Fractal structure of the crystalline-nuclei boundaries in 2D colloidal crystallization: Computer simulations

By performing 2D kinetic Monte Carlo simulations of colloidal crystallization we found that the boundaries of the crystalline nuclei are not only rough, as obtained by experimentalists, but fractal, whose value (_df$d_f$) we calculated. The corresponding boundary for the crystals, above the critical size (_Nc$N_c$), is also fractal but smoother. A knowledge of the particles coordinates during the crystallization process allows us to calculate the _Nc$N_c$, the line tension (γ) and the chemical potential difference (Δμ) between the two phases. However, different from the experimentalists procedure, we found that the boundary fractalities are needed to derive γ and Δμ.     

Enhanced suppression of superconductivity in amorphous films with nano-scale patterning

We have measured the thickness dependence of the superconducting critical temperature, T_c(d_Bi)$T_c(d_{\text{Bi}})$, in amorphous Bi/Sb films patterned with a regular array of holes as well as nanoscale thickness variations. We find that the mean field T_c$T_c$ is suppressed relative to simultaneously produced unstructured films of the same thickness. Surprisingly, however, the functional form for T_c(d_Bi)$T_c(d_{\text{Bi}})$, remains unaffected. The role of the thickness variations in suppressing T_c$T_c$ is compared to the role of the holes, through parameterization of the surface, as measured through AFM/SEM and a proximity effect calculation. These results suggest that these two nanoscale modifications suppress T_c$T_c$ about equally and are consistent with T_c$T_c$ being determined on a microscopic length scale.     

Microwave performance enhancement in Double and Single Gate HEMT with channel thickness variation

In Single Gate HEMT (SGHEMT) shortening of gate length (_Lg)$\left(L_g\right)$ below 100 nm leads to reduction in Transconductance (_gm)$\left(g_m\right)$, which reduces the unloaded voltage gain (_gm/_gd)$(g_m/g_d)$ of the device, thereby reducing the maximum frequency of oscillation (_fmax)$\left(f_{max}\right)$. The main reason for this reduction in _gm$g_m$ with _Lg$L_g$ in the Single Gate HEMT (SGHEMT) is its inability to maintain the desired channel aspect ratio (α$\alpha$). At such a miniaturization level, α$\alpha$ not only depends on the channel depth (d)$\left(d\right)$ but also on the channel thickness (_dc)$\left(d_c\right)$ of the device [5]. Moreover, the variation of _dc$d_c$ may switch the device characteristics from quantum regime to classical regime  and . The Double Gate HEMT (DGHEMT)  and  has emerged as a solution for further reduction in _Lg$L_g$ and provides enhancements over SGHEMT by virtue of its double gate and also for same _dc$d_c$ due to double heterojunctions, which virtually increases the value of α$\alpha$. In the present work, extensive simulation work has been carried out using ATLAS device simulator [35] in order to study the effect of _dc$d_c$ and _Lg$L_g$ on DGHEMT and SGHEMT. An analytical model has also been proposed for SGHEMT and DGHEMT to incorporate the effect of variation of _dc$d_c$ and _Lg$L_g$.     

Shot noise in HTc superconductor quantum point contact system

We study the electrical transport properties of a quantum point contact between a lead and a high Tc superconductor. For this, we use the Hamiltonian approach and non-equilibrium Green functions of the system. The electrical current and the shot noise are calculated with this formalism. We consider d_x²−y²${\mathrm{d}}_{x^2-y^2}$, d_xy${\mathrm{d}}_{\mathit{xy}}$, d_x²−y²+is${\mathrm{d}}_{x^2-y^2}+\mathrm{is}$ and d_xy+is${\mathrm{d}}_{\mathit{xy}}+\mathrm{is}$ symmetries for the pair potential. Also we explore the s₊₋${\mathrm{s}}_{+-}$ and s₊₊${\mathrm{s}}_{++}$ symmetries describing the behavior of the ferropnictides superconductors. We found that for d_xy${\mathrm{d}}_{\mathit{xy}}$ symmetry there is not a zero bias conductance peak and for d+is$\mathrm{d}+\mathrm{is}$ symmetries there is a displacement of the transport properties. From shot noise and current, the Fano factor is calculated and we found that it takes values of effective charge between e and 2e  , this is explained by the diffraction of quasiparticles in the contact. For the s₊₋${\mathrm{s}}_{+-}$ and s₊₊${\mathrm{s}}_{++}$ symmetries the results show that the electrical current and the shot noise depend on the mixing coefficient, furthermore, the effective electric charge can take values between 0 and 2e, in contrast with the results obtained for s wave superconductors.     

