Table of contents

The molecular compound [Fe₂(μ₂-oxo)(C₃H₄N₂)₆(C₂O₄)₂] was designed and synthesized for the first time and its structure was determined using single-crystal X-ray diffraction. The magnetic susceptibility of this compound was measured from 2 to 300 K. The analysis of the susceptibility data using protocols developed for other spin singlet ground-state systems indicates that the quantum entanglement would remain at temperatures up to 732 K, significantly above the highest entanglement temperature reported to date. The large gap between the ground state and the first-excited state (282 K) suggests that the spin system may be somewhat immune to decohering mechanisms. Our measurements strongly suggest that molecular magnets are promising candidate platforms for quantum information processing.

Quantum and classical correlations are investigated during quantum brachistochrone evolution (QBE) in this paper. We found some typical properties of the pair of quantum states sampled randomly by use of the Harr measure in this special kind of quantum evolution. This kind of evolution of a three-qubit system between two distinct states cannot be implemented without classical correlations (including bipartite J₂ and tripartite J₃) and quantum correlations (including bipartite D₂ and tripartite D₃). We also found that some QBEs between two distinct GHZ states do not follow the typical behaviour, and that this kind of evolution can be implemented without bipartite quantum correlations. Although the probability density function of the time-averaged bipartite classical correlation, time-averaged bipartite quantum correlations and time-averaged genuine tripartite correlations become more and more uniform with the decrease of angles of separation between an initial state and a final state, the features of their most probable values exhibit a different trend.

We study the superfluidity of a spin-orbit coupled Bose-Einstein condensate (BEC) by computing its Bogoliubov excitations, which are found to have two branches: one is gapless and phonon-like at long wavelength; the other is typically gapped. These excitations imply superfluidity that has two new features: i) due to the absence of the Galilean invariance, one can no longer define the critical velocity of superfluidity independent of the reference frame; ii) the superfluidity depends not only on whether the speed of the BEC exceeds a critical value, but also on cross-helicity that is defined as the direction of the cross-product of the spin and the kinetic momentum of the BEC.

We generalize the scale-free network model of Barabási and Albert (Science, 286 (1999) 509) by proposing a class of stochastic models for scale-free interdependent networks in which interdependent nodes are not randomly connected but rather are connected via preferential attachment (PA). Each network grows through the continuous addition of new nodes, and new nodes in each network attach preferentially and simultaneously to a) well-connected nodes within the same network and b) well-connected nodes in other networks. We present analytic solutions for the power-law exponents as functions of the number of links both between networks and within networks. We show that a cross-clustering coefficient vs. size of network N follows a power law. We illustrate the models using selected examples from the Internet and finance.

We study the diffusion of particles exposed to the external constant force in the periodic spatial potential by means of computer modelling. It has been shown that in the underdamped case the diffusion coefficient is increased exponentially with a drop in temperature in a certain interval of F. The physical reasons for such an abnormal behavior have been analyzed.

Light-emitting atoms or material in a cavity are well known to generate thermal or coherent fields depending on, e.g., the quality of the cavity and the strength of the excitation of the emitting material. We investigate the possibilities of generating antibunched photons and nonclassical light by adding nonlinear elements in the cavity and show that it is indeed feasible. In particular, we focus on cavities with emitters formed by macroscopic many-body systems like gas of excitons in semiconductor quantum wells where different nonlinearities in the emitters and absorbers will be shown to lead to bunched, Poissonian, and antibunched photon statistics. We show that, while the single two-state system with suitable coupling parameters can produce antibunched photons, macroscopic systems need an additional nonlinearity to generate antibunching phenomenon. Our results demonstrate that a nonclassical cavity field can be generated by a conventional laser when a suitable nonlinear absorbing element is added to the cavity structure. Thus, the proposed device configuration can be manufactured using standard optoelectronic materials and processing techniques.

The behavior of weakly deformed drops on non-wetting surfaces is usually described using linear models. We show that these simple pictures cannot account for measurements of the dynamics of droplets that oscillate or bounce on super-hydrophobic substrates. We demonstrate that several peculiar experimental observations observed in previous works can be understood through a logarithmic correction of the linear model.

We study the propagation of linear waves and solitons in an array of optical waveguides with an embedded defect created by a pair of waveguides with gain and loss satisfying the so-called parity-time ($\mathcal {PT}$) symmetry condition. We demonstrate that in the case of small soliton amplitudes, the linear theory describes the scattering of solitons with a good accuracy. We find that the incident high-amplitude solitons can excite the mode localized at the $\mathcal {PT}$-symmetric defect. We also show that by exciting the localized mode of a large amplitude, it is possible to perform phase-sensitive control of soliton scattering and amplification or damping of the localized mode.

