Showing papers on "Resonance published in 2001"

Probing phonon dispersion relations of graphite by double resonance Raman scattering.

Abstract: The phonon dispersion relations of graphite can be probed over a wide range of the Brillouin zone by double resonance Raman spectroscopy. The double resonance Raman process provides us with new assignments for the dispersive and nondispersive features observed in the Raman spectra of disordered graphite and carbon nanotubes, some features having been incorrectly assigned previously, or not assigned at all.

Plasmon resonances of silver nanowires with a nonregular cross section

Abstract: We investigate numerically the spectrum of plasmon resonances for metallic nanowires with a nonregular cross section, in the 20‐50 nm range. We first consider the resonance spectra corresponding to nanowires whose cross sections form different simplexes. The number of resonances strongly increases when the section symmetry decreases: A cylindrical wire exhibits one resonance, whereas we observe more than five distinct resonances for a triangular particle. The spectral range covered by these different resonances becomes very large, giving to the particle-specific distinct colors. At the resonance, dramatic field enhancement is observed at the vicinity of nonregular particles, where the field amplitude can reach several hundred times that of the illumination field. This near-field enhancement corresponds to surface-enhanced Raman scattering~SERS! enhancement locally in excess of 10 12 . The distance dependence of this enhancement is investigated and we show that it depends on the plasmon resonance excited in the particle, i.e., on the illumination wavelength. The average Raman enhancement for molecules distributed on the entire particle surface is also computed and discussed in the context of experiments in which large numbers of molecules are used.

A theoretical investigation of average H/V ratios

Abstract: SUMMARY The mode summation method and a finite difference technique are applied to investigate the spectral ratio between the horizontal and vertical components (H/V ratio) of ambient vibrations and to explore the variation of the resonance frequency and the amplitude and shape of polarization as a function of the structure and the source positions. Layered structural models are used by assuming a large number of sources distributed around a receiver, with shallow source depths that are randomly assigned. We identify stable parts of the H/V ratios that are independent of the source distance and are dominated by the ellipticity of the fundamental-mode Rayleigh wave in the frequency band between the fundamental frequency of resonance of the unconsolidated sediments and the first minimum of the average H/V ratio. The ellipticity in this frequency band is determined by the layering of the sediments. The numerical simulations are compared with observations at a site where the thickness and velocity structure of the unconsolidated sediments are known from S-wave and surface wave measurements. Two methods are applied to compute the H/V ratio, the classical method in the frequency domain and a method based on frequency–time analysis that allows us to locate P–SV wavelets in the time-series. The main problem in comparing synthetics with observations is the contribution of SH waves in the observed H/V ratios. We propose a method to minimize these effects and the effects of the superposition of different incoming P–SV waves. An inversion scheme is applied to the stable parts of the observed H/V ratio, based on a genetic algorithm, to retrieve the S-wave velocity structure from a single ambient vibration record.

Plasmon resonant coupling in metallic nanowires

Abstract: We investigate the plasmon resonances of interacting silver nanowires with a 50 nm diameter. Both non–touching and intersecting configurations are investigated. While individual cylinders exhibit a single plasmon resonance, we observe much more complex spectra of resonances for interacting structures. The number and magnitude of the different resonances depend on the illumination direction and on the distance between the particles. For very small separations, we observe a dramatic field enhancement between the particles, where the electric field amplitude reaches a hundredfold of the illumination. A similar enhancement is observed in the grooves created in slightly intersecting particles. The topology of these different resonances is related to the induced polarization charges. The implication of these results to surface enhanced Raman scattering (SERS) are discussed.

