Vol. 44 No.5 - Highlights

Hydrodynamics of the fractional quantum Hall effect (Vol. 44 No. 6)

Many features of the Fractional Quantum Hall effect (FQHE) can be understood by considering electrons on the surface of semiconductors as a very peculiar, charged, two-dimensional fluid in the presence of a strong magnetic field. In this paper a classical hydrodynamic model of such a fluid is presented.

The model incorporates a relation between the vorticity and density of the fluid specific for FQHE and exhibits the Hall viscosity and Hall conductivity found in FQHE liquids. The relation of the model to previous effective models such as the Chern–Simons–Ginzburg– Landau theory of FQHE is explained. It is also shown how the Laughlin’s wavefunction is annihilated by the quantum velocity operator.

A. G. Abanov, ‘On the effective hydrodynamics of the fractional quantum Hall effect’ J. Phys. A: Math. Theor., 46, 292001 (2013)

A multi-object spectral imaging instrument (Vol. 44 No. 6)

Fluorescent microbeads are tracked using a camera. A series of slits displayed on a DMD deflect light into a spectrometer and are updated to follow the microbeads. Spectra are obtained in real time from a CCD image of the slits after dispersion from a prism.

Imaging spectrometers acquire three-dimensional spectral data cubes (x, y, λ) to enable chemical imaging in fields ranging from microscopy and biomedicine to remote sensing. Traditional systems, employing time-sequential recording of the complete data cube, cannot record time-varying phenomena and are optically highly inefficient. Our technique enables spectra to be recorded in real time from a programmable sparse array of spots within a microscope sample. Real-time computer-controlled manipulation of the spot array enables tracking and video-rate spectroscopy of these points with the very high optical efficiency. This is achieved with a digital micro-mirror device (DMD) that deflects light from the sparse multiple points in the sample into a slitless spectrometer consisting of a dispersive prism and CCD camera. We demonstrate real-time spectra of multiple fluorescent microbeads in aqueous solution as they diffuse in the sample.

G. M. Gibson, M. Dienerowitz, P. A. Kelleher, A. R. Harvey and M. J. Padgett, ‘A multi-object spectral Imaging Instrument’, J. Opt. 15, 085302 (2013)

X-rays emitted in neutron-induced fission of 238U(n,f) (Vol. 44 No. 6)

Charge distribution determined from the x-ray yield measurements (symbols) for different incident neutron intervals. For more detail see text.

Prompt x-rays emitted in neutron-induced fission help unveil the evolution of fission fragment charge yields as a function of incident neutron energy.

Nuclear fission is accompanied by the prompt emission of neutrons, gamma rays and x-rays. It has been known since the sixties that fission prompt x-rays originate essentially as a consequence of the internal conversions occurring in the prompt gamma deexcitation cascades of fission fragments.

This work presents for the first time a measurement of the prompt fission x-ray yields in 238U(n,f) for average incident neutron energies ranging from 3 to 200 MeV. Fission fragment charge distributions are derived from the measured x-ray yields using x-ray emission probabilities per fragment obtained in an earlier work on low energy fission. The results are found to be in a remarkable agreement with the Wahl phenomenological systematics for fission product yields, as well as with the more sophisticated GEF fission model. More detailed comparisons demonstrate that x-ray emission evolution with increasing incident neutron energy tends to be dominated by the transition towards lighter fragments which on average are closer to closed-shell nuclei and are thus less subject to internal conversion.

T. Granier, R.O. Nelson, T. Ethvignot, M. Devlin, N. Fotiades, P.E. Garrett and W. Younes, ‘Measurement of prompt X-rays in 238U(n,f) from threshold to 400MeV’, Eur. Phys. J. A 49, 114 (2013)

Atom-based analogues to electronic devices (Vol. 44 No. 6)

Spectral functions of the first (left panel) and the second (right panel) quantum dot.

New research gives a theoretical explanation as to how transport of single atoms that may be applied to optical lattices is made possible through a chain of quantum dots.

The authors have pushed back the boundaries of atom-based transport, creating a current by characterising the many-body effects in the transport of the atoms along a periodic lattice. This work has adopted a new analytical approach before comparing it to approximate numerical simulations, and is reported in the present paper.

