Table of contents

The effect of a single inactive site is studied on spatiotemporal dynamics in a ring of locally coupled Stuart-Landau oscillators. An inactive site is where a damped oscillator is placed instead of an active (i.e. spontaneous) one. It is numerically shown that such a site suppresses spatiotemporal chaos (STC) in some region of a parameter plane. How STC recovers as a parameter is varied is elucidated by introducing some order parameters quantifying the degree of phase-locking and frequency synchronization between each active oscillator and the inactive one.

By means of multicanonical computer simulations, we investigate thermodynamic properties of the aggregation of interacting semiflexible polymers. We analyze a mesoscopic bead-stick model, where nonbonded monomers interact via Lennard-Jones forces. Aggregation turns out to be a process, in which the constituents experience strong structural fluctuations, similar to peptides in coupled folding-binding cluster formation processes. In contrast to a recently studied related proteinlike hydrophobic-polar heteropolymer model, aggregation and crystallization are separate processes for a homopolymer with the same small bending rigidity. Rather stiff semiflexible polymers form a liquid-crystal–like phase, as expected. In analogy to the heteropolymer study, we find that the first-order–like aggregation transition of the complexes is accompanied by strong system-size–dependent hierarchical surface effects. In consequence, the polymer aggregation is a phase-separation process with entropy reduction.

We investigate optimal control of a single qubit coupled to an ohmic heat bath. For the weak bath coupling regime, we derive a Bloch-Redfield master equation describing the evolution of the qubit state parameterized by vectors in the Bloch sphere. By use of the optimal control methodology, we determine a field that generates a single-qubit rotation. We use techniques of automatic differentiation to compute the gradient for the cost functional.

By the algebraic method we study the approximate solution to the Dirac equation with scalar and vector modified Pöschl-Teller (MPT) potentials carrying pseudospin symmetry. The transcendental energy equation and spinor wave functions with arbitrary spin-orbit coupling quantum number k are presented. It is found that there exist only negative-energy states for bound states under pseudospin symmetry, and the energy levels will approach a constant when the potential parameter α goes to zero. There also exist the corresponding degenerate states between (n+1, k- 2) and (n, k) in the case of pseudospin symmetry.

We investigate localization and mass spectrum of spin-1/2 fermionic field on a scalar thick brane configuration with Poincaré symmetry, which arises in a 5-dimensional theory of gravity coupling to a self-interacting scalar field in a regular Riemannian manifold. It is shown that, in the case of no Yukawa coupling of scalar-fermion, there is no existence of localized massless zero mode for both left and right chiral fermions. By introducing appropriate coupling, we can obtain the localized modes as well as a continuous spectrum of massive Kaluza-Klein (KK) modes for left and right chiral fermions. It is found that there exists a mass gap between the massless zero mode and the excited modes for left chiral fermions or right chiral fermions. The total number of bound states is determined by the Yukawa coupling constant and the parameter of the thick brane solution. Depending on the sign of the coupling constant, only one category of zero modes, namely, either the left or right chiral zero mode can be trapped on the brane.

We investigate the induced geodesic deviation equations in the brane world models, in which all the matter forces except gravity are confined on the 3-brane. Also, the Newtonian limit of induced geodesic deviation equation is studied. We show that in the first Randall-Sundrum model the Bohr–Sommerfeld quantization rule is as a result of consistency between the geodesic and geodesic deviation equations. This indicates that the path of test particle is made up of integral multiples of a fundamental Compton-type unit of length h/mc.

The dendrite growth velocity during solidification is measured on liquid drops of the intermetallic compound Ni₅₀Al₅₀ undercooled by levitation up to 265 K. A sharp increase of the growth velocity is found at a critical undercooling ΔT^*≈250 K. In situ diffraction of synchrotron radiation on levitation-processed samples unambiguously shows a transition from ordered to disordered growth at ΔT^*. The sharp interface model is extended to describe the transition from ordered to disordered dendrite growth by taking into account the velocity dependence of the order parameter and the kinetic growth coefficient.

