Transitions on perpendicular rigid disks in quasi-contact

Results of quasi-contact recording on rigid perpendicular disks are presented in this paper. Footprints of Various recording heads on different recording media, obtained at quasi-static contact conditions using a high resolution magnetoresistive transducer, provide useful information such as sharpness of the magnetization transition and the effect of the demagnetizing field. Very high density magnetization transitions, up to 12500 fc/mm, on single-layer perpendicular rigid disks were recorded and read back by thin film ring heads at a velocity of 5-8 m/s under quasi-contact conditions. At high linear densities the amplitudes on the experimental density response curve are higher than those obtained by the linear superposition of experimental isolated pulses.     

Optical-disk media fabrication for high data-transfer rate

Two media fabrication methods are developed in order to improve the data-transfer rate of optical disks. One is a beam-scanning method for multispiral pregroove recording. A sixfold spiral pregroove with a 1.6-μm spacing can be recorded with this method. The other is a method for fabrication of phase-change optical-disk media that provides signal reading at 670 nm, writing at 830 nm, and high erasability at high linear velocity. The linear recording density of disks fabricated in this way is 1.2 times higher than that of optical disks read at 830 nm, and the erasability is more than 25 dB at 40 m/s. These methods offer the possibility of a data transfer rate 10 times higher than with conventional methods.     

Computer modeling of the write process in perpendicular recording

A self-consistent computer simulation model utilizing a ring head and single layer recording medium has been developed to study perpendicular recording. In the model it is assumed that the medium switches uniformly following the volume averaged field. The optimum recording field obtained from the model calculation at various head-medium spacings permits an estimation of head saturation limitations. The roll-off curve, which is the linear superposition of the alternating voltage pulses, is in good agreement with experimental data. The write spacing loss is spacing dependent and is 120d/λ (dB) at contact for a typical recording case. For contact recording the imaging effect is important in the write process. Optimum recording at contact is shown to produce a high overshoot and sharp leading edge in the transition while leaving the magnetization level below the coercivity; this yields an apparent "negative" transition length in terms of the arctangent transition model.     

Effect of quantum interference on a three-level atom driven by two laser fields

Abstract

We study the effect of quantum interference on the population distribution and absorptive properties of a V-type three-level atom driven by two lasers of unequal intensities and different angular frequencies. Three coupling configurations of the lasers to the atom are analysed: (a) both lasers coupled to the same atomic transition, (b) each laser coupled to different atomic transition and (c) each laser coupled to both atomic transitions. Dressed states for the three coupling configurations are identified, and the population distribution and absorptive properties of the weaker field are interpreted in terms of transition dipole moments and transition frequencies among these dressed states. In particular, we find that in the first two cases there is no population inversion between the bare atomic states, but the population can be trapped in a superposition of the dressed states induced by quantum interference and the stronger field. We show, that the trapping of the population, which results from the cancellation of transition dipole moments, does not prevent the weaker field to be coupled to the cancelled (dark) transitions. As a result, the weaker field can be strongly amplified on transparent transitions. In the case of each laser coupled to both atomic transitions the population can be trapped in a linear superposition of the excited bare atomic states leaving the ground state unpopulated in the steady state. Moreover, we find that the absorption rate of the weaker field depends on the detuning of the strong field from the atomic resonances and the splitting between the atomic excited states. When the strong field is resonant to one of the atomic transitions a quasi-trapping effect appears in one of the dressed states. In the quasi-trapping situation all the transition dipole moments are different from zero, which allows the weaker field to be amplified on the inverted transitions. When the strong field is tuned halfway between the atomic excited states, the population is completely trapped in one of the dressed states and no amplification is found for the weaker field.     

