Table of contents

We introduce a model to study the effect of degree-frequency correlations on synchronization in networks of coupled oscillators. Analyzing this model, we find several remarkable characteristics. We find a stationary synchronized state that is i) universal, i.e., the degree of synchrony, as measured by a global order parameter, is independent of network topology, and ii) fully phase-locked, i.e., all oscillators become simultaneously phase-locked despite having different natural frequencies. This state separates qualitatively different behaviors for two other classes of correlations where, respectively, slow and fast oscillators can remain unsynchronized. We close by presenting an analysis of the dynamics under arbitrary degree-frequency correlations.

We propose a method based on the Wang-Landau algorithm to numerically generate the spectral densities of random matrix ensembles. The method employs Dyson's log-gas formalism for random matrix eigenvalues and also enables one to simultaneously investigate the thermodynamic properties. This approach is a powerful alternative to the conventionally used Monte Carlo simulations based on the Boltzmann sampling, and is ideally suited for investigating β-ensembles.

We generalize Huberman-Rudnick universal scaling law for all periodic windows of the logistic map and show the robustness of q-Gaussian probability distributions in the vicinity of chaos threshold. Our scaling relation is universal for the self-similar windows of the map which exhibit period-doubling subharmonic bifurcations. Using this generalized scaling argument, for all periodic windows, as chaos threshold is approached, a developing convergence to q-Gaussian is numerically obtained both in the central regions and tails of the probability distributions of sums of iterates.

It is shown that the energy in the centre-of-mass frame of colliding particles in free fall at any point of the ergosphere of the rotating black hole can grow without limit for fixed energy values on infinity. The effect takes place for large negative values of the angular momentum of one of the particles.

In a pendular Fabry-Perot interferometer the system placed inside one of the minimum of the optomechanical potential undergoes an escape if it crosses the point of sudden change of reflectivity near the top of the potential well. We demonstrate that the loss of information that occurs retaining only the sequence of escapes, rather than the full trajectory, is mild if suitable signal processing techniques are applied to reveal the noise intensity or the presence of a coherent signal.

Using exact calculations, we elucidate the significance of surface states for the Casimir interactions in an Ising strip with a finite width. The surface states are responsible for the strong asymmetry between the super- and sub-critical regimes of the Casimir forces. We introduce an enhanced version of Fisher-Privman theory and justify it using another exact calculation. This gives a rather accurate account of the Casimir scaling function in the sub-critical regime and its exact asymptotic behaviour. We apply analogous ideas in three dimensions and obtain the Casimir scaling function which is in striking agreement with Monte Carlo simulations results for large negative scaling variable.

Recurrence-plot–based time series analysis is widely used to study changes and transitions in the dynamics of a system or temporal deviations from its overall dynamical regime. However, most studies do not discuss the significance of the detected variations in the recurrence quantification measures. In this letter we propose a novel method to add a confidence measure to the recurrence quantification analysis. We show how this approach can be used to study significant changes in dynamical systems due to a change in control parameters, chaos-order as well as chaos-chaos transitions. Finally we study and discuss climate transitions by analysing a marine proxy record for past sea surface temperature.

This paper is dedicated to the 25th anniversary of the introduction of recurrence plots.

Recommender systems recommend objects regardless of potential adverse effects of their overcrowding. We address this shortcoming by introducing crowd-avoiding recommendation where each object can be shared by only a limited number of users or where object utility diminishes with the number of users sharing it. We use real data to show that contrary to expectations, the introduction of these constraints enhances recommendation accuracy and diversity even in systems where overcrowding is not detrimental. The observed accuracy improvements are explained in terms of removing potential bias of the recommendation method. We finally propose a way to model artificial socio-economic systems with crowd avoidance and obtain first analytical results.

The heteroclinic cycles and channels are mathematical images of a sequential switching activity in neural ensembles. In this paper we present a phenomenological model of such activity. The model is based on coupled Poincaré systems. The existence of heteroclinic cycles and channels is shown.

Active Brownian particles (ABPs, such as self-phoretic colloids) swim at fixed speed v along a body-axis u that rotates by slow angular diffusion. Run-and-tumble particles (RTPs, such as motile bacteria) swim with constant u until a random tumble event suddenly decorrelates the orientation. We show that when the motility parameters depend on density ρ but not on u, the coarse-grained fluctuating hydrodynamics of interacting ABPs and RTPs can be mapped onto each other and are thus strictly equivalent. In both cases, a steeply enough decreasing v(ρ) causes phase separation in dimensions d = 2,3, even when no attractive forces act between the particles. This points to a generic role for motility-induced phase separation in active matter. However, we show that the ABP/RTP equivalence does not automatically extend to the more general case of u-dependent motilities.

