Green photon beams more agile than optical tweezers (Vol. 44 No. 6)

Charge-density computation of butane molecules

A new manipulation tool exploits the fact that when light interacts with matter, it creates a force that produces material properties in macromolecules and biological cells.

Romanian scientists have discovered a novel approach for the optical manipulation of macromolecules. The authors had the idea to use green photon beams. With them, it is possible to perform optical manipulation of macrostructures, such as biological proteins, with greater precision than with optical tweezers.

The authors used high-density green photon beams (HDGP) capable of inducing a polarisation effect within complex macrostructures. They found that the effect of the beam leads to ‘biological optical matter’. This includes newly-organised material structures, such as molecular aggregates and micro-particles, and can feature new characteristics such as antioxidant properties. The authors realised that this approach covers a larger area than focused tweezers and is capable of organising mesoscopic matter into a new 3D molecular architecture.

S. Comorosan, S. Polosan, I. Popescu, I. Stamatin, E. Ionescu, S. Avramescu, L. Cristian Cune and M. Apostol, 'Optical manipulation of complex molecular systems by high density green photons: experimental and theoretical evidence', Eur. Phys. J. B, 86, 232 (2013)
[Abstract]

Antimony variations in GaAs/GaAsSb heterostructured nanowires (Vol. 44 No. 6)

Schematic band diagram of a zinc blende GaAsSb insert in a wurtzite GaAs nanowire. The model is based on the structural and compositional analysis by scanning transmission electron microscopy and optical characterization by micro-photoluminescence.

Semiconductor nanowires have attracted huge attention recently due to their unique and often superior properties compared to bulk or planar counterparts. Complex heterostructures can be made and several nanowire-based devices (e.g. solar cells) have been realized. GaAsSb is an interesting ternary compound semiconductor because of its tunable bandgap and the possibility for both type I and type II band alignment with GaAs. In the present study 20-80 nm long zinc blende GaAsSb segments in wurtzite GaAs bare-core and GaAs/AlGaAs core-shell nanowires were studied. The work established the presence of both axial and radial compositional variations in the GaAsSb segments and their effect on the optical properties of these nanowires. The Sb concentration profiles within the inserts were determined using energy dispersive X-ray spectroscopy and quantitative scanning transmission electron microscopy and related directly to micro-photoluminescence measurements for the same single nanowires. The results of the article are relevant for further growth optimization and tailoring of the optical properties of GaAs/GaAsSb heterostructured nanowires.

J. Todorovic, H. Kauko, L. Ahtapodov, A. F. Moses, P. Olk, D. L. Dheeraj, B. O. Fimland, H. Weman and A. T. J. van Helvoort, ‘The effects of Sb concentration variation on the optical properties of GaAsSb/GaAs heterostructured nanowires’, Semicond. Sci. Technol. 28, 115004 (2013)
[Abstract]

Semiclassical propagation up to the Heisenberg time (Vol. 44 No. 6)

Long time propagation of a resonance in the hyperbola billiard. The Ehrenfest time is 2.2, and the Heisenberg time is 35.3

Semiclassical propagation of waves is a fruitful approach to understand and evaluate a wide set of physical processes. This is performed by associating quantum states with Lagrangian manifolds in phase space, and the propagation is accomplished by the evolution of manifolds. However, long time propagation in Hamiltonian systems with chaotic dynamics is a longstanding unsolved problem; the reason being that Lagrangian manifolds evolve into very complex objects.

Recently, we have shown that by using the stable and unstable manifolds of periodic orbits, the propagation is simplified enormously. For this reason, in this paper we study in detail the manifolds of a periodic orbit of the hyperbola billiard, finding that they are organized by a simple tree structure. Then, we compute a complete set of homoclinic orbits (resulting from the intersection of the manifolds), which is required to evaluate the autocorrelation function of a quantum state constructed in the neighborhood of the periodic orbit (resonance). Finally, we compare the quantum and semiclassical autocorrelation up to the Heisenberg time, finding a relative error of the order of the Planck constant.

E. G. Vergini, ‘Semiclassical propagation up to the Heisenberg time’, EPL, 103, 20003 (2013)
[Abstract]

Calculation of the connective constant for self-avoiding walks (Vol. 44 No. 5)

A typical self-avoiding walk of 100 million steps on the square lattice, generated using the pivot algorithm.