Combatting partial erasure and transition jitter in magneticrecording

Practical recording channels deviate significantly from the oft-used model based on linearity and additive Gaussian noise. Among the well-known anomalies are partial erasure and transition jitter. In this paper, we first describe a precompensation technique to combat the partial erasure effect. It utilizes the unique property of the maximum transition run (MTR) code wherein all dibits are isolated. The idea is to increase the spacing between the two transitions that make up each dibit. This tends to mitigate the nonlinear amplitude reduction associated with partial erasure. Next, we consider a recording channel that is linear but subject to transition jitter. We modify the hyperplane-based detector known as 3D-110 by introducing data-dependent threshold terms. The overall complexity of the detector is small, yet this modification provides almost 2 dB of improvement relative to the original 3D-110 structure and the extended partial response maximum likelihood (EPRML) method tuned to additive Gaussian noise     

Model for demagnetization-induced noise in thin-film magneticrecording media

A linear, statistical model is described which predicts the power spectrum of measured noise in bulk-demagnetized (i.e. AC-erased) thin-film magnetic recording media. It is shown that the noise is the result of magnetic flux which is ascribed to erasure-induced transitions along the track length in the medium. The noise power spectrum for a rigid disk medium is shown to correspond to the power spectrum of Poisson-distributed induced transitions along the track length, while noise along the track width is sufficiently described in terms of a uniform, average magnetization with small variance. Experimental data from two thin-film disks are used with the model to estimate the Poisson parameter for each disk. It is demonstrated that AC-erased noise from particulate media can be considered as a limiting case of the Poisson model     

Polarization dependence of the C 1s X-ray absorption spectra of individual multi-walled carbon nanotubes

The polarization dependence (linear dichroism) of the C 1s X-ray absorption spectrum of individual multi-walled carbon nanotubes (MWCNTs) is measured using scanning transmission X-ray microscopy. A very strong dichroic effect is found in the C 1s --> pi* transition, with almost complete disappearance of this transition when the electric-field (E)-vector is aligned parallel to high-quality (low-defect) MWCNTs and maximum intensity when the E-vector is orthogonal to the tube axis. In contrast, there is very little dichroism in the C 1s --> sigma* transitions. The origin of this polarization effect is explained. The magnitude of the polarization dependence is found to differ in MWCNTs synthesized by different methods (arc discharge versus chemical vapor deposition). This is ascribed to differences in densities of sp(2)-type defects. The potential for use of this signal to characterize defects in single-carbon-nanotube devices is discussed.     

When should we superpose M/G/1 queues?

Conventional wisdom suggests that a fast single-server queue is preferable to a number of slower single-server queues. The rationale is that if the service rate of the superposed queue is the sum of the service rates of the individual queues, then the average waiting time for the superposed queue is smaller than the weighted average waiting time of the individual queues. This argument, however, assumes that the service time of the customers is perfectly scalable. Unfortunately, this assumption rarely holds in reality. In this paper we analyze the superposition of M/G/1 queues. We assume that there are scalable and unscalable components in the service time. On the basis of the criteria of average waiting time and average response time, we derive necessary and sufficient conditions for whether the superposition of queues is preferable. Examples of magnetic rotational disks are given to illustrate our findings.     

Vortex core size in submonolayer superfluid4He films

The superfluid transition of⁴He films adsorbed on 500 Å diameter slip-cast alumina powder is measured with a torsion oscillator technique for films with transition temperatures between 50 and 700 mK. The transitions are found to be controlled by the universal Kosterlitz- Thouless critical line, but a strong increase in the broadening of the transition is observed for the thinnest films. Analysis in terms of vortex pair excitations shows that this broadening results from a vortex core size that increases rapidly as the film is thinned, scaling roughly with the interparticle spacing of the superfluid submonolayer. Third sound modes are identified in the dissipation, in addition to a broad peak from the vortex unbinding. This system provides evidence of a three-dimensional superfluid transition mediated by vortex excitations.     

Self-consistent modeling of morphology evolution during unidirectional solidification

A self-consistent model is developed to describe the morphology evolution during unidirectional solidification, which shows that, for a given temperature gradient, the interface morphology will go planar → shallow cell → deep cell → dendrite → cell → planar with increasing growth velocity. By examining the interaction of adjacent cells/dendrites, a wide allowable range of primary spacing for given growth conditions is determined, which shows a good agreement with experimental results. Numerical results show that cellular/dendritic and dendritic/cellular transitions appear not at a unique velocity but over a range of velocities, the critical velocity for the transition being dependent on the primary spacing before the transition.     