Detrended cross-correlation analysis provides a scaling exponent that should characterize the power-law cross-correlation of two simultaneously recorded series. This exponent by itself is not able to guarantee the presence of cross-correlation, being strongly influenced by the auto-correlation properties of the single series. Through the use of σDCCA coefficients and simulation with ARFIMA models we built families of curves that can be used as templates to correctly detect the power-law behaviour and quantify the degree of coupling between series with any degree of auto-correlation.

We have investigated anomalous chromomagnetic and chromoelectric dipole moments of the top quark via top pair production in a γp collision at the LHC. We obtain 95% CL bounds on the anomalous couplings with various values of the integrated luminosity. Improved constraints on the anomalous couplings have been obtained compared to current limits.

At the LHC energy of $\sqrt s = 7\,{\mathrm { TeV}}$, under various beam and background conditions, luminosities, and Roman Pot positions, TOTEM has measured the differential cross-section for proton-proton elastic scattering as a function of the four-momentum transfer squared t. The results of the different analyses are in excellent agreement demonstrating no sizeable dependence on the beam conditions. Due to the very close approach of the Roman Pot detectors to the beam center (≈5σ_beam) in a dedicated run with β* = 90 m, |t|-values down to 5·10⁻³ GeV² were reached. The exponential slope of the differential elastic cross-section in this newly explored |t|-region remained unchanged and thus an exponential fit with only one constant B = (19.9 ± 0.3) GeV⁻² over the large |t|-range from 0.005 to 0.2 GeV² describes the differential distribution well. The high precision of the measurement and the large fit range lead to an error on the slope parameter B which is remarkably small compared to previous experiments. It allows a precise extrapolation over the non-visible cross-section (only 9%) to t = 0. With the luminosity from CMS, the elastic cross-section was determined to be (25.4 ± 1.1) mb, and using in addition the optical theorem, the total pp cross-section was derived to be (98.6 ± 2.2) mb. For model comparisons the t-distributions are tabulated including the large |t|-range of the previous measurement (TOTEM Collaboration (Antchev G. et al), EPL, 95 (2011) 41001).

The TOTEM experiment at the LHC has measured the inelastic proton-proton cross-section at $\sqrt {s}= 7\,{\mathrm { TeV}}$ in a β* = 90 m run with low inelastic pile-up. The measurement was based on events with at least one charged particle in the T2 telescope acceptance of 5.3 < |η| < 6.5 in pseudorapidity. Combined with data from the T1 telescope, covering 3.1 < |η| < 4.7, the cross-section for inelastic events with at least one |η| ⩽ 6.5 final-state particle was determined to be (70.5 ± 2.9) mb. This cross-section includes all central diffractive events of which maximally 0.25 mb is estimated to escape the detection of the telescopes. Based on models for low mass diffraction, the total inelastic cross-section was deduced to be (73.7 ± 3.4) mb. An upper limit of 6.31 mb at 95% confidence level on the cross-section for events with diffractive masses below 3.4 GeV was obtained from the difference between the overall inelastic cross-section obtained by TOTEM using elastic scattering and the cross-section for inelastic events with at least one |η| ⩽ 6.5 final-state particle.

The TOTEM experiment at the LHC has performed the first luminosity-independent determination of the total proton-proton cross-section at $\sqrt {s} = 7\,{\mathrm { TeV}}$. This technique is based on the optical theorem and requires simultaneous measurements of the inelastic rate – accomplished with the forward charged-particle telescopes T1 and T2 in the range 3.1 < |η| < 6.5 – and of the elastic rate by detecting the outcoming protons with Roman Pot detectors. The data presented here were collected in a dedicated run in 2011 with special beam optics (β* = 90 m) and Roman Pots approaching the beam close enough to register elastic events with squared four-momentum transfers |t| as low as 5·10⁻³ GeV². The luminosity-independent results for the elastic, inelastic and total cross-sections are σ_el = (25.1 ± 1.1) mb, σ_inel = (72.9 ± 1.5) mb and σ_tot = (98.0 ± 2.5) mb, respectively. At the same time this method yields the integrated luminosity, in agreement with measurements by CMS. TOTEM has also determined the total cross-section in two complementary ways, both using the CMS luminosity measurement as an input. The first method sums the elastic and inelastic cross-sections and thus does not depend on the ρ parameter. The second applies the optical theorem to the elastic-scattering measurements only and therefore is free of the T1 and T2 measurement uncertainties. The methods, having very different systematic dependences, give results in excellent agreement. Moreover, the ρ-independent measurement makes a first estimate for the ρ parameter at $\sqrt {s} = 7\,{\mathrm { TeV}}$ possible: |ρ| = 0.145 ± 0.091.

