bluuebunny/crossref_metadata_2025 huggingface dataset:
Brachial artery diameter and vasodilator response to nitroglycerine, but not flow-mediated dilatation, are associated with the presence and quantity of coronary artery calcium in asymptomatic adults
Authors: Kullo Iftikhar J. , Malik A. Rauoof , Bielak Lawrence F. , Sheedy Patrick F. , Turner Stephen T. , Peyser Patricia A.
Published: March 2007

In the present study, we investigated whether measures of brachial artery reactivity were associated with the presence and extent of subclinical coronary atherosclerosis in asymptomatic adults. Electron beam computed tomography was employed to assess the presence and quantity of CAC (coronary artery calcium) in 441 participants (mean age, 61 years; 49% men) without prior history of CHD (coronary heart disease) or stroke, and CAC score was calculated using the method described by Agatston and co-workers [(1990) J. Am. Coll. Cardiol. 15, 827–832] High-resolution ultrasound was employed to measure BAD (brachial artery diameter), FMD (flow-mediated dilatation) and NMD (nitroglycerine-mediated dilatation). CAC score and FMD were log-transformed after adding 1 to reduce skewness. Multivariable logistic and linear regression models based on generalized estimating equations were used to assess whether BAD, FMD and NMD were each independently associated with the presence and quantity of CAC after adjustment for CHD risk factors and use of statin and hypertension medication. CAC was detectable in 64% of participants. After adjustment for age and sex, FMD was not correlated (r=−0.06; P=0.27), BAD was positively correlated (r=0.16; P=0.004) and NMD was inversely correlated in a borderline significant manner (r=−0.10; P=0.084) with log(CAC+1). In multivariable logistic regression analyses, FMD was not associated, whereas higher BAD (P=0.021) and lower NMD (P=0.030) were independently associated with the presence of CAC. In multivariable linear regression analyses, higher BAD (P=0.004) and lower NMD (P=0.016), but not FMD, were independently associated with log(CAC+1). We conclude that greater diameter of the brachial artery and lower vasodilator response to nitroglycerine, but not FMD, are associated with subclinical coronary atherosclerosis.
C-reactive protein modulates vagal heart rate control in patients with coronary artery disease
Authors: Nolan Robert P. , Reid Graham J. , Seidelin Peter H. , Lau Herbert K.
Published: March 2007

Systemic inflammation is associated with sympathetic cardiac activation and decreased HRV (heart rate variability) in subjects at high risk of CAD (coronary artery disease). In the present study, we examined the influence of systemic inflammation, measured by CRP (C-reactive protein), on vagal HR (heart rate) control during behavioural relaxation in patients with CAD. It was hypothesized that CRP would be associated with decreased vagal HR modulation. Consecutive patients were screened 2 weeks prior to elective PTCA (percutaneous transluminal coronary angioplasty). The study was comprised of 29 subjects who represented the first and fourth quartiles of the CRP distribution: Low (0.47±0.07 μg/ml)- and High (8.19±1.95 μg/ml)-CRP groups respectively. Vagal HR control was quantified as RR high-frequency spectral power (0.15 to 0.40 Hz), and was assessed in log-transformed absolute units (logHF power). Near-IR particle immunoassay was used to determine high-sensitivity CRP concentration. Assessment entailed 5 min of silent reading and self-guided behavioural relaxation. RR logHF power was decreased in the High-CRP group across both assessment procedures (P=0.032). Behavioural relaxation increased RR logHF power for both the Low- and High-CRP groups (P=0.033). Hierarchical linear regression determined that CRP accounted for 18.9% of the variance in RR logHF power during behavioural relaxation (P=0.03), independent of baseline RR interval, cardiac medication, respiratory logHF power and body mass index. In conclusion, patients with CAD had augmented vagal HR control with behavioural relaxation, but this effect was moderated by the severity of CRP. Therefore it may be advisable to assess systemic inflammation in interventions aimed at improving neurocardiac regulation in patients with CAD.
Angiogenic growth factors in maternal and fetal serum in pregnancies complicated by intrauterine growth restriction
Authors: Wallner Wenzel , Sengenberger Ruth , Strick Reiner , Strissel Pamela L. , Meurer Britta , Beckmann Matthias W. , Schlembach Dietmar
Published: March 2007

The present study was performed to compare serum concentrations of maternal and fetal angiogenic growth factors in IUGR (intrauterine growth restriction) and normal pregnancy at the time of delivery. VEGF (vascular endothelial growth factor), PlGF (placental growth factor), sFlt-1 (soluble fms-like tyrosine kinase 1), sKDR (soluble kinase domain receptor) and bFGF (basic fibroblast growth factor) were measured by ELISA in serum from a maternal peripheral vein, the umbilical vein and the umbilical arteries in 15 women with pregnancies complicated by IUGR and 16 controls (women with normal pregnancies). In IUGR, sFlt-1 was increased, and PlGF and sKDR were decreased, in both maternal serum and serum from the umbilical vein. Additionally, bFGF was increased in serum from the umbilical vein of women with pregnancies complicated by IUGR. No significant differences in growth factor concentrations between the groups were found in serum from the umbilical artery. In both groups, levels of VEGF were higher and levels of sFlt-1 were lower in serum from the umbilical vein and umbilical artery compared with maternal serum. PlGF levels were found to be lower in serum from the umbilical vein compared with maternal serum in both groups, whereas PlGF levels in serum from the umbilical artery were significantly lower only in the control group. These findings suggest an imbalance of angiogenic and anti-angiogenic factors in IUGR, with formation of an anti-angiogenic state in maternal and, to a lesser extent, umbilical vein blood. The placenta appears to play a central role in the release of sFlt-1 into maternal and umbilical blood. Umbilical artery blood was unaffected in IUGR, indicating that the fetus does not contribute to changes in angiogenic growth factor concentrations.
Aging is associated with an increased susceptibility to ischaemic necrosis due to microvascular perfusion failure but not a reduction in ischaemic tolerance
Authors: Harder Yves , Amon Michaela , Georgi Mirko , Scheuer Claudia , Schramm Rene , Rücker Martin , Pittet Brigitte , Erni Dominique , Menger Michael D.
Published: March 2007

