Table of contents

The largest eigenvalue of a network provides understanding to various dynamical as well as stability properties of the underlying system. We investigate the interplay of inhibition and multiplexing on the largest eigenvalue statistics of networks. Using numerical experiments, we demonstrate that the presence of the inhibitory coupling may lead to a behaviour of the largest eigenvalue statistics of multiplex networks very different from that of isolated networks depending upon the network architecture of the individual layer. We demonstrate that there is a transition from the Weibull to the Gumbel or to the Fréchet distribution as networks are multiplexed. Furthermore, for denser networks, there is a convergence to the Gumbel distribution as network size increases indicating higher stability of larger systems.

Under investigation in this work is a generalized (2+1)-dimensional Boussinesq equation, which can be used to describe the propagation of small-amplitude, long wave in shallow water. By virtue of Bell's polynomials, an effective way is presented to succinctly construct its bilinear form. Furthermore, based on the bilinear formalism and the extended homoclinic test method, the breather wave solution, rogue-wave solution and solitary-wave solution of the equation are well constructed. Our results can be used to enrich the dynamical behavior of the generalized (2+1)-dimensional nonlinear wave fields.

Fully characterizing the steerability of a quantum state of a bipartite system has remained an open problem ever since the concept of steerability was first defined. In this paper, using our recent geometrical approach to steerability, we suggest a necessary and sufficient condition for a two-qubit state to be steerable with respect to projective measurements. To this end, we define the critical radius of local models and show that a state of two qubits is steerable with respect to projective measurements from Alice's side if and only if her critical radius of local models is less than 1. As an example, we calculate the critical radius of local models for the so-called T-states by proving the optimality of a recently suggested local hidden state model.

The aim of this letter is threefold: First is to show that nonlinear generalizations of electrodynamics support various types of knotted solutions in vacuum. The solutions are universal in the sense that they do not depend on the specific Lagrangian density, at least if the latter gives rise to a well-posed theory. Second, is to describe the interaction between probe waves and knotted background configurations. We show that the qualitative behaviour of this interaction may be described in terms of Robinson congruences, which appear explicitly in the causal structure of the theory. Finally, we argue that optical arrangements endowed with intense background fields could be the natural place to look for the knots experimentally.

A nonlinear oscillator model with negative time-delayed feedback is studied numerically under external deterministic and stochastic forcing. It is found that in the unforced system complex partial synchronization patterns like chimera states as well as salt-and-pepper–like solitary states arise on the route from regular dynamics to spatio-temporal chaos. The control of the dynamics by external periodic forcing is demonstrated by numerical simulations. It is shown that one-cluster and multi-cluster chimeras can be achieved by adjusting the external forcing frequency to appropriate resonance conditions. If a stochastic component is superimposed to the deterministic external forcing, chimera states can be induced in a way similar to stochastic resonance, they appear, therefore, in regimes where they do not exist without noise.

We evaluate self-interaction effects on the quantum correlations of field modes of opposite momenta for scalar $\lambda \phi^4$ theory in a two-dimensional asymptotically flat Robertson-Walker spacetime. Such correlations are encoded both in the von Neumann entropy defined through the reduced density matrix in one of the modes and in the covariance expressed in terms of the expectation value of the number operators for each mode in the evolved state. The entanglement between field modes carries information about the underlying spacetime evolution.

We consider a theoretical model describing the "anomalous heating" of charged grains due to their stochastic motion in the volume of a spatially inhomogeneous plasma. On the basis of this model for the first time we propose the analytical relations for conditions of the heating of grains due to the gradient of their charge in the electric field of a trap. The obtained relations were tested by numerical simulations of the problem for one and two charged particles.

We suggest a universal phenomenological description for the collective access patterns in the Internet traffic dynamics both at local and wide area network levels that takes into account erratic fluctuations imposed by cooperative user behaviour. Our description is based on the superstatistical approach and leads to the q-exponential inter-session time and session size distributions that are also in perfect agreement with empirical observations. The validity of the proposed description is confirmed explicitly by the analysis of complete 10-day traffic traces from the WIDE backbone link and from the local campus area network downlink from the Internet Service Provider. Remarkably, the same functional forms have been observed in the historic access patterns from single WWW servers. The suggested approach effectively accounts for the complex interplay of both "calm" and "bursty" user access patterns within a single-model setting. It also provides average sojourn time estimates with reasonable accuracy, as indicated by the queuing system performance simulation, this way largely overcoming the failure of Poisson modelling of the Internet traffic dynamics.

