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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)
[Abstract]

Field effect transistors of epitaxial graphene on SiC (Vol. 41, No. 6)

image This figure shows the so-called transfer characteristics of a Solution Gated Field Effect Transistor (SGFET) fabricated on epitaxial graphene on silicon carbide. The current through the transistor channel from the source to the drain contact, driven with constant source-drain voltage, is plotted vs. the control voltage applied between the source and the gate. The minimum of each curve belongs to the situation where the Fermi level in the graphene coincides with the Dirac energy. The control voltage corresponding to this condition shows a pH dependent shift due to varying protonation of the graphene surface. Insert: Schematics of the device and the electrical circuitry adopted in the experiment.

Graphene is at date the most recognized new candidate for the electronic material of the future, the 2010 physics Nobel prize being only the ultimate indication of this fact to the public. As a two-dimensional electronic system of only one atomic layer with a correspondingly low number of charge carriers per area, it makes the material extremely sensitive to charge exchange with the outside world. This is blessing and curse at the same time: high electronic sensitivity for adsorbates is useful for chemical sensing if it can be selectively controlled; it is extremely annoying, however, for stable switching devices. In any case, understanding the interfaces between graphene, its (unavoidable) substrate onto which it is prepared, and the ambient in contact with its surface is vital for developing electronic devices based on the material.

The article at hand is focussing on this topic, adopting a special transistor design, the so-called solution gated field effect transistor, for investigating epitaxial graphene on silicon carbide in contact with an aqueous medium. The detailed analysis of the device’s transfer characteristics yields a surprisingly high charge carrier mobility in the graphene sheet, but also reveals the important role of silicon carbide surface states at the graphene interface.

Characteristics of solution gated field effect transistors on the basis of epitaxial graphene on silicon carbide
J. Ristein, Wenying Zhang, F. Speck, M. Ostler, L. Ley and T. Seyller, J. Phys. D: Appl. Phys. 43, 345303 (2010).
[Abstract]

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)
[Abstract]

Finite temperature entanglement in many body systems (Vol. 46 No. 2)

Entanglement negativity at finite temperature between an interval A (red) and the remainder (green) obtained through particular correlation functions on an infinite cylinder whose radius is proportional to the inverse temperature.

Entanglement is a key feature of quantum mechanics setting it apart from the classical world. In the last decade, entanglement also became a practical tool to characterise the various phases of matter of many-body quantum systems in a pure state, in particular in connection with topological and critical phases. However, the quantification of entanglement for a bipartite many-body system in a mixed state, such as at finite temperature, is a harder task. Various measures of entanglement for mixed states have been introduced and the most practical one is the so-called negativity. Focusing on one-dimensional many-body systems at criticality, for a bipartition of the system into a finite interval and its remainder (see figure), we find an expression for the negativity at finite temperature, which turns out to depend only on the ratio between the temperature and the length of the interval. This universal function encodes the full operator content of the theory.

P. Calabrese, J. Cardy and E. Tonni,, “Finite temperature entanglement negativity in conformal field theory”, J. Phys. A: Math. Theor. 48, 015006 (2015)
[Abstract]

First aid kit in some living organisms helps fix DNA after lengthy sun exposure (Vol. 48, No. 5-6)

Important photolyase residues for DNA binding.”

New study unveils the binding mechanisms of enzymes capable of repairing DNA damaged by UV light before any risk of cellular malfunction sets in

Sunburn in living organisms is caused by ultraviolet (UV) light from the sun damaging the DNA in the cells. Many organisms, however, have an in-built mechanism for repairing the sun damage. This is possible thanks to an enzyme called DNA photolyase, which is so specialised that cryptochrome, a structurally similar molecule, is unable to do the same job. By comparing both types of molecule, physicists can understand precisely how the ability of our enzymes to repair DNA boils down to the most minute structural details. In a study published recently, the authors pinpoint the mechanism by which repair enzymes bind to the damaged site.

K. Aalbæk Jepsen and I. A. Solov'yov, On binding specificity of (6-4) photolyase to a T(6-4)T DNA photoproduct, Eur. Phys. J. D 71, 155 (2017)
[Abstract]

First measurement of 60Ge β decay (Vol. 47 No. 3)

A sample image recorded by the OTPC detector’s CCD camera showing the particle trajectories. The vertical track corresponds to the 60Ge ion entering the detector and the other track corresponds to the (β-delayed) proton.

