International Journal of Modern Physics B

Pinning force data, $F_{\mathrm{\text{p}}}$, of a variety of Fe-based high-$T_c$ superconductors (11-, 111-, 122- and 1111-type) were analyzed by means of a scaling approach based on own experimental data and an extensive collection of literature data. The literature data were mostly replotted, but also converted from critical current measurements together with data for the irreversibility line when available from the same authors. Using the scaling approaches of Dew-Hughes [J. Appl. Phys. 44, 1360 (1973)] and Kramer [Philos. Mag. 30, 293 (1974)], we determined the scaling behavior and the best fits to the theory. The data of most experiments analyzed show a good scaling behavior at high temperatures when plotting the normalized pinning force $F_{\mathrm{\text{p}}}/F_{\mathrm{\text{p}},\mathrm{\text{max}}}$ versus the irreversibility field, $H_{\mathrm{\text{irr}}}$. The resulting peak positions, $h_0$, were found at $\approx 0.3$ for the 11-type materials, at $\approx 0.48$ for the 111-type materials, between 0.32 and 0.5 for the 1111-type materials and between 0.25 and 0.71 for the 122-type materials. Compared to the typical results of Bi₂Sr₂CaCu₂O${}_{8+\delta }$ $(h_0\approx 0.22)$ and YBa₂Cu₃O${}_{7-\delta }$ $(h_0\approx 0.33)$, most of the 122 and 1111 samples investigated show peak values higher than 0.4, which is similar to the data obtained on the light-rare earth 123-type HTSC like NdBa₂Cu₃O_y. This high peak position ensures a good performance of the materials in high applied magnetic fields and is, therefore, a very promising result concerning the possible applications of the Fe-based high-$T_c$ superconductors.

The importance of fluoro-perovskites, especially KMgF₃ and NaMgF₃, for the industrial technology is indisputable. The flagrant lack of adequate information on K${}_{0.5}$Na${}_{0.5}$MgF₃ motivated us to undertake a comprehensive study of their behavior under the effect of hydrostatic pressure. The scope of the study includes theoretical prediction in order to analyze and provide more information on the structural, mechanical, electronic and optical properties under the effect of pressure and their mechanical stability using all-electron self-consistent full-potential linearized augmented plane waves plus local orbital (FP-(L)APW$+$lo) method. Structural and elastic properties such as the lattice parameter, bulk modulus, its pressure derivative and the cohesive energy of KMgF₃, K${}_{0.5}$Na${}_{0.5}$MgF₃ and NaMgF₃ as well as the effect of pressure on the values of lattice parameter, elastic constants and their related parameters have been predicted using GGA-PBE and GGA-PBEsol. These functionals have shown great success in the prediction of such properties and the related found results are closer to those available. The modification of the Becke–Johnson potential developed by Tran and Blaha has remedied the problem of underestimation of bandgap calculation by the GGA. This gave us an opportunity to predict bandgaps of these materials using this potential. For KMgF₃, the found result (using this potential) is very close to that of experimental and we firmly believe that our values for K${}_{0.5}$Na${}_{0.5}$MgF₃ and NaMgF₃ (using this same potential) will also be closer once their bandgaps will be calculated. Dielectric functions of KMgF₃, K${}_{0.5}$Na${}_{0.5}$MgF₃ and NaMgF₃ under hydrostatic pressure have been studied with several functionals (GGA-PBE, GGA-PBEsol and GGA-EV).

We study the interaction and merging of optical solitons in normal-dispersion fiber lasers within the framework of the cubic-quintic complex Ginzburg–Landau equation (CGLE). We start finding homogeneous analytic solutions to this equation that are used in numerical simulations to build interacting kinks that lead to the formation of stable solitons. We then study numerically the interaction and merging of these solitons and characterize this process using moments such as the energy and momentum along the fiber. We have found that the merging process occurs only for small enough values of the initial distance (that depends on the phase difference) between solitons, otherwise they repel each other monotonically. The structure of the merging process shows a double peaked energy burst and (depending on the phase difference) a gain and/or loss of momentum. Finally, we propose a fitting model for isolated solitons in order to find a suitable analytical representation for the attractive interaction law before their merging.

An analytic technique for calculating the correlation function and correlation radius of systems belonging to the universality class of the three-dimensional Ising model is elaborated within the collective variables (CV) method. The calculations of these nonuniversal characteristics are performed for the temperatures $T>T_{\mathrm{\text{c}}}$, $T<T_{\mathrm{\text{c}}}$ and $T=T_{\mathrm{\text{c}}}$ ($T_{\mathrm{\text{c}}}$ is the phase transition temperature) on the microscopic level in the approximation of the quartic distribution for modes of spin moment density oscillations (the ${\rho }^4$ model). Proceeding from the expression for the Fourier transform of the correlation function, we have also obtained the susceptibility of the system. The correction to scaling and the potential averaging correction are taken into account in our calculations.

