Graphite/CdMnTe Schottky diodes and their electrical characteristics (Vol. 45 No.2)

Comparison of the calculation results (solid lines) with the measured I-V curves (circles) of the graphite/CdMnTe diode at different temperatures.

CdTe is a basic material for X- and γ-ray detectors, which are widely used in various areas. However, the leakage current in these devices at room temperature is too large, which precludes a high energy resolution in the measured spectra. In the 1990s, Cd1-xZnxTe alloy with a wider band gap was proposed as a solution, but hopes pinned on it were not fully fulfilled. The search for new materials for the detectors continues and Cd1-xMnxTe is considered a promising material. The main obstacle hindering the application of Cd1-xMnxTe in the detectors is the lack of Cd1-xMnxTe-based diodes. In this paper we show that Schottky diodes fabricated by the deposition of colloidal graphite have good rectifying properties and low reverse currents. Their I-V curves are described analytically in terms of the generation-recombination theory based on the Shockley-Read-Hall statistics. It is shown that tunneling is responsible for the increase of the leakage currents at higher voltages and ways of its elimination are proposed.

L. A. Kosyachenko, R. Yatskiv, N. S. Yurtsenyuk, O. L. Maslyanchuk and J. Grym, “Graphite/CdMnTe Schottky diodes and their electrical characteristics”, Semicond. Sci. Technol., 29, 015006 (2014)

Einstein’s conversion from a static to an expanding universe (Vol. 45 No.2)

Einstein and Lemaître photographed around 1933.
Credit: Archives Lemaître, Université Catholique, Louvain

Albert Einstein accepted the modern cosmological view that the universe is expanding, only long after several of his contemporaries had demonstrated it with astrophysical observations.

Until 1931, physicist Albert Einstein believed that the universe was static, in line with his 1917 model. Now, the author explains how Einstein changed his mind and adopted the notion of an expanding universe following many encounters with some of the most influential astrophysicists of his generation.

He then fiercely resisted the view that the universe was expanding. For example, in 1922, Alexander Friedman showed that Einstein’s equations were viable for dynamical worlds. And, in 1927, Georges Lemaître, concluded that the universe was expanding by combining general relativity with astronomical observations.

It is only by April 1931 that Einstein finally adopted a model of an expanding universe. In 1932 he teamed up with Willem de Sitter, to propose an eternally expanding universe. This became the cosmological model generally accepted until the middle of the 1990s.

H. Nussbaumer, “Einstein’s conversion from his static to an expanding universe”, Eur. Phys. J. H, 39, 37 (2014)

Multifractal analysis of breast cancer IR thermograms (Vol. 45 No.2)

Multifractal analysis of temperature time-series (A-C) of the cancerous (red) and intact (black) breasts of a patient, and of a healthy volunteer breast (green). D(h) singularity spectra (D): the multifractal wide spectrum of healthy breasts reduces to a single point (monofractality) in the presence of a tumor.

Breast cancer is a common type of cancer among women and despite recent advances in the medical field there are still some inherent limitations in current screening techniques. The radiological interpretation of X-ray mammograms often leads to over-diagnoses and to unnecessary traumatic and painful biopsies. In this paper, the authors propose a computer-aided multifractal analysis of dynamic infrared imaging as an efficient method for preliminary screening in asymptomatic women, in order to identify those with a higher risk of breast cancer. Using a wavelet-based multi-scale method to analyze the temporal fluctuations of breast skin temperature, collected both from patients with breast cancer, and from healthy volunteers, they show that the multifractal complexity of temperature fluctuations observed in intact breasts is lost in mammary glands with a malignant tumor. Besides potential clinical application, these results underline the informative content of physiological changes that may precede anatomical alterations in breast cancer development.

