Table of contents

Using integrate-and-fire networks, we study the relationship between the architectural connectivity of a network and its functional connectivity as characterized by the network's dynamical properties. We show that dynamics on a complex network can be controlled by the topology of the network, in particular, scale-free functional connectivity can arise from scale-free architectural connectivity, in which the architectural degree correlation plays a crucial role.

We develop a concept of entanglement percolation for long-distance singlet generation in quantum networks with neighboring nodes connected by partially entangled bipartite mixed states. We give a necessary and sufficient condition on the class of mixed network states for the generation of singlets. States beyond this class are insufficient for entanglement percolation. We find that neighboring nodes are required to be connected by multiple partially entangled states and devise a rich variety of distillation protocols for the conversion of these states into singlets. These distillation protocols are suitable for a variety of network geometries and have a sufficiently high success probability even for significantly impure states. In addition to this, we discuss possible further improvements achievable by using quantum strategies including generalized forms of entanglement swapping.

We consider a photo-Carnot engine that consists of a single-mode radiation field in an optical cavity. One the heat reservoirs is made of a beam of thermally entangled pairs of two-level atoms that interact resonantly with the cavity. We express the thermodynamic efficiency of the engine in terms of the quantum discord of the atomic pair and find that it can exceed its classical value. Our results show that useful work can be extracted from quantum correlations, indicating that the latter are a valuable resource in quantum thermodynamics.

It is well known that for unflavored leptogenesis there is in general no connection between low- and high-energy CP violation. We stress that for a non-unitary lepton mixing matrix this may not be the case. We give an illustrative example for this connection and show that the non-standard CP phases that are induced by non-unitarity can be responsible for observable effects in neutrino oscillation experiments as well as for the generation of the baryon asymmetry of the Universe. Lepton flavor violation in decays such as τ→μγ can also be induced at an observable level. We also comment on the neutrino mass limits from leptogenesis, which get barely modified in case of a non-unitary mixing matrix.

It is known that the measured and calculated g-factors of 2₁⁺ states in even-even N=Z nuclei have values g≅0.5. Both the collective model and the LS limit of the shell model (with L=2, S=0) can explain this. Here we note that the isoscalar g-factors of low-lying 1⁺, 3⁺, and 5⁺T=0 states in medium-mass odd-odd N=Z nuclei have also been measured to have values close to 0.5. An up-to-date compilation of the experimental values is presented. Also included are the results of large-scale shell model calculations which agree very well with the experimental data. We show that the isoscalar g-factors in N=Z nuclei also approach 0.5 in the large-l limit for a single-j shell model.

Electron-hydrogen scattering in Debye plasma environments has been investigated using the close-coupling approximation. Three different models, viz. 3-state CCA, 6-state CCA, and 9-state CCA, have been employed in the present investigation. The positions and respective widths of the 2s^2 1S^e resonance of hydrogen negative ion have been investigated for various plasma conditions. The inter-particle interactions have been taken to be of Debye-Hückel type. The present results are in close agreement with those of Kar and Ho (New J. Phys., 7 (2005) 141).

The emission spectrum produced at 300 K in dense plasmas of helium excited by a corona discharge shows an asymmetric atomic line at 706 nm that may be associated with quasi-molecular states of He₂. We present here the first calculations of He-He collisional profiles of this line done in a unified line shape semi-classical theory using ab initio molecular potentials. The excellent agreement between experimental and theoretical determinations of the near wing of the line profiles establishes the accuracy of the interaction potentials. The strong pressure dependence of the collision profiles can then be used as a diagnostic of the physical parameters of the source.

We study a one-dimensional Yukawa model where the coupling of a spinless fermionic field to a neutral bosonic scalar field becomes exactly soluble in the recoil-free limit. We demonstrate how the correlation between two spatially separated quantum sources can be transferred onto the created bosons. The presence of these correlations manifests itself in the absence of any quantum interference in the spatial densities when two bosons pass through each other. The interference is recovered if the sources are uncorrelated. We compare the evolution of the bosons' spatial probability with the one obtained from classical sources that also leads to interference. The classical description is inappropriate even for situations in which the bosons interfere.

The temporal coherence of Free-Electron Laser (FEL) sources, which exhibit, in the self-amplified spontaneous-emission mode, spiking spectral and temporal distributions, can be drastically improved by seeding with an external laser or high-order harmonics. Here, experiments at 160 nm put in evidence that the improvement of spectral properties (and thus temporal coherence) of the FEL radiation takes place for a larger seed intensity than the one required to overcome the shot noise.

