional materials consists of a ring of states in the Brillouin zone. The energy-momentum relation has the form of a `Mexican hat' or a Rashba dispersion. The two-dimensional density of states is singular at or near the band edge, and the band-edge density of modes turns on nearly abruptly as a step function. The large band-edge density of modes enhances the Seebeck coefficient, the power factor, and the thermoelectric figure of merit ZT. Electronic and thermoelectric properties are determined from ab initio calculations for few-layer III-VI materials GaS, GaSe, InS, InSe, for Bi$_{2}$Se$_{3}$, for monolayer Bi, and for bilayer graphene as a function of vertical field. The effect of interlayer coupling on these properties in few-layer III-VI materials and Bi$_{2}$Se$_{3}$ is described. Analytical models provide insight into the layer dependent trends that are relatively consistent for all of these few-layer materials. Vertically biased bilayer graphene could serve as an experimental test-bed for measuring these effects.
TL;DR: In this article, the effects of different mirror symmetry-breaking perturbations on the (001) surface band structure were studied and the Landau level spectrum has an unusual pattern of degeneracy and interesting features.
Abstract: In the recently discovered topological crystalline insulators SnTe and ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}$(Te, Se), crystal symmetry and electronic topology intertwine to create topological surface states with many interesting features including Lifshitz transition, Van-Hove singularity, and fermion mass generation. These surface states are protected by mirror symmetry with respect to the (110) plane. In this work we present a comprehensive study of the effects of different mirror-symmetry-breaking perturbations on the (001) surface band structure. Pristine (001) surface states have four branches of Dirac fermions at low energy. We show that ferroelectric-type structural distortion generates a mass and gaps out some or all of these Dirac points, while strain shifts Dirac points in the Brillouin zone. An in-plane magnetic field leaves the surface state gapless, but introduces asymmetry between Dirac points. Finally, an out-of-plane magnetic field leads to discrete Landau levels. We show that the Landau level spectrum has an unusual pattern of degeneracy and interesting features due to the unique underlying band structure. This suggests that Landau level spectroscopy can detect and distinguish between different mechanisms of symmetry breaking in topological crystalline insulators.
TL;DR: A molecular absorption cell was introduced into the virtually imaged phased array (VIPA) based Brillouin spectroscopy setup to absorb the Rayleigh component, which resulted in a substantial improvement of the BrillouIn spectrum quality.
Abstract: Brillouin spectroscopy is an emerging tool for microscopic optical imaging as it allows for non-contact, non-invasive, direct assessment of the elastic properties of materials. However, strong elastic scattering and stray light from various sources often contaminate the Brillouin spectrum. A molecular absorption cell was introduced into the virtually imaged phased array (VIPA) based Brillouin spectroscopy setup to absorb the Rayleigh component, which resulted in a substantial improvement of the Brillouin spectrum quality.
TL;DR: Brillouin spectroscopy is an optical technique that allows one to determine the directional dependence of acoustic velocities in minerals and materials subject to a wide range of environmental conditions.
Abstract: Brillouin spectroscopy is an optical technique that allows one to determine the directional dependence of acoustic velocities in minerals and materials subject to a wide range of environmental conditions. It is based on the inelastic scattering of light by spontaneous collective motions of particles in a material in the frequency range between 10−2 to 10 GHz.
Brillouin spectroscopy is used to determine acoustic velocities and elastic properties of a number of crystalline solids, glasses, and liquids. It is most commonly performed on transparent single crystals where the complete elastic tensor of the sample material can be derived. However, Brillouin spectra can be also measured from opaque materials, from which partial or complete information on the elastic tensor can be determined. It is a very flexible technique with many possible areas of application in research disciplines from condensed matter physics to biophysics to materials sciences to geophysics.
Brillouin scattering can be performed on very small samples and it can be easily combined with the diamond anvil cell and carried out at high pressures and temperatures (see reviews by Grimsditch and Polian 1989 and Eremets 1996). This makes this technique the method of choice to study the elastic properties of deep Earth materials, relevant to construct a mineralogical model of the interior of our planet that is consistent with the constraints from seismology.
Several of the candidate minerals of the Earth’s interior are not stable at ambient conditions, and only recently has there been substantial progress in their synthesis. Unfortunately, those deep earth minerals that can be metastably preserved at ambient pressure and temperature are only available as single crystals with sizes of the order of several tens of microns at most. However, crystals of this size are large enough for Brillouin scattering to be performed. In addition, more sophisticated methods …
TL;DR: This article improved the linear tetrahedron method to overcome systematic errors due to overestimations (underestimations) in integrals for convex (concave) functions, respectively.
