Space-charge mechanism of aging in ferroelectrics: an exactly solvable two-dimensional model

A mechanism of point defect migration triggered by local depolarization fields is shown to explain some still inexplicable features of aging in acceptor doped ferroelectrics. A drift-diffusion model of the coupled charged defect transport and electrostatic field relaxation within a two-dimensional domain configuration is treated numerically and analytically. Numerical results are given for the emerging internal bias field of about 1 kV/mm which levels off at dopant concentrations well below 1 mol%; the fact, long ago known experimentally but still not explained. For higher defect concentrations a closed solution of the model equations in the drift approximation as well as an explicit formula for the internal bias field is derived revealing the plausible time, temperature and concentration dependencies of aging. The results are compared to those due to the mechanism of orientational reordering of defect dipoles.Comment: 8 pages, 4 figures. accepted to Physical Review

Incremental expansions for Hubbard-Peierls systems

The ground state energies of infinite half-filled Hubbard-Peierls chains are investigated combining incremental expansion with exact diagonalization of finite chain segments. The ground state energy of equidistant infinite Hubbard (Heisenberg) chains is calculated with a relative error of less than $3 \cdot 10^{-3}$ for all values of $U$ using diagonalizations of 12-site (20-site) chain segm ents. For dimerized chains the dimerization order parameter $d$ as a function of the onsite repulsion interaction $U$ has a maximum at nonzero values of $U$, if the electron-phonon coupling $g$ is lower than a critical value $g_c$. The critical value $g_c$ is found with high accuracy to be $g_c=0.69$. For smaller values of $g$ the position of the maximum of $d(U)$ is approximately $3t$, and rapidly tends to zero as $g$ approaches $g_c$ from below. We show how our method can be applied to calculate breathers for the problem of phonon dynamics in Hubbard-Peierls systems.Comment: 4 Pages, 3 Figures, REVTE

Aging of poled ferroelectric ceramics due to relaxation of random depolarization fields by space-charge accumulation near grain boundaries

Migration of charged point defects triggered by the local random depolarization field is shown to plausibly explain aging of poled ferroelectric ceramics providing reasonable time and acceptor concentration dependences of the emerging internal bias field. The theory is based on the evaluation of the energy of the local depolarization field caused by mismatch of the polarizations of neighbor grains. The kinetics of charge migration assumes presence of mobile oxygen vacancies in the material due to the intentional or unintentional acceptor doping. Satisfactory agreement of the theory with experiment on the Fe-doped lead zirconate titanate is demonstrated.Comment: theory and experiment, 22 pages, 3 figure

Evaluation of the CELL-DYN® 3500 haematology instrument for the analysis of the mouse and rat blood

The objective of this study was to evaluate the performance of the CELL-DYN® 3500 for rat and mouse blood analysis in a routine environment. The WBC (white blood cells), RBC (red blood cells), PLT (platelets) counts and the WBC differential were determined. In addition, the following aspects were studied: within-run precision, day-to-day precision, biasfree paired difference precision; extended ranges of linearity for RBC, HCT (haematocrit), WBC, PLT; carry-over, the fffect of blood ageing, cell stability with different anticoagulants; and the normal ranges, the out of range flagging and some typical pathology cases. The CELL-DYN® 3500 is a multiparameter flow cytometer which counts and differentiates WBC, based on the principle of multi-angle polarised light scatter separation. RBC and PLT are determined by the impedance method. The WBC count is evaluated by both, optical and impedance methods. Reference methods used were according to the ICSH recommendations on blood cell analysis, including manual counts of WBC and platelets, a centrifugal microhaematocrit method and a haemoglobin measurement by spectrophotometry using the WHO haemoglobin standard. All cell counts were compared with the results obtained by our routine blood cell analyser (Contraves AL820), and the WBC differential was compared with the manual microscopic differentiation of the 400 WBC (200 cells differentiated by two technicians). The following coefficients of variation were obtained: within-run precision was 1.2% and 2.7% for WBC; 1.0% and 1.0% for RBC; 1.3% and 0.9% for haematocrit; 2.1% and 2.7% for platelets (rats and mice respectively). Day-to-day precision was performed using human trilevel control blood, and the CVs were found to be <1.7% for WBC, <1.4% for RBC, <1.2% for haemoglobin and <6.3% for platelets. The following ranges of measurement were found to be linear in the rat: WBC: 0.10-20.20×103/μl; RBC: 0.016-14.3×106/μl; haemoglobin: 0.08-26.8 g/dl; haematocrit: 5.0%-77%; platelets: 14.0-1670.0×103/μl. Equal ranges were observed for mouse blood. Carry-over in rat blood was found to be 0.12% for WBC, 0.05% for RBC, 0.15% for haemoglobin and 0.46% for platelets. In mice, similar carry-over results were obtained. The correlation coefficients (Pearson, correlation coefficient) between the CELL-DYN® 3500 and Contraves AL 820 using linear regression analysis were as follows: 0.988 and 0.997 for WBC; 0.986 and 0.920 for RBC; 0.995 and 0.984 for haemoglobin; 0.958 and 0.85 for haematocrit; 0.958 and 0.963 for platelets, for rats and mice, respectively. Correlation coefficients between the CELL-DYN® 3500 and the manual differential of NEU (neutrophils) and LYM (lymphocytes) were higher than 0.8 in rats and higher than 0.9 in mice. Due to the relatively low absolute counts of MONO (monocytes), EOS (eosinophils) and BASO (basophils), only moderate correlation of methods was found. The CELL-DYN® 3500 was judged to be reliable, accurate and easy-to-use for counting and identifying normal and most of the pathological blood specimens obtained from mice and rats. By using the CELL-DYN® 3500, the time for blood sample analysis can be shortened significantly and provides extensive opportunities to characterise pathological sample