The Study on Ferromagnetic Resonance Linewidth for NM/80NiFe/NM (NM=Cu, Ta, Pd and Pt) Films

Abstract: The out-of-plane angular dependence of ferromagnetic resonance (FMR) was measured for NM/80NiFe(Py)/NM (NM=Cu, Ta, Pd and Pt) films with various Py, Cu and Ta thicknesses fabricated by magnetron sputtering. The out-of-plane angular dependences of FMR resonance field and linewidth were analyzed using Landau-Lifshitz-Gilbert equation taking account of broadening of linewidth due to magnetic inhomogeneities in a film. Magnetic inhomogeneities were assumed to be the fluctuation of magnitude and direction of the effective demagnetization field which contains both demagnetization and perpendicular anisotropy field for a film. The calculations of the angular variations of linewidth agreed with the experimental ones quantitatively. The fluctuations of magnitude and direction of the effective demagnetization field, which are represented as Δ(4πMeff.) and ΔθH, respectively, increased with decreasing Py thickness for all NM/Py/NM films. ΔθH increased as the thicknesses of the buffer layers increased for Cu/Py(40 A)/Cu films and was almost constant with increasing buffer layer thickness for Ta/Py(40 A)/Ta films. Only in the case of NM=Pd and Pt films, the Gilbert damping parameter, which is the speed of decay of magnetization precession, was enhanced significantly as compared with that for the bulk sample and was dependent on Py thickness.

Long-range surface plasmons for high-resolution surface plasmon resonance sensors

Abstract: We present the application of long-range surface plasmons to a wavelength-modulated surface plasmon resonance sensor. Theoretical design parameters and experimental data are presented for two sensor designs, using either magnesium fluoride or Teflon AF-1600 as a dielectric buffer layer. The demonstrated sensitivity of the long-range surface plasmon resonance sensor in refractometric experiments is up to seven times higher than that of an equivalent conventional surface plasmon resonance (SPR) sensor, while the measured resolution is comparable. According to theoretical design calculations presented, further optimization of materials and layer thickness could reduce the resonance width while achieving even higher sensitivities, thereby creating a sensor with significantly better resolution than conventional SPR sensors.

Controlling the Interaction between Light and Gold Nanoparticles: Selective Suppression of Extinction

Abstract: The interaction of visible light with the particle-plasmon resonance of metallic nanoparticles can be controlled by geometrical arrangement of nanoparticle arrays. These arrays are placed on a substrate that supports guided modes in the wavelength range of the particle plasmon. Coupling of this particle-plasmon resonance to the directly incident light and to the waveguide modes results in almost complete suppression of light extinction within narrow spectral bands due to destructive interference. Variation of the structure parameters allows continuous tuning of these high-transmission bands across the particle-plasmon resonance.

Evolution of the resonance and incommensurate spin fluctuations in superconducting YBa(2)Cu(3)O(6+x)

Abstract: Polarized and unpolarized neutron triple-axis spectrometry was used to study the dynamical magnetic susceptibility chi"(q,omega) as a function of energy ((h) over bar omega) and wave vector (q) in a wide temperature range for the bilayer superconductor YBa(2)Cu(3)O(6+x) with oxygen concentrations, x, of 0.45, 0.5, 0.6, 0.7, 0.8, 0.93, and 0.95, The most prominent features in the magnetic spectra include a spin gap in the superconducting state, a pseudogap in the normal state, the much-discussed resonance, and incommensurate spin fluctuations below the resonance. We establish the doping dependence of the spin gap in the superconducting state, the resonance energy, and the incommensurability of the spin fluctuations. The magnitude of the spin gap (E(sg)) up to the optimal doping is proportional to the superconducting transition temperature T(c) with E(sg)/k(B)T(c)=3.8. The resonance, which exists exclusively below T(c) for highly doped YBa(2)Cu(3)O(6+x) with x = 0.93 and 0.95, appears above T(c) for underdoped compounds with x less than or equal to0.8. The resonance energy (E(r)) also scales with k(B)T(c), but saturates at E(r)approximate to 40 meV for x close to 0.93. The incommensurate spin fluctuations at energies below the resonance have structures similar to that of the single-layer superconducting La(2-x)Sr(x)CuO(4). However, there are also important differences. While the incommensurability (delta) of the spin fluctuations in La(2-x)Sr(x)CuO(4) is proportional to T(c) for the entire hole-doping range up to the optimal value, the incommensurability in YBa(2)Cu(3)O(6+x) increases with T(c) for low-oxygen doping and saturates to delta =0.1 for x greater than or equal to0.6. In addition, the incommensurability decreases with increasing energy close to the resonance. Finally, the incommensurate spin fluctuations appear above T(c) in underdoped compounds with x less than or equal to0.6 but for highly doped materials they are only observed below T(c). We discuss in detail the procedure used for separating the magnetic scattering from phonon and other spurious effects. In the comparison of our experimental results with various microscopic theoretical models, particular emphasis was made to address the similarities and differences in the spin fluctuations of the two most studied superconductors. Finally, we briefly mention recent magnetic-field-dependent studies of the spin fluctuations and discuss their relevance in understanding the microscopic origin of the resonance.