We show that time-delayed feedback can induce an optimal regularity in the pulsating dynamics of a nonlinear system without the need for an additional noise source. This deterministic delay-induced coherence resonance is reported experimentally in the chaotic dynamics of a laser diode subject to a phase-conjugate optical feedback, when varying the amount of feedback light. Qualitatively similar resonance is found theoretically in the model of a time-delayed class-A laser. The resonance is therefore not related to an interplay between time-delayed dynamics and faster undamped relaxation oscillations and is thought to be generic for a large class of delayed nonlinear systems.

We here examine the structure of turbulence in the case of a complex fluid made up of water and surfactants. This fluid has the particular property of shear thickening when driven at shear rates above a certain threshold. Through a study of the spectral properties and the structure function scalings, important differences arise with respect to the reference case, i.e., water. The surfactant solution shows strong intermittency at small scales. The large scales are, on the other hand, free of intermittency. While this transition is observed in the structure function scalings, no sign of this transition is seen in the power spectrum of velocity fluctuations which shows a single scaling range. The strongly intermittent small-scale region, despite the scaling of the power spectrum, exhibits properties reminiscent of the near-dissipative range.

We present a unified treatment of temporal and spatial depolarization in a quasi-monochromatic electromagnetic wave using Stokes parameters analysis. In our approach the Stokes parameters depend on the physical properties of light in a certain space volume. In this volume light can be divided into different light components that have the property of being homogenously polarized and that occupy different volume portions. In particular the obtained Stokes parameters are the weighted average of the Stokes parameters of these light components.

A scheme of terahertz (THz) generation by the cross-focusing of two collinear Gaussian lasers is investigated with frequency difference in spatially periodic density plasma (rippled density). For the resonant excitation of THz radiation the ripple wave number is suitably chosen to satisfy the phase matching condition. In this process, the lasers exert a ponderomotive force which imparts an oscillatory velocity to the electrons that couple with the density ripple to generate a stronger transient transverse current due to the spatial variation of their fields, driving THz radiation (with the frequency of the order of the plasma frequency). Various laser and plasma parameters were optimized and we report an efficiency of the order of ∼8 × 10⁻³ for the current scheme.

We utilize a quantitative phase-field model to simulate three-phase eutectic growth and the oscillatory instabilities at large spacings. We analyze the effect of symmetry in the selection of the oscillatory mode for the case of directional solidification. Going further from our previous article (Choudhury A.et al., Phys. Rev. E, 83 (2011) 051608), we manipulate the symmetry elements in a given configuration through the use of solid-solid anisotropy. Characteristic modes are compared between the case of isotropic surface energies and those retrieved in the presence of solid-solid anisotropy. We end with certain general arguments with regards to the growth rate of oscillatory modes and symmetry elements of the obtained microstructure depending on the symmetry elements of the starting configuration.

We present results on neutron scattering in solid ⁴He in the range of parameters where supersolidity is observed. The measurements address, among other questions, the viability of one possible mechanism of supersolidity: via a metastable amorphous phase. We have attempted to observe a glassy phase by neutron scattering. We have found that it is impossible to do this by total scattering, as it would be common in a classical solid, due to an extremely large inelastic diffuse signal related to the anomalously strong zero-point motion of helium atoms. This raises a general question on the interpretation of such scattering as the signature of an amorphous phase. Results from energy-resolved elastic scattering are heavily influenced by multiple scattering of neutrons which may be the major contribution to the measured elastic signal, but allow to put the limit on the concentration of an amorphous phase to ≲5% in a polycrystal with millimeter-size crystallites and to ≲2% in a single crystal. The values of NCRIf, expected from these limits should be much lower, although exact values depend strongly on a particular model of glass-related supersolidity.

