AG Draconis, a mystery older than the Republic

Interacting variable stars, to which symbiotic binary stars belong, are  among the most exciting objects that a variety of variable stars offer. Physical mechanisms related to transfer and accretion of matter cause observable activity by manifesting increases of brightness (about 2 – 5 mag) and significant changes in the spectra of these objects. The brightenings of symbiotic systems have been recorded since the early 20th…

Deformable spin-1/2 antiferromagnetic XXZ Heisenberg chain in a magnetic field

We consider a deformable spin-1/2 antiferromagnetic XXZ Heisenberg chain and examine the spin-chain lattice compressibility as well as the sound velocity as a function of an external magnetic field applied either along the z axis or along the x axis. For strong fields, we observe a noticeable increase (with jump and hysteresis) of the lattice compressibility. Our findings are obtained by several complimentary approaches (Jordan-Wigner…

Localised quasiclassical states in asymptotic theory of ion-atom interactions

The interest to the study of localised states in atomic physics is, on the one hand, evoked by the fact, that in the numerous cases their analysis can be carried out analytically. Such method belongs to not much approaches, which allow investigating the system with nonfactorised variables without the use of perturbation theory or cumbersome numerical methods. On the other hand, the aforementioned states often…

Effects of higher-order exchange interactions in classical XY model

A standard two-dimensional (2D) XY model is well known to show a topological Berezinskii-Kosterlitz-Thouless (BKT) phase transition with the quasi-long-range-order (QLRO) BKT phase characterized by an algebraically decaying correlation function. It will be shown by spin-wave theory and Monte Carlo simulations that including higher-order terms can lead to dramatic changes in its critical behavior, including phase transition order or universality class and even the phase…