International Journal of Modern Physics B

There are many community organizations in social and biological networks. How to identify these community structure in complex networks has become a hot issue. In this paper, an algorithm to detect community structure of networks is proposed by using spectra of distance modularity matrix. The proposed algorithm focuses on the distance of vertices within communities, rather than the most weakly connected vertex pairs or number of edges between communities. The experimental results show that our method achieves better effectiveness to identify community structure for a variety of real-world networks and computer generated networks with a little more time-consumption.

In addition to the gauge charges, a new charge degree of freedom is identified in the deconfined phase in the lattice Ising gauge theory. While applying to the hydrogen-bonded ferroelectrics, the new charge is essentially the electric charge, leading to the divergent dielectric susceptibility. The new degree of freedom paves an experimentally accessible way to identify the deconfined phase in the lattice Ising gauge theory.

First-principles investigations are employed to provide a fundamental understanding of the structural features, phase stability, mechanical properties, Debye temperature, and hardness of manganese tetraboride. Eight candidate structures of known transition-metal tetraborides are chosen to probe. The calculated lattice parameters, elastic properties, Poisson’s ratio, and $B/G$ ratio are derived. It is observed that the monoclinic structure with $P{2}_1/c$ symmetry (MnB₄–MnB₄) is the most stable in energy. The mechanical and thermodynamic stabilities of seven possible phases are confirmed by the calculated elastic constants and formation enthalpy. Moreover, the analysis on density of states demonstrates semiconducting behavior of MnB₄–MnB₄ and different metallic behaviors of other phases. The estimated hardness of MnB₄–MnB₄ is 38.3 GPa, which is in good agreement with experimental value. Furthermore, the relationship between hardness and Debye temperature is investigated and verifies that MnB₄–MnB₄ is a newly potential semiconducting ultrahard material with high melting point. It provides a new perspective of searching for semiconducting superhard materials to be applied in extreme conditions.

We develop a comprehensive approach of stabilizer codes and provide a scheme generating equientangled basis interpolating between the product basis and maximally entangled basis. The key ingredient is the theory of phase states for finite-dimensional systems (qudits). In this respect, we derive entangled phase states for a multiqudit system whose dynamics is governed by a two-qudit interaction Hamiltonian. We construct the stabilizer codes for this family of entangled phase states. The stabilizer phase states are defined as the common eigenvectors of the stabilizer group generators which are explicitly specified. Furthermore, we construct equally entangled bases from bipartite as well as multipartite entangled qudit phase states.

Ca${}_{1-3x/2}$Bi${}_x$Cu₃Ti₄O${}_{12}$ ($x$ = 0.0, 0.1, 0.2 and 0.3; BCCTO) ceramics were prepared by traditional solid-state sintering method. All samples had pure cubic perovskite-like structure. A drastic grain size reduction was observed with bismuth doping. Dielectric spectra showed two obvious relaxation steps corresponding to two series of peaks in the imaginary part of electric modulus spectra and dielectric loss spectra too. Activation energy fitting by electric modulus spectra reflected different conducting segments in BCCTO ceramics of grains and oxygen vacancies below room temperature. Normalized capacitance dependent of extra bias showed different voltage–capacitance coefficients at special frequencies that suggested multirelaxation mechanisms related with grain and oxygen vacancy. A positive capacitance curvature implies dipolar relaxations in CCTO grains. Whereas a negative curvature suggests oxygen-related relaxations at the interface.

The mixing properties of liquid Al–Au alloys with respect to the concentration of each constituent is determined using a method based on hard sphere system and pseudo-potential perturbation. These models were used to get relevant information on mixing properties of the Al–Au alloys like the Gibbs energy and the entropy of mixing. The concentration fluctuations, chemical short range order for the hard sphere mixture (quasi-lattice theory) and the activity are calculated to know the extent of order in the liquid alloys. The results revealed that there is a degree of ordering in liquid Al–Au alloy (hetero-coordinated).

An ab initio method, based on the plane wave pseudopotential and the generalized gradient approximation (GGA), is performed to investigate the physical properties such as structural, elastic, electronic and bonding properties of newly synthesized Li₂RhSi₃ and predicted Li₂OsSi₃ ternary silicide superconductors for the first time. Both of these compounds are mechanically stable and are brittle in nature. They also have good machinability. Electronic band structures reveal that these compounds have metallic characteristics. They possess complex bonding nature (metallic, covalent and ionic). According to theoretical Vickers hardness, Li₂RhSi₃ is softer than Li₂OsSi₃.

Inspired by the discovery of topological crystalline insulators (TCIs) in three-dimensional materials such as Pb${}_{1-x}$Sn${}_x$Se(Te), the classification of topological insulators has been extended to other discrete symmetry classes such as crystal point group symmetries. In this paper, we construct and study a simple model of weak TCIs, which will serve as a more viable project in the experimental probe for such new type of topological phases.

