arXiv subjects

M. Sperl

Publications and source records attributed to M. Sperl.

16 recordsLinked to original sources

arXiv2021

Dynamical yield criterion for granular matter from first principles

We investigate, using a recently developed model of liquid state theory describing the rheology of dense granular flows, how a yield stress appears in granular matter at the yielding transition. Our model allows us to predict an analytical equation of the corresponding dynamical yield surface, which is compared to usual models of solid fracture. In particular, this yield surface interpolates between the typical failure behaviors of soft and hard materials. This work also underlines the central role played by the effective friction coefficient at the yielding transition.

cond-mat.soft↗

arXiv2020

Temperature Expansions in the Square-Shoulder Fluid I: the Wiener-Hopf Function

We investigate the spatial structure of dense square-shoulder fluids. To this end we derive analytical perturbative solutions of the Ornstein-Zernike equation in the low- and high-temperature limits as expansions around the known hard sphere solutions. We then discuss the suitability of perturbative approaches in relation to the Ornstein-Zernike equation.

cond-mat.stat-mech↗

arXiv2020

Temperature Expansions in the Square Shoulder Fluid II: Thermodynamics

In a companion paper, we derived analytical expressions for the structure factor of the square-shoulder potential in a perturbative way around the high- and low-temperature regimes. Here, various physical properties of these solutions are derived. In particular, we investigate the large wave number sector, and relate it to the contact values of the pair-correlation function. Then, thermoelastic properties of the square-shoulder fluids are discussed.

cond-mat.soft↗

arXiv2013

Identifying the electronic character and role of the Mn states in the valence band of (Ga,Mn)As

We report high-resolution hard x-ray photoemission spectroscopy results on (Ga,Mn)As films as a function of Mn doping. Supported by theoretical calculations we identify, over the entire 1% to 13% Mn doping range, the electronic character of the states near the top of the valence band. Magnetization and temperature dependent core-level photoemission spectra reveal how the delocalized character of the Mn states enables the bulk ferromagnetic properties of (Ga,Mn)As.

cond-mat.str-el↗

arXiv2012

Transversal Magnetic Anisotropy in Nanoscale PdNi-Strips

We investigate submicron ferromagnetic PdNi thin-film strips intended as contact electrodes for carbon nanotube-based spintronic devices. The magnetic anisotropy and micromagnetic structure are measured as function of temperature and aspect ratio. Contrary to the expectation from shape anisotropy, magnetic hysteresis measurements of Pd0.3Ni0.7 on arrays containing strips of various width point towards a magnetically easy axis in the sample plane, but transversal to the strip direction. Anisotropic magnetoresistance measured on individual Pd0.3Ni0.7 contact strips and magnetic force microscopy images substantiate that conclusion.

cond-mat.mes-hall↗

arXiv2012

The Glass Transition in Driven Granular Fluids: A Mode-Coupling Approach

We consider the stationary state of a fluid comprised of inelastic hard spheres or disks under the influence of a random, momentum-conserving external force. Starting from the microscopic description of the dynamics, we derive a nonlinear equation of motion for the coherent scattering function in two and three space dimensions. A glass transition is observed for all coefficients of restitution, epsilon, at a critical packing fraction, phi_c(epsilon), below random close packing. The divergence of timescales at the glass-transition implies a dependence on compression rate upon further increase of the density - similar to the cooling rate dependence of a thermal glass. The critical dynamics for coherent motion as well as tagged particle dynamics is analyzed and shown to be non-universal with exponents depending on space dimension and degree of dissipation.

cond-mat.stat-mech↗

arXiv2008

Metal-insulator transition and giant anisotropic magnetoresistance in ultra thin (Ga,Mn)As

MBE-grown, 5 nm-thick annealed Ga0.95Mn0.05As films with Tc~90K demonstrate transition from metallic to insulating state below To~10K, where sheet resistances Rsh~h/e2 and both longitudinal Rxx and transverse Rxy components become comparable. Below metal-insulator transition we found giant anisotropic magnetoresistance (GAMR), which depends on orientation of magnetization to crystallographic axes and manifests itself in positive magnetoresistance near 50% for Rxx at T=1.7K, H//[110] crystallographic direction and parallel to current in contrast to smaller and negative magnetoresistance for H// direction. We connect GAMR with anisotropic spin-orbit interaction resulting in formation of high- and low- resistance states with different localization along non-equivalent easy axes.

