Showing papers on "Resonance published in 2004"

Condensation of pairs of fermionic atoms near a Feshbach resonance.

Abstract: We have observed Bose-Einstein condensation of pairs of fermionic atoms in an ultracold $^{6}\mathrm{Li}$ gas at magnetic fields above a Feshbach resonance, where no stable $^{6}\mathrm{Li}_{2}$ molecules would exist in vacuum. We accurately determined the position of the resonance to be $822\ifmmode\pm\else\textpm\fi{}3\text{ }\text{ }\mathrm{G}$. Molecular Bose-Einstein condensates were detected after a fast magnetic field ramp, which transferred pairs of atoms at close distances into bound molecules. Condensate fractions as high as 80% were obtained. The large condensate fractions are interpreted in terms of preexisting molecules which are quasistable even above the two-body Feshbach resonance due to the presence of the degenerate Fermi gas.

Mechanism of stochastic resonance

Abstract: Stochastic resonance in an overdamped oscillator is considered theoretically. It has been shown that a seeming resonance is actually caused by a noise-induced change in the effective stiffness and damping factor with respect to a signal. For a certain noise intensity, the effective stiffness is minimal, which leads to a nonmonotonic variation of the output-signal amplitude as a function of noise intensity. It is substantial that the position of the minimum of the effective stiffness and its value depend strongly on the signal frequency. The results are compared with similar processes for vibrational resonance. Considerable differences between these phenomena are indicated.

Coupled-Resonator-Induced Transparency

Abstract: We demonstrate that a cancellation of absorption occurs on resonance for two (or any even number of) coupled optical resonators, due to mode splitting and classical destructive interference, particularly when the resonator finesse is large and the loss in the resonator farthest from the excitation waveguide is small. The linewidth and group velocity of a collection of such coupled-resonator structures may be decreased by using larger resonators of equal size, by using larger resonators of unequal size where the optical path length of the larger resonator is an integer multiple of that of the smaller one, or by using a larger number of resonators per structure. We explore the analogy between these effects and electromagnetically-induced transparency in an atomic system.

DNA strand breaks induced by 0-4 eV electrons: the role of shape resonances.

Abstract: Collisions of 0--4 eV electrons with thin DNA films are shown to produce single strand breaks. The yield is sharply structured as a function of electron energy and indicates the involvement of ${\ensuremath{\pi}}^{*}$ shape resonances in the bond breaking process. The cross sections are comparable in magnitude to those observed in other compounds in the gas phase in which ${\ensuremath{\pi}}^{*}$ electrons are transferred through the molecule to break a remote bond. The results therefore support aspects of a theoretical study by Barrios et al. [J. Phys. B 106, 7991 (2002)] indicating that such a mechanism could produce strand breaks in DNA.

Elastic modulus and resonance behavior of boron nitride nanotubes

Abstract: The effective elastic modulus (E) of boron nitride nanotubes (BNNTs) was measured using the electric-field-induced resonance method inside a transmission electron microscope. The average value of E from the measurements of 18 individually cantilevered BNNTs was 722 GPa, comparable to the theoretical estimate of ∼850 GPa. No strong variation of E with the diameter of the BNNTs, which spanned from 34 to 94 nm, was observed. Low quality factors (<680) obtained from the mechanical resonance is attributed to the layered structural nature of BNNT.

Resonant leaky-mode spectral-band engineering and device applications

Abstract: Single-layer subwavelength periodic waveguide films with binary profiles are applied to design numerous passive guided-mode resonance elements. It is shown that the grating profile critically influences the spectral characteristics of such devices. In particular, the symmetry of the profile controls the resonance spectral density. Symmetric profiles generate a single resonance on either side of the second stopband whereas two resonances arise, one on each side of the band, for asymmetric structures. Moreover, the profile's Fourier harmonic content, along with the absolute value of the grating modulation strength, affects the resonance linewidths and their relative locations. Computed Brillouin diagrams are presented to illustrate many key properties of the resonant leaky-mode spectra in relation to modulation strength and profile symmetry at the second stopband. Associated mode plots elucidate the spatial distribution of the leaky-mode field amplitude at resonance and show that, for small modulation, the mode shape may be simple whereas at higher modulation, the shape appears as a complex mixture of modes. By computing device spectra as function of the modulation strength, the buildup of the final spectral properties is illustrated and the contributions of the various leaky modes clarified. The results presented include wavelength and angular spectra for several example devices including narrow linewidth bandpass filters with extended low sidebands for TE and TM polarization, wideband reflectors for TE and TM polarization, polarizer, polarization-independent element, and a wideband antireflector, all with only a single binary layer with one-dimensional periodicity. These results demonstrate new dimensions in optical device design and may provide complementary capability with the field of thin-film optics.

