Highlighted Papers / Colloquium Papers

Highlighted Papers are articles selected by the Editorial Board to increase the visibility of what are deemed to be especially important papers. The full PDF of the Highlighted Papers can be viewed and downloaded from this page free of charge. Every Highlighted Paper is introduced by a short paragraph explaining the novelty and particular importance of the published work.

A further subset of Highlighted Papers is "Colloquium Papers". They describe the development of new areas of research or the impact of new and promising experimental or theoretical methods in the fields that are within the spectrum of topics covered by the journal. While not as extensive and complete as reviews in the usual sense, they are intended to suitably introduce new research directions and techniques in their early stage of development to a wider audience.

January 2012

EPJB - New model for epidemic contagion

Improved estimates on the geographical spread of infectious diseases are achieved by studying human mobility networks
Humans are considered the hosts for spreading epidemics. The speed at which an epidemic spreads is now better understood thanks to a new model accounting for the provincial nature of human mobility, according to a study published in EPJB. The research was conducted by a team lead by Vitaly Belik from the Massachusetts Institute of Technology, USA, who is also affiliated with the Max Planck Institute for Dynamics and Self-Organization, Germany.
The authors modelled human mobility as recurrent trips centred around a home base. The model accounted for the bi-directional travels around a central node, representing their home location and forming a star-shaped network. Previous models were based on diffusion and would imply that people travel randomly in space, not necessarily returning to their home location. These do not accurately describe the high degree of predictability in human mobility.
The researchers found that older diffusion-based models overestimated the speed at which epidemics spread. The speed of epidemics spreading through bi-directional travel, which is dependent on the travel rate, is significantly lower than the speed of epidemics spreading by diffusion.
In addition, the authors discovered that the time individuals spend outside their home locations influences the speed of epidemics spreading and whether an outbreak goes global. This contrasts with previous findings based on diffusion models, which suggested that the rate of travel between locations is the key factor influencing the global outbreak of epidemics.
This model must be tested against real data on human mobility before it can be used as a risk analysis and decision-making tool for epidemics such as avian flu. This model could also be used in areas such as population dynamics and evolutionary biology.

Recurrent hostmobility in spatial epidemics: beyond reaction-diffusion. V. Belik, T. Geisel, D. Brockmann, Eur. Phys. J. B (2011) 84, 579–587, DOI: 10.1140/epjb/e2011-20485-2

January 2012

EPJB - Towards high-temperature superconductors

Scientists produce a new type of superconductor by manipulating graphene, the study of which led to a Nobel Prize
Chinese scientists have manipulated the charge and the degree of freedom, known as spin, of electrons and their associated magnetic properties in a single-layer carbon material called graphene, making it suitable for applications involving superconductivity, a quantum mechanical phenomenon in which electrons travel in a material with no electrical resistance. These findings have recently been published in an article in EPJB by Chunxu Bai from Anyang Normal University and colleagues from the Henan Institute of Science and Technology in Xinxiang.
The authors investigated the means of exploiting a superconducting graphene flake. They looked at how unconventional electron pairing – known as d-wave pairing symmetries – would affect coherent subatomic – i.e., quantum – level transport within the material. In particular, they focused on electrical conductance in a system called a spin valve, which consists of two conducting magnetic materials, including a normal metal and graphene superconductor, the electrical resistance of which alternates from so-called giant magnetic resistance to none, depending on the alignment of its magnetic layers.
By reversing the magnetisation direction of one of the layers of the valve, researchers found that it is possible to achieve a spin-switch effect, whereby a normal current is converted into a superconducting current. The authors also found that the spin-switch effect is sensitive to the incident energy level and the orientation of the material’s superconducting gap.
The authors hope that their theoretical results can provide the basis to design a spin-switch electron device able to operate at so-called high temperature, where liquid nitrogen can be used as a refrigerant at 77 Kelvins (-196 °C) instead of the liquid helium (−269 °C) traditionally used in conventional superconductors. This approach opens doors for the wider use of an all-graphene spin-switch circuit in quantum information applications in the future.

Spin-switch effect in a graphene d-wave superconductor spin valve. C. Bai, J. Wang, H. Tang, and Y. Yang, Eur. Phys. J. B (2011) 84, 1, DOI 10.1140/epjb/e2011-20507-1

December 2011

EPJB - Random noise helps make signals clearer

Model shows that signal clarity only improves if specific energy conditions are met
Scientists have shown the energy conditions, under which a weak signal supplied to a physical system emerges as a stronger signal at the output thanks to the presence of random noise (a process known as stochastic resonance), in a paper that has just been published in EPJB.
Stochastic resonance goes against the intuitive idea that where noise is present, the signal tends to fade. It occurs in systems where the response is not proportional to the applied input signal, known as nonlinear systems.
The authors, Shubhashis Rana, Sourabh Lahiri and Arun M. Jayannavar from the Institute of Physics, in Bhubaneswar, India, used a model consisting of a symmetric double-well energy potential in which a particle moves randomly. They studied the effect of the steepness of the walls of the confining energy potential by observing the movement of the particle, which they subjected to an external sinusoidal signal that alternately lowers either of the wells.
The authors selected a quantifier – the average work done on the system by the signal – to determine the conditions under which the particle moving from one well to the opposite side well and back at every cycle of the signal reaches stochastic resonance. They found that it only occurs when the potential is “hard”, meaning that it has sufficiently steep walls, but breaks down otherwise. Previous work used different quantifiers and found similar results, confirming their findings using numerical simulations.
This study contributes to improving scientists’ understanding of stochastic resonance. It could, ultimately, contribute to gaining deeper insights into physics-related phenomena such as the processing of unclear images to increase their resolution* and biological systems, including mechanoreceptor cells in crayfish and the functioning of sensory neurons in humans.

