Vol. 42 No. 2 - Highlights

Graphene/ferroelectric hybrid devices (Vol. 42, No. 2)

image The figure shows the device operations of GFeFETs on PZT (Fig. a) with different local ferroelectric polarization magnitude. In the linear regime of PZT (Fig. b), ultra-low-voltage operation of graphene field effect transistors within +/-1 V with maximum doping exceeding 1013 cm-2 and on-off ratios larger than 10 times is demonstrated. After polarizing PZT (Fig. c), switching of graphene field effect transistors are characterized by pronounced resistance hysteresis, suitable for ultra-fast non-volatile electronics.

The soaring demands on non-volatile memory for ultra-portable electronic devices have grown NAND flash into a multi-billion dollar business. As a Si-CMOS based technology, NAND flash provides the most aggressive scalability, closely following the-state-of-art semiconductor manufacturing process. NAND flash also takes advantage of its relatively simple floating-gate structure and seamless integration with Si-CMOS logics, leading to significant lower production cost over other competing non-volatile technologies such as FeRAM and MRAM.

Graphene, with its ultra-high mobility and almost unlimited scalability down to atomic scale, is considered now as one of the most promising candidates for replacing Si. Graphene-based non-volatile memory has also been demonstrated very recently. However, a seamless solution for integrating graphene transistors and non-volatile memory remains a challenge.

Here, Zheng et al. demonstrate the wafer-scale patterning and device operations of Cu-CVD graphene-ferroelectric field effect transistors (GFeFETs) on ferroelectric Pb(Zr0.3Ti0.7)O3 (PZT) substrates, integrating both transistor and non-volatile memory functionalities on the same chip.

In the linear regime of PZT, ultra-low-voltage operations of GFeFETs within +/-1V can be used as controlling transistors for addressing and reading/writing of memory unit cells. After polarizing PZT, the hysteretic switching of GFeFETs is ideal for ultra-fast non-volatile data storage. The combination of high-quality Cu-CVD graphene and functional substrates will not only greatly speed up the studies of all graphene-based electronics but also open up a new route in exploring new graphene physics and functionalities.

Wafer-scale graphene/ferroelectric hybrid devices for low-voltage electronics
Yi Zheng, Guang-Xin Ni, Sukang Bae, Chun-Xiao Cong, Orhan Kahya, Chee-Tat Toh, Hye Ri Kim, Danho Im, Ting Yu, Jong Hyun Ahn, Byung Hee Hong and Barbaros Özyilmaz, EPL, 93, 17002 (2011)
[Abstract] | [PDF]

A tunnelling probe for molecular currents (Vol. 42, No. 2)

image Laser-induced tunnelling ionization of a multi-electron system (blue wavefunction) triggers charge oscillations of the created electron vacancy (shown in red). A second tunnelling step probes the temporal rearrangement of the vacancy, enabling its characterization.

Internal electron currents in molecules play a crucial role in chemical and biological processes, like charge transport in cellular respiration and in photosynthesis. Since electron currents can be ultrafast and escape most traditional probes, they are hard to capture. We show that laser-induced tunnel ionization is a powerful probe of internal currents.

An intense infrared laser field acts on a molecule essentially as the tip of a scanning tunnelling microscope (STM): it extracts a weakly bound electron through a tunnelling barrier. The electron is not equally likely to tunnel out in any direction when the orbital has an asymmetric shape - this has led to the development of the molecular STM, a probe of the static electronic structure.

Unlike in the STM, tunnelling in a laser field is an attosecond phenomenon and therefore potentially launches attosecond electron dynamics. Such dynamics has also been inferred from high-harmonic generation, but that method is insensitive to the actual degree of electronic coherence. Here, we show that laser-induced tunnelling directly probes time-dependent deformations of the electron cloud and maps them on two complementary observables: the total tunnelling current and its momentum distribution.