In this letter, we investigate the wrinkling instability of a stiff thin film bonded on a soft substrate, induced by the gravity of densely packed pillars adhered on the surface of the film. By using linear perturbation analysis, we show that the gravity of the pillars can induce wrinkling instability of the system when the gravitational force of the pillars is large enough. Our calculation results give the instability criterion and illustrate how the wavelength of the wrinkles varies with several parameters of the system. The results of this article may be useful in the applications of similar pillar structures.

We develop recursion equations to describe the three-dimensional shape of a sheet upon which a series of concentric curved folds have been inscribed. In the case of no stretching outside the fold, the three-dimensional shape of a single fold prescribes the shape of the entire origami structure. To better explore these structures, we derive continuum equations, valid in the limit of vanishing spacing between folds, to describe the smooth surface intersecting all the mountain folds. We find that this surface has negative Gaussian curvature with magnitude equal to the square of the fold's torsion. A series of open folds with constant fold angle generate a helicoid.

Levitating a liquid over a vapor film was limited to droplets. Here we show that on curved substrates a larger quantity of fluid can be suspended. This opens a new possibility for exploring free-liquid-surface phenomena without any contact with a solid. In one of the simplest possible situations, a large fluid torus is levitated over a circular trough. A poloidal flow inside the ring generates a wave on its inner side, making it polygonal. This wave is described by a solitonic model which balances surface tension and the pressure depletion due to the distortion of the poloidal flow.

The generation of Rossby rogue waves (Rossby rogons), as well as the excitation of bright and dark Rossby envelpe solitons are demonstrated on the basis of the modulational instability (MI) of a coherent Rossby wave packet. The evolution of an amplitude-modulated Rossby wave packet is governed by a one-dimensional (1D) nonlinear Schrödinger equation (NLSE). The latter is used to study the amplitude modulation of Rossby wave packets for fluids in Earth's atmosphere and in the solar photosphere. It is found that an ampitude-modulated Rossby wave packet becomes stable (unstable) against quasi-stationary, long-wavelength (in comparision with the Rossby wavelength) perturbations, when the carrier Rossby wave number satisfies k² < 1/2 or $\sqrt {2}+1<k^2<3$ (k² > 3 or $1/2<k^2<\sqrt {2}+1$). It is also shown that a Rossby rogon or a bright Rossby envelope soliton may be excited in the shallow-water approximation for the Rossby waves in solar photosphere. However, the excitation of small- or large-scale perturbations may be possible for magnetized plasmas in the ionosphereic E-layer.

The relativistic electron/positron particle beam propagation in overdense magnetized plasmas is studied theoretically, using a fluid plasma model and accounting for the quantum properties of individual particles. The collective character of the particle beam manifests through the macroscopic, beam created, plasma wake field. The transverse dynamics is described by the quantum Schrödinger equation for the single-particle wave function, within the Hartree mean-field approximation, coupled with the Poisson equations for the wake potential. The resulting nonlinear nonlocal Schrödinger equation is solved analytically in the strongly nonlocal regime, yielding breathing/wiggling Hermite-Gauss ring solitons. The nonstationary rings may be parametrically unstable. The conditions for instability and the growth rates are estimated analytically.

Spatial correlations of microscopic fluctuations are investigated via real-space experiments and computer simulations of colloidal glasses under steady shear. It is shown that while the distribution of one-particle fluctuations is always isotropic regardless of the relative importance of shear as compared to thermal fluctuations, their spatial correlations show a marked sensitivity to the competition between shear-induced and thermally activated relaxation. Correlations are isotropic in the thermally dominated regime, but develop strong anisotropy as shear dominates the dynamics of microscopic fluctuations. We discuss the relevance of this observation for a better understanding of flow heterogeneity in sheared amorphous solids.

We report on investigations of the static structure factors of glass-forming Zr-Cu alloy melts by combination of the containerless processing technique of electrostatic levitation with diffraction of neutron and synchrotron radiation. The partial Bhatia-Thornton structure factors S_NN and S_NC were determined from the two total structure factors. While it is widely assumed in literature that the good glass-forming ability of Zr-Cu is related to an icosahedral short-range order prevailing in the melt, our partial structure factors demonstrate that the liquid Zr-Cu is not characterized by a dominant icosahedral short-range order.

