Showing papers on "Brillouin zone published in 2002"
TL;DR: In this paper, the band anticrossing model is extended over the entire Brillouin zone to explain the pressure behaviour of the lowest conduction band minimum in GaP1−xNx.
Abstract: In this paper we review the basic theoretical aspects as well as some important experimental results of the band anticrossing effects in highly electronegativity-mismatched semiconductor alloys, such as GaAs1−xNx and InyGa1−yAs1−xNx. The many-impurity Anderson model treated in the coherent potential approximation is applied to these semiconductor alloys, in which metallic anion atoms are partially substituted by a highly electronegative element at low concentrations. Analytical solutions of the Green's function provide dispersion relations and state broadenings for the restructured conduction bands. The solutions also lead to the physically intuitive and widely used two-level band anticrossing model. Significant experimental observations, including large bandgap reduction, great electron effective mass enhancement and unusual pressure behaviour of the bandgap, are compared with the predictions of the band anticrossing model. The band anticrossing model is extended over the entire Brillouin zone to explain the pressure behaviour of the lowest conduction band minimum in GaP1−xNx. Finally, we show that the band anticrossing can also account for the large bandgap bowing parameters observed in GaAsxSb1−x, InAsySb1−y and GaPxSb1−x alloys.
321 citations
TL;DR: In this article, the dispersion properties of dielectric slabs perforated with two-dimensional photonic crystals (PCs) of square symmetry were analyzed, in three dimensions, for all k-vectors in the first Brillouin zone, and not only along the characteristic high symmetry directions.
Abstract: We analyze, in three dimensions, the dispersion properties of dielectric slabs perforated with two-dimensional photonic crystals (PCs) of square symmetry. The band diagrams are calculated for all k-vectors in the first Brillouin zone, and not only along the characteristic high-symmetry directions. We have analyzed the equal-frequency contours of the first two bands, and we found that the square lattice planar photonic crystal is a good candidate for the self-collimation of light beams. We map out the group velocities for the second band of a square lattice planar PC and show that the group velocity is the highest in the region of maximum self-collimation. Such a self-collimated beam is predicted to show beating patterns due to the specific shape of the equal-frequency contours. A geometrical transformation maps the region of the first and second photonic bands where self-collimation takes place one onto the other and gives additional insights on the structural similarities of self-collimation in those two bands.
303 citations
TL;DR: In this article, the Brillouin scattering behavior in single-mode fibers with different waveguide characteristics in terms of their longitudinal mode structures in the gain spectrum, linewidth narrowing, and stimulated SBS threshold levels is investigated.
Abstract: We study the Brillouin scattering behavior in several single-mode fibers with different waveguide characteristics in terms of their longitudinal mode structures in the gain spectrum, linewidth narrowing, and stimulated Brillouin scattering (SBS) threshold levels. Evolution from spontaneous to SBS is investigated by monitoring the Brillouin line-shape and the behavior of the longitudinal acoustooptic resonance modes that exist in the core. We compare our results with the current theory of Brillouin scattering generated from noise in the undepleted pump approximation. We also present experimentally Brillouin gain spectra in the highly depleted pump regime, where there is no analytical solution, by showing the evolution of the gain spectrum as a function of the injected laser intensity.
265 citations
TL;DR: Angle-resolved photoemission is employed to measure the band structure of TiSe2 in order to clarify the nature of the charge density wave transition, showing a very small indirect gap in the normal phase transforming into a larger indirect gap at a different location in the Brillouin zone.
Abstract: Angle-resolved photoemission is employed to measure the band structure of TiSe2 in order to clarify the nature of the ( 2 x 2 x 2) charge density wave transition. The results show a very small indirect gap in the normal phase transforming into a larger indirect gap at a different location in the Brillouin zone. Fermi surface topology is irrelevant in this case. Instead, electron-hole coupling together with a novel indirect Jahn-Teller effect drives the transition.
177 citations
TL;DR: In this paper, the elastic constants of PbTe and other Pb chalcogenide compounds (PbSe, PbS) were calculated self-consistently using the scalar-relativistic full-potential linearized augmented plane wave method.
146 citations
TL;DR: The Fourier spectra of longitudinal and transverse waves corresponding to random particle motion were measured in a two-dimensional plasma crystal and the phonons were found to obey a dispersion relation that assumes a Yukawa interparticle potential.
