Broad Spectrum Of Relaxation Times Research Articles

The origin of charge transients in Al/undoped diamond/p-Si diodes

An experimental study of relaxation behaviour of Al/undoped diamond/p-Si diodes was conducted using charge deep-level transient spectroscopy (QDLTS) and feedback charge capacitance method (FCM) in the time domain. Prior to vacuum deposition of Al onto 5-μm-thick diamond films, the latter were exposed to a hydrogen discharge at 150 °C for 30 min. When measuring capacitance at delays of a few microseconds with respect to the trailing edge of the probing pulse ΔU at ambient temperatures, only the instantaneous capacitance C(0) corresponding to the thickness of the diamond film was detected. After shifting the excitation and processing of the charge transients to the region of tens of milliseconds, excess capacitance (charge) is observed, its value depending on temperature. Both isothermal and thermal-scan QDLTS measurements revealed a broad spectrum of relaxation times indicating a Debye-type dielectric relaxation, i.e. the amplitude of the QDLTS peak is a linear function of ΔU. The thermal activation energy ΔE≈0.13 eV of the spectral components corresponding to the maximum of the signal matches perfectly the activation energy of dc conductivity at equivalent temperatures. The origin of the dielectric relaxation is explained in terms of a Maxwell–Wagner polarisation due to a conducting phase embedded in the semi-insulating matrix of the diamond films, the width of the spectrum of relaxation times reflecting the spread of the sizes of microcrystallites. The results may also be useful for finding the demarcation temperature of detecting response from deep traps located within the depletion region at the Al/diamond interface.

The Dynamic Properties of 5Cb Filled with Aerosil Particles Investigated by Pcs

ABSTRACTNematic liquid crystal filled with Aerosil particles, a prospective composite material for optoelectronic application, has been investigated by static light scattering and Photon Correlation Spectroscopy (PCS). The Aerosil particles in filled nematic liquid crystals (FN) form a network structure with LC domains about 2500 Å in size with a random distribution of the director orientation of each domain.We found that the properties of 5CB are considerably affected by the network. The N-I phase transition in filled 5CB was found to be smeared out and depressed. PCS experiments show that two new relaxation processes appear in filled 5CB in addition to the director fluctuation process in bulk. The slow relaxation process, with a broad spectrum of relaxation times, is somewhat similar to the slow decay, which is observed in confined nematic liquid crystal.The middle frequency process was assigned to the director fluctuations in the surface layer formed at the particle-LC interface. The decay function describing this relaxation process is a stretched exponential (β ≍ 0.7). The temperature dependence of the relaxation times of the middle frequency obeys the Vogel-Rilcher law. Such a temperature dependence, accompanied by a broad spectrum of relaxation times suggests that the dynamics of the director fluctuations near the Aerosil particle-LC interface is glass-like.

Linear viscoelastic behavior of aggregated colloidal dispersions

The viscoelastic behavior of a depletion-flocculated dispersion of colloidal spheres is investigated at different volume fractions of the spheres, using a controlled stress and a dynamic rheometer. Combining the results, we obtain the storage G′ and loss G′′ moduli over a frequency range of 0.02<ω<200rad/s. The measured G′ gradually increases with increasing frequency, while G′′ almost remains constant, indicating a broad spectrum of relaxation times. To describe and explain the observed behavior of the moduli as a function of frequency and volume fraction in terms of microscopic parameters, a microrheological model based on the fractal concept is proposed. Comparing experimental results with model calculations, we find a good agreement between the two, with physically plausible parameter values.

Slow Dynamics and Glass-Like Behavior of Liquid Crystals Dispersed in Nanoporous Media

ABSTRACTWe have investigated the dynamic behavior of liquid crystals (LC), which are not glass formers when in bulk form, confined in porous matrices with randomly oriented, interconnected pores as well as in parallel cylindrical pores with different pore sizes by photon correlation (time range: 20 ns-103s) and dielectric spectroscopies (frequencies: 0.1 Hz-1.5GHz). We observed that in random pores (pore size is 10 nm and 100 nm) LC does not crystallize at temperatures about 25° C below bulk crystallization temperature and the non-Debye relaxational processes studied by both photon correlation and dielectric experiments were found not to be frozen. Slow relaxational process which does not exist in bulk LC and a broad spectrum of relaxation times (10−8− 10)s appear not only for LC in random pores but in cylindrical pores as well. However in matrices with random pores of 100 Å, glass-like behavior of slow mode (τ&gt; 1ms) was observed. The relaxation time (determined in photon correlation experiment) of this slow process strongly increases when temperature decreases from 300 K up to 270 K varying from 0.2ms to 14 s and it's temperature dependence is described by the Vogel-Fulcher law.

