Search Results (22 total)

Beam breakup instability in superconducting cavities is a serious problem. In this work, a four-cell LEP cavity installed in the KAERI linear accelerator is considered as an example. Dependence of the breakup instability threshold currents on the characteristics of a dipole mode was determined both analytically and numerically. An efficient technique to suppress breakup instability using rf beam focusing within a cavity is suggested. The technique involves applying TE-type monopole higher-order modes and is useful for multicell superconducting cavities with many trapped high-Q dipole modes.

Different applications of TE modes in accelerator physics are discussed. In this discussion, the Fourier transform of the squared axial component of magnetic field, B_z²(z), plays an important role. If it turns out to be zero, the rf field-particle energy transfer is negligible and the focal length of the TE mode lens is phase independent. Such rf lens focuses continuous beams just as a solenoid. In order to compensate spherical aberrations and emittance growth caused by field nonlinearities of a focusing solenoid, the rf focusing is used also even if the beam space charge is taken into account. If the transform has its maximum, excitations of TE mode resonances and electron beam self-focusing are possible. Furthermore, the rf field of the TE mode can be used to expand the radial acceptance of a FEL for the THz region.

The physics of confined water has stimulated extensive research in recent years, in particular, regarding the role of hydrogen bonding as a significant factor in the observed dynamics. In this work, two-dimensional infrared spectroscopy was employed to investigate the response of the OH moiety of water in phospholipid membrane samples. The results show strong evidence for three distinct hydrogen bonding motifs (H₂O with zero, one, or both OH moieties hydrogen bonded), whose relative proportions at the membrane interface are estimated.

The spectroscopic properties of Ho³⁺ laser channels in locally disordered tetragonal NaBi(WO₄)₂ (NaBiW), NaBi(MoO₄)₂ (NaBiMo), and LiBi(MoO₄)₂ (LiBiMo) single crystals grown by the Czochralski method have been studied in the 5–300-K temperature range using several holmium concentrations [Ho]≈0.05–0.6×10²⁰ cm⁻³. Here 5-K polarized optical absorption and photoluminescence measurements have been used to determine the energy position of 85, 56, and 39 Ho³⁺ Stark levels in NaBiW, NaBiMo, and LiBiMo crystals, respectively. These energy levels were labeled with irreducible representations corresponding to the S₄ local symmetry of an average optical center. Single-electron Hamiltonians combining together free-ion and crystal-field interactions have been used in the fit of experimental energy levels and in the simulation of the corresponding 4f¹⁰ Ho³⁺ configuration for NaBiW and NaBiMo crystals. Very satisfactory correlations were obtained between experimental and calculated crystal-field levels, with rms deviations σ=8.8 and 7.3 cm⁻¹ for NaBiW and NaBiMo, respectively. The radiative properties and emission cross sections of Ho³⁺ laser channels in these hosts were calculated by the Judd-Ofelt theory and compared with experimental results. The emission cross sections of Ho³⁺ in NaBiW are similar to those observed in other crystal laser hosts, and positive gain cross sections can be achieved in extended spectral ranges. These properties make the Ho³⁺-doped double tungstates and double molybdates feasible materials for tunable and short-pulse laser operation.

A long-standing problem for ZnSe (and related alloys) has been to obtain good p-type doping. Recent work has given about an order-of-magnitude improvement in such doping by use of Te as a “codopant” to facilitate the introduction of an acceptor dopant (N), since it is known that p-ZnTe can be obtained quite readily; the Te was introduced in submonolayer quantities via planar (δ) doping during molecular beam epitaxy. Here, we examine the mechanism of this improved doping. We show that it resides in the formation of ZnTe-rich nanoislands, with the N embedded in these. This result is obtained by studies involving transmission electron microscopy, high-resolution x-ray diffraction, secondary-ion mass spectroscopy, and temperature quenching of photoluminescence. We note that these nanoislands appear quite unique, in providing doping of semiconductors, and thus are of great interest of their own.

We report low-temperature transport measurements through molecules of Gd metallofullerenes between superconducting suspended electrodes. The presence and number of molecules in the 2-nm-wide gap between electrodes was determined by high resolution transmission electron microscopy. We find that a junction containing a single metallofullerene dimer between superconducting electrodes displays signs of proximity-induced superconductivity. In contrast, no proximity effect develops in junctions containing a larger cluster of metallofullerenes. These results can be understood by taking into account multiple Andreev reflections, and the spin states of the Gd atoms.

We report on realization of channel plasmon-polariton (CPP) propagation along a subwavelength metal groove. Using imaging with a near-field microscope and end-fire coupling with a tapered fiber connected to a tunable laser at telecommunication wavelengths (1425–1620 nm), we demonstrate low-loss (propagation length ∼100  μm) and well-confined (mode width ≅1.1  μm) CPP guiding along a triangular 0.6   μm-wide and 1  μm-deep groove in gold. We develop a simple model based on the effective-index method that accounts for the main features of CPP guiding and provides a clear physical picture of this phenomenon.

