Anomalous Dynamic Behavior Research Articles

Heterogeneous structure of poly(vinyl chloride) as the origin of anomalous dynamical behavior

We have investigated the thermal evolution of the structure and the dynamics of poly(vinyl chloride) (PVC) in a wide temperature range. Corroborating earlier findings, small angle neutron scattering revealed the presence of structural heterogeneities. On the other hand, the single chain form factor corresponds to that of Gaussian chains. Gradually with increasing temperature the system becomes homogeneous. A simple description of the heterogeneities in terms of microcrystallites is forwarded. The dynamical behavior of PVC has been investigated combining broadband dielectric spectroscopy (DS) with coherent and incoherent neutron scattering. In a wide temperature range broadband DS facilitated a precise determination of the dynamic response related to the segmental relaxation. Close to the glass transition temperature the line shape strongly deviates from the usual Kohlrausch–Williams–Watts functional form of common glassforming systems. Moreover, the characteristic relaxation time observed by incoherent scattering displays an anomalous dependence on momentum transfer indicating the possible existence of heterogeneities in the sample. Based on the structural and dynamical results, a model is proposed, that considers the coexistence of regions with different dynamical properties leading to a distribution of characteristic relaxation times. The model accounts for the experimental observations, assuming for all regions the same functional form for the α-relaxation. It may be univocally determined from the coherent scattering data at the first static structure peak. The distribution of relaxation times found is compatible with the distribution of only one variable, the glass transition temperature.

Anomalous magnetic and dynamic behavior in magnetoresistive compounds: origin of bulk colossal magnetoresistivity

We present the results of our extensive Mössbauer effect studies carried out on a wide variety of mixed valence manganites as well as other types of magnetoresistive materials, including pyrochlore Tl2Mn2O7 and the chalcospinels Fe0.5Cu0.5Cr2S4 and FeCr2S4 with absolutely different natures of the magnetism, in a search for similarities linked to their magnetoresistive behavior. The double exchange electron transfer and coupling between the electrons and Jahn–Teller lattice distortions invoked by most theories to explain the colossal magnetoresistivity and associated metal–insulator transition in manganites are not applicable to pyrochlore nor to chalcospinels. Nevertheless, we find intriguing similarities in the anomalous magnetic and dynamic behavior among these widely different systems at, above, and below the Curie temperature TC, which shed light on the origin of bulk magnetoresistivity in general. All these compounds share the following features. The long-range ferromagnetic order breaks down even below the Curie temperature, with the formation of nano-size spin clusters. Softening of the lattice was observed near TC. The short-range interactions in these spin clusters survive well above TC. When an external magnetic field is applied, the spin clusters coalesce to form large clusters, with considerable lowering of the resistivity. There is a strong evidence that the existence of nano-size spin clusters with superparamagnetic-like behavior near TC is a prerequisite for the occurrence of bulk magnetoresistivity.

Magnon damping by magnon-phonon coupling in manganese perovskites

Inelastic neutron scattering was used to systematically investigate the spin-wave excitations (magnons) in ferromagnetic manganese perovskites. In spite of the large differences in the Curie temperatures ($T_C$s) of different manganites, their low-temperature spin waves were found to have very similar dispersions with the zone boundary magnon softening. From the wavevector dependence of the magnon lifetime effects and its correlation with the dispersions of the optical phonon modes, we argue that a strong magneto-elastic coupling is responsible for the observed low temperature anomalous spin dynamical behavior of the manganites.

Inulin hydrogels. I. Dynamic and equilibrium swelling properties

Inulin hydrogels were developed as potential new carriers for colonic drug targeting. This work describes the dynamic and equilibrium swelling properties of the prepared inulin hydrogels. The influence of various parameters on these properties were assessed, in particular the degree of substitution and feed concentration of methacrylated inulin and varying concentrations of the initiators of the polymerisation reaction. As these hydrogels were developed as colon-specific drug delivery systems, also the effect of pH, ionic strength and esterases were investigated. The results suggest that the rate of water transport into the inulin hydrogels is quite high (mean swelling time <1.2 h) and that the hydrogels exhibit anomalous dynamic swelling behaviour. The equilibrium swelling of the hydrogels is influenced by the degree of substitution and feed concentration of methacrylated inulin, by the initiator concentration, and by the ionic strength and an acidic pH of the swelling solvent. Esterase activity and pH values of the swelling solvent commonly encountered in the small intestine and the colon on the contrary do not affect equilibrium swelling. By means of differential scanning calorimetry, the glass transition temperature of dry methacrylated inulin hydrogel samples was determined. The mobility of the hydrogel chains seemed to be affected by both the degree of substitution and the feed concentration of methacrylated inulin.

Populated Domain Walls

Several experiments suggest that the charge carriers in the normal state of certain cuprate superconductors reside on domain walls. In an earlier paper, we suggested that several aspects of the anomalous dynamical behavior of these materials could be explained, at least qualitatively, on this basis. Here, using results on the ground state energy of the one-dimensional Hubbard model (soluble by Bethe ansatz techniques) as a function of charge density, we argue that a nonzero charge density localized to domain walls is a very plausible consequence of strong short-range electron-electron repulsion. We also suggest a method to suppress meandering of the walls and thereby enhance their signature in neutron diffraction.