Ultra-cold atoms trapped in optical potentials offer solutions for the transport of particles capable of producing a current. In this study, the authors extended previous single-atoms transport approaches to a model reflecting the many-body setting of bosonic atoms transport. Their challenge was to develop an analytical approach that allows particles to jump in and out and therefore produce a controlled current through the sample under study. They used a chain of quantum dots coupled to two bosonic reservoirs that keep the system far from equilibrium.

G. Ivanov, G. Kordas, A. Komnik and S. Wimberger, ‘Bosonic transport through a chain of quantum dots’, Eur. Phys. J. B, 86, 345 (2013)

Electron wave packet after tunnel ionization (Vol. 44 No. 6)

Coulomb potential tilted by the laser field. More accurate theoretical descriptions are necessary to account for the longitudinal spread

The dynamics of electron ionization are an important topic in attosecond science. Applying a strong laser pulse, electrons can quantum mechanically tunnel through the potential barrier created by the combined Coulomb field of the atom and the laser field. At the tunnel exit, it is commonly assumed that the electron velocity parallel to the electric field is zero, contrary to the well-described distribution of transverse momenta.

After ionization, electrons propagate in the remainder of the laser pulse, where they acquire a momentum spread due to the different phases of the field at their individual exit time. However, the longitudinal momentum spread measured in experiments on helium is considerably larger than that.

Monte Carlo simulations with zero initial longitudinal momentum agree with the theoretical predictions of acquired spread, while simulations that include a longitudinal momentum spread at the tunnel exit are compatible with experimental data. The authors introduced a new method to investigate electron velocity spreads after ionization. Applying this method to experimental data lead to a more accurate reconstruction of the electron wave packet immediately after tunnelling.

C. Hofmann, A. S. Landsman, C Cirelli, A. N. Pfeiffer and U. Keller, ‘Comparison of different approaches to the longitudinal momentum spread after tunnel ionization’, J. Phys. B: At. Mol. Opt. Phys. 46, 125601 (2013)

Universality in the symmetric exclusion process (Vol. 44 No. 6)

A system connected to two sources of heat or particles reaches, in the long time limit, a non-equilibrium steady state characterized by a non-vanishing and fluctuating current. Its study is an active topic in both classical and quantum systems. A relevant observable is the number Qt of particles flowing through the system during a time t. It can be calculated for simple models such as the symmetric simple exclusion process (SSEP), which describes two reservoirs at fixed densities connected by an L-site chain on which particles diffuse with a same site hard core repulsion. The corresponding cumulants of Qt are exactly known in one dimension and they coincide with those computed for the transport of free fermions through a mesoscopic conductor. We have generalized these results to arbitrary large but finite d-dimensional domains or graphs. Our numerical results indicate that, for large enough lattices and contacts to the reservoirs, the ratios of the cumulants of Qt take universal values, independent of the domain dimension and shape.

E. Akkermans, T. Bodineau, B. Derrida and O. Shpielberg, ‘Universal current fluctuations in the symmetric exclusion process and other diffusive systems, EPL, 103, 20001 (2013)

Copenhagen interpretation as an emergent phenomenon (Vol. 44 No. 6)

The diagram illustrates the effectiveness of coarse graining on two particular external states as decoherence proceeds. The final states at the top are suitably localised.

This work shows that a successful interpretation of quantum mechanics can be seen to emerge by taking the actual, or internal, states of a sub-system to correspond to one the eigenvectors of its reduced density matrix. Previous work has highlighted serious objections to such a modal type interpretation because it apparently leads to macroscopic superpositions and physically unacceptable instabilities near degeneracies. We show that both these problems are solved if the sub-system consists of a large number of coarse-grained degrees of freedom which is natural as real measuring devices have both finite spatial and temporal resolution. What results is an interpretation in which both decoherence and coarse graining play key roles and from which the rules of the Copenhagen Interpretation are seen to emerge in realistic situations. In this interpretation a measurement process is smooth but results in internal states that corresponds to the distinct outcomes ones expects. Previous work has suggested that the internal states of a device measuring the position of a particle would be spread out macroscopically.

