Stability of Magneto-optical Traps with Large Field Gradients: Limits on the Tight Confinement of Single Atoms

We report measurements of the stability of magneto-optical traps (MOTs) for neutral atoms in the limit of tight confinement of a single atom. For quadrupole magnetic field gradients at the trap center greater than ∼1 kG/cm, we find that stochastic diffusion of atoms out of the trapping volume becomes the dominant particle loss mechanism, ultimately limiting the MOT size to greater than ∼5 μm. We measured and modeled the diffusive loss rate as a function of laser power, detuning, and field gradient for trapped cesium atoms. In addition, for as few as two atoms, the collisional loss rates become very high for tightly confined traps, allowing the direct observation of isolated two-body atomic collisions in a MOT

Electrically detected magnetic resonance using radio-frequency reflectometry

The authors demonstrate readout of electrically detected magnetic resonance at radio frequencies by means of an LCR tank circuit. Applied to a silicon field-effect transistor at milli-kelvin temperatures, this method shows a 25-fold increased signal-to-noise ratio of the conduction band electron spin resonance and a higher operational bandwidth of > 300 kHz compared to the kHz bandwidth of conventional readout techniques. This increase in temporal resolution provides a method for future direct observations of spin dynamics in the electrical device characteristics.Comment: 9 pages, 3 figure

Dynamics of Human Walking

The problem of biped locomotion at steady speeds is discussed through a Lagrangian formulation developed for velocity-dependent, body driving forces. Human walking on a level surface is analyzed in terms of the data on the resultant ground-reaction force and the external work. It is shown that the trajectory of the center of mass is due to a superposition of its rectilinear motion with a given speed and a backward rotation along a shortened hypocycloid. A stiff-to-compliant crossover between walking gaits is described and the maximum speed for human walking, given by an instability of the trajectory, is predicted. Key words: locomotion, integrative biology, muscles, bipedalism, human walking, biomechanics.Comment: 9 pages, 4 figure

A Numerical Study of Fuel Stratification, Heat Transfer Loss, Combustion, and Emissions Characteristics of a Heavy- Duty RCCI Engine Fueled by E85/Diesel

Reactivity-controlled compression ignition is a new advanced combustion strategy developed to reach cleaner and more efficient combustion by controlling fuel stratification inside the engine cylin-der and reducing heat loss. While its potential to produce high efficiency and low emissions and to reach higher loads than other Low-Temperature Combustion strategies (LTC) has been confirmed numerous times, its operating range is still limited to moderate loads. One potential solution to in-crease the operating range is using E85 fuel as the premixed fuel due to the potential of providing a longer combustion duration. This work will focus on developing a computational fluid dynamics (CFD) model for a reactivity-controlled compression ignition (RCCI) engine fueled by E85/diesel with a double step piston bowl geometry. The model is used to investigate the effects of four differ-ent design parameters, namely injection timing, boost pressure, initial temperature, and spray in-cluded angle, to identify their impact on all crucial parameters describing combustion i.e. the strati-fication level, heat loss, and emissions characteristics. It has been found that the start of injection affects the fuel stratification levels inside the cylinder, with the optimum location for efficiency lo-cated in the moderate stratified region. The boost pressure mainly influences the mean gas tem-perature, the start of combustion, combustion duration, and the recession time of the Heat Release Rate (HRR) curve. It is found that the boost pressure does not have an influence on the heat loss of the engine and the heat loss is more correlated to flame temperature than the average tempera-ture. It is also proven that the boost pressure could assist in the suppression of NOx, but when the intake pressure is too high, the thermal efficiency drops. Furthermore, the results show that the ini-tial temperature is preferred to be as low as possible but sufficiently high enough to burn all the in-troduced fuel. Intake temperature alters the HRR shape and combustion duration significantly. Last-ly, it is found that the combination of the spray included angle and piston bowl geometry can sub-stantially determine the way the flame is formed and its location. The study on the effect of spray angle provides essential insights on the origin of unburned hydrocarbon emission, HRR shape, and heat loss

Distinguishing syntactic operations in the brain: Dependency and phrase-structure parsing

Finding the structure of a sentence — the way its words hold together to convey meaning — is a fundamental step in language comprehension. Several brain regions, including the left inferior frontal gyrus, the left posterior superior temporal gyrus, and the left anterior temporal pole, are supposed to support this operation. The exact role of these areas is nonetheless still debated. In this paper we investigate the hypothesis that different brain regions could be sensitive to different kinds of syntactic computations. We compare the fit of phrase-structure and dependency structure descriptors to activity in brain areas using fMRI. Our results show a division between areas with regard to the type of structure computed, with the left ATP and left IFG favouring dependency structures and left pSTG favouring phrase structures

Forming simulation of a thermoplastic commingled woven textile on a double dome

This paper presents thermoforming experiments and FE simulations of a commingled glass-PP woven composite on a double dome geometry, with the aim of assessing the correspondence of predicted and experimental shear angles. Large local deformations - especially in-plane shear, i.e. relative rotation between the two yarn families – occur when draping a textile on a three dimensional part and eventually unwanted phenomena like wrinkling or tearing may occur. The macroscopic drape behaviour of a weave is generally subdivided into: 1) The high tensile resistance along the yarn directions, expressed as non-linear stress-strain curves, and 2) The shear resistance, expressed as non-linear shear force versus shear angle curves. The constitutive model is constituted of a dedicated non-orthogonal hypo-elastic shear resistance model, previously described in [1, 2], combined with truss elements that represent the high tensile resistance along the yarn directions. This model is implemented in a user subroutine of the ABAQUS explicit FE solver. The material parameters have been identified via textile biaxial tensile tests at room temperature and bias extension tests at 200°. Thermoforming experiments are performed on a rectangular blank with the warp direction along the second symmetry plane of the tool, with a preheating temperature of 200°C, a constant mold temperature of about 70°C, and a blankholder ring. It was concluded that the shear angles were fairly well predicted for this particular case study, which could be expected in view of the fact that no wrinkles had formed during the thermoforming experiment