We consider a particle moving with equation of motion $\dot x=f(t)$, where f(t) is a random function with statistics which are independent of x and t, with a finite drift velocity $v=\langle f\rangle$ and in the presence of a reflecting wall. Far away from the wall, translational invariance implies that the stationary probability distribution is $P(x)\sim \exp(\alpha x)$. A classical example of a problem of this type is sedimentation equilibrium, where α is determined by temperature. In this work we do not introduce a thermal reservoir and α is determined from the equation of motion. We consider a general approach to determining α which is not always in agreement with Einstein's relation between the mean velocity and the diffusion coefficient. We illustrate our results with a model inspired by the Boltzmann equation.

We postulate a general Lagrangian density which leads to the equations of motion of the well-known cases of the electromagnetic field, the theory of elasticity, and some other particular problems in classical physics. When the mentioned cases are studied in multilayer systems, i.e., when the constitutive parameters depend on one of the Cartesian coordinates, all of them lead us to a matrix Sturm-Liouville problem whose equation of motion, in its most general form, can be derived from the postulated Lagrangian density too. These results demonstrate the ubiquitous character of the Sturm-Liouville matrix problem and consequently the relevance of its study from the mathematical, physical and numerical point of view. The consequences of this curious result are briefly analyzed.

A new class of forces, approximately dispersionless forces, were recently predicted as part of a semiclassical description of the Aharonov-Bohm effect. Electron time-of-flight measurements have been performed that test for such forces. Magnetized iron cores used in the previous time-of-flight experiment may affect potential back-action forces and have, therefore, been eliminated. We report that no forces were detected. This finding supports the local and nonlocal, quantum descriptions of the AB effect and rules out local, semiclassical descriptions.

Using the boundary string field theory (BSFT) techniques we study the boundary state and partition function for a dynamical (rotating-moving) Dp-brane coupled to the electromagnetic and tachyonic background fields in superstring theory. By making use of the created partition function, the super BSFT action with a tachyonic field and Dirac-Born-Infeld–type action will be constructed. By analyzing the obtained action interesting features will be revealed.

This letter presents a study of the inclusive production of $H\rightarrow b\bar{b}$ plus a recoil, using simulated samples of pp collisions at $\sqrt{s}=14\ \text{TeV}$ for an integrated luminosity in the range between $30\ \text{fb}^{-1}$ and $3\ \text{ab}^{-1}$. The case for experiments to include un-prescaled b-tag multijet triggers for this topology is made and the ideal jet thresholds are discussed. The sensitivity to the Standard Model Higgs boson with a transverse momentum of at least 200 GeV is evaluated with respect to a continuous background, dominated by multijet processes. The mass of b-jet-pairs is analysed, quoting sensitivity to cross-sections in the range from 1 to 2 pb, for $100\ \text{fb}^{-1}$, covering the total Higgs-boson production cross-section of 1.8 pb. The trigger strategy presented in this letter is compared to triggers already in use, showing an increase on the signal efficiency for masses below 200 GeV and a performance comparable to a logical OR of all the currently available akin triggers for higher masses. The robustness of the expected sensitivity against systematic uncertainties is estimated by considering various typical sources, such as those on the fitting parameters of the continuous background, shape uncertainties affecting the signal acceptance and the background modelling. The accuracy of the Higgs-boson production cross-section measurements is also discussed, quoting sensitivity to deviations of 50% for $100\ \text{fb}^{-1}$ and 10% for $3\ \text{ab}^{-1}$.

Premelting of ice within pores in earth materials is shown to depend on the presence of vapor layers. For thick vapor layers between ice and pore surfaces, a nanosized water sheet can be formed due to repulsive Lifshitz forces. In the absence of vapor layers, ice is inhibited from melting near pore surfaces. In between these limits, we find an enhancement of the water film thickness in silica and alumina pores. In the presence of metallic surface patches in the pore, the Lifshitz forces can dramatically widen the water film thickness, with potential complete melting of the ice surface.

We experimentally study an optomechanical cavity that is formed between a mechanical resonator, which serves as a movable mirror, and a stationary on-fiber dielectric mirror. A significant change in the behavior of the system is observed when the distance between the fiber's tip and the mechanical resonator is made smaller than about $1\ \mu \text{m}$. The combined influence of Casimir force, Coulomb interaction due to trapped charges, and optomechanical coupling is theoretically analyzed. The comparison between experimental results and theory yields a partial agreement.