A scanning near-field optical microscope is used to directly map the propagation of light in the wavelength range of 1500–1630 nm along straight photonic crystal waveguides (PCWs) fabricated on silicon-on-insulator wafers. The PCWs were formed by removing a single row of holes in the triangular 428-nm-period lattices with different filling factors (0.76 and 0.82) and connected to access ridge waveguides. Using the near-field optical images we investigate the light propagation along PCWs for TM and TE polarizations (the electric field is perpendicular/parallel to the sample surface). Efficient guiding (for both samples) of the TM-polarized radiation is observed in the whole range of laser tunability. For TE polarization, the efficient guiding is limited to the wavelengths shorter than 1552 or 1570 nm for the PCW with the filling factor of 0.76 or 0.82, respectively. For longer wavelengths, we observe drastic and rapid deterioration of the waveguiding and PCW mode confinement, ending up with the complete disappearance of the PCW mode once the wavelength exceeds the cutoff value by merely 2 nm. Using averaged cross sections of the intensity distributions along the PCW axis, the propagation loss is evaluated and found to be in good agreement with the corresponding transmission spectra. Considering spatial frequency spectra of the intensity variations along the PCW axis, we determine (for both polarizations) the dispersion of the PCW mode propagation constant and, thereby, the mode group and phase velocities.

External fields are necessary for emittance compensation and beam focusing in rf photoelectron guns. For rf guns with superconducting cavities two fields have been discussed up to now. The first field is a specially designed radial component of the rf field immediately after the cathode and the second is a static magnetic field downstream of the superconducting cavity. In this paper we discuss a third possibility. Inside the cavity the magnetic rf field of a TE mode focuses the electron beam and prevents the increase of the transverse emittance. The results depend only weakly on the phase of the TE mode. For a bunch charge of 1 nC, an emittance of 0.7 mm mrad has been obtained with a surface field strength of the magnetic field below the quench limit.

A method is presented to reconstruct the structure of a scattering object from data acquired with a photon scanning tunneling microscope . The data may be understood to form a Gabor type near-field hologram and are obtained at a distance from the sample where the field is defocused and normally uninterpretable. Object structure is obtained by the solution of the inverse scattering problem within the accuracy of a perturbative, two-dimensional model of the object.

Small-scale quasicoherent oscillations of the x-ray emissivity and magnetic field perturbations are observed in the T-10 tokamak during abrupt growth of the m=2, n=1 magnetohydrodynamic modes at the density limit disruption. Analysis indicates a possible link between the small-scale oscillations and nonthermal electron beams induced around the X points of the m=2, n=1 magnetic island during reconnection of magnetic field lines at the disruption instability.

We derive the magnetic transport-of-intensity equation (MTIE) that links defocused contrast of magnetic nanoobjects imaged by partially coherent electron waves to their micromagnetic parameters. This provides Maxwell’s explanation for observable contrast in terms of the Z component of currents existing around vortices in superconductors and domain vortices and walls in ferromagnets. The solution of the MTIE via Fourier transform is used for quantitative mapping of magnetic flux and projected induction in magnetic and superconducting materials imaged by Lorentz microscopy.

We employ a collection scanning near-field optical microscope (SNOM) to image the propagation of light at telecommunication wavelengths along straight and bent regions of silicon-on-insulator photonic crystal waveguides (PCWs) formed by removing a single row of holes in the triangular 410-nm-period lattice along the ΓM direction of the irreducible Brillouin zone. We obtain high quality SNOM images of PCWs excited in the wavelength range of 1520–1570 nm, which indicate good PCW mode confinement and low propagation loss. Using averaged cross sections of the intensity distributions before and after PCW bends, bend loss is evaluated and found to noticeably increase with the increase of the light wavelength from ∼1 dB at 1520 nm to ∼6 dB at 1570 nm. We analyze light intensity variations along PCWs measured with the SNOM at different distances from the sample surface. Considering the interference between a quasihomogeneous background field and Bloch harmonics of the PCW mode, we account for spatial frequency spectra of the intensity variations and determine the propagation constant of the PCW mode at 1520 nm. The possibilities and limitations of SNOM imaging for the characterization of PCWs are discussed.

We propose to use channels in strongly scattering nonabsorbing random media for guiding electromagnetic waves, and demonstrate this concept using near-field microscopy of surface plasmon polaritons (SPP’s) propagating along the gold film surface covered with randomly located scatterers. In the wavelength range of 725–765 nm, we observe complete inhibition of the SPP propagation inside the random structures composed of ∼50-nm-wide gold bumps and their clusters with the density of 50  μm^-2, as well as well-defined SPP guiding along corrugation-free 2- and 4-μm-wide channels.

