Macroscopic Separation Research Articles

Wave–particle duality and quantum erasure in polarized–neutron interferometry

An interference experiment with polarized neutrons is proposed to check the duality relation between the fringe visibility and the which-way information for massive fermions with macroscopic spatial separation of the interfering sub-beams. The connection between polarization-sensitive post-selection of sub-ensembles and the concept of quantum erasure is discussed. This neutron experiment will supplement those recently performed with photons and atoms.

Effect of staphylococcal delta-lysin on the thermotropic phase behavior and vesicle morphology of dimyristoylphosphatidylcholine lipid bilayer model membranes. Differential scanning calorimetric, 31P nuclear magnetic resonance and Fourier transform infrared spectroscopic, and X-ray diffraction studies.

We investigated the effects of various concentrations of staphylococcal delta-lysin on the thermotropic phase behavior of large multilamellar dimyristoylphosphatidylcholine (DMPC) vesicles by differential scanning calorimetry (DSC), 31P nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy, and X-ray diffraction. The DSC studies revealed that at all concentrations, the addition of delta-lysin progressively decreases the enthalpy of the pretransition of DMPC bilayers without significantly affecting its temperature or cooperativity. Similarly, the addition of smaller quantities of peptide has little effect on the temperature of the main phase transition of DMPC bilayers but does reduce the cooperativity and enthalpy of this transition somewhat. However, at higher peptide concentrations, a second phase transition with a slightly increased temperature and a markedly reduced cooperativity and enthalpy is also induced, and this latter phase transition resolves itself into two components at the highest peptide concentrations that are tested. Moreover, our 31P NMR spectroscopic studies reveal that at relatively low delta-lysin concentrations, essentially all of the phospholipid molecules produce spectra characteristic of the lamellar phase, whereas at the higher peptide concentrations, an increasing proportion exhibit an isotropic signal. Also, at the highest delta-lysin concentrations that are studied, the isotropic component of the 31P NMR spectrum also resolves itself into two components. At the highest peptide concentration that was tested, we are also able to effect a macroscopic separation of our sample into two fractions by centrifugation, a pellet containing relatively smaller amounts of delta-lysin and a supernatant containing larger amounts of peptide relative to the amount of lipid present. We are also able to show that the more cooperative phase transition detected calorimetrically, and the lamellar phase 31P NMR signal, arise from the pelleted material, while the less cooperative phase transition and the isotropic 31P NMR signal arise from the supernatant. In addition, we demonstrate by X-ray diffraction that the pelleted material corresponds to delta-lysin-containing large multilamellar vesicles and the supernatant to a mixture of delta-lysin-containing small unilamellar vesicles and discoidal particles. We also show by FTIR spectroscopy that delta-lysin exists predominantly in the alpha-helical conformation in aqueous solution or when interacting with DMPC, and that a large fraction of the peptide bonds undergo H-D exchange in D(2)O. However, upon interaction with DMPC, the fraction of exchangeable amide protons decreases. We also demonstrate by this technique that both of the phase transitions detected by DSC correspond to phospholipid hydrocarbon chain-melting phase transitions. Finally, we show by several techniques that the absolute concentrations of delta-lysin and the thermal history, as well as the lipid:peptide ratio, can affect the thermotropic phase behavior and morphology of peptide-lipid aggregates.

Transition from Rydberg to giant-dipole-moment states of hydrogen atoms in crossed fields: A suggestion for an experiment

The pseudoseparation of the center of mass and relative motion of a hydrogen atom in crossed electric and magnetic fields led to a theoretical prediction of highly extended states localized in the outer potential well. These states correspond to an almost macroscopic separation between a proton and an electron, and give rise to giant dipole moments. In the present work we describe a simple, experimentally feasible, scheme for the preparation of a hydrogen atom in a superposition of such states. The preparation scheme is based on a two-step switching on of an external electric field, in the course of which the relative motion is trapped in the outer well. On the basis of classical simulations we suggest the experimental parameters at which trapping is achieved extremely efficiently.

Microphase Separation in a Mixture of Block Copolymers in the Strong Segregation Regime

Microphase separation in a blend of A-B and C-B block copolymers with strong A-B and C-B incompability is considered in the strong segregation regime. It is assumed that the length of the B block is always much larger than the lengths of A and C blocks, so the complex spherical micelles containing shorter A and longer C chains are formed. The inner structure of the complex A-C micelle is considered, and the distribution of volume fractions of A and C monomer units is calculated. It is shown that shorter A chains are distributed mainly in the periphery of the micelle, while C chains are coiled in the center of this micelle. Macroscopic separation in this blend is studied. It is shown that, in the mixtures with a small fraction of C-B block copolymer, a two-phase structure should be observed: one phase consists of practically pure A micelles, while the other phase the complex A-C micelles, with enriched contents of longer C chains, are formed. For chemically different A and C blocks, even slight A-C repulsion leads to phase separation between the phases with pure A and pure C micelles.