The dielectric cylinder covered with graphene (DCCG) is found to be a promising platform for studying multi-qubit collective effects. The plasmons supported by DCCG have huge wave numbers and low loss. Under some conditions, the zeroth- and first-order modes even have the same wavelength. Qubits along DCCG can interact with each other strongly by coupling with the plasmons within tens to hundreds of plasmonic wavelengths. Additionally, the electro-optical tunability of graphene means that we can manipulate the plasmonic wavelength and subsequently the dynamic evolutions of the qubits mediated by the DCCG conveniently. These properties make the graphene be a potential choice for quantum information device.

In this letter, we show that dilute granular systems can exhibit a type of intermittency that has no analogue in gas dynamics. We consider a simple system in which a very dilute set of granular particles falls under gravity through a nozzle. This setting is analogous to the classical problem of high-speed nozzle flow in the study of compressible gases. It is well known that very dilute granular systems exhibit behavior qualitatively similar to gases, and that gas flowing through a nozzle does not exhibit intermittency. Nevertheless, we show that the intermittency in dilute granular nozzle flows can occur and corresponds to complicated transitions between supersonic and subsonic regimes. We also provide detailed explanations of the mechanism underlying this phenomenon.

Recently we have shown that the asymmetric lateral coherence of betatron radiation is characterized by peculiar properties that are evidenced with the analysis of the coherence factor of radiation. We extend such results to radiation emitted by ultra-relativistic proton beams in a 2-periods undulator. Results of a 2-dimensional simulation show that the real analysis of the asymmetric lateral coherence substantially improves the resolving power, thus transverse beam non-uniformities and the beam size can be measured at large distance.

We propose a scheme to realize parity-time ($\mathcal {PT}$)-symmetry in an ensemble of strongly interacting Rydberg atoms, which act as superatoms due to the dipole blockade mechanism. We show that Rydberg-dressed ⁸⁷Rb atoms in a four-level inverted Y-type configuration is highly efficient to generate the refractive index for a probe field, with a symmetric (antisymmetric) profile spatially in the corresponding real (imaginary) part. Comparing with earlier investigations, the present scheme provides a versatile platform to control the system from $\mathcal {PT}$-symmetry to non-$\mathcal{PT}$-symmetry via different external parameters, i.e., coupling field detuning, probe field intensity and control field intensity.

We construct explicit spatiotemporal or light bullet (LB) solutions to the (3 + 1)-dimensional nonlinear Schrödinger equation (NLSE) with inhomogeneous diffraction/dispersion and nonlinearity in the presence of parity-time (PT) symmetric potential with competing nonlinearities. The solution is based on the similarity transformation, by which the initial inhomogeneous problem is reduced to the standard NLSE with constant coefficients but with redefined variables and potential. Transmission characteristics of LB solutions, such as the phase change, half width and chirp, are studied in the media with exponentially decreasing diffraction/dispersion and with periodic modulation. Our outcomes demonstrate that diffraction/dispersion and nonlinearity management can prolong the stability of LBs in a PT potential.

We report evidence of a new mechanism able to damp very efficiently geodesic acoustic mode (GAM) in the presence of a nonuniform temperature profile in a toroidally confined plasma. This represents a particular case of a general mechanism that we have found and that can be observed whenever the phase-mixing acts in the presence of a damping effect that depends on the wave number k_r. Here, in particular, the combined effect of the Landau and continuum damping is found to quickly redistribute the GAM energy in phase-space, due to the synergy of the finite orbit width of the passing ions and the cascade in wave number given by the phase-mixing. This damping mechanism is investigated analytically and numerically by means of global gyrokinetic simulations. When realistic parameter values of plasmas at the edge of a tokamak are used, damping rates up to 2 orders of magnitude higher than the Landau damping alone are obtained. We find in particular that, for temperature and density profiles characteristic of the high confinement mode, the so-called H-mode, the GAM decay time becomes comparable to or lower than the nonlinear drive time, consistently with experimental observations (Conway G. D. et al., Phys. Rev. Lett., 106 (2011) 065001).

