High-resolution magnetic imaging by local tunneling magnetoresistance

We give an overview over our recent efforts of high-resolution magnetic imaging using scanning tunneling microscopy with a ferromagnetic tip. Magnetic sensitivity is obtained on the basis of local tunneling magnetoresistance between a soft magnetic tip and the sample. The magnetisation of the tip is switched periodically with a small coil, leading to variations of the tunneling current due to the tunneling magnetoresistance effect. These variations are detected with a lock-in amplifier to separate spin-dependent parts from the topographic parts of the tunneling current such that the topography and the magnetic structure of the sample can be recorded simultaneously. Crucial for this method is to avoid mechanical vibrations of the tip, that may also lead to variations in the tunneling current. Exemplary studies of polycrystalline Ni and the closure domain pattern of Co(0001) are presented, showing high contrast at acquisition times as low as 3 ms/pixel and a lateral resolution of the order of 1 nm. Further it is demonstrated that besides topography and magnetisation, also local information about the magnetic susceptibility can be obtained. Received: 28 April 2000 / Accepted: 15 May 2000 / Published online: 7 March 2001     

Symmetrized general hopping current equation

We show that within the validity range of local thermal equilibrium--therein, however, irrespective of the magnitude of the driving force--a simple current equation can be formulated that expresses the current in terms of a product of a local nonequilibrium conductivity and a sinh function of half the electrochemical potential drop (normalized with respect to kBT) over the local hopping distance. This local current/driving force relation takes account of both electrical and compositional effects and can be generalized as to include interactions and structural variations.     

On the origin of the anomalous sign reversal in the Hall effect in Nb thin films

We re-visit the anomalous sign reversal problem in the Hall effect of the sputtered Nb thin films. We find that the anomalous sign reversal in the Hall effect is extremely sensitive to a small tilting of the magnetic field and to the magnitude of the applied current. Large anomalous variations are also observed in the symmetric part of the transverse resistance R_xy. We suggest that the surface current loops on superconducting grains at the edges of the superconducting thin films may be responsible for the Hall sign reversal and the accompanying anomalous effects in the symmetric part of R_xy.     

Testing Newtonian gravity in space

We propose to measure the universal constant of gravity G in space by means of a small scale planetary system in geosynchronous orbit; test particle satellites of different nuclear composition and with different distances from the central mass would allow one to check possible variations with material and distance. With current technology G can be measured with a relative accuracy of 10⁻⁵ or better depending on the capability of controlling some perturbations which act at the level of 10⁻⁶ (i.e. ≈ 10⁻¹¹ cm s⁻²; however, particular dynamical configurations mutuated from celestial mechanics should allow detection of differential effects with composition and distance (if any) to at least 10⁻⁶. Distance variations will be detected only if acting on a distance scale of 10 m.     

Si4团簇电子输运性质的第一性原理计算

采用密度泛函理论和非平衡格林函数相结合的方法对Si₄ 团簇与Au(100)电极空位相连的纳米结点的电子输运性质进行了理论模拟计算, 得到了纳米结点在不同距离下的几何结构、电子结构、电导、透射谱、电荷转移量; 讨论了当距离d_z=12.004 Å时纳米结点的电导、电流随电压的变化关系. 关键词： 密度泛函理论 非平衡格林函数 4团簇')" href="#">Si₄团簇 电子输运     

Identifying contact effects in electronic conduction through C60 on silicon

We present a theory of current conduction through buckyball (C(60)) molecules on silicon by coupling a density functional treatment of the molecular levels embedded in a semiempirical treatment of the silicon surface with a nonequilibrium Green's function treatment of quantum transport. Several experimental variations in conductance-voltage characteristics are quantitatively accounted for by varying the detailed molecule-silicon bonding geometries. We identify how variations in contact surface microstructure influence the number, positions, and shapes of the conductance peaks, while varying separations of the scanning probe from the molecules influence their peak amplitudes.     

Space and time variations of the order parameter and other relevant quantities within a short superconducting weak link

We have numerically solved the microscopic equations of Golub for the space and time variations of the order parameter, supercurrent, and normal current within a current biased short superconducting weak link. Two limiting cases are considered. We also use a perturbation method to solve for the I–V characteristics of a weak link, and show that they are cosistent with experiment.     