In this paper we characterize the superconductor-insulator phase transition on a network of 2d percolation clusters. Sufficiently close to the percolation threshold, for p ≃ p_c, this network has a broad degree distribution, and at p = p_c the degree distribution becomes scale free. We study the transverse Ising model on this complex topology in order to characterize the superconductor-insulator transition in a network formed by 2d percolation clusters of a superconductor material. We show, by a mean-field treatment, that the critical temperature of superconductivity depends on the maximal eigenvalue Λ of the adjacency matrix of the network. At the percolation threshold, p = p_c, we find that the maximal eigenvalue Λ of the adjacency matrix of the network of 2d percolation clusters has a maximum. In correspondence of this maximum the superconducting critical temperature T_c is enhanced. These results suggest the design of new superconducting granular materials with enhanced critical temperature.

The pressure-induced formation of a hexagonal phase (P6/m, Z = 2) of LiN₃ is predicted by particle swarm optimization (PSO) structural search at zero temperature and pressure of > 34.7 GPa. Compared to the ground-state insulator C2/m phase, the P6/m-LiN₃ is found to possess a metallic feature dominated by the 2p_z bands. Electron topological analysis revealed an intriguing phase change of nitrogen, from azide to novel pseudo-benzene "N₆" molecules at the formation of the P6/m phase. The occurrence of this hexagonal phase follows the polymerization of the typical linear N₃⁻ anion molecular chains in the low-pressure molecular phases of LiN₃ and is originated from the volume reduction favorable for a denser structure packing under compression.

By the spin-fermion formula, the Hubbard model on the honeycomb lattice is represented by a U(2) gauge theory using the mean-field method; non-Abelian vortex solutions are constructed based on this theory. The quantization condition shows that the magnetic flux quanta are half-integer. There are 2k bosonic zero modes for the k-winding vortex. For the fermions, there are 2 zero-energy states (ZESs) corresponding to the single elementary vortex. In the vortex core and on the edge, the system is in the semi-metal phase with a spin gap and in the insulator phase with a Néel order, and can be mapped to the superconductor in the class A and CI, respectively.

We design, from first-principles calculations, a novel family of thallium halide-based compounds as candidates for new high-temperature superconductors, whose superconductivity is mediated by the recently proposed mechanism of non-local correlation-enhanced strong electron-phonon coupling. Two prototype compounds namely CsTlF₃ and CsTlCl₃ are studied with various hole doping levels and volumes. The critical superconducting temperatures (T_c's) are predicted to be about 30 K and 20 K with ∼0.35/f.u. hole doping and require only modest pressures (∼10 and ∼2 GPa), respectively. Our procedure of designing this class of superconductors is quite general and can be used to search for other "other high-temperature superconductors".

Motivated by the unveiled complexity of nonmagnetic insulating behavior in the pentavalent post-perovskite NaIrO₃, we have studied its electronic structure and phase diagram in the plane of Coulomb repulsive interaction and spin-orbit coupling by using the newly developed local density approximation plus Gutzwiller method. Our theoretical study proposes that the metal-insulator transition can be generated by two different physical pictures: renormalized band insulator or Mott insulator regime. For the realistic material parameters in NaIrO₃, Coulomb interaction U = 2.0 eV (J = U/4) and spin-orbit coupling strength η = 0.33 eV, it tends to favor the renormalized band insulator picture as revealed by our study.

Topological surface states, while protected by time-reversal symmetry in the bulk limit, can be missing in films with thicknesses much greater than the decay lengths of the surface states. This novel effect is demonstrated theoretically in Bi₂Se₃, the best known topological insulator. When the spin-orbit-coupling strength is tuned through the quantum critical point (realizable experimentally by low-Z element substitution), there is a wide dead zone where the film is topological but without topological surface states within the projected bulk gap. This dead zone can be suppressed by interfacial bonding.

Armchair silicene nanoribbons with width of 9–39 silicon atoms are investigated by using the self-consistent field crystal orbital method based on density functional theory. The carrier mobilities obtained from deformation potential theory oscillate with respect to the width and the values are comparable with those the graphene nanoribbons have. The buckled structure, hydrogen saturation, edge reconstruction as well as edge roughness decrease the carrier mobilities which are explained with the aid of crystal orbitals.

On the basis of first-principles calculations we have systematically investigated the energetics of hydrogen desorption from the MgH₂ (001) surface. Based on total energy and electronic structure calculations, two modes namely strain and doping of selected dopants (Al, Si, Ti) and the combined effect of both on the dehydrogenation energies (ΔH) of MgH₂ (001) systems have been analyzed. The maximum improvement in ΔH has been obtained with the combined effect of doping and strain. Among all the dopants, Al gives the lowest value of ΔH when the system Al-MgH₂ is subjected to a 7.5% biaxial symmetric strain whereas the Si-MgH₂ systems show the least improvement in ΔH. The doping of Ti on MgH₂ (001) is also very beneficial even without strain. The reduction in ΔH is caused by the charge localization on the metal atoms, destabilization and the weakening of metal-hydrogen bonds.

