Showing papers on "Brillouin zone published in 2007"
TL;DR: In this article, a technique based on the spatial localization of electron and phonon Wannier functions was proposed to perform first-principles calculations of the electron-phonon interaction with an ultradense sampling of the Brillouin zone.
Abstract: We introduce a technique based on the spatial localization of electron and phonon Wannier functions to perform first-principles calculations of the electron-phonon interaction with an ultradense sampling of the Brillouin zone. After developing the basic theory, we describe the practical implementation within a density-functional framework. The proposed method is illustrated by considering a virtual crystal model of boron-doped diamond. For this test case, we first discuss the spatial localization of the electron-phonon matrix element in the Wannier representation. Then, we assess the accuracy of the Wannier-Fourier interpolation in momentum space. Finally, we study the convergence of the electron-phonon self-energies with the sampling of the Brillouin zone by calculating the electron and phonon linewidths, the Eliashberg spectral function, and the mass enhancement parameter of B-doped diamond. We show that more than ${10}^{5}$ points in the irreducible wedge of the Brillouin zone are needed to achieve convergence.
612 citations
TL;DR: In this paper, the authors presented the full in-plane phonon dispersion of graphite obtained from inelastic x-ray scattering, including the optical and acoustic branches, as well as the midfrequency range between the $K$ and $M$ points in the Brillouin zone, where the experimental data have been unavailable so far.
Abstract: We present the full in-plane phonon dispersion of graphite obtained from inelastic x-ray scattering, including the optical and acoustic branches, as well as the midfrequency range between the $K$ and $M$ points in the Brillouin zone, where the experimental data have been unavailable so far. The existence of a Kohn anomaly at the $K$ point is further supported. We fit a fifth-nearest neighbor force-constant model to the experimental data, making improved force-constant calculations of the phonon dispersion in both graphite and carbon nanotubes available.
391 citations
TL;DR: A new high-resolution angle-resolved photoemission study of 1T-TiSe2 in both its room-temperature, normal phase and its low-tem temperature, charge-density wave phase gives strong evidence in favor of the excitonic insulator scenario as a driving force for the charge- density wave transition in 1T.
Abstract: We present a new high-resolution angle-resolved photoemission study of 1T-TiSe2 in both its room-temperature, normal phase and its low-temperature, charge-density wave phase. At low temperature the photoemission spectra are strongly modified, with large band renormalizations at high-symmetry points of the Brillouin zone and a very large transfer of spectral weight to backfolded bands. A calculation of the theoretical spectral function for an excitonic insulator phase reproduces the experimental features with very good agreement. This gives strong evidence in favor of the excitonic insulator scenario as a driving force for the charge-density wave transition in 1T-TiSe2.
345 citations
01 Jan 2007
TL;DR: In this paper, the authors review the physical basis of the modern theory of polarization, emphasizing how the polarization can be defined in terms of the accumulated adiabatic flow of current occurring as a crystal is modified or deformed.
Abstract: In this Chapter we review the physical basis of the modern theory of polarization, emphasizing how the polarization can be defined in terms of the accumulated adiabatic flow of current occurring as a crystal is modified or deformed. We explain how the polarization is closely related to a Berry phase of the Bloch wavefunctions as the wavevector is carried across the Brillouin zone, or equivalently, to the centers of charge of Wannier functions constructed from the Bloch wavefunctions. A resulting feature of this formulation is that the polarization is formally defined only modulo a “quantum of polarization” – in other words, that the polarization may be regarded as a multi-valued quantity. We discuss the consequences of this theory for the physical understanding of ferroelectric materials, including polarization reversal, piezoelectric effects, and the appearance of polarization charges at surfaces and interfaces. In so doing, we give a few examples of realistic calculations of polarization-related quantities in perovskite ferroelectrics, illustrating how the present approach provides a robust and powerful foundation for modern computational studies of dielectric and ferroelectric materials.
339 citations
TL;DR: In this article, an efficient formula for Z 2 topological invariants characterizing the quantum spin Hall effect is derived in a lattice Brillouin zone, which enables us to implement numerical calculations for realistic models even in three dimensions.
Abstract: We derive an efficient formula for Z 2 topological invariants characterizing the quantum spin Hall effect. It is defined in a lattice Brillouin zone, which enables us to implement numerical calculations for realistic models even in three dimensions. Based on this, we study the quantum spin Hall effect in Bi and Sb in quasi-two and three dimensions using a tight-binding model.