E. Gerasimova, B. Audit, S.-G. Roux, A. Khalil, F. Argoul, O. Naimark and A. Arneodo, "Multifractal analysis of dynamic infrared imaging of breast cancer", EPL 104, 68001 (2013)

Optimising custody is child’s play for physicists (Vol. 45 No.2)

A graph depicting the children’s custody problem, where grey nodes represent couples

Ensuring that parents in recomposed families see their children regularly is a complex network problem that models developed to study materials may help to solve.

As a diversion from his normal duties as a theoretical physicist, one of the authors set out to finding a suitable weekend for both partners in his recomposed family to see all their children at the same time. This resulted in a study showing that solving this problem equates to minimising the energy in a material model.

The authors assume that people in the network, who are connected, as current or ex-partners, are willing to cooperate and communicate. They attempt to verify whether all parents could see all of their children together every other weekend. The answer is that it is not possible.  However, authors found an algorithm to maximise the number of parents spending time with their own children and those of their current partners. It was akin to minimising the energy of a particular magnetic material called a spin glass.

A. Gomberoff, V. Muñoz and P. P. Romagnoli, “The physics of custody”, Eur. Phys. J. B87, 37 (2014)

Hamiltonian walks and applications to protein folding (Vol. 45 No.2)

Elementary 2x2x2 building block of 4x4x4 Hamiltonian walks

Hamiltonian walks on lattices are model systems for compact polymers such as proteins. The authors present a new enumeration algorithm that exactly counts the number of Hamiltonian walks on the 4x4x4 cube, which before was out of reach by several orders of magnitude of computing power. In addition, the authors present a new related Monte Carlo algorithm for counting the number of Hamiltonian walks, up to the 7x7x7 cube.

The exact enumeration algorithm proved to be very fast, taking only several hours on a single PC. The number of Hamiltonian walks on the 4x4x4 cube was found to be 27,747,833,510,015,886.

The number of Hamiltonian walks grows faster with system size than previously anticipated. The authors attribute this to severe surface corrections. These effects could only be shown using the new Monte Carlo algorithm, which greatly extended the range of system sizes, compared to previous results.

Finally, the paper discusses the uniqueness of ground states in the HP model for protein folding. It provides a bound on the uniqueness, and gives an intuitive picture for the uniqueness in the thermodynamic limit.

R. D. Schram and H. Schiessel, “Exact enumeration of Hamiltonian walks on the 4 × 4 × 4 cube and applications to protein folding”, J. Phys. A: Math. Theor., 46, 485001 (2013)

MINOS: A vertex tracker for in-beam spectroscopy of exotic nuclei (Vol. 45 No.2)

View of the MINOS device inside the DALI2 gamma array at the RIKEN Radioactive Isotope Beam Factory

MINOS is a new apparatus dedicated to in-beam nuclear structure experiments with low-intensity exotic beams at energies above 150 MeV/nucleon. It is intended to provide increased luminosity compared to standard solid-target experiments in hydrogen-induced studies, while simultaneously improving experimental resolution. This work exposes the concept of the device developed at the CEA in France and reviews in detail the associated recent technical advances. MINOS is composed of a thick finger-shaped liquid hydrogen target, from 50 to 200 mm thick, combined with a compact time projection chamber serving as a vertex tracker, the first of its kind in low-energy nuclear physics. This innovative setup offers access to the first spectroscopy of a new range of very exotic nuclei beyond our current reach. An exciting program on the search for new 21+ states in neutron-rich even-even nuclei, spectroscopy of unbound oxygen nuclei and di-neutron correlations in Borromean nuclei will be performed with MINOS at the RIKEN Radioactive Isotope Beam Factory in Japan over the next few years. MINOS is funded by the European Research Council.