Abstract: The Fourier spectra of longitudinal and transverse waves corresponding to random particle motion were measured in a two-dimensional plasma crystal. The crystal was composed of negatively charged microspheres immersed in a plasma at a low gas pressure. The phonons were found to obey a dispersion relation that assumes a Yukawa interparticle potential. The crystal was in a nonthermal equilibrium, nevertheless phonon energies were almost equally distributed with respect to wave number over the entire first Brillouin zone.
143 citations
TL;DR: In this article, the authors theoretically investigated the performance of the photonic crystal superprism, that is, the propagating beam quality, the wavelength sensitivity, and the resolution as a narrow band filter at 1.5μm-wavelength range.
Abstract: We theoretically investigated the performance of the photonic crystal superprism, that is, the propagating beam quality, the wavelength sensitivity, and the resolution as a narrow band filter at 1.5-μm-wavelength range. First, we defined the equi-incident-angle curve in the Brillouin zone. Next, we mapped three parameters that represented the abovementioned performance over the Brillouin zone. As a result, we found a narrow design window that allows a high resolution of 0.4 nm along an equi-incident-angle curve but requires an incident beam width of over 100 μm and a device length of centimeter order. It can be an essential high efficiency filter if the input end of the crystal is optimized and the propagation loss is suppressed.
132 citations
TL;DR: In this article, the effect of resonant interface states on the conductance of epitaxial tunnel junctions is discussed based on model and ab initio calculations of symmetrical barriers of the Brillouin zone.
Abstract: Based on model and ab initio calculations we discuss the effect of resonant interface states on the conductance of epitaxial tunnel junctions. In particular we show that the ``hot spots'' found by several groups in ab initio calculations of symmetrical barriers of the ${\mathbf{k}}_{\ensuremath{\Vert}}$-resolved conductance can be explained by the formation of bonding and antibonding hybrids between the interface states on both sides of the barrier. If the resonance condition for these hybrid states is met, the electron tunnels through the barrier without attenuation. Even when both hybrid states move together and form a single resonance, strongly enhanced transmission is still observed. The effect explains why, for intermediate barrier thicknesses, the tunneling conductance can be dominated by interface states, although hot spots only occur in a tiny fraction of the surface Brillouin zone.
128 citations
TL;DR: In this article, the authors show that the phenomenon of modulational instability in arrays of Bose-Einstein condensates confined to optical lattices gives rise to coherent spatial structures of localized excitations.
Abstract: We show that the phenomenon of modulational instability in arrays of Bose–Einstein condensates confined to optical lattices gives rise to coherent spatial structures of localized excitations. These excitations represent thin discs in 1D, narrow tubes in 2D, and small hollows in 3D arrays, filled in with condensed atoms of much greater density compared to surrounding array sites. Aspects of the developed pattern depend on the initial distribution function of the condensate over the optical lattice, corresponding to particular points of the Brillouin zone. The long-time behaviour of the spatial structures emerging due to modulational instability is characterized by the periodic recurrence to the initial low-density state in a finite optical lattice. We propose a simple way to retain the localized spatial structures with high atomic concentration, which may be of interest for applications. A theoretical model, based on the multiple-scale expansion, describes the basic features of the phenomenon. Results of numerical simulations confirm the analytical predictions.
123 citations
TL;DR: The second-order elastic constants of CaF2 (fluorite) have been determined by Brillouin scattering to 9.3 GPa at 300 K as discussed by the authors.
Abstract: The second-order elastic constants of CaF2 (fluorite) have been determined by Brillouin scattering to 9.3 GPa at 300 K. Acoustic velocities have been measured in the (111) plane and inverted to simultaneously obtain the elastic constants and the orientation of the crystal. A notable feature of the present inversion is that only the density at ambient condition was used in the inversion. We obtain high-pressure densities directly from Brillouin data by conversion to isothermal conditions and iterative integration of the compression curve. The pressure derivative of the isentropic bulk modulus and of the shear modulus determined in this study are 4.78 ± 0.13 and 1.08 ± 0.07, which differ from previous low-pressure ultrasonic elasticity measurements. The pressure derivative of the isothermal bulk modulus is 4.83 ± 0.13, 8% lower than the value from static compression, and its uncertainty is lower by a factor of 3. The elastic constants of fluorite increase almost linearly with pressure over the whole investigated pressure range. However, at P ≥ 9 GPa, C
11 and C
12 show a subtle structure in their pressure dependence while C
44 does not. The behavior of the elastic constants of fluorite in the 9–9.3 GPa pressure range is probably affected by the onset of a high-pressure structural transition to a lower symmetry phase (α-PbCl2 type). A single-crystal Raman scattering experiment performed in parallel to the Brillouin measurements shows the appearance of new features at 8.7 GPa. The new features are continuously observed to 49.2 GPa, confirming that the orthorhombic high-pressure phase is stable along the whole investigated pressure range, in agreement with a previous X-ray diffraction study of CaF2 to 45 GPa. The high-pressure elasticity data in combination with room-pressure values from previous studies allowed us to determine an independent room-temperature compression curve of fluorite. The new compression curve yields a maximum discrepancy of 0.05 GPa at 9.5 GPa with respect to that derived from static compression by Angel (1993). This comparison suggests that the accuracy of the fluorite pressure scale is better than 1% over the 0–9 GPa pressure range.