A theory of non-Markovian rotational Brownian motion is developed for axisymmetric particles moving in a Maxwell fluid in the presence of an external field. Both the inertial and viscoelastic effects are taken into account. A kinetic equation for the joint probability distribution of orientation, angular velocity, and acceleration of a particle without spin is derived starting from the rotational Langevin equation with relaxed hydrodynamic and random torques. A third-order stochastic differential equation for the particle orientation vector is also derived. Directly from this equation, the set of nonlinear evolution equations for one-time moments is derived in a noninertial approximation. The expressions for a linear response to a time-dependent external field and dynamic susceptibility of particle are obtained by direct averaging of particle orientation equation. Appendices derive the rotational mobility of axisymmetric particles in a general linear viscoelastic fluid, and the evolution equations for one-time moments of the orientation vector for axisymmetric particles moving in a Maxwell fluid in the presence of an external field.

We report measurements on ropes of single-walled carbon nanotubes (SWNT) in low-resistance contact to nonsuperconducting (normal) metallic pads, at low voltage and at temperatures down to 70 mK. In one sample, we find a 2 orders of magnitude resistance drop below 0.55 K, which is destroyed by a magnetic field of the order of 1 T, or by a dc current greater than 2.5 μA. These features strongly suggest the existence of superconductivity in ropes of SWNT.

We report observations of acoustoelectric effects in carbon nanotubes. We excite sound in μm long ropes of single walled carbon nanotubes suspended between two metallic contacts by applying radio-frequency electric field. The sound is detected by measuring either the dc resistance of the tubes in a region of strong temperature dependence (in the vicinity of superconducting or metal-insulator transition), or their critical current. We show that, depending on the excitation power, the vibrations produce either electron heating or phase coherence breaking.

We investigate spatial instabilities (induced by small angular perturbations) of counterpropagating waves in nonlinear distributed feedback (DFB) structures. We determined the DFB-structure threshold length at which an absolute instability occurs and a nonhomogeneous spatial intensity distribution is generated. The evolution of the transverse intensity distribution is studied for counterpropagating waves as a function of the control parameters.

The local-field effects are considered which are feasible in a dense ensemble of resonant atoms modeled by multilevel quantum systems. Our approach is based on the generalized two-level system [V.S. Butylkin, A.E. Kaplan, and Yu.G. Khronopulo, Zh. Éksp. Teor. Fiz. 59, 921 (1970)]. Making use of this model, we take account of the nonresonant polarization and Stark shift of the absorption line due to the difference in linear polarizabilities of atoms in ground and excited states. With near dipole-dipole interaction and quadratic Stark effect included simultaneously, new features of the hysteresis behavior of the population difference as a function of the external-field intensity are predicted.

The theory of collective excitations in a single antidot and in a system of interacting antidots is presented. The problem is solved within the framework of classical electrodynamics neglecting the nonlocal and retardation effects. It is shown that the spectrum of collective excitations in a single antidot consists of two branches. The first mode coincides with the single-particle cyclotron resonance ω=ω_c; the second one is the edge magnetoplasmon (EMP) mode. The EMP mode has the vanishing damping (in the collisionless approximation) only at ω<ω_c. At ω>ω_c it decays on account of emission of two-dimensional (2D) bulk magnetoplasmons to the surrounding 2D medium. The induced electric potential and charge density of the EMP mode have the form of outgoing cylindrical waves at ω>ω_c. As a consequence, the interantidot interaction cannot be neglected in an array of antidots at ω>ω_c. Collective excitations in an array of interacting antidots are considered in the modified-dipole and effective-medium approximations. The results obtained explain the main features of the antidot excitation spectrum observed in recent experiments.

The evaporation residue cross sections σ_ER in reactions between massive nuclei have been analyzed within different models of complete fusion. The calculations in the framework of the optical model, the surface friction model, and the macroscopic dynamic model can give the results which are by few orders of magnitude different from experimental data. This takes place due to neglect of the competition between complete fusion and quasifission. A possible mechanism of compound nucleus formation in heavy-ion-induced reactions has been suggested. The analysis of the complete fusion of nuclei on the basis of dinuclear system approach has allowed one to reveal an important feature of the fusion process of massive nuclei, that is, the appearance of the fusion barrier during dinuclear system evolution to a compound nucleus. As a result, the competition between complete fusion and quasifission arises and strongly reduces the cross section of the compound nucleus formation. A model is proposed for calculation of this competition in a massive symmetric dinuclear system. This model is applied for collision energies above the Coulomb barrier. The σ_ER values calculated in the framework of dinuclear system approach seem to be close to the experimental data. For illustration the reactions ¹⁰⁰Mo+¹⁰⁰Mo, ¹¹⁰Pd+¹¹⁰Pd, and ¹²⁴Sn+⁹⁶Zr have been considered.