Abstract: We improve the linear tetrahedron method to overcome systematic errors due to overestimations (underestimations) in integrals for convex (concave) functions, respectively. Our method is applicable to various types of calculations such as the total energy, the charge (spin) density, response functions, and the phonon frequency, in contrast with the Bl\"ochl correction, which is applicable to only the first two. We demonstrate the ability of our method by calculating phonons in MgB${}_{2}$ and fcc lithium.
TL;DR: In this article, a phononic crystal (PC) composed of a square array of densely packed square iron rods in air is used to construct a near zero-refractive index (ZRI) material.
Abstract: Zero-refractive-index materials may lead to promising applications in various fields. Here, we design and fabricate a near Zero-Refractive-Index (ZRI) material using a phononic crystal (PC) composed of a square array of densely packed square iron rods in air. The dispersion relation exhibits a nearly flat band across the Brillouin zone at the reduced frequency f = 0.5443c/a, which is due to Fabry-Perot (FP) resonance. By using a retrieval method, we find that both the effective mass density and the reciprocal of the effective bulk modulus are close to zero at frequencies near the flat band. We also propose an equivalent tube network model to explain the mechanisms of the near ZRI effect. This FP-resonance-induced near ZRI material offers intriguing wave manipulation properties. We demonstrate both numerically and experimentally its ability to shield a scattering obstacle and guide acoustic waves through a bent structure.
TL;DR: In this article, the effect of the substrate on the lattice dynamics in molybdenum disulfide was investigated and the experimental results were discussed within the frames of the recently proposed model of electron-phonon coupling involving transverse acoustic phonons.
TL;DR: In this article, the authors measured the exciton binding energy of monolayer transition metal dichalcogenides (TMDC) with linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopic (TP-PLE) measurements.
Abstract: The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach toward measuring the exciton binding energy of monolayer WS2 with linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE). TP-PLE measurements show the exciton binding energy of 0.71eV around K valley in the Brillouin zone. The trion binding energy of 34meV, two-photon absorption cross section 4X10^{4}cm^{2}W^{-2}S^{-1} at 780nm and exciton-exciton annihilation rate around 0.5cm^{2}/s are experimentally obtained.
TL;DR: In this paper, a double Dirac cone is realized at the center of the Brillouin zone of a two-dimensional phononic crystal (PC) consisting of a triangular array of core-shell-structure cylinders in water.
Abstract: A double Dirac cone is realized at the center of the Brillouin zone of a two-dimensional phononic crystal (PC) consisting of a triangular array of core-shell-structure cylinders in water. The double Dirac cone is induced by the accidental degeneracy of two double-degenerate Bloch states. Using a perturbation method, we demonstrate that the double Dirac cone is composed of two identical and overlapping Dirac cones whose linear slopes can also be accurately predicted from the method. Because the double Dirac cone occurs at a relatively low frequency, a slab of the PC can be mapped onto a slab of zero refractive index material by using a standard retrieval method. Total transmission without phase change and energy tunneling at the double Dirac point frequency are unambiguously demonstrated by two examples. Potential applications can be expected in diverse fields such as acoustic wave manipulations and energy flow control.
01 Dec 2014
TL;DR: In this article, the effect of spin-orbit coupling (SOC) on the band structure of single-layer and bulk TMDs, including explicitly the role of the chalcogen orbitals and their hybridization with the transition metal atoms, was studied.
Abstract: One of the main characteristics of the new family of two-dimensional crystals of semiconducting transition metal dichalcogenides (TMDs) is the strong spin–orbit interaction, which makes them very promising for future applications in spintronics and valleytronics devices. Here we present a detailed study of the effect of spin–orbit coupling (SOC) on the band structure of single-layer and bulk TMDs, including explicitly the role of the chalcogen orbitals and their hybridization with the transition metal atoms. To this aim, we combine density functional theory (DFT) calculations with a Slater–Koster tight-binding (TB) model. Whereas most of the previous TB models have been restricted to the K and K’ points of the Brillouin zone (BZ), here we consider the effect of SOC in the whole BZ, and the results are compared to the band structure obtained by DFT methods. The TB model is used to analyze the effect of SOC in the band structure, considering separately the contributions from the transition metal and the chalcogen atoms. Finally, we present a scenario where, in the case of strong SOC, the spin/orbital/valley entanglement at the minimum of the conduction band at Q can be probed and be of experimental interest in the most common cases of electron-doping reported for this family of compounds.
TL;DR: In this paper, the authors studied the time-evolution of wave-packets, which are spectrally concentrated near conical points in the Brillouin zone, and proved that the large but finite, time dynamics is governed by the Dirac equations.
Abstract: In a recent article (Fefferman and Weinstein, in J Am Math Soc 25:1169–1220, 2012), the authors proved that the non-relativistic Schrodinger operator with a generic honeycomb lattice potential has conical (Dirac) points in its dispersion surfaces. These conical points occur for quasi-momenta, which are located at the vertices of the Brillouin zone, a regular hexagon. In this paper, we study the time-evolution of wave-packets, which are spectrally concentrated near such conical points. We prove that the large, but finite, time dynamics is governed by the two-dimensional Dirac equations.
