J. Sznajd

Discontinuous phase transition in U3As4 under magnetic field

Abstract We present the magnetization results obtained for a single crystal of U 3 As 4 along the easy and hard axes, in static external fields up to 20.5 T at various temperatures. The magnetization jumps have been observed in the hard direction for temperatures below about 105 K and external magnetic fields H c >140 T. From these measurements the temperature dependence of the anisotropy constants K 1 and K 2 have been calculated. By using the Landau theory and renormalization group approach the tricritical point, from which the magnetization jump vanishes, has been determined.

Real space renormalization group study of the anisotropic quantum Heisenberg model on a square lattice

Abstract Within a real space renormalization group framework, we study the critical behaviour of the anisotropic spin- 1 2 Heisenberg model on a square lattice. We use the rotationally invariant transformation which preserves all symmetries of the original problem. The results suggest that in contrast to the isotropic Heisenberg model, the two-dimensional XY model undergoes a phase transition. The inverse critical temperature of this transition, obtained in this paper, is in satisfactory agreement with that found by high temperature series analysis.

On the real-space renormalization-group study of some 2D quantum spin systems

Abstract Within a real-space renormalization-group framework, the critical behaviour of the spin- 1 2 anisotropic quantum Heisenberg model on the three planar lattices has been studied. To examine the stability of the results with respect to changes in the approximation scheme, various divisions of the lattices and various truncation procedures have been used. It has been shown that there is a non-trivial fixed point and corresponding critical temperature for the nearest-neighbour XY model for each two-dimensional lattice irrespective of the approximation used. Concerning the Heisenberg model, to first order in the cumulant expansion there is no non-trivial fixed point, however, it appears in the second- and third-order calculations. This suggests that the two-dimensional Heisenberg model also exhibits some kind of a critical behaviour. In addition, the phase transitions to the many-sublattice structure have been discussed.

Magnetic and quadrupolar order in a one-dimensional ferromagnet with cubic crystal-field anisotropy

The zero-temperature phase diagram of a one-dimensional

On the spin dynamics of 2D ferromagnets with Dzyaloshinsky-Moriya interaction

S=2

Tricritical points in ferromagnets with cubic single-ion anisotropy

Heisenberg ferromagnet with single-ion cubic anisotropy is studied numerically using the density-matrix renormalization-group method. Evidence is found that although the model does not involve quadrupolar couplings, there is a purely quadrupolar phase for large values of the anisotropy. The phase transition between the magnetic and quadrupolar phases is continuous and it seems to be characterized by Ising critical exponents.

Phase front motion in uniaxial ferromagnet

Using molecular dynamics (MD) simulation to a classical 2D easy plane ferromagnet with Dzualoshinksy-Moriya (D-M) interaction we have found the dependence on the external magnetic field of the ground state configuration as well as the stability of an initial in-plane “vortex”.

Renormalization of ferrimagnetic alternating spin chains

Abstract Certain physical systems with n ⩾2 component Hamiltonians exhibit first order phase transitions, even though Landau theory predicts a continuous transition and an appropriate stable fixed point exists. Such a situation can occur if the LGW Hamiltonian parameters lie outside the domain of attraction of the stable fixed point. These parameters are calculated as functions of microscopic constants and temperature ( T ) for ferromagnets with single-ion cubic anisotropy ( K ). The phase diagram in the plane ( K, T ) is constructed and the domain of attraction of the isotropic n =3 stable fixed point, and then the coordinates of the tricritical points for this system are found. It is shown that in both cases, K > 0 and K K / J exists for which the phase transition is expected to be of the first order. The usefulness of the model under consideration to the description of the transitions between phases with distinct propagation vectors are discussed.

Spinless fermion chains with weak interchain hopping

It is well known that in the uniaxial ferromagnet in the presence of an external magnetic field perpendicular to the easy axis (hx) a continuous phase transition occurs for a critical value of this field. There are metastable and stable states if one includes a small field parallel to the easy axis (hz). The motion of the relaxation front of the metastable state is investigated. It is found that an “interphase wall of Neel-type” exists, its velocity is proportional to hz and increases when the critical point is approached.

Low temperature phase of the quantum triangular lattice XY model with Dzyaloshinsky-Moriya interaction

The linear real space renormalization group is applied to study alternating spin (1/2,1) chains. The temperature dependences of the specific heat and spin–spin correlation functions for the quantum easy-axis mixed-spin chain are found. The role of single-ion anisotropy is discussed. The phase diagram of Ising spin chains with antiferromagnetic intrachain interactions and weak ferromagnetic interchain interactions is presented. As one expects, the weak interchain coupling leads to antiferrimagnetic long-range order in several chains. The threshold value of single-ion anisotropy below which the system does not exhibit a phase transition is found.