Decoherence and Schrödinger-cat states in a Stern-Gerlach-type experiment

A Stern-Gerlach setup for the measurement on a quantum system (spin-$$$\frac{1}{2}$) with a macroscopic quantum apparatus (the particle trajectory) produces a Schr\"odinger-cat-like superposition in which the spin states correlate with wave packets centered around macroscopically distinguishable positions and momenta. When an interaction with an environment is included, the pure density matrix of the system-apparatus combination reduces to a statistical mixture in the spin space, and exact solutions show that the decoherence time for this reduction goes inversly as the macroscopic separation between the two parts of the superposition correlating with up- and down-spin states. This is consistent with Zurek's approximate result for the decoherence time, and the persistence of system-meter correlations at large times makes it an interesting candidate to look at experimentally.

Imaging the nonlinear grating in frequency-doubling fibres

THE second-order response of a transparent material to intense light creates an oscillatory electromagnetic field at twice the driving frequency. Materials with a strong second-order response can therefore be used for frequency-doubling, for example to convert infrared laser light to visible light1. Although amorphous materials have no significant intrinsic second-order response, glass fibres can nevertheless exhibit second-harmonic generation after exposure to intense laser irradiation2. Beating between the electromagnetic fields of the laser light at the fundamental frequency and a weak second-harmonic signal (externally applied or intrinsic to the fibre) permanently modifies the glass and enhances the second-order response; the high efficiency of the response points to the formation of a periodic electric-field grating within the fibre3–7. High electric fields have been detected in fibres8 and the existence of a grating has been confirmed indirectly9. Here we present direct images of this grating in germanosilicate optical fibres, obtained by exposing the fibres to chemical attack by hydrofluoric acid while the grating is in place. The rate of etching is sensitive to the intensity of the internal electric field in the fibres. Our results are consistent with the idea that the grating results from macroscopic separation of charge at the boundary between the fibre core and cladding, rather than from a microscopic reorientation of dipoles throughout the material. (This article was corrected on 21 October 2015.)

Macroscopic Isotope Separation of 13C by a CO2 Laser

Abstract By photochemical dissociation of the rare carbon isotope component of CHClF2 by means of a CO2 laser with an average power of 150 W, Q-switched at 10 kHz, we have demonstrated the separation of more than 1 mol of 13C, enriched to 50% (2 mol of total carbon). It is contained in about 1 mol (101 g) of the product C2F4. The total throughput of the starting material was 29 kg. The experiment was run day and night for 2 weeks, almost only controlled by a computer. We obtained production rates of 5 mmol/h, corresponding to about 0.5 kg 13C per year.

Dember effect in C 60 thin films

Photoinduced diffusion of charge carriers is observed in the form of a potential difference across thin films of photoexcited C 60. The threshold photon energy for macroscopic separation of mobile charge carriers in C 60 is 2.3 ± 0.1 eV. This is also a conclusive demonstration of the fact that the optical absorption at 2.3 eV is responsible for electronic excitations across the C 60 “bandgap”. Experimental evidence suggests a bimolecular process for charge transport and carrier dynamics in C 60 solids. The polarity of the observed photovoltage confirms earlier published results that the negative charge carriers (electrons) are significantly more mobile than the positive charge carriers (holes) in C 60 solids. The estimated mobility of the electrons in C 60 is ≈ 1 cm 2 V −1. The diffusion length for electrons in C 60 is estimated to be ≈ 10 microm.

Electron emission from ferroelectrics - a review

The strong pulsed emission of electrons from the surface of ferroelectric (FE) materials was discovered at CERN in 1987. Since then many aspects and properties of the method of generation and propagation of electron beams from FE have been studied experimentally. The method is based on macroscopic charge separation and self-emission of electrons under the influence of their own space-charge fields. Hence, this type of emission is not limited by the Langmuir-Child law as are conventional emission methods. Charge separation and electron emission can be achieved by rapid switching of the spontaneous, ferroelectric polarization. Polarization switching may be induced by application of electrical-field or mechanical-pressure pulses, as well as by thermal heating or laser illumination of the ferroelectric emitter. At higher emission intensities plasma formation assists the FE emission and leads to a strong growth of emitted current amplitude, which is no longer limited by the FE material and the surface properties. The most attractive features of FE emission are robustness and ease of manipulation of the emitter cathodes which can be transported through atmospheric air and used without any problems in vacuum, low-pressure gas or plasma environments. Large-area arrangements of multiple emitters, switched in interleaved mode, can produce electron beams of any shape, current amplitude or time structure. The successful application of FE emission in accelerator technology has been demonstrated experimentally in several cases, e.g. for triggering high-power gas switches, for photocathodes in electron guns, and for electron-beam generators intended to generate, neutralize and enhance ion beams in ion sources and ion linacs. Other applications can be envisaged in microwave power generators and in the fields of electronics and vacuum microelectronics.

