Table of contents

We show that amplitude chimeras in ring networks of Stuart-Landau oscillators with symmetry-breaking nonlocal coupling represent saddle-states in the underlying phase space of the network. Chimera states are composed of coexisting spatial domains of coherent and of incoherent oscillations. We calculate the Floquet exponents and the corresponding eigenvectors in dependence upon the coupling strength and range, and discuss the implications for the phase-space structure. The existence of at least one positive real part of the Floquet exponents indicates an unstable manifold in phase space, which explains the nature of these states as long-living transients. Additionally, we find a Stuart-Landau network of minimum size N = 12 exhibiting amplitude chimeras.

Continuous unitary transformations are a powerful tool to extract valuable information out of quantum many-body Hamiltonians, in which the so-called flow equation transforms the Hamiltonian to a diagonal or block-diagonal form in second quantization. Yet, one of their main challenges is how to approximate the infinitely-many coupled differential equations that are produced throughout this flow. Here we show that tensor networks offer a natural and non-perturbative truncation scheme in terms of entanglement. The corresponding scheme is called "entanglement-CUT" or eCUT. It can be used to extract the low-energy physics of quantum many-body Hamiltonians, including quasiparticle energy gaps. We provide the general idea behind eCUT and explain its implementation for finite 1d systems using the formalism of matrix product operators. We also present proof-of-principle results for the spin-(1/2) 1d quantum Ising model and the 3-state quantum Potts model in a transverse field. Entanglement-CUTs can also be generalized to higher dimensions and to the thermodynamic limit.

We investigate the collective dynamics of bi-stable elements connected in different network topologies, ranging from rings and small-world networks, to random and deterministic scale-free networks. We focus on the correlation between network properties and global stability measures of the synchronized state, in particular the average critical coupling strength $\langle \epsilon_c \rangle$ yielding transition to synchronization. Further, we estimate the robustness of the synchronized state by finding the minimal fraction of nodes f_c that need to be perturbed in order to lose synchronization. Our central result from these synchronization features is the following: while networks properties can provide indicators of synchronization within a network class, they fail to provide consistent indicators across network classes. For instance, the heterogeneity of degree does not consistently impact synchronization, as is evident through the stark difference in the synchronizability of rings vis-à-vis small-world and star networks, all of which have same average degree and deviation around the mean degree in the limit of large networks. Further we demonstrate that clustering coefficient is also not a consistent feature in determining synchronization. This is clear through the similarity of synchronization properties in rings with significantly different clustering coefficients, and the striking difference in synchronization of a star network and a ring having the same clustering coefficient. Even the characteristic path length, which is of paramount importance in determining synchronization, does not provide a one-to-one correspondence with synchronization properties across classes. Namely, synchronization is significantly favoured in networks with low path lengths within a network class. However, the same characteristic path length in different types of networks yields very different $\langle \epsilon_c \rangle$ and f_c.

The study of quantum correlations in solid-state systems is a large avenue for research and their detection and manipulation are an actual challenge to overcome. In this context, we show by using first-principles calculations on the prototype material KNaCuSi₄O₁₀ that the degree of quantum correlations in this spin cluster system can be managed by external hydrostatic pressure. Our results pave the way for research in detection and manipulation of quantum correlations in magnetic systems with promising applications in quantum information science.

We derive the reciprocal relation between the linear mechanical and thermal response functions in a driven diffusive model. In this model, we consider the violation of the fluctuation-response relation (FRR) of the velocity of a particle to the thermal perturbation. In addition, we also consider the FRR violation of the heat current of the system to a mechanical perturbation. We show the equality for the extent of these FRR violations using the Fokker-Planck equation.

Conventional models for the thermal conductivity of nanocomposites neglect the particle size distribution by assuming all nanoparticles to be uniform in size, although in reality most particles in a nanocomposite are not uniform and the size distribution of the nanoparticles might strongly affect the thermal conductivity of nanocomposites as demonstrated in this letter. By including the lognormal distribution of the nanoparticle size into the thermal conductivity model of nanocomposites embedded with spherical particles, we show that the mean value and the standard deviation (SD) of the nanoparticle radius significantly affect the thermal conductivity of particulate nanocomposites. At the same mean radius of embedded particles, the thermal conductivity of the nanocomposite with lognormal size distribution at $\text{SD}=56$ can be twice that with a uniform particle size. Our improved model on effective thermal conductivity of nanocomposites with nonuniform particles can provide a more accurate prediction on the thermal conductivity of realistic particulate nanocomposites.

