Renormalized field theory for the static crossover in dipolar ferromagnets

A field theoretical description for the static crossover in dipolar ferromagnets is presented. New non leading critical exponents for the longitudinal static susceptibility are identified and the existence and magnitude of the dip in the effective critical exponent of the transverse susceptibility found by matching techniques is scrutinized

Incommensurate phases in ferromagnetic spin-chains with weak antiferromagnetic interchain interaction

We study planar ferromagnetic spin-chain systems with weak antiferromagnetic inter-chain interaction and dipole-dipole interaction. The ground state depends sensitively on the relative strengths of antiferromagnetic exchange and dipole energies kappa=J'a^2c/(g_L\mu_B)^2. For increasing values of \kappa, the ground state changes from a ferromagnetic via a collinear antiferromagnetic and an incommensurate phase to a 120^o structure for very large antiferromagnetic energy. Investigation of the magnetic phase diagram of the collinear phase, as realized in CsNiF_3, shows that the structure of the spin order depends sensitivly on the direction of the magnetic field in the hexagonal plane. For certain angular domains of the field incommensurate phases appear which are separated by commensurate phases. When rotating the field, the wave vector characterizing the structure changes continuously in the incommensurate phase, whereas in the commensurate phase the wave vector is locked to a fixed value describing a two-sublattice structure. This is a result of the competition between the exchange and the dipole-dipole interaction.Comment: 12 pages, ReVTeX, 13 figures, to be published in Z. Physi

Critical dynamics of ferromagnets

The crossover in the dynamics from isotropic to dipolar critical behaviour has been a matter of debate over many years. We review a mode coupling theory for dipolar ferromagnets which gives a unified explanation of the seemingly contradictory experimental situation. The shape functions, the scaling functions for the damping coefficients and the precise position of the crossover are computed. Below Tc only the exchange interaction is taken into account

On the critical dynamics of ferromagnets

The dynamic scaling functions for ferromagnets above and below the critical temperature are determined using mode coupling theory. Below the critical temperature we study isotropic ferromagnets taking into account the exchange interaction only and give the first numerical solution of the resulting mode coupling equations. In the paramagnetic phase we examine how the critical dynamics is modified by the addition of the dipoledipole interaction. On the basis of this theory we are able to explain in a unifying fashion the results of different experimental methods; i.e.: neutron scattering, hyperfine interaction and electron-spin resonance. Predictions for new experiments are made

Phase diagram and magnons in quasi-one-dimensional dipolar antiferromagnets

We investigate antiferromagnetic spin chains, which are coupled by a weak antiferromagnetic exchange interaction on a hexagonal lattice. We particulary study the role of the dipole-dipole interaction within the framework of a Heisenberg model with nearest-neighbor exchange and additional dipolar interaction. We find several commensurate and incommensurate phases depending on the ratio of dipolar energy to interchain-exchange energy due to their competing qualtity. The ground-state analysis is supplemented by a stability analysis by means of a linear spin-wave theory. In comparison with experiments (CsMnBr_3, RbMnBr_3) we obtain good agreement for the energy gaps. From this we conclude, that the dipolar interaction is the most important source of anisotropy in these Mn-compounds.Comment: 3 pages, 3 Postscript figure

Hyperfine induced spin and entanglement dynamics in Double Quantum Dots: A homogeneous coupling approach

We investigate hyperfine induced electron spin and entanglement dynamics in a system of two quantum dot spin qubits. We focus on the situation of zero external magnetic field and concentrate on approximation-free theoretical methods. We give an exact solution of the model for homogeneous hyperfine coupling constants (with all coupling coefficients being equal) and varying exchange coupling, and we derive the dynamics therefrom. After describing and explaining the basic dynamical properties, the decoherence time is calculated from the results of a detailed investigation of the short time electron spin dynamics. The result turns out to be in good agreement with experimental data.Comment: 10 pages, 8 figure

Perturbative regimes in central spin models

Central spin models describe several types of solid state nanostructures which are presently considered as possible building blocks of future quantum information processing hardware. From a theoretical point of view, a key issue remains the treatment of the flip-flop terms in the Hamiltonian in the presence of a magnetic field. We systematically study the influence of these terms, both as a function of the field strength and the size of the spin baths. We find crucial differences between initial states with central spin configurations of high and such of low polarizations. This has strong implications with respect to the influence of a magnetic field on the flip-flop terms in central spin models of a single and more than one central spin. Furthermore, the dependencies on bath size and field differ from those anticipated so far. Our results might open the route for the systematic search for more efficient perturbative treatments of central spin problems.Comment: 7 pages, 3 figure