The role of soft versus hard bistable systems on stochastic resonance using average cycle energy as a quantifier. S. Rana, S. Lahiri, A.M. Jayannavar, Eur. Phys. J. B (2011) 84, 2, DOI 10.1140/epjb/e2011-20802-9

November 2011

EPJB - No extraordinary effects from microwave and mobile phone heating

Study quantifies effects of electric field-induced versus conventional heating
The effect of microwave heating and cell phone radiation on sample material is no different than a temperature increase, according to scientists from the Department of Chemistry and Biochemistry, Arizona State University, in Tempe, as published in a recent issue of EPJB.
Abidah Khalife, Ullas Pathak and Ranko Richert attempted for the first time to systematically quantify the difference between microwave-induced heating and conventional heating using a hotplate or an oil-bath, with thin liquid glycerol samples. The authors measured molecular mobility and reactivity changes induced by electric fields in these samples, which can be gauged by what is known as configurational temperature.
By conducting experiments at varying field frequencies and sample thicknesses, they realised that thin samples exposed to low-frequency electric field heating can have a considerably higher mobility and reactivity than samples exposed to standard heating, even if they are at the exact same sample temperature. They also found that at frequencies exceeding several megahertz and for samples thicker than one millimetre, the type of heating used does not have a significant impact on the level of molecular mobility and reactivity, which is mainly dependent on the sample temperature. In effect, the configurational temperatures will only be marginally higher than the real measurable temperature.
Previous studies were mostly fundamental in nature and did not establish a connection between microwaves and mobile phone heating effects. These findings imply that for heating with microwave or cell phone radiation operating in the gigahertz frequency range, no other effect than a temperature increase should be expected.
Since the results are based on averaged temperatures, future work will be required to quantify local overheating, which can, for example, occur in biological tissue subjected to a microwave field, and better assess the risks linked to using both microwaves and mobile phones.

Heating liquid dielectrics by time dependent fields. A. Khalife, U. Pathak, and R. Richert, Eur. Phys. J. B (2011) 83, 429 – 435, DOI 10.1140/epjb/e2011-20599-5

November 2011

EPJB - Can metals remember their shape at nanoscale, too?

How nickel-titanium nanometric-size particles change back to their memorised shape
University of Constance physicists Daniel Mutter and Peter Nielaba have visualised changes in shape memory materials down to the nanometric scale in an article published in EPJB.
Metallic alloys can be stretched or compressed in such a way that they stay deformed once the strain on the material has been released. Only shape memory alloys, however, can return to their original shape after being heated above a specific temperature.
For the first time, the authors determine the absolute values of temperatures at which shape memory nanospheres start changing back to their memorised shape – undergoing so-called structural phase transition, which depends on the size of particles studied. To achieve this result, they performed a computer simulation using nanoparticles with diameters between 4 and 17 nm made of an alloy of equal proportions of nickel and titanium.
To date, research efforts to establish structural phase transition temperature have mainly been experimental. Thanks to a computerised method known as molecular dynamics simulation, the authors were able to visualise the transformation process of the material during the transition. As the temperature increased, they showed that the material’s atomic-scale crystal structure shifted from a lower to a higher level of symmetry. They found that the strong influence of the energy difference between the low- and high-symmetry structure at the surface of the nanoparticle, which differed from that in its interior, could explain the transition.
Most of the prior work on shape memory materials was in macroscopic scale systems and used for applications such as dental braces, stents or oil temperature-regulating devices for bullet trains. Potential new applications include the creation of nanoswitches, where laser irradiation could heat up such shape memory material, triggering a change in its length that would, in turn, function as a switch.

Simulation of the thermally induced austenitic phase transition in NiTi nanoparticles. D. Mutter, P. Nielaba, Eur. Phys. J. B (2011), DOI 10.1140/epjb/e2011-20661-4

November 2011 / Colloquium Paper

EPJB - The secrets of tunneling through energy barriers

Electrons moving in graphene behave in an unusual way, as demonstrated by 2010 Nobel Prize laureates for physics Andre Geim and Konstantin Novoselov, who performed transport experiments on this one-carbon-atom-thick material. A review article, just published in EPJB, explores the theoretical and experimental results to date of electrons tunneling through energy barriers in graphene.
As good an electrical conductor at room temperature as copper graphene is, it also outperforms all other known materials as a heat conductor. It is both very dense due to its honeycomb lattice structure and almost completely transparent, making it suitable, among other applications, for touch screens and light panels.
What could partly explain graphene’s properties is that electrons travelling inside the material behave as if they were massless. Their behavior is described by the so-called massless Dirac equation that is normally used for high-energy particles such as neutrinos nearing the speed of light. However, electrons in graphene move at a constant speed 300 times smaller than that of light.
In this review, P.E. Allain and J.N. Fuchs, both from the Université Paris-Sud, focus on the tunneling effect occurring when Dirac electrons found in graphene are transmitted through different types of energy barriers. Contrary to the laws of classical mechanics, which govern larger scale particles that cannot cross energy barriers, electron tunneling is possible in quantum mechanics – though only under restricted conditions, depending on the width and energy height of the barrier.
However, the Dirac electrons found in graphene can tunnel through energy barriers regardless of their width and energy height; a phenomenon called Klein tunneling, described theoretically for 3D massive Dirac electrons by the Swedish physicist Oskar Klein in 1929. Graphene was the first material in which Klein tunneling was observed experimentally, as massive Dirac electrons required energy barriers too large to be observed.

Klein tunneling in graphene: optics with massless electrons. P.E. Allain, J.N. Fuchs, Eur. Phys. J. B (2011), DOI 10.1140/epjb/e2011-20351-3

August 2011

EPJ B – Online activity grows in a similar pattern to those of real-life networks

The activity of online communities does not grow in line with the number of users, according to a model recently published in the European Physical Journal B.
The Internet has given rise to its own sorting devices. Among these, tagging consists in assigning user-chosen keywords to a piece of information (such as a digital image) to facilitate searches. Lingfei Wu, a researcher at the City University of Hong Kong’s Department of Media and Communication, used the tagging behaviour of social media application users to study the growth of online communities’ activity.
Wu focused on two social media sites: Flickr and Delicious, in which a faster growth of overall tagging activity than of user population was observed. This phenomenon is called accelerated growth and confirms that tagging activity is not correlated in a linear way to the number of social media users using tagging. In this study, Wu suggests that the accelerating growth pattern originates from the effect of the community size on individual tagging behaviour. He found that despite the fluctuation in the number of tags and of the population, communities have a heterogeneity (in terms of individual tagging activity) that remains constant over time, but differs across systems. Given this time-invariant heterogeneity, the average individual activity will grow as the system expands, leading to the accelerating growth of overall activity.
Previous studies focusing on real-world examples such as cities and biological networks exhibited similar growth pattern. This study shows that there are also accelerating growth patterns in the virtual world.
Immediate applications of modelling the online activity growth include predicting the server capacity required for social media sites on the basis of historical data. Future work will focus on devising a unified model that explains the regularity governing the scaling up of both real-life systems (e.g. biological species and cities) and virtual communities.