cond-mat.mtrl-sci↗

arXiv2007

Bond formation and slow heterogeneous dynamics in adhesive spheres with long--ranged repulsion: Quantitative test of Mode Coupling Theory

A colloidal system of spheres interacting with both a deep and narrow attractive potential and a shallow long-ranged barrier exhibits a prepeak in the static structure factor. This peak can be related to an additional mesoscopic length scale of clusters and/or voids in the system. Simulation studies of this system have revealed that it vitrifies upon increasing the attraction into a gel-like solid at intermediate densities. The dynamics at the mesoscopic length scale corresponding to the prepeak represents the slowest mode in the system. Using mode coupling theory with all input directly taken from simulations, we reveal the mechanism for glassy arrest in the system at 40% packing fraction. The effects of the low-q peak and of polydispersity are considered in detail. We demonstrate that the local formation of physical bonds is the process whose slowing down causes arrest. It remains largely unaffected by the large-scale heterogeneities, and sets the clock for the slow cluster mode. Results from mode-coupling theory without adjustable parameters agree semi-quantitatively with the local density correlators but overestimate the lifetime of the mesoscopic structure (voids).

cond-mat.soft↗

arXiv2007

Dynamic Glass Transition in Two Dimensions

The question about the existence of a structural glass transition in two dimensions is studied using mode coupling theory (MCT). We determine the explicit d-dependence of the memory functional of mode coupling for one-component systems. Applied to two dimensions we solve the MCT equations numerically for monodisperse hard discs. A dynamic glass transition is found at a critical packing fraction phi_c^{d=2} = 0.697 which is above phi_c^{d=3} = 0.516 by about 35%. phi^d_c scales approximately with phi^d_{\rm rcp} the value for random close packing, at least for d=2, 3. Quantities characterizing the local, cooperative 'cage motion' do not differ much for d=2 and d=3, and we e.g. find the Lindemann criterion for the localization length at the glass transition. The final relaxation obeys the superposition principle, collapsing remarkably well onto a Kohlrausch law. The d=2 MCT results are in qualitative agreement with existing results from MC and MD simulations. The mean squared displacements measured experimentally for a quasi-two-dimensional binary system of dipolar hard spheres can be described satisfactorily by MCT for monodisperse hard discs over four decades in time provided the experimental control parameter Gamma (which measures the strength of dipolar interactions) and the packing fraction phi are properly related to each other.

cond-mat.soft↗

arXiv2006

The Jamming Transition in Granular Systems

Recent simulations have predicted that near jamming for collections of spherical particles, there will be a discontinuous increase in the mean contact number, Z, at a critical volume fraction, phi_c. Above phi_c, Z and the pressure, P are predicted to increase as power laws in phi-phi_c. In experiments using photoelastic disks we corroborate a rapid increase in Z at phi_c and power-law behavior above phi_c for Z and P. Specifically we find power-law increase as a function of phi-phi_c for Z-Z_c with an exponent beta around 0.5, and for P with an exponent psi around 1.1. These exponents are in good agreement with simulations. We also find reasonable agreement with a recent mean-field theory for frictionless particles.

cond-mat.soft↗

arXiv2004

Critical Decay at Higher-Order Glass-Transition Singularities

Within the mode-coupling theory for the evolution of structural relaxation in glass-forming systems, it is shown that the correlation functions for density fluctuations for states at A_3- and A_4-glass-transition singularities can be presented as an asymptotic series in increasing inverse powers of the logarithm of the time t: $ϕ(t)-f\propto \sum_i g_i(x)$, where $g_n(x)=p_n(\ln x)/x^n$ with p_n denoting some polynomial and x=ln (t/t_0). The results are demonstrated for schematic models describing the system by solely one or two correlators and also for a colloid model with a square-well-interaction potential.