111 citations
TL;DR: The gap enhancement in the vicinity of the hot spots emphasizes the role of antiferromagnetic fluctuations and the similarity in the origin of superconductivity for electron- and hole-doped cuprates.
Abstract: Low energy polarized electronic Raman scattering of the electron-doped superconductor Nd2-x Ce x CuO4 ( x = 0.15, T(c) = 22 K) has revealed a nonmonotonic d(x(2)-y(2)) superconducting order parameter. It has a maximum gap of 4.4k(B)T(c) at Fermi surface intersections with an antiferromagnetic Brillouin zone (the "hot spots") and a smaller gap of 3.3k(B)T(c) at fermionic Brillouin zone boundaries. The gap enhancement in the vicinity of the hot spots emphasizes the role of antiferromagnetic fluctuations and the similarity in the origin of superconductivity for electron- and hole-doped cuprates.
TL;DR: In this paper, a new set of 12 force constants of 2D graphite up to the fourth neighbor are determined by a self-consistent fitting procedure, combined with double-resonance Raman theory.
Abstract: Phonon dispersion relations of a two-dimensional (2D) graphite are obtained by fitting dispersive Raman modes that originate from nonzone center phonons near the $\ensuremath{\Gamma}$ or K point in the Brillouin zone (BZ). A new set of 12 force constants of 2D graphite up to the fourth neighbor are determined by a self-consistent fitting procedure, combined with double-resonance Raman theory. Analytical expressions for eigenvalues and eigenvectors at high symmetry points of the BZ are presented.
TL;DR: In this paper, the authors investigated analytically and numerically Bloch waves for a Bose-Einstein condensate in a sinusoidal external potential and showed that at low densities the dependence of the energy on the quasimomentum is similar to that for a single particle, but at densities greater than a critical one the lowest band becomes triple valued near the boundary of the first Brillouin zone and develops the structure characteristic of the swallowtail catastrophe.
Abstract: We investigate analytically and numerically Bloch waves for a Bose-Einstein condensate in a sinusoidal external potential. At low densities the dependence of the energy on the quasimomentum is similar to that for a single particle, but at densities greater than a critical one the lowest band becomes triple valued near the boundary of the first Brillouin zone and develops the structure characteristic of the swallowtail catastrophe. We comment on the experimental consequences of this behavior.
TL;DR: It is clearly demonstrated that stimulated Brillouin scattering is directly responsible for both fast transient dynamics of the laser and reduction of the lasers' pulse width.
Abstract: We have designed and performed an experiment that permitted direct observation of Brillouin backscattering in an Yb-doped double-clad fiber laser. Fifteen Brillouin-shifted frequencies were observed for the first time to our knowledge. We clearly demonstrate that stimulated Brillouin scattering is directly responsible for both fast transient dynamics of the laser and reduction of the laser's pulse width.
TL;DR: In this paper, the photonic bands of a 2D lattice patterned in a planar waveguide are calculated by expanding the magnetic field on the basis of waveguide modes, yielding both the truly guided modes of the structure as well as the quasi-guided modes (or guided resonances) which lie above the light line in the first Brillouin zone.
Abstract: The photonic bands of a two-dimensional (2-D) lattice patterned in a planar waveguide are calculated by expanding the magnetic field on the basis of waveguide modes. The method yields both the truly guided modes of the structure as well as the quasi-guided modes (or guided resonances) which lie above the light line in the first Brillouin zone. Representative results for the photonic bands are shown in the cases of strong- and weak-confinement waveguides patterned with a triangular lattice of holes. The gap maps as a function of hole radius are calculated and show significant differences with respect to the ideal 2-D case. A comparison of the photonic bands with those extracted from the calculated surface reflectance shows very good agreement, thereby indicating the reliability of the approach.