A natural question arising when observing crack networks in brittle layers such as, e.g., paints, muds, skins, pottery glazes, coatings, ceramics, is what determines the distance between cracks. This apparently simple question received a wealth of more or less complex and appropriate answers, but no consensus has emerged. Here, we show that the cracks interact mutually as soon as the spacing between them is smaller than ten times the thickness of the layer. Then, a simple Griffith-type balance between the elastic deformation energy and the fracture bulk and debonding costs captures a broad number of observations, going from the square-root or linear increase of the spacing with the thickness, to its decrease with loading until saturation. The adhesion strength is identified as playing a key role in these behaviour changes. As illustration, we show how the model can be applied to study the influence of the layer thickness on crack patterns. We believe that the versatility of the approach should permit wide applicability, from geosciences to engineering.

Density waves were studied in a phase-separated binary complex plasma under microgravity conditions. For the big particles, waves were self-excited by the two-stream instability, while for small particles, they were excited by heartbeat instability with the presence of reversed propagating pulses of a different frequency. By studying the dynamics of wave crests at the interface, we recognize a "collision zone" and a "merger zone" before and after the interface, respectively. The results provide a generic picture of wave-wave interaction at the interface between two "mediums".

Determining the equilibrium configuration and shape of curved two-dimensional films with (generalized) liquid-crystalline order is a difficult infinite-dimensional problem of direct relevance to the study of generalized polymersomes, soft matter and the fascinating problem of understanding the origin and formation of shape (morphogenesis). The symmetry of the free energy of the LC film being considered and the topology of the surface to be determined often requires that the equilibrium configuration possesses singular structures in the form of topological defects such as disclinations for nematic films. The precise number and type of defect plays a fundamental role in restricting the space of possible equilibrium shapes. Flexible closed vesicles with spherical topology and nematic or smectic order, for example, inevitably possess four elementary strength $+1/2$ disclination defects positioned at the four vertices of a tetrahedral shell. Here we address the problem of determining the equilibrium shape of flexible vesicles with generalized liquid-crystalline order. The order parameter in these cases is an element of $S^1/Z_p$, for any positive integer p. We will focus on the case $p =3$, known as triatic liquid crystals (LCs). We construct the appropriate order parameter for triatics and find the associated free energy. We then describe the structure of the elementary defects of strength $+1/3$ in flat space. Finally, we prove that sufficiently floppy triatic vesicles with the topology of the 2-sphere equilibrate to octahedral shells with strength $+1/3$ defects at each of the six vertices, independently of the scale.

The reactivity of Xe is important in both fundamental chemistry and geological science. The discovery of the reductive reactivity of Xe extended the doctrinal boundary of chemistry for which a completed shell is inert to reaction. The oxidation of Xe by various elements has been explored. On the other hand, the opposite chemical inclination, i.e., gaining electrons and forming anions, has not been thoroughly studied for Xe or other noble-gas elements. In this work, we demonstrate, using first-principles calculations and an efficient structure prediction method, that Xe can form stable $\text{XeLi}_{n}\ (n=1\text{--}5)$ compounds under high pressure. These compounds are intermetallic and Xe are negatively charged. The stability of these compounds indicates that atoms or ions with completely filled shell may still gain electrons in chemical reactions.

Within a one-dimensional tight-binding model, the dynamic dissociation processes of trions under an external electric field have been investigated using a nonadiabatic evolution method. We demonstrate that the dissociation of the trion under an applied electric field is more difficult than in the case for either the polaron or the bipolaron. Considering the effect of the electron-electron interactions, it is found that the on-site Coulomb interactions reduce while the nearest-neighbor interactions enhance the stability of the trion. Furthermore, due to the competition between the on-site Coulomb and the nearest-neighbor interactions, the static trion presents two typical configurations in general, i.e., a bound state of an exciton and a polaron or a bound state of a bipolaron and an opposite charged polaron. Accordingly, the outcome of the trion dissociation is closely related to its static configuration.