198 citations
01 Jan 2007
TL;DR: In this article, the authors investigate slow light via stimulated Brillouin scattering (SBS) in a room temperature optical fiber that is pumped by a spectrally broadened laser and find that partial overlap of the Stokes and anti-Stokes resonances can actually lead to an enhancement of the slow light delay-bandwidth product when Deltaomegapsime 1.3OmegaB.
Abstract: In this paper, we investigate slow light via stimulated Brillouin scattering (SBS) in a room temperature optical fiber that is pumped by a spectrally broadened laser. Broadening the spectrum of the pump field increases the linewidth Deltaomegap of the Stokes amplifying resonance, thereby increasing the slow-light bandwidth. One physical bandwidth limitation occurs when the linewidth becomes several times larger than the Brillouin frequency shift OmegaB so that the anti-Stokes absorbing resonance substantially cancels out the Stokes amplifying resonance and, hence, the slow-light effect. We find that partial overlap of the Stokes and anti-Stokes resonances can actually lead to an enhancement of the slow-light delay-bandwidth product when Deltaomegapsime1.3OmegaB. Using this general approach, we increase the Brillouin slow-light bandwidth to over 12 GHz from its nominal linewidth of ~30 MHz obtained for monochromatic pumping. We controllably delay 75-ps-long pulses by up to 47 ps and study the data-pattern dependence of the broadband SBS slow-light system
183 citations
TL;DR: In this article, the authors used direct laser writing to fabricate high-quality polymeric 3D spiral photonic crystals using a 2D network of bars at or close to the top of the 3D crystal.
Abstract: Chiral 3D photonic crystals are an interesting subclass of 3D photonic crystals. For example, large complete 3D photonic bandgaps have been predicted for high-index-contrast silicon square-spiral structures; corresponding experiments using glancing-incidence deposition, interference lithography, or direct laser writing have been published. In addition to complete gaps or stop bands, theory also predicts polarization stop bands, i.e., stop bands for just one of the two circular polarizations. Such polarization stop bands can give rise to strong circular dichroism, which can potentially be used for constructing compact “thin-film” optical diodes. In this report, we fabricate high-quality polymeric 3D spiral photonic crystals via direct laser writing. The measured transmittance spectra of these low-index-contrast structures reveal spectral regions where the transmittance is below 5 % for one circular incident polarization and above 95 % for the other—for just eight lattice constants along the propagation direction. The experimental data are compared with scattering-matrix calculations for the actual finite structures, leading to good agreement. For what conditions do we expect strong circular dichroism? For circular polarization of light, the tip of the electricfield vector simply follows a spiral. The pitch of this spiral is just the material wavelength k. Thus, intuitively, we expect a chiral resonance from spiral photonic crystals if the pitch of circularly polarized light matches the pitch of the dielectric spirals, i.e., the lattice constant az. This condition, k/az = 1, corresponds to the edge of the second Brillouin zone, i.e., to a wave number kz = 2p/k= 2p/az. Recall that the edge of the first Brillouin zone is at kz = p/az. Thus, one does not anticipate a strong chiral response around and below the fundamental stop band (or bandgap), but rather at higher frequencies. Theory for high-index silicon-based structures confirms this intuitive reasoning. We have repeated similar calculations for low-index-contrast polymeric structures, revealing essentially the same trends. The parameters of the 3D spiral photonic crystals to be discussed below are the result of an optimization with respect to circular dichroism. The samples in our experiments are made by direct laser writing, which essentially allows for the fabrication of almost arbitrarily shaped 3D photoresist structures. Details of our process based on the commercial thick-film resist SU-8 can be found in the Experimental section and in earlier work. Our structures are mechanically supported by a 2D network of bars at, or close to, the top of the 3D crystal. As the spirals are not at all mechanically connected to their neighbors, very unstable low-quality structures would result without this grid. Furthermore, all the structures for optical experiments are surrounded by a thick massive wall (see Fig. 1a), which aims at reducing the effects of strain on the 2D grid caused by photoresist shrinkage during development. Here, we use a round (rather than a rectangular) wall in order to evenly distribute strain inside the wall. Through numerical calculations (see below), we have confirmed that the distortion of the optical properties by the 2D network is only marginal. Most importantly, the network does not introduce any chirality. A small gallery of selected electron microscopy images is shown in Figure 1, which gives first evidence that the sample quality is very good. Figure 1a gives an overview of the sample to be optically characterized below. The sample parameters are: in-plane lattice constant axy = 1.3 lm, pitch az = 1.3 lm, spiral diameter d = 0.78 lm, volume filling fraction 34.7 %, lateral diameter of the spiral arms darm = 380 nm, ratio between the axial and the lateral diameter 2.7, and N = 8 lattice constants along the z-direction. These parameters were extracted from the close-up cross-sectional view in Figure 1a. To demonstrate the versatility of our approach, Figure 1b exhibits a cut of a structure with axy = 1.5 lm, az = 1.5 lm, and N = 4. Because the focused-ion-beam cut was stopped in between two rows of spirals, the stabilizing network mentioned C O M M U N IC A IO N
143 citations
TL;DR: In this article, the fine structure of the lowest energy excitonic manifold of two nearly spherical PbSe quantum dots of radius R = 15.3 and 30.6 A was computed using atomistic single-particle wave functions.