The integration of III-V semiconductors on functional perovskite-oxide can lead to new material properties and new device applications by combining the rich properties of perovskite-oxides together with the superior optical and electronic properties of III-V semiconductors. The structural and electronic properties of the surface and interface of the GaAs/BaTiO₃ are studied using first-principles calculations. We point out the energetically favorable GaAs/BaTiO₃ interfaces according to the GaAs initial adsorption on the BaTiO₃(001) substrate. Our calculations predict the existence of the metallic behavior at the GaAs/BaTiO₃ interfaces.

The size of air bubbles in nematic liquid crystals can be continuously decreased through the absorption of air molecules into the host liquid crystal. A bubble and its accompanying hyperbolic hedgehog point defect undergo a continuous asymmetric motion, while the bubble decreases in size. In this study, a mechanism is proposed to theoretically explain both the motion of the air bubble and the point defect observed experimentally. Anisotropic evaporation of air molecules may occur because of the symmetry breaking of the director configuration near the point defect. The motion of the center of the air bubble to the hyperbolic hedgehog point defect is induced by the anisotropic force due to evaporation of air molecules and Stokes drag force.

We report in situ Raman scattering studies of electrochemically top gated VO₂ thin film to address metal-insulator transition (MIT) under gating. The room temperature monoclinic insulating phase goes to metallic state at a gate voltage of 2.6 V. However, the number of Raman modes do not change with electrolyte gating showing that the metallic phase is still monoclinic. The high-frequency Raman mode A_g(7) near 616 cm⁻¹ ascribed to V-O vibration of bond length 2.06 Å in VO₆ octahedra hardens with increasing gate voltage and the B_g(3) mode near 654 cm⁻¹ softens. This shows that the distortion of the VO₆ octahedra in the monoclinic phase decreases with gating. The time-dependent Raman data at fixed gate voltages of 1 V (for 50 minutes, showing enhancement of conductivity by a factor of 50) and 2 V (for 130 minutes, showing further increase in conductivity by a factor of 5) show similar changes in high-frequency Raman modes A_g(7) and B_g(3) as observed in gating. This slow change in conductance together with Raman frequency changes show that the governing mechanism for metalization is more likely due to the diffusion-controlled oxygen vacancy formation due to the applied electric field.

We demonstrate that an in-plane uniaxial anisotropy may be induced in non-amorphous soft CoFeZr films. We used broadband ferromagnetic resonance spectroscopy and complex permeability spectra to investigate the spin dynamics in CoFeZr films. We report a systematic study of the FM thickness on the fundamental dynamic parameters such as the effective magnetisation, the g-factor and relaxation mechanisms. Our study reveals that the decrease of the effective magnetisation mesured with FMR with thickness is not due to perpendicular anisotropy but to low dimentionality. Moreover, we observed a decrease of the g-factor with thickness and a modification of the ratio of the orbital to the spin magnetic moment. These films exhibit good high-frequency performance red (i.e. high permeability in a broad frequency range and a low damping) at low thickness of about a few nanometers.

Cobalt-chromia interface exchange interactions in the Co-on-Cr₂O₃(0001) system are investigated. Density-functional theory predicts the exchange coupling at the interface to be antiferromagnetic, in agreement with earlier experimental results. The spin-polarized photoemission spectra reveal both perpendicular and in-plane magnetization components, in the cobalt adlayer on chromia. A magnetization canted with respect to the surface normal, inferred from the presence of remnant spin polarization both in the plane of the cobalt film and along the surface normal may be understood as a micromagnetic canting effect involving magnetostatic self-interaction and exchange coupling between Co and Cr₂O₃.

The research on the electrocaloric effect (ECE), the materials and their application has significantly increased in the last years, which resulted in several different concepts and demonstrators of electrocaloric (EC) cooling devices. The aim of this paper is to give a systematic overview of possible design concepts of EC cooling devices and to provide a method for their classification. Nine different device types could be distinguished. Each device type is being specified according to function principle, characteristic properties, technical challenges and technical readiness level. This classification and state of the art reveal areas requiring deeper research and can help researchers and engineers to select appropriate concepts for further investigation, improvement and application.

Two-dimensional electron gases with very high mobility show a huge or giant negative magnetoresistance at low temperatures and low magnetic fields. We present an experimental and theoretical work on the influence of the applied current on the negative huge magnetoresistance of these systems. We obtain an unexpected and strong nonlinear behavior consisting in an increase of the negative huge magnetoresistance with increasing current, in other words, for increasing current the magnetoresistance collapses at small magnetic fields. This nonlinearity is explained by the subtle interplay of elastic scattering within Landau levels and between Landau levels.