We study spin-orbit dynamics in a rare gas ion, the simplest example of an internal electron current launched by ionization. Such currents are ubiquitous in molecules, where the sudden departure of an electron triggers an internal rearrangement. Laser-induced tunnelling is a powerful probe of such fundamental events.

A new aspect offered by the proposed concept is the control over electronic dynamics and double ionization. We show that the spin state (i.e. the entanglement) of the ejected electron pair can be controlled offering interesting opportunities for quantum control.

Imaging and controlling multi-electron dynamics by laser-induced tunnel ionization
H. J. Wörner and P.B. Corkum, J. Phys. B: At. Mol. Opt. Phys. 44, 041001 (2011) [Abstract] | [PDF]

Graphene transport with a polymer electrolyte gate (Vol. 42, No. 2)

image The figure shows the temperature dependence of graphene resistivity at high (red) and intermediate (blue) carrier densities. Since phonon modes follow the Bose-Einstein law, the scattering of electrons from acoustic phonons is strongly suppressed at low temperatures, thus leading to power-law dependence. The temperature scale that determines this suppression increases linearly with the Fermi level. At high doping, the resistivity is weakly density-dependent. A detailed analysis shows there is an important contribution from neutral weak scatterers. Top panel: Optical image and schematics of graphene device coated with polymer electrolyte gate.

The electronic properties of graphene have received wide recognition, most notably the award of the 2010 physics Nobel Prize. In graphene's two-dimensional hexagonal lattice, the electrons behave as massless chiral Fermions obeying the Dirac equation. The exotic nature of these quasi-particles and their low carrier density is at the heart of several interesting phenomena, including the unconventional quantum Hall effect and Klein tunneling. Raising the density of charge carriers in graphene can have several implications including the possibility for van Hove singularity driven high-Tc superconductivity. From a fundamental point of view, the high carrier densities substantially modify the interactions between quasi-particles in this two-dimensional crystal.

In this report, graphene devices in Hall-bar geometry are gated with a polymer electrolyte to realize carrier densities an order of magnitude higher than achieved by conventional methods. At these densities, the carriers sufficiently screen the long-range interactions. This allows fully appreciating the importance of various neutral defects on the graphene lattice. In contrast to metals, the temperature scale for quantum suppression of acoustic-phonon scattering in graphene is significantly gate-tunable. The phenomenon is readily observed to high temperatures at high carrier densities.

Graphene transport at high carrier densities using a polymer electrolyte gate
A. Pachoud, M. Jaiswal, P. K. Ang, K. P. Loh and B. Özyilmaz, EPL, 92, 27001 (2010)
[Abstract] | [PDF]

Two-site Bose-Hubbard model in waveguides (Vol. 42, No. 2)

image Optical realization of the two-site Bose-Hubbard model based on light transport in a waveguide array with engineered refractive index profile. The figure shows an example of Josephson oscillations in Fock space, in which light trapped in the various waveguides gives the occupation probability of the Fock states.

Light transport in waveguide lattices has provided over the past decade a test bench to visualize the classical analogues of a wide variety of coherent single-particle quantum phenomena generally encountered in condensed matter or matter wave systems. Since photons in linear optical structures do not interact, it is a common belief that the use of photonics as a model system for quantum physics carries the intrinsic drawback of being limited to visualize single-particle phenomena, missing the possibility to simulate the richer physics of interacting many-particle systems.

In this paper, the author has now pushed the realm of quantum-optical analogies beyond the single-particle phenomena, demonstrating that linear photonics can provide an accessible laboratory system to visualize in a purely classical setting the very basic dynamical aspects embodied in the physics of interacting quantum particles. In particular, a classical realization of the famous Bose-Hubbard Hamiltonian, which provides a paradigmatic model to describe the physics of strongly interacting bosons, has been theoretically proposed for light transport in engineered waveguide lattices. The author has shown that spatial propagation of light waves along the photonic crystal structure mimics in the Fock space the temporal dynamics of ultracold bosonic atoms trapped in a double-well potential, the so-called bosonic junction. While for ultracold atoms the full multi-particle dynamics is typically not accessible, the present work proposes a new route to simulate the Bose-Hubbard model which overcomes such a limitation, thus opening a new route to the realization of classical simulators of many-particle quantum physics.