In this work the finite-difference method (FDM) is presented for the undoped Hg${}_{1-x}$Cd_xTe/HgTe quantum well (QW) which allows to calculate the band structures based on Kane’s 8 × 8 kp model including all second-order terms representing the remote-band contributions. In particular the common central-difference form is employed in the discretization procedure. The FDM is applied in the envelope function approach (EFA) for the [001]-oriented system for two cases: without a magnetic field ($B$ = 0) and with a magnetic field perpendicular to the layer ($B\parallel z$). A proposed presented method can solve all the states simultaneously and can be used for a wide range of temperatures and widths of QW for different values of $x$ for Hg${}_{1-x}$Cd_xTe/HgTe QW as well as for more complex structures, e.g., asymmetric QW, double quantum wells (DQWs), multiple quantum wells (MQWs) and for the so-called 3D topological insulator with a strained HgTe layer. The results obtained by this method are in a complete agreement with the previous ones. Based on that it is shown that the different $x$–Cd compounds in the barrier as well as in the QW make the critical width different than 6.4 nm for HgTe QW. What is also very interesting from the application point of view of the strained mixed HgCdTe QW is that for a different width and a different mismatch lattice it is possible to observe the influence of the upper and lower parts of the Dirac cone in, e.g., a magneto-transport experiment.

In this paper, rapid quenching of Ni from crystal to metallic glass (MG) at different external pressures is simulated by molecular dynamics. The pair distribution functions (PDFs), mean-square displacement, glass transition temperature ($T_g$) and elastic property are calculated and compared with each other. The split of the second PDF peak means the liquid’s transition to glass state starts as previously reported for other MGs. And the $R_i/R_1$ ratio rule is found to hold very well in Ni MG and reveals the SPO structural feature in the configurations. Moreover, with high external pressure, $T_g$ values are more approximated by density–temperature and enthalpy–temperature curves. At last, the elastic modulus and mechanics modulus of quenching models produced a monotonous effect with increasing external pressure and temperature.

Effects of Lévy noise on self-propelled particles in a two-dimensional potential is investigated. The current reversal phenomenon appears in the system. $V$ ($x$-direction average velocity) changes from negative to positive with increasing asymmetry parameter $\beta$, and changes from positive to negative with increasing self-propelled velocity $v_0$. $V$ has a maximum with increasing modulation constant $\lambda$.

The dynamics and the energy spectrum of an ultracold gas of bosonic atoms in an optical lattice can be described by a Bose–Hubbard model for which the system parameters can be controlled by laser light. We study by means of the perturbation theory in addition to the numerical diagonalization, the energy spectrum and the related features of the band structures of the ultracold bosons in optical lattices containing a few number of quanta interacting with next-nearest neighbor interactions (NNNI) modeled by the Bose–Hubbard Hamiltonian. The energy spectra of such system display the bound states signature, which are analyzed in the first Brillouin zone for different wave numbers. The finding, i.e., quantum breathers, shows that their probabilities’ weight depends on the wave vector. The influence of NNNI on both the probabilities’ amplitude and the correlation function is also realized in case of a system with a small number of sites, respectively.

DNA methylation patterns have profound impacts on genome stability, gene expression and development. The molecular base of DNA methylation patterns has long been focused at single CpG sites level. Here, we construct a kinetic model of DNA methylation with collaborations between CpG sites, from which a correlation function was established based on experimental data. The function consists of three parts that suggest three possible sources of the correlation: movement of enzymes along DNA, collaboration between DNA methylation and nucleosome modification, and global enzyme concentrations within a cell. Moreover, the collaboration strength between DNA methylation and nucleosome modification is universal for mouse early embryo cells. The obtained correlation function provides insightful understanding for the mechanisms of inheritance of DNA methylation patterns.

The approach to equilibrium of a quantum mechanical system in interaction with a bath is studied from a practical as well as a conceptual point of view. Explicit memory functions are derived for given models of bath couplings. If the system is a harmonic oscillator representing a molecule in interaction with a reservoir, the generalized master equation derived becomes an extension into the coherent domain of the well-known Montroll–Shuler equation for vibrational relaxation and unimolecular dissociation. A generalization of the Bethe–Teller result regarding energy relaxation is found for short times. The theory has obvious applications to relaxation dynamics at ultra-short times as in observations on the femtosecond time scale and to the investigation of quantum coherence at those short times. While vibrational relaxation in chemical physics is a primary target of the study, another system of interest in condensed matter physics, an electron or hole in a lattice subjected to a strong DC electric field that gives rise to well-known Wannier–Stark ladders, is naturally addressed with the theory. Specific system–bath interactions are explored to obtain explicit details of the dynamics. General phenomenological descriptions of the reservoir are considered rather than specific microscopic realizations.