Vol. 46 No.5-6 - Highlights

Revealing the microscopic origin of φ0 Josephson junctions (Vol. 46 No. 5-6)

The dependence of φ0 on the length L of the N bridge (see inset) with an intrinsic SOC and spin-splitting field

A spontaneous dissipationless current (supercurrent) can flow in a superconducting ring even in the absence of a magnetic flux, if the ring is interrupted by a so-called φ0 junction. In the present work the authors present a full microscopic theory that explains the appearance of the anomalous φ0 phase in junctions with an intrinsic spin-orbit coupling (SOC) and a spin-splitting field like the exchange field in ferromagnets. The SOC generates the spin precession of moving particles, and, in addition, it causes a spin-dependent deflection of electron trajectories. The latter can be interpreted in terms of an effective spin- dependent SU(2) magnetic field that in normal systems is the origin of the intrinsic spin Hall effect and the existence of spin currents in the equilibrium state. A finite φ0 in a Josepson junction is directly related to the appearance of an equilibrium spin current with a spin projection parallel to the exchange field. These findings are the first steps towards spin-orbitronics with superconductors by making a natural connection between charge-spin conversion in dissipative and superconducting structures.

F. S. Bergeret and I. V. Tokatly, Theory of diffusive φ0 Josephson junctions in the presence of spin-orbit coupling, EPL, 110, 57005 (2015)

Identifying ever-growing disturbances leading to freak waves (Vol. 46 No. 5-6)

A schematic one-dimensional illustration of the spatiotemporal evolution of the envelope of wave-train in the absolutely unstable case.

Physicists now better understand wave systems exhibiting unusual disturbances by identifying growing localised patterns as early indicators of such disturbances.

Physicists like to study unusual kinds of waves, like freak waves found in the sea. Such wave movements can be studied using models designed to describe the dynamics of disturbances. The authors have focused on finding ways of best explaining how wave disturbance occurs under very specific initial conditions that are key to the genesis of these disturbances. They looked for solutions to this puzzle by resolving a type of equation, called the nonlinear Schrödinger equation. It is solved by applying a method designed for studying instabilities tailored to these initial conditions. Their approach makes it possible to locate exactly where and how pertinent information used to identify disturbance patterns can be extracted from localised disturbances' characteristics. The findings have been published recently. They therefore contribute to a better understanding of the complex dynamics of systems subjected to such disturbances. For example, they could be used to better understand waves appearing on fluid surfaces, whose evolution is influenced by gravity, or light waves propagated in optical fibres.

S. Coulibaly, E. Louvergneaux, M. Taki and L. Brevdo, Spatiotemporal wave-train instabilities in nonlinear Schrödinger equation: revisited, Eur. Phys. J. D 69, 186 (2015)

Activity–driven fluctuations in living cells (Vol. 46 No. 5-6)

Mean square displacement of tracers in living cells

Living cells operate far from equilibrium due to the permanent injection of energy provided by ATP supply. The dynamics of the intracellular components is driven by both thermal equilibrium fluctuations, and active stochastic forces generated by the molecular motors.

To sort out genuine nonequilibrium fluctuations from purely thermal effects, we inject tracer particles in ATP depleted cells. By testing the fluctuation-dissipation theorem (FDT), we identify these cells as an equilibrium-like reference in which the tracers remain locally confined by the elastic cytoskeletal network that permeates the cytoplasm. In contrast, we highlight a violation of the FDT and a diffusion-like motion at long time scales in untreated and selective motor inhibited cells. Removing the thermal contribution in the tracer fluctuations, we estimate the spectrum of the active forces. Eventually, we report non-Gaussian tails in the tracer displacement distribution as a result of directed motion events.

We recapitulate theoretically the observed fluctuations by modeling the dynamics with a con- fining harmonic potential which experiences random bursts as a result of motor activity. This minimal model allows us to quantify the time scales of the active forces, along with the energy injected by the ensuing fluctuations.