Study of magnetization process in ordered Fe nanowire arrays

Fe polycrystalline nanowires were electrodeposited in nanoporous anodized alumina membranes. Their magnetic properties were studied at 10 K. The behavior of the isothermal remanence _Mr$M_{\mathrm{r}}$ and the demagnetization remanence _Md$M_{\mathrm{d}}$ was determined. The corresponding ΔM$\mathrm{\Delta }M$ plot revealed the character demagnetizing of the dominant interactions. A simple analysis suggests that curling is the reversal magnetization mode in these samples. The forms of the _Mrev(_Mirr)Hi$M_{\mathrm{rev}}(M_{\mathrm{irr}}{)}_{H_{\mathrm{i}}}$ curves were different from those encountered until now, and it seems that these forms are associated with the curling reversal mode.     

Phase diagrams of a spin-1 Ising superlattice

The three-dimensional spin-1 Ising superlattice consisting of two different ferromagnetic materials with two different crystal fields _Δ1${\Delta }_1$ and _Δ2${\Delta }_2$ is considered in the mean field approximation. The phase diagrams are considered in the (t,_d2$t,d_2$) plane for different ranges of variation of _d1(t=T/J,_d1=_Δ1/J$d_1(t=T/J,d_1={\Delta }_1/J$, _d2=_Δ2/J$d_2={\Delta }_2/J$ are the reduced temperature and crystal fields respectively). The phase diagrams exhibit a variety of multicritical points and reentrant and double reentrant behaviours. They are found to depend qualitatively and/or quantitatively on the thicknesses of the materials in a supercell. This has direct consequences on the nature of the magnetic states of superlattices with different thicknesses.     

Fractal structure of equipotential curves on a continuum percolation model

We numerically investigate the electric potential distribution over a two-dimensional continuum percolation model between the electrodes. The model consists of overlapped conductive particles on the background with an infinitesimal conductivity. Using the finite difference method, we solve the generalized Laplace equation and show that in the potential distribution, there appear quasi-equipotential clusters   which approximately and locally have the same values as steps and stairs. Since the quasi-equipotential clusters have the fractal structure, we compute the fractal dimension of equipotential curves and its dependence on the volume fraction over [0,1]$[0,1]$. The fractal dimension in [1.00, 1.246] has a peak at the percolation threshold _pc$p_c$.     

Giant spin–orbit splitting in ultrathin Ir(111) film

Spin–orbit (SO) splitting in ultrathin Ir(111) film was examined on the basis of density functional theory. The states with giant SO splitting can be found in few-monolayer Ir film terminated by H atoms. They distribute in several bands. From projected wave function study, the giant SO splitting is mainly induced by the _pz$p_z$ and _dyz$d_{yz}$ states. The origin of this orbital dependence is analyzed by the distribution of states on the film. The stability of states is also discussed when the Ir surface is covered with graphene or graphone (half-hydrogenated graphene). The hybridization of energy bands greatly influences the large SO splitting and the persistence of states.     