The accelerating growth of online tagging systems
L. Wu, Eur. Phys. J. B (2011) DOI: 10.1140/epjb/e2011-20187-9

August 2011 / Colloquium Paper

EPJB - Atomistic details over longer time scales

The conventional method for atomistic simulation, namely molecular dynamics (MD), is not well suited to resolve slow dynamical processes, that is processes associated with a system that gets trapped from time to time in deep local energy minima. In a Colloquium paper in the European Physical Journal B, A. Kushima, J. Eapen, Ju Li, S. Yip and T. Zhu review the capabilities of biased molecular simulation methods such as metadynamics, autonomous basin climbing (ABC), strain-boost and adaptive boost simulations – methods designed to probe at the atomic level mechanisms that drive system-level behavior observable on macroscopic time scales.
The authors discuss adaptations of these methods applied to the study of glassy dynamics, creep deformation in stressed solids and diffusion. As these are rather different areas of condensed matter science, the aim is to draw attention to other complex processes involving anthropological or geological time scales, where behavior can be simulated at atomic resolution and understood in terms of micro-scale processes of molecular rearrangements and collective interactions.

Time scale bridging in atomistic simulation of slow dynamics: viscous relaxation and defect activation
A. Kushima et al., Eur. Phys. J. B (2011) DOI: 10.1140/epjb/e2011-20075-4

July 2011

EPJB - Disordered networks synchronise faster than small world networks

A study recently published in European Physical Journal B presents observations of how complex systems synchronise over time.
Synchronisation occurs when individual elements in a complex network behave in line with each other. This applies to real-life examples such as the way neurons fire during an epileptic seizure or the phenomenon of crickets falling into step with one another.
In this study, Carsten Grabow and colleagues from the Max Planck Institute for Dynamics and Self-Organization in Göttingen, Germany, created a model to test the speed of synchronisation of complex networks in collaboration with the Warwick Complexity Centre, UK. They tested this model using three very different oscillators acting on complex networks, which are known as Kuramoto, Rössler and pulse-coupled oscillators. As a result, for all tested networks they showed that the structure of the coupling between network nodes determines the speed of synchronisation.
In short: the higher the disorder in the network, the faster the synchronisation.
They subsequently verified this observation in real-life networks including an air-transported network, a social network and a human travel network. Given the great variety of networks used, the increase in the speed of synchronisation in line with increased disorder can be considered universal.
This result goes against previous observations, which showed that so-called small-world networks, which consist of an intermediate structure of fully ordered and fully disordered networks, favour synchronisation. The small-world effect was famously applied to analysing social networks and gave rise to the theory that there are only six degrees of separation between people in a given country.
The authors are currently working on deriving a mathematical formula to predict which complex network would synchronise and how fast. Such an approach would require integrating parameters, including the network size and typical number of links per node, as well as the spread of the disorder introduced. This work could have real-life applications, for example, in measuring the robustness of the relaxation process in gene regulatory networks.

Speed of complex network synchronization C. Grabow, S. Grosskinsky and M. Timme
Eur. Phys. J. B (2011) DOI: 10.1140/epjb/e2011-20038-9

June 2011
EPJ B - Deciphering complex games
Game theory has changed our way of thinking about socio-economic interaction, shedding light on the consequences of leaving individuals take their choices for the sake of their self-interest. As exemplified by the prisoner's dilemma, the prediction of this approach can be quite far from what welfare optimization would predict. Still, most of the intuition of game theory has been limited to either simple games or to games with few players, that, in many cases, fall short of capturing the complexity of the ``games'' which are played in our societies. Typically, individuals are different and their incentives are different, and they are not only involved in playing games with their neighbors, but their choices have also to take into account the global games they are involved in. Ramezanpour, Realpe-Gomez and Zecchina show how the statistical mechanics approach can be extended to cope with the complexity of these games. This not only shows how to characterize the set of possible (Nash) equilibria of the society, but also helps finding those equilibria, which are typically hard to compute, which have optimal welfare properties.

Statistical physics approach to graphical games: local and global interactions A. Ramezanpour, J. Realpe-Gomez and R. Zecchina
Eur. Phys. J. B 81, 327-339 (2011) DOI: 10.1140/epjb/e2011-10963-x

May 2011 / Colloquium Paper
Coherent electron transport in quasi one-dimensional carbon-based systems
In this colloquium paper, I. Deretzis and A. La Magna seek to elucidate the potentiality and possible drawbacks of future all-carbon-based electronics. They focus their study on the coherent electron transport properties of graphene nanoribbons, carbon nanotubes and linear chains, as well as the role played by the modification of their structural and electronic symmetries. To read the full paper 'Coherent electron transport in quasi one-dimensional carbon-based systems', by I. Deretzis and A. La Manga, Eur. Phys. J. B 81, 15-36 (2011), click here.

May 2011 / Colloquium Paper
Exotic high pressure behavior of light alkali metals, lithium and sodium
At ambient conditions, lithium and sodium behave as free-electron metals and adopt highly symmetric close-packed structures. Under high pressure, however, these simple metals undergo a series of complex phase transitions to structures of lower symmetry, along with significant changes of their physical properties. In this invited colloquium paper, A. Bergara and co-workers provide a comprehensive description of these light alkali metals, with a focus on the structural and electronic properties under pressure.
To read the full paper 'Exotic high pressure behavior of light alkali metals, lithium and sodium', by B. Rousseau et al., Eur. Phys. J. B 81, 1-14 (2011), click here.

April 2011 / Colloquium Paper
Dispersion of solutes in porous media
In this colloquium paper, A. Hunt and co-workers test a new theory of solute transport in porous media by comparison with experiment. The predictions of their theory in the absence of diffusion are verified by comparing 2200 experiments over length scales from a few microns to 100km. The comparison focuses on the dispersivity. The agreement between their theory and the experiments requires rethinking the relevance of diffusion and multi-scale heterogeneity. It would also signal the inappropriateness of the classical advection-dispersion equation or any of its derivatives to model solute transport.
To read the full paper 'Dispersion of solutes in porous media', by A.G. Hunt et al., Eur. Phys. J. B (2011), click here.