TL;DR: The extended effective length for the relevant nonlinear processes from the distributed Raman gain and the reduced Brillouin threshold from the seeding effect of Rayleigh backscattered waves are considered to be the key factors that explain the operation of this structure.
Abstract: We investigate the dynamics of a novel multiwavelength generator in which cascaded-stimulated Brillouin scattering and Rayleigh scattering are automatically balanced to given an evenly spaced (9.4-GHz), highly flattened (<3-dB) optical frequency comb over a 57.2-nm span. The extended effective length for the relevant nonlinear processes from the distributed Raman gain and the reduced Brillouin threshold from the seeding effect of Rayleigh backscattered waves are considered to be the key factors that explain the operation of this structure.
TL;DR: In this paper, a quantitative analysis of the phonon line shapes obtained by neutron inelastic scattering methods in the relaxor ferroelectric Pb(Mg 1 / 3 Nb 2 / 3 )O 3 (PMN) was presented.
Abstract: We present a quantitative analysis of the phonon line shapes obtained by neutron inelastic scattering methods in the relaxor ferroelectric Pb(Mg 1 / 3 Nb 2 / 3 )O 3 (PMN). Differences in the shapes and apparent positions ofthe transverse acoustic- (TA) and transverse optic- (TO) phonon peaks measured in the (300) and (200) Brillouin zones at 690 K are well described by a simple model that couples the TA and soft TO modes in which the primary parameter is the wave vector and temperature-dependent TO linewidth Γ(q,T). This mode-coupling picture provides a natural explanation for the uniform displacements of the polar nanoregions (PNR's), discovered by Hirota et al. as the PNR result from the condensation of a soft TO mode that also contains a large acoustic component.
Journal Article•
TL;DR: In this article, the electron-doped superconductor Nd 2 - x Ce x CuO 4 (x = 0.15, T c = 22 K) has revealed a nonmonotonic d x 2 - y 2 superconducting order parameter.
Abstract: Low energy polarized electronic Raman scattering of the electron-doped superconductor Nd 2 - x Ce x CuO 4 (x = 0.15, T c = 22 K) has revealed a nonmonotonic d x 2 - y 2 superconducting order parameter. It has a maximum gap of 4.4k B T c at Fermi surface intersections with an antiferromagnetic Brillouin zone (the "hot spots") and a smaller gap of 3.3k B T c at fermionic Brillouin zone boundaries. The gap enhancement in the vicinity of the hot spots emphasizes the role of antiferromagnetic fluctuations and the similarity in the origin of superconductivity for electron- and hole-doped cuprates.
TL;DR: In this paper, infrared and Raman phonon modes in the normal cubic MgAl2O4 spinel are calculated at the first Brillouin zone center using density functional theory with plane-wave basis and norm-conserving pseudopotentials.
Abstract: Ab initio infrared and Raman phonon modes in the normal cubic MgAl2O4 spinel are calculated at the first Brillouin zone centre using density functional theory with plane-wave basis and norm-conserving pseudopotentials. Good agreement with available experimental infrared and Raman spectra is found for both natural and synthetic spinel. A new set of infrared measurements on synthetic spinels shows extra vibrational infrared modes which are not predicted by calculations on normal spinel. The ionic character of spinels is illustrated by comparison between calculated and experimental Born effective charges.
TL;DR: In this paper, the authors introduce the fundamentals of optical carrier Brillouin processing and discuss its applications to the field of microwave photonics, which is based on the control of the amplitude as well as the phase of the optical carrier of a modulated lightwave signal using the stimulated BrillouIN scattering effect.
TL;DR: In this paper, a detailed lattice dynamics study involving experimental Raman scattering measurements and theoretical rigid ion model calculations of the rare earth aluminum garnets (RE 3 Al 5 O 12 ).