Chaos dynamics in vertical-cavity surface-emitting semiconductor lasers with polarization-selected optical feedback

We study experimentally and numerically the dynamic states of chaotic oscillations in vertical-cavity surface-emitting semiconductor lasers (VCSELs) with polarization-selected optical feedback. We identify the regimes of fully-developed chaotic states, low-frequency fluctuations (LFFs), and coexistent states of LFFs and stable oscillation for the variations of the bias injection current and the optical feedback ratio. In particular, coexistent states of LFFs and stable oscillations are observed at higher optical feedback ratio and lower bias injection current. We draw maps of dynamic states in the space of the bias injection current and the optical feedback ratio. The qualitative agreement between the theory and the experiment is found.     

Study of a circuit-breaker arc with self-generated flow. II. Theflow phase

For pt.I see ibid., vol.16, no.6, p.606-14 (1988). The experimental results presented concern the variations of the mean electric field and gas mass flow during this phase. The most important part consists of a modeling of the evolution of the interruption arc during the decrease of the current from 1000 A to 0. In this modeling, based on the conservation equations of mass and energy, the boundary conditions are determined by an approximate separate modeling of the arc whereas turbulence is treated through Prandtl's approximation. This theoretical study has been developed in the case of SF₆ and nitrogen. The computed values of the electric field and temperature show that the arc has a quasi-stationary behaviour as long as the current intensity is greater than a few tens of amperes, for a decay rate of 1.35 A/μs. The energy losses are governed by radiation at high current and by turbulence conduction at low current. The most important results concern the conductance, whose evolution time constant, immediately prior to current zero, is 3.5 μs in SF₆ and 15 μs in nitrogen. The difference is essentially due to variations with temperature of thermal conductivity and specific heat in two gases     

Demultiplexing of OTDM-DPSK signals based on a single semiconductor optical amplifier and optical filtering

We propose and demonstrate the use of a single semiconductor optical amplifier (SOA) and optical filtering to time demultiplex tributaries from an optical time division multiplexing-differential phase shift keying (OTDM-DPSK) signal. The scheme takes advantage of the fact that phase variations added to the target channel by cross-phase modulation from the control signal are effectively subtracted in the differential demodulation scheme employed for DPSK signals. Demultiplexing from 80 to 40 Gbit/s is demonstrated with moderate power penalty using an SOA with recovery time twice as long as the bit period at 80 Gbit/s. Large dynamic ranges for the input power and SOA current are experimentally demonstrated. The scheme is expected to be scalable toward higher bit rates.     

Simulation of the Metal-Semiconductor-Metal photodetector based on InGaAs for the photodetection at the wavelength 1.55 μm

In this paper we present the simulation of Metal-Semiconductor-Metal photodetector (MSM-PD) of interdigitated planar structure based on InAlAs/InGaAs adapted for photodetection at the wavelength 1.55 μm. We use the theoretical models to plot the variations of the dark current, the photocurrent, the capacity, and the cut-off frequency of the photodetector as a function of bias voltage and the interelectrode distance. The obtained results show a very low dark current, mainly due to the introduction of a thin layer to increase the Schottky barrier based on In_0.52Al_0.48As in the epitaxial structure of component. The obtained photocurrent and cut-off frequencies are very appreciable, these latter are mainly limited by the transit time of the photo-generated carriers given the low component capacity obtained by simulation.     

Observation of 77 K coulomb staircases in GaAs/AlGaAs heterostructures implanted by a focused ion beam

We report evidence of single electron tunneling at 77 K in GaAs/AlGaAs heterostructures implanted with a single line scan of a focused ion beam (FIB). We observe clear staircases in the current-voltage characteristics when the current is measured across the implanted area. These staircases exist in all the fabricated devices but show variations in step width and step height. The observed staircases most likely originate from single electron tunneling through a specific Coulomb island in the random distribution of potential fluctuations which results from implantation damage. Analysis of the observed step widths results in an estimate of the spatial extent of the island which is consistent with the correlation length of the potential fluctuations estimated from the defect density.     

Detection of current-induced resonance of geometrically confined domain walls

Magnetic domain walls are found to exhibit quasiparticle behavior when subjected to geometrical variations. Because of the spin torque effect such a quasiparticle in a potential well is excited by an ac current leading to a dip in the depinning field at resonance for current densities as low as 2 x 10(10) A/m2. Independently the resonance frequencies of transverse walls and vortex walls are determined from the dc voltage that develops due to a rectifying effect of the resonant domain wall oscillation. The dependence on the injected current density reveals a strongly nonharmonic oscillation.     