Inelastic electron tunneling via molecular vibrations in single-molecule transistors.

Abstract: In single-molecule transistors, we observe inelastic cotunneling features that correspond energetically to vibrational excitations of the molecule, as determined by Raman and infrared spectroscopy. This is a form of inelastic electron tunneling spectroscopy of single molecules, with the transistor geometry allowing in situ tuning of the electronic states via a gate electrode. The vibrational features shift and change shape as the electronic levels are tuned near resonance, indicating significant modification of the vibrational states. When the molecule contains an unpaired electron, we also observe vibrational satellite features around the Kondo resonance.

Unshielded three-axis vector operation of a spin-exchange-relaxation-free atomic magnetometer

Abstract: We describe a vector alkali–metal magnetometer that simultaneously and independently measures all three components of the magnetic field. Using a feedback system, the total field at the location of the magnetometer is kept near zero, suppressing the broadening due to spin-exchange collisions. The resonance linewidth and signal strength of the magnetometer compare favorably with two different scalar operation modes in which spin-exchange relaxation is only partially suppressed. Magnetic field sensitivity on the order of 1pT∕Hz is demonstrated in a laboratory environment without magnetic shields.

Observation of heteronuclear Feshbach resonances in a mixture of bosons and fermions.

Abstract: Three magnetic-field induced heteronuclear Feshbach resonances were identified in collisions between bosonic 87Rb and fermionic 40K atoms in their absolute ground states. Strong inelastic loss from an optically trapped mixture was observed at the resonance positions of 492, 512, and 543+/-2 G. The magnetic-field locations of these resonances place a tight constraint on the triplet and singlet cross-species scattering lengths, yielding (-281+/-15)a(0) and (-54+/-12)a(0), respectively. The width of the loss feature at 543 G is 3.7+/-1.5 G wide; this broad Feshbach resonance should enable experimental control of the interspecies interactions.

Fano resonance in a quantum wire with a side-coupled quantum dot

Abstract: We report a transport experiment on the Fano effect in a quantum connecting wire with a side-coupled quantum dot (QD). The Fano resonance occurs between the QD and the ``T-shaped'' junction in the wire, and the transport detects antiresonance or a forward-scattered part of the wave function. While it is more difficult to tune the shape of the resonance in this geometry than in the previously reported Aharonov-Bohm-ring-type interferometer, the resonance purely consists of the coherent part of transport. Utilizing this advantage, we have quantitatively analyzed the temperature dependence of the Fano effect by including the thermal broadening and the decoherence. We have also proven that this geometry can be a useful interferometer for measuring the phase evolution of electrons at a QD.

Collective and individual plasmon resonances in nanoparticle films obtained by spin-assisted layer-by-layer assembly.

Abstract: Nanoscale uniform films containing gold nanoparticle and polyelectrolyte multilayer structures were fabricated by the using spin-assembly or spin-assisted layer-by-layer (SA-LbL) deposition technique. These SA-LbL films with a general formula [Au/(PAH-PSS)nPAH]m possessed a well-organized microstructure with uniform surface morphology and high surface quality at a large scale (tens of micrometers across). Plasmon resonance peaks from isolated nanoparticles and interparticle interactions were revealed in the UV-visible extinction spectra of the SA-LbL films. All films showed the strong extinction peak in the region of 510-550 nm, which is due to the plasmon resonance of the individual gold nanoparticles redshifted because of a local dielectric environment. For films with sufficient density of gold nanoparticles within the layers, the second strong peak was consistently observed between 620 and 660 nm, which is the collective plasmon resonance from intralayer interparticle coupling. Finally, we suggested that, for certain film designs, interlayer interparticle resonance might be revealed as an independent contribution at 800 nm in UV-visible spectra. The observation of independent and concurrent individual, intralayer, and interlayer plasmon resonances can be critical for sensing applications, which involve monitoring of optomechanical properties of ultrathin optically active compliant membranes.