Yne-diamond —a new carbon allotrope constructed by inserting two carbon atoms into the carbon-carbon bonds of diamond was expected to have super-hardness comparable to diamond because of the three-dimensional network of strong sp-sp³ and sp-sp bonds. However, from a theoretical point of view, this idea has never been validated carefully. Based on first-principles calculations, we present the first theoretical evidence that, in contrast to the early expectation, yne-diamond possesses low ideal tensile strength, low shear strength and small Pugh's modulus ratio, owning to the large void network in the covalent bond skeleton. Combined with a simple model, we predict that the yne-diamond family will not be a super-hard material family. This provides a new understanding of the mechanical properties of carbon allotropes containing sp-hybridized carbon atoms.

Polymer nanocomposites (PNCs) hold great promise for designing novel materials. Current challenges in PNCs are achieving nanosized dispersion of the inorganic component as well as robust control of nanoparticle orientation. We show that a gel of multi-block polymers with functionalized end groups that have specific affinity towards nanorods provides a general example of a PNC where the inorganic component is dispersed at the nanometer scale and displays long range as well as orientation order. We find a novel type of liquid crystalline (LC) order consisting of thin nanorod strips, i.e., "flexible ladders" (FL). Depending on concentration and affinity, FL display positional long-range order and patch together into 2D smectic phases. We discuss implications for designing new PNCs and address possible realizations of our systems via DNA linkers. The problem of LC order on curved geometries is also discussed.

We study the ground-state structures of identical classical point charges with Coulomb interactions, confined between two symmetric parallel charged walls. For the well-understood homogeneous dielectric case with no electrostatic images, the charges evenly condense on the opposite walls, thereby forming a bilayer Wigner crystal; five structures compete upon changing the inter-wall separation. Here, we consider a dielectric jump between the walls and a solvent in which charges are immersed, implying repulsive images. Using recently developed series representations of lattice sums for Coulomb law, we derive the complete phase diagram. In contrast to the homogeneous dielectric case, the particles remain in a hexagonal Wigner monolayer up to a certain distance between the walls. Beyond this distance, a bifurcation occurs to a sequence of Wigner bilayers, each layer having a nonzero spacing from the nearest wall. Another new phenomenon is that the ground-state energy as a function of the wall separation exhibits a global minimum.

We prove the existence of discrete solitons in infinite parity-time ($\cal {PT}$) symmetric lattices by means of analytical continuation from the anticontinuum limit. The energy balance between dissipation and gain implies that in the anticontinuum limit the solitons are constructed from elementary $\cal {PT}$-symmetric blocks such as dimers, quadrimers, or more general oligomers. We consider in detail a chain of coupled dimers, analyze bifurcations of discrete solitons from the anticontinuum limit and show that the solitons are stable in a sufficiently large region of the lattice parameters. The generalization of the approach is illustrated on two examples of networks of quadrimers, for which stable discrete solitons are also found.

The electronic and transport properties of the molecular devices of finite-sized carbon nanotubes (CNTs) have been studied by combining the density-functional theory and Green's function method. Different edge-passivated types are considered for the edge carbon atoms at the two open ends. It is shown that the electronic and transport properties of the finite-size CNTs are sensitive to the passivation types of edge atoms. An asymmetry passivation method which can tune the spin density of states of CNTs has been proposed. The electron transmission can be manipulated by modifying the CNTs, such as edge passivation types and concentrations.

We explore a new type of domain wall structure in ultrathin films with perpendicular anisotropy, that is influenced by the Dzyaloshinskii-Moriya interaction due to the adjacent layers. This study is performed by numerical and analytical micromagnetics. We show that these walls can behave like Néel walls with very high stability, moving in stationary conditions at large velocities under large fields. We discuss the relevance of such walls, that we propose to call Dzyaloshinskii domain walls, for current-driven domain wall motion under the spin Hall effect.

Heterojunctions composed of n-type oxygen-deficient SrTiO_3−δ and p-type GaAs were fabricated using pulsed laser deposition method. The good crystallinity of SrTiO_3−δ was confirmed by X-ray diffraction, reflection high-energy electron diffraction, and transmission electron microscopy. These heterojunctions exhibited excellent rectifying behavior from 40 K to room temperature. The photocarrier injection effect and a large photo-resistance were observed in a wide temperature range. The photo-resistance is nearly 100% at low temperatures and ∼40% at room temperature under −0.5 V bias. Strong dependences on both temperature and bias voltage were found as well, which might be understood by considering the band structure of the formed p-n junction.