We study the electronic properties of the BiAlO₃/SrTiO₃ (BAO/STO) interface and compare them to the well-studied LaAlO₃/STO system. Due to the ferroelectricity of BAO it is possible to manipulate the carrier density at the interface. Using density functional theory, we investigate the spin-orbit coupling (SOC) effects in the two-dimensional electron gas (2DEG) in BAO/STO and its magnetic counterparts, the BAO/EuTiO₃ and BAO/Sr₂NiWO₆ heterostructures. In the latter two structures, the proximity to ferroelectric insulators breaks the time-reversal symmetry in the 2DEG and can lead, in combination with inversion symmetry breaking and SOC to a single Fermi surface, analogous to the situation in topological insulators.

We studied the suppression of the weak antilocalization (WAL) effect and the dependence of spin dynamics for a two-dimensional electron gas in the inversion layers of two different Hg_1−xCd_xTe samples in the presence of in-plane magnetic field $(B_{//})$. The WAL magnetoconductance is fitted by the Golub model to acquire the variations of phase coherence time with increasing $B_{//}$. The effective g-factors in the form of $\vert m_{r}^{\ast} g^{\ast}\vert$ ($m_{r}^{\ast}$ is the relative effective mass) at zero magnetic field and high magnetic field are obtained by investigating the electron dephasing with varying $B_{//}$ and measuring the spin splitting of the Shubnikov-de Hass oscillations, respectively. As the obtained g-factors are in accordance with the reported results, the suppression of the WAL effect can be attributed to the competition between Zeeman splitting and spin-orbit interaction rather than to the microroughness scattering.

A common occurrence in everyday human activity is where people join, leave and possibly rejoin clusters of other individuals —whether this be online (e.g. social media communities) or in real space (e.g. popular meeting places such as cafes). In the steady state, the resulting interaction network would appear static over time if the identities of the nodes are ignored. Here we show that even in this static steady-state limit, a non-zero nodal mobility leads to a diverse set of outbreak profiles that is dramatically different from known forms, and yet matches well with recent real-world social outbreaks. We show how this complication of nodal mobility can be renormalized away for a particular class of networks.

Migration is one of the most dramatic and vast human processes in modern times. Migration is defined as people that leave their home and home-land and move to a new country. In this research we address the pattern of this massive human movement with the tools of network theory. The undirected global flow migration network (2006–2010) was identified as an exclusive disassortative network which combines two types of defined groups of large- and small-degree (D) countries with betweeness (Be) of Be∼D³. This structure was modeled and simulated with synthetic networks of similar characteristics as the global flow migration network, and the results suggest that small-degree nodes have the topology of random networks, but the dominant part of the large-degree hubs controls this topology and shapes the network into an ultra-small world. This exclusive topology and the difference of the global flow migration network from scale-free and from Erdös-Rényi networks may be a result of two defined and different topologies of large- and small-degree countries.

Bulk heterojunction (BHJ) organic photovoltaic cells are analysed within a simple efficient model that includes the important physical properties of such photovoltaic systems. In this model, in contrast with most of the previous studies, we take into account the motion of both the electron and the hole in the separation process at the donor-acceptor interface. We theoretically examine the exciton dissociation yield under the influences of charge Coulomb interaction and non-radiative recombination. We find that the electron-hole local Coulomb attraction and charge carriers' coupling parameters play an important role in the system performance and in the optimal energy conversion efficiency of the BHJ photocell. We show that the fixed-hole models tend to underestimate the yield.

We point out that a successful inflationary magnetogenesis could be realised if we break the local U(1) gauge symmetry during inflation. The effective electric charge is fixed as a fundamental constant, which allows us to obtain an almost scale-invariant magnetic spectrum avoiding both the strong coupling and back reaction problems. We examine the corrections to the primordial curvature perturbation due to these stochastic electromagnetic fields and find that, at both linear and non-linear orders, the contributions from the electromagnetic field are negligible compared to those created from vacuum fluctuations. Finally, the U(1) gauge symmetry is restored at the end of inflation.

Original article: EPL, 114 (2016) 68002.

Expanding frontiers of cosmic-ray muon imaging On the dependency of friction on load: Theory and experiment Stochastic thermodynamics of resetting Unusual transport properties of the topological Dirac metal Na₃Bi