We study theoretically the electronic excitation within semiconductors under irradiation with an ultrashort VUV laser pulse as provided by the new free-electron laser FLASH in Hamburg, Germany. Applying Monte Carlo technique we obtain the transient distribution of the excited electrons within solid silicon. We find the statistical nature of an effective energy gap for multiple electronic excitation, providing the fundamental understanding of the experimentally accessible pair creation energy measured as a long time limit. Considering photoabsorbtion, impact ionizations and Coster-Kroning transitions, we estimate the pair creation energy and give a general formula to calculate the effective energy gap for semiconductors.

The results of investigations on the simultaneous extraction of positive ions and electrons from a single-grid ICP source are reported. It is shown that the single-grid ion source is capable of generating coincident ion and electron flows in contrast to the more common two- or three-grid sources. Electron and ion emission characteristics of the source are presented for the cases of DC and RF acceleration bias. A method of ion/electron current ratio control is proposed allowing to change the current compensation state of the ion beam from non-compensated to highly over-compensated. The researches were conducted in the ion energy range 10–250 eV with a high ion current density of 5 mA/cm², so the presented results may be useful for modern technology applications.

Using large-scale molecular-dynamics simulations, we investigate the mechanical behavior of thermoset/thermoplastic polymer alloys. We focus here on the effect of linear-chain mass fraction Γ_l, for different chain lengths N_l, and strain rates . Our results show that tensile strain (i.e., strain to break) decreases with increasing Γ_l up to a threshold value Γ^*_l, beyond which it increases with Γ_l. This non-monotonic behavior, which we call "anomalous ductility", is qualitatively independent of and N_l, so long as fracture occurs in bulk. However, for larger strain rates (i.e., ) adhesive fracture occurs and no anomalous ductility is observed. Γ^*_l decreases with increasing N_l and we observe microscopic evidence that Γ^*_l signifies the onset of interchain interactions. A simple scaling argument suggests that Γ^*_l is related to the overlap concentration c^* of the thermoplastic homopolymer in the cured thermoset matrix.

The size and morphological instability of a heteroepitaxially stressed ring of constant height and constant area is theoretically investigated through a static calculation of the edge and elastic energy terms. It is shown that above a critical value of stress, the formation of a ring-shaped nanostructure of selected width is favoured compared to a circular island of the same area. It is then found that in-phase fluctuations of both ring edges may preferentially develop leading to serpentine shape. When the ring width increases and its radii decrease, the possibility of development of anti-phase fluctuations leading to the formation of more complicated shape is discussed.

The optical-Kerr-effect configuration is used to study the enhancement of photo-induced molecular reorientation in a twisted nematic liquid crystal due to the presence of metal nanoparticles. The system consisting of a liquid crystal with photoisomerizable dye Disperse Orange 3 shows an enhancement of the Kerr-like response at 532 nm excitation light when doped with 15 nm in size gold nanoparticles. A simple theoretical model predicting the behavior and magnitude of the expected response has been formulated. The contribution of localized surface plasmon of gold nanoparticles to light-induced torque is discussed.

We studied the dynamics in the chiral mesogenic phases observed in binary mixtures of 5CB and bent-core molecules by using dynamic light scattering. In the lower-temperature phase, a clear orientational fluctuation was observed, which was not observed in the higher-temperature one. The wave number dependence of the relaxation frequency strongly indicates that nanoscale phase separation occurs and that the fluctuations of 5CB with nematic order are suppressed by the bent-core–rich domains. The spatial heterogeneity of the nematic dynamic motion yields exact information on the characteristic size of the nanostructure, which cannot be obtained by static structural analysis.

We present inelastic neutron scattering measurements of the dynamics of helium adsorbed on carbon nanotube bundles. The goal is to determine the vibrational properties of the 1D and 2D quantum solids that are stabilized on the nanotube bundle surfaces at different fillings of ⁴He. The mean square vibrational amplitude in the 1D solid is large with a Lindemann ratio of γ_1D=(⟨u²⟩)^1/2/a₁=0.25 comparable to bulk solid ⁴He. The γ_2D is significantly smaller. The frequency density of states of the 2D solid, g(ω), has a gap at ω≃ 0.75 meV consistent with a commensurate lattice. The 1D solid has no gap or a gap smeared by disorder. The 1D and 2D g(ω) are well described by dispersion curves having no gap and a gap, respectively, with some vibration along additional dimensions indicated.

We present a microscopic theory for interacting graphene armchair nanoribbon quantum dots. Long-range interaction processes are responsible for Coulomb blockade and spin-charge separation. Short-range ones, arising from the underlying honeycomb lattice of graphene smear the spin-charge separation and induce exchange correlations between bulk electrons —delocalized on the ribbon— and single electrons localized at the two ends. As a consequence, entangled end-bulk states where the bulk spin is no longer a conserved quantity occur. Entanglement's signature is the occurrence of negative differential conductance effects in a fully symmetric set-up due to symmetry-forbidden transitions.