March 2011 / Colloquium Paper
The insulating state of matter: a geometrical theory
In 1964 Kohn published the milestone paper "Theory of the insulating state", according to which insulators and metals differ in their ground state, the key difference being the organization of their electrons. However, the theory of the insulating state remained somewhat incomplete until the late 1990s.
This review by Rafaelle Resta addresses the recent developments: characterization of the many-body ground wavefunction by means of geometrical concepts, theoretical studies of several kinds of insulators: band insulators, Mott insulators, Anderson insulators, quantum Hall insulators, Chern and topological insulators...
To read the full paper 'The insulating state of matter: a geometrical theory', by R. Resta, Eur. Phys. J. B 79, 121-137 (2011), click here.

March 2011 / Colloquium Paper
EPJB - Atomistic simulations of pressure-induced structural transformations in solids
Constant-pressure molecular dynamics simulations allow the study of systems where external pressure is a driving force for a structural transformation. In this colloquium paper, Roman Martonak reviews various approaches to constant pressure simulations with focus on the recent developments in simulation methodology, such as metadynamics and transition path sampling. The application of the techniques to bulk and finite systems is illustrated by several examples. To read the full paper 'Atomistic simulations of pressure-induced structural transformations in solids', by R. Martoňák, Eur. Phys. J. B 79, 241-252 (2011), click here

December 2010
EPJ B – Au and Cu as building blocks of high-density memory devices
Gold and copper atoms adsorbed on a NaCl surface behave as isolated atoms and complex electronic interactions with the surface are negligible. A study by a group of Brazilian researchers uses first-principles simulations to measure the electronic and magnetic properties of gold and copper atoms adsorbed on NaCl(001) surfaces, as well as the modifications in these properties upon charge injection.
The results presented in EPJ B show that neutral Au and Cu adatoms on NaCl(001) interact weakly with the ionic substrate. Magnetization values are close to those of the corresponding isolated atoms, with spatial distributions concentrated mainly around the adatoms. The magnetization comes from the unfilled s valence orbital of the adatoms, and its value drops to zero when a single electron is injected in the adatom and fills its s shell.
Quantifying and manipulating the magnetic moments of the adsorbed atoms is essential to exploit the these systems for producing ultra-high density magnetic memory devices. Experimentalists have already shown the feasibility of manipulating the adatom charge and magnetization with an STM tip.
To read the full paper “Au and Cu Atoms on NaCl(001): a single-atom based memory device prototype?” by A.S. Martins et al., Eur. Phys. J. B 78, 543–546 (2010) click here.

October 2010 / Colloquium Paper
Modelling non-adiabatic processes using correlated electron-ion dynamics
The paradigmatic method to investigate the non-adiabatic exchange of energy between electrons and nuclei is Ehrenfest Dynamics, which is however unable to reproduce the correct heating of nuclei by current-carrying electrons.
In this invited Colloquium paper, McEniry et al. survey the theory and applications of a new family of methods (referred to as Correlated Electron-Ion Dynamics), which can be applied to improve the modelling of non-adiabatic processes.
Click here to view the full paper: [Modelling non-adiabatic processes using correlated electron-ion dynamics by E.J. McEniry et al., Eur. Phys. J. B 77, 305-330 (2010)]

September 2010
One-dimensional neutron-polarization analysis on magnetic nanostructures
Small-angle neutron scattering (SANS) is a prominent and powerful method to investigate the bulk of magnetic nanostructures on a length scale between a few and a few hundred nanometers. The recent development of efficient ³He spin filters (for cold neutrons) now allows to routinely perform one-dimensional neutron-polarization analysis (POLARIS) in a SANS experiment.
The first experiments by Honecker et al. on an FeCr based two-phase nanocrystalline alloy demonstrate the power of this technique for the investigation of magnetic nanostructures.
Click here to view the full paper: [Longitudinal polarization analysis in small-angle neutron scattering by D. Honecker et al., Eur. Phys. J. B 76, 209-213 (2010)]

September 2010
"The Geyser effect in the expansion of solid helium into vacuum" by Giorgio Benedek, Pablo Nieto and J. Peter Toennies
The vacuum expansion of solid helium through a micrometric orifice was suggested as a mean to inject excess vacancies into the solid bulk [R. Grisenti et al, J. Electr. Spectr. 129 (2003) 201]. Unexpectedly these vacuum expansion experiments exhibited spectacular periodic intensity bursts (geyser effect) on the uniform He flow out of the orifice.The results presented in this Highlight paper indicate that the geyser collapse does not occur near the orifice, as previously suggested, but at a plug in the feed line upstream of the source chamber. Each collapse is triggered by the increasing vacancy concentration which makes the solid behave much as a liquid. On this basis, Benedek, Nieto and Toennies argue that vacuum expansion provides a novel approach for investigating exotic non-equilibrium phases of quantum solids such as helium.
Click here to view the full paper: [G. Benedek et al., Eur. Phys. J. B 76, 237–249 (2010)]

June 2010
"Superfluidity of a perfect quantum crystal " by V. Golovko
Only one liquid exhibits Bose-Einstein condensation in nature: Helium II. At such temperatures, all other substances are solid. In these two papers, Vitaly Golovko demonstrates that Bose-Einstein condensation can also occur in the solid state. Moreover, it is shown that at 0 K, a condensate crystal is energetically preferable with respect to the same quantum crystal without condensate. Therefore, on lowering the temperature of the crystal there must somewhere happen Bose-Einstein condensation, as in liquid helium. This opens a huge field for experimental investigations of Bose-Einstein condensation and of its influence on properties of solids.
Click here to view the full papers: [V. Golovko, Eur. Phys. J. B 71 1 (2009) 85-95] and [V. Golovko, Eur. Phys. J. B 74 3 (2010) 345-356]