圆筒形预应力结构锚索间距的确定方法

圆筒形预应力结构的锚索间距s和单束锚索张拉力P如设计不当,往往会导致预应力轴向分布不均匀,预应力施加过程中会出现环向裂缝。对单束锚索作用下结构内力M、Q、w、q的计算方法进行了简单分析,提出了确定锚索间距的弹性力学叠加公式,并根据工程实例对锚索作用的影响范围和内力变化规律进行了对比验证。结果表明,理论分析成果和现场实测结果是比较吻合的,二者相差不超过5%。提出的叠加公式可用于实际工程的计算分析。     

Self-consistent modeling of morphology evolution during unidirectional solidification

A self-consistent model is developed to describe the morphology evolution during unidirectional solidification, which shows that, for a given temperature gradient, the interface morphology will go planar→shallow cell→deep cell→dendrite→cell→planar with increasing growth velocity. By examining the interaction of adjacent cells/dendrites, a wide allowable range of primary spacing for given growth conditions is determined, which shows a good agreement with experimental results. Numerical results show that cellular/dendritic and dendritic/cellular transitions appear not at a unique velocity but over a range of velocities, the critical velocity for the transition being dependent on the primary spacing before the transition.     

Thin-film technology and dc-SQUID instrumentation for a superheated superconducting detector

We report on improvements of the dc-SQUID instrumentation developed to study superheating-supercooling transitions of thin-film indium dots. In previous measurements on ordered arrays of dots with diameters larger than 100µm the flip signal of an individual dot in perpendicular magnetic field has been detected. In order to use these dots as superheated superconducting detectors it is necessary to reduce the dot diameter to lower the energy threshold and to avoid intermediate states. For these reasons, our effort has been to develop small and thick dots with diameter down to10 µm and thickness up to3.5 µm. At the same time the coupling with the dc-SQUID readout was improved using a thin-film niobium pickup coil integrated on the same chip containing the dots. Phase transitions of individual dots were observed. The superheating phase transition spread in temperature of the samples was about 0.15 %.     

Application of the boundary element method to dense dispersions

The boundary element method (BEM) is used to find the effective conductivity of a random dispersion of disks in a second material. Small separations between inclusions give rise to difficulties not usually associated with the use of the BEM. We find that the integration point spacing on a given surface element has to be of the order of the shortest distance between disks. The spacing requirement becomes a limitation at high densities, even when the ratio of the conductivity of the disks to that of the host material is close to one. This limitation is overcome by increasing the order of the integration scheme used rather than by increasing the number of surface elements. A more conventional concern is that the surface mesh itself must be sufficiently fine to represent the temperature and flux profiles. As with all simulations of dense dispersions, periodic boundary conditions have to be used and further, because random dispersions are of interest, averages of calculations over many configurations are needed. Taking account of the above considerations we calculate the first exact results for the effective conductivity of a dispersion of cylinders at a high (0.6) density. The influence of vectorization on the performance of the code is briefly mentioned, as are other possible improvements.     

Interpretation of spectral response in perpendicular recording

A previously published voltage spectrum in perpendicular recording is analyzed here utilizing the exact fields of a probe head and keepered medium. It is concluded that sharp transitions have been written at a negligible spacing. Previous analysis which utilized a duality between a ring head and a probe-keeper head yielded a non-zero spacing or recorded transition length. Measurements of D50for varying record geometries are analyzed in terms of scaling laws.     