The nature of a puzzling high-temperature ferromagnetism of doped mixed-valent vanadium oxide nanotubes reported earlier by Krusin-Elbaum et al., Nature, 431 (2004) 672, has been addressed by static magnetization, muon spin relaxation, nuclear magnetic and electron spin resonance spectroscopy techniques. A precise control of the charge doping was achieved by electrochemical Li intercalation. We find that it provides excess electrons, thereby increasing the number of interacting magnetic vanadium sites, and, at a certain doping level, yields a ferromagnetic-like response persisting up to room temperature. Thus we confirm the surprising previous results on the samples prepared by a completely different intercalation method. Moreover our spectroscopic data provide first ample evidence for the bulk nature of the effect. In particular, they enable a conclusion that the Li nucleates superparamagnetic nanosize spin clusters around the intercalation site which are responsible for the unusual high-temperature ferromagnetism of vanadium oxide nanotubes.

We study resonant transport through a superconducting double-barrier structure. At each barrier, due to the proximity effect, an incident electron can either reflect as an electron or a hole (Andreev reflection). Similarly, transport across the barrier can occur via direct tunneling as electrons as well as via the crossed Andreev channel, where a hole is transmitted. In the subgap regime, for a symmetric double-barrier system (with low transparency for each barrier), we find a new T=1/4 resonance (T is the transmission probability for electrons incident on the double-barrier structure) due to interference between electron and hole wave functions between the two barriers, in contrast to a normal double-barrier system which has the standard transmission resonance at T=1. We also point out as an application that the resonant value of T=1/4 can produce pure spin current through the superconducting double-barrier structure.

In order to investigate the possibility of "beating the superparamagnetic limit with exchange bias" (Skumryev V. et al., Nature, 423 (2003) 850) we perform atomistic modeling on Co/CoO magnetic nanoparticles varying the strength of the ferromagnet/antiferromagnet interfacial coupling. Our results show that exchange-biased systems exhibit an increased energy barrier along the bias direction, with a corresponding decrease of the barrier in the opposite direction. For systems with large values of the interfacial coupling, the ferromagnetic core is found to be unconditionally stable in the bias direction. Such a system is ideal for thermal stability since there is no energetically stable reversed state, providing the antiferromagnetic shell is unchanged. In order to permit magnetic recording, which essentially requires a bi-stable system, we propose a heat-assisted recording method, whereby both the ferromagnet (FM) and antiferromagnet (AF) are switched. Upon heating to the Néel temperature of CoO, the AF magnetic order is destroyed allowing switching of the FM core with a small applied field, with thermal stability reappearing on cooling through T_N.

The J₁-J₂ model on a square lattice exhibits a rich variety of different forms of magnetic order that depend sensitively on the ratio of exchange constants J₂/J₁. We use bulk magnetometry and polarized neutron scattering to determine J₁ and J₂ unambiguously for two materials in a new family of vanadium phosphates, Pb₂VO(PO₄)₂ and SrZnVO(PO₄)₂, and we find that they have ferromagnetic J₁. The ordered moment in the collinear antiferromagnetic ground state is reduced, and the diffuse magnetic scattering is enhanced, as the predicted bond-nematic region of the phase diagram is approached.

Resistive transitions have been measured on a perforated Nb thin film with a lattice of holes with period of the order of ten nanometers. Bumps in the dR/dH-vs.-H curves have been observed at the first matching field and its fractional values, 1/4, 1/9 and 1/16. This effect has been related to different vortex lattice configurations made available by the underlying lattice of holes.

Although semiconductor Bloch equations have been widely used for decades to address ultrafast optical phenomena in semiconductors, they have a few important drawbacks: i) Coulomb terms between free electron-hole pairs require a Hartree-Fock treatment which, in its usual form, preserves excitonic poles but loses biexcitonic resonances. ii) The resulting coupled differential equations impose heavy numerics which completely hide the physics. This can be completely avoided if, instead of free electron-hole pairs, we use correlated pairs, i.e., excitons. Their interactions are easy to handle through the recently constructed composite-boson many-body theory. This allows us to obtain the time evolution of the polarization induced by a laser pulse analytically. Polarization is shown to come from Coulomb interactions between virtual excitons, but also from Coulomb-free fermion exchanges, these being dominant at large detuning.

We investigate the time evolution of the excitonic polarization in semiconducting carbon nanotubes by a quantum theory, and present a modulation scheme of the excitonic polarization by applying a weak laser field. In calculation, we consider the many-body interaction between the electrons and the holes. Our results indicate that there is an intermittency of excitonic polarization. A correlation effect of the excitons induced by the weak laser field may be responsible for this phenomenon. We discuss the effect of the nanotube radius, laser field strength, and laser field frequency on the excitonic polarization.

