Quantum localization of hydrogen atoms in solids

Quantum distributions of hydrogen-like atoms in solids were investigated theoretically by the first-principles path-integral molecular dynamics (FP-PIMD) method, in which interatomic forces are calculated precisely based on the density functional theory. Some distributions are qualitatively different from those obtained by conventional simulations with classical treatment of protons. Interestingly, quantum fluctuations sometimes cause localization of the nuclei. The mechanism of this ‘quantum localization’ is intuitively explained by considering characteristic potential energy surfaces.     

Longitudinal quantum beam tomography

Abstract

We propose an experiment to determine the density operator for the longitudinal quantum state of an atomic beam. The method is based on tomographic reconstruction of the Wigner function via a set of measured probability distributions for the phase-space rotated position operator. The time evolution of the Wigner function in free space effectively performs the required phase-space rotation. We propose a state-selective time-dependent technique for measuring longitudinal probability distributions.     

Quantum-dot-assisted characterization of microtubule rotations during cargo transport

Owing to their wide spectrum of in vivo functions, motor proteins, such as kinesin-1, show great potential for application as nanomachines in engineered environments. When attached to a substrate surface, these motors are envisioned to shuttle cargo that is bound to reconstituted microtubules--one component of the cell cytoskeleton--from one location to another. One potentially serious problem for such applications is, however, the rotation of the microtubules around their longitudinal axis. Here we explore this issue by labelling the gliding microtubules with quantum dots to simultaneously follow their sinusoidal side-to-side and up-and-down motion in three dimensions with nanometre accuracy. Microtubule rotation, which originates from the kinesin moving along the individual protofilaments of the microtubule, was not impeded by the quantum dots. However, pick-up of large cargo inhibited the rotation but did not affect the velocity of microtubule gliding. Our data show that kinesin-driven microtubules make flexible, responsive and effective molecular shuttles for nanotransport applications.     

Experimental microdosimetric distributions of protons, from 19 to 65 MeV, using an A150-walled Low Pressure Proportional Counter

Microdosimetric distributions of protons, from 19 to 65 MeV, were measured using an A150-walled Low Pressure Proportional Counter at the cyclotron facility in the National Institute of Radiological Sciences. The measured distributions were found to be very different from the spectra assumed by the Continuous Slowing Down Approximation. The measurements consisted of determining the energy deposition by direct incident protons, by secondary electrons produced in the wall of the detector and by scattered protons at the wall. The secondary electron and the scattered proton distributions were studied using analytical functions and the MCNPX Monte Carlo code, respectively.     

A Minimalist Single-Layer Metasurface for Arbitrary and Full Control of Vector Vortex Beams

Vector vortex beams (VVBs) possess ubiquitous applications from particle trapping to quantum information. Recently, the bulky optical devices for generating VVBs have been miniaturized by using metasurfaces. Nevertheless, it is quite challenging for the metasurface-generated VVBs to possess arbitrary polarization and phase distributions. More critical is that the VVBs' annular intensity profiles demonstrated hitherto are dependent on topological charges and are hence not perfect, posing difficulties in spatially shared co-propagation of multiple vortex beams. Here, a single-layer metasurface to address all those aforementioned challenges in one go is proposed, which consists of two identical crystal-silicon nanoblocks with varying positions and rotation angles (i.e., four geometric parameters throughout). Those four geometric parameters are found to be adequate for independent and arbitrary control of the amplitude, phase, and polarization of light. Perfect VVBs with arbitrary polarization and phase distributions are successfully generated, and the constant intensity profiles independent of their topological charges and polarization orders are demonstrated. The proposed strategy casts a distinct perception that a minimalist design of just one single-layer metasurface can empower such robust and versatile control of VVBs. That provides promising opportunities for generating more complex vortex field for advanced applications in structural light, optical micromanipulation, and data communication.     

Mechanisms behind the generation of protonated ions for polyaromatic hydrocarbons by atmospheric pressure photoionization

In this study, the mechanism behind the generation of protonated polyaromatic hydrocarbon (PAH) ions without heteroatoms by atmospheric pressure photoionization (APPI) is investigated. Comparing data obtained by APPI of anthracene dissolved either in toluene or perdeuterated toluene suggests that toluene acts as a source of protons and that breakage of C-H bonds in the toluene molecule is important for the overall protonation reaction. Our data describing an Arrhenius-type temperature-dependent relationship between the signal intensities of molecular and protonated ions suggest a mechanistic relation between the generated molecular and protonated ions. The APPI protonation mechanism that best explains the observed phenomena is composed of two reactions: electron transfer followed by hydrogen transfer. This two-step mechanism for APPI was originally suggested by Syage (Syage, J. A. J. Am. Soc. Mass Spectrom. 2004, 15 , 1521-1533). Further quantum mechanical study shows that an energetically favorable ion-molecular complex can be generated as a result of electron transfer from toluene to PAH, which subsequently facilitates hydrogen transfer. This suggests that both electron transfer and hydrogen transfer can occur as a "concerted" reaction through the ion-molecular complex precursor state, which is consistent with experimental results. To our best knowledge, this is the first time that the dynamic nature of the APPI process is clearly revealed by combined experimental and quantum mechanical studies.     