A. Obertelli et al., “MINOS: A vertex tracker coupled to a thick liquid-hydrogen target for in-beam spectroscopy of exotic nuclei”, Eur. Phys. J. A, 50, 8 (2014)

Quasi-effective medium theory for multilayered magneto-dielectric structures (Vol. 45 No.2)

Object-independent cloaking

Maxwell–Garnett theory is the most widely used effective medium theory for the determination of the permittivity/permeability of nano-composite materials. However, it places a serious restriction on the physical dimensions of the constituents, that is, the feature sizes must be smaller than the incident wavelength. Thus, its applicability is limited to the quasi-static regime. An alternative theory has now been proposed by the authors which uses mode-dependent quasi-effective impedances to allow exact calculations of the far field scattering/extinction of complex multi-shell structures regardless of the object physical dimensions.

To demonstrate the physical insights that are to be gained based on this quasi-effective medium theory the authors have studied two practical examples: (i) they consider the problem of surface plasmon hybridization in concentric multi-shell particles, and (ii) they have applied the theory to design an object-independent cloak which consists of an arbitrary shaped object enclosed by a set of concentric shells. The most significant advantages of this new theory are its simplicity, ease of implementation and the important insights the theory provides into the optical properties of complex systems.

D. A. Genov and P. C. Mundru, "Quasi-effective medium theory for multi-layered magneto-dielectric structures", J. Opt. 16, 015101 (2014)

Elucidating biological cells’ transport mechanisms (Vol. 45 No.2)

Image of mitochondria observed by transmission electron microscopy.
Credit: K. Hayashi et al.

A new study focuses on the motion of motor proteins in living cells, applying a physicist’s tool called non-equilibrium statistical mechanics used to study diffusion.

Using an ingenious setup, the authors have, for the first time, calculated the force of molecular motors —called kinesin and dynein— acting on the inner components of biological cells, called mitochondria. These findings could contribute to elucidating the transport mechanism in biological cells by multiple motors.

The authors compared evaluations of the diffusion coefficient obtained via the so-called Einstein relation—which stems from non-equilibrium statistical mechanics—applied to both mitochondria and the random motion of beads artificially incorporated into a cell. They found that the medium’s viscosity obtained using the beads, was slightly lower than that obtained using the mitochondria motion. This means that physical laws such as the Einstein relation are not sufficient to fully describe the mitochondria’s motion, which is subjected to complex biological processes.

K. Hayashi, C. G. Pack, M. K. Sato, K. Mouri, K. Kaizu, K. Takahashi and Y. Okada, “Viscosity and drag force involved in organelle transport: Investigation of the fluctuation dissipation theorem”, Eur. Phys. J. E, 36, 136 (2013)

Sharpening the focus in quantum photolithography (Vol. 45 No.2)

Photolithography is used to etch circuits onto microelectronic devices.
Credit: Tambako the Jaguar/Flickr

A new protocol, exploiting the quantum properties of materials, makes it possible to improve the accuracy of photolithography by addressing its physical limitations due to diffraction.

Photolithography uses light beams to design thin geometric patterns on the substrates of semiconductors used in microelectronic devices. This is achieved using a chemical reaction on a light-sensitive chemical, called photoresist. The trouble is that the phenomenon of light diffraction does not permit highly accurate patterns. Now, the author has developed a quantum lithography protocol designed to improve the resolution of this technology. The author establishes the formula for the probability of a single, and no longer multiple, photon transition from a bound state of a quantum system to a state of continuous spectrum, using the so-called Markov approximation. This makes it possible to select the exposure time and the beam’s intensity to obtain a narrow stripe in the photoresist on the substrate.

G. P. Miroshnichenko, “Quantum lithography on bound-free transitions”, Eur. Phys. J. D, 67, 257 (2013)

Carbon dating uncovers forged Cubist painting (Vol. 45 No.2)

The alleged Contraste de formes, an oil on canvas from the Peggy Guggenheim Collection, Venice, was proven to be a fake.
Credit: Caforio et al.

Physicists use carbon dating to confirm alleged Fernand Léger painting was definitely a fake, thus corroborating the doubts about its authenticity previously expressed by art historians.

For the first time, it has been possible to identify a fake painting by relying on the anomalous behaviour of the concentration of 14C in the atmosphere after 1955 to date its canvas. These findings were recently obtained by the authors.