We study the electronic properties of GaV₄S₈ (GVS) and GaTa₄Se₈ (GTS), two distant members within the large family of chalcogenides AM₄X₈, with A = {Ga, Ge}, M = {V, Nb, Ta, Mo} and X = {S, Se}. While all these compounds are Mott insulators, their ground states show many types of magnetic order, with GVS being ferromagnetic and GTS non-magnetic. Based on their band structures, calculated with density functional theory methods, we compute an effective tight-binding Hamiltonian in a localised Wannier basis set, for each of the two compounds. The localised orbitals provide a very accurate representation of the band structure, with hopping amplitudes that rapidly decrease with distance. We estimate the superexchange interactions and show that the Coulomb repulsion with Hund's coupling may account the for the different ground states observed in GVS and GTS. Our localised Wannier basis provides a starting point for realistic dynamical mean-field theory studies of strong-correlation effects in this family compounds.

By employing an adaptive time-dependent density matrix renormalization group (t-DMRG) method, we investigate the electron-electron correlation effects on the inelastic-scattering processes of oppositely charged-soliton anti-soliton pairs in conjugated polymers under the influence of an external electric field. The simulations are performed based on the Su-Schrieffer-Heeger (SSH) model modified to include electron-electron interactions and an external electric field. The percent yield of product is numerically calculated by a projection method. Our results show that the on-site Coulomb interaction is of fundamental importance and favors the recombination between the pairs of soliton. Additionally, the effects of external electric field and nearest-neighbor electron-electron interaction on the collision processes are also discussed.

We interpret a recent pioneering experiment (Zgirski M.et al., Phys. Rev. Lett., 106 (2011) 257003) on quasiparticle manipulation in a superconducting break junction in terms of spin blockade drawing analogy with spin qubits. We propose a novel qubit design that exploits the spin state of two trapped quasiparticles. We detail the coherent control of all four spin states by resonant quantum manipulation and compute the corresponding Rabi frequencies. The read-out technique is based on the spin blockade that inhibits quasiparticle recombination in triplet states. We provide extensive microscopic estimations of the parameters of our model.

We report the temperature evolution of coherently excited acoustic and optical phonon dynamics in the superconducting iron pnictide single crystal Ca(Fe_0.944Co_0.056)₂As₂ across the spin density wave transition at T_SDW ∼ 85 K and the superconducting transition at T_SC ∼20 K. The strain pulse propagation model applied to the generation of the acoustic phonons yields the temperature dependence of the optical constants, and longitudinal and transverse sound velocities in the temperature range from 3.1 K to 300 K. The frequency and dephasing times of the phonons show anomalous temperature dependence below T_SC indicating a coupling of these low-energy excitations with the Cooper-pair quasiparticles. A maximum in the amplitude of the acoustic modes at T ∼ 170 is seen, attributed to spin fluctuations and strong spin-lattice coupling before T_SDW.

The upper critical field, H_c2, in quasi-1D superconductors is investigated by the weak coupling renormalization group technique. It is shown that H_c2 greatly exceeds not only the Pauli limit, but also the conventional paramagnetic limit of the Flude-Ferrell-Larkin-Ovchinnikov (FFLO) state. This increase is mainly due to the quasi-1D fluctuations effect as triggered by interference between unconventional superconductivity and density-wave instabilities. Our results give a novel viewpoint on the large H_c2 observed in TMTSF-salts in terms of a d-wave FFLO state that is predicted to be verified by H_c2 measurements under pressure.

We measured the thermopower of LaFe_11.6Si_1.4 and its hydride LaFe_11.6Si_1.4H_y to investigate changes in the electronic structure induced by hydriding. Using a model based on a density-of-states (DOS) function we can accurately describe a non-linear temperature dependence of the thermopower over a wide temperature range. The fit of the model to experimental data yields a significantly broader maximum in the DOS function near the Fermi energy of the hydride as compared to LaFe_11.6Si_1.4. Additionally, a new scattering mechanism leading to a decreased thermopower is observed in LaFe_11.6Si_1.4H_y at low temperatures which is attributed to scattering of electrons on magnetic excitation.

The two-dimensional conducting properties of the Si(111) $\sqrt {3} \times \sqrt {3}$ surface doped by the charge surface transfer mechanism have been calculated in the frame of a semiclassical Drude-Boltzmann model considering donor scattering mechanisms. To perform these calculations, the required values of the carrier effective mass were extracted from reported angle-resolved photoemission results. The calculated doping dependence of the surface conductance reproduces experimental results reported and reveals an intricate metallization process driven by disorder and assisted by interband interactions. The system should behave as an insulator even at relatively low doping due to disorder. However, when doping increases, the system achieves to attenuate the inherent localization effects introduced by disorder and to conduct by percolation. The mechanism found by the system to conduct appears to be connected with the increasing of the carrier effective mass observed with doping, which seems to be caused by interband interactions involving the conducting band and deeper ones. This mass enhancement reduces the donor Bohr radius and, consequently, promotes the screening ability of the donor potential by the electron gas.

