Europhysics News

Highlights

Anti-hydrogen origin revealed by collision simulation (Vol. 47 No. 2)

Scientists studying the formation of antihydrogen ultimately hope to explain why there is more matter than antimatter in the universe. © vpardi / Fotolia

Numerical model takes us one step closer to understanding anti-hydrogen formation, to explain the prevalence of matter and antimatter in the universe

Anti-hydrogen is a particular kind of atom, made up of the antiparticle of an electron—a positron—and the antiparticle of a proton—an antiproton. Scientists hope that studying the formation of anti-hydrogen will ultimately help explain why there is more matter than antimatter in the universe. In a new study published recently, the authors demonstrate that the two different numerical calculation approaches they developed specifically to study collisions are in accordance. As such, their numerical approach could therefore be used to explain antihydrogen formation. The authors employed two very different calculations —using a method dubbed coherent close-coupling — for both one- and two-centre collisions respectively in positron scattering on hydrogen and helium. Interestingly, they obtained independently convergent results for both approaches. Such convergence matters, as it is a way to ascertain the accuracy of their calculations for anti-hydrogen formation.

I. Bray, J. J. Bailey, D. V. Fursa, A. S. Kadyrov and R. Utamuratov, Internal consistency in the close-coupling approach to positron collisions with atoms, Eur. Phys. J. D 70, 6 (2016)
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Antimatter study to benefit from recipe for ten-fold spatial compression of plasma (Vol. 49 No.5-6)

Example of raw images from the detector for identical particle operations with antiproton detection (left) and electron detection (right)

Improving the spatial compression of a mixed matter-antimatter trapped plasma brings us one step closer to grasping the acceleration of antimatter due to Earth’s gravity.

An international team of physicists studying antimatter have now derived an improved way of spatially compressing a state of matter called non-neutral plasma, which is made up of a type of antimatter particles, called antiprotons, trapped together with matter particles, like electrons. The new compression solution, which is based on rotating the plasma in a trapped cavity using centrifugal forces like a salad spinner, is more effective than all previous approaches. In this study published recently, the team shows that — under specific conditions — a ten-fold compression of the size of the antiproton cloud, down to a radius of only 0.17 millimetres, is possible. These findings can be applied in the field of low-energy antimatter research, charged particle traps and plasma physics. Further, this work is part of a larger research project, called AEgIS, which is intended to achieve the first direct measurement of the gravitational effect on an antimatter system. The ultimate goal of the project, which is being pursued at CERN, the Particle Physics Laboratory in Geneva, Switzerland, is to measure the acceleration of antimatter — namely antihydrogen — due to Earth’s gravity with a precision of 1%.

S. Aghion and 61 co-authors, Compression of a mixed antiproton and electron non-neutral plasma to high densities, Eur. Phys. Jour. D 72, 76 (2018)
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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)
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Appearance of deformation in the yttrium isotopic chain (Vol. 48, No. 3)

Evolution of deformation across Y isotopic chain.

In the isotopes of rubidium (Rb), strontium (Sr), ittrium (Y), zirconium (Zr) and niobium (Nb) (i.e., with Z=37-41), a sudden change of the nuclear structure occurs when the number of neutrons reaches N=60. While the nuclei with N<60 exhibit spherical shape in their ground states, the isotopes with N≥60 are significantly deformed. This phenomenon is considered the most dramatic shape change in the nuclear chart. A question was raised of whether the deformed structures appear just at N=60 or they reside also in the lighter isotopes. Indeed, deformed rotational bands built on the excited isomeric states are placed in ^{95Rb, ⁹⁶Sr, ⁹⁸Y, ^98-99Zr, i.e., at N=58 and 59, however, nothing was known about location of such collective excitations at N<<58. In our work, it was possible to significantly develop the level scheme of ⁹⁶Y57 via gamma-coincidence spectroscopy technique. During the analysis, a new 201(30)-ns isomeric state at 1655 keV excitation energy was located and the existence of a rotational band built on it was suggested. This result points to the presence of deformed structures already at N=57 which, with the increasing number of neutrons, gradually decrease in energy, to become dominant at N≥60.}

