Narrow Results

Using the double-time Green’s function method in combination with the random phase approximation and Anderson–Callen approximation, we investigate the magnetic properties of the ferrimagnetic Heisenberg model on a decorated square lattice. The effects of nearest-neighbor interactions and single-ion anisotropy on the compensation temperature are explored. Our results indicate that the next-nearest-neighbor interaction and/or single-ion anisotropy are not a necessary condition of the appearance of compensation temperature. When considering only the nearest-neighbor interaction, the system can also exhibit a compensation temperature. By comparing the impact of the nearest-neighbor interaction and single-ion anisotropy on the sublattice magnetizations of the system, a general condition of the appearance of compensation temperature is claimed.

Using the Monte Carlo Simulation (MCS) method combined with the Metropolis algorithm, we explore the magnetic properties of a two-dimensional (2D) spin system with spins 1, 2 and 3. The adapted simulation to the Ising model aims to study the impact of the correction term a of the interaction with second neighbors and the crystal field D on the critical temperature $T_c$ of the 2D anisotropic square lattice for different integer values of spins. The obtained results demonstrate that the critical temperature $T_c$ is significantly affected by the competing interactions between the anisotropy parameter, second-neighbor interaction and the crystal field. The considered model exhibits a critical phase transition regardless of the spin S value where the critical temperature increases with the value of S. Note that this phase transition occurs under the condition $D\ge -2$. Moreover, the obtained results show that the complex interactions give rise to a variety of phase diagrams, thereby revealing the richness of possible magnetic behaviors in the studied system.

We study translation-invariant quasi-free states for a system of fermions with two-particle interactions. The associated energy functional is similar to the BCS functional but also includes direct and exchange energies. We show that for suitable short-range interactions, these latter terms only lead to a renormalization of the chemical potential, with the usual properties of the BCS functional left unchanged. Our analysis thus represents a rigorous justification of part of the BCS approximation. We give bounds on the critical temperature below which the system displays superfluidity.

It is a remarkable property of BCS theory that the ratio of the energy gap at zero temperature $\mathrm{\Xi }$ and the critical temperature $T_c$ is (approximately) given by a universal constant, independent of the microscopic details of the fermionic interaction. This universality has rigorously been proven quite recently in three spatial dimensions and three different limiting regimes: weak coupling, low density and high density. The goal of this short note is to extend the universal behavior to lower dimensions $d=1,2$ and give an exemplary proof in the weak coupling limit.

Galam reshuffling introduced in opinion dynamics models, is investigated under the nearest neighbor Ising model on a square lattice using Monte Carlo simulations. While the corresponding Galam analytical critical temperature T_C≈3.09 [J/k_B] is recovered almost exactly, it is proved to be different from both values, not reshuffled (T_C =2/arcsinh (1)≈2.27 [J/k_B]) and mean-field (T_C =4 [J/k_B]). On this basis, gradual reshuffling is studied as function of 0≤p≤1 where p measures the probability of spin reshuffling after each Monte Carlo step. The variation of T_C as function of p is obtained and exhibits a nonlinear behavior. The simplest Solomon network realization is noted to reproduce Galam p =1 result. Similarly to the critical temperature, critical exponents are found to differ from both, the classical Ising case and the mean field values.

We have analyzed low and high temperature series expansions for the three-dimensional Ising model on the simple cubic lattice. Our analysis of Butera and Comi's new 21-term high temperature series yields and from the 32-term low temperature series of Vohwinkel we find K_c=0.22167±0.00002, consistent with the high temperature series but with larger error bars. We discuss the reasons for the larger error bars on the low temperature side and compare these values with estimates from other series analyses and from simulations.

We discuss the asymptotic properties of quantum states density for fundamental p-branes which can yield a microscopic interpretation of the thermodynamic quantities in M-theory. The matching of the BPS part of spectrum for superstring and supermembrane gives the possibility of getting membrane's results via string calculations. In the weak coupling limit of M-theory, the critical behavior coincides with the first-order phase transition in the standard string theory at temperature less than the Hagedorn's temperature T_H. The critical temperature at large coupling constant is computed by considering M-theory on manifold with topology . Alternatively we argue that any finite temperature can be introduced in the framework of membrane thermodynamics.

Experimental evidences of a first-order phase transition from a superfluid to a non-interacting Fermi gas system are studied for ${}^{97,98}$Mo isotopes. The experimental observations are compared with the semi-empirical macroscopic model and superfluid formalism. We have shown that the entropy excess ratio introduced in our previous publications within the superfluid model can describe the first-order phase transition due to pair-breaking in atomic nuclei.