Momentum transfer from incoming magnons to a Bloch domain wall is calculated using one-dimensional continuum micromagnetic analysis. Due to the confinement of the wall in space, the dispersion relation of magnons is different from that of a single domain. This mismatch of dispersion relations can result in reflection of magnons upon incidence on the domain wall, whose direct consequence is a transfer of momentum between magnons and the domain wall. The corresponding counteraction force exerted on the wall can be used for the control of domain wall motion through magnonic linear-momentum transfer, in analogy with the spin transfer torque induced by magnonic angular-momentum transfer.

The atomic and electronic structures of Cl-intercalated epitaxial graphene on SiC are studied by first-principles calculations. By increasing the Cl concentration, doping levels from n-type to slightly p-type are achieved on the SiC(0001) surface, while a wider range of doping levels is possible on the SiC(000$\overline {1}$) surface. We find that the Cl atoms prefer bonding to the substrate rather than to the graphene. By varying the Cl concentration the doping level can be tailored. Consideration of van der Waals forces improves the distance between the graphene and the substrate as well as the binding energy, but it is not essential for the formation energy. For understanding the doping mechanism the introduction of non-local van der Waals contributions to the exchange correlation functional is shown to be essential.

In this paper, X-ray diffraction, X-ray energy dispersive spectrometer, photoluminescence, electron paramagnetic resonance were performed to examine the evolution of intrinsic defect states during the annealing treatment for understanding the origin of ferromagnetism in undoped ZnO granular films deposited by the magnetron sputtering method. A strong link between the ferromagnetism and the zinc vacancies was established, indicating zinc vacancies may be more effective in modulating ferromagnetism than oxygen vacancies in ZnO. This novel observation is significant not only to get further insight into the ferromagnetic origin but also to tune the magnetic properties for spintronic materials.

We propose a Markov jump process with the three-state herding interaction. We see our approach as an agent-based model for the financial markets. Under certain assumptions this agent-based model can be related to the stochastic description exhibiting sophisticated statistical features. Along with power-law probability density function of the absolute returns we are able to reproduce the fractured power spectral density, which is observed in the high-frequency financial market data. The given example of consistent agent-based and stochastic modeling will provide a background for further developments in the research of complex social systems.

Computer simulations are presented of colloids, bidisperse in size, suspended in a shear-thinning viscoelastic fluid with the flow characteristics of a surfactant solution. The worm-like micelles are modeled in Responsive Particle Dynamics (RaPiD) as single soft particles obeying a generalized Brownian equation of motion including transient forces that effectively account for the entanglements of the polymeric chains. The colloids mix homogeneously in the quiescent fluid, but in a shear flow they string together and form colloidal trains. Besides alignment, we also observe simultaneous segregation of the colloids by size. Experimental studies have reported on separation by size occurring near the walls of the rheometer, while in the current study the colloids segregate in the bulk of the fluid.

In order to obtain the moments of different theoretical distributions and compare them with those obtained from distributions of true rotational rates, we derive the moments of the generalized distribution function of the projected rotational velocities V sin i and the true equatorial rotational velocities V from the Tsallis q-statistical formalism, then we compare such moments with others obtained via the conventional method. We show that the moments are constrained to intervals of validity for q larger than those for standard moments, thus it is possible to obtain distribution function broader than the standard cases. The variances show that the distributions become narrower around the mean as q decreases. We now should highlight that one can derive averages of sin i smaller than unity even if q_V ≠ q_V sin i.

We apply the nonextensive statistical theory given by Landau and Lifshitz to study the stability condition of convection in a fluid. When introducing Du's equation (Du J. L., Europhys. Lett., 67 (2004) 893), a relationship between the temperature gradient and the potential field of a nonequilibrium complex system by the nonextensive parameter, into the stability condition, we find that the nonextensive parameter can be used as the stability criterion of the convection taking place in a fluid. With the data of the standard solar models for BP2000 and BS2005, we apply this criterion to the solar interior and show a stability range from the center of the Sun to 0.74 R_⊙,which is in excellent agreement with the helioseismological measurements inside the Sun. With the same data, the lower limit of the adiabatic index, when stability is needed, has been calculated.