arXiv subjects

F. Schwabl

Publications and source records attributed to F. Schwabl.

18 recordsLinked to original sources

arXiv2003

Electron-Polarization Coupling in Superconductor-Ferroelectric Superlattices

We present a phenomenological model of periodic ferroelectric-superconductor (FE-S) heterostructures containing two alternating ferroelectric and superconducting layers. The interaction at the FE-S contacts is described as a coupling of the local carrier density of the superconductor with the spontaneous ferroelectric polarization near the FE-S interface. We obtain a stable symmetric domain-type phase exhibiting a contact-induced polarization and the ferroelectric domain structure at temperatures above the bulk ferroelectric transition temperature. With an increasing coupling energy, we find the appearance of the ferroelectric phase coexisting with the suppressed superconductivity in the S-film. The system is analyzed for different thicknesses of the FE- and S-films demonstrating the dramatic change of the topology of the phase diagrams with a variation of the layers thickness. The results are expected to shed light on the processes occurring in high-temperature superconducting films grown on perovskite alloy-substrates exhibiting ferroelectric properties at lower temperatures.

cond-mat.stat-mech↗

arXiv2000

Influence of defects on the critical behaviour at the \boldmath{105} K structural phase transition of SrTiO$_3$: I. The broad component

The critical fluctuations in SrTiO$_3$ near its $105 K$ structural phase transition were studied with triple axis diffractometry using high energy ($\ge 100 keV$) synchrotron radiation in different SrTiO$_3$ crystals with different oxygen vacancy concentrations. Due to the presence of oxygen vacancies the critical behaviour is changed compared to defect-free systems. In our experiments a smearing out of the squared order parameter and a crossover of the critical exponents $ν$ and $γ$ above the phase transition temperature is observed, with the crossover temperature strongly depending on the concentration of the defects. To understand the experimental findings, e.g. the unusual values for the critical exponents found near the critical temperature, the Ginzburg-Landau-Wilson functional for structural phase transitions in disordered systems was analyzed using renormalization group theory and the replica trick. Considering the effects of defects which locally increase the transition temperature leads to a qualitative understanding of the observed behaviour. The crossover behaviour of the critical exponents can be modeled and a quantitative analysis of the observed experimental data is presented.

cond-mat.mtrl-sci↗

arXiv1999

Critical dynamics of a uniaxial and dipolar ferromagnet

We study the critical dynamics of three-dimensional ferromagnets with uniaxial anisotropy by taking into account exchange and dipole-dipole interaction. The dynamic spin correlation functions and the transport coefficients are calculated within a mode coupling theory. It is found that the crossover scenario is determined by the subtle interplay between three length scales: the correlation length, the dipolar and uniaxial wave vector. We compare our theoretical findings with hyperfine interaction experiments on Gd and find quantitative agreement. This analysis allows us to identify the universality class for Gd. It also turns out that the $μ$SR relaxation rate can be best fitted if it is assumed that muons occupy octahedral interstitials sites within the Gd lattice.

cond-mat.stat-mech↗

arXiv1998

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.

cond-mat.stat-mech↗

arXiv1997

Determination of the universality class of Gadolinium

We resolve a longstanding puzzle for the static and dynamic critical behavior of Gadolinium by a combined theoretical and experimental investigation. It is shown that the spin dynamics of a three dimensional ferromagnet with hcp lattice structure and a spin-spin interaction given by both exchange and dipole-dipole interaction belongs to a new dynamic universality class, model J$^*$. Comparing results from mode coupling theory with results from three different hyperfine interaction probes we find quantitative agreement. The crossover scenario for the wavevector dependence of the hyperfine relaxation rate is determined by a subtle interplay between three length scales: the correlation length, the dipolar and the uniaxial wave vector.

cond-mat.mtrl-sci↗

arXiv1997

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_Lμ_B)^2. For increasing values of κ, 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.

cond-mat.stat-mech↗

arXiv1997

Spin waves and phase diagram of one-dimensional, dipolar antiferromagnets

We report on the properties of a dipolar, antiferromagnetic chain in the framework of linear spin-wave theory. The phase diagram is calculated for an arbitrary ratio of dipolar interaction to exchange interaction and for fields both parallel and perpendicular to the chain direction. The calculated magnetization is compared with experimental measurements for RbMnBr_3, a quasi one-dimensional antiferromagnet.

cond-mat.stat-mech↗

arXiv1997

Alternating commensurate-incommensurate structures in the magnetic phase diagram of CsNiF3