Previously, art historians called upon scientists to compare the alleged Léger painting from the Peggy Gugenheim Collection (PGC), in Venice, Italy, with an authentic painting of the ‘Contrastes de formes’ series belonging to the Solomon Guggenheim Foundation (SGF) in New York, USA. They showed that the canvas fibres and the paint pigments differed, evidence was inconclusive. Using accelerator mass spectrometry, the authors definitely proved that the canvas sample contains a level of radioactive carbon found in 1959, years after Léger’s death in 1955.

L. Caforio et al., "Discovering forgeries of modern art by the 14C Bomb Peak", Eur. Phys. J. Plus, 129, 6 (2014)

Subrecoil cavity cooling towards degeneration (Vol. 45 No.2)

Single-particle momentum distribution for bosons in a ring cavity. The initially broad distribution is cavity-cooled to |p| = 0, ±1hk momentum states.

Cavity assisted cooling has become a valuable tool to implement cavity-QED with ultra cold quantum gases, trapped ions and optomechanical elements. Injecting a red-detuned laser extracts kinetic energy from the particles to create cavity photons, which leak out of the resonator, carrying away energy and effectively cooling the system. In contrast to conventional laser cooling, this method works without resonant excitation and spontaneous emission, eliminating photon re-absorption and making it applicable to a wide class of polarisable particles with final temperatures only limited by the cavity line-width.

We present a detailed numerical analysis of the cooling dynamics involving a cavity with energy uncertainty below the recoil energy. Motivated by a recent Hamburg experiment demonstrating targeted cooling on the subrecoil scale, we embrace a tailored sequence of laser pulses transferring the particles from a thermal state towards the ground state reaching subrecoil kinetic energies. The few particle simulations give encouraging prospects to implement condensation of a quantum gas via cavity cooling and exhibit genuine quantum correlations distinguishing fermions and bosons.

A broad momentum distribution is cooled to generate a large ground state population. Each step of a laser pulse sequence with optimized detunings transfers specific momentum states irreversibly towards lower momenta.

R. M. Sandner, W. Niedenzu and H. Ritsch, “Subrecoil cavity towards degeneration: a numerical study”, EPL, 104, 43001 (2013)

How hypergravity impacts electric arcs (Vol. 45 No.2)

Glide arc discharge under normal gravity conditions.
Credit: J. Sperka et al.

A new study focused on electric discharge behaviour under intense gravitational forces shows that its dynamic changes as gravity increases.

Arc discharges are common in everyday conditions like welding or in lightning storms. But in altered gravity, not as much is known about their behaviour. For the first time, the authors studied the behaviour of a special type of arc discharge, so-called glide arc, in varying hypergravity conditions, up to 18 G. In this work they demonstrate how the plasma channel of this glide arc discharge moves due to external forces of buoyancy in varying gravity conditions. These results could have implications for improved safety precautions in manned space flights and in the design of ion thrusters used for spacecraft propulsion.

The authors performed measurements on atmospheric pressure glide arc helium plasma under the forces of hypergravity. They established a model showing that gravity strongly influences the glide arc discharge. These effects stem from thermal buoyancy, which increases with gravity, they conclude.

J. Sperka, P.Souček, J. J.W.A. Van Loon, A. Dowson, C. Schwarz, J. Krause, G. Kroesen and V.Kudrle, “Hypergravity Effects on Glide Arc Plasma”, Eur. Phys. J. D, 67, 261 (2013)

Finding and verifying Quantumness in “Classical” States (Vol. 45 No.2)

Changes in conditional marginal distributions as signature of discord.

Separable states were previously treated as “classical” states due to the lack of entanglement. Recently, quantum discord was proposed as a general quantification of quantumness that is able to reveal nonclassical correlations beyond entanglement. This measure suggests that for bipartite Gaussian states, quantum correlations are nonzero for all but product states. This implies that even a non-entangled bipartite state prepared by splitting a thermal state on beamsplitter could display nonzero quantumness. To examine such nonclassical correlations, experimental methods to verify the presence of discord are of particular interest.