TL;DR: In this article, a perfluorinated graded-index polymer optical fiber (POF) with 50-μm core diameter was used for distributed temperature measurements based on stimulated Brillouin scattering.
Abstract: Distributed temperature measurements in a perfluorinated graded-index polymer optical fiber (POF) with 50- μm core diameter are reported for the first time, to the best of our knowledge. Brillouin optical frequency-domain analysis is shown to be able to resolve spatially the temperature-dependent Brillouin frequency shift profile along a 20-m POF fiber sample, at a nominal spatial resolution of 4 m. The results indicate that POFs are potentially useful for distributed temperature measurements based on stimulated Brillouin scattering.
TL;DR: In this paper, the authors presented a comprehensive multiphonon Raman and complementary infrared analysis for bulk and monolayer MoS2 with the main contributions coming from four phonons.
Abstract: We present a comprehensive multiphonon Raman and complementary infrared analysis for bulk and monolayer MoS2.For the bulk the analysis consists of symmetry assignment from which we obtain a broad set of allowed second order transitions at the high symmetry M,K and gamma Brillouin zone points. The attribution of about 80 transitions of up to fifth order Raman processes are proposed in the low temperature(95K)resonant Raman spectrum measured with the excitation energy of 1.96 eV,which is slightly shifted from the A exciton. We propose that the main contributions come from four phonons:A1g(M),E12g(M2),E22g(M1)(TA'(M))and E22g (M2)(LA'(M)). The last three are single degenerate phonons at M with an origin of the E12g(gamma)and E22g(gamma)phonons. Among the four phonons, we identify in the resonant Raman spectra all(but one) of the second order overtones,combination and difference bands and many of the third order bands. Consistent with the expectation that at the M point only combinations with the same inversion symmetry (g or u)are Raman allowed, the contribution of combinations with the LA(M)phonon can not be considered with the above four phonons. Although minor,contribution from K point and possibly gamma point phonons are also evident. The "2LA band",measured at ~460 cm-1 is reassigned.Supported by the striking similarity between this band, measured under off resonant conditions, and recently published two phonon density of states, we propose that the lower part of the band,previously attributed to 2LA(M),is due to a van Hove singularity between K and M. The higher part,previously attributed exclusively to the A2u(gamma)phonon,is mostly due to the LA and LA' phonons at M. For the monolayer MoS2, the second order phonon processes from M and gamma Brillouin zone points are also analyzed and are discussed within similar framework to that of the bulk.
TL;DR: In this article, the topological edge states of zigzag Bi(111) nanoribbons can be significantly tuned by edge H edge adsorption, and the Fermi velocity is increased by 1 order of magnitude.
Abstract: Based on first-principles and tight-binding calculations, we report that the topological edge states of zigzag Bi(111) nanoribbon can be significantly tuned by H edge adsorption. The Fermi velocity is increased by 1 order of magnitude, as the Dirac point is moved from the Brillouin zone boundary to the Brillouin zone center, and the real-space distribution of Dirac states are made twice more delocalized. These intriguing changes are explained by an orbital filtering effect of edge H atoms, which pushes certain components of the p orbital of edge Bi atoms out of the band gap regime that reshapes the topological edge states. In addition, the spin texture of the Dirac states is also modified, which is described by introducing an effective Hamiltonian. Our findings not only are of fundamental interest but also have practical implications in potential applications of topological insulators.
TL;DR: In this article, it was shown that both intranodal finite momentum pairing and internodal BCS superconductivity are allowed for Weyl semimetals, and that for local attractive interaction, the finite-momentum pairing state with chiral $p$-wave symmetry is the most favorable at finite chemical potential.
Abstract: Unconventional superconducting states of matter are realized in the presence of strong spin-orbit coupling. In particular, nondegenerate bands can support odd-parity superconductivity with rich topological content. Here we study whether this is the case for Weyl semimetals. These are systems whose low-energy sector, in the absence of interactions, is described by linearly dispersing chiral fermions in three dimensions. The energy spectrum has nodes at an even number of points in the Brillouin zone. Consequently both intranodal finite momentum pairing and internodal BCS superconductivity are allowed. For local attractive interaction the finite momentum pairing state with chiral $p$-wave symmetry is found to be most favorable at finite chemical potential. The state is an analog of the superfluid ${}^{3}$He $A$ phase, with Cooper pairs having finite center-of-mass momentum. For chemical potential at the node the state is preempted by a fully gapped charge density wave. For nonlocal attraction the BCS state wins out for all values of the chemical potential.