L. W. Iskra and 35 co-authors, New isomer in ⁹⁶Y marking the onset of deformation at N = 57, EPL 117, 12001 (2017)
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Approximate quantum cloning: the new way of eavesdropping in quantum cryptography (Vol. 49, No. 3)

New approximate cloning method facilitates quantum computing
Credit Markus Spiske via Unsplash

New approximate cloning method avoids the previous limitations of quantum cloning to enhance quantum computing and quantum cryptography leaks

Cloning of quantum states is used for eavesdropping in quantum cryptography. It also has applications in quantum computation based on quantum information distribution. Uncertainty at the quantum scale makes exact cloning of quantum states impossible. Yet, they may be copied in an approximate way—with a certain level of probability—using a method called probabilistic quantum cloning, or PQC. In a new study published recently, the authors demonstrate that partial PQC of a given quantum state secretly chosen from a certain set of states, which can be expressed as the superposition of the other states, is possible. This cloning operation is very important with regard to classical computing. It allows scientists to make many copies of the output of computations—which take the form of unitary operations. These can, in turn, be used as input and fed into various further processes. In quantum computing, for example, previous studies have shown that PQC can help to enhance performance compared to alternative methods. This means that when unitary operations generate some linearly-dependent states, partial PQC can be helpful.

P. Rui, W. Zhang, Y. Liao and Z. Zhang, Probabilistic quantum cloning of a subset of linearly dependent states, Eur. Phys. J. D 72, 26 (2018)
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Arbitrarily slow, non-quasistatic, isothermal transformations (Vol. 47 No. 5-6)

A thermodynamically irreversible cycle for single-particle and classical engines

Joule or free expansion of an ideal gas into a volume at lower pressure is an example of an irreversible isothermal process. This nonequilibrium example is often used in thermodynamics texts to demonstrate that an arbitrarily slow process need not be reversible. Cyclic operation of engines that involve a free expansion therefore requires work.

Here, the authors explore experimentally the origin of thermodynamic irreversibility at the level of a single-particle “gas”. A feedback trap confines a silica particle in a virtual bistable potential, creating a system analogous to two vessels connected by a valve, where the volume of one vessel is adjustable via piston. The authors operate two types of cyclic transformations; both start and end in the same equilibrium state, and both use the same basic operations—but in different order. One transformation required no work, while the other required work, no matter how slowly it was carried out.

Why the difference? As the illustration shows, the result of carrying out a protocol backwards in time may not match the initial state. This property is not possible to notice in a single repetition, unlike in a macroscopic system where free expansion is followed by a “whoosh”.

M. Gavrilov and J. Bechhoefer, Arbitrarily slow, non-quasistatic, isothermal transformations, EPL 114, 50002 (2016)
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Astronauts to bring asteroid back into lunar orbit (Vol. 48, No. 5-6)

How to bring an asteroid back nearer to lunar orbit

Italian Space Agency presents plans to develop a robotic solar-powered spacecraft capable of displacing a near-Earth asteroid towards lunar orbit for ease of study

Future space exploration aims to fly further from Earth than ever before. Now, Italian Space Agency scientists have expressed an interest in contributing to the development of robotic technologies to bring an asteroid from beyond lunar orbit back into closer reach in order to better study it. In a paper published recently, the authors make the case for taking part in the robotic phase of the Asteroid Redirect Mission (ARM). In addition to taking manned spaceflights deeper into space than ever before, the proposed mission would also bring some benefit for planetary science. Further, the mission has potential implications for a field called planetary defence. The next step for human space exploration after the International Space Station is to send astronauts on a Near Earth Asteroid by 2025, as planned by NASA. This constitutes an intermediate step towards future manned missions to Mars. The planned ARM mission has been part of the NASA program since 2013.The robotic spacecraft would cruise in deep space towards a near-Earth asteroid, using a technology called advanced Solar Electric Propulsion. Under the proposed plan, Italy would contribute by enhancing the carrying capacity of that spacecraft.