In this paper, the authors demonstrated a simple yet efficient technique for certifying quantum discord in continuous variable states. By checking the difference between conditioned marginal distributions, such as peak separation, the authors were able to reveal discord in bipartite Gaussian states and a certain class of non-Gaussian states. Hence, the presence of informational contents greater than that attributed to classical correlations in these separable states was verified. With some prior knowledge about the bipartite states, the proposed method could detect quantum correlations with minimal resources, thus serves as an indispensable tool for the testing of quantumness.

S. Hosseini, S. Rahimi-Keshari, J. Y. Haw, S. M. Assad, H. M. Chrzanoswki, J. Janousek, T. Symul, T. C. Ralph and P. K. Lam, "Experimental verification of quantum discord in continuous-variable states" J. Phys. B: At. Mol. Opt. Phys., 47 025503 (2014)

Ferroelectric tunnel junction for memory and logic design (Vol. 45 No.2)

The structure of Co/BaTiO3/La0.67Sr0.33MnO3 FTJ and DC simulation curves with proposed model

Ferroelectric tunnel junction (FTJ) is an emerging nonvolatile binary data storage device. Unlike conventional tunnel junctions, FTJ is switched via a pure electronic mechanism, and it exhibits higher OFF/ON resistance ratio and larger resistance-area product. Considering great potential of FTJ as next generation memory, the authors aimed to develop the first compact model of FTJ for associated circuits design and simulation.

They presented a SPICE-compatible model of Co/BaTiO3/La0.67Sr0.33MnO3 FTJ through investigating a variety of physical theories including Brinkman model, JKD semi-empirical scaling law, KAI model, and Merz’s law. These theories quantitatively explain the experimental data of tunnel resistance and switching process, and hence verify the accuracy of proposed model. This model has been programmed with Verilog-A language and integrated on Cadence platform. With the proposed model, the authors researched the reading reliability and power dissipation of FTJ based on CMOS 40nm technology node. Simulation results demonstrated the advantages of FTJ over magnetic tunnel junction (MTJ) in high reliability and ultralow power.

The authors have added this model into the open source device library SPINLIB to allow IC designers to simulate FTJ-based circuits efficiently.

Z. H. Wang, W. S. Zhao, W. Kang, A. Bouchenak-Khelladi, Y. Zhang, J.-O. Klein, D. Ravelosona and C. Chappert, “A physics-based compact model of ferroelectric tunnel junction for memory and logic design”, J. Phys. D: Appl. Phys. 47, 045001 (2014)

Einstein’s forgotten model of the universe (Vol. 45 No.2)

An image of the blackboard used in Einstein’s 2nd Rhodes lecture at Oxford in April 1931.
Credit: Museum of the History of Science, University of Oxford, UK.

New insights into Einstein’s view of the cosmos from the translation and study of one of his least known papers.

This work provides the first English translation and an analysis of one of Albert Einstein’s little-known papers, “On the cosmological problem of the general theory of relativity.” Published in 1931, it features a forgotten model of the universe, while refuting Einstein’s own earlier static model of 1917. In this paper, Einstein introduces a cosmic model in which the universe undergoes an expansion followed by a contraction. This interpretation contrasts with the monotonically expanding universe of the widely known Einstein-de Sitter model of 1932.

The authors provide insights into Einstein’s view of cosmology. At that time, the first pieces of evidence for an expanding universe emerged, among others, stemming from Hubble’s observations of the expanding universe. In this paper, the authors also discuss Einstein’s view of issues such as the curvature of space and the timespan of the expansion, while also uncovering some anomalies in Einstein’s calculations.

C. O’Raifeartaigh and B. McCann (2014), “Einstein’s cosmic model of 1931 revisited”, Eur. Phys. J. H, 39, 63 (2014)