60Ge, with its 28 neutrons and 32 protons, is an extremely exotic nucleus, discovered about 10 years ago when only three ions were produced. Its decay properties were measured for the first time in this work. In this experiment, performed at the National Superconducting Cyclotron Laboratory (MSU, USA), the 60Ge ions were produced in 78Kr beam fragmentation reactions and separated from the other reaction products in the A1900 separator. The ions were detected in the active volume of the gaseous time-projection chamber with optical readout (OTPC), where they later decayed. This detector allows exotic decay modes to be identified, even with very small statistics present. The decay of about 20 60Ge ions was observed by β-delayed proton emission yielding a branching ratio of ~100% and a half-life of 20+7-5 ms. This value agrees well with theoretical predictions.

A. A. Ciemny +25 co-authors, First measurement of 60Ge β decay, Eur. Phys. J. A 52, 89 (2016)
[Abstract]

First quantum machine to produce four clones (Vol. 43 No. 1)

image These intertwinned rays of light are an artistic metaphore for quantum cloning

This article presents a theory for a quantum cloning machine able to produce several copies of the state of a particle at atomic or sub-atomic scale, or quantum state. It could have implications for quantum information processing methods used, for example, in message encryption systems.

Quantum cloning is difficult because quantum mechanics laws only allow for an approximate copy-not an exact copy-of an original quantum state to be made, as measuring such a state prior to its cloning would alter it. The present work shows that it is theoretically possible to create four approximate copies of an initial quantum state, in a process called asymmetric cloning. The authors have extended previous work that was limited to quantum cloning providing only two or three copies of the original state. One key challenge was that the quality of the approximate copy decreases as the number of copies increases. It appears possible to optimise the quality of the cloned copies, thus yielding four good approximations of the initial quantum state. The present quantum cloning machine is shown to have the advantage of being universal and therefore able to work with any quantum state, ranging from a photon to an atom.

Asymetric quantum cloning has applications in analysing the security of messages encryption systems, based on shared secret quantum keys. Two people will know whether their communication is secure by analysing the quality of each copy of their secret key. Any third party trying to gain knowledge of that key would be detected as measuring it would disturb the state of that key.

Optimal asymmetric 1 4 quantum cloning in arbitrary dimension
X.J. Ren, Y. Xiang and H. Fan, Eur. Phys. J. D (2011)
[Abstract]

Flat inductive plasma for large area plasma processing (Vol. 48, No. 3)

Schematic of the 1.2 x 1.2 m2 planar antenna. Capacitors join the ends of copper leg inductors to form a LC resonant network.”

Low temperature plasma generated by a resonant network antenna.

1 m2) is of fundamental importance for the industrial production of solar cells, flat panel displays, packaging, surface treatment, large area electronics, etc. Magnetic induction by RF oscillating currents in parallel legs is often used to drive the plasma in large inductive sources. In this work, the novel plasma source is a multiple LC resonant network antenna as shown in the figure. An electromagnetic model describes the antenna-plasma coupled system as a multi-conductor transmission line. Inspired by the “complex image” model for power transmission lines, this theory is used for the first time to calculate the induced image currents in the plasma. This approach could be applied generally to ICP antennas for large area plasma processing.

Ph. Guittienne, R. Jacquier, A. A. Howling and I. Furno, Electromagnetic, complex image model of a large area RF resonant antenna as inductive plasma source, Plasma Sources Sci. Technol. 26, 035010 (2017).
[Abstract]

Flexibility and phase transitions in zeolite frameworks (Vol. 42, No. 4)

image Detail of a zeolite structure built from corner-sharing tetrahedral units.

The zeolites are a group of minerals whose complex and beautiful atomic structures are formed by different arrangements of a very simple building block- a group of four oxygen atoms forming a tetrahedron, with a silicon or aluminium atom at the centre. Each oxygen atom belongs to two tetrahedra, so the structure can be viewed as a network of tetrahedra linked at the corners.

Zeolites have found widespread applications in chemical industry, particularly as catalysts. Their chemical properties depend on the shape of the pores and channels that run through the structure, containing water molecules, ions and even small organic molecules. More than a hundred different frameworks are known to exist in natural minerals or have been synthesised by chemists.