Abstract: An exciton evolving from an m-fold degenerate hole level and an n-fold degenerate electron level has a nominal m × n degeneracy, which is often removed by electron−hole interactions. In PbSe quantum dots, the degeneracy of the lowest-energy exciton is m × n = 64 because both the valence-band maximum and the conduction-band minimum originate from the 4-fold degenerate (8-fold including spin) L valleys in the Brillouin zone of bulk PbSe. Using a many-particle configuration-interaction approach based on atomistic single-particle wave functions, we have computed the fine structure of the lowest-energy excitonic manifold of two nearly spherical PbSe quantum dots of radius R = 15.3 and 30.6 A. We identify two main energy splittings, both of which are accessible to experimental probe: (i) The intervalley splitting δ is the energy difference between the two near-edge peaks of the absorption spectrum. We find δ = 80 meV for R = 15.3 A and δ = 18 meV for R = 30.6 A. (ii) The exchange splitting Δx is the energy dif...
132 citations
TL;DR: Broadband slow light is demonstrated by using stimulated Brillouin scattering in optical fibers based on a double BrillouIn pump, the peaks of which are spectrally separated by twice the Brillou in frequency.
Abstract: Broadband slow light is demonstrated by using stimulated Brillouin scattering in optical fibers based on a double Brillouin pump, the peaks of which are spectrally separated by twice the Brillouin frequency The loss spectrum generated by one of the pump waves is fully compensated by the gain spectrum of the other one, which permits the enlargement of the bandwidth to 25 GHz and a variable time delay of up to 109 ps with 37 ps pulses
132 citations
TL;DR: The calculated phonon self-energy and Eliashberg spectral function show that superconductivity cannot be explained without taking into account the finite-wave-vector Fourier components of the vibrational modes introduced by boron, as well as the breaking of the diamond crystal periodicity induced by doping.
Abstract: We present a first-principles technique for investigating the electron-phonon interaction with millions of $k$ points in the Brillouin zone, which exploits the spatial localization of electronic and lattice Wannier functions. We demonstrate the effectiveness of our technique by elucidating the phonon mechanism responsible for superconductivity in boron-doped diamond. Our calculated phonon self-energy and Eliashberg spectral function show that superconductivity cannot be explained without taking into account the finite-wave-vector Fourier components of the vibrational modes introduced by boron, as well as the breaking of the diamond crystal periodicity induced by doping.
123 citations
TL;DR: In this paper, the dispersion relation for collective modes in patterned arrays through the numerical solution of an eigenvalue problem for an integral operator has been investigated and quantitatively explained by a theoretical model.
Abstract: Magnetization dynamics of dipolarly coupled nanowire arrays has been studied by Brillouin light scattering. Measurements performed in uniformly magnetized wires as a function of the transferred wave vector demonstrated the existence of several discrete collective modes, propagating through the structure with a periodic dispersion curve encompassing several Brillouin zones relative to the artificial spatial periodicity. This experimental evidence has been quantitatively explained by a theoretical model which permits the calculation of the dispersion relation for collective modes in patterned arrays through the numerical solution of an eigenvalue problem for an integral operator.