A monolayer of orthorhombic arsenic (arsenene) is a promising candidate for nano-electronic devices due to the uniquely electronic properties. To further extend its practical applications, an additional layer is introduced to tune the electronic structures. Four layer-stacking manners, namely AA-, AB-, AB'-, and AC-stacking, are constructed and studied through using first-principles calculations. Compared with monolayer, an indirect-direct gap transition is realized in AB-stacking. More importantly, a semimetal feature appears in the AC- and AB'-stacked bilayers, leaving the electronic structure of AA-stacking trivial. In addition, the energy dispersion around Γ is largely tuned from the layer-stacking effect. To understand the underlying physics, the $\textbf{k}\cdot\textbf{p}$ approximation is taken to address this issue. Our results show that the level repulsion from the additional layer domaintes the anisotropy of energy dispersion around Γ. The works like ours would shed new light on the tunability of the electronic structure in layered arsenene.

The Dirac equation in curved space is used to study the optical transmittance of deformed graphene along a given direction. Our theoretical analysis of the available experimental data for the light transmittance suggests that the periodic ripple associated with the out-of-plane deformation observed in unstrained graphene explains the observations. Furthermore, the experimental uniaxial strained graphene for light transmittance shows two features, namely the modification of the $\cos^2\theta$ law and the decrease of the amplitude of the oscillations with the polarization angle θ, which can be well accommodated within the theoretical analysis used here and provide further evidence of the validity of using QFT in curved space to understand two-dimensional materials.

The infrared (IR) phonon vibration of the narrow t_2g band system YTiO₃ and its effect on electronic properties are studied using the generalized gradient approximation (GGA) and its combination with Coulomb correlation U. Coulomb U is found to improve the GGA results not only in electronic properties which has been researched frequently, but also in its lattice dynamics. The phonon frequency is increased effectively, especially in high-frequency region, which corrects the gap between experimental and theoretical GGA result. More importantly, IR-active modes are found to affect electronic properties significantly, and we find vibration-sensitive magnetic exchange interaction and strong electron-phonon coupling.

Friedel oscillations are ubiquitous features seen in all impurity-doped metallic structures but in the case of graphene-like materials they are not so evident because the relevant wavelengths are perfectly commensurate with the lattice parameter. Here we demonstrate that this commensurability effect leads to a slow convergence of supercell-based total energy calculations in impurity-doped carbon nanotubes. We derive a mathematically transparent expression for the formation energy and identify a very distinctive dependence on the size of the supercell unit. We make use of this dependence through a simple extrapolation scheme to obtain density functional theory results with accuracy levels that would otherwise require enormously large unit cells.

We have conducted a temperature-dependent angle-resolved photoemission spectroscopy (ARPES) study of the electronic structures of PbTe, PbSe and PbS. Our ARPES data provide direct evidence for the light-hole upper valence bands (UVBs) and hitherto undetected heavy-hole lower valence bands (LVBs) in these materials. An unusual temperature-dependent relative movement between these bands leads to a monotonic decrease in the energy separation between their maxima with increasing temperature, which is known as band convergence and has long been believed to be the driving factor behind extraordinary thermoelectric performances of these compounds at elevated temperatures.

We present results on low-temperature magnetization approaching the saturation law in aligned bundles of CNTs with ferromagnetic nanoparticles embedded inside inner channels of nanotubes for two directions of the magnetic field, parallel and perpendicular to the CNT axes. Elaborating experimental data, we were able to extract the explicit form of the correlation functions describing the orientation of the magnetic anisotropy axes in real space. In the parallel field the long-range coherence in the magnetic anisotropy axes is the characteristic feature. In the perpendicular direction the peculiar feature is the 2D exchange coupling. The nature of the exchange interaction and the role of the CNT medium in it is also discussed.

We consider tunneling from the spin-polarized tip into the Luttinger liquid edge state of quantum spin Hall system. This problem arose in the context of the spin and charge fractionalization of an injected electron. Renormalization of the dc conductances of the system is calculated in the fermionic approach and scattering states formalism. In the lowest order of the tunneling amplitude we confirm previous results for the scaling dependence of conductances. Going beyond the lowest order we show that the interaction affects not only the total tunneling rate, but also the asymmetry of the injected current. The helical edge state forbids the backscattering, which leads to the possibility of two stable fixed points in the renormalization group sense, in contrast to the Y-junction between the usual quantum wires.