A fundamental geometric question is whether it is possible for the tetrahedra of the framework to exist in an undistorted, geometrically ideal form, or whether distortions are inevitably caused by the linking together of the tetrahedral units to form the structure. A new study links this question to the compression behaviour of zeolites in the analcime group. Four different structures display a common behaviour: they exist in a high-symmetry form at low pressures when the tetrahedra can exist without distortions, but transform to low-symmetry forms under pressure when distortions become inevitable. A deeper understanding of the rules governing the formation of zeolite structures may one day allow us to synthesise structures with specific properties on demand. New insights into the physics and geometry of frameworks are an important step in this direction.

Flexibility windows and phase transitions of ordered and disordered ANA framework zeolites
S. A. Wells, A. Sartbaeva and G. D. Gatta, EPL, 94, 56001 (2011)
[Abstract]

Fluctuational electrodynamics for nonlinear media (Vol. 48 No. 2)

Nonlinear coupling of incident waves (blue) and fluctuation-induced waves (black)

Nonlinear optics gives rise to a lot of interesting phenomena like frequency mixing, the optical Kerr effect, the Raman effect, and many others. With the advent of metamaterials, (nonlinear) optical properties can nowadays be tuned, controlled, and designed, allowing for the exploration of new physics. In this article, the fluctuations of the electromagnetic field in the presence of such materials are investigated.

Fluctuational electrodynamics, combining classical electrodynamics with quantum and thermal noise, is a powerful framework to study effects which appear in equilibrium (such as Casimir forces) as well as those found out of equilibrium (such as radiative heat transfer). So far, this concept relies on the optical linearity of the involved objects.

In this article, fluctuational electrodynamics is adapted to describe also objects with nonlinear optical response, including the amendment of the noise (so called Rytov currents). Most notably, electric currents fluctuating because of noise and induced currents due to incident waves become coupled, giving rise to new phenomena. As an example, the Casimir force between two plates with nonlinear optical properties is computed, which has a different distance dependence at close proximity compared to the linear case.

H. Soo and M. Krüger, Fluctuational electrodynamics for nonlinear media, EPL 115, 41002 (2016)
[Abstract]

Focusing surface acoustic waves into an optomechanical nanobeam (Vol. 51, No. 1)

Focusing and conversion efficiency of SAW into the modes of a straight nanobeam and to cavity modes

Cavity optomechanics (OM) is a topical issue of growing interest due to the potential applications ranging from sensing and communications to quantum information technology. However, the number of phonons created in a cavity by an external optical fibre remains relatively low. An alternative way to induce large phonon population is the use of a phonon pump by transforming an RF signal into surface acoustic waves (SAW). In a simulation work, we demonstrated an efficient focusing and conversion of SAW generated by focusing interdigital transducers (IDT) on piezoelectric aluminum nitride film deposited on silicon on insulator (SOI) into the guided modes of a silicon nanobeam. For a straight nanobeam, we achieve an efficiency of -22dB and displacements about 50pm when applying a voltage of 1V to the IDT. When the nanobeam is structured to form a phononic crystal with a cavity, the guided modes excite some localized modes around 2 GHz inside the cavity with a magnitude of the vibrational motions around 1nm. The preliminary experiments confirm the simulations.

Work is supported by the European Commission through project PHENOMEN, Grant Agreement No. 713450.

Alexander V Korovin et al, Conversion between surface acoustic waves and guided modes of a quasi-periodic structured nanobeam, J. Phys. D: Appl. Phys 52, 32LT01 (2019)
[Abstract]

Football displays fractal dynamics (Vol. 45 No.3)

Players’ and ball’s positions captured on video.

Physicists reveal that the real-time dynamics in a football game are subject to self-similarity characteristics in keeping with the laws of physics, regardless of players’ psychology and training.

Football fascinates millions of fans. Despite their seemingly arbitrary decisions, each player obeys certain rules, as they constantly adjust their positions in relation to their teammates, opponents, the ball and the goal. In this work the authors have now analysed the time-dependent fluctuation of both the ball and all players’ positions throughout an entire match.

The authors considered two previous football matches. Thanks to their analysis of the time-series variation in the ball versus the front-line movements of the players, they were the first to discover that these dynamics have a fractal nature. This finding implies that the movement of the ball/front-line at any given time has a strong influence on subsequent actions. This is due to the so-called memory effect, linked to the game’s fractal nature.