TL;DR: In this paper, an ab initio approach for computing the anomalous Hall conductivity was proposed by converting the integral over the Fermi sea into a more efficient integral on the fermi surface only.
Abstract: While the intrinsic anomalous Hall conductivity is normally written in terms of an integral of the electronic Berry curvature over the occupied portions of the Brillouin zone, Haldane has recently pointed out that this quantity (or more precisely, its ``nonquantized part'') may alternatively be expressed as a Fermi-surface property. Here we present an ab initio approach for computing the anomalous Hall conductivity that takes advantage of this observation by converting the integral over the Fermi sea into a more efficient integral on the Fermi surface only. First, a conventional electronic-structure calculation is performed with spin-orbit interaction included. Maximally localized Wannier functions are then constructed by a postprocessing step in order to convert the ab initio electronic structure around the Fermi level into a tight-binding-like form. Working in the Wannier representation, the Brillouin zone is sampled on a large number of equally spaced parallel slices oriented normal to the total magnetization. On each slice, we find the intersections of the Fermi-surface sheets with the slice by standard contour methods, organize these into a set of closed loops, and compute the Berry phases of the Bloch states as they are transported around these loops. The anomalous Hall conductivity is proportional to the sum of the Berry phases of all the loops on all the slices. Illustrative calculations are performed for Fe, Co, and Ni.
TL;DR: In this article, the authors showed that the elastic wave velocities for a nearadiabatic lower mantle with a bulk composition dominated by magnesium silicate perovskite are consistent with the average lower mantle seismic velocity structure.
TL;DR: This work provides a complete experimental characterization of stimulated Brillouin scattering in a 160 m long solid-core photonic crystal fiber, including threshold and spectrum measurements as well as position-resolved mapping of the BrillouIn frequency shift.
Abstract: We provide a complete experimental characterization of stimulated Brillouin scattering in a 160 m long solid-core photonic crystal fiber, including threshold and spectrum measurements as well as position-resolved mapping of the Brillouin frequency shift. In particular, a three-fold increase of the Brillouin threshold power is observed, in excellent agreement with the spectrally-broadened Brillouin gain spectrum. Distributed measurements additionally reveal that the rise of the Brillouin threshold results from the broadband nature of the gain spectrum all along the fiber and is strongly influenced by strain. Our experiments confirm that these unique fibers can be exploited for the passive control or the suppression of Brillouin scattering.
TL;DR: In this article, the authors reported the highest speed distributed sensing of dynamic strain based on stimulated Brillouin scattering in optical fibers, achieving a sampling rate of 1 kHz, more than an order of magnitude higher than the former best result.
Abstract: We report the highest speed distributed sensing of dynamic strain based on stimulated Brillouin scattering in optical fibers. A sampling rate of 1 kHz, more than an order of magnitude higher than the former best result, is achieved by applying a simplified Brillouin optical correlation domain analysis with optimized time gates and an unbalanced Mach-Zehnder delay line. In experiments, we present the measurement of various dynamic strains at the maximum frequency of 200 Hz with 10-cm spatial resolution and 20-m measurement range.
TL;DR: It is found that the boson peak variation under pressure cannot be explained by the elastic continuum transformation only, and surprisingly, the shape of the bosons peak remains unchanged even at such high compression.
Abstract: We study the changes in the low-frequency vibrational dynamics of poly(isobutylene) under pressure up to 1.4 GPa, corresponding to a density change of 20%. Combining inelastic neutron, x-ray, and Brillouin light scattering, we analyze the variations in the boson peak, transverse and longitudinal sound velocities, and the Debye level under pressure. We find that the boson peak variation under pressure cannot be explained by the elastic continuum transformation only. Surprisingly, the shape of the boson peak remains unchanged even at such high compression.
TL;DR: In this paper, the authors used the 246 Landau potential with saturation terms to describe the elastic anomalies associated with octahedral tilting transitions in perovskites that are associated with the M and R points of the Brillouin zone.
