Searcharxiv⌕ Search

arXiv subjects

S. Trimper

Publications and source records attributed to S. Trimper.

At least 19 recordsLinked to original sources
arXiv2013

Thermally activated in-plane magnetization rotation induced by spin torque

We study the role of thermal fluctuations on the spin dynamics of a thin permalloy film with a focus on the behavior of spin torque and find that the thermally assisted spin torque results in new aspects of the magnetization dynamics. In particular, we uncover the formation of a finite, spin torque-induced, in-plane magnetization component. The orientation of the in-plane magnetization vector depends on the temperature and the spin-torque coupling. We investigate and illustrate that the variation of the temperature leads to a thermally-induced rotation of the in-plane magnetization.

cond-mat.mes-hall↗
arXiv2013

Fokker-Planck approach to the theory of magnon-driven spin Seebeck effect

Following the theoretical approach by Xiao et al [Phys. Rev. B 81, 214418 (2010)] to the spin Seebeck effect, we calculate the mean value of the total spin current flowing through a normalmetal/ ferromagnet interface. The spin current emitted from the ferromagnet to the normal metal is evaluated in the framework of the Fokker-Planck approach for the stochastic Landau-Lifshitz-Gilbert equation. We show that the total spin current depends not only on the temperature difference between the electron and the magnon baths, but also on the external magnetic field and magnetic anisotropy. Apart from this, the spin current is shown to saturate with increasing magnon temperature, and the saturation temperature increases with increasing magnetic field and/or magnetic anisotropy.

cond-mat.mes-hall↗
arXiv2007

Fluctuation effects in the theory of microphase separation of diblock copolymers in the presence of an electric field

We generalize the Fredrickson-Helfand theory of the microphase separation in symmetric diblock copolymer melts by taking into account the influence of a time-independent homogeneous electric field on the composition fluctuations within the self-consistent Hartree approximation. We predict that electric fields suppress composition fluctuations, and consequently weaken the first-order transition. In the presence of an electric field the critical temperature of the order-disorder transition is shifted towards its mean-field value. The collective structure factor in the disordered phase becomes anisotropic in the presence of the electric field. Fluctuational modulations of the order parameter along the field direction are strongest suppressed. The latter is in accordance with the parallel orientation of the lamellae in the ordered state.

cond-mat.soft↗
arXiv2006

Size and doping effects on the coercive field of ferroelectric nanoparticles

A microscopic model for describing ferroelectric nanoparticles is proposed which allows us to calculate the polarization as a function of an external electric field, the temperature, the defect concentration and the particle size. The interaction of the constituents of the material, arranged in layers, depends on both the coupling strength at the surface and that of defect shells in addition to the bulk values. The analysis is based on an Ising model in a transverse field, modified in such a manner to study the influence of size and doping effects on the hysteresis loop of the nanoparticles. Using a Green function technique in real space we find the coercive field, the remanent polarization and the critical temperature which differ significantly from the bulk behavior. Depending on the varying coupling strength due to the kind of doping ions and the surface configuration, the coercive field and the remanent polarization can either increase or decrease in comparison to the bulk behavior. The theoretical results are compared with a variety of different experimental data.

cond-mat.stat-mech↗
arXiv2005

Persistence Probabilities of the German DAX and Shanghai Index

We present a relatively detailed analysis of the persistence probability distributions in financial dynamics. Compared with the auto-correlation function, the persistence probability distributions describe dynamic correlations non-local in time. Universal and non-universal behaviors of the German DAX and Shanghai Index are analyzed, and numerical simulations of some microscopic models are also performed. Around the fixed point $z_0=0$, the interacting herding model produces the scaling behavior of the real markets.