May 2010 / Colloquium Paper
"Simulating the mechanical response of amorphous solids using atomistic methods"
by M.L. Falk and C.E. Maloney
The study of elasticity, plasticity and failure in non-crystalline solids has greatly benefitted from the application of atomic scale simulation. This "colloquium paper" reviews the ways in which a variety of computational approaches have been used to elucidate the atomic scale phenomena that control the mechanics of amorphous solids. The constitutive theories that have been developed for describing mechanical response are briefly reviewed, as well as the prospects for testing the assumptions of these theories using simulation. M.L. Falk and C.E. Maloney list what they believe to be the most pressing open questions for substantiating these theoretical approaches, and ultimately for understanding and predicting the mechanical responses of amorphous solids.
Click here to view the full text: [M.L. Falk and C.E. Maloney, Eur. Phys. J. B (2010)]

May 2010 / Colloquium Paper
"sp³ domain in graphite induced by visible light" by K. Nasu
Models of photo-induced structural phase transitions are reviewed and are related to recent experiments. Two key concepts, the hidden multi-stability of the ground state and proliferations of optically excited states are discussed. Taking the ionic to neutral phase transition in an organic charge-transfer crystal as example, Keiichiro Nasu documents the fundamental nature of photo-induced structural phase transitions in terms of charge-transfer exciton and neutral domain proliferation. Further, Dr. Nasu provides evidence for the discovery of a new photo-induced phase, "diaphite", located in between graphite and diamond. The mechanism of this photo-induced structural phase transition is discussed in terms of the proliferation of photo-generated inter-layer charge-transfer excitations in the visible regime.
Click here to view the full text: [K. Nasu, Eur. Phys. J. B (2010)]

December 2009 / Colloquium Paper
"Front-end process modeling in silicon" by L. Pelaz et al.
Front-end processing mostly deals with technologies associated to junction formation in semiconductor devices. Ion implantation and thermal anneal models are key to predict active dopant placement and activation.
Lourdes Pelaz and co-workers review the main models involved in process simulation, including ion implantation, evolution of point and extended defects, amorphization and regrowth mechanisms, and dopant-defect interactions. Hierarchical simulation schemes, going from fundamental calculations to simplified models, are emphasized in this Colloquium paper. Although continuum modeling is the mainstream in the semiconductor industry, atomistic techniques are starting to play an important role in process simulation for devices with nanometer size features. The paper illustrates the use of atomistic modeling techniques to gain insight and provide clues for process optimization.
Click here to view the full text: [L. Pelaz et al., Eur. Phys. J. B 72, 323-359 (2009)]

November 2009 / Colloquium Paper
Microscopic modeling of electronic quantum nanodevices reviewed in a Colloquium paper by D. Taj, R.C. Iotti and F. Rossi
Quantum devices represent an important topic of modern nanoscience, characterized by its multi-disciplinary flavor where condensed matter physics, quantum theory, and information technology merge into a unique body of knowledge. In this Colloquium paper Taj and co-workes review and discuss how to work out a microscopic modeling of state-of-the-art electronic quantum devices. The emphasis is on the description of energy-relaxation and decoherence phenomena. Finally, the authors propose an alternative formulation of the problem in terms of a generalized Fermi's Golden Rule is proposed and discussed. Click here to view the full text: [D. Taj et al., Eur. Phys. J. B 72 (2009)]

October 2009 / Colloquium Paper
Electronic properties and quantum transport in Graphene-based nanostructures
Carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) represent a novel class of low-dimensional materials. All these graphene-based nanostructures are expected to display the extraordinary electronic, thermal and mechanical properties of graphene and are thus promising candidates for a wide range of nanoscience and nanotechnology applications. In this paper, the electronic and quantum transport properties of these carbon nanomaterials are reviewed. Although these systems share the similar graphene electronic structure, confinement effects are playing a crucial role. Indeed, the lateral confinement of charge carriers could create an energy gap near the charge neutrality point, depending on the width of the ribbon, the nanotube diameter, the stacking of the carbon layers regarding the different crystallographic orientations involved. After reviewing the transport properties of defect-free systems, doping and topological defects (including edge disorder) are also proposed as tools to taylor the quantum conductance in these materials. Their unusual electronic and transport properties promote these carbon nanomaterials as promising candidates for new building blocks in a future carbon-based nanoelectronics, thus opening alternatives to present silicon-based electronics devices.
[S.M.-M. Dubois et al., Eur. Phys. J. B 72/1 (2009)]

November 2009
Excess of financial tools against market stability
The proliferation of financial instruments – assets, derivatives, securitized loans, etc – provides more means for risk diversification, thus making the market more efficient and closer to the theoretical limit of complete markets, where risk can be eliminated altogether. This conclusion relies on the assumptions of perfect competition and full information. We contrast this picture with a simple model of the market as an interacting system, where prices are affected by trading on the underlying and on derivatives. In this setting, the application of statistical mechanics of disordered systems shows that the proliferation of derivatives brings a market closely resembling the efficient arbitrage-free, complete market described by Asset Pricing Theory. However, the same region of the phase space is also characterized by a phase transition between a supply-limited equilibrium to a demand-limited one.Close to the transition, small perturbations in the risk perception of banks can provoke dramatic changes in the volume of traded derivatives, and large fluctuations are observed in response functions. The uncontrolled proliferation of financial instruments has two main consequences: i) it erodes systemic stability, driving the market to a critical state characterized by large susceptibility, strong fluctuations and enhanced correlations, and ii) it provokes a sharp rise in trading volumes in derivative markets. This suggests that market completeness may not be compatible with a stablemarket dynamics. Therefore, financial stability acquires the properties of a common good, which suggests that appropriate measures should be introduced in derivative markets, to preserve stability.
[F. Caccioli, M. Marsili and P-P. Vivo, "Eroding market stability by proliferation of financial instruments", Eur. Phys. J. B 71, 467 (2009) ]

November 2009
Systemic Risk beyond financial systems
Systemic Risk denotes the risk that a whole system, consisting of many interacting elements, fails. This is not only a problem in financial systems but also in social systems (e.g., spreading of an epidemic disease), in technical systems (e.g., blackout of power grids) or in materials (e.g., rupture of a bundle of fibres). These systems may exhibit a switch to collective failure, which is reminiscent of phase transitions in physical systems. As in many problems in statistical physics, the question is how such a macroscopic state may emerge from local interactions of the subunits, given some specific boundary conditions. The fraction X of failed elements can be regarded as a measure of systemic risk. How can one predict the value of X over time or asymptotically? In a recent paper, the authors were able to solve this question by developing a general framework for models of cascade and contagion processes on networks. They could identify three model classes, which differ in their microscopic interaction mechanism and cover most phenomena of collective breakdown. For each model class, a phase diagram was derived to show the critical conditions where small changes in the initial conditions lead to global failure.
[J. Lorenz, S. Battiston and F. Schweitzer, "Systemic risk in a unifying framework for cascading processes on networks", Eur. Phys. J. B 71, 441 (2009) ]