Wide temperature range operation of micrometer-scale silicon electro-optic modulators

We demonstrate high bit rate electro-optic modulation in a resonant micrometer-scale silicon modulator over an ambient temperature range of 15 K. We show that low bit error rates can be achieved by varying the bias current through the device to thermally counteract the ambient temperature changes. Robustness in the presence of thermal variations can enable a wide variety of applications for dense on chip electronic photonic integration.     

Mode locking features of driven vortex matter in an amorphous MoGe film detected by rf impedance technique

We have investigated mode locking resonance of a driven vortex lattice in an amorphous MoGe film by means of rf impedance measurements. This technique allows us to detect directly the lattice motion in response to a rf current superimposed on top of a dc current. At low rf currents the resonance appears as jumps (dips) in real (imaginary) part of rf impedance Z. On increasing rf current, the jump height in the real part of Z at a given resonant condition decreases monotonically, whereas the imaginary part of Z exhibits a transformation from the dip to peak behaviors. Namely, the resonant feature changes from capacitive to inductive responses, on increasing rf current. This behavior implies variations in the phase of rf current relative to that of the lattice velocity modulated by the pinning potential.     

Scattering from nonuniform tilted fiber gratings

Using volume current analysis, we describe the scattering from intracore fiber gratings with various nonuniformities and find that effects of cross-sectional and longitudinal variations can be roughly decoupled. We show that cross-sectional asymmetry affects the azimuthal pattern of scattering around the fiber and find good agreement with real gratings exhibiting UV-induced grating asymmetry across the core. We also show that longitudinal variations affect the spread of scatter angles with respect to the fiber axis, and this angular spread depends on the Fourier transform of the profiles of the grating period, the average index, and the index modulation of the grating.     

Nanobubbles in solid-state nanopores

From conductance and noise studies, we infer that nanometer-sized gaseous bubbles (nanobubbles) are the dominant noise source in solid-state nanopores. We study the ionic conductance through solid-state nanopores as they are moved through the focus of an infrared laser beam. The resulting conductance profiles show strong variations in both the magnitude of the conductance and in the low-frequency noise when a single nanopore is measured multiple times. Differences up to 5 orders of magnitude are found in the current power spectral density. In addition, we measure an unexpected double-peak ionic conductance profile. A simple model of a cylindrical nanopore that contains a nanobubble explains the measured profile and accounts for the observed variations in the magnitude of the conductance.     

Current-density imaging using ultra-low-field MRI with adiabatic pulses

Magnetic resonance imaging (MRI) allows measurement of electric current density in an object. The measurement is based on observing how the magnetic field of the current density affects the associated spins. However, as high-field MRI is sensitive to static magnetic field variations of only the field component along the main field direction, object rotations are typically needed to image three-dimensional current densities. Ultra-low-field (ULF) MRI, on the other hand, with B₀ on the order of 10–100 μT, allows novel MRI sequences. We present a rotation-free method for imaging static magnetic fields and current densities using ULF MRI. The method utilizes prepolarization pulses with adiabatic switch-off ramps. The technique is designed to reveal complete field and current-density information without the need to rotate the object. The method may find applications, e.g., in conductivity imaging. We present simulation results showing the feasibility of the sequence.     

Transverse charge-carrier transfer in selectively doped GaAs/AlGaAs semiconductor heterostructures with longitudinal current flow

Photoreflectance spectra of selectively doped GaAs/AlGaAs heterostructures are studied under the conditions of direct current flow along the structure layers. Using a model developed for the spectra, variations of the internal transverse electric fields are calculated for longitudinal current flow. It is proved experimentally that even a weak heating of electrons in such structures leads to a spatial redistribution of electrons in the direction transverse to the heterostructure layers.     

Imaging of spatial structures in superconducting tunnel junctions by electron-beam scanning

The voltage change caused in a current-biased superconducting tunnel junction by scanning the junction surface with an electron beam can serve to generate a two-dimensional image of spatial structures within the specimen. Depending upon the bias point during the scanning process, an image of local variations in the tunneling current density due to variations of the tunnel barrier resistance or in the energy gaps of the electrodes is obtained. PbAuIn/PbBi cross-line and SiO-window junctions have been used to demonstrate this imaging technique.