We propose a nonlinear sigma-model for the description of quantum transport in a mesoscopic metallic conductor with magnetic impurities frozen in a spin glass phase. It accounts for the presence of both the corresponding scalar and magnetic random potentials. In a spin glass, this magnetic random potential is correlated between different realizations. As the strength of the magnetic potential is varied, this model describes the crossover between orthogonal and unitary universality classes of the nonlinear sigma-model. We apply this technique to the calculations of the correlation of conductance between two frozen spin configurations in terms of dephasing rates for the usual low-energy modes of weak-localization theory.

We point out the existence of finite charge and spin Hall conductivities of graphene in the presence of a spin orbit interaction (SOI) and localized magnetic impurities. The SOI in graphene results in different transverse forces on the two spin channels yielding the spin Hall current. The magnetic scatterers act as spin-dependent barriers, and in combination with the SOI effect lead to a charge imbalance at the boundaries. As indicated here, the charge and spin Hall effects should be observable in graphene by changing the chemical potential close to the gap.

We present measurements of the thermal fluctuations of the free surface of an aging colloidal suspension, Laponite. The technique consists in measuring the fluctuations of the position of a laser beam that reflects from the free surface. Analysing the data statistics, we show that, as the fluid ages, the dynamics becomes heterogeneous. The intermittent events correspond to large changes in the local slope of the free surface over a few milliseconds. We show that those quakes are uncorrelated, although they are kept in memory by the surface over short time scales.

We analyze the influence of the anchoring energy strength on the relaxation of the nematic deformation, when the distorting field is removed, at t=0. The analysis is performed by assuming that the nematic sample is in the shape of a slab, the anchoring energy can be approximated by the form proposed by Rapini and Papoular, and the surface dissipation, responsible for the surface viscosity, is negligible with respect to the bulk one. The switching time of the distorting field is supposed finite, to avoid the non-physical discontinuity of the time derivative of the tilt angle at t=0. We show that the relaxation time of the nematic deformation is a multi-relaxation phenomenon. For large t, the relaxation phenomenon is simple, and the relaxation time is proportional to the diffusion time τ_D=ηd²/k, where d is the thickness of the sample, η the viscosity and k the elastic constant of the nematic liquid crystal. For t≪τ_D, the time dependence of the tilt angle is well approximated by few exponential terms. The relation between the effective relaxation time and the anchoring energy strength is deduced. A critical discussion on the standard analysis based on the diffusion equation is also reported.

For models whose evolution takes place on a network it is often necessary to augment the mean-field approach by considering explicitly the degree dependence of average quantities (heterogeneous mean field). Here we introduce the degree dependence in the pair approximation (heterogeneous pair approximation) for analyzing voter models on uncorrelated networks. This approach gives an essentially exact description of the dynamics, correcting some inaccurate results of previous approaches. The heterogeneous pair approximation introduced here can be applied in full generality to many other processes on complex networks.

Using molecular simulations, we identify microscopic relaxation events of individual particles in ageing structural glasses, and determine the full distribution of relaxation times. We find that the memory of the waiting time t_w elapsed since the quench extends only up to the first relaxation event, while the distribution of all subsequent relaxation times (persistence times) follows a power law completely independent of history. Our results are in remarkable agreement with the well-known phenomenological trap model of ageing. A continuous-time random walk (CTRW) parametrized with the atomistic distributions captures the entire bulk diffusion behavior and explains the apparent scaling of the relaxation dynamics with t_w during ageing, as well as observed deviations from perfect scaling.

We consider the conditional galaxy density around each galaxy, and study its fluctuations in the newest samples of the Sloan Digital Sky Survey Data Release 7. Over a large range of scales, both the average conditional density and its variance show a non-trivial scaling behavior, which resembles criticality. The density depends, for 10⩽r⩽80 Mpc/h, only weakly (logarithmically) on the system size. Correspondingly, we find that the density fluctuations follow the Gumbel distribution of extreme-value statistics. This distribution is clearly distinguishable from a Gaussian distribution, which would arise for a homogeneous spatial galaxy configuration. We also point out similarities between the galaxy distribution and critical systems of statistical physics.

Possible empirical evidences in favor of the hypothesis that the speed of light decreases by a few centimeters per second each year are examined. Lunar laser ranging data are found to be consistent with this hypothesis, which also provides a straightforward explanation for the so-called Pioneer anomaly, that is, a time-dependent blue-shift observed when analyzing radio tracking data from distant spacecrafts, as well as an alternative explanation for both the apparent time-dilation of remote events and the apparent acceleration of the Universe. The main argument against this hypothesis, namely, the constancy of fine-structure and Rydberg constants, is discussed. Both of them being combinations of several physical constants, their constancy implies that, if the speed of light is indeed time-dependent, then at least two other "fundamental constants" have to vary as well. This puts severe constraints on the development of any future varying–speed-of-light theory.