November 2009
Spin-waves in yttrium-iron garnet
Recently spin-waves in thin films of the ferrimagnetic insulator yttrium-iron garnet (YIG) have been excited via microwave pulses and detected through optical Brillouin light-scattering. Using the fact that the spin-wave (magnon) dispersion in thin films of YIG has aminimumat finitewave-vectors, Demokritov et al. (2006) showed that magnons can condense into a strongly correlated coherent state even at room temperature. The existence of a minimum in the magnon spectrum of YIG is the result of a subtle interplay between finite-size effects of the thin film, the short-range exchange and the long-range dipole-dipole interactions. Although the main features of the magnon spectrum of YIG can be understood within the phenomenological Landau-Lifshitz equation, we have used a microscopic approach based on the usual 1/S-expansion, a successful tool to understand the quantum excitations in ordered magnets. Combining the 1/S-expansion with Ewald summation techniques to perform the dipolar sums, we have calculated the magnon-spectrum of experimentally relevant thin YIG films. Our approach provides a straightforward method to determine the interactions between magnons, which is a starting point for further investigations on the non-equilibrium behaviour of themagnon gas in YIG.
[A. Kreisel, F. Sauli, L. Bartosch and P. Kopietz, "Microscopic spin-wave theory for yttrium-iron garnet films", Eur. Phys. J. B 71, 59 (2009) ]

November 2009
Theory of the optical spectrum of Na2 on 4He droplets: effects of the zero-point energy of the nearest atoms
Collective excitations in confined quantum systems can be probed by the optical spectroscopy of doping chromophores [Szalewicz: Superfluid He nano-droplets:the Franklin Physics Medal to G. Scoles and J.P. Toennies, doi:10.1016/j.jfranklin. 2008.04.008]. The lucky circumstance that alkalimolecules can be trapped at the surfaceof 4He droplets offers a unique tool to explore their surface excitations. In Na2 an optical transition between triplet states [F. Stienkemeier et al, J. Chem. Phys. 102, 615 (1995)] determines a spectacular contraction of the molecule, with a sudden weakening of the molecule-surface interaction and a conversion of the 4He-droplet collective excitations localized around the sodiumdimer intomuch softer, delocalized excitations of the droplet surface, similar to tidalwaves.The authors showthat in thismodel case the spectral function can be derived analytically within the Lax formalism, and that the localized phonon zero-point energy released in this process yields a peculiar asymmetric triangular shape of the surface-phonon sideband of themolecular vibronic lines, with suppression of the zero-phonon line. The good agreement with experiment shows that the structure of the vibronics line carries information on the dynamics of the droplet surface, with a negligible perturbation of the probemolecule. Thus a surface-excitation optical spectroscopy of quantum droplets appears to be feasible.
[V. Hizhnyakov, I. Tehver and G. Benedek, Eur. Phys. J. B 70, 507-512 (2009)]

September 2009 / Colloquium Paper
A practical first-principles band-theory approach to the study of correlated materials
The ever growing applicative importance of correlated materials, such as e.g. magnetic and superconducting oxides, urgently calls for a realistic and affordable description based on first-principles approaches. In this colloquium we review the formulation of pseudo-self-interaction-corrected local-density-functional theory (pSIC), and discuss its relation with comparable methods (LDA+U and hybrid functionals). We extend the approach proposing a practical way to calculate quantum forces, which were previously unavailable in this framework. We then consider a number of recent applications demonstrating the usefulness and accuracy of the method.
[A. Filippetti and V. Fiorentini, Eur. Phys. J. B 71, 139-183 (2009)]

August 2008
Dynamical stability with long-range interactions
Systems with long-range interactions have the remarkable property to organize spontaneously into large-scale coherent structures. These quasi-stationary states (QSS) correspond to galaxies in the universe and vortices in two-dimensional turbulence. In many physical situations, the collisional relaxation time is huge so these QSS are not Boltzmannian equilibria. In fact, they result from a violent collisionless relaxation and they are steady states of the Vlasov equation on some coarse-grained scale.
On general grounds, it is important to study the dynamical stability of steady states of the Vlasov equation. This problem was first considered in plasma physics where the interaction between charges is repulsive. A powerful method to determine the linear stability of a spatially homogeneous distribution function f(v) has been devised by Nyquist (1932). To apply the Nyquist method, we just have to plot the hodograph of the dielectric function. If this curve encircles the origin then the system is unstable, otherwise it is stable. The number of tours gives the number of unstable modes. From this graphical method, one can prove very simply that the Maxwellian distribution is always stable in a plasma.
For attractive interactions, the sign changes and the Nyquist curve is reversed.To illustrate the Nyquist method for attractive interactions, we have considered the Hamiltonian Mean Field (HMF) model. This is a one dimensional model in which the particles interact via a cosine potential.This model shares many analogies with self-gravitating systems but is simpler to study (self-gravitating systems are generally spatially inhomogeneous precluding the application of the Nyquist method). In that case, the Nyquist method shows that a Maxwellian distribution becomes unstable below a critical temperature Tc. We have performed an exhaustive study of more complex distributions leading to rich stability diagrams.
[P.H. Chavanis and L. Delfini, Eur. Phys. J. B 69, 389-429 (2009)]

July 2008
Magnetic ordering in (110) Eu films in a magnetic field
Below its ordering temperature (90K), bulk bcc Eu has a helical magnetic state with propagation vectors along the three equivalent <100> directions. In contrast, epitaxial (110)Eu films exhibit a unique magnetic ordering: the domain with a magnetic helix propagating along the in-plane [001] direction vanishes on cooling (below Td), at the expense of other domains with helices propagating along [100] and [010]. In addition, the two remaining propagation vectors continuously rotate towards the [110] growth direction. Both the temperature Td and the rotation of wave vectors exhibit a pronounced dependence on film thickness, as a consequence of a thickness- and temperature- dependent lattice clamping effect that distorts the Eu lattice at low temperature. The helix propagating along the [001] direction can be restored by the application of an external field along this direction. On the contrary, when a magnetic field is applied along an intermediate direction, [10], the domain with a helix propagating along [001] is suppressed. Both effects depend on film thickness. They are explained if one considers that, because of the low magnetic anisotropy of Eu, a helix with a propagation vector parallel to (or close to) the applied magnetic field is energetically more favorable than cycloidal structures with unchanged propagation vectors. Finally, the amplitudes of the propagation vectors and their directions do not vary under magnetic field.
[S. Soriano et al., Eur. Phys. J. B 63, 469 (2008)]