Observation of Molecules Produced from a Bose-Einstein Condensate

Abstract: Molecules are created from a Bose-Einstein condensate of atomic $^{87}\mathrm{R}\mathrm{b}$ using a Feshbach resonance. A Stern-Gerlach field is applied, in order to spatially separate the molecules from the remaining atoms. For detection, the molecules are converted back into atoms, again using the Feshbach resonance. The measured position of the molecules yields their magnetic moment. This quantity strongly depends on the magnetic field, thus revealing an avoided crossing of two bound states at a field value slightly below the Feshbach resonance. This avoided crossing is exploited to trap the molecules in one dimension.

Measuring the uniaxial strain of individual single-wall carbon nanotubes: resonance Raman spectra of atomic-force-microscope modified single-wall nanotubes.

Abstract: Raman spectroscopy is used to measure the strain in individual single-wall carbon nanotubes, strained by manipulation with an atomic-force-microscope tip. Under strains varying from 0.06%--1.65%, the in-plane vibrational mode frequencies are lowered by as much as 1.5% ($40\text{ }\text{ }{\mathrm{c}\mathrm{m}}^{\ensuremath{-}1}$), while the radial breathing mode (RBM) remains unchanged. The RBM Stokes/anti-Stokes intensity ratio remains unchanged under strain. The elasticity of these strain deformations is demonstrated as the down-shifted Raman modes resume their prestrain frequencies after a nanotube is broken under excessive strain.

Perturbation approach to resonance shift of whispering gallery modes in a dielectric microsphere as a probe of a surrounding medium

Abstract: A first-order perturbation theory similar to the one widely used in quantum mechanics is developed for transverse-electric and transverse-magnetic photonic resonance modes in a dielectric microsphere. General formulas for the resonance frequency shifts in response to a small change in the exterior refractive index and its radial profile are derived. The formulas are applied to two sensor applications of the microsphere to probe the medium in which the sphere is immersed: a refractive index detector; and a refractive index profile sensor.

Microwave magnetoelectric effects in single crystal bilayers of yttrium iron garnet and lead magnesium niobate-lead titanate

Abstract: The first observation of microwave magnetoelectric (ME) interactions through ferromagnetic resonance (FMR) in bilayers of single crystal ferromagnetic-piezoelectric oxides and a theoretical model for the effect are presented. An electric field $E$ produces a mechanical deformation in the piezoelectric phase, resulting in a shift $\ensuremath{\delta}{H}_{E}$ in the resonance field for the ferromagnet. The strength of ME coupling is obtained from data on $\ensuremath{\delta}{H}_{E}$ vs $E$. Studies were performed at $9.3\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$ on bilayers of (111) yttrium iron garnet (YIG) films and (001) lead magnesium niobate-lead titanate (PMN-PT). The samples were positioned outside a ${\mathrm{TE}}_{102}$-reflection type cavity. Resonance profiles were obtained for $E=0\char21{}8\phantom{\rule{0.3em}{0ex}}\mathrm{kV}∕\mathrm{cm}$ for both in-plane and out-of-plane magnetic fields $H$. Important results are as follows. (i) The ME coupling in the bilayers is an order of magnitude stronger than in polycrystalline composites and is in the range $1\char21{}5.4\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}\phantom{\rule{0.3em}{0ex}}\mathrm{cm}∕\mathrm{kOe}$, depending on the YIG film thickness. (ii) The coupling strength is dependent on the magnetic field orientation and is higher for out-of-plane $H$ than for in-plane $H$. (iii) Estimated ME constant and its dependence on volume ratio for the two phases are in good agreement with the data.

Three-boson problem near a narrow Feshbach resonance.

Abstract: We consider a three-boson system with resonant binary interactions and show that for sufficiently narrow resonances three-body observables depend only on the resonance width and the scattering length. The effect of narrow resonances is qualitatively different from that of wide resonances revealing novel physics of three-body collisions. We calculate the rate of three-body recombination to a weakly bound level and the atom-dimer scattering length and discuss implications for experiments on Bose-Einstein condensates and atom-molecule mixtures near Feshbach resonances.

Spin wave contributions to the high-frequency magnetic response of thin films obtained with inductive methods

Abstract: The high-frequency magnetic response of Permalloy thin films have been measured using network-analyzer ferromagnetic resonance. We demonstrate that the excitation of spin waves by the coplanar wave-guide modify the magnetic response appreciably, in particular, by causing a frequency shift and broadening of the resonance peak. An analytic theory is presented to account for the experimental observations and provides a quantitative tool to accurately determine the Gilbert damping constant.