Self-avoiding walks are walks on a lattice, which are not allowed to self-intersect. Despite the apparent simplicity of the self-avoiding walk model, it is an important model of polymers, and over the past 60 years it has resisted all attempts to find an exact solution.

One of the basic features of self-avoiding walks is the number of walks for a given number of steps. The number of walks grows exponentially with length, and the rate of exponential growth, called the connective constant, is a quantity of fundamental interest.

Using a novel divide and conquer Monte Carlo algorithm, the number of self-avoiding walks on the simple cubic lattice for selected lengths of up to 38 million steps were estimated to high precision. For instance, the number of walks with 606 207 steps is 7.7 × 10406 535! Using these estimates the connective constant was found to be 4.684 039 931 ± 0.000 000 027, which is significantly more accurate than estimates obtained via alternative methods.

A key open question is whether similarly powerful enumeration methods can be found for other models in statistical mechanics.

Nathan Clisby, ‘Calculation of the connective constant for self-avoiding walks via the pivot algorithm’, J. Phys. A: Math. Theor. 46, 245001 (2013)
[Abstract]

Self-passivation of vacancies in α-PbO (Vol. 44 No. 5)

Migration of oxygen vacancy towards lead vacancy followed by pair formation

Polycrystalline Lead Oxide (PbO) is one of the most promising materials for application in radiation medical imaging. At the current stage of technology, electronic grade PbO is not achievable because of large defect concentration. Defects act as traps for x-ray generated charge carriers during their transit across PbO layers: average distance drifted before trapping is smaller than layer thickness. Therefore, suppression of the effect of defects on carrier transport is an important challenge in PbO technology.

In metal oxides, vacancies are the main source of traps. The authors have shown that in thermally deposited PbO layers both Pb and O vacancies appear primarily in charged states of opposite sign. As a result, neighbouring vacancies can form neutral pair, which is no longer act as trapping centre. This finding offers a practical way to improve the transport properties. The post-growth annealing would initiate migration of the O vacancies towards Pb vacancies and facilitate their merging and neutral pair formation. The reduction in an amount of ionized centres increases carrier mobility and suppresses recombination thus improving x-ray generated charge collection.

J. Berashevich, J. A. Rowlands and A. Reznik, ‘Self-passivation of vacancies in α-PbO’, EPL, 102 (2013) 47002
[Abstract]

Dynamic Screening acts on radiating GaAs/AlGaAs quantum wells (Vol. 44 No. 5)

Dependencies of the THz amplitude on the excitation fluence at different bias voltages. The solid curves are results of calculations by the developed model with the following parameters: me = 0.065m0, mh = 0.5m0, ε = 12.9, α0 = 1.8x104 cm−1 (electron and hole effective masses, dielectric constant and bulk optical absorption coefficient for ħωex = 1.56 eV, respectively). The initial electric field F0 in the QWs was supposed to be equal to the averaged electric field in the structure part containing QWs.

The article reports results of investigations of generation of coherent terahertz (THz) radiation from 29 nm thick GaAs/Al0.37Ga0.63As quantum wells (QWs) with transverse electric bias under interband femtoseconds laser photoexcitation at room temperature. The detected THz radiation is attributed to the excitation of time-varying dipole moment induced by polarization of non-equilibrium electron–hole pairs in QWs. Noticeable sub-linearity in the dependence of THz amplitude on excitation density is observed. A theoretical model, which accounts for the dynamic screening of the electric field in wide GaAs QWs by nonequilibrium carriers, has been developed. The model describes well the properties of the observed THz signal.

A.V. Andrianov, P.S. Alekseev, G.V. Klimko, S.V. Ivanov, V.L. Tscheglov, I.V. Sedova, and A.O. Zakhar’in, ‘Influence of dynamic screening effect on coherent terahertz radiation from biased GaAs/AlGaAs quantum wells’, Semicond. Sci. Technol. 28, 105012 (2013)
[Abstract]

Passivated Tunneling Contacts for c-Si Solar Cells by Al2O3 and ZnO Nanolayers (Vol. 44 No. 5)

Band diagram of the proposed novel front contact

Al2O3 nanolayers are well-known for their ability to reduce recombination losses at crystalline silicon surfaces, making Al2O3 an attractive material for passivation of the next-generation high efficiency solar cells. In this work, we try to take the application of Al2O3 one step further: when Al2O3 is deposited on n-type silicon, a high concentration of holes accumulates at the surface due to the high density of negative charges in the Al2O3. Consequently a pn-junction is formed which can replace the traditional front side p-doped region made by high temperature diffusion.