Molecular iodine fluorescence spectra generated with helium-neon lasers for spectrometer calibration

Gas-phase molecular iodine laser-induced fluorescence (LIF) spectra were recorded out to 815 nm at 1 cm(-1) resolution using green, yellow, and red helium-neon (HeNe) lasers as excitation sources. Nine previously unreported I(2) B←X absorption transitions accessed by these lasers were identified, and specific rovibronic transition assignments were made for two hundred LIF peaks--more than sixty per laser. These I(2) LIF peaks can be used to calibrate the vacuum wavenumber coordinate of spectrometers to better than 0.1 cm(-1) accuracy. In particular, green HeNe excitation of the I(2) R(106) 28-0 transition leads to strong fluorescence well suited for calibration, with a rotational doublet spacing of 15 cm(-1) and a doublet-to-doublet spacing of 190 cm(-1). Calibration by HeNe I(2) LIF may be an especially valuable technique for Raman spectroscopy applications.     

The effect of dislocations on the roughening transition in the weak coupling approximation

The analysis of the roughening transition proposed by Nozières and Gallet is extended to interfaces threaded with either randomly placed screw dislocations or dislocation loops. In both cases we consider a low density of dislocations which intersect normally with the surface leading to a local deformation of the crystal planes and to the formation of small steps in the interface. We show that dislocation loops do not change the roughening transition significantly: it still remains within the Kosterlitz-Thouless universality class of phase transitions. In contrast, randomly placed screw dislocations modify the phase transition: the Kosterlitz-Thouless transition is rounded on a length scale of order the spacing of dislocations. However there is a transition at a slightly lower temperature for randomly placed screw dislocations: the superroughening transition. This conclusion is only valid if the dislocations are randomly placed, not if they are distributed as closely spaced pairs of opposite sign.     

A parametric study of ultrasonic beam profiles for a linear phased array transducer

A numerical simulation model is presented to investigate the influences of design parameters of linear phased array transducers on beam focusing and steering performance. The characteristic of ultrasonic beam profiles has been simulated on the basis of the Huygen's superposition principle. For the simulation, a linear phased array is considered as the composition of finite number of elements separated by equidistance. Individual elements are considered as two-dimensional point sources. The waves generated from piezoelectric elements are considered as simplified transient ultrasonic waves that are constructed with the cosine function enveloped with a Hanning window. The characteristic of ultrasonic wave propagation into a medium from the phased array transducer is described. The effects of the number, the interelement spacing, steering angle, the focal length, and frequency bandwidth of the piezoelectric elements on beam directivity and ultrasonic pressure field in a linear phased array transducer are systematically discussed.     

Fabrication of Flyable Perpendicular Discrete Track Media

Discrete track media offers many potential recording advantages over conventional continuous media in hard disk drives. In this study, we present a novel fabrication process for discrete track perpendicular magnetic media via electron beam lithography, ion milling, and the use of a protective Al sacrificial layer. Physical characterization of the media confirms the process is able to produce patterned tracks with no damage to the media. Spin stand analysis verifies the disks are flyable and capable of recording sharp transitions without any degradation in the magnetic signal     

一般线性粘弹性阻尼器耗能结构瞬态响应的非正交振型叠加精确解

对一般线性粘弹性阻尼器(含线性橡胶隔震支座)耗能结构的非正交振型叠加精确解法进行了系统研究。首先采用最一般的线性粘弹性阻尼器的积分型精确分析模型,用微分积分方程组实现一般粘弹性阻尼器耗能结构的时域非扩阶精确建模;然后采用传递矩阵法,直接在耗能结构原始空间上获得了一般线性粘弹性耗能变频结构在任意激励和非零初始条件下位移与速度时域瞬态响应的非正交振型叠加精确解;通过与3种典型结构的对比分析,验证了该精确解的正确性、简易性和普适性。该非扩阶精确解具有明确的物理意义,可视为现有比例粘滞阻尼定常结构的经典正交振型叠加精确解在一般线性粘弹性阻尼耗能变频结构的推广,能从本质上精确揭示耗能结构的振动机理,即尽管耗能结构的振型不具有正交性,但耗能结构响应仍然可精确分解为各振型响应的线性组合。此振动机理将为建立耗能结构精确的振型分解反应谱法提供分析路径,同时可将现有用于一般粘滞阻尼定常结构的参数识别、动力修改、最优控制及优化设计等方法推广到一般粘弹性阻尼变频非定常结构。