Squeezing out the last 1 nanometer of water: A detailed nanomechanical study

In this study, we present a detailed analysis of the squeeze-out dynamics of nanoconfined water confined between two hydrophilic surfaces measured by small-amplitude dynamic atomic force microscopy (AFM). Explicitly considering the instantaneous tip-surface separation during squeezeout, we confirm the existence of an adsorbed molecular water layer on mica and at least two hydration layers. We also confirm the previous observation of a sharp transition in the viscoelastic response of the nanoconfined water as the compression rate is increased beyond a critical value (previously determined to be about 0.8 nm/s). We find that below the critical value, the tip passes smoothly through the molecular layers of the film, while above the critical speed, the tip encounters "pinning" at separations where the film is able to temporarily order. Pre-ordering of the film is accompanied by increased force fluctuations, which lead to increased damping preceding a peak in the film stiffness once ordering is completed. We analyze the data using both Kelvin-Voigt and Maxwell viscoelastic models. This provides a complementary picture of the viscoelastic response of the confined water film

Optimization of double drive pulse pumping in Ne-like Ge x-ray lasers

Pumping of the Ne-like Ge x-ray laser with two 100 ps duration pulses (a prepulse and main pulse) is investigated using a fluid and atomic physics code coupled to a 3D ray tracing postprocessor code. The modeling predicts the optimum ratio of the irradiance of the two pulses for the maximum x-ray laser output resulting from the balance between the relative lower electron density gradients and wider gain region which is produced with a larger prepulse and the higher peak gain coefficients produced with a small prepulse. With a longer pulse interval between prepulse and main pulse, a relatively lower optimum pulse ratio is found. The threshold irradiance of the main driving pulse with a prepulse required to make an order of magnitude enhancement of laser output compared to irradiation without a prepulse is also found at 3-4x10(13) W/cm(2) for Ne-like Ge. (C) 1998 American Institute of Physics

One-Dimensional Variational Ionospheric Retrieval Using Radio Occultation Bending Angles:Part 1 -Theory

A new one-dimensional variational (1D-Var) retrieval method for ionospheric GNSS radio occultation (GNSS-RO) measurements is described. The forward model implicit in the retrieval calculates the bending angles produced by a one-dimensional ionospheric electron density profile, modeled with multiple “Vary-Chap” layers. It is demonstrated that gradient based minimization techniques can be applied to this retrieval problem. The use of ionospheric bending angles is discussed. This approach circumvents the need for Differential Code Bias (DCB) estimates when using the measurements. This new, general retrieval method is applicable to both standard GNSS-RO retrieval problems, and the truncated geometry of EUMETSAT's Metop Second Generation (Metop-SG), which will provide GNSS-RO measurements up to about 600 km above the surface. The climatological a priori information used in the 1D-Var is effectively a starting point for the 1D-Var minimization, rather than a strong constraint on the final solution. In this paper the approach has been tested with 143 COSMIC-1 measurements. We find that the method converges in 135 of the cases, but around 25 of those have high “cost at convergence” values. In the companion paper (Elvidge et al., 2023), a full statistical analysis of the method, using over 10,000 COSMIC-2 measurements, has been made.Key Points• A new method of deriving ionospheric electron densities, using the difference between bending angles at two different frequencies• It is based on a 1D variational retrieval, the solution of which is the best fit to the a priori background and the observations• The forward model assumes the ionosphere to consist of several idealized “Vary-Chap” electron density layer

One‐Dimensional Variational Ionospheric Retrieval Using Radio Occultation Bending Angles::2. Validation

Culverwell et al. (2023, https://doi.org/10.1029/2023SW003572) described a newone-dimensional variational (1D-Var) retrieval approach for ionospheric GNSS radio occultation (GNSS-RO)measurements. The approach maps a one-dimensional ionospheric electron density profile, modeled withmultiple “Vary-Chap” layers, to bending angle space. This paper improves the computational performance ofthe 1D-Var retrieval using an improved background model and validates the approach by comparing with theCOSMIC-2 profile retrievals, based on an Abel Transform inversion, and co-located (within 200 km) ionosondeobservations using all suitable data from 2020. A three or four layer Vary-Chap in the 1D-Var retrievalshows improved performance compared to COSMIC-2 retrievals in terms of percentage error for the F2 peakparameters (NmF2 and hmF2). Furthermore, skill in retrieval (compared to COSMIC-2 profiles) throughout thebottomside (∼90–300 km) has been demonstrated. With a single Vary-Chap layer the performance is similarbut this improves by approximately 40% when using four-layers

Efficiency and Stability Issues in the Numerical Computation of Fourier Transforms and Convolutions on the 2-Sphere

Earlier work by Driscoll and Healy has produced an efficient algorithm for computing the Fourier transform of band-limited functions on the sphere. In this paper we present a greatly improved inverse transform, and consequent improved convolution algorithm for such functions. We also discuss implementational considerations and give heuristics for allowing reliable floating point implementations of a slightly modified algorithm at little cost in either theoretical or actual performance. This discussion is supplemented with numerical experiments from our implementation in C on a DecStation 5000. These results give strong indications that the algorithm is both reliable and efficient for a large range of useful problem sizes

Radiation from low-momentum zoom-whirl orbits

We study zoom-whirl behaviour of equal mass, non-spinning black hole binaries in full general relativity. The magnitude of the linear momentum of the initial data is fixed to that of a quasi-circular orbit, and its direction is varied. We find a global maximum in radiated energy for a configuration which completes roughly one orbit. The radiated energy in this case exceeds the value of a quasi-circular binary with the same momentum by 15%. The direction parameter only requires minor tuning for the localization of the maximum. There is non-trivial dependence of the energy radiated on eccentricity (several local maxima and minima). Correlations with orbital dynamics shortly before merger are discussed. While being strongly gauge dependent, these findings are intuitive from a physical point of view and support basic ideas about the efficiency of gravitational radiation from a binary system.Comment: 9 pages, 6 figures, Amaldi8 conference proceedings as publishe

Ground State Wave Function of the Schr\"odinger Equation in a Time-Periodic Potential

Using a generalized transfer matrix method we exactly solve the Schr\"odinger equation in a time periodic potential, with discretized Euclidean space-time. The ground state wave function propagates in space and time with an oscillating soliton-like wave packet and the wave front is wedge shaped. In a statistical mechanics framework our solution represents the partition sum of a directed polymer subjected to a potential layer with alternating (attractive and repulsive) pinning centers.Comment: 11 Pages in LaTeX. A set of 2 PostScript figures available upon request at [email protected] . Physical Review Letter

Built-in field control in alloyed c-plane III-N quantum dots and wells

We investigate the degree to which the built-in electric field can be suppressed by employing polarization-matched barriers in III-N quantum well and dot structures grown along the c axis. Our results show that it is possible to take advantage of the opposite contributions to the built-in potential arising from the different possible combinations of wurtzite GaN, InN, and AlN when alloying the materials. We show that, for a fixed bandgap of the dot/well, optimal alloy compositions can be found that minimize the built-in field across the structure. We discuss and study the impact of different material parameters on the results, including the influence of nonlinear effects in the piezoelectric polarization. Structures grown with unstrained barriers and on GaN epilayers are considered, including discussion of the effects of constraints such as strain limits and alloy miscibility. (C) 2011 American Institute of Physics. [doi:10.1063/1.3563568