Diffusion-limited aggregates grown on nonuniform substrates

In the present paper, patterns of diffusion-limited aggregation (DLA) grown on nonuniform substrates are investigated by means of Monte Carlo simulations. We consider a nonuniform substrate as the largest percolation cluster of dropped particles with different structures and forms that occupy more than a single site on the lattice. The aggregates are grown on such clusters, in the range the concentration, p$p$, from the percolation threshold, _pc$p_c$ up to the jamming coverage, _pj$p_j$. At the percolation threshold, the aggregates are asymmetrical and the branches are relatively few. However, for larger values of p$p$, the patterns change gradually to a pure DLA. Tiny qualitative differences in this behavior are observed for different k$k$ sizes. Correspondingly, the fractal dimension of the aggregates increases as p$p$ raises in the same range _pc≤p≤_pj$p_c\le p\le p_j$. This behavior is analyzed and discussed in the framework of the existing theoretical approaches.     

Exploring self-similarity of complex cellular networks: The edge-covering method with simulated annealing and log-periodic sampling

Song et al. [Self-similarity of complex networks, Nature 433 (2005) 392–395] have recently used a version of the box-counting method, called the node-covering method, to quantify the self-similar properties of 43 cellular networks: the minimal number _NV$N_V$ of boxes of size ?$?$ needed to cover all the nodes of a cellular network was found to scale as the power-law _NV∼(?+1)^-_DV$N_V\sim (?+1{)}^{-D_V}$ with a fractal dimension _DV=3.53±0.26$D_V=3.53\pm 0.26$. We implement an alternative box-counting method in terms of the minimum number _NE$N_E$ of edge-covering boxes which is well-suited to cellular networks, where the search over different covering sets is performed with the simulated annealing algorithm. The method also takes into account a possible discrete scale symmetry to optimize the sampling rate and minimize possible biases in the estimation of the fractal dimension. With this methodology, we find that _NE$N_E$ scales with respect to ?$?$ as a power-law _NE∼?^-_DE$N_E\sim {?}^{-D_E}$ with _DE=2.67±0.15$D_E=2.67\pm 0.15$ for the 43 cellular networks previously analyzed by Song et al. [Self-similarity of complex networks, Nature 433 (2005) 392–395]. Bootstrap tests suggest that the analyzed cellular networks may have a significant log-periodicity qualifying a discrete hierarchy with a scaling ratio close to 2.     

Specific heat of magnetic and semiconductor quasiperiodic structures

We have performed a theoretical study of the specific heat C(T)$C(T)$, as a function of temperature, of magnetic and semiconductor quasiperiodic structures. The quasiperiodic structures considered here are constructed according to the Fibonacci, double-period and Thue–Morse quasiperiodic sequences. On one hand, we assume the magnetic structures composed of ferromagnetic films, each one described by the Heisenberg model. On the other hand, we consider semiconductor structures composed of slabs of AlN and GaN, which are characterized by the dielectric functions _εA(ω)${\epsilon }_A(\omega )$ and _εB(ω)${\epsilon }_B(\omega )$, and have thicknesses _da$d_a$ and _db$d_b$, respectively. Our results illustrate the effects of disorder on the oscillatory behavior of the specific heat in the low temperature regime.     

Strange attractor in the Potts spin glass on hierarchical lattices

The spin-glass q-state Potts model on d  -dimensional diamond hierarchical lattices is investigated by an exact real space renormalization group scheme. Above a critical dimension _dl(q)$d_l(q)$ for q>2$q>2$, the coupling constants probability distribution flows to a low-temperature strange attractor   or to the high-temperature paramagnetic fixed point, according to the temperature is below or above the critical temperature _Tc(q,d)$T_c(q,d)$. The strange attractor was investigated considering four initial different distributions for q=3$q=3$ and d=5$d=5$ presenting strong robustness in shape and temperature interval suggesting a condensed phase with algebraic decay.     