In this paper, we study the magnetic properties of a tri-decorated $(\sigma =1/2,S=1,q=3/2)$ graphene structure using Monte Carlo simulations (MCS). Indeed, we first elaborate the ground state phase diagrams and then, we found that from $2\times 3\times 4=24$ phases, the only stable configurations are: $(\sigma ,S,q)=(-1/2,+1,-3/2)$, $(+1/2,-1,+3/2)$, $(-1/2,-1,-3/2)$, $(+1/2,+1,+3/2)$, $(-1/2,-1,+3/2)$, $(+1/2,+1,-3/2)$, $(-1/2,+1,+3/2)$, $(+1/2,-1,-3/2)$, $(+1/2,0,+3/2)$, $(-1/2,0,-3/2)$, $(+1/2,0,+1/2)$ and $(-1/2,0,-1/2)$. For low reduced temperature values, the partial magnetizations are found to be in good agreement with the corresponding ground state phase diagrams. The corresponding partial susceptibilities show a notable peak around the reduced temperature value 2.0 in the absence of the external magnetic field $(h/J_{\sigma S}=0.0)$ and crystal field $(\mathrm{\Delta }/J_{\sigma S}=0.0)$. To complete this study, we present and discuss the magnetic hysteresis loops.

We study the indirect exchange interaction between two localized magnetic moments, known as Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction, in a one-dimensional (1D) spin-polarized electron gas. We find explicit expressions for each term of this interaction, study their oscillatory behaviors as a function of the distance between two magnetic moments, $R$, and compare them with the known results for RKKY interaction in the case of 1D standard electron gas. We show this interaction can be written in an anisotropic Heisenberg form, $E(R)={\lambda }^2{\chi }_{xx}(S_{1x}{S}_{2x}+S_{1y}{S}_{2y})+{\lambda }^2{\chi }_{zz}{S}_{1z}{S}_{2z}$, coming from broken time-reversal symmetry of the host material.

Motivated by recent developments in the field of 1D topological superconductors (TSCs), we investigate the phase transition properties of p-wave superconductor with 50 sites. In this paper, we first map the p-wave fermion model into the transverse $XY$ model with an incommensurate modulated transverse field. We study the phase transition from TSC phase to Anderson localization phase by using imaginary time-evolving block decimation (TEBD) algorithm. By calculating the correlation function $C_{ij}=〈{\sigma }_i^x{\sigma }_j^x〉$, we numerically calculate the average correlation function in different p-wave pairing amplitude $\mathrm{\Delta }$. By plotting the average correlation function versus the strength of incommensurate $V$ for $\mathrm{\Delta }=0.5$ and $\mathrm{\Delta }=1.5$, we identify the phase transition from TSC phase to Anderson localization phase [X. M. Cai et al., Phys. Rev. Lett. 110, 176403 (2013)]. Last but not least, we give an average measure of the entanglement of a single site with the rest of the system and von Neumann entropy of two boundary sites for the same parameters showing that the system possesses very strong entanglement at the critical point. The finite size effect and Majorana fermions are discussed in this paper.

First-principles calculations have been used to comparatively investigate electronic and magnetic properties of nitrogen-doped (N-doped) nonmagnetic semiconductor perovskite-type stannate (MSnO₃, M = Ca, Sr, Ba). A total magnetic moment of 1.0 ${\mu }_{\mathrm{\text{B}}}$ induced by N is found in MSnO₃ supercell with one N dopant. The spontaneous polarization mainly originates from spin splitting on $2p$ state of N. The medium-sized formation energy shows that the N-doped MSnO₃ can be realized experimentally under the metal-rich environments, but the clustering tendency and short-range coupling imply that the stannate matrices are unsuitable for magnetizing by substituting N for O. Our study offers a fresh sight of spontaneous spin polarization in $d^0$ magnetism. The FM coupling in N-doped MSnO₃ should be attributed to the hole-mediated $p$–$p$ coupling mechanism.

The paper uses empirical data, including 42 globally main stock indices in the period 1996–2014, to systematically study the evolution of fluctuation modes and inner structures of global stock markets. The data are large in scale considering both time and space. A covariance matrix-based principle fluctuation mode analysis (PFMA) is used to explore the properties of the global stock markets. It has been ignored by previous studies that covariance matrix is more suitable than the correlation matrix to be the basis of PFMA. It is found that the principle fluctuation modes of global stock markets are in the same directions, and global stock markets are divided into three clusters, which are found to be closely related to the countries’ locations with exceptions of China, Russia and Czech Republic. A time-stable correlation network constructing method is proposed to solve the problem of high-level statistical uncertainty when the estimated periods are very short, and the complex dynamic network (CDN) is constructed to investigate the evolution of inner structures. The results show when the clusters emerge and how long the clusters exist. When the 2008 financial crisis broke out, the indices form one cluster. After these crises, only the European cluster still exists. These findings complement the previous studies, and can help investors and regulators to understand the global stock markets.

I propose a model of a material that exhibits negative thermal expansion (NTE) properties and criteria for the occurrence of linear and volumetric NTE. I derived the criteria for an arbitrary force between rigid units in the material. These criteria are also discussed specifically for the Lennard–Jones (6–12) potential and in more detail for metal–organic framework (MOF) materials comprising rigid units connected by organic linkers. Qualitatively, the model predictions can explain some observed results. Surprisingly, the model can produce equations for the transition temperature from NTE to positive thermal expansion (PTE), $T_c\approx T_0-50$ K, which is exactly the same as the temperature at which the glass transition begins to occur in most polymers, i.e., $T_c\approx T_g-50$ K.