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Browsing by Author "Bulenda, M."

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    Defect-induced condensation and the central peak at elastic phase transitions
    Bulenda, M.; Schwabl, Franz; Täuber, Uwe C. (1996-09-01)
    Static and dynamical properties of elastic phase transitions under the influence of short–range defects, which locally increase the transition temperature, are investigated. Our approach is based on a Ginzburg–Landau theory for three–dimensional crystals with one–, two– or three–dimensional soft sectors, respectively. Systems with a finite concentration nD of quenched, randomly placed defects display a phase transition at a temperature Tc(nD), which can be considerably above the transition temperature T0c of the pure system. The phonon correlation function is calculated in single–site approximation. For T > Tc(nD) a dynamical central peak appears; upon approaching Tc(nD), its height diverges and its width vanishes. Using an appropriate self–consistent method, we calculate the spatially inhomogeneous order parameter, the free energy and the specific heat, as well as the dynamical correlation function in the ordered phase. The dynamical central peak disappears again as the temperatur is lowered below Tc(nD). The inhomogeneous order parameter causes a static central peak in the scattering cross section, with a finite k width depending on the orientation of the external wave vector k relative to the soft sector. The jump in the specific heat at the transition temperatur of the pure system is smeared out by the influence of the defects, leading to a distinct maximum instead. In addition, there emerges a tiny discontinuity of the specific heat at Tc(nD). We also discuss the range of validity of the mean–field approach, and provide a more realistic estimate for the transition temperature.
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    Dimensional crossover in dipolar magnetic layers
    Bulenda, M.; Täuber, Uwe C.; Schwabl, Franz (IOP, 2000-01-14)
    We investigate the static critical behaviour of a uniaxial magnetic layer, with finite thickness L in one direction, yet infinitely extended in the remaining d dimensions. The magnetic dipole-dipole interaction is taken into account. We apply a variant of Wilson’s momentum shell renormalisation group approach to describe the crossover between the critical behaviour of the 3-D Ising, 2-d Ising, 3-D uniaxial dipolar, and the 2-d uniaxial dipolar universality classes. The corresponding renormalisation group fixed points are in addition to different effective dimensionalities characterised by distinct analytic structures of the propagator, and are consequently associated with varying upper critical dimensions. While the limiting cases can be discussed by means of dimensional ǫ expansions with respect to the appropriate upper critical dimensions, respectively, the crossover features must be addressed in terms of the renormalisation group flow trajectories at fixed dimensionality d.