A. Kijima, K. Yokoyama, H.Shima and Y. Yamamoto (2014), “Emergence of self-similarity in football dynamics”, Eur. Phys. J. B, 87, 41 (2014)
[Abstract]

Force transmission bottlenecks as determinants of shear bands (Vol. 46 No. 5-6)

Particle rotations and minimum cut from start to end of loading history (columns (a) to (h)): simple shear DEM simulation (row A) and triaxial compression of Caicos Ooid (row B) and Ottawa (row C) sand. Particles with high (low) rotations are coloured red (blue). The minimum cut lies at the blue-red interface; green particles identify source/sink nodes.

The formation of shear bands (or 'strain localisation') is a key attribute of degradation and failure in soil, rocks, and other amorphous and crystalline materials. Their deleterious effects on material performance is well known, though on the other hand their rich pores provide important conduits for flow in petroleum and natural gas recovery from shale and tight rock formations. Despite intense research efforts, their origin and mechanisms of evolution have proved elusive. Here, patterns discovered from data on sand and discrete element simulations suggest that the early localization of bottlenecks in force transmission is the root cause of shear bands in dense granular media. This mechanism was shown to initiate early in the loading history for initially (globally) homogeneous samples. The finding paves the way for early prediction of failure and highlights promising avenues to explore ways to change its course from inception.

A. Tordesillas, S. Pucilowski, S. Tobin, M. R. Kuhn, E. Andò, G. Viggiani, A. Druckrey and K. Alshibli, Shear bands as bottlenecks in force transmission, EPL, 110, 58005 (2015)
[Abstract]

Formation of alkali clusters attached to helium nano-droplets (Vol. 42, No. 2)

image Illustration of cluster formation upon successive pick-up of atoms by a helium nano-droplet. The inset shows the simulated formation probability of a Na3 cluster as a function of droplet size and the density of sodium atoms

Helium nanodroplets provide a unique matrix for the spectroscopy of embedded species. The ability to form clusters inside the droplet by successive pick-up of single atoms provides a novel method for the study of small clusters isolated in a 370 mK cold, weakly interacting environment. However, the formed clusters exist in a wide size distribution and cannot be size selected. This fact often hinders the interpretation of the experimental data.

A common technique to determine the size distribution is recording the pick-up statistics. Helium droplets collect atoms or molecules via inelastic collisions when passing a pick-up cell. Monitoring the intensity of a cluster correlated signal as a function of the pressure in the pick-up cell gives access to the pick-up statistics. Experiments have shown that the size distribution often deviates from the expected Poissonian statistics, in particular in the case of alkali atoms. In this paper the influence of the effects of droplet shrinking, momentum transfer and cluster desorption on the pick-up statistics are simulated. Our results compare well with measured pick-up statistics of alkali clusters and demonstrate the different effects on the terminal size distributions. In addition information on the spin statistics of formed clusters can be derived from the presented data.

Modelling the formation of alkali clusters attached to helium nano-droplets and the abundance of high-spin states
O. Bünermann and F. Stienkemeier, Eur. Phys. J. D 61, 645 (2011)
[Abstract] | [PDF]

Fractal agglomerates fragment into dissimilar fragments (Vol. 50, No. 5-6)

Fractal agglomerates fragment into dissimilar fragments
A fragmentation event and the distribution of fragment sizes upon random bond removal in a Diffusion Limited Cluster Aggregation (DLCA) cluster

Fragmentation occurs everywhere in nature: polymers degrade, soot particles break up, cells divide, volcanic ash fragments, droplets break up in turbulent flow, lung fluid fragments to generate droplets. Nevertheless, little is known about the distribution of fragment sizes when a fractal agglomerate breaks up. The fragment-size distribution upon random bond removal in a linear chain composed of identical units is uniform. How does the distribution change as the morphology of the fragmenting structure changes?

More generally, fragmentation kernels, which depend on the size distribution and the fragmentation rate, have been extensively used in population balance equations. Usually, their analytical form is dictated by homogeneity requirements (as suggested by coagulation kernels) or physical arguments. In this work, the morphology-dependent fragment-size distribution is determined from numerical simulations of fragmenting in silico fractal-like agglomerates. The overarching idea is to map the agglomerate onto a graph via the adjacency matrix, the matrix that specifies the monomer-monomer bonds. Fragmentation occurs via random bond removal. The simulations showed that the distribution is U-shaped, fragmentation into dissimilar fragments, accurately reproduced by a symmetric beta distribution.

Y. DrossinoS, A. D. Melas, M. Kostoglou and L. Isella, Morphology-dependent random binary fragmentation of in silico fractal-like agglomerates, EPL 127, 46002 (2019)
[Article]

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