Modern Physics Letters A

In this paper, the density matrix is obtained using the non-Markovian master equation method for a system consisting of two-qubit modeling with the Heisenberg XXZ chain, which involves the Dzyaloshinskii–Moriya interaction and is exposed to the bosonic baths. Also, using the proposed formula for calculating the specific heat through the density matrix of the open quantum system, the specific heat is calculated. The quantum entanglement behavior with time and coupling constant is investigated. It is observed that the specific heat at low temperatures becomes negative when the system is exposed to the environment. The time behavior of quantum entanglement for this model showed that the entanglement decreases to zero with increasing time.

In this paper, the periodic solutions of the equation of Friedmann–Robertson–Walker cosmology with a cosmological constant are investigated. Using variable transformation, the original second-order ordinary differential equation is converted to a planar dynamical system with cosmic time t. Numerical simulations indicate that period function T(h) of this dynamical system is monotonically increasing. However, a new planar dynamical system could be deduced by using conformal time variable $\tau$. We prove that the new planar dynamical system has two isochronous centers under certain parameter conditions by using Picard–Fuchs equation. Explicitly, we find that there exist two families of periodic solutions with equal period for the new planar dynamical system which is derived from the Friedmann–Robertson–Walker model.

In this research, we have investigated the behavior of massive and massless scalar field (SF) models (normal and phantom) for Kaluza–Klein universe in $f(R,T)$ gravity with cosmological term ($\mathrm{\Lambda }$). To obtain field equations, we have used $f(R,T)=R+2h(T)$ model given by Harko et al. [Phys. Rev. D 84, 024020 (2011)] and anisotropy feature of the universe. Finally, we have discussed our results in $f(R,T)$ and General Relativity Theory (GRT) with various graphics.

The purpose of this paper is to find conformal vector fields of some perfect fluid Kantowski–Sachs and Bianchi type III spacetimes in the $f(R)$ theory of gravity using direct integration technique. In this study, there exist only eight cases. Studying each case in detail, we found that in two cases proper conformal vector fields exist while in the rest of the cases, conformal vector fields become Killing vector fields. The dimension of conformal vector fields is either 4 or 6.

This paper studies the viable regions of some cosmic models in a higher derivative $f(R,\square R,T)$ theory with the help of energy conditions (where $R$, $\square$ and $T$ are the Ricci scalar, d’Alembert’s operator and trace of energy–momentum tensor, respectively). For this purpose, we assume a flat Friedmann–Lemaître–Robertson–Walker metric which is assumed to be filled with perfect fluid configurations. We take two distinct realistic models that might be helpful to explore stable regimes of cosmological solutions. After taking some numerical values of cosmic parameters, like crackle, snap, jerk (etc.) as well as viable constraints from energy conditions, the viable zones for the under observed $f(R,\square R,T)$ models are examined.

In this note, we discuss the hypothesis of a presence of the negative mass matter in the Schwarzschild’s spacetime and demonstrate, that the discrete PT transformation of the coordinates and mass inversion in the Schwarzschild’s metric solution is equivalent to the inversion of the Kruskal–Szekeres coordinates [M. D. Kruskal, Phys. Rev. 119, 1743 (1960); G. Szekeres, Publ. Mat. Debrecen 7, 285 (1960)], in the full region of the solution’s spacetime. As the consequence of the result, it is argued that the whole Schwarzschild spacetime can be described in terms of regions of positive and negative masses interconnected by the discrete transforms.

Large-N expansions are usually applied in single-well setups. We claim that this technique may offer an equally efficient constructive tool for potentials with more than one deep minimum. In an illustrative multi-well model, this approach enables us to explain the phenomenon of an abrupt relocalization of ground state caused by a minor change of the couplings.

Using the non-extensive Tsallis entropy and the holographic hypothesis, we propose a new dark energy (DE) model with timescale as infrared (IR) cutoff. Considering the age of the Universe as well as the conformal time as IR cutoffs, we investigate the cosmological consequences of the proposed DE models and study the evolution of the Universe filled by a pressureless matter and the obtained DE candidates. We find that although this model can describe the late time acceleration and the density, deceleration and the equation of state parameters show satisfactory behavior by themselves, these models are classically unstable unless the interaction between the two dark sectors of the Universe is taken into account. In addition, the results of the existence of a mutual interaction between the cosmos sectors are also addressed. We find out that the interacting models are stable at the classical level which is in contrast to the original interacting agegraphic dark energy models which are classically unstable [K. Y. Kim, H. W. Lee and Y. S. Myung, Phys. Lett. B 660, 118 (2008)].

Here, we have studied the sensitivity of fusion excitation functions of reactions induced by weakly bound projectiles ${}^6\mathrm{\text{He}}$ and ${}^7\mathrm{\text{Li}}$ on ${}^{209}\mathrm{\text{Bi}}$ target on nuclear potential parameters and on number of partial waves. The Kemble version of WKB approximation and Hill–Wheeler formula has been used to predict the fusion transmission probability below and above the Coulomb barrier, respectively, and the optimum values for radius (r₀) = 1.17 fm, diffuseness (a) = 0.5 fm and for partial waves (l) = up to 60 are proposed. The coupled channel calculations have also been performed and it is found that the matching between data and predictions have been enhanced on inclusion of coupling effects. Further, the breakup effects are also taken into account through the dynamic polarization potential (DPP) approach. It further improves matching between data and predictions.