Abstract: Landau theory has been used to develop expressions for the elastic anomalies that accompany octahedral tilting transitions in perovskites that are associated with the M and R points of the Brillouin zone. The master equation is a 246 Landau potential with saturation terms that provides phenomenological descriptions of transition sequences from a parent cubic structure through tetragonal or rhombohedral intermediates to orthorhombic or monoclinic product structures. Data from the literature have been used to determine values for all the coefficients required to generate a quantitative description of the Pm 3 m ↔ I 4/ mcm transition in SrTiO 3 , which is taken as a model system. Solutions to the Landau expansion have been adapted to include the general influence of hydrostatic pressure and non-hydrostatic stress on transition temperature and the evolution of the order parameter. Critical examination of elastic constant data from the literature reveals inconsistencies between the results of measurements on tetragonal samples using ultrasonic rather than Brillouin scattering methods. An internally consistent data set has, nevertheless, been assembled. Good qualitative agreement was obtained between the general pattern of calculated and observed variations of all the single crystal elastic constants, and semi-quantitative agreement was obtained for C 11 , C 33 , C 12 , and C 13 . Some inconsistencies remain in relation to the temperature dependence of the square of the soft mode frequencies in the tetragonal phase, which follow the square of the order parameter rather than its inverse susceptibility, but the 246 potential seems to provide a good description of the structural evolution of SrTiO 3 over a wide temperature interval up to the cubic-tetragonal transition point.
TL;DR: In this article, the phonon frequencies for the Raman-active and the infrared-active modes are assigned, and phonon dispersion curves along high symmetry lines in the Brillouin zone are also calculated.
Abstract: Lattice dynamical, dielectric, and thermodynamic properties of β-Ga2O3 are investigated by first principles. The calculated phonon frequencies for the Raman-active and the infrared-active modes are assigned. The phonon dispersion curves along high symmetry lines in the Brillouin zone and the phonon density of states are also calculated. The electronic and static dielectric tensors are calculated. The calculated static dielectric constants are much larger than the electronic constants, showing the rather strong ionic contributions to static dielectric constants. These calculated results are in a good agreement with available experimental values. The thermodynamic functions are calculated by using the phonon density of states.
TL;DR: The trigonal warping of the dispersion relation around the valleys in the Brillouin zone is effective only for low-energy excitations, and its role cannot be neglected in the zero-energy minimal conductivity of bilayer graphene.
Abstract: Using a reformulated Kubo formula we calculate the zero-energy minimal conductivity of bilayer graphene taking into account the small but finite trigonal warping. We find that the conductivity is independent of the strength of the trigonal warping and it is 3 times as large as that without trigonal warping and 6 times larger than that in single layer graphene. Although the trigonal warping of the dispersion relation around the valleys in the Brillouin zone is effective only for low-energy excitations, our result shows that its role cannot be neglected in the zero-energy minimal conductivity.
TL;DR: In this article, the topological insulator is defined as an electronic phase stabilized by spin-orbit coupling that supports propagating edge states and is not adiabatically connected to the ordinary insulator.
Abstract: The topological insulator is an electronic phase stabilized by spin-orbit coupling that supports propagating edge states and is not adiabatically connected to the ordinary insulator In several ways it is a spin-orbit-induced analog in time-reversal-invariant systems of the integer quantum Hall effect (IQHE) This paper studies the topological insulator phase in disordered two-dimensional systems, using a model graphene Hamiltonian introduced by Kane and Mele [Phys Rev Lett 95, 226801 (2005)] as an example The nonperturbative definition of a topological insulator given here is distinct from previous efforts in that it involves boundary phase twists that couple only to charge, does not refer to edge states, and can be measured by pumping cycles of ordinary charge In this definition, the phase of a Slater determinant of electronic states is determined by a Chern parity analogous to Chern number in the IQHE case Numerically, we find, in agreement with recent network model studies, that the direct transition between ordinary and topological insulators that occurs in band structures is a consequence of the perfect crystalline lattice Generically, these two phases are separated by a metallic phase, which is allowed in two dimensions when spin-orbit coupling is present The same approach can be used to study three-dimensional topological insulators
TL;DR: The results of the numerical simulations reveal that this high-frequency phonon is indeed trapped within the fiber core by the air-hole microstructure, in good agreement with experimental measurements.
Abstract: We experimentally investigate guided acoustic wave Brillouin scattering in several photonic crystal fibers by use of the so-called fiber loop mirror technique and show a completely different dynamics with respect to standard all-silica fibers. In addition to the suppression of most acoustic phonons, we show that forward Brillouin scattering in photonic crystal fibers is substantially enhanced only for the fundamental acoustic phonon because of efficient transverse acousto-optic field overlap. The results of our numerical simulations reveal that this high-frequency phonon is indeed trapped within the fiber core by the air-hole microstructure, in good agreement with experimental measurements.