Strength of the electric field in apertureless near-field optical microscopy

Abstract: Enhancement γ of the electrical field at the end of a tip relative to the incident field in a focused radiation beam is calculated by the finite-element time-domain (FETD) method. First, the reliability of the FETD method is established by calculating the electric field on simple structures like thin cylinders, spheres, and ellipsoids, and comparing the results with analytical solutions. The calculations on these test structures also reveal that phase retardation effects substantially modify γ when the size of the structure is larger than approximately λ/4, λ being the radiation wavelength. For plasmon resonance, in particular, phase retardation severely reduces the resonance and the expected field enhancement for a gold tip. The small value of γ=4 calculated by FETD is about an order of magnitude smaller than the value found in recent published work. Resonance effects can be recovered for special tips, which have a discontinuity or a different material composition at the end of the tip. Some tuning of the discontinuity dimension is needed to maximize the resonance. Under optimal conditions for plasmon resonance, an enhancement in the electric field of about 50 is calculated at the end of a small gold protrusion mounted on a wider silicon or glass tip.

Simultaneous liquid viscosity and density determination with piezoelectric unimorph cantilevers

Abstract: We have examined both experimentally and theoretically a piezoelectric unimorph cantilever as a liquid viscosity-and-density sensor. The fabricated piezoelectric unimorph consisted of a PbO⋅ZrO2⋅TiO2 (PZT) layer on a thin stainless-steel plate. In addition to a driving electrode, a sensing electrode was placed on top of the PZT layer, permitting the direct measurement of the resonance frequency. The cantilever was tested using water–glycerol solutions of different compositions. In all three of the tested modes, the resonance frequency decreased while the width of the resonance peak increased with increasing glycerol content. To account for the liquid effect, we consider the cantilever as a sphere of radius R oscillating in a liquid. By including the high and low frequency terms in the induced mass and the damping coefficient of the liquid, we show that for a given liquid density and viscosity the oscillating-sphere model predicts a resonance frequency and peak width that closely agree with experiment. Furthermore, the viscosity and the density of a liquid have been determined simultaneously using the experimentally measured resonance frequency and peak width as inputs to the oscillating-sphere model. The calculated liquid viscosity and density closely agreed with the known values, indicating that our cantilever-based sensor is effective in determining viscosity and density, simultaneously. We also show that scaling analysis predicts an increase in the width of the resonance peak with decreasing cantilever size, an observation in agreement with the large peak widths observed for microcantilevers.

Cavity-quantum electrodynamics using a single InAs quantum dot in a microdisk structure

Abstract: We investigate cavity-quantum electrodynamics (QED) effects in an all-semiconductor nanostructure by tuning a single self-assembled InAs quantum dot (QD) into resonance with a high quality factor microdisk whispering gallery mode (WGM). The stronger temperature dependence of the QD single-exciton (1X) resonance allows us to change the relative energy of the WGM and the 1X transitions by varying the sample temperature. The two coupled resonances exhibit crossing behavior due to the weak coupling cavity-QED regime. We demonstrate exciton lifetime reduction by 6 due to the Purcell effect by tuning the QD into resonance with the WGM. Our experiments also show that single-exciton lifetime is independent of temperature up to 50 K.