Small-size sonic crystals with strong attenuation bands in the audible frequency range

Abstract: Recent experiments have proved that sonic crystals containing locally resonant structures exhibit strong sound attenuation bands at frequencies about two orders of magnitude smaller than predicted by Bragg’s theory. The effect is well reproduced here by means of two-dimensional numerical simulations of the elastic wave propagation in a 13 cm slab of locally resonant sonic crystals. Three strong attenuation bands are found in the frequency range from 0.3 to 6.0 kHz. A heuristic model is proposed, which allows one to predict the resonance frequencies in good agreement with the numerical simulations.

Effect of fluids on the Q factor and resonance frequency of oscillating micrometer and nanometer scale beams.

Abstract: Resonance oscillations of micrometer and nanometer scale beams in gases and liquids have increasingly important applications in physics and biology. In this work, we calculate fluid damping and its effect on damped resonance frequency omega(d), and quality factor Q, for oscillating long beams at micrometer and submicrometer scales. For beams of nanometer scale, which are smaller than the mean free path of air molecules at standard conditions, the continuum limit breaks down and the commonly used Stokes drag calculation must be replaced by the appropriate calculation for rarefied gas flow. At scales where the continuum limit holds, this quasisteady Stokes solution is often still inapplicable due to the high resonant frequency associated with small beams, typically 10(2) MHz. The unsteady drag can be over two orders of magnitude higher than that predicted by the quasisteady Stokes solution and the added mass is non-negligible. Here we calculate Q factors as a function of gas pressure over the range from 10(-5) torr to 10(5) torr, corresponding to free molecular to continuum limit. The comparison of the Q factors for two typical beams at various pressures suggests an advantage of using submicrometer scale over micrometer scale beams for applications near ambient pressure.

P-wave Feshbach resonances of ultracold ⁶Li

Abstract: We report the observation of three p-wave Feshbach resonances of 6 Li atoms in the lowest hyperfine state f =1/2. Thepositions of the resonances are in good agreement with theory. We study the lifetime of the cloud in the vicinity of the Feshbach resonances and show that, depending on the spin states, two- or three-body mechanisms are at play. In the case of dipolar losses, we observe a nontrivial temperature dependence that is well explained by a simple model. DOI: 10.1103/PhysRevA.70.030702 In the presence of a magnetic field, it is possible to obtain a quasidegeneracy between the relative energy of two colliding atoms and that of a weakly bound molecular state. This effect, known as a Feshbach resonance, is usually associated with the divergence of the scattering length and is the key ingredient that led to the recent observation of superfluids from fermion atom pairs of 6 Li [1‐4] and 40 K [5] .U p to now these pairs were formed in s-wave channels but it is known from condensed matter physics that fermionic superfluidity can arise through higher angular momentum pairing: p-wave Cooper pairs have been observed in 3 He [6] and d-wave cooper pairs in high-Tc superconductivity [7]. Although Feshbach resonances involving p or higher partial waves have been found in cold atom systems [8‐10], p-wave atom pairs have never been directly observed. In this paper we report the observation of three narrow p-wave Feshbach resonances of 6 Li in the lowest hyperfine state f =1/2. We measure the position of the resonance as well as the lifetime of the atomic sample for all combinations uf =1/2, mfl + uf =1/2, m fl, henceforth denoted smf , m fd .W e show that the position of the resonances are in good agreement with theory. In the case of atoms polarized in the ground state s 1/2,1/2 d, the atom losses are due to threebody processes. We show that the temperature dependence of the losses at resonance cannot be described by the threshold law predicted by [11] on the basis of the symmetrization principle for identical particles. In the case of atoms polarized in s ˛1/2,˛1/2 d or that of a mixture s 1/2,˛1/2 d, the losses are mainly due to two-body dipolar losses. These losses show a nontrivial temperature dependence that can nevertheless be understood by a simple theoretical model with only one adjustable parameter. In the s 1/2,˛1/2 d channel, we take advantage of a sharp decrease of the two-body loss rate below the Feshbach resonance to present a first evidence for the generation of p-wave molecules.

Terahertz plasma wave resonance of two-dimensional electrons in InGaP/InGaAs/GaAs high-electron-mobility transistors

Abstract: We have observed the frequency dependence of the plasma resonant intensity in the terahertz range for a short gate-length InGaP∕InGaAs∕GaAs pseudomorphic high-electron-mobility transistor. The plasma resonance excitation was performed by means of interband photoexcitation using the difference-frequency component of a photomixed laser beam. Under sufficient density of two-dimensional (2D) conduction electrons (>1012cm−2) and a moderate modulation index (the ratio of the density of photoexcited electrons to the initial density of the 2D electrons) we clearly observed the plasma-resonant peaks at 1.9 and 5.8THz corresponding to the fundamental and third-harmonic resonance at room temperature, which is in good agreement with theory.