cond-mat.soft↗

arXiv2004

Nearly-logarithmic decay of correlations in glass-forming liquids

Nearly-logarithmic decay of correlations, which was observed for several supercooled liquids in optical-Kerr-effect experiments [G. Hinze et al. Phys. Rev. Lett. 84, 2437(2000), H. Cang et al. Phys. Rev. Lett. 90, 197401 (2003)], is explained within the mode-coupling theory for ideal glass transitions as manifestation of the $β$-peak phenomenon. A schematic model, which describes the dynamics by only two correlators, one referring to density fluctuations and the other to the reorientational fluctuations of the molecules, yields for strong rotation-translation coupling response functions in agreement with those measured for benzophenone and Salol for the time interval extending from 2 picoseconds to about 20 and 200 nanoseconds, respectively.

cond-mat.dis-nn↗

arXiv2003

Dynamics in Colloidal Liquids near a Crossing of Glass- and Gel-Transition Lines

Within the mode-coupling theory for ideal glass-transitions, the mean-squared displacement and the correlation function for density fluctuations are evaluated for a colloidal liquid of particles interacting with a square-well potential for states near the crossing of the line for transitions to a gel with the line for transitions to a glass. It is demonstrated how the dynamics is ruled by the interplay of the mechanisms of arrest due to hard-core repulsion and due to attraction-induced bond formation as well as by a nearby higher-order glass-transition singularity. Application of the universal relaxation laws for the slow dynamics near glass-transition singularities explains the qualitative features of the calculated time dependence of the mean-squared displacement, which are in accord with the findings obtained in molecular-dynamics simulation studies by Zaccarelli et. al [Phys. Rev. E 66, 041402 (2002)]. Correlation functions found by photon-correlation spectroscopy in a micellar system by Mallamace et. al [Phys. Rev. Lett. 84, 5431 2000)] can be interpreted qualitatively as a crossover from gel to glass dynamics.

cond-mat.soft↗

arXiv2002

Higher-order glass-transition singularities in systems with short-ranged attractive potentials

Within the mode-coupling theory for the evolution of structural relaxation, the A_4 glass-transition singularities are identified for systems of particles interacting with a hard-sphere repulsion complemented by different short-ranged potentials: Baxter's singular potential regularized by a large-wave-vector cutoff, a model for the Asakura-Oosawa depletion attraction, a triangular potential, a Yukawa attraction, and a square-well potential. The regular potentials yield critical packing fractions, critical Debye-Waller factors and critical amplitudes very close to each other. The elastic moduli and the particle's localization lengths for corresponding states of the Yukawa system and the square-well system may differ by up to 20% and 10%, respectively.

cond-mat.soft↗

arXiv2002

Logarithmic Relaxation in Glass-Forming Systems

Within the mode-coupling theory for ideal glass transitions, an analysis for the correlation functions of glass-forming systems for states near higher-order glass-transition singularities is presented. It is shown that the solutions of the equations of motion can be asymptotically expanded in polynomials of the logarithm of time t. In leading order, an ln(t)-law is obtained, and the leading corrections are given by a fourth-order polynomial. The correlators interpolate between three scenarios. First, there are planes in parameter space where the dominant corrections to the ln(t)-law vanish, so that the logarithmic decay governs the structural relaxation process. Second, the dynamics due to the higher-order singularity can describe the initial and intermediate part of the alpha-process thereby reducing the range of validity of von Schweidler's law and leading to strong alpha-relaxation stretching. Third, the ln(t)-law can replace the critical decay law of the beta-process leading to a particularly large crossover interval between the end of the transient and the beginning of the alpha-process. This may lead to susceptibility spectra below the band of microscopic excitations exhibiting two peaks. Typical results of the theory are demonstrated for models dealing with one and two correlation functions.

cond-mat.soft↗

arXiv2000

Higher order glass-transition singularities in colloidal systems with attractive interactions

The transition from a liquid to a glass in colloidal suspensions of particles interacting through a hard core plus an attractive square-well potential is studied within the mode-coupling-theory framework. When the width of the attractive potential is much shorter than the hard-core diameter, a reentrant behavior of the liquid-glass line, and a glass-glass-transition line are found in the temperature-density plane of the model. For small well-width values, the glass-glass-transition line terminates in a third order bifurcation point, i.e. in a A_3 (cusp) singularity. On increasing the square-well width, the glass-glass line disappears, giving rise to a fourth order A_4 (swallow-tail) singularity at a critical well width. Close to the A_3 and A_4 singularities the decay of the density correlators shows stretching of huge dynamical windows, in particular logarithmic time dependence.

cond-mat.soft↗