A Measurement of the Branching Fraction for the Inclusive B --> X(s) gamma Decays with the Belle Detector

Abstract: We have measured the branching fraction of the inclusive radiative B meson decay B --> X(s) gamma to be Br(B->X(s)gamma)=(3.36 +/- 0.53(stat) +/- 0.42(sys) +0.50-0.54(th)) x 10^{-4}. The result is based on a sample of 6.07 x 10^6 BBbar events collected at the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric e^+e^- storage ring.

Retardation-induced plasmon resonances in coupled nanoparticles.

Abstract: We study the coupling induced by retardation effects when two plasmon-resonant nanoparticles are interacting. This coupling leads to an additional resonance, the strength of which depends on a subtle balance between particle separation and size. The scattering cross section and the near field associated with this coupled resonance are studied for cylindrical particles in air and in water. Implications for surface-enhanced Raman scattering and nano-optics are discussed.

Compact sensor using microcavity structures

Abstract: A compact sensor for detection of chemical and/or biological compounds in low concentration. The sensor comprises electro-magnetic microcavities. The agent to be detected passes the microcavities, is absorbed and/or absorbed by the microcavities, and modifies the electromagnetic field inside the microcavities. After the agent has been adsorbed and/or absorbed, a probe beam is applied to the microcavities. The change of electromagnetic field is detected by the detector, and the frequency of the probe beam at which the resonance is observed, is indicative of a particular agent being present. A method for detecting chemical and/or biological compounds using the sensor.

Local spectroscopy of a Kondo impurity: Co on Au(111)

Abstract: We present a detailed study of the local electronic properties of the Kondo system formed from cobalt adatoms deposited onto Au(111) at a temperature of 6.6 K. Cryogenic scanning-tunneling spectroscopy was used to observe impurity-induced resonances at the Fermi energy and at the Au(111) surface-state band edge. The line shape of the Fermi-energy resonance, identified as a Kondo resonance, is observed to vary with lateral position from the impurity center and with impurity binding position on the reconstructed Au(111) surface. Little vertical dependence is seen in the resonance line shape for positions above the center of the impurity. Interaction effects between Kondo impurities are observed to remain small as cobalt coverage is increased up to 1 ML on the gold surface. The Kondo resonance is shown theoretically to be a member of a general class of Fano resonances arising from the interaction of a discrete impurity state with a conduction-electron continuum. The asymmetric line shape of the resonance thus reflects quantum interference between the d orbital and continuum conduction electron channels, as well as their coupling to the STM tip.

Identifying framework titanium in TS-1 zeolite by UV resonance Raman spectroscopy

Abstract: Framework titanium in Ti-silicalite-1 (TS-1) zeolite was selectively identified by its resonance Raman bands using ultraviolet (UV) Raman spectroscopy. Raman spectra of the TS-1 and silicalite-1 zeolites were obtained and compared using continuous wave laser lines at 244, 325, and 488 nm as the excitation sources. It was only with the excitation at 244 nm that resonance enhanced Raman bands at 490, 530, and 1125 cm-1 appeared exclusively for the TS-1 zeolite. Furthermore, these bands increased in intensity with the crystallization time of the TS-1 zeolite. The Raman bands at 490, 530, and 1125 cm-1 are identified as the framework titanium species because they only appeared when the laser excites the charge-transfer transition of the framework titanium species in the TS-1. No resonance Raman enhancement was detected for the bands of silicalite-1 zeolite and for the band at 960 cm-1 of TS-1 with any of the excitation sources ranging from the visible to UV regions. This approach can be applicable for the ide...

Nonequilibrium theory of scanning tunneling spectroscopy via adsorbate resonances: Nonmagnetic and Kondo impurities