Optical realization of the two-site Bose-Hubbard model in waveguide lattices
S. Longhi, J. Phys. B: At. Mol. Opt. Phys. 44 051001 (2011)
[Abstract] | [PDF]

Photoproduction of n'-mesons off the deuteron (Vol. 42, No. 2)

image From left to right of panels in Fig. 1, we have the inclusive reaction data first, then the world free and, this experiment, quasi-free proton data and last quasi-free neutron data from coincidence and an extraction of proton from deuteron inclusive data. Solid lines: solution (I) NH model, dashed lines: η'-MAID for neutron and dotted lines: CLAS proton data.

A key to our understanding of Quantum Chromodynamics (QCD) in the strong regime is our ability to reproduce the hadronic excitation spectrum. Up to now, and due to their limited predictive power, quark models forecast of this spectrum at high excitation energies is unsatisfactory and is dubbed "the missing resonances problem". To explore the high excitation energies in the hadron spectrum production or scattering of heavier mesons from a nucleon target is essential.

In a recent tour-de-force experiment, I. Jaegle et al. report on an impressive first measurement of η' photoproduction off a deuteron target at beam energies between 1.47 - 2.45 GeV at the tagged photon beam of the ELSA electron accelerator. Differential cross sections with a wide angular coverage were derived for quasi-free production both on protons and neutrons validating the quasi-free picture. And the first estimate of the coherent γd -> dη' contribution is found consistent with an impulse approximation, pointing to a viable isospin composition of model amplitudes and weak final state interactions.

Legendre polynomials coefficients from angular distributions fits of this experiment and world data are reported in the Fig. where proton and neutron cross sections for photon energies above 2 GeV, in a region where contributions from t-channel exchange are important, display a similar behaviour. At lower photon energies from where the proton cross-section peaks, the behaviour points to different resonance contributions and would require polarization observables for future investigation.

Photoproduction of η'-mesons off the deuteron
I. Jaegle et al. (the CBELSA/TAPS Collaboration), Eur. Phys. J. A, 47, (2011)
[Abstract] | [PDF]

Mean-field theory and stochastic evolution (Vol. 42, No. 2)

image Opposing pairs (A-C, B-D) of equal strength evolve deterministically on a closed orbit (blue loop), oscillating endlessly (upper left). An example of stochastic trajectories is shown in red (dashed line). Lower panels display systems with identical initial conditions but different strengths, ending at opposite edges.

Population dynamics is a venerable and widely applicable subject. Over the last two centuries, many studies provided valuable insights into various phenomena, e.g., the emergence of biodiversity and fitness/extinction, while novelties are continually being discovered. Specifically, recent investigations of three species competing cyclically (e.g., rock-paper-scissors game) revealed rich and complex behaviours, whether the populations are well-mixed or dwelling on one- or two-dimensional lattices. Indeed, the well-mixed system displayed surprising survival probabilities: The species with the slowest consumption rate wins, leading to a popularized headline "Survival of the Weakest." Fascinating properties were also found in systems with spatial structure, including formation of complex patterns and mobility effects. Many aspects can be understood by exploiting techniques from statistical physics and non-linear dynamics.

Our work focuses on four cyclically competing species. Unlike the 3-species case, ours allows final states with coexisting pairs. The reason is simple: Resembling the game of Bridge, the four form two opposing teams of ally-pairs. For each pair, the product of their consumption rates determines if it wins or loses. From a master equation for the full stochastic problem, we derive an approximate set of rate equations (ODE's). Predictions from the latter typically agree well with simulations. Instead of the weakest surviving, our observations support a different maxim: "The prey of the prey of the weakest is the least likely to survive." Intuitively reasonable, this principle also applies to the special 3-species case! Meanwhile, a variety of intriguing extinction probabilities, discovered through simulations, provides numerous challenges for future research.