The idea presented in the article is to deposit a stack consisting of an Al2O3 and ZnO layer on the silicon. The holes separated from the electrons at the junction can tunnel through the Al2O3 into the transparent conductive ZnO where they are collected with minimal energy loss when the Al2O3 charge density and ZnO doping density are properly tailored. Using atomic layer deposition, it was demonstrated that Al2O3 and Al-doped ZnO films deposited with sub-nanometer precision can be used for this purpose with sufficiently high tunneling currents when the Al2O3 is about 1-2 nm thick.

D. Garcia-Alonso, S. Smit, S. Bordihn and W. M. M. Kessels, ‘Silicon passivation and tunneling contact formation by atomic layer deposited Al2O3/ZnO stacks’, Semicond. Sci. Technol. 28, 082002 (2013)
[Abstract]

Dissecting Deuteron Compton Scattering I (Vol. 44 No. 5)

Sensitivity of a double-polarisation-asymmetry observable to the E1-M2 spin polarisability.

The electromagnetic polarisabilities of the nucleons characterise their responses to external fields. The simplest are the electric and magnetic polarisabilities that describe the induced dipole moments. For spin-1/2 particles there are also four spin polarisabilities, analogous to rotations of the polarisation of light by optically active media. The best experimental window on them is Compton scattering of photons, which has provided good determinations of the electric and magnetic polarisabilities of the proton. Future experiments with polarised protons will give access to its spin polarisabilities. In contrast, much less is known of about the neutron since it does not exist as a stable target. Nonetheless, its properties can be obtained from Compton scattering on light nuclei, most notably the deuteron -- a weakly bound proton and neutron. A new generation of experiments is planned to provide beams of polarised photons on targets of polarised deuterons. If the spins of the final particles are not observed, there are 18 independent observables. This work provides, for the first time, the complete set of these, which will be needed for the experimental analyses. More importantly, it also examines their sensitivities to the various polarisabilities, which will be crucial for the design of the experiments.

H.W. Grießhamme, 'Dissecting Deuteron Compton Scattering I: The Observables with Polarised Initial States', Eur. Phys. J. A, 49, 100 (2013)
[Abstract]

Removing complexity layers from the universe’s creation (Vol. 44 No. 5)

A schematic depiction of the combined motion that a Brownian particle executes in a background polycrystalline space.

Understanding complexity in the early universe may require combining simpler models to interpret cosmological observations. The authors publish results pertaining to theoretical predictions of cosmological systems’ dynamics.

They found that the combination of Einstein’s special relativity and quantum-mechanical dynamics is mathematically identical to a complex dynamical system akin to two interlocked processes with different energy scales. To model it, the authors consider a quantum mechanical dynamics in a background polycrystalline space where particles exhibit a Brownian motion. The observed relativistic dynamics then comes solely from a particular grain distribution in this space. In cosmology, such distribution might stem from early universe’s formation.

This new interpretation focuses on the interaction of a quantum particle with gravity. The non-existence of the relativistic dynamics leads to a natural mechanism for the formation of particles-antiparticles asymmetry. When coupled with cosmology, the authors’ approach predicts that a charge asymmetry should have been produced at ultra-minute fractions of seconds after the Big Bang, in agreement with constraints born out of recent cosmological observations.

P. Jizba and F. Scardigli, ‘Special relativity induced by granular space’, Eur. Phys. J. C 73, 2491 (2013)
[Abstract]

How cells get a skeleton (Vol. 44 No. 5)

Microscopy image of a cell.

Stress generated by nano-motors within animal cells can lead to the creation of a condensed layer of filaments beneath the outer cell membrane.

The authors have found that a well-defined layer beneath animal cells’ outer membrane forms beyond a certain critical level of stress generated by motor proteins within the cellular system. They have created hydrodynamic models of active gels to model the cell cortex. They first derived the equations providing a coarse-grained description of cortical dynamics, then calculated the configuration in which their model was in a steady state.

They found that for sufficiently high levels of contractile stress it consisted of a dense layer near the membrane, which abruptly cut off beyond a certain thickness. The key advance in their model is the inclusion of gel disassembly throughout the system, and the contractility due to molecular motors. 

J.-F. Joanny, K. Kruse, J. Prost and S. Ramaswamy, ‘The actin cortex as an active wetting layer’, Eur. Phys. J. E 36, 52 (2013)
[Abstract]