T. J. Hollowood, ‘The Copenhagen interpretation as an emergent phenomenon’, J. Phys. A: Math. Theor. 46, 325302 (2013)

Multilayer Memristive/Memcapacitive Devices (Vol. 44 No. 6)

Device 1 is the conventional memristor design: electrodes are separated by a single ionic conductor insulator. Device 2 utilizes the multi-layer design, which forms a quasi-uniform conduction front. The simulated resistive switching of the multilayer memristor occurs largely within the linear regions where tuning can occur.

Memristive devices are reshaping computing paradigms as one of the leading candidates for the future of computer memory. In conventional memristors, metallic filaments form and extend stochastically under applied electrical bias, producing a highly non-uniform conduction front. This produces devices with large variations in electrical properties that are difficult to tune.

Here, we introduce and simulate a specialized multi-layered device structure with alternating ionic conductor layers that enables the development of a quasi-uniform conduction front. This reduces catastrophic switching (Fig. 1b) in which devices rapidly exit the linear-tuning region and switch state. In our simulations of a single layer memristor, the majority of the resistance change during a switching event occurs catastrophically, while in the multilayer device the majority of switching occurs in the linear tuning region. This ability to fine-tune switching events in devices is an important property for multi-bit memory and neuromorphic computing applications.

P. R. Mickel and C. D. James, ‘Multilayer Memristive/Memcapacitive devices with engineered conduction fronts’, Eur. Phys. J. Appl. Phys., 62, 30102 (2013)

Magnetic hyperthermia for tumour reduction (Vol. 44 No. 6)

Schematic diagram of the mechanisms leading to heat dissipation in magnetic nanoparticles

Magnetic hyperthermia is the process by which cycling magnetic nanoparticles in an alternating magnetic field leads to heat dissipation. It is a very attractive approach for the treatment of cancer because it generates no side effects unlike more conventional therapies such as radiotherapy or chemotherapy. The development of this therapy has been hampered by the lack of a clear understanding of the physical mechanisms leading heat generation. At the present time it is not possible for clinicians to be given details of the dosage and field conditions required for a given therapeutic outcome.

There are three mechanisms by which exposing magnetic nanoparticles to a cycling field can generate heat: susceptibility loss, hysteresis loss and viscous heating. We have found that these mechanisms are highly particle size dependent as shown schematically in the figure and will also depend upon the degree of aggregation of the particles. In experiments of magnetic nanoparticles of different sizes dispersed in solvents of varying viscosities, hysteresis heating has been shown to be the dominant mechanism. Although the contribution arising from viscous heating is significant its effects are uncontrollable and will not occur in vivo due to the high viscosity of tumour tissue.

G. Vallejo-Fernandez, O. Whear, A. G. Roca, S. Hussain, J. Timmis, V. Patel and K. O’Grady, ‘Mechanisms of hyperthermia in magnetic nanoparticles’, J. Phys. D: Appl. Phys. 46, 312001 (2013)

Pulsating dust cloud dynamics modelled (Vol. 44 No. 6)

Different phases of a self-gravitational potential

New research outlines a new design of spatio-temporal models of astrophysical plasmas.

It is the collapse of dense molecular clouds under their own weight that offers the best sites of star formation. In the present work, the authors have proposed a new model for investigating molecular clouds fluctuations at sites of star formation and thus study their pulsational dynamics. They study the pulsating dynamics of inhomogeneous molecular clouds that periodically undergo both self-gravitational contraction due to the weight of the massive dust grains, and electrostatic expansion resulting from the interaction of dust grains of the same electric charge.

They designed a model for investigating the cloud fluctuations with charge-varying grains, as a function of weight and charge interaction (referred to as nonlinear gravito-electrostatic coupling). They then carried out a detailed shape analysis to characterize these clouds on the astrophysical scale.

P. K. Karmakar and B. Borah, ‘Nonlinear Pulsational Eigenmodes of a Planar Collisional Dust Molecular Cloud with Grain-charge Fluctuation’, Eur. Phys. J. D, 67, 187 (2013)

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)

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)

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)

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)

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