Abstract: We report on a fully nonequilibrium theory of scanning tunneling microscopy (STM) through resonances induced by impurity atoms adsorbed on metal surfaces The theory takes into account the effect of tunneling current and finite bias on the system, and is valid for arbitrary intra-adsorbate electron correlation strength It is thus applicable to recent STM experiments on Kondo impurities We discuss the finite-temperature effects and the consequences of atomic scale resolution of the STM for the spectral property of such systems We find that the tip position affects the resonance line shapes in two ways As a function of the distance from the surface, the line shapes vary due to the different extents of the adsorbate and metal wave functions into the vacuum However, we do not expect large variations in line shapes unless tunneling into the tightly bound adsorbate states is considerable, or nonequilibrium effects are significant As a function of the lateral tip position, line shapes should not change significantly on length scales of ${R}_{\ensuremath{\parallel}}l~10\AA{}$ under typical experimental conditions when the electrons tunnel into the perturbed bulk conduction states hybridized with the outer shell $\mathrm{sp}$ adsorbate orbitals Tunneling into surfaces states on (111) surfaces of noble metals should be important for an observation of resonance at larger distances (g10 \AA{}), and oscillatory variations in the line shape should develop This long-range behavior was not resolved in recent experiments with Kondo impurities The temperature dependence of the Kondo resonance cannot be deduced directly from the differential conductance, as the thermal broadening of the tip Fermi surface produces qualitatively similar effects of comparable and larger magnitudes A careful deconvolution is necessary to extract the temperature dependence of the Kondo resonance The finite-bias current-induced nonequilibrium effects in tunneling through Kondo impurities should produce a characteristic broadening of the resonance in the case of strong hybridization of the discrete state with the STM tip

Sum-Frequency Generation in Chiral Liquids near Electronic Resonance

Abstract: We demonstrate experimentally that visible-visible sum-frequency generation in the bulk of a chiral liquid is observable near electronic resonant transitions. Although the process is electric dipole allowed, it is rather weak because the orientational average over molecules effectively reduces the bulk chiral nonlinearity.

Solvent effects on ground and excited electronic state structures of p-nitroaniline

Abstract: Resonance Raman intensities of p-nitroaniline, a prototypical “push–pull” chromophore with a large first hyperpolarizability (β), have been measured in dilute solution in five solvents having a wide range of polarities (cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol) at excitation wavelengths spanning the strong near-ultraviolet charge-transfer absorption band. The absolute Raman excitation profiles and absorption spectra are simulated using time-dependent wave packet propagation techniques to determine the excited-state geometry changes along the five or six principal Raman-active vibrations as well as estimates of the solvent reorganization energies. The total vibrational reorganization energy decreases and the solvent reorganization energy increases with increasing solvent polarity in all solvents except methanol, where specific hydrogen-bonding interactions may be important. The dimensionless normal coordinate geometry changes obtained from the resonance Raman analysis are conve...

Is strong modal resonance a precursor to power system oscillations

Abstract: We suggest a new mechanism for interarea electric power system oscillations in which two oscillatory modes interact near a strong resonance to cause one of the modes to subsequently become unstable. The possibility of this mechanism for oscillations is shown by theory and computational examples. Theory suggests that passing near strong resonance can be expected in general power system models. The mechanism for oscillations is illustrated in 3- and 9-bus examples with detailed generator models.

Resonance frequency and Q factor mapping by ultrasonic atomic force microscopy

Abstract: We developed an improved ultrasonic atomic force microscopy (UAFM) for mapping resonance frequency and Q factor of a cantilever where the tip is in linear contact with the sample. Since the vibration amplitude at resonance is linearly proportional to the Q factor, the resonance frequency and Q factor are measured in the resonance tracking mode by scanning the sample in the constant force mode. This method enables much faster mapping of the resonance frequency and Q factor than the previous one using a network analyzer. In this letter, we describe the principle and instrumentation of the UAFM and show images of carbon-fiber-reinforced plastic composites.

Ferromagnetic resonance studies of Ni nanowire arrays

Abstract: Using ferromagnetic resonance, the angular dependence of the uniform precession mode of infinite cylinders is investigated at room temperature for low density Ni nanowire arrays, embedded in a polycarbonate membrane, with wire diameters ranging from 35 mn to 500 nm. All wires reveal a very similar behavior of the resonance field vs angle, independent of the wire diameter and wire density, corresponding to the uniform precession mode of an infinite cylinder including the shape demagnetization anisotropy and a small uniaxial anisotropy contribution. From the analysis of the angular dependence of the linewidth, the distribution of the wire orientation and the effective anisotropy field can be estimated. The latter is broadened due to the presence of a sub-structure in the absorption spectra.