We study the dynamics of first-order confinement-deconfinement phase transition through nucleation of hadronic bubbles in an expanding quark–gluon plasma in the context of heavy ion collisions for interacting quark and hadron gas and by incorporating the effects of curvature energy. We find that the interactions reduce the delay in the phase transition whereas the curvature energy has a mixed behavior. In contrast to the case of early Universe phase transition, here lower values of surface tension increase the supercooling and slow down the hadronization process. Higher values of bag pressure tend to speed up the transition. Another interesting feature is the start of the hadronization process as soon as the QGP is created.

Holographic s-wave superconductors in the framework of nonlinear Born–Infeld-type electrodynamics are investigated in the background of Schwarzschild anti-de Sitter black holes. As particular cases, at some model parameters, we obtain results for Born–Infeld and exponential electrodynamics. We explore the analytical Sturm–Liouville eigenvalue problem in the probe limit where the scalar and electromagnetic fields do not affect the background metric. The critical temperatures of phase transitions and the order parameter are calculated which depend on the model parameters. We show that the critical exponent near the critical temperature is 1/2. Making use of the matching method, we derive analytical expressions for the condensation values and the critical temperature. The conductivity by the analytical method is calculated.

A discussion is made on the appearance of strong effective magnetic field with a new quantum state induced by the zero-point plasma oscillation in semi-localized half-filling electron system in the 2D network of circular molecular orbits. With the help of numerical calculation we study the appearance of superconductivity in the Cu and Ru oxide superconductor employing the field-induced quantum state and a gauge-field pairing model, and succeed in giving a unified explanation to the two apparently different types of oxide superconductivity. It is found that in the Cu oxide superconductor singlet pairing of d_x²-y² symmetry occurs in each CuO₂ layer at the decoupling of the zero-spin-sum singlet FQHE state by small carrier doping, while in the Ru oxide superconductor triplet electron pairing is caused by two neighboring RuO₂ layers at the decoupling of the spin-polarized FQHE Ψ_1,1,1 state where the pairs have d_3z²-r² symmetry with horizontal node on the Fermi surface. Numerical calculation gives critical temperature consistent with experimental observation.

The behavior of the compensation of a mixed spin-1 and spin-1/2 Heisenberg ferrimagnetic system on a square lattice is investigated theoretically by a two-time Green-function technique which takes into account the quantum nature of Heisenberg spins. The model includes the nearest and next-nearest neighbor interactions between spins, a crystal field and an external magnetic field. This model can be relevant for understanding the magnetic behavior of bimetallic molecular ferrimagnets that are currently being synthesized by several experimental groups. We study the spin-wave spectra of the ground state and investigate the effects of the next-nearest neighbor interactions, a crystal field and an external magnetic field on the compensation temperature. It is found that a compensation point appears and it is basically unchanged when the next-nearest-neighbor interaction between spin-1/2 is taken into account and exceeds a minimum value for other values in Hamiltonian fixed. The compensation temperature is influenced by the next-nearest-neighbor interaction between spin-1 and the external magnetic field and it is disappearing as these parameters exceed the trivial values.

We investigate the thermal entanglement in the two-qubit anisotropic XXZ model in the presence of an external inhomogeneous magnetic field along the z axis. Concurrence, the measurement of entanglement is calculated. The effects of the anisotropic parameter Δ on the thermal entanglement are studied carefully for both the antiferromagnetic and ferromagnetic cases. The critical temperature T_c can also be controlled by the anisotropic parameter Δ.

The crossover of dilution in the critical exponent associated with the thermodynamic properties in the magnetic materials via Monte Carlo simulations is observed. The obtained results of critical exponent associated with the magnetic susceptibility, specific heat and correlation length for the ZnCr_2xAl_2-2xS₄, Cd_1-yCr_2-2xIn_y+2xSe₄ and Zn_1-xMn_xTe systems are comparable with those given by the experiment results.

The structural, electronic thermodynamic and thermal properties of Ba_xSr_1-xTe ternary mixed crystals have been studied using the ab initio full-potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). In this approach, the Perdew–Burke–Ernzerhof-generalized gradient approximation (PBE-GGA) was used for the exchange-correlation potential. Moreover, the recently proposed modified Becke Johnson (mBJ) potential approximation, which successfully corrects the band-gap problem was also used for band structure calculations. The ground-state properties are determined for the cubic bulk materials BaTe, SrTe and their mixed crystals at various concentrations (x = 0.25, 0.5 and 0.75). The effect of composition on lattice constant, bulk modulus and band gap was analyzed. Deviation of the lattice constant from Vegard's law and the bulk modulus from linear concentration dependence (LCD) were observed for the ternary Ba_xSr_1-xTe alloys. The microscopic origins of the gap bowing were explained by using the approach of Zunger and co-workers. On the other hand, the thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing, ΔH_m as well as the phase diagram. It was shown that these alloys are stable at high temperature. Thermal effects on some macroscopic properties of Ba_xSr_1-xTe alloys were investigated using the quasi-harmonic Debye model, in which the phononic effects are considered.