nlin.AO↗
arXiv2005

Influence of layer defects on the damping in ferroelectric thin films

A Green's function technique for a modified Ising model in a transverse field is applied, which allows to calculate the damping of the elementary excitations and the phase transition temperature of ferroelectric thin films with structural defects. Based on an analytical expression for the damping function, we analyze its dependence on temperature, film thickness and interaction strength numerically. The results demonstrate that defect layers in ferroelectric thin films, layers with impurities or vacancies as well as layers with dislocations are able to induce a strong increase of the damping due to different exchange interactions within the defect layers. The results are in good agreement with experimental data for thin ferroelectric films with different thickness.

cond-mat.stat-mech↗
arXiv2004

Impact of layer defects in ferroelectric thin films

Based on a modified Ising model in a transverse field we demonstrate that defect layers in ferroelectric thin films, such as layers with impurities, vacancies or dislocations, are able to induce a strong increase or decrease of the polarization depending on the variation of the exchange interaction within the defect layers. A Green's function technique enables us to calculate the polarization, the excitation energy and the critical temperature of the material with structural defects. Numerically we find the polarization as function of temperature, film thickness and the interaction strengths between the layers. The theoretical results are in reasonable accordance to experimental datas of different ferroelectric thin films.

cond-mat.mtrl-sci↗
arXiv2001

Two harmonically coupled Brownian particles in random media

We study the behaviour of two Brownian particles coupled by an elastic harmonic force in a quenched disordered medium. We found that to first order in disorder strength, the relative motion weakens (with respect to the reference state of a Brownian particle with the double mass) the effect of the quenched forces on the centre of mass motion of the Brownian particles, so that the motion will become less subdiffusive (superdiffusive) for potential (solenoidal) disorder. The mean-square relative distance between the particles behaves in a different way depending of whether the particles are free to move or one particle is anchored in the space. While the effect of nonpotential disorder consists in increasing the mean-square distance in both cases, the potential disorder decreases the mean-square distance, when the particles are free to move, and increases it when one particle is anchored in the space.

cond-mat.stat-mech↗
arXiv2001

An Extended Network Model with a Packages Diffusion Process

The dynamics of a packages diffusion process within a selforganized network is analytically studied by means of an extended $f$% -spin facilitated kinetic Ising model (Fredrickson-Andersen model) using a Fock-space representation for the master equation. To map the three component system (active, passive and packages cells) onto a lattice we apply two types of second quantized operators. The active cells correspond to mobile states whereas the passive cells correspond to immobile states of the Fredrickson-Andersen model. An inherent cooperativity is included assuming that the local dynamics and subsequently the local mobilities are restricted by the occupation of neighboring cells. Depending on a temperature-like parameter $h^{-1}$ (interconnectivity) the diffusive process of the packages (information) can be almost stopped, thus we get a well separation of the time regimes and a quasi-localization for the intermediate range at low temperatures.

cond-mat.stat-mech↗
arXiv2001

Phase Ordering Dynamics of $ϕ^4$ Theory with Hamiltonian Equations of Motion

Phase ordering dynamics of the (2+1)- and (3+1)-dimensional $ϕ^4$ theory with Hamiltonian equations of motion is investigated numerically. Dynamic scaling is confirmed. The dynamic exponent $z$ is different from that of the Ising model with dynamics of model A, while the exponent $λ$ is the same.

cond-mat.stat-mech↗
arXiv2001

Corrections to Scaling for the Two-dimensional Dynamic XY Model

With large-scale Monte Carlo simulations, we confirm that for the two-dimensional XY model, there is a logarithmic correction to scaling in the dynamic relaxation starting from a completely disordered state, while only an inverse power law correction in the case of starting from an ordered state. The dynamic exponent $z$ is $z=2.04(1)$.