July 2008
Nanosized superparamagnetic precipitates in cobalt-doped ZnO
The rapidly emerging field of spintronics requires material systems combining ferromagnetism (FM) with the versatile electronic properties of semiconductors. Diluted magnetic semiconductors (DMS) such as Mn-doped InAs or GaAs are very attractive in this regard. Unfortunately, these well established DMS have Curie temperatures TC = 170 K, preventing room temperature (RT) applications. In contrast, TC > 300 K has been predicted for cobalt-doped ZnO and ferromagnetic behavior has been reported. However, there is an ongoing debate on whether this material is really a DMS or the observed behavior is caused by magnetic nanometer-sized precipitates of the Co dopant atoms embedded in a nonmagnetic ZnO matrix. To unambiguously clarify the nature of FM in ZnO:Co thin films we have combined SQUID magnetometry, X-ray magnetic circular dichroism (XMCD), and AC susceptibility measurements with careful X-ray and high resolution TEM studies. We simultaneously recorded XMCD spectra in both the total electron (TEY) and the fluorescence yield (FY) modes, allowing for an element-specific distinction between surface and bulk magnetic properties. Our data provide clear evidence that our ZnO:Co thin films are not homogeneous DMS. Rather the observed RT magnetic behavior is caused by nanometer-sized superparamagnetic Co precipitates, which are directly evidenced by XMCD and energy- filtering transmission electron microscopy (EFTEM). Of course, our data do not prove that the realization of a DMS is impossible for ZnO:Co. However, more effort is required to unambiguously determine the nature of FM, and conclusions based on superficial studies should be considered with care.
[M. Opel et al., Eur. Phys. J. B 63, 437 (2008)]

July 2008 / Colloquium Paper
Phase diagram of the Holstein polaron in one dimension
The behavior of the 1D Holstein polaron is described, with emphasis on lattice coarsening effects, by distinguishing between adiabatic and nonadiabatic contributions to the local correlations and dispersion properties. The original and unifying systematization of the crossovers between the different polaron behaviors, usually considered in the literature, is obtained in terms of quantum to classical, weak coupling to strong coupling, adiabatic to nonadiabatic, itinerant to self-trapped polarons and large to small polarons. It is argued that the relationship between various aspects of polaron states can be specified by five regimes: the weak-coupling regime, the regime of large adiabatic polarons, the regime of small adiabatic polarons, the regime of small nonadiabatic (Lang-Firsov) polarons, and the transitory regime of small pinned polarons for which the adiabatic and nonadiabatic contributions are inextricably mixed in the polaron dispersion properties. The crossovers between these five regimes are positioned in the parameter space of the Holstein Hamiltonian.
[O.S. Barisic and S. Barisic, Eur. Phys. J. B 64, 1-18 (2008)]

April 2008
Slowly rocking symmetric, spatially periodic Hamiltonians: The role of escape and the emergence of giant transient directed transport
Non-integrable dynamics of driven Hamiltonian systems may provide rich diversity of transport phenomena.We illustrate the emergence of a transient giant directed flow of particles evolving in a symmetric, spatially periodic potential. Starting with an ensemble of particles that are trapped in one potential well, escape necessitates chaotic dynamics. The latter is generated by time-periodic alternations of the inclination of the potential by an external ac-field. It has to be emphasized that the system is unbiased in the sense that the force averaged over a period length in time and space respectively vanishes. Trajectories that become embraced by the arising chaotic layer around the broken separatrix may escape from its trapping region. Interestingly, for adiabatic modulations of the potentials inclination there results a substantial directed flow. Otherwise, for intermediate and fast modulations, the chaotic trajectories are swept across the separatrix layer corresponding to repeated trapping-detrapping transitions. Most importantly, as we demonstrate for adiabatic modulations all particles that manage to escape from the trapping region fly subsequently in a unique direction that is determined by the phase of the ac-field. The unidirectional flow proceeds then over an extremely long time interval corresponding to 15 x 10<3> period durations of the ac-field and during this transient the particles cover giant distances. Strikingly, the slower the modulation the larger is the gain in momentum of the escaped particles and thus the emerging asymptotic current that is inversely proportional to the modulation frequency. Explanation of this phenomenon are given in terms of the underlying phase space geometry. In particular trapping of the trajectories in ballistic channels contained in the non-uniform chaotic layer serves for long-lasting ballistic motion.
[D. Hennig, L. Schimansky-Geier and P. Hänggi, Eur. Phys. J. B 62, 493 (2008)]

April 2008 / Colloquium Paper
Quantum thermal transport in nanostructures
In this colloquia review we discuss methods for thermal transport calculations for nanojunctions connected to two semi-infinite leads served as heat-baths. Our emphases are on fundamental quantum theory and atomistic models. We begin with an introduction of the Landauer formula for ballistic thermal transport and give its derivation from scattering wave point of view. Several methods (scattering boundary condition, mode-matching, Piccard and Caroli formulas) of calculating the phonon transmission coefficients are given. The nonequilibrium Green's function (NEGF) method is reviewed and the Caroli formula is derived. We also give iterative methods and an algorithm based on a generalized eigenvalue problem for the calculation of surface Green's functions, which are starting point for an NEGF calculation. A systematic exposition for the NEGF method is presented, starting from the fundamental definitions of the Green's functions, and ending with equations of motion for the contour ordered Green's functions and Feynman diagrammatic expansion. In the later part, we discuss the treatments of nonlinear effects in heat conduction, including a phenomenological expression for the transmission, NEGF for phonon-phonon interactions, molecular dynamics generalized Langevin) with quantum heat-baths, and electron-phonon interactions. Some new results are also shown. We briefly review the experimental status of the thermal transport measurements in nanostructures.
[J.-S. Wang, J. Wang and J.T. Lü, Eur. Phys. J. B 62, 391-404 (2008)]