Comparison of optical VCSEL models on the simulation of oxide-confined devices

Abstract: We compare the results of different optical vertical-cavity surface-emitting laser models on the position-dependent effects of thin oxide apertures Both scalar and vectorial models as well as hybrid models are considered Physical quantities that are compared are resonance wavelength, threshold material gain, and modal stability For large device diameters and low-order modes, the agreement between the different models is quite good Larger differences occur when considering smaller devices and higher order modes It is also observed that the spread in the resonance wavelengths is smaller than that for the threshold material gain

The ep→e′pη Reaction at and above the S11(1535) Baryon Resonance

Abstract: New cross sections for the reaction e p-->e p eta are reported for total center of mass energy W = 1.5--1.86 GeV and invariant momentum transfer Q2 = 0.25--1.5 (GeV/c)(2). This large kinematic range allows extraction of important new information about response functions, photocouplings, and eta N coupling strengths of baryon resonances. Newly observed structure at W approximately 1.65 GeV is shown to come from interference between S and P waves and can be interpreted with known resonances. Improved values are derived for the photon coupling amplitude for the S11(1535) resonance.

Electronic transition energy E ii for an isolated ( n , m ) single-wall carbon nanotube obtained by anti-Stokes/Stokes resonant Raman intensity ratio

Abstract: A resonant Raman study of the anti-Stokes and Stokes spectra for individual isolated single-wall carbon nanotubes is presented. The observed asymmetry between the anti-Stokes and Stokes spectra is analyzed within the framework of resonant Raman scattering theory, thereby providing a method for accurately determining the transition energy between van Hove singularities ${E}_{\mathrm{ii}}$ in the electronic density of states and unambiguously assigning the $(n,m)$ nanotube indices. Furthermore, resonant Raman theory allows us to determine whether the resonance is with the incident or scattered photon, and to estimate the relative magnitudes of the matrix elements for the G-band and the radial breathing mode Raman processes.

Superheavy hydrogen (5)H.

Abstract: An experimental search for 5H using a secondary beam of 6He was performed The transfer reaction p(6He,2He)5H was studied by detecting two protons emitted from the decay of 2He A peak consistent with the 5H resonance at 17±03 MeV above the n + n + t threshold was observed, with a width of 19±04 MeV The angular distribution of the p(6He,2He)5H reaction was measured as well as the energy correlation of two protons

Infrared spectroscopic study of CuO: Signatures of strong spin-phonon interaction and structural distortion

Abstract: ~Received 24 January 2000; revised manuscript received 26 June 2000; published 2 February 2001! Optical properties of single-crystal monoclinic CuO in the range 70‐6000 cm 21 were studied at temperatures from 7 to 300 K. Normal reflection spectra were obtained from the ~001! and ~010! crystal faces thus giving separate data for the Au and Bu phonon modes excited in the purely transverse way ~TO modes!. Mode parameters, including polarizations of the Bu modes not determined by the crystal symmetry, were extracted by the dispersion analysis of reflectivity curves as a function of temperature. Spectra of all the components of the optical conductivity tensor were obtained using the Kramers-Kronig method recently extended to the case of the low-symmetry crystals. The number of strong phonon modes is in agreement with the factor-group analysis for the crystal structure currently accepted for the CuO. However, several ‘‘extra’’ modes of minor intensity are detected; some of them are observed in the whole studied temperature range, while existence of others becomes evident at low temperatures. Comparison of frequencies of ‘‘extra’’ modes with the available phonon dispersion curves points to possible ‘‘diagonal’’ doubling of the unit cell $a,b,c%!$a1c,b,a-c% and formation of the superlattice. The previously reported softening of the Au mode (;400 cm 21 ) with cooling at TN is found to be ;10% for the TO mode. The mode is very broad at high temperatures and strongly narrows in the antiferromagnetic phase. We attribute this effect to strong resonance coupling of this mode to optical or