TaTe₂ is a quasi-2D charge density wave (CDW) compound with distorted-1T–type structure exhibiting double-zigzag chains. Here we report the Fermi surface topology of a low-temperature phase of TaTe₂ (LT-TaTe₂) by anisotropic magneto-transport and magnetic measurements on high-quality single crystals. An anomalous large linear magnetoresistance up to 140% at 3 K in 9 T was observed, suggesting the existence of a small Fermi pocket in the Dirac cone state in quantum transport models. Moreover, strong magnetic anisotropy was observed for B⊥(001) and B∥(001). Angle-dependent magnetoresistance and de Hass-van Alphen oscillations suggest the anisotropy of the normal Fermi surface and the small Fermi pocket in Dirac cone state.

Extreme Light Infrastructure-Nuclear Physics (ELI-NP), to become operational in 2019, is a new Research Center built in Romania that will use extreme electromagnetic fields for nuclear physics research. The ELI-NP facility will combine two large equipments with state-of-the-art parameters, namely a 2 × 10 PW high-power laser system and a very brilliant gamma-beam system delivering beams with energies up to 19.5 MeV. The laser and gamma-beam systems under construction and typical proposed first-phase experiments are described.

Link prediction is the problem of predicting the existence and location of unknown links from uncertain structural information of a network. Most existing accuracy measures do not consider the role of time evolution within the network. Moreover, almost all existing methods use indirect links to infer and evaluate the validity of links. We introduce time as a parameter for link prediction accuracy measures, and we modify the structure of the link prediction algorithms to exploit information of the known direct links for link prediction. We find that the direct link algorithm performs better than the indirect link algorithm for a range of time varying networks. We show that the network structure plays a more important role than weights for links prediction. In addition, our analysis finds that the number of common neighbours also plays an important role for the so-called weak-ties phenomenon.

The similarity between neural and (adaptive) immune networks has been known for decades, but so far we did not understand the mechanism that allows the immune system, unlike associative neural networks, to recall and execute a large number of memorized defense strategies in parallel. The explanation turns out to lie in the network topology. Neurons interact typically with a large number of other neurons, whereas interactions among lymphocytes in immune networks are very specific, and described by graphs with finite connectivity. In this paper we use replica techniques to solve a statistical mechanical immune network model with "coordinator branches" (T-cells) and "effector branches" (B-cells), and show how the finite connectivity enables the coordinators to manage an extensive number of effectors simultaneously, even above the percolation threshold (where clonal cross-talk is not negligible). A consequence of its underlying topological sparsity is that the adaptive immune system exhibits only weak ergodicity breaking, so that also spontaneous switch-like effects as bi-stabilities are present: the latter may play a significant role in the maintenance of immune homeostasis.

We study a generic one-dimensional model for an intracellular cargo driven by N motor proteins against an external applied force. The model includes motor-cargo and motor-motor interactions. The cargo motion is described by an over-damped Langevin equation, while motor dynamics is specified by hopping rates which follow a local detailed balance condition with respect to the change in energy per hopping event. Based on this model, we show that the stall force, the mean external force corresponding to zero mean cargo velocity, is completely independent of the details of the interactions and is, therefore, always equal to the sum of the stall forces of the individual motors. This exact result is arrived on the basis of a simple assumption: the (macroscopic) state of stall of the cargo is analogous to a state of thermodynamic equilibrium, and is characterized by vanishing net probability current between any two microstates, with the latter specified by motor positions relative to the cargo. The corresponding probability distribution of the microstates under stall is also determined. These predictions are in complete agreement with numerical simulations, carried out using specific forms of interaction potentials.

We study thermal radiation of a warm neutron star with a variable shell-like heater located in its crust. The heater and the star are taken to be initially in a stationary state. Then the heat power is increased or decreased for some period of time producing a peak or a dip of the thermal surface emission; afterwards the stationary state is restored. Only a small fraction of the generated heat is thermally emitted through the surface. Time variation of the surface luminosity is weakened and distorted with respect to the variation of the generated heat power; the former variation can be observable only under special conditions —neutron stars are "hiding" their internal temperature variations. These results can be useful for the interpretation of the observations of neutron stars with variable thermal surface emission, particularly, magnetars and transiently accreting neutron stars in low-mass X-ray binaries.