Quantum Turbulence in Trapped Atomic Bose-Einstein Condensates

Generally there are two kinds of cooperative phenomena comprised of quantized vortices. One is a vortex lattice under rotation, and the other is a vortex tangle (quantum turbulence) made by some flow. Both have been studied in the field of superfluid helium through the long research history. On the other hand, the research of atomic Bose-Einstein condensates (BECs) has been limited to the former case, namely a vortex lattice. In this work, we address for the first time quantum turbulence in atomic BECs theoretically and numerically. We propose how to make quantum turbulence in a trapped BEC by combining rotation around two axes, and confirm the Kolmogorov spectra by the Gross-Pitaevskii model.      

Ultrafast spin dynamics in colloidal ZnO quantum dots

Time-resolved Faraday rotation measurements in the ultraviolet have been performed to reveal the ultrafast spin dynamics of electrons in colloidal ZnO quantum dots. Oscillating Faraday rotation signals are detected at frequencies corresponding to an effective g factor of g = 1.96. Biexponential oscillation decay is observed that is due to (i) rapid depopulation of the fundamental exciton (tau = 250 ps) and (ii) slow electron spin dephasing ( T 2 = 1.2 ns) within a metastable state formed by hole-trapping at the quantum dot surface.     

Effect of accelerated crucible rotation on the axial impurity distribution in proustite single crystals

The effect of modulated crucible rotation on the axial distributions of Cu and Mg impurities in proustite single crystals grown by the Stockbarger method using accelerated crucible rotation is studied in a broad range of Taylor numbers: from 1.9 × 10⁵ to 7.12 × 10⁷. The axial impurity distributions in the tail portion (containing the highest impurity concentration) of the crystals are measured by electron probe x-ray microanalysis and are shown to correlate well with the periodic modulation of the crucible rotation rate.Translated from Neorganicheskie Materialy, Vol. 41, No. 2, 2005, pp. 143–147.Original Russian Text Copyright © 2005 by Distanov, Kirdyashkin.     

A fine structure in angular distributions of protons transmitted through insulating capillaries

The processes of interaction of accelerated protons with the surface of insulating capillaries have been investigated to determine the character of motion of particles during such an interaction. The angular distributions of 240-keV protons transmitted through glass capillaries have been measured, and splitting of the distribution into a series of lines equally spaced from each other is revealed.     

Straggling effects in a proton recoil neutron spectrometer

The influence of proton straggling on the efficiency and resolution of a combined proton recoil and time-of-flight neutron spectrometer, TANSY, is studied. The spectrometer is designed for 14 MeV neutron diagnostics at JET (Joint European Torus). Energy and angular distributions for 14 MeV protons slowing down in polyethylene foils with various thicknesses are presented. The results indicate that for a foil thickness of 1.24 mg cm⁻² about 10% of the protons lose more than twice the average energy loss. Furthermore, the angular straggling leads to a coincidence loss of 4.6 ± 0.4%.     

Thickness distribution of films fabricated by the molecular beam epitaxy technique

The thickness distribution of the film deposited on a rotating substrate holder depends on both the geometrical parameters of the molecular-beam-epitaxy apparatus and the angular distribution of molecular flux from the evaporation furnace. First, Deppisch's formulation for calculation of the thickness distribution is generalized so that it can be applied to a more practical case when the furnace is inclined relative to the axis of rotation of the holder. Next, the angular distributions are calculated for several types of furnaces of practical use by means of the test-particle Monte Carlo method. They are expressed in simple analytical forms. Lastly, example calculations are made to show how to determine such geometrical parameters as are optimum for yielding uniformly thick films.     

Above threshold dissociation in HD+ using frequency chirped laser pulses

We have theoretically studied the dynamics of above threshold dissociation (ATD) in molecular ions HD⁺ using frequency chirped femtosecond laser pulses from numerical solutions of the time-dependent Schrödinger equation by using the three-dimensional time-dependent quantum wave packet method. Energy-dependent distributions of ATD fragments are analyzed by an asymptotic-flow expression in momentum space. Linearly positive and negative frequency chirped laser pulses are adopted. It is found that varying frequency chirped parameters can change branching ratios of the 1sσ _g and 2pσ _u dissociations channels. The concept of a light-induced potential is used to interpret the ATD process. The angular resolved energy distributions of the photofragments are also illustrated.     