In the present work, we investigate the quantum thermal entanglement in molecular magnets composed of dimers of spin S, using an Entanglement Witness built from measurements of magnetic susceptibility. An entanglement temperature, T_e, is then obtained for some values of spin S. From this, it is shown that T_e is proportional to the intradimer exchange interaction J and that entanglement appears only for antiferromagnetic coupling. The results are compared to experiments carried on three isostructural materials: KNaMSi₄O₁₀ (M=Mn, Fe or Cu).

In this letter we propose discrete wave turbulence (DWT) as a counterpart of classical statistical wave turbulence (SWT). DWT is characterized by resonance clustering, not by the size of clusters, i.e. it includes, but is not reduced to, the study of low-dimensional systems. Clusters with integrable and chaotic dynamics co-exist in different sub-spaces of the k-space. NR-diagrams are introduced, a graphical representation of an arbitrary resonance cluster allowing to reconstruct uniquely dynamical system describing the cluster. DWT is shown to be a novel research field in nonlinear science, with its own methods, achievements and application areas.

Recently Parigi et al. (Science, 317 (2007) 1890) measured the statistics of a photon-subtracted and a photon-added thermal field. They showed that the measurements agree with the theoretical predictions of the beam splitter (BS) model. We show the equivalence of the quantum trajectory approach and the BS model in photon subtraction by deriving the BS model from the generalized photon counting operators. Our photon counting operators, corresponding to measurements using a resolving detector and a nonresolving detector, are generalization of the standard quantum jump approach. Our model is exact from weak quantum fields to the classical limit of strong fields. We show that our generalized photon counting operators reproduce exactly the results of the BS model when the reflection probability of the beam splitter is equated with the absorption probability of a photon in the standard quantum jump model of cavity field damping. Our theory can explain the recent experimental results of Parigi et al. and shows how similar experiments can be made to test the photon subtraction for the whole range of electromagnetic field intensities ranging from quantum to classical limit. We propose new experiments to test the generalized photon counting operators in the intermediate regime between the quantum and classical limits.

We have examined the excitation on coherent states of the pseudoharmonic oscillator which are obtained by repeated action of the raising operator on the usual coherent states. By using the density matrix approach, we have examined some interesting properties (including the nonclassicality) of these states, both in pure and also in mixed (thermal) cases.

The thermal discrete element method (TDEM), a coupling of the discrete element model (DEM) with the inter-particle heat transfer models, has been applied to the simulation of the particulate systems with heat transfer. Additionally, small normal spring stiffness coefficient has often been adopted instead of the real value in the softening treatment to improve the calculation efficiency. However, present research has indicated that the heat transfer thus simulated is exaggerated even though such softening treatment has almost no effect on the movement of particles. In this letter, we propose a restoration method to restore the inter-particle contact time and contact area in the real heat transfer process even when particle stiffness is softened by several orders. This restoration method keeps the merit of the softening treatment in the DEM simulation of the particulate systems with heat transfer and avoids the unreasonable heat transfer calculations using the softening treatment method.

The transition between the class-B and class-A dynamical behaviors of a semiconductor laser is directly observed by continuously controlling the lifetime of the photons in a cavity of sub-millimetric to centimetric length. It is experimentally and theoretically proved that the transition from a resonant to an overdamped behavior occurs progressively, without any discontinuity. In particular, the intermediate regime is found to exhibit features typical from both the class-A and class-B regimes. The laser intensity noise is proved to be a powerful probe of the laser dynamical behavior.

In 1997, a new turbulent regime has been observed in a Rayleigh-Bénard cell and has been interpreted as the "Ultimate Regime" of convection. This observation was based on global heat transfer measurements at very high Rayleigh numbers (Ra). Using a set-up similar to the one used in 1997, we examine the signature of this regime from within the flow itself. A systematic study of probe-size corrections shows that the earlier local temperature measurements within the flow were altered by an excessive size of thermometer, but not according to a theoretical model proposed in the literature. Using a probe one order of magnitude smaller than the one used previously, we find evidence that the transition to the very-high-Ra regime is indeed accompanied with a clear change in the statistics of temperature fluctuations in the flow.