We report that the crossover from normal martensite to strain glass in the TiNi-Fe alloy system does not proceed directly as indicated by previous studies but is actually mediated by an intermediate state named crossover martensite, appearing in the crossover composition of Ti₅₀Ni₄₅Fe₅. The martensitic transition from the parent phase to the crossover martensite is a thermodynamic phase transition, being similar to a normal martensitic transition. However, its produced crossover martensite shows an obvious kinetic feature such as a time-dependent behavior in internal friction (aging effect), resembling that of the strain glass. The microstructure of the crossover martensite exhibits a special morphology of coexisting macro-sized martensite plates and nano-sized martensitic domains. The displaying of both thermodynamic and kinetic features in the crossover martensite demonstrates that it is an intermediate state between normal martensite and strain glass; this finding provides a complete physical picture for the crossover phenomenon from normal martensite to strain glass.

Methods from statistical physics, such as those involving complex networks, have been increasingly used in the quantitative analysis of linguistic phenomena. In this paper, we represented pieces of text with different levels of simplification in co-occurrence networks and found that topological regularity correlated negatively with textual complexity. Furthermore, in less complex texts the distance between concepts, represented as nodes, tended to decrease. The complex networks metrics were treated with multivariate pattern recognition techniques, which allowed us to distinguish between original texts and their simplified versions. For each original text, two simplified versions were generated manually with increasing number of simplification operations. As expected, distinction was easier for the strongly simplified versions, where the most relevant metrics were node strength, shortest paths and diversity. Also, the discrimination of complex texts was improved with higher hierarchical network metrics, thus pointing to the usefulness of considering wider contexts around the concepts. Though the accuracy rate in the distinction was not as high as in methods using deep linguistic knowledge, the complex network approach is still useful for a rapid screening of texts whenever assessing complexity is essential to guarantee accessibility to readers with limited reading ability.

Using infrared spectroscopy, we investigate bottom gated ABA-stacked trilayer graphene subject to an additional environment-induced p-type doping. We find that the Slonczewski-Weiss-McClure tight-binding model and the Kubo formula reproduce the gate voltage-modulated reflectivity spectra very accurately. This allows us to determine the charge densities and the potentials of the π-band electrons on all graphene layers separately and to extract the interlayer permittivity due to higher-energy bands.

The gene co-expression networks of many organisms including bacteria, mice and man exhibit scale-free distribution. This heterogeneous distribution of connections decreases the vulnerability of the network to random attacks and thus may confer the genetic replication machinery an intrinsic resilience to such attacks, triggered by changing environmental conditions that the organism may be subject to during evolution. This resilience to random attacks comes at an energetic cost, however, reflected by the lower entropy of the scale-free distribution compared to the more homogenous, random network. In this study we found that the cell cycle-regulated gene expression pattern of the yeast Saccharomyces cerevisiae obeys a power-law distribution with an exponent α = 2.1 and an entropy of 1.58. The latter is very close to the maximal value of 1.65 obtained from linear optimization of the entropy function under the constraint of a constant cost function, determined by the average degree connectivity 〈k〉. We further show that the yeast's gene expression network can achieve scale-free distribution in a process that does not involve growth but rather via re-wiring of the connections between nodes of an ordered network. Our results support the idea of an evolutionary selection, which acts at the level of the protein sequence, and is compatible with the notion of greater biological importance of highly connected nodes in the protein interaction network. Our constrained re-wiring model provides a theoretical framework for a putative thermodynamically driven evolutionary selection process.

The recommender system is a very promising way to address the problem of overabundant information for online users. Although the information filtering for the online commercial systems has received much attention recently, almost all of the previous works are dedicated to design new algorithms and consider the user-item bipartite networks as given and constant information. However, many problems for recommender systems such as the cold-start problem (i.e., low recommendation accuracy for the small-degree items) are actually due to the limitation of the underlying user-item bipartite networks. In this letter, we propose a strategy to enhance the performance of the already existing recommendation algorithms by directly manipulating the user-item bipartite networks, namely adding some virtual connections to the networks. Numerical analyses on two benchmark data sets, MovieLens and Netflix, show that our method can remarkably improves the recommendation performance. Specifically, it not only improves the recommendations accuracy (especially for the small-degree items), but also helps the recommender systems generate more diverse and novel recommendations.