M. Tantardini and E. Flamini, Synergies between human space exploration and science in the asteroid redirect mission and the potential Italian participation in the asteroid redirect robotic mission phase, Eur. Phys. J. Plus 132, 314 (2017)
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Astrophysics in lab via collisions of heavy systems (Vol. 43 No. 4)

High resolution X-ray spectra of Ar¹⁷⁺ -> Ar

Collisions between slow highly charged ions and atoms are one of the most common fundamental processes in space. The consequent emitted light is used to diagnose the relative abundance of constituents in intergalactic clouds and comets. During the collision, the projectile-ions capture, in a highly excited-state, from one to many target-electrons. By a series of atomic cascades the electrons "tumble" from the very high atomic levels onto the ground state through multiple and complex pathways. These cascades lead to photon and/or electron emissions. The accurate analysis of the light (from UV to hard X) emitted during the interaction provides direct insights into the early stages of capture mechanisms.

Until now, for systems involving a large number of electrons, only low-resolution X-ray spectra recorded with solid state detectors were available. In the present work, the contribution of single-electron capture from multiple-capture processes in the X-ray emission have been successfully disentangled for an Ar¹⁷⁺ projectile colliding with N₂ or Ar gaseous target at v=0.53 a.u.

Thanks to an accurate calibration of the spectrometers and a complete determination of the ion beam-gas target overlap, absolute X-ray emission cross section has been extracted with a significant improvement in uncertainty. Using a mosaic crystal spectrometer, 2 orders of magnitude in resolving power have been reached. The whole He-like Ar¹⁶⁺ Lyman series from n = 2 to 10 has been resolved as well as the fine structure of 1s2l → 1s2 transitions. The role of single-electron capture, leading to transitions from n = 7 to 10 levels, has been clearly discriminated from multiple capture processes that populate lower lying states. Furthermore, a precise determination of the influence of metastable states emphasizes that transposition of the measurements via ’laboratory ion-atom collisions’ towards interpretation of astrophysical spectra should be made with caution.

Investigation of slow collisions for (quasi) symmetric heavy systems: what can be extracted from high-resolution X-ray spectra
M. Trassinelli et al. (8 co-authors), J. Phys. B 45, 085202 (2012). [Abstract]

Asymmetrical magnetic microbeads transform into micro-robots (Vol. 47 No. 5-6)

Transformation of particle clusters while exposed to an oscillating external magnetic field

Thanks to the ordering effects of two-faced magnetic beads, they can be turned into useful tools controlled by a changing external magnetic field

Janus was a Roman god with two distinct faces. Thousands of years later, he inspired material scientists working on asymmetrical microscopic spheres—with both a magnetic and a non-magnetic half—called Janus particles. Instead of behaving like normal magnetic beads, with opposite poles attracting, Janus particle assemblies look as if poles of the same type attract each other. A new study reveals that the dynamics of such assemblies can be predicted by modelling the interaction of only two particles and simply taking into account their magnetic asymmetry. These findings were recently published by the authors. It is part of a topical issue entitled "Nonequilibrium Collective Dynamics in Condensed and Biological Matter." The observed effects were exploited in a lab-on-a-chip application in which microscopic systems perform tasks in response to a changing external magnetic field, such as, for instance, to create a zipper-style micro-muscle on a chip.

G. Steinbach, S. Gemming and A. Erbe, Non-equilibrium dynamics of magnetically anisotropic particles under oscillating fields, Eur. Phys. J. E, 39 69 (2016)
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Atom distributions in a low-pressure He-Xe discharge (Vol. 43 No. 2)

Visible emission of the discharge in the mixture He+2%Xe, pressure - 5 Torr, discharge current 40mA. Top: overview of the cathode region; bottom: magnified view of the spot plasma and the scale for the characteristic sizes.