The drag fluctuations of a disk placed on the axis of a turbulent incompressible jet are studied at Re=70000. Statistics and spectra have been obtained for different disk sizes. A significant spatial averaging effect is observed in the symmetrization of probability distribution function and in the low-pass filtering of spectra. These effects are associated with a redistribution of the high-frequencies energy to the low frequencies. It is shown that this redistribution is done in such a way that the rms value of the drag fluctuations increases linearly with the disk surface. These results concerning the drag fluctuations are compared and found to be consistent with fluctuations of a global kinetic energy extracted from the turbulent field in front of the disk.

The structural and magnetic properties of hydrogen-doped Ge_1−xMn_x diluted magnetic semiconductors are investigated using a first-principles pseudopotential method. Our results show that hydrogen impurities intend to bond to Mn ions in the Mn-doped Ge system and strongly influence the magnetic properties of this system. Hydrogen impurities actually reduce the Curie temperature of this system and might be one of the reasons for the existence of paramagnetic samples of Ge_1−xMn_x. Alternatively, hydrogenation can provide an easy and nonvolatile way to control and pattern the ferromagnetic properties of Mn-doped Ge diluted magnetic semiconductors, which has been achieved in the Mn-doped GaAs system (Goennenwein S. T. B. et al., Phys. Rev. Lett., 92 (2004) 227202).

We investigate a scattering of electron which is injected individually into an empty ballistic channel containing a cavity that is Coulomb coupled to a quantum ring charged with a single electron. We solve the time-dependent Schrödinger equation for the electron pair with an exact account for the electron-electron correlation. Absorption of energy and angular momentum by the quantum ring is not an even function of the external magnetic field. As a consequence we find that the electron backscattering probability is asymmetric in the magnetic field and thus violates Onsager symmetry.

Very-high-frequency surface acoustic waves, generated and transmitted along single-crystal lithium niobate, are used to drive homogeneous aqueous suspensions of polystyrene nanoparticles along microchannels. At a few hundred milliwatts, uniform and mixing flows with speeds of up to 10 mm/s were obtained in centimetres-long rectangular channels with cross-sectional dimensions of tens to a few hundreds of microns. A transition from uniform to mixing flow occurs as the channel width grows beyond the wavelength of sound in the fluid at the chosen excitation frequency. At far lower input powers, the suspension agglomerates into equally spaced, serpentine lines coincident with nodal lines in the acoustic pressure field. We expose the physics underlying these disparate phenomena with experimental results aided by numerical models.

We propose to use the quantum interference effect in a Rashba ring to detect the pure spin current driven by a spin bias. By means of the Keldysh Green's function method, we demonstrated that a tunneling charge current could be induced flowing through the ring due to the combined quantum interference effect of the electron spin precession phase from Rashba spin-orbit coupling (RSOC) and the electron traveling phase difference in the two arms of the ring. The nonzero charge current can be used to deduce not only the magnitude of the spin bias but also its spin polarization. This charge current depends on some system parameters such as RSOC strength and the asymmetry of two arms. Our proposal may provide a practical and all-electrical way to indirectly detect the pure spin current (spin bias) by measuring the induced charge current/bias in a Rashba ring.

A graphene pn junction is studied theoretically in the presence of both intrinsic and Rashba spin-orbit couplings. We show that a crossover from perfect reflection to perfect transmission is achieved at normal incidence by tuning the perpendicular electric field. By further studying angular-dependent transmission, we demonstrate that perfect reflection at normal incidence can be clearly distinguished from trivial band gap effects. We also investigate how spin-orbit effects modify the conductance and the Fano factor associated with a potential step in both nn and np cases.

We report on the anomalous bias dependence of tunneling magnetoresistance (TMR) in (CrO₂/SnO₂/Co)-based magnetic tunnel junctions as a function of barrier thickness. For a relatively thin SnO₂ barrier, the TMR is asymmetric and exhibits sign reversal at a specific bias voltage with varying thickness due to defect mediated resonant tunneling. On the other hand, diffusive transport dominates for sufficiently thick SnO₂ barriers, and a diverging TMR is observed close to zero bias.