Cyclic competition of four species: Mean-field theory and stochastic evolution
S. O. Case, C. H. Durney, M. Pleimling and R. K. P. Zia, EPL, 92, 58003 (2010)
[Abstract] | [PDF]

Crystal nucleation on polymer droplets (Vol. 42, No. 2)

image a) A Si substrate with a polystyrene (PS) layer forms an unfavourable surface for a thin PE film. b) Upon heating, the unstable film dewets to form droplets. c) Optical microscopy image (500 µm wide). Amorphous droplets appear dark, while crystalline droplets become bright.

A nucleation site initiates the birth of a crystal. In most cases, take for example the dust particle in a snowflake, nucleation starts from a heterogenous defect. Homogenous nucleation is more elusive because of the prevalence of defects in any bulk sample. Crystallisation in tiny droplets alleviates this difficulty in a manner that is conceptually simple: subdivide the system into more domains than the number of defects. If the domains greatly outnumber the defects then only the homogenous mechanism can induce nucleation in a defect free compartment.

Such an approach has been used here to investigate nucleation in polyethylene (PE) droplets. At high temperatures, a thin PE film dewets from an unfavourable surface forming tiny droplets, much like water beading up on a waxy leaf (Fig. (b)). The resulting sample geometry is ideal: thousands of droplets ranging in size can be monitored simultaneously with optical microscopy, with a nucleation event easily distinguishable by the rapid growth of the crystal (Fig. (c)). Each droplet becomes an isolated independent nucleation experiment. By investigating thousands of droplets supercooled well below the melting temperature, studies of homogenous nucleation become straightforward. Relating the probability of homogenous nucleation to the size of the droplet, the authors show that nucleation is surface activated. Stated most simply, a droplet with twice the surface area is twice as likely to nucleate, indicating that the perturbation induced by the interface reduces the intrinsic activation barrier to crystal nucleation.

Surface nucleation in the crystallisation of polyethylene droplets
J.L. Carvalho and K. Dalnoki-Veress, Eur. Phys. J. E 34, 6 (2011)
[Abstract] | [PDF]

Cosmic rays: A (partly) untold story (Vol. 42, No. 2)

image Domenico Pacini in May 1910 (32 years old) while making a measurement. (Courtesy of the Pacini family.)
The work behind the discovery of cosmic rays, a milestone in science, involved many scientists in Europe and the New World fascinated by the puzzling penetrating radiation, and took place during a period characterized by lack of communication and by nationalism caused primarily by World War I.

It took eventually from the turn of the century until 1926 before the extraterrestrial nature of the penetrating radiation was generally accepted.

In the work that culminated with high altitude balloon flights, many important contributions have been forgotten and in particular those of Domenico Pacini, who, in June 1911, demonstrated by studying the decrease of radioactivity with an electroscope immersed in water that the radiation today called "cosmic rays" could not come from the crust of the Earth. This was the first time in which the technique of comparison of undersea measurements with measurements at sea level has been used to obtain a result in fundamental physics; this technique will be used in neutrino experiments of the near future. This article carefully retraces the history of the discovery of cosmic rays and puts the unfolding story in both the political and scientific contexts. With the help of material previously unknown to the history of science, for example the nominations for the Nobel prizes related to cosmic ray research and the relevant internal reports of the Swedish Royal Academy of Science, and letters exchanged between Victor Hess and Pacini, a more complete view of this fascinating discovery is possible.

Nationalism and internationalism in science: the case of the discovery of cosmic rays
P. Carlson and A. De Angelis, Eur. Phys. J. H 36, 309 (2010)
[Abstract] | [PDF]

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]