Abstract: This paper reports detailed lattice dynamics studies involving experimental Raman scattering measurements and theoretical rigid ion model calculations of the rare earth aluminum garnets (RE 3 Al 5 O 12 ). The studies are fairly involved as these garnets have complex crystal structure with 80 atoms/primitive cell. Our calculations have provided a theoretical understanding of the mode eigenvectors, phonon dispersion relations, density of states, and effective charges of these materials. The calculated Raman mode eigenvectors reveal that they correspond to mixtures of molecular modes of the basic polyhedra, implying thus strongly coupled polyhedra. The assignment of the Raman and infrared active modes has been elucidated and the frequencies of some modes, which do not appear in the vibrational spectrum, have been calculated. Our calculations show a differentiation of the effective charges at the various symmetry sites, which leads to a mostly covalent and to an almost ionic character for the tetrahedral Al-O and the dodecahedral RE-O bonds, respectively. Finally, the dispersion curves along the [100] direction of the Brillouin zone as well as the one and two-phonon density of states have been calculated and discussed.
TL;DR: The results point to the potential utility of momentum-resolved inelastic x-ray scattering in providing valuable information about electronic structure of strongly correlated insulators.
Abstract: We report momentum-resolved charge excitations in a one-dimensional (ID) Mott insulator studied using high resolution inelastic x-ray scattering over the entire Brillouin zone for the first time. Excitations at the insulating gap edge are found to be highly dispersive (momentum dependent) compared to excitations observed in two-dimensional Mott insulators. The observed dispersion in ID cuprates (SrCuO 2 and Sr 2 CuO 3 ) is consistent with charge excitations involving holons which is unique to spin-½ quantum chain systems. These results point to the potential utility of momentum-resolved inelastic x-ray scattering in providing valuable information about electronic structure of strongly correlated insulators.
TL;DR: In this article, the disorder-induced D-band and some other non-zone center Raman modes of graphite and single wall carbon nanotubes are assigned to phonon modes in their respective Brillouin zones.
Abstract: The disorder-induced D-band and some other non-zone center Raman modes of graphite and single wall carbon nanotubes are assigned to phonon modes in their respective Brillouin zones. In disordered graphite, the weak, dispersive phonon modes, which have been known but never assigned so far, are well described by the double resonance Raman process. All weak Raman peaks observed for sp2 carbons are useful for determining the phonon dispersion relations of graphite. In carbon nanotubes, all semiconducting nanotubes and some metallic nanotubes have van Hove singular k points for their electronic and phonon energy dispersion curves at the Γ point of the Brillouin zone. A corresponding Raman process is relevant to explain the observed D-band and intermediate frequency spectra.
TL;DR: In this paper, the SBS threshold energy, and damage threshold were measured in multimode glass fibers with core diameters between 25 and 200 μm, and high SBS reflectivities over 80% were obtained with 30 ns laser pulses at 1.06 μm wavelength.
TL;DR: It is shown that the discrete diffraction properties of a nonlinear optical zigzag waveguide array can be significantly modified, by exploiting the topological arrangement of the lattice itself, to generate altogether different families of discrete soliton solutions, which are stable over a wide range of parameters.
Abstract: We show that the discrete diffraction properties of a nonlinear optical zigzag waveguide array can be significantly modified, by exploiting the topological arrangement of the lattice itself. This introduces extended interactions (beyond nearest neighbors), which, in turn, affect the lattice dispersion relation within the Brillouin zone. As a result of this band alteration, we demonstrate that altogether different families of discrete soliton solutions are possible, which are stable over a wide range of parameters. In the regime where instabilities occur, all scenarios are considered in detail. By appropriately engineering the geometrical configuration of the array we find both standing and traveling diffraction-free beams. Our method opens opportunities for diffraction management that can be employed to generate low-power spatial discrete optical solitons.
TL;DR: In this paper, an experimental and theoretical investigation of phonon dispersion relations in zinc blende (3C) SiC is presented. But the experimental data were obtained for the entire Brillouin zone by inelastic x-ray scattering (IXS) using a synchrotron radiation source.
Abstract: We present an experimental and theoretical investigation of the phonon dispersion relations in zinc blende (3C) SiC. The experimental data were obtained for the entire Brillouin zone by inelastic x-ray scattering (IXS) using a synchrotron radiation source. Eigenvector analysis is performed with the aid of state-of-the-art linear response first principles calculations based on density functional theory. The theoretical predictions reproduce the experimental phonon dispersion remarkably well. These results are compared with data obtained previously for the 〈111〉 direction by Raman spectroscopy using several SiC polytypes and the backfolding technique. IXS data for 4H modification along the c axis are also reported.
TL;DR: Using the standard tight binding model of 2D graphite with short range electron repulsion, a gapless spin-1, neutral collective mode branch below the particle-hole continuum with energy vanishing linearly with momenta at the Gamma and K points in the Brillouin zone is predicted.