We report on photoluminescence (PL) measurements at 85 K for GaN/Al_0.2Ga_0.8N surface quantum wells (SQWs) with a width in the range of 1.51–2.9 nm. The PL spectra show a redshift with decreasing SQW width, in contrast to the blueshift normally observed for conventional GaN QWs of the same width. The effect is attributed to a strong coupling of SQW confined exciton states with surface acceptors. The PL hence originates from the recombination of surface-acceptor-bound (A⁰_sX_A) excitons. Two types of acceptors were identified.

Zero-modes, their topological degeneracy and relation to index theorems have attracted attention in the study of single-layer and bilayer graphene. For negligible scalar potentials, index theorems can explain why the degeneracy of the zero-energy Landau level of a Dirac Hamiltonian is not lifted by gauge field disorder, for example due to ripples, whereas other Landau levels become broadened by the inhomogenous effective magnetic field. That also the bilayer Hamiltonian supports such protected bulk zero-modes was proved formally by Katsnelson and Prokhorova to hold on a compact manifold by using the Atiyah-Singer index theorem. Here we complement and generalize this result in a pedestrian way by pointing out that the simple argument by Aharonov and Casher for degenerate zero-modes of a Dirac Hamiltonian in the infinite plane extends naturally to the multilayer case. The degeneracy remains, though at non-zero energy, also in the presence of a gap. These threshold modes make the spectrum asymmetric. The rest of the spectrum, however, remains symmetric even in arbitrary gauge fields, a fact related to supersymmetry. Possible benefits of this connection are discussed.

We introduce a model for nonlinear viscoelastic solids, for which travelling shear waves with compact support are possible. Using analytical and numerical methods, we investigate the general case of this model, and an exact, kink-type travelling-wave solution is obtained as a special case result. Additionally, we derive and examine a new Burgers' type evolution equation based on the introduced constitutive equations.

It has been proved that the spanning tree from a given network has optimal synchronizability, which means the index R=λ_N/λ₂ reaches the minimum 1. Although the optimal synchronizability is corresponding to the minimal critical overall coupling strength to reach synchronization, it does not guarantee a shorter converging time from disorder initial configuration to synchronized state. In this letter, we find that the depth of the tree is the only factor that affects the converging time. The relation between the depth and the converging time is given as well. In addition, we present a simple and universal way to get such an effective oriented tree from a given network to reduce the converging time significantly by minimizing the depth of the tree. The shortest spanning tree has both maximal synchronizability and minimal converging time.

Aggregation of noisy observations involves a difficult tradeoff between observation quality, which can be increased by increasing the number of observations, and aggregation quality which decreases if the number of observations is too large. We clarify this behavior for a prototypical system in which arbitrarily large numbers of observations exceeding the system capacity can be aggregated using lossy data compression. We show the existence of a scaling relation between the collective error and the system capacity, and show that large-scale lossy aggregation can outperform lossless aggregation above a critical level of observation noise. Further, we show that universal results for scaling and critical value of noise can be obtained when the system capacity increases toward infinity.

The influence of a network configuration on its vulnerability to cascade spreading is investigated. In our model, a failure cascade is initiated by sequential disturbances that impose additional loads on each node that lead to local failure. The resulting redistribution of these additional loads on neighboring nodes in turn leads to a cascade of local failures along the network connections. The extent of these failures on the network depends on the reachability of its constituent parts and on its specific link-node configuration, properties that are quantitatively characterized by a global connection efficiency and a coefficient of variability, respectively. The results show that lower values of the global efficiency make the network more resilient to cascading failures by increasing the critical failure load. Yet, once a critical load is exceeded, the transition to complete failure occurs more rapidly. The converse is true when this efficiency is lower. The analysis provides parameters that are relevant to the design of networks.