cond-mat.stat-mech↗
arXiv2000

Reaction-controlled diffusion

The dynamics of a coupled two-component nonequilibrium system is examined by means of continuum field theory representing the corresponding master equation. Particles of species A may perform hopping processes only when particles of different type B are present in their environment. Species B is subject to diffusion-limited reactions. If the density of B particles attains a finite asymptotic value (active state), the A species displays normal diffusion. On the other hand, if the B density decays algebraically ~t^{-a} at long times (inactive state), the effective attractive A-B interaction is weakened. The combination of B decay and activated A hopping processes gives rise to anomalous diffusion, with mean-square displacement < x_A(t)^2 > ~ t^{1-a} for a < 1. Such algebraic subdiffusive behavior ensues for n-th order B annihilation reactions (n B -> 0) with n >=3, and n = 2 for d < 2. The mean-square displacement of the A particles grows only logarithmically with time in the case of B pair annihilation (n = 2) and d >= 2 dimensions. For radioactive B decay (n = 1), the A particles remain localized. If the A particles may hop spontaneously as well, or if additional random forces are present, the A-B coupling becomes irrelevant, and conventional diffusion is recovered in the long-time limit.

cond-mat.stat-mech↗
arXiv2000

Numerical studies of the extended two-chain model of friction

We investigate numerically a simple microscopic model to describe wearless dry friction between atomically flat contact interfaces without thermal fluctuations (T=0K). The analysis of the incommensurate ground state shows a breaking of analyticity when the amplitude $ε$ of the Lennard-Jones interaction between the both harmonic chains increased beyond a critical value $ε_c$. By the introduction of a suitable order parameter and using a finite size scaling we could show that the breaking of analyticity is a second order phase transition. This transition is often called Aubry transition. By applying of an uniform external force $F^{ex}$ we have determined the critical force of static friction $F_c$ above a sliding motion occurs.

cond-mat.stat-mech↗
arXiv1999

Deterministic Equations of Motion and Dynamic Critical Phenomena

Taking the two-dimensional $ϕ^4$ theory as an example, we numerically solve the deterministic equations of motion with random initial states. Short-time behavior of the solutions is systematically investigated. Assuming that the solutions generate a microcanonical ensemble of the system, we demonstrate that the second order phase transition point can be determined already from the short-time dynamic behavior. Initial increase of the magnetization and critical slowing down are observed. The dynamic critical exponent z, the new exponent $θ$ and the static exponents $β$ and $ν$ are estimated. Interestingly, the deterministic dynamics with random initial states is in a same dynamic universality class of Monte Carlo dynamics.

cond-mat.stat-mech↗
arXiv1998

Monte Carlo Simulations of a Generalized n--spin facilitated kinetic Ising Model

A kinetic Ising model is analyzed where spin variables correspond to lattice cells with mobile or immobile particles. Introducing additional restrictions for the flip processes according to the n-spin facilitated kinetic Ising model and using Monte Carlo methods we study the freezing process under the influence of an additional nearest-neighbor interaction. The stretched exponential decay of the auto-correlation function is observed and the exponent $γ$ as well as the relaxation time are determined depending on the activation energy $h$ and the short range coupling $J$. The magnetization corresponding to the density of immobile particles is found to be the controlling parameter for the dynamic evolution.

cond-mat.dis-nn↗
arXiv1998

Dynamic Simulations of the Kosterlitz-Thouless Phase Transition

Based on the short-time dynamic scaling form, a novel dynamic approach is proposed to tackle numerically the Kosterlitz-Thouless phase transition. Taking the two-dimensional XY model as an example, the exponential divergence of the spatial correlation length, the transition temperature $T_{KT}$ and all critical exponents are computed. Compared with Monte Carlo simulations in equilibrium, we obtain data at temperatures nearer to $T_{KT}$.

cond-mat.stat-mech↗
arXiv1998

Short-time Dynamic Behaviour of Critical XY Systems

Using Monte Carlo methods, the short-time dynamic scaling behaviour of two-dimensional critical XY systems is investigated. Our results for the XY model show that there exists universal scaling behaviour already in the short-time regime, but the values of the dynamic exponent $z$ differ for different initial conditions. For the fully frustrated XY model, power law scaling behaviour is also observed in the short-time regime. However, a violation of the standard scaling relation between the exponents is detected.

cond-mat.stat-mech↗