The magnetic phase diagram of the quasi one-dimensional spinchain system CsNiF$_3$ below the Néel temperature is determined. For magnetic fields perpendicular to the spin chains incommensurate phases are predicted. From linear spin-wave theory we obtain the instability line of the paramagnetic phase as a function of the strength and the direction of the field. The system undergoes a transition to a commensurate or an incommensurate phase depending on the direction of the magnetic field. In the commensurate phase the characterizing wave vector is locked to values describing a two-sublattice structure, whereas in the incommensurate phase the wave vector changes continuously between the corresponding two-sublattice wave vectors.

cond-mat.stat-mech↗

arXiv1996

The effect of magnetic dipolar interactions on the interchain spin wave dispersion in CsNiF_3

Inelastic neutron scattering measurements were performed on the ferromagnetic chain system CsNiF_3 in the collinear antiferromagnetic ordered state below T_N = 2.67K. The measured spin wave dispersion was found to be in good agreement with linear spin wave theory including dipolar interactions. The additional dipole tensor in the Hamiltonian was essential to explain some striking phenomena in the measured spin wave spectrum: a peculiar feature of the dispersion relation is a jump at the zone center, caused by strong dipolar interactions in this system. The interchain exchange coupling constant and the planar anisotropy energy were determined within the present model to be J'/k_B = -0.0247(12)K and A/k_B = 3.3(1)K. This gives a ratio J/J' \approx 500, using the previously determined intrachain coupling constant J/k_B = 11.8$. The small exchange energy J' is of the same order as the dipolar energy, which implies a strong competition between the both interactions.

cond-mat↗

arXiv1996

Continuous Elastic Phase Transitions in Pure and Disordered Crystals

We review the theory of second--order (ferro--)elastic phase transitions, where the order parameter consists of a certain linear combination of strain tensor components, and the accompanying soft mode is an acoustic phonon. In three--dimensional crystals, the softening can occur in one-- or two--dimensional soft sectors. The ensuing anisotropy reduces the effect of fluctuations, rendering the critical behaviour of these systems classical for a one--dimensional soft sector, and classical with logarithmic corrections in case of a two--dimensional soft sector. The dynamical critical exponent is $z = 2$, and as a consequence the sound velocity vanishes as $c_s \propto | T - T_c |^{1/2}$, while the phonon damping coefficient is essentially temperature--independent. Disorder may lead to a variety of precursor effects and modified critical behaviour. Defects that locally soften the crystal may induce the phenomenon of local order parameter condensation. When the correlation length of the pure system exceeds the average defect separation $n_{\rm D}^{-1/3}$, a disorder--induced phase transition to a state with non--zero average order parameter can occur at a temperature $T_c(n_{\rm D})$ well above the transition temperature $T_c^0$ of the pure crystal. Near $T_c^0$, the order--parameter curve, susceptibility, and specific heat appear rounded. For $T < T_c(n_{\rm D})$ the spatial inhomogeneity induces a static central peak with finite $q$ width in the scattering cross section, accompanied by a dynamical component that is confined to the very vicinity of the disorder--induced phase transition.

cond-mat↗

arXiv1996

Defect-induced condensation and central peak at elastic phase transitions

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 $n_{\rm D}$ of quenched, randomly placed defects display a phase transition at a temperature $T_c(n_{\rm D})$, which can be considerably above the transition temperature $T_c^0$ of the pure system. The phonon correlation function is calculated in single--site approximation. For $T>T_c(n_{\rm D})$ a dynamical central peak appears; upon approaching $T_c(n_{\rm D})$, 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 $T_c(n_{\rm D})$. 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 ${\bf k}$ 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 $T_c(n_{\rm D})$. We also discuss the range of validity of the mean--field approach, and provide a more realistic estimate for the transition temperature.

cond-mat↗

arXiv1994

Spin Wave Frequency and Critical Dynamics of Ferromagnets Below T_C

Employing mode-coupling theory we show that the amplitude of the spin wave frequency scaling function for the isotropic Heisenberg Hamiltonian is universal. Theoretical and experimental values for Fe, Ni, Co, EuO, and EuS agree quite well. Recent measurements of the longitudinal line width in Ni are explained quantitatively.

cond-mat↗

arXiv1994

Scaling laws and simulation results for the self--organized critical forest--fire model

