International Journal of Modern Physics B

Superconductivity in a novel class of layered materials, Ti-based pnictide oxides, was recently discovered. These compounds have attracted interest since they combine features of copper oxide and iron pnictide superconductors. Here the transition metal (titanium) forms two-dimensional Ti₂O layers (anti structure to the CuO₂ planes), capped by pnictogen ions (similar to Fe₂As₂ layers). The pnictide oxide compounds show a spin or charge density wave phase which coexists with superconductivity in some members of the family. Unlike the cuprates, but similar to iron pnictides, the parent compounds of pnictide oxides are metals with specific nesting properties of the Fermi surface which leads to the density wave instability. The nature of the superconductivity, coexisting with the density wave order, and the possible competition or mutual interaction between both states is one of the central questions of recent studies. This short review summarizes the current knowledge from an experimental as well as theoretical point of view and discusses some of the open questions and possible future developments.

We put forward the photon-added squeezing-enhanced thermal states (PASETS) theoretically by adding photon to the squeezed enhancing thermal states (SETS) repeatedly. Based on the normally ordered density operator of PASETS, we investigate the nonclassical behavior of the PASETS by evaluating, both analytically and numerically, Mandel's Q-parameter, photon-number distribution (PND), and Wigner function (WF). It is found that smaller squeezing parameter r and thermal photon number n_c can lead to more chance of the appearance of sub-Poissonian statistics. And it is shown that the PND of PASETS exhibit more remarkable oscillations than that of SETS in stronger squeezing case. The WF exhibit partial negativity in phase space and the squeezing parameter r can result in both squeezing and rotating effect. By investigating the fidelity between PASETS and SETS shows that the fidelity tender to steady values in the high value of squeezing parameter or thermal photon number. In addition, the decoherence effect on the PASETS is examined by the time-evolution of the analytical WF in thermal channel. The results show that the PASETS shall lose nonclassicality and non-Gaussianity and reduce to classical states with Gaussian distribution after sufficient time interaction with the thermal noise. And larger photon-added number or thermal photon number shall render shorter decoherence time.

In this paper, multistability is discussed for delayed recurrent neural networks with ring structure and multi-step piecewise linear activation functions. Sufficient criteria are obtained to check the existence of multiple equilibria. A lemma is proposed to explore the number and the cross-direction of purely imaginary roots for the characteristic equation, which corresponds to the neural network model. Stability of all of equilibria is investigated. The work improves and extends the existing stability results in the literature. Finally, two examples are given to illustrate the effectiveness of the obtained results.

In this paper, we report a density functional study of the structural, electronic and pressure-induced solid–solid phase transitions of SrTiO₃. These first-principles calculations have been performed using the full potential linearized augmented plane wave method (FP-LAPW) within the generalized gradient approximation (GGA) developed by Perdew–Burke–Ernzerhor for solids (PBEsol). The calculated structural parameters like the lattice parameters, the bulk modulus B and their pressure derivative B′ are used to analyze the relative stability and phase transitions under pressure of SrTiO₃. Calculations were done for the cubic (Pm-3m), tetragonal (I4/mcm, P4/mbm, P4mm) and orthorhombic (Cmcm, Pnma) structures where we found that the tetragonal I4/mcm phase is the most stable structure compared to the other structures at T = 0 K and P = 0 GPa. For the electronic properties calculations, the exchange and correlation effects were treated by the Tran–Blaha modified Becke–Johnson (TB-mBJ) potential to prevent the shortcoming of the underestimation of the energy gaps in both LDA and GGA approximations. The obtained results are compared to available experimental data and to other theoretical calculations.

Achieving phonon–atom couplings in cavity optomechanical system could lead to fundamentally new regimes of phonon–matter interaction and the development of sensitive mass and force sensors. Here we derive a master equation to describe the phonon–atom system and numerically simulate the equation. The phonon–atom coupling is mediated by a driven cavity mode. A crossover from the bad cavity limit, which validates the master equation, to the limit of small cavity loss rate is studied.

A comprehensive study of structural, morphological, hydrogen absorption and magnetic properties of MmNi_4.22 Co_0.48Mn_0.15Al_0.15 alloy as a promising hydrogen storage media was investigated. The X-ray diffraction (XRD) profiles show that the alloy maintains its crystal structure (hexagonal LaNi₅-type) even after 30 hydrogenation/dehydrogenation (H/D) cycles. However, the XRD peaks are found to be slightly broadened after cycling. SEM images reveal that particles size of the cycled sample decreases, with more uniform particle size distribution compared to noncycled ones. The pressure-composition (PC) isotherms and kinetics curves of hydrogen absorption reaction were obtained at different working temperatures by using a homemade Sievert apparatus. The enthalpy and entropy of hydride formation of the alloy were evaluated. Furthermore, the Jander diffusion and Johnson–Mehl–Avrami models as the fitting models were employed to study the kinetic mechanism of hydriding reaction and its activation energy. The room temperature magnetic measurements indicate that the milling and H/D cycling change the magnetic properties of the as-annealed alloy.

Community detection is a fundamental work to analyze the structural and functional properties of complex networks. There are many algorithms proposed to find the optimal communities of network. In this paper, we focus on how vertex order influences the results of community detection. By using consensus clustering, we discover communities and get a consensus matrix under different vertex orders. Based on the consensus matrix, we study the phenomenon that some nodes are always allocated in the same community even with different vertex permutations. We call this group of nodes as constant community and propose a constant community detection algorithm (CCDA) to find constant communities in network. We also further study the internal properties of constant communities and find constant communities play a guiding role in community detection. Finally, a discussion of constant communities is given in the hope of being useful to others working in this field.

Li⁺ and Er³⁺ codoped Y₂O₃ powders are synthesized via sol–gel method. The effects of Li⁺ doping on the optical properties of Er³⁺: Y₂O₃ are investigated under 980 nm excitation. The emission intensity of Er³⁺ is obviously improved after Li⁺ doping. Moreover, the quenching concentration of ²H_11/2/⁴S_3/2 level of Er³⁺ is 3.0 mol% for the samples without Li⁺ but it decreases to 2.0 mol% after introducing 5.0 mol% Li⁺. This is because Li⁺ doping shrinks the host lattice and decreases the Er³⁺–Er³⁺ distance, which contribute to the formation of Er³⁺ clusters and thus induces the quenching concentration of ²H_11/2/⁴S_3/2 level to decrease.

The quantum vacuum fluctuation i.e., Casimir attraction can induce mechanical instability in ultra-small devices. Previous researchers have focused on investigating the instability in structures with planar or rectangular cross-section. However, to the best knowledge of the authors, no attention has been paid for modeling this phenomenon in the structures made of nanowires with cylindrical geometry. In this regard, present work is dedicated to simulate the Casimir force-induced instability of freestanding nanoactuator and nanotweezers made of conductive nanowires with circular cross-section. To compute the quantum vacuum fluctuations, two approaches i.e., the proximity force approximation (for small separations) and scattering theory approximation (for large separations), are considered. The Euler-beam model is employed, in conjunction with the size-dependent modified couple stress continuum theory, to derive governing equations of the nanostructures. The governing nonlinear equations are solved via three different approaches, i.e., using lumped parameter model, modified variation iteration method (MVIM) and numerical solution. The deflection of the nanowire from zero to the final stable position is simulated as the Casimir force is increased from zero to its critical value. The detachment length and minimum gap, which prevent the instability, are computed for both nanosystems.