Strong nonlinearity under weak pumping. New approach to the problem of optical damage

It is shown that even at homogeneous pumping of glass-like materials macroscopic charge separation takes place. In spite of the strong Coulomb repulsion the pumped volume divides into electron and hole domains. As a result, a strong macroscopic electric field and a contribution to the refractive index appear. 1/ƒ noise in the refractive index fluctuation is predicted.

The kinetics of defect accumulation under irradiation: many-particle effects

The kinetics of Frenkel defect accumulation under permanent particle source (irradiation) is discussed with special emphasis on many-particle effects. Defect accumulation is restricted by their diffusion and annihilation, A + B → 0, if the relative distance is less than the critical distance r0. A novel formalism of many-point particle densities based on Kirkwood's superposition approximation is developed to take into account aggregation of similar defects (A−A, B−B). The dependence of the saturation concentration after a prolonged irradiation upon spatial dimension (d̄ = 1, 2, 3), defect mobility and the initial correlation within geminate pairs is analyzed. It is shown that the defect concentration really saturates only for d̄ = 3 whereas for low dimensions (d̄ = 1, 2) it is not the case because of the macroscopic defect separation into alternating domains containing defects A or B only.

Oxygen tracer study of the high-temperature oxidation of pure and impure cobalt

Oxygen tracer experiments were used to examine the role of oxygen movement in the development of CoO scales at 1273 K and 1373 K. Samples were oxidized in various sequences of16O2 and18O2, and the resulting isotope distributions imaged in polished sections of scales using a Camec a SIMS. Scales that were simple (i.e., plane parallel and composed of large, pore-free columnar grains) grew without a significant contribution by movement of oxygen. However, oxygen circulation within the scale was essential to the growth of duplex scales that developed after extended periods of oxidation. The relatively coarsegrained inner oxide on the purer Co grew without macroscopic separation of the scale from the metal. The oxygen for this inner scale came from the development of large pores in the columnar part of the scale. The fine-grained inner oxide on the impure Co had a crudely layered structure due to repeated sequences of scale separation followed by growth of porous oxide on the metal back to the main scale. The oxygen for these inner layers came in part from the development of large pores, but also from a more general dissociation of the base of the columnar scale. Scale failures, which occurred primarily at corners of specimens, did allow penetration of oxygen from the atmosphere to the inner scale. However, these failures sealed and were not essential for continued scale growth.

Pressure-dependent measurements on n+ GaAs (Si, Sn): the effect of deep donor (DX) states on the electrical properties and persistent photoconductivity

Shubnikov-de Haas and persistent photoconductivity measurements are used to study the mobility, free electron density (n) and the occupancy of the DX centre in heavily doped n-GaAs as a function of doping level and hydrostatic pressure. The results show that the DX centre produces a resonant donor level between the Γ - and L-conduction band minima at a concentration comparable with the doping level. For the Si-doped samples, comparison with local vibration mode measurements indicates that the DX level can be identified with SiGa. The level acts to pin the Fermi energy at electron concentrations around 1.8 × 1019 cm−3. Analysis of the results indicates that macroscopic charge separation is not responsible for persistent photoconductivity in these samples.

Photophysical and photochemical switches based on twisted intramolecular charge transfer (TICT) states

TICT states, accessible in twisted multichromophoric systems, are nonradiative funnels to the ground state in many dyes. By controlling these funnels, faster saturable absorbers for subpicosecond laser pulses can be developed. Oriented assemblies of TICT molecules, as in liquid crystalline polymers, are expected to exhibit light-induced macroscopic charge separation. Chemical approaches to supramolecular bistable species are also shown.