TL;DR: In this article, the cosine energy-momentum relation was used to derive a formula for the photoionization rate of crystals, which is characteristic of tight binding approximation and directly takes into account the influence of Bragg-type reflections of oscillating electrons at the edges of the first Brillouin zone.
Abstract: The Keldysh model of the photoionization [L. V. Keldysh, Sov. Phys. JETP 20, 1307 (1965)] is extended by deriving a formula for the photoionization rate of crystals based on the cosine energy-momentum relation. The relation is characteristic of tight-binding approximation and directly takes into account the influence of Bragg-type reflections of oscillating electrons at the edges of the first Brillouin zone. Due to the reflections and oscillations, the dependence of the photoionization rate on laser and material parameters takes form of a multibranch function with the branches separated by singularity points with unlimited increasing of the rate. Each of the singularities is coupled to the flattening of the effective-band structure. The laser intensity corresponding to the first singularity is found to be about $10\phantom{\rule{0.3em}{0ex}}\mathrm{TW}∕{\mathrm{cm}}^{2}$ for most wide band-gap crystals. We also show that the lowest-order branch of the photoionization rate completely corresponds to the multiphoton regime, and the first singularity takes place before the tunneling regime starts to dominate. Analysis of the ionization-rate asymptotic in the vicinity of the first-singularity point suggests possibility of ionization suppression by high-intensity radiation for certain ranges of laser wavelength.
TL;DR: In this article, it was shown that the spectral edges of Schrodinger, Maxwell, and waveguide operators lie inside the Brillouin zone, rather than at the corners of high symmetry points.
Abstract: The paper discusses the following frequently arising question on the spectral structure of periodic operators of mathematical physics (e.g., Schrodinger, Maxwell, waveguide operators, etc). Is it true that one can obtain the correct spectrum by using the values of the quasimomentum running over the boundary of the (reduced) Brillouin zone only, rather than the whole zone? Or, do the edges of the spectrum occur necessarily at the set of 'corner' high symmetry points? This is known to be true in 1D, while no apparent reasons exist for this to be happening in higher dimensions. In many practical cases, though, this appears to be correct, which sometimes leads to the claims that this is always true. There seems to be no definite answer in the literature, and one encounters different opinions about this problem in the community. In this paper, starting with simple discrete graph operators, we construct a variety of convincing multiply-periodic examples showing that the spectral edges might occur deeply inside the Brillouin zone. On the other hand, it is also shown that in a 'generic' case, the situation of spectral edges appearing at high symmetry points is stable under small perturbations. This explains to some degree why in many (maybe even most) practical cases the statement still holds.
TL;DR: In this paper, a theory for the determination of the collective spin-wave modes of regular arrays of magnetic particles, taking into account the dipolar interaction among particles, was developed, and the frequencies and profiles of the spin modes of arrays of permalloy cylindrical particles with different interparticle separation were calculated with a numerical implementation of this model, using a three-dimensional representation of the magnetic particles in their actual nonuniform fundamental state.
Abstract: We have developed a theory for the determination of the collective spin-wave modes of regular arrays of magnetic particles, taking into account the dipolar interaction among particles. The frequencies and profiles of the spin modes of arrays of permalloy cylindrical particles with different interparticle separation have been calculated with a numerical implementation of this model, using a three-dimensional representation of the magnetic particles in their actual nonuniform fundamental state. The results show a very good agreement with recently published experimental data, and allow us to discuss the dispersion curves and some relevant properties of the Brillouin light-scattering intensity from spin modes in periodic arrays.
TL;DR: In this article, a method for zone unfolding when the primitive cells in a rectangular unit cell are not all identical is presented, which is applied to a Si! 4" Ge 4" superlattice using a set of optimized Si and Ge tight binding strain parameters.