Inspired from the connection between Lie symmetries and two-dimensional materials, we propose a new statistical lattice model based on a double hexagonal structure appearing in the G₂ symmetry. We first construct an Ising-1/2 model, with spin values σ = ±1, exhibiting such a symmetry. The corresponding ground state shows the ferromagnetic, the antiferromagnetic, the partial ferrimagnetic and the topological ferrimagnetic phases depending on the exchange couplings. Then, we examine the phase diagrams and the magnetization using the mean field approximation (MFA). Among others, it has been suggested that the present model could be localized between systems involving the triangular and the single hexagonal lattice geometries.

We synthesized the YBa_mCu_1+mO_y superconductors; m = 2, 3, 4, 5 that were Y123 (YBa₂ Cu₃O_7-x), Y134 (YBa₃Cu₄O_9-x), Y145 (YBa₄Cu₅O_11-x), Y156 (YBa₅Cu₆O_13-x), by solid state reaction with the Y₂O₃, BaCO₃ and CuO as the beginning materials. The calcination temperature was 950°C and varied the sintering temperature to be 950°C and 980°C. The resistivity measurement by four-point-probe technique showed that the T_c^onset of Y123, Y134, Y145, Y156 were at 97, 93, 91, 85 K, respectively. The XRD and Rietveld full-profile analysis method were used and found that the crystal structure was in the orthorhombic with Pmmm space group with the ratio c/a were 3.0, 4.0, 5.0 and 6.0 for Y123, Y134, Y145 and Y156, respectively. The oxygen content was characterized by Iodometric titration. The (Cu³⁺/Cu²⁺ and Oxygen content) were (0.28, 6.83), (0.19, 8.81), (0.13, 10.79), (0.16, 12.92) of Y123, Y134, Y145, Y156, respectively. We also found that the increasing of sintering temperature has reduced the oxygen content and the critical temperature of all samples.

Single domain YBCO bulk superconductors with different additions of ZnO have been successfully fabricated by RE$+$011 TSIG process with a new solid phase of [(100-$x$)(Y₂O${}_3+1.2$BaCuO₂)+$x$ZnO] and a new liquid phase of (Y₂O₃+6CuO+10BaCuO₂). The effects of ZnO additions on the growth morphology, microstructure, critical temperature ($T_{\mathrm{\text{c}}}$), the levitation force and trapped field of the YBCO bulks have been investigated. It is found that within the range of ZnO additions $x$=0–1.0 wt.%, all the samples are of the typical characteristic of single-domain YBCO bulk; the $T_{\mathrm{\text{c}}}$ of the samples decreases from 92 K to 80 K when the ZnO addition $x$ increases from $x$=0 wt.% to $x$=1.0 wt.%; the levitation force and trapped field of the samples firstly increase and then decrease with increase of ZnO additions after going through a maximum, which is closely related with the ZnO addition and the resulting flux pinning force caused by lattice distortion due to the substitution of Zn${}^{2+}$ for Cu${}^{2+}$ site in the YBCO crystal; the largest levitation force 36.8 N (77 K, 0.5 T) and trapped field 0.416 T (77 K, 0.5 T) of the samples are obtained when $x$=0.1 wt.%, respectively. This result is significantly important and helpful for us to improve the properties of YBCO bulk superconductors.

In this paper, we study the influence of the physical parameters on the thermal and magnetic properties of a nano-particle of bi-fullerene-like structure X${}_{60}$-Y${}_{60}$ separated by nonmagnetic spherical surfaces, using the Metropolis Monte Carlo (MC) simulations. The coupling between the two spheres containing the spins $\sigma$ = 3/2 and the spins S = 7/2 belongs to the core and shell, respectively. This study is based on the RKKY (Ruderman–Kittel–Kasuya–Yoshida) interaction type. To complete this work, we carry out the hysteresis cycles of the studied system for different nonmagnetic surface (NMS) numbers.

The magnetic properties of NiAl${}_x$Fe${}_{2-x}$O₄ (NAFO) spinels were studied. Influence of Al doping on magnetic properties of NiFe₂O₄ spinel were examined. The exchange interactions in NAFO were obtained. The general expression of saturation magnetization and the critical temperature were obtained using mean field theory. The high-temperature series expansions combined with the Padé approximant are given to determine the critical temperature of NAFO. The critical exponent associated with the magnetic susceptibility $\gamma$ was also deduced. The obtained results were in good agreement with magnetic measurement.