Photon Correlation Spectroscopy of Liquid Crystals Confined in Porous Matrices with Different Structure and Pore Size

ABSTRACTWe present the results of photon correlation spectroscopy investigations of the influence of confinement, interface, porous matrix structure, pore size and shape on the dynamic behavior of nematic liquid crystals (LC) dispersed in porous matrices with randomly oriented, interconnected pores (porous glasses) and parallel cylindrical pores (Anopore membranes). Investigations of LC in cylindrical pores together with studies in random porous matrices, makes it possible to separate the role of random structure and domain formation from the contributions due to existence of LC - solid pore wall interface and pure finite size effect in relaxation of order parameter or director fluctuations. In the temperature range below nematic - isotropie phase transition temperature we observed two overlapping relaxational processes which are satisfactorily described by the decay functionf(q,t) =a·exp(–t/τ1) + (1–a)·exp(–xz), wherex=ln(t/τ0)/ln(τ2/τ0) and τ0= 10−8s. For LC in 100 Å random pores the second term describing the slow process dominates, whereas for 200 Å and 2000 Å cylindrical pores as well as 1000 Å random pores the contribution from the first term (fast process) is more visible. Since the slow relaxational process which does not exist in the bulk LC and broad spectrum of relaxation times (10−6- 10)s appear not only for LC in random pores but in cylindrical as well, we conclude that differences in dynamical behavior of confined LC from that in the bulk are mainly due to the existence of the interface.

The Role of Stochastic Noise on the Glass Transition

ABSTRACTGlasses are amorphous solids that exhibit an intricate structural relaxation. A broad relaxation time spectrum always emerges when these systems are perturbed. By using a Langevin-type differential equation to describe the structure dynamicsof these materials, it is depicted how the broad relaxation time spectrum arises due to the stochastic noise and how this affects the system's structure evolution as it is cooled down into the glass transition region. This stochastic model provides a macroscopic as well a microscopic view of the glass relaxation process.

Dielectric relaxation dynamics in C 60 and C 70

The electrical response of polycrystalline fullerites has been studied after annealing them under various conditions. Dielectric measurements have been carried out in the frequency range 100 Hz < ν < 1 MHz and for temperatures 10K < T < 300 K. For C 60, electronic conduction, a phase transition and a dielectric relaxation process are clearly resolved. The latter is described by simple thermally activated behaviour but a broad spectrum of relaxation times, the origin of which is discussed. For C 70, no clear experimental evidence for a similar dielectric relaxation process could be found in the temperature and frequency regime investigated.

Ordering of microscopic Dzyaloshinskii-Moriya vectors in spin glasses: Statics and low-frequency dynamics.

The mean-field theory of static and dynamic properties of the Dzyaloshinskii-Moriya (DM) interaction in Heisenberg spin glasses is developed. The main idea of the paper is based on the property of microscopic DM vectors ${\mathbf{D}}_{\mathit{i}\mathit{j}}$ describing this interaction: each vector ${\mathbf{D}}_{\mathit{i}\mathit{j}}$ is defined up to a sign. It is a reason for the existence of additional degrees of freedom connected with ordering of vectors ${\mathbf{D}}_{\mathit{i}\mathit{j}}$ on orientations below the freezing temperature. The theory is formulated on the basis of the eigenmodes of the exchange integrals matrix ${\mathit{J}}_{\mathit{i}\mathit{j}}$. There are two additional important assumptions to define the model. The first one is the independence of the sum ${\mathit{tsum}}_{\mathit{i}}$${\mathit{J}}_{\mathit{i}\mathit{j}}$ from the index j that restricts possible distributions of random exchange integrals. However, due to this assumption the total magnetization appears to be a hard eigenmode of the exchange part of the Hamiltonian and it is possible to a great extent to escape a detailed consideration of the spin-glass transition as well as to separate terms containing magnetization vectors in an equation for the thermodynamic potential in explicit form. The description of ordering of DM vectors ${\mathbf{D}}_{\mathit{i}\mathit{j}}$ is carried out in terms of some effective paramagnetic pseudospin, of which the state depends on the thermodynamic path of the spin-glass sample. This property displays itself in the distinction between the field and zero-field-cooling susceptibility and in an appearance of the macroscopical unidirectional anisotropy field (the DM field). The second assumption is to consider the DM field as a slow dynamical variable with a broad spectrum of relaxation times which, however, can behave as a quenched quantity at low enough temperatures. Motion equations of a dissipative character for pseudospins are examined. Pseudospins corresponding to localized eigenmodes with characterizing size \ensuremath{\xi}\ensuremath{\sim}${10}^{2}$--${10}^{3}$ \AA{} effectively contribute to observable slow relaxation. Using the dynamic susceptibility calculated the frequency dependence of freezing temperature and the magnetization decay law are found.