Aggregation behaviour of stereoregular poly(methyl methacrylates) in dilute solution: light scattering and proton n.m.r. study

The aggregation behaviour of syndiotactic and isotactic poly(methyl methacrylate) in methyl ethyl ketone, n-butyl acetate, and 2-ethoxyethanol was investigated by light scattering and 1H n.m.r. spectroscopy. Syndiotactic poly(methyl methacrylate) remains in its molecular form in solution at temperatures above 60°–70°C; on cooling it undergoes aggregation followed by macroscopic separation (precipitation) of the polymer from solution. The rate of these processes depends on temperature, concentration of the polymer, and solvent. Molecular solutions of isotactic poly(methyl methacrylate) can be prepared only by long-term heating at temperatures above 100°–130°C. During cooling, isotactic macromolecules prior to separation form stable associates in the region limited by discrete temperatures, and below this region polymer precipitates. The individual stereo forms of poly(methyl methacrylate) are separated from solution at different temperatures which are above the θ-temperatures of the atactic polymer.

Pyroelectric induced optical damage in LiNbO3

Laser-induced changes in the extraordinary index of refraction (optical damage) in ferroelectrics are caused by macroscopic charge separation produced during laser illumination. If the sample is heated in the vicinity of the laser beam, pyroelectric fields are produced in ferroelectrics which can also produce macroscopic charge separation. We report the observation of the continuous transition in LiNbO3 between macroscopic charge separation associated with optical damage and pyroelectricity. Macroscopic charge separation due to pyroelectricity represents a second source of optical damage in ferroelectrics which can occur whenever the temperature of the sample is raised significantly (32 °C for LiNbO3).

Electrostatic observations of laser-induced optical damage in LiNbO3

A new technique for observing optical damage is reported. It is based on detecting electrostatic potentials which result from the laser-induced macroscopic charge separation responsible for optical damage. This new technique has been used to determine, for the first time, the charge distribution of the displaced charges. For uniform circular illumination of diameter D, the charges appear to reside in a thin cylindrical shell also of diameter D, with the negative charge on the positive ferroelectric axis. For a Gaussian illumination, we observe the charge distribution to change with increasing exposure time, approaching a cylindrical-shell charge distribution, a result which may provide an explanation for ’’optical clean-up’’. This new technique for probing optical damage is as sensitive as standard optical techniques and is unaffected by temperature variations and mechanical vibrations.

Thermodynamics of two-dimensional plasmas or discrete line vortex fluids

The equilibrium properties of a two-dimensional plasma are examined theoretically within the framework of the random phase approximation. The guiding-center case is of most interest because of the formal equivalence to two-dimensional discrete line vortex fluids and because of the formal admissibility of negative temperatures. Using a judicious treatment of the periodic boundary conditions, extensive thermodynamics for nonnegative temperatures and a new value for the energy at which the temperature becomes negative are found. An explicit form for the structure function is derived and compared with the result of a Monte Carlo calculation. Good agreement is found for the negative temperature threshold prediction and for the general shape of the structure function. An equilibrium energy spectrum is calculated, which for positive temperatures, is the Debye–Hückel result; for negative temperatures it predicts an enhanced excitation of the lowest mode, corresponding to macroscopic charge separation.

Macroscopic isotope separation of uranium by selective photoionization

Macroscopic isotope separation of uranium by selective photoionization

Mutual solubility of polymers and properties of their mixtures

AbstractHeats of mixing of polymers with each other have been measured, the behavior of the mixtures of solutions of various polymers has been studied, and the dependence of mechanical properties of polymer mixtures on the ratio of components has been investigated. It has been shown that mixing of polymers with each other is usually an endothermic process and, therefore, leads to formation of macroscopically homogenous, but actually microheterogenous, systems with an extremely high degree of dispersion. These microheterogenous polymer mixtures are formed because of the enormous viscosity of polymer mixtures, which prevents macroscopic separation into phases but does not hinder the considerable mobility of the segments of flexible chain molecules. It has been shown that the dependence of mechanical properties of microheterogenous polymer mixtures on the ratio of polymers in the mixture have sharp maxima or minima which cannot be found in the case of true polymers in polymer solutions. It has been found that the behavior of some polymer pairs is anomalous, in that exothermal mixing is supplemented by separation of the solution mixture into phases and by the appearance of maxima or minima in the dependences of the properties of polymer mixtures on the ratio of polymers in the mixture. This anomaly has been attributed to the effect of loose packing of the molecules of the polymers which show anomalous behavior. It has been shown that, in these systems, there necessarily exists a lower critical temperature of mixing whose value can be decreased by adding low‐molecular solvents to the loosely packed polymer. Attention has been drawn to the fact that, although mixing of amorphous polymers should be considered on a thermodynamic basis to be a mutual solution of two liquid phases, the large dimensions and the flexibility of polymer chain molecules require a critical revision of the possibility of formal application of the basis thermodynamic concepts and relations to a theoretical analysis of the behavior of polymer mixtures.