Linewidth of a quantum-cascade laser assessed from its frequency noise spectrum and impact of the current driver

We report on the measurement of the frequency noise properties of a 4.6-μm distributed-feedback quantum-cascade laser (QCL) operating in continuous wave near room temperature using a spectroscopic set-up. The flank of the R(14) ro-vibrational absorption line of carbon monoxide at 2196.6cm−1 is used to convert the frequency fluctuations of the laser into intensity fluctuations that are spectrally analyzed. We evaluate the influence of the laser driver on the observed QCL frequency noise and show how only a low-noise driver with a current noise density below {\approx} 1~\mbox{nA/}\sqrt{}\mbox{Hz} allows observing the frequency noise of the laser itself, without any degradation induced by the current source. We also show how the laser FWHM linewidth, extracted from the frequency noise spectrum using a simple formula, can be drastically broadened at a rate of {\approx} 1.6~\mbox{MHz/}(\mbox{nA/}\sqrt{}\mbox{Hz}) for higher current noise densities of the driver. The current noise of commercial QCL drivers can reach several \mbox{nA/}\sqrt{}\mbox{Hz} , leading to a broadening of the linewidth of our QCL of up to several megahertz. To remedy this limitation, we present a low-noise QCL driver with only 350~\mbox{pA/}\sqrt{}\mbox{Hz} current noise, which is suitable to observe the ≈550kHz linewidth of our QC

Internal states of model isotropic granular packings. III. Elastic properties

In this third and final paper of a series, elastic properties of numerically simulated isotropic packings of spherical beads assembled by different procedures and subjected to a varying confining pressure P are investigated. In addition P, which determines the stiffness of contacts by Hertz's law, elastic moduli are chiefly sensitive to the coordination number, the possible values of which are not necessarily correlated with the density. Comparisons of numerical and experimental results for glass beads in the 10kPa-10MPa range reveal similar differences between dry samples compacted by vibrations and lubricated packings. The greater stiffness of the latter, in spite of their lower density, can hence be attributed to a larger coordination number. Voigt and Reuss bounds bracket bulk modulus B accurately, but simple estimation schemes fail for shear modulus G, especially in poorly coordinated configurations under low P. Tenuous, fragile networks respond differently to changes in load direction, as compared to load intensity. The shear modulus, in poorly coordinated packings, tends to vary proportionally to the degree of force indeterminacy per unit volume. The elastic range extends to small strain intervals, in agreement with experimental observations. The origins of nonelastic response are discussed. We conclude that elastic moduli provide access to mechanically important information about coordination numbers, which escape direct measurement techniques, and indicate further perspectives.Comment: Published in Physical Review E 25 page

Noise properties of an optical frequency comb from a SESAM-mode-locked 1.5-μm solid-state laser stabilized to the 10−13 level

We present a detailed investigation of the noise properties of an optical frequency comb generated from a femtosecond diode-pumped solid-state laser operating in the 1.5-μm spectral region. The stabilization of the passively mode-locked Er:Yb:glass laser oscillator, referred to as ERGO, is achieved using pump power modulation for the control of the carrier envelope offset (CEO) frequency and by adjusting the laser cavity length for the control of the repetition rate. The stability and the noise of the ERGO comb are characterized in free-running and in phase-locked operation by measuring the noise properties of the CEO, of the repetition rate, and of a comb line at 1558nm. The comb line is analyzed from the heterodyne beat signal with a cavity-stabilized ultra-narrow-linewidth laser using a frequency discriminator. Two different schemes to stabilize the comb to a radio-frequency (RF) reference are compared. The comb properties (phase noise, frequency stability) are limited in both cases by the RF oscillator used to stabilize the repetition rate, while the contribution of the CEO is negligible at all Fourier frequencies, as a consequence of the low-noise characteristics of the CEO-beat. Alinewidth of ≈150kHz and a fractional frequency instability of 4.2×10−13 at 1s are obtained for an optical comb line at 1558nm. Improved performance is obtained by stabilizing the comb to an optical reference, which is a cavity-stabilized ultra-narrow linewidth laser at 1558nm. The fractional frequency stability of 8×10−14 at 1s, measured in preliminary experiments, is limited by the reference oscillator used in the frequency compariso