We discuss the properties of a self--organized critical forest--fire model which has been introduced recently. We derive scaling laws and define critical exponents. The values of these critical exponents are determined by computer simulations in 1 to 8 dimensions. The simulations suggest a critical dimension $d_c=6$ above which the critical exponents assume their mean--field values. Changing the lattice symmetry and allowing trees to be immune against fire, we show that the critical exponents are universal.

cond-mat↗

arXiv1994

Crossover from Isotropic to Directed Percolation

Percolation clusters are probably the simplest example for scale--invariant structures which either are governed by isotropic scaling--laws (``self--similarity'') or --- as in the case of directed percolation --- may display anisotropic scaling behavior (``self--affinity''). Taking advantage of the fact that both isotropic and directed bond percolation (with one preferred direction) may be mapped onto corresponding variants of (Reggeon) field theory, we discuss the crossover between self--similar and self--affine scaling. This has been a long--standing and yet unsolved problem because it is accompanied by different upper critical dimensions: $d_c^{\rm I} = 6$ for isotropic, and $d_c^{\rm D} = 5$ for directed percolation, respectively. Using a generalized subtraction scheme we show that this crossover may nevertheless be treated consistently within the framework of renormalization group theory. We identify the corresponding crossover exponent, and calculate effective exponents for different length scales and the pair correlation function to one--loop order. Thus we are able to predict at which characteristic anisotropy scale the crossover should occur. The results are subject to direct tests by both computer simulations and experiment. We emphasize the broad range of applicability of the proposed method.

cond-mat↗

arXiv1993

Formation of Space-Time Structure in a Forest-Fire Model

We present a general stochastic forest-fire model which shows a variety of different structures depending on the parameter values. The model contains three possible states per site (tree, burning tree, empty site) and three parameters (tree growth probability $p$, lightning probability $f$, and immunity $g$). We review analytic and computer simulation results for a quasideterministic state with spiral-shaped fire fronts, for a percolation-like phase transition and a self-organized critical state. Possible applications to excitable systems are discussed.

cond-mat↗

arXiv1993

Neel Temperature for Quasi-Two-Dimensional Dipolar Antiferromagnets

We calculate the Néel temperature $T_N$ for two-dimensional isotropic dipolar Heisenberg antiferromagnets via linear spin-wave theory and a high temperature expansion, employing the method of Callen. The theoretical predictions for $T_N$ for K$_2$MnF$_4$, Rb$_2$MnF$_4$, Rb$_2$MnCl$_4$ and (CH$_3$NH$_3$)$_2$MnCl$_4$ are in good agreement with the measured values.

cond-mat↗

arXiv1993

Influence of cubic and dipolar anisotropies on the static and dynamic coexistence anomalies of the time-dependent Ginzburg-Landau models

In isotropic systems below the transition temperature, the massless Goldstone modes imply critical infrared singularities in the statics and dynamics along the entire coexistence curve. We examine the important question whether these coexistence anomalies are of relevance also in more realistic systems displaying anisotropies. By applying a generalized renormalization scheme to the time-dependent Ginzburg-Landau models, we treat two quite different but characteristic cases, namely the influence of (i) weak cubic anisotropies, and (ii) long-range dipolar interactions. In the presence of cubic terms, the transverse excitations acquire a mass and thus one expects the theory to approach an uncritical "Gaussian" regime in the limit ${\vec q} \rightarrow 0$ and $ω\rightarrow 0$. Therefore, we first consider the one-component case in order to show that our formalism also provides a consistent description of the crossover into an asymptotically Gaussian theory. In the case of (weak) cubic anisotropies, the fact that for $n < 4$ the fluctuations tend to restore the $O(n)$-symmetry at the critical point proves to be most important, since under these circumstances coexistence-type singularities may be found in an intermediate wavenumber and frequency range. The dipolar interaction induces an anisotropy in momentum space, which does not completely destroy the massless character of the transverse fluctuations, but only effectively reduces the number of Goldstone modes by one. Remarkably, similar to the isotropic case the asymptotic theory can be treated exactly. For $n \geq 3$ we find coexistence anomalies governed by the isotropic power laws. However, the amplitudes of the respective scaling functions depend on the angle between order parameter and external wavevector.

cond-mat↗

arXiv1993

Order of Two-Dimensional Isotropic Dipolar Antiferromagnets

The question of the existence of order in two-dimensional isotropic dipolar Heisenberg antiferromagnets is studied. It is shown that the dipolar interaction leads to a gap in the spin-wave energy and a nonvanishing order parameter. The resulting finite Néel-temperature is calculated for a square lattice by means of linear spin-wave theory.

cond-mat↗