Darko Kapor

Effects of quantum lattice fluctuations on multiquanta Davydov-like solitons in a molecular chain

The dressing effect due to the quantum nature of phonons may cause the creation of bound states of several vibrons in the molecular chain. The possibility of creating soliton states of this type is discussed for the simple Frohlich one-dimensional model. The regions of the system parameter space where different mechanisms dominate the behaviour of such entities are characterized.

The effect of a surface on the dynamic and thermodynamic properties of S=1 Heisenberg ferromagnet with biquadratic exchange

Properties of semi-infinite (S=1) Heisenberg ferromagnet with biquadratic exchange were studied in terms of surface exchange (e=IS/I) and biquadratic coupling (a). It was shown that a strict correlation exists, depending on e, between the type of surface spin waves (acoustic or optical) and the mean-field (MF) critical temperature, bulk (Tc) and surface TcS>Tc (for e>54). Within the framework of the Landau–Ginsburg theory for semi-infinite simple cubic ferromagnet, a detailed study is presented of the critical behaviour of the system, in particular in the vicinity of the tricritical point which is the consequence of the biquadratic interaction. It is shown that tricritical exponents satisfy exactly the scaling relations for d=3. The analysis of the spin–spin correlation function within the framework of the same theory, shows that there occurs the critical magnetic scattering of low-energy electrons (LEED) from the surface in the case e⩾54, when the ordering temperature TcS is approached from above (from paramagnetic phase). In the opposite case, e<54, there occurs no surface critical scattering. It was also shown that in the vicinity of the tricritical point, the biquadratic interaction increases the range of validity of the MF approximation.

On the mean-field theory of magnetic multilayers with bilinear and biquadratic Heisenberg exchange

A system consisting of several layers of magnetic ions interacting by both bilinear and biquadratic Heisenberg exchange is studied within the framework of the mean-field approximation. It is shown that for S=1 there exist two types of ordering: ferromagnetic and ferroquadrupolar. The stability of phases as the function of temperature, biquadratic exchange and surface exchange is discussed analytically and numerically and it was shown that similar to bulk samples there appear first- and second-order transitions and a tricritical point may appear depending on system parameters.

Phase diagram of spin-12 quantum Heisenberg J1–J2 antiferromagnet on the body-centered-cubic lattice in random phase approximation

Abstract Magnetic properties of spin 1 2 J 1 – J 2 Heisenberg antiferromagnet on the body centered cubic lattice are investigated. By using two-time temperature Greens functions, sublattice magnetization and critical temperature depending on the frustration ratio p = J 2 / J 1 are obtained in both stripe and Neel phases. The analysis of ground state sublattice magnetization and phase diagram indicates the critical end point at J 2 / J 1 = 0.714 , in agreement with previous studies.

Low-temperature properties of the Ising model in the transverse field and the influence of kinematical effects

We present the low-temperature method for the Ising model in the transverse field based on the approximate boson expression for Pauli-operators. Kinematic effects, which arise through special decoupling of Greens functions, lead to appearance of additional energy levels.

New approach to surface magnon states in the semi-infinite antiferromagnetic Heisenberg model with biquadratic exchange and single-ion anisotropy

Abstract We present a new approach to the study of excitations in a semi-infinite antiferromagnet described by the Heisenberg Hamiltonian with biquadratic exchange and single-ion anisotropy.

Solitons in models of an alpha -helix: influence of symmetry

Solitons in an alpha -helix are studied within the framework of a modified Davydov model which takes into account third-order axis symmetry. It is shown that antisymmetric solitons with one vanishing amplitude do not satisfy this demand. The result is confirmed by perturbational calculation for the static solitons.

INFLUENCE OF TRANSLATIONAL SYMMETRY OF FERROMAGNETIC SUPERLATTICES IN THE STUDY OF ELEMENTARY EXCITATIONS

Elementary excitations in ferromagnetic superlattices are analyzed theoretically using the transfer matrix method. The method is extended to enable the study of the cases when the boundaries are not consisting of a single plane, but of a complete cell. In this way, semi-infinite superlattice, the interface between two superlattices and the film built of a superlattice are treated. However, it is shown that the choice of the transfer matrix strongly depends on the translational symmetry of the remaining part of the system. Energies of localized excitations are derived and compared both with bulk and the results of standard transfer matrix treatment proposed by Barnas. Also, a special Ansatz for spin amplitudes is proposed leading to identical results with the transfer matrix method and introducing large simplifications.

THE INFLUENCE OF BIQUADRATIC COUPLING AND UNIAXIAL ANISOTROPY ON THE SURFACE MODES IN A SIMPLE CUBIC ANTIFERROMAGNET

Abstract We study the surface spin excitations in a simple cubic two-sublattice Heisenberg antiferromagnet with both biquadratic coupling (a) and uniaxial anisotropy (D, DS). The combined effect is a lowering or narrowing of the energies, and changes of the range of the system parameters for which these modes can exist. For certain values of the system parameters, the surface antiferromagnetic resonance (SAFMR) mode at the Brillouin zone center can be well resolved from the bulk AFMR mode.

Magnetic excitations in the semi-infinite Heisenberg model with biquadratic exchange

Abstract We study the excitations in the semi-infinite ferromagnet described by the Heisenberg Hamiltonian with biquadratic exchange and single-ion anisotropy. The spectrum of excitations is shown to be more complex than the one previously discussed.