Power-law-like stress relaxation of block copolymers: disentanglement regimes

We consider stress relaxation in a strongly segregated lamellar mesophase, where block copolymers are in the state with junction points confined to the interface between the adjacent lamellae and blocks stretched out away from it. If the molecular weight of the blocks is large enough, they entangle with their neighbors as well as with blocks from the opposite brush. The number of entanglements of one particular chain with the opposite brush varies from chain to chain even in the monodisperse system. We demonstrate that this dispersion of the number of entanglements leads to a very broad spectrum of relaxation times and to an effectively power-law-like stress relaxation function log G - (log t) with CY = 1/2 (a = 2) in the strong (weak) chain-stretching limit. We analyze various disentanglement mechanisms for diblocks and triblocks and the onset of the diffusion of copolymers along the interface. Another relaxation mechanism is due to the displacement of a block across the interface into the enemy phase. We conclude that for highly entangled copolymers it will be an important mode of stress relaxation (especially for triblock copolymers). For asymptotically long times the relaxation of stress along the perfectly ordered lamella will be liquidlike. In a system with defects in domain structure the relaxation of stress is controlled by the processes of equilibration of excess density along the layers. For the lamellar mesophase we found G(t) - t 4. 2, while for the cylindrical mesophase, G(t) - t

Low-frequency dielectric response of PbMg1/3Nb2/3O3

An investigation was carried out on both the linear and nonlinear parts of the complex dielectric susceptibility in the frequency and temperature ranges 10-3-103 Hz and 100-450 K, respectively. The analysis of the imaginary part of the dielectric susceptibility has revealed the existence of a broad spectrum of relaxation times in the above temperature range. The temperature dependence of the most probable frequency of the spectrum was obtained. For temperatures T

Dynamic bulk moduli of several elastomers

Dynamic hulk moduli are given as functions of frequency for several types of elastomers over a temperature range of − 10 °C to + 40 °C. Measurements were made by a modification of the acoustic coupler method of McKinney et al. [J. Appl. Phys. 27, 425 (1956)]. Values of K′, the real or storage part of the complex modulus, fell into the range 2 to 4 GPa, typical of values for organic liquids. The loss tangents fell into the range below 5% and were mostly less than 2%, also typical of many liquids. This similarity to liquids suggests that similar short‐range dynamic mechanisms dominate the compressional free‐volume models. One neoprene sample exhibited the full transition from low to high limiting values of K″ over the experimental temperature range, thereby providing a good test for the applicability of WLF‐type temperature “shifting” in these materials. A master curve was constructed for this sample. It gave evidence for a relatively broad spectrum of relaxation times.

Experimental and linear viscoelastic stress distribution measurements of high density polyethylene flowing into and within a slit

We report experimental measurements fo the centreline stress build up and relaxation as molten polyethelene flows into a slit die. The time-dependent extensional stress distribution is obtained using flow birefringence techniques and these observations complement the corresponding velocity field measurements already reported. Experimental measurements of the linear viscoelastic storage and loss modulus are obtained and, from these results, the polymers are characterized in terms of a modulus spectrum. Using this modulus spectrum together with a Maxwell-type constitutive equation and the experimental centreline kinematics, we find that it is possible to simulate successfully the experimentally observed stress distributions. Our results indicate that it is essential to include the polymers' broad spectrum of relaxation times when considering time dependent flow problems.

Low-frequency current noise with a 1/fspectrum in solids

The problem of low-frequency current noise with a 1/f spectrum in solids (flicker noise) is reviewed. The fundamental general properties of this noise are described: frequency-dependence of the spectral density, time-dependence of the correlation function, form of the distribution function of the fluctuations, dependence of the spectral density on the potential applied to the specimen, anisotropy of the fluctuations of the resistivity tensor, correlation length of the fluctuations, dependence of the noise on the dimensions of the specimen and on the concentration of current carriers, the empirical Hooge relationship, etc. A model of 1/f noise is presented and discussed that associates its spectrum with the presence in solids of an extensive hierarchy of relaxation times. Concrete models of systems having an exponentially broad spectrum of relaxation times are described (the McWhorter model, two-level tunneling systems, the disordered Ising kinetic model). The theory and experimental data on current noise caused by temperature fluctuations are analyzed. The problem is treated of surface noise in semiconductors and 1/f noise in metal films.