Low-pressure glow discharges in mixtures of xenon with rare gases are used for lighting purposes in mercury-free fluorescent lamps. Over the last decade investigations have been carried out on their output characteristics in dependence on parameters such as mixture composition, pressure, discharge current and operation mode. However, the lifetime of these types of fluorescent lamps is limited by the performance of the electrodes and their interaction with the surrounding plasma. When using a flat oxide cathode a hot spot and a plasma can be generated. This spot mode changes the plasma-cathode interaction and prolongs the lifetime. The source of the plasma is localized in a very narrow region, but the plasma size is much bigger than the hot spot size. Outside of the cathode spot the plasma is sustained by transport of excited species, namely xenon atoms in the metastable and resonance states. Therefore, the discharge in spot mode can be used for investigations of the role of various transport mechanisms.

In the present work the method of laser atom absorption spectroscopy was used to measure the spatial distribution of excited xenon atoms in radial and axial dimensions. The analysis of the experimental results have been performed by means of numerical model which comprises the solution of the transport equations for metastable and the resonance atoms. Various approximations for the description of particle and radiation transport have been considered. It has been shown that the expansion of the plasma is mainly caused by transport of the resonance radiation. Suggested numerical method can be further used for precise description of density distribution of excited atoms and predictions of output characteristics of luminescent lamps.

Spatial distribution of metastable and resonance atoms in a low-pressure He-Xe discharge in spot mode
Yu. B. Golubovskii, S. Gorchakov, H. Lange, A. Timofeev, D. Uhrlandt and J. Winter, J. Phys. D: Appl. Phys. 45, 055205 (2012)
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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)
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Atomic photoionization: When does it actually begin? (Vol. 42, No. 4)

The crest position of the electron wave packet after the end of the XUV pulse is fitted with the straight line, which corresponds to the free propagation. In the inset, extrapolation of the free propagation inside the atom is shown. The XUV pulse is over-plotted with the black dotted line.

Among other spectacular applications of the attosecond streaking technique, it has become possible to determine the time delay between subjecting an atom to a short XUV pulse and subsequent emission of the photoelectron. This observation opened up a question as to when does atomic photoionization actually begin.

We address this question by solving the time dependent Schrödinger equation and by carefully examining the time evolution of the photoelectron wave packet. In this way we establish the apparent "time zero" when the photoelectron leaves the atom. At the same time, we provide a stationary treatment to the photoionization process and connect the observed time delay with the quantum phase of the dipole transition matrix element, the energy dependence of which defines the emission timing.

As an illustration of our approach, we consider the valence shell photoionization of Ne and double photoionization (DPI) of He. In Ne, we relate the opposite signs of the time delays t₀(2s)<0 and t₀(2p)<0 (Figure) with energy dependence of the p and d scattering phases which is governed by the Levinson-Seaton theorem. In He, we demonstrate that an attosecond time delay measurement can distinguish between the two leading mechanisms of DPI: the fast shake-off (SO) and the slow knockout (KO) processes. The SO mechanism is driven by a fast rearrangement of the atomic core after departure of the primary photoelectron. The KO mechanism involves repeated interaction of the primary photoelectron with the remaining electron bound to the singly charged ion.

Timing analysis of two-electron photoemission
A.S. Kheifets, I.A. Ivanov and Igor Bray, J. Phys. B: At. Mol. Opt. Phys. 44, 101003 (2011)
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Atoms crystallised by light for precision measurement (Vol. 46 No. 2)

Trajectories and intensities of two beam splitters. Credit: D. Holzmann et al.

A new study focuses on the collective dynamics of illuminated atoms coupled to photons travelling in a nanofibre.