January 2008
Thermalisation by a boson path in a pure state
Statistical physics textbooks usually start with the equal a priori probability postulate. This postulate states that an isolated system in equilibrium is equally likely to be in any of its accessible states. Temperature is then defined as a statistical concept for this micro-canonical mixed state. Of particular interest are isolated composite systems consisting of a small system S weakly coupled to a large heat bath. In this case, S is found to be described by the familiar thermal equilibrium Boltzmann distribution.
In this article, the following questions are addressed: Does S relax to thermal equilibrium if the composite system is in a pure quantum state? What temperature does the thermometer S show in this case? One considers an arbitrary system S in contact with a bath of harmonic oscillators and assumes that initially S is in any state and that the bath is in a typical pure state of macroscopically well-defined energy E. It is shown that the thermometer S relaxes to thermal equilibrium and indicates the bath micro-canonical temperature at energy E. The fraction of bath states of macroscopic energy E which do not thermalise S vanishes in the limit of a large bath. A key step in this derivation is the proof that, whereas the bath is not at equilibrium, its eigenmodes obey the Boltzmann distribution.
[S. Camalet, Eur. Phys. J. B 61, 193 (2008)]

January 2008 / Colloquium Paper
Heat and fluctuations from order to chaos
The Heat theorem reveals the second law of equilibrium Thermodynamics (i.e. existence of Entropy) as a manifestation of a general property of Hamiltonian Mechanics and of the Ergodic Hypothesis, valid for 1 as well as 1023 degrees of freedom systems, i.e. for simple as well as very complex systems, and reflecting the Hamiltonian nature of the microscopic motion. In Nonequilibrium Thermodynamics theorems of comparable generality do not seem to be available. Yet it is possible to find general, model independent, properties valid even for simple chaotic systems (i.e. the hyperbolic ones), which acquire special interest for large systems: the Chaotic Hypothesis leads to the Fluctuation Theorem which provides general properties of certain very large fluctuations and reflects the time-reversal symmetry. Implications on Fluids and Quantum systems are briefly hinted. The physical meaning of the Chaotic Hypothesis, of SRB distributions and of the Fluctuation Theorem is discussed in the context of their interpretation and relevance in terms of Coarse Grained Partitions of phase space. This review is written taking some care that each section and appendix is readable either independently of the rest or with only few cross references.
[G. Gallavotti, Eur. Phys. J. B 61, 1-24 (2008)]

November 2007
Role Models for Complex Networks
The functional roles played by interactive agents lead to specific patterns in the link structure of their interaction network. Understanding complex multi-agent systems from the social life, or biosciences requires understanding of the complex topology of the underlying network. To identify sets of roleequivalent agents we combine ideas from spin glass physics and social network analysis to develop a framework for automatically decomposing ("block-modeling") the functional classes of agents in a (multi-relational) network. The functional classes and their patterns of connectivity are represented in a resulting image graph, depicting a large network as a small one in a quasi isomorphic way. Our cost function finds the optimal image graph and simultaneously maps agents into functional classes. The method handles directed and undirected two- and onemode data, weighted networks, finds an optimal number of roles, and is computationally efficient and nonparametric. Applied to the world trade network, countries are grouped into classes with similar commodity bundles of trade relations with others. The image graph shows preferred links where the trade volume exceeds the expectation value given countries' total import and export volume.
[J. Reichardt and D.R. White, Eur. Phys. J. B 60, 217 (2007)]

November 2007 / Colloquium Paper
Kinetic exchange models for income and wealth
Increasingly, a huge amount of statistics have been gathered which clearly indicates that income and wealth distributions in various countries or societies follow a robust pattern, close to the Gibbs distribution of energy in an ideal gas in equilibrium. However, it also deviates in the low income and more significantly for the high income ranges. Application of physics models provides illuminating ideas and understanding, complementing the observations.
[A. Chatterjee and B.K. Chakrabarti, Eur. Phys. J. B 60, 135-149 (2007)]

August 2007
Magnetohydrodynamic properties of incompressible Meissner fluids
Superconducting samples lying around in the lab are all solids, and theirmelting temperature far exceeds the critical temperature. Nevertheless, there is no a priori requirement that superconductivity must occur only against a crystalline background. Phonons, the key to the BCS mechanism, are also present in liquid metals, and recent work by Babaev and co-workers surmise that liquid metallic hydrogen might be superconducting. This paper describes how a hypothetical liquid that exhibits the Meissner- Ochsenfeld effect is influenced by an applied magnetic field. Clearly, both the hydrodynamical (flow) and the hydrostatic properties can be altered by the applied field. Droplets of the Meissner fluid deform due to the interplay between surface tension and demagnetization, and the applied magnetic field can be used to tune the droplet shape and oscillation frequency. A change in the frequency also occurs for surface waves (see Figure). A magnetic field applied parallel to the surface and orthogonal to the wave vector typically affects the dispersion relation most strongly for wave lengths of the order of microns to centimeters. To demonstrate the predicted phenomena that mix superconductivity and hydrodynamics, it is proposed to use a colloidal suspension of coated superconducting nanoparticles, such as the samples investigated by Tao and co-workers. It would be hard to achieve perfect conductivity in these suspensions, but they do exhibit the Meissner effect and its hydrodynamic properties will be influenced as described in this work.
[A. Maeyens and J. Tempere, Eur. Phys. J. B 58, 231 (2007)]

October 2005 / Colloquium Paper
Resonant inelastic X-ray scattering in d and f electron
Recent progress in the study of resonant inelastic X-ray scattering (RIXS) spectroscopy in d and f electron systems is described. The main space is devoted to the theoretical investigations, some typical experimental data and the comparison of calculated and experimental results, putting emphasis on the underlying physical mechanisms. We confine ourselves mainly to the studies performed since 2000, and discuss the following topics: (1) RIXS in high Tc cuprates, (2) f0 and d0 systems, (3) other transition metal compounds, (4) RIXS by electric quadrupole excitation, and (5) magnetic circular dichroism in RIXS of ferromagnetic systems. Some brief description is also given on the future prospect of the RIXS study.
[A. Kotani, Eur. Phys. J. B 47, 3-27 (2005)]