E. Fodor, M. Guo, N. S. Gov, P. Visco, D. A. Weitz and F. van Wijland, Activity-driven fluctuations in living cells, EPL, 110, 48005 (2015)

Shaping the hilly landscapes of a semi-conductor nanoworld (Vol. 46 No. 5-6)

Redeposition on hexagonally arranged dots

A new study reveals how hexagonal-patterned, self-organised hill structures emerge in 2D at the nanoscale due to redeposition following semi-conductor bombardment with low-energy ions.

Nanoscale worlds sometimes resemble macroscale roller-coaster style hills, placed at the tip of a series of hexagons. Surprisingly, these nanohills stem from the self-organisation of particles – the very particles that have been eroded and subsequently redeposited following the bombardment of semi-conductors with ion beams. Now, a new theoretical study constitutes the first exhaustive investigation of the redeposition effect on the evolution of the roughening and smoothing of two-dimensional surfaces bombarded by multiple ions. The results demonstrate that the redeposition can indeed act as stabilising factor during the creation of the hexagonally arranged dot patterns observed in experiments. These findings have been published recently.

C. Diddens and S. J. Linz, Continuum modeling of particle redeposition during ion-beam erosion, Eur. Phys. J. B, 88, 190 (2015)

Single-photon observables and preparation uncertainty relations (Vol. 46 No. 5-6)

Increase of uncertainty relations for circularly polarized Gaussian states as a function of the momentum spread. In the paraxial limit (ΔP→0) ħ/2 is retrieved

The escalating requests for highly accurate manipulation of single photons call for an appropriate description of their observables. The authors provide a unified procedure for treating all single-photon observables in terms of Positive Operator-Valued Measures (POVMs), allowing for the evaluation of corresponding probability distributions.

The suppression of longitudinal (or equivalently 0-helicity) photon states is identified as a projection from an extended Hilbert space onto the physical one, carrying an irreducible spin-1 mass-0 representation of the Poincaré group.

POVMs are naturally obtained by applying such projections to Projection-Valued Measures (PVMs) associated to operators well-defined on the extended Hilbert space. Such operators are inherited from the familiar relativistic description of spin-1 massive particles and simply adapted to photons. Results show that PVMs of momentum and helicity remain unaltered, while those of position and spin are turned into POVMs by the projection, reflecting their intrinsic unsharpness. Finally, evaluation of uncertainty relations for position and momentum and probability distribution of spin over a broad class of physically relevant states is done, leading to new quantitative and experimentally measurable results.

G. Guarnieri, M. Motta and L. Lanz, Single-photon observables and preparation uncertainty relations, J. Phys. A: Math.Theor., 48, 265302 (2015)

Law governing anomalous heat conduction revealed (Vol. 46 No. 5-6)

Heat conductance as the function of temperature T for different lattice size N = 50; 100; 200; 400; 800 and 1600

Study finds the law governing how heat transport scales up with temperature.

How heat travels, matters. Yet, there is still no consensus on the exact physical mechanism that causes anomalous heat conduction—despite the existence of previous numerical simulation, theoretical predictions and experimental observations. Now, the authors have demonstrated that electron transport depends on temperature. It follows a scaling governed by a power law—and not the exponential scaling previously envisaged. These findings were published recently. Heat conduction depends on the internal energy transferred by microscopic diffusion and collisions of particles, such as electrons, within a given body. Anomalous heat conduction can be best studied in a particular kind of model: one that accounts for the thermal transport in a one-dimensional (1D) lattice. In this study, the chosen 1D model is dubbed the coupled rotator lattice model. The authors systematically investigated how heat conductivity changes with temperature. This approach led them to the thesis that heat conductivity correlates with a power law, instead of an exponential scaling as previously predicted. Further, this phenomenon occurs without a transition temperature above which the heat conduction is normal and below which it is anomalous.