Theoretical physicists have uncovered the existence of self-organised dynamics of atoms, bound by light into a crystal, with long range atom-atom interactions. These findings were recently obtained by the authors. This approach could, among others, help to better understand the process of crystallisation in new materials, and help implement efficient photon storage and precision measurements.

Their study focuses on atoms trapped in the leaked light very close to a tapered optical nano-fibre. Such fibres are too thin to confine all the light within. These atoms are exposed to a transverse laser beam, whose light becomes partially redirected, or scattered, into the nanofibre at each atom’s position, before propagating along the chain of atoms. It thus mediates a strong effective atom-atom interaction. This approach yields a stable chain of atoms, bound by light, forming a crystal.

D. Holzmann, M. Sonnleitner and H. Ritsch, “Self-ordering and collective dynamics of transversely illuminated point-scatterers in a 1D trap”, Eur. Phys. J. D 68, 352 (2014)
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Attosecond control of electron correlation (Vol. 43 No. 5)

The photo-recombination dipole phase ΔΦ in a correlation-assisted channel is proportional to electric field strength F_IR at the time of recombination tr.

Electron correlation is ubiquitous in one-photon ionization and its time reverse, photo-recombination. It redistributes transition probabilities between open ionization channels in an atom or molecule. The photo-ionization and photo-recombination cross-sections are commonly assumed to be a fixed property of the target, impervious to experimental manipulation.

This article develops an analytical model of correlated photo-ionization and photo-recombination in the presence of a strong, near-infrared (IR) laser field. It shows that the characteristic time of the electron-electron interaction differs slightly from the time of the ionization (or recombination). As the result, correlation channels acquire an additional phase, proportional to the instantaneous IR laser electric field. Interferences between the direct and correlation channels then lead to either suppression or enhancement of overall cross-sections. These interferences are under direct experimental control and can be used to adjust probabilities of ionization and recombination.

Numerical estimates suggest that the desired control conditions are attainable in strong-field experiments. If the prediction is confirmed by experiment, it will open new avenues for strong-field investigations of electron correlation and for the design of high-harmonic radiation sources.

S. Patchkovskii, O. Smirnova and M. Spanner, ‘Attosecond control of electron correlations in one-photon ionization and recombination’, J. Phys. B: At. Mol. Opt. Phys. (2012) 45, 131002
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Auger electron energy spectrum from N2: new visit (Vol. 43 No. 4)

The top panel shows the experimental setup. The bottom panels show the angle resolved electron yield for different Auger electron energies (circles) along with the calculated angular distributions (dashed) for different final state symmetries. The bottom right panel shows the molecular distribution.

This article presents a study of electronic relaxation of core-excited molecules. Relaxation occurs through the Auger process where a valence electron fills the core vacancy, created by x-ray ionization, and a second valence electron is released from the molecule. The kinetic energy of this second electron then depends on the final state of the core-relaxed molecule. The Auger electron energy spectrum can therefore be viewed as a kind of fingerprint for molecular composition.

In this study, an infrared laser pulse was used to align an ensemble of nitrogen molecules relative to the laboratory frame. We then photo-ionized the molecules with a ~60 fs x-ray laser pulse. Rotating the polarization of the infrared laser, as shown in the Fig., changed the orientation between the aligned molecules and the laboratory frame electron detector. In this way we measured the angular pattern of the entire Auger electron spectrum in the molecular frame as shown in the Fig.

This experiment demonstrates a new way to measure the molecular-frame angular emission pattern for every Auger electron feature. Adding angular information to the spectral information allows incorporating electronic symmetry in feature identification. These findings suggest reordering some previous Auger feature assignments in the seemingly well-known N₂, showcasing the power of this method to measure transient changes in electronic symmetry as a molecule undergoes a chemical reaction.

Molecular frame Auger electron energy spectrum from N₂
J. P. Cryan, R. N. Coffee and 32 co-authors, J. Phys. B: At. Mol. Opt. Phys. 45 (2012) 055601
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