Measurement of complex susceptibility on a metallic spin glass with broad relaxation spectrum

AC susceptibility measurements (0.625 H z⩽ v ⩽625 H z) have been performed in a SQUID magnetometer on the amorphous metallic spin glass (Fe 0.06Ni 0.94) 75P 16B 6Al 3. The in-phase component of the susceptibility ( x') shows a sharp peak at the spin glass freezing temperature T g≈8 K with a frequency dependence of ▵ T g/ T g≈0.015 per decade of frequency increase. The out-of-phase component ( x″) exhibits a small anomaly at the spin glass transition with the magnitude of the anomaly almost independent of frequency. In particular we find the relation x″=( π;/2)ϑ x'/ϑ lnω to be valid in the vicinity of T g. This relation and a frequency independence of x″ are fundamental characteristics of a spin glass having a broad spectrum of relaxation times. We argue that similar results from ac susceptibility measurements should be found in many spin glass systems, metallic as well as non-metallic.

Dielectric relaxation in hydrated elastin

Abstract The dielectric properties of water absorbed in elastin are measured for different water, NaCl and HCl contents over the frequency range 102–105 Hz from 20 to—60°C. For all samples, both the dielectric constant and the loss factor decrease monotonically with increasing frequency or decreasing temperature. The absence of a maximum in the loss curves is indicative of an extremely broad spectrum of relaxation times. By shifting the data along the log frequency axis, all the curves obtained for the dielectric constant and the loss factor at different temperatures are represented on master curves at an arbitrarily chosen temperature. The results are explained on the basis that hydrogen-bonded water clusters with a wide size distribution exist in contact with elastin and that their diffusion is the rate determining step for dipolar orientation. These water molecules are nonfreezable and quite mobile at low temperatures until a glassy state results at approximately 130°K.

Effect of interdiffusion on the elasticity and internal friction of compositionally modulated copper-nickel thin films

Copper-nickel thin films with composition modulation wavelengths of 9 Å and 35 Å have been prepared on thin reed substrates of fused silica. The films have been annealed at temperatures up to 550 °C to produce interdiffusion. The virtual absence of any change in the low temperature natural vibration frequencies shows that the Young's modulus of the film is essentially unaffected by homogenization, and thus no anomaly is apparent in the elastic behavior of the initial modulated structure. Above 300 °C a strong thermally activated internal friction is observed, which decreases irreversibly on annealing near 500 °C in a manner that correlates with homogenization of the film by lattice diffusion. The internal friction involves a broad spectrum of relaxation times with activation energies between 2 and 3 eV. It is concluded that grain boundary relaxation is probably the dominant cause of the internal friction, but this is only possible if the relaxation time is proportional to the grain size, as predicted by the theory of viscous sliding. Because of their much finer grain size, it is expected that grain boundary peaks may be exhibited in thin films at appreciably lower temperatures than in bulk samples.

The structure of water absorbed in collagen. I. The dielectric properties.

AbstractThe dielectric constant and the loss factor of water in collagen are measured for various water, NaCl, and HCl contents at frequencies ranging from 102 to 105 Hz and at temperatures ranging from −70° to +23°C. For all measurements, both the dielectric constant and the loss decrease monotonically as the frequency increases, or the temperature decreases; the absence of a maximum in the loss curves as a function of temperature and frequency indicates an extremely broad spectrum of relaxation times. By shifting the curves obtained for the dielectric constant and the loss factor along the log–frequency axis, all data, obtained at different temperatures, can be represented on master curves valid for 23°C.In order to explain these results, water molecules are assumed to be hydrogen bonded to each other in long chains. All water molecules in a chain can, cooperatively, be oriented in two different directions along the channel, resulting in large, reversible, dipole moments. These chains are not rigid, but are flexible liquid‐like structures. Diffusion of chains as entities is assumed to be the rate‐limiting step for dipole reorientation. Although the rate of diffusion decreases inversely proportional to chain length, the activation energy is independent of chain length. At lower temperatures, diffusion becomes slower, until at the glass point, approximately −100°C, it ceases.

Application of the Gevers–Du Pre Theory of Amorphous Dielectrics

A review is given of the Gevers–du Pre theory of amorphous dielectrics and its findings are examined for applicability to amorphous films of silicon monoxide (SiO). In the context of the theory amorphous dielectrics are defined as those insulators having irregular structures, a wide range of activation energies, and a broad spectrum of relaxation times. Inasmuch as polycrystalline film dielectrics display a distribution of relaxation times, the Gevers–du Pre theory should apply to films of SiO and many other dielectrics. For the present study, SiO was deposited onto boro-silicate glass substrates at rates ranging from 54 to 133 Å/min. The values of the temperature coefficient of capacitance (TCC) showed a strong dependence upon the deposition rate, varying from −23 to +300 ppm/°C. The negative values were obtained for the slower deposition rates, and the positive values for the faster rates. All of the TCC data agreed qualitatively with the Gevers–du Pre theory, suggesting that the negative TCC’s are due to large expansion coefficients and small film densities obtained at slow deposition rates. The results of the present study also suggest that zero TCC’s can be obtained for SiO films by precise control of the deposition rate.