Abstract: Numerical calculations of nanostructure electronic properties are often based on a nonprimitive rectangular unit cell, because the rectangular geometry allows for both highly efficient algorithms and ease of debugging while having no drawback in calculating quantum dot energy levels or the one-dimensional energy bands of nanowires. Since general nanostructure programs can also handle superlattices, it is natural to apply them to these structures as well, but here problems arise due to the fact that the rectangular unit cell is generally not the primitive cell of the superlattice, so that the resulting E! k" relations must be unfolded to obtain the primitivecell E! k" curves. If all of the primitive cells in the rectangular unit cell are identical, then the unfolding is reasonably straightforward; if not, the problem becomes more difficult. Here, we provide a method for zone unfolding when the primitive cells in a rectangular cell are not all identical. The method is applied to a Si! 4" Ge! 4" superlattice using a set of optimized Si and Ge tight-binding strain parameters.
TL;DR: This work investigates the amplitude flatness of Rayleigh-assisted Brillouin-Raman comb laser in a linear cavity in which feedbacks are formed by high-reflectivity mirror and obtains an average signal-to-noise ratio of 17 dB.
Abstract: We investigate the amplitude flatness of Rayleigh-assisted Brillouin-Raman comb laser in a linear cavity in which feedbacks are formed by high-reflectivity mirror. The optimization of Brillouin pump power and wavelength is very crucial in order to obtain a uniform power level between Stokes lines. The Brillouin pump must have a relatively large power and its wavelength must be located closer to the Raman peak gain region. The flat-amplitude bandwidth is also determined by the choice of Raman pump wavelengths. A flat-amplitude bandwidth of 30.7 nm from 1527.32 to 1558.02 nm is measured when Raman pump wavelengths are set to 1435 and 1450 nm. 357 uniform Brillouin Stokes lines with 0.086 nm spacing are generated across the wavelength range. The average signal-to-noise ratio of 17 dB is obtained for all the Brillouin Stokes lines.
TL;DR: The k-space spectral evolution directly demonstrates, for the first time, that the CDW order parameter microscopically competes with superconductivity in the same band.
Abstract: A superconducting state (T(c) approximately 4.2 K) has very recently been observed upon successful doping of the charge-density-wave (CDW) ordered triangular lattice TiSe(2), with copper. Using state-of-the-art photoemission spectroscopy we identify, for the first time, momentum-space locations of doped electrons that form the Fermi sea of the superconductor. With doping, we find that kinematic nesting volume increases, whereas coherence of the CDW collective order sharply drops. In superconducting doping, as chemical potential rises, we observe the emergence of a large density of states in the form of a narrow electron pocket near the L point of the Brillouin zone with d-like character. The k-space spectral evolution directly demonstrates, for the first time, that the CDW order parameter microscopically competes with superconductivity in the same band.
TL;DR: In this paper, the first application of Brillouin light scattering (BLS) to probe elastic waves in transparent-substrate supported polymer thin films was demonstrated, and the results showed that BLS can probe elastic wave propagation in thin films.
Abstract: The present work demonstrates the first application of Brillouin light scattering (BLS) to probe film-guided elastic waves in transparent-substrate supported polymer thin films. In comparison with ...
TL;DR: In this article, the energy band structures, density of states (DOS) and optical properties of paraelectric cubic crystal KTaO3 have been calculated using density functional theory in its local density approximation.
Abstract: The energy band structures, density of states (DOS) and the optical properties of paraelectric cubic crystal KTaO3 have been calculated by the first principle pseudopotential using density functional theory in its local density approximation. The calculated band structure shows a direct band gap of 2.987 eV at the Γ point in the Brillouin zone. The real and imaginary parts of the dielectric function and hence the optical constants such as eeff (the optical dielectric constant) and Neff (the effective number of electrons) per unit cell are calculated. The calculated spectra are compared with the experimental results for KTaO3 and are found to be in good agreement with the experimental results.
TL;DR: In this article, a new Brillouin diagram is presented that accounts for different types of physical leakage, namely, leakage into one or more surface waves or also simultaneously into space.
Abstract: Dispersion and radiation properties for bound and leaky modes supported by 1-D printed periodic structures are investigated. A new type of Brillouin diagram is presented that accounts for different types of physical leakage, namely, leakage into one or more surface waves or also simultaneously into space. This new Brillouin diagram not only provides a physical insight into the dispersive behavior of such periodic structures, but it also provides a simple and convenient way to correctly choose the integration paths that arise from a spectral-domain moment-method analysis. Numerical results illustrate the usefulness of this new Brillouin diagram in explaining the leakage and stopband behavior for these types of periodic structures.