Modelling and calculations of the response of tissue equivalent proportional counter to charged particles

Space activities in earth orbit or in deep space pose challenges to the estimation of risk factors for both astronauts and instrumentation. In space, risk from exposure to ionising radiation is one of the main factors limiting manned space exploration. Therefore, characterising the radiation environment in terms of the types of radiations and the quantity of radiation that the astronauts are exposed to is of critical importance in planning space missions. In this paper, calculations of the response of TEPC to protons and carbon ions were reported. The calculations have been carried out using Monte Carlo track structure simulation codes for the walled and the wall-less TEPC counters. The model simulates nonhomogenous tracks in the sensitive volume of the counter and accounts for direct and indirect events. Calculated frequency- and dose-averaged lineal energies 0.3 MeV-1 GeV protons are presented and compared with the experimental data. The calculation of quality factors (QF) were made using individual track histories. Additionally, calculations of absolute frequencies of energy depositions in cylindrical targets, 100 nm height by 100 nm diameter, when randomly positioned and oriented in water irradiated with 1 Gy of protons of energy 0.3-100 MeV, is presented. The distributions show the clustering properties of protons of different energies in a 100 nm by 100 nm cylinder.     

The information of ambiguity

The phase space characteristics of a quantum state are best captured by the Wigner distribution. This displays transparently the diagonality information of the density matrix. The complementary function offering transparently the off-diagonal elements is captured by a function called the S-function, or the ambiguity. In carrying the maximal information about the quantum coherences it represents the uncertainties or ambiguity of the diagonal information. Mathematically this is manifested in its role as the phase space moment generating function. Formally it complements the information in the Wigner function. These formal relations provide the starting point for the present investigations. As a measure of quantum uncertainties, ambiguity may be used to define a probability measure on the off-diagonality. The mathematical and physical consistency of this view is presented in this paper. For a pure state, we find the extraordinary result that such distributions are their own Fourier transforms. The physical interpretation of this distribution as a carrier of classical signal fuzziness suggests the introduction of heuristic approximations to the observational uncertainties. We illustrate the properties and interpretation of the ambiguity function by some specific examples. We find that for smooth, ‘Gaussian-like’ distributions, the heuristic considerations provide good approximations. On the other hand, representing quantum interferences, the ambiguity serves as the most positive probe for the ultimate quantum structures which have been called sub-Planckian. They are interesting because it has been argued that such structures are physically observable.     

Influence of Quantum Interference on the Tunnel Magnetoresistance in a Molecular Junction in the Presence of In-Plane Electric Field

Quantum interference effects on the spin-dependent transport in a molecular spin-valve, as FM/Benzene/FM model junction, in which a benzene molecule attached to ferromagnetic (FM) three-dimensional leads is numerically investigated. Using a generalized Green’s function method and in the framework of Landauer–Büttiker formalism, the variation of interference conditions is determined by rotation of the benzene molecule, and replacement of the connection configurations of the molecule to the FM leads from ortho to meta and then para configuration. We have found that transport characteristics, including the spin-dependent current and tunnel magnetoresistance (TMR) are strongly influenced by the quantum interference effects. Besides, the effect of an in-plane electric field on the spin-dependent transport characteristics is treated.     

Quantum diffusion of positive muons and muonium atoms

The diffusion properties of the positive muon (μ⁺) and muonium (Mu) as light isotopes of protons and atomic hydrogen in crystalline solids are reported. It is demonstrated that they are excellently described as quantum tunneling of small polarons interacting with a phonon/electron bath. As one of the most dramatic manifestations of the quantum nature, the tunneling probability increases with decreasing temperature, which is in sharp contrast with the case of thermally activated diffusion. Moreover, the recent experimental study at very low temperature (below 0.1 K) strongly suggests that Mu is in a delocalized state analogous to the Bloch state for conduction electrons.     

Mathematical analysis of whirled turbulent flow through a pipe

The effect of rotation of the stream on the development of turbulent flow in a pipe is analyzed by a numerical method. Calculated distributions of average turbulence velocity and energy are compared with experimental data.     

Effect of spanwise rotation on centrifugal instability in rotating curved non-isothermal flows

 The effect of spanwise rotation on the centrifugal instability is examined numerically in terms of flow structure transitions by a finite volume method. The specific problem considered is a steady, hydrodynamically and thermally fully developed flow in a square duct with a strong streamwise curvature, a spanwise rotation in both positive and negative directions and wall cooling. The rotation is found to be of considerable effect on flow structures. As well the effect is more complex for non-isothermal flows due to a buoyancy force which increases with the square of the rotation speed. Several hitherto unknown flow structures are revealed. The results cover both transitions in flow structures and effects of the flow transitions on temperature distributions.