Abstract: Using the standard tight binding model of 2D graphite with short range electron repulsion, we predict a gapless spin-1, neutral collective mode branch below the particle-hole continuum with energy vanishing linearly with momenta at the Gamma and K points in the Brillouin zone This spin-1 mode has a wide energy dispersion, 0 to approximately 2 eV, and is not Landau damped The "Dirac cone spectrum" of electrons at the chemical potential of graphite generates our collective mode, so we call this "spin-1 zero sound" of the "Dirac sea" Epithermal neutron scattering experiments and spin polarized electron energy loss spectroscopy can be used to confirm and study our collective mode
TL;DR: In this article, the dispersion relation for surface-plasmon polaritons propagating across an interface was obtained by using the homogeneous form of the reduced Rayleigh equation for the electric field above and on the rough surface of a metal in contact with a vacuum.
Abstract: By the use of the homogeneous form of the reduced Rayleigh equation for the electric field above and on the rough surface of a metal in contact with a vacuum, we have obtained the dispersion relation for surface-plasmon polaritons propagating across such an interface. This dispersion relation is exact within the domain of validity of the Rayleigh hypothesis upon which it is based. It is applied to a two-dimensional vacuum-metal interface formed from a periodic array of hemiellipsoids on the planar surface of the same metal. Nonperturbative numerical solutions of the resulting dispersion relation are obtained for wave vectors along the boundary of the irreducible element of the first Brillouin zone for a square and a triangular array of the hemiellipsoids. Absolute band gaps in the frequency spectrum, where the density of states vanishes, are found, and the dependence of the position and width of these gaps on various geometrical parameters of each array is investigated. We also study the influence of radiative damping and ohmic losses on the dispersion curves. Our theoretical results are compared with existing experimental data.
TL;DR: In this article, Brillouin light scattering and surface acoustic wave spectroscopy are used to measure density, porosity and stiffness properties of nanoporous methylsilsesquioxane films of low-k value.
Abstract: Thin porous films with nanometer pore sizes are the subject of intense interest, primarily because of their reduced dielectric constant k. The lack of useful characterization tools and the reduction in film mechanical properties with increasing porosity have severely hindered their development and application. We show that both Brillouin light scattering and surface acoustic wave spectroscopy allow one to measure density, porosity and stiffness properties of nanoporous methylsilsesquioxane films of low-k value. Excellent correlations are observed among independent measurements of density, porosity and the Young’s modulus which show that the results obtained are reliable and reveal properties of the films which are difficult or impossible to obtain using other techniques.
TL;DR: In this paper, the authors present theoretical studies of the spin excitations in ultrathin free standing Fe(110) films, and such films adsorbed on a semi-infinite W(110)-surface.
Abstract: We present theoretical studies of the nature of the spin excitations in ultrathin free standing Fe(110) films, and such films adsorbed on a semi-infinite W(110) substrate. The calculations are carried out within the framework of an itinerant electron theory, with a realistic underlying electronic structure. The energy bands of the film and substrate are described by a nine band empirical tight binding picture, to include the relevant d bands and overlapping sp complex. Ferromagnetism in the Fe film is driven by on site Coulomb interactions between the $3d$ electrons, treated in mean field theory while a description of the spin wave excitations is generated through use of the random phase approximation. Results are reported and discussed for Fe films three, four, and five layers in thickness. A principal conclusion which emerges from these studies is that the ``frozen magnon'' or adiabatic description of spin wave excitations proves inadequate in a qualitative sense, for systems such as those studied here. The spin waves are embedded in the Stoner continuum, with the consequence they are heavily damped by decay to Stoner excitations, save for the lowest lying mode at long wavelengths. A consequence is that throughout much of the surface Brillouin zone, the spectrum of spin excitations at each wave vector consists of a broad feature which shows dispersion, with no evidence of the sequence of standing spin wave modes predicted by a theory based on the adiabatic approximation. For the five layer film, even near the center of the surface Brillouin zone, while we find a weakly damped low lying acoustic spin wave, the first standing wave is quite broad, and the second standing wave structure is so broad it cannot be viewed as a well defined excitation. At large wave vectors, we find a single broad feature in the spectral density which shows dispersion, very similar to a spin wave, rather than a sequence of standing modes. The results are very similar in character to recent SPEELS data on the Co/Cu system. The physical reasons for the breakdown of the adiabatic method are discussed.