Low dark current p-i-n photodiodes with an anomalous dynamic behavior

Up to now, InGaAs/InP photodiodes are the main detectors for the long-wavelength region. The dynamic behavior of these devices limits the maximum bitrate of communication systems. We have investigated the dynamic behavior of these heterojunction devices. If the heterojunction lies outside of the space charge region, the heterointerface shows an additional response due to a storage of carriers in the valley of the conduction band. At higher biases the interface lies inside the space charge region and the time response of the interface is negligible and no additional time response was observed.

Anomalous dynamical behavior of the Cu(110) surface.

The thermal vibrations of surface atoms on Cu(110) have been studied as a function of temperature with ion-scattering spectroscopy in the impact-collision mode. By comparing the experimental data to Monte Carlo simulations of the scattering process, mean-square displacements of surface atoms are obtained that agree well with the results of helium diffraction and inverse photoemission experiments. Below 550 K the atom vibrations can be described quite well by a Debye temperature of 224 K obtained from phonon slab calculations. Above 550 K, strong anharmonic vibrations are observed. In this temperature region we find thermally activated adatoms to be produced with an activation energy of 0.27\ifmmode\pm\else\textpm\fi{}0.07 eV. Both effects lead above 1000 K to structural changes of the surface that can be described in terms of a rough surface or a quasiliquid layer on the surface.

Anomalous dynamic behaviour of a gas of quasiparticles in superfluid 3He-B

Anomalous dynamic behaviour of a gas of quasiparticles in superfluid 3He-B

Anomalous dynamic behaviour of a gas of quasiparticles in superfluid 3HeB

In 3 HeB it is possible to probe the unusual dynamics arising from the non-Newtonian dispersion relation of the excitations. Since the dispersion relation is tied to the rest frame of the condensate, it is distorted by condensate motion. A container of superfluid with a velocity gradient has a range of excitation dispersion curves, depending on velocity and thus the density of states available to the excitations varies with position. We show that, in consequence the behaviour of a moving object in the excitation gas shows unusual and often counter-intuitive behaviour. At low enough temperature, the force on a moving object becomes independent of velocity.

Dynamics of random spin systems

We present inelastic neutron scattering experiments on three prototypical random magnets. For the dilute, insulating antiferromagnet Rb2CoxMgt-xF4, the randomness has purely geometrical consequences, and the anomalous dynamical behavior which we observe for x close to the magnetic percolation threshold is due to the fractal nature of percolating networks. Comparison of inelastic and quasielastic scattering data demonstrate the dynamical nature of the spin glass transition in amorphous, metallic MnSi. Finally, we show the coexistence of spin waves and static spin fluctuations near the crossover from ferromagnetic to spin glass behaviors in amorphous (Fe, Mn1-x)75P16B6Al3.

Dynamics of Random Spin Systems

We present inelastic neutron scattering experiments on three prototypical random magnets. For the dilute, insulating antiferromagnet Rb 2 Co x Mg t- x F 4 , the randomness has purely geometrical consequences, and the anomalous dynamical behavior which we observe for x close to the magnetic percolation threshold is due to the fractal nature of percolating networks. Comparison of inelastic and quasielastic scattering data demonstrate the dynamical nature of the spin glass transition in amorphous, metallic MnSi. Finally, we show the coexistence of spin waves and static spin fluctuations near the crossover from ferromagnetic to spin glass behaviors in amorphous (Fe, Mn 1- x ) 75 P 16 B 6 Al 3 .

Experimental tests of non-thermal effect for pulsed-laser annealing by time-resolved reflectivity and EPR measurements

Experimental tests of non-thermal effect for pulsed laser annealing (PLA) of semiconductors have been done by means of two techniques. One is time-resolved reflectivity measurement during single 30-ps PLA of amorphous GaAs. An anomalous dynamic behavior is observed at an energy-density window, i.e., a reflectivity dip appears after the disappearance of the high reflectivity phase, concomitant with the final production of a new amorphous GaAs state. This result cannot be interpreted in terms of the simple thermal effect. The other is EPR measurement of Si samples which are implanted and then annealed by 40-ns pulsed laser. No EPR results of N donors in Si support positively a non-thermal effect, while it is difficult to explain EPR results of laser-induced paramagnetic defects only by the simple thermal annealing model.

Anomalous dynamic mechanical behavior of poly(vinyl chloride)

AbstractIn this paper, we present a study of the dynamic mechanicall behavior of a quenched poly(vinyl chloride). A new relaxation between the glass transition and the β transition is reported. We have related this new relaxation to an absence of physical aging.

Dynamic Properties of "Hard" Magnetic Bubbles

A theory is developed to explain the anomalous dynamic behavior of a new type of cylindrical magnetic domain called a hard bubble. This theory, which is based on the Landau-Lifshitz-Gilbert equation of motion, is able to explain satisfactorily all the salient features of the motion of hard bubbles.