The promotion of cooperation on spatial lattices is an important issue in evolutionary game theory. This effect clearly depends on the update rule: it diminishes with stochastic imitative rules whereas it increases with unconditional imitation. To study the transition between both regimes, we propose a new evolutionary rule, which stochastically combines unconditional imitation with another imitative rule. We find that, surprisingly, in many social dilemmas this rule yields higher cooperative levels than any of the two original ones. This nontrivial effect occurs because the basic rules induce a separation of timescales in the microscopic processes at cluster interfaces. The result is robust in the space of 2×2 symmetric games, on regular lattices and on scale-free networks.

Using fluorescence microscopy, we directly visualize the condensed structures of individual semi-flexible actin filaments in a poor solvent. The condensation of filaments into either ring-like or racquet-like structures is driven by non-adsorbing polymers which induce attractive interactions between filaments via the well-known depletion mechanism. A quantitative analysis of the racquet structures yields a direct measurement of the adhesion strength between a pair of filaments. We also compare our experimental data with a theoretical model, demonstrating that in the limit of weak binding, thermal fluctuations can renormalize the effective strength of the attractive depletion interactions. Our experimental methods can be applied to other filamentous structures to directly measure their attractive intermolecular potentials.

We propose a method to reconstruct and analyze a complex network from data generated by a spatio-temporal dynamical system, relying on the nonlinear mutual information of time series analysis and betweenness centrality of the complex network theory. We show that this approach reveals a rich internal structure in complex climate networks constructed from reanalysis and model surface air temperature data. Our novel method uncovers peculiar wave-like structures of high-energy flow, that we relate to global surface ocean currents. This points to a major role of the oceanic surface circulation in coupling and stabilizing the global temperature field in the long-term mean (140 years for the model run and 60 years for reanalysis data). We find that these results cannot be obtained using classical linear methods of multivariate data analysis, and have ensured their robustness by intensive significance testing.

In a recent paper (Abe S. and Suzuki N., Europhys. Lett., 65 (2004) 581), the concept of earthquake network has been introduced in order to describe the complexity of seismicity. There, the cell size, which is the scale of coarse graining needed for constructing an earthquake network, has remained as a free parameter. Here, a method is presented for determining it based on the scaling behavior of the network. Quite remarkably, both the exponent of the power law connectivity distribution and the clustering coefficient are found to approach the respective universal values and remain invariant as the cell size becomes larger than a certain value, l_*, which depends on the number of events contained in the analysis, in general. This l_* fixes the scale of coarse graining. Universality of the result is demonstrated for all of the networks constructed from the data independently taken from California, Japan and Iran.

Standard DNA melting curves record the separation of the two strands vs. temperature, but they do not provide any information on the location of the opening. We introduce an experimental method which adds a new dimension to the melting curves of short DNA sequences by allowing us to record the degree of opening in several positions along the molecule all at once. This adds the spatial dimension to the melting curves and allows a precise investigation of the role of the base pair sequence on the fluctuations and denaturation of the DNA double helix. We illustrate the power of the method by investigating the influence of an AT-rich region on the fluctuations of neighboring domains.

We analyze complex networks under the random matrix theory framework. Particularly, we show that Δ₃ statistics, which gives information about the long-range correlations among eigenvalues, provides a measure of randomness in networks. As networks deviate from the regular structure, Δ₃ follows the random matrix prediction of logarithmic behavior (i.e., ) for longer scale.

We determine and relate the characteristic velocity, length, and time scales for bacterial motion in swarming colonies of Paenibacillus dendritiformis growing on semi-solid agar substrates. The bacteria swim within a thin fluid layer, and they form long-lived jets and vortices. These coherent structures lead to anisotropy in velocity spatial correlations and to a two-step relaxation in velocity temporal correlations. The mean squared displacement of passive tracers exhibits a short-time regime with nearly ballistic transport and a diffusive long-time regime. We find that various definitions of the correlation length all lead to length scales that are, surprisingly, essentially independent of the mean bacterial speed, while the correlation time is linearly proportional to the ratio of the correlation length to the mean speed.