Quantum spherical model with competing interactions

We analyse the phase diagram of a quantum mean spherical model in terms of the temperature T$T$, a quantum parameter g$g$, and the ratio p=−_J2/_J1$p=-J_2/J_1$, where _J1>0$J_1>0$ refers to ferromagnetic interactions between first-neighbour sites along the d$d$ directions of a hypercubic lattice, and _J2<0$J_2<0$ is associated with competing antiferromagnetic interactions between second neighbours along m≤d$m\le d$ directions. We regain a number of known results for the classical version of this model, including the topology of the critical line in the g=0$g=0$ space, with a Lifshitz point at p=1/4$p=1/4$, for d>2$d>2$, and closed-form expressions for the decay of the pair correlations in one dimension. In the T=0$T=0$ phase diagram, there is a critical border, _gc=_gc(p)$g_c=g_c\left(p\right)$ for d≥2$d\ge 2$, with a singularity at the Lifshitz point if d<(m+4)/2$d<(m+4)/2$. We also establish upper and lower critical dimensions, and analyse the quantum critical behavior in the neighborhood of p=1/4$p=1/4$.     

Mutually unbiased projectors and duality between lines and bases in finite quantum systems

Quantum systems with variables in the ring Z(d)$\mathbb{Z}\left(d\right)$ are considered, and the concepts of weak mutually unbiased bases and mutually unbiased projectors are discussed. The lines through the origin in the Z(d)×Z(d)$\mathbb{Z}\left(d\right)\times \mathbb{Z}\left(d\right)$ phase space, are classified into maximal lines (sets of d$d$ points), and sublines (sets of d_i$d_i$ points where d_i|d$d_i|d$). The sublines are intersections of maximal lines. It is shown that there exists a duality between the properties of lines (resp., sublines), and the properties of weak mutually unbiased bases (resp., mutually unbiased projectors).     

The mechanism of anisotropic exchange interaction in superconducting iron arsenides

Using a combination of linear response theory and constrained orbital hybridization approach, we study the mechanism of magnetic exchange interaction of iron-based superconductor. We reproduce the observed highly anisotropic exchange interaction, and our constrained orbital calculation unambiguously identifies that the anisotropic feature of exchange interaction is not sensitive to the unequal _dxz/_dyz$d_{xz}/d_{yz}$ orbital population.     

Dust ion-acoustic rogue waves in a three-species ultracold quantum dusty plasmas

Dust ion-acoustic (DIA) rogue waves are reported for a three-component ultracold quantum dusty plasma comprised of inertialess electrons, inertial ions, and negatively charged immobile dust particles. The nonlinear Schrödinger (NLS) equation appears for the low frequency limit. Modulation instability (MI) of the DIA waves is analyzed. Influence of the modulation wave number, ion-to-electron Fermi temperature ratio ρ$\rho$ and dust-to-ion background density ratio N_d$N_d$ on the MI growth rate is discussed. The first- and second-order DIA rogue-wave solutions of the NLS equation are examined numerically. It is found that the enhancement of N_d$N_d$ and carrier wave number can increase the envelope rogue-wave amplitudes. However, the increase of ρ$\rho$ reduces the envelope rogue-wave amplitudes.     

Strong contribution to octet baryon mass splittings

We calculate the _md−_mu$m_d-m_u$ contribution to the mass splittings in baryonic isospin multiplets using SU(3) chiral perturbation theory and lattice QCD. Fitting isospin-averaged perturbation theory functions to PACS-CS and QCDSF-UKQCD Collaboration lattice simulations of octet baryon masses, and using the physical light-quark mass ratio _mu/_md$m_u/m_d$ as input, allows _Mn−_Mp$M_n-M_p$, _MΣ−−_MΣ+$M_{{\Sigma }^{-}}-M_{{\Sigma }^{+}}$ and _MΞ−−_MΞ0$M_{{\Xi }^{-}}-M_{{\Xi }^0}$ to be evaluated from the full SU(3) theory. The resulting values for each mass splitting are consistent with the experimental values after allowing for electromagnetic corrections. In the case of the nucleon, we find _Mn−_Mp=2.9±0.4 MeV$M_n-M_p=2.9\pm 0.4\text{MeV}$, with the dominant uncertainty arising from the error in _mu/_md$m_u/m_d$.