Y. Li, N. Li and B. Li, Temperature dependence of thermal conductivities of coupled rotator lattice and the momentum diffusion in standard map, Eur. Phys. J. B, 88, 182 (2015)

Improving insulation materials, down to wetting crossed fibres (Vol. 46 No. 5-6)

Evolution of the morphology of a drop of silicone oil on two touching crossed fibres.

Scientists model the manner in which a liquid wets fibres, gaining useful insights for improving glass wool properties

Sandcastles are a prime example of how adding a small amount of liquid to a granular material changes its characteristics. But understanding the effect of a liquid wetting randomly oriented fibres in a fibrous medium remains a mystery. Relevant to the building industry, which uses glass wool, for instance, this phenomenon can be better understood by studying the behaviour of a liquid trapped between two parallel fibres. It can either remain in the shape of a drop or spread between the fibres into a long and thin column of liquid. Now, the authors have demonstrated that the spreading of the liquid is controlled by three key parameters: the amount of liquid on the fibres, the fibres’ orientation and the minimum distance between them. These findings, based on experimental and modelling work, were published recently.

A. Sauret, F. Boulogne, B. Soh, E. Dressaire and H. A. Stone, Wetting morphologies on randomly oriented fibers, Eur. Phys. J. E 38, 62 (2015)

Vibrationally assisted quantum engines – a new scheme for directed coherent transport (Vol. 46 No. 5-6)

Schematic of the vibrationally rocked excitonic quantum ratchet

Energy transport at the nano/quantum scale has a long history of research, with significant interest being paid in the debate over whether quantum coherence plays a role in the efficiency of exciton transport in photosynthetic complexes. Much attention has also turned to improving energy transport for man-made energy harvesting systems and nanodevices, such as in solar cells and quantum dot arrays.

Achieving directed quantum transport permits far superior collection of the deposited energy. The study of quantum ratchets shows how directed energy transport is achievable in quantum dot arrays. Recent experimental work on light harvesting molecules have implicated the role of discrete mechanical modes in enhancing the energy transport through dipole arrays, but say less about directed transport. Here the authors bring together these two apparently unrelated models to present a scheme for a new type of quantum engine. Utilising both excitonic and vibrational motions it is shown that the resulting coherent mechanical dynamics causes directed enhanced energy transport towards one end of the exciton chain. The quantum engine is autonomous, requiring no external pumping or modulation but works off the initial charge on the exciton chain which excites the vibrational motion.

C. R. Myers, G. J. Milburn and J. Twamley, Vibrationally assisted quantum energy pumps, New J. Phys., 17, 093030 (2015)

Gold-diamond nanodevice for hyperlocalised cancer therapy (Vol. 46 No. 5-6)

Colocalisation studies with confocal fluorescence microscopy and acidotropic probes show particles trapped in the lysosomes of the living HeLa cells

Gold nanorods can be used as remote controlled nanoheaters delivering the right amount of thermal treatment to cancer cells, thanks to diamond nanocrystals used as temperature sensors.

Precise targeting biological molecules, such as cancer cells, for treatment is a challenge, due to their sheer size. Now, the authors have proposed an advanced solution, based on a novel combination of previously used techniques, which can potentially be applied to thermal cancer therapy. The authors presented in this work an improved sensing technique for nanometre-scale heating and temperature sensing. Using a chemical method to attach gold nanorods to the surface of a diamond nanocrystal, they have invented a new biocompatible nanodevice. It is capable of delivering extremely localised heating from a near-infrared laser aimed at the gold nanorods, while accurately sensing temperature with the nanocrystals.The novelty of this study is that it shows that it is possible to use diamond nanocrystals as hypersensitive temperature sensors with a high spatial resolution—ranging from 10 to 100 nanometres—to monitor the amount of heat delivered to cancer cells.

.-Ch. Tsai, O. Y. Chen, Y.-K. Tzeng, Y. Y. Hui, J. Y. Guo, Ch.-Ch. Wu, M.-Sh. Chang and H.-Ch. Chang, Gold/diamond nanohybrids for quantum sensing applications, EPJ Quantum Technology, 2, 19 (2015)

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)