Vibrational Spectroscopy and Density Functional Theory of Transition-Metal Ion-Benzene and Dibenzene Complexes in the Gas Phase

Abstract: Metal−benzene complexes of the form M(benzene)n (M = Ti, V, Fe, Co, Ni) are produced in the gas-phase environment of a molecular beam by laser vaporization in a pulsed nozzle cluster source. These complexes are photoionized with an ArF excimer laser, producing the corresponding cations. The respective mono- and dibenzene complex ions are isolated in an ion-trap mass spectrometer and studied with infrared resonance enhanced multiple-photon dissociation (IR-REMPD) spectroscopy using a tunable free electron laser. Photodissociation of all complexes occurs by the elimination of intact neutral benzene molecules, and this process is enhanced on resonances in the vibrational spectrum, making it possible to measure vibrational spectroscopy for size-selected complexes. Vibrational bands in the 600−1700 cm-1 region are characteristic of the benzene molecular moiety with systematic shifts caused by the metal bonding. The spectra in this solvent-free environment exhibit periodic trends in band shifts and intensities ...

Multiplet structure of Feshbach resonances in nonzero partial waves

Abstract: We report a unique feature of magnetic-field Feshbach resonances in which atoms collide with nonzero orbital angular momentum. p-wave (l=1) Feshbach resonances are split into two components depending on the magnitude of the resonant state's projection of orbital angular momentum onto the field axis. This splitting is due to the magnetic dipole-dipole interaction between the atoms and it offers a means to tune anisotropic interactions of an ultracold gas of atoms. Furthermore this splitting in the p-wave Feshbach resonance has been experimentally observed and is reported. A parametrization of the p-wave resonance in terms of an effective-range expansion is given.

Single-electron-phonon interaction in a suspended quantum dot phonon cavity.

Abstract: An electron-phonon cavity consisting of a quantum dot embedded in a freestanding GaAs/AlGaAs membrane is characterized using Coulomb blockade measurements at low temperatures. We find a complete suppression of single electron tunneling around zero bias leading to the formation of an energy gap in the transport spectrum. The observed effect is induced by the excitation of a localized phonon mode confined in the cavity. This phonon blockade of transport is lifted at discrete magnetic fields where higher electronic states with nonzero angular momentum are brought into resonance with the phonon energy.

Dynamic stripes and resonance in the superconducting and normal phases of YBa2Cu3O6.5 ortho-II superconductor

Abstract: We describe the relation between spin fluctuations and superconductivity in a highly ordered sample of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.5}$ using both polarized and unpolarized neutron inelastic scattering. The spin susceptibility in the superconducting phase exhibits one-dimensional incommensurate modulations at low energies, consistent with hydrodynamic stripes. With increasing energy the susceptibility curves upward to a commensurate, intense, well-defined, and asymmetric resonance at 33 meV with a precipitous high-energy cutoff. In the normal phase, which we show is gapless, the resonance remains surprisingly strong and persists clearly in Q scans and energy scans. Its similar asymmetric spectral form above ${T}_{c}=59\mathrm{K}$ suggests that incoherent superconducting pairing fluctuations are present in the normal state. On cooling, the resonance and the stripe modulations grow in well above ${T}_{c}$ below a temperature that is comparable to the pseudogap temperature where suppression occurs in local and low-momentum properties. The spectral weight that accrues to the resonance is largely acquired by transfer from suppressed low-energy fluctuations. We find the resonance to be isotropically polarized, consistent with a triplet carrying $\ensuremath{\sim}2.6%$ of the total spectral weight of the Cu spins in the planes.

Electromagnetic meson production in the nucleon resonance region

Abstract: Recent experimental and theoretical advances in investigating electromagnetic meson production reactions in the nucleon resonance region are reviewed. We give a description of current experimental facilities with electron and photon beams and present a unified derivation of most of the phenomenological approaches being used to extract the resonance parameters from the data. The analyses of π and η production data and the resulting transition form factors for the Δ(1232)P33, N(1535)S11, N(1440)P11, and N(1520)D13 resonances are discussed in detail. The status of our understanding of the reactions with production of two pions, kaons, and vector mesons is also reviewed.