In the present study in a murine model of chronic ischaemia, we analysed: (i) whether aging was associated with an increased susceptibility to ischaemic necrosis, and (ii) whether this was based on microvascular dysfunction or reduced ischaemic tolerance. An ischaemic pedicled skin flap was created in the ear of homozygous hairless mice. The animals were assigned to three age groups, including adolescent (2±1 months), adult (10±2 months) and senescent (19±3 months). Microvascular perfusion of the ischaemic flap was assessed over 5 days by intravital microscopy, evaluating FCD (functional capillary density), capillary dilation response and the area of tissue necrosis. Expression of the stress-protein HO (haem oxygenase)-1 was determined by immunohistochemistry and Western blotting. Induction of chronic ischaemia stimulated a significant expression of HO-1 without a significant difference between the three age groups. This was associated with capillary dilation, which, however, was more pronounced in adolescent (10.5±2.8 μm compared with 3.95±0.79 μm at baseline) and adult (12.1±3.1 μm compared with 3.36±0.45 μm at baseline) animals compared with senescent animals (8.5±1.7 μm compared with 3.28±0.69 μm at baseline; P value not significant). In senescent animals, flap creation further resulted in complete cessation of capillary flow in the distal area of the flap (FCD, 0±0 cm/cm2), whereas adult (11.9±13.5 cm/cm2) and, in particular, adolescent animals (58.4±33.6 cm/cm2; P<0.05) were capable of maintaining residual capillary perfusion. The age-associated microcirculatory dysfunction resulted in a significantly increased flap necrosis of 49±8% (P<0.05) and 42±8% (P<0.05) in senescent and adult animals respectively, compared with 31±6% in adolescent mice. Of interest, functional inhibition of HO-1 by SnPP-IX (tin protoporphyrin-IX) in adolescent mice abrogated capillary dilation, decreased functional capillary density and aggravated tissue necrosis comparably with that observed in senescent mice. Thus aging is associated with an increased susceptibility to tissue necrosis, which is due to a loss of vascular reactivity to endogenous HO-1 expression, rather than a reduction in ischaemic tolerance.
Isometric handgrip training does not improve flow-mediated dilation in subjects with normal blood pressure
Authors: Mcgowan Cheri L. , Levy Andrew S. , Mccartney Neil , Macdonald Maureen J.
Published: March 2007

Isometric HG (handgrip) training lowers resting arterial BP (blood pressure), yet the mechanisms are elusive. In the present study, we investigated improved systemic endothelial function as a mechanism of arterial BP modification following isometric HG training in normotensive individuals. This study employed a within-subject repeated measures design primarily to assess improvements in BA FMD (brachial artery flow-mediated dilation; an index of endothelium-dependent vasodilation), with the non-exercising limb acting as an internal control. Eleven subjects performed four 2-min unilateral isometric HG contractions at 30% of maximal effort, three times per week for 8 weeks. Pre-, mid- and post-training resting ABP and BA FMD (exercised arm and non-exercised arm) were measured via automated brachial oscillometry and ultrasound respectively. BA FMD (normalized to the peak shear rate experienced in response to the reactive hyperaemic stimulus) remained unchanged [exercised arm, 0.029±0.003 to 0.026±0.003 to 0.029±0.004%/s−1 (pre- to mid- to post-training respectively); non-exercised arm, 0.023±0.003 to 0.023±0.003 to 0.024±0.003%/s−1 (pre- to mid- to post-training respectively); P=0.22]. In conclusion, improved systemic endothelial function is unlikely to be responsible for lowering arterial BP in this population.
Vascular inflammation in hypertension and diabetes: molecular mechanisms and therapeutic interventions
Authors: Savoia Carmine , Schiffrin Ernesto L.
Published: March 2007

More than 80% of patients with type 2 diabetes mellitus develop hypertension, and approx. 20% of patients with hypertension develop diabetes. This combination of cardiovascular risk factors will account for a large proportion of cardiovascular morbidity and mortality. Lowering elevated blood pressure in diabetic hypertensive individuals decreases cardiovascular events. In patients with hypertension and diabetes, the pathophysiology of cardiovascular disease is multifactorial, but recent evidence points toward the presence of an important component dependent on a low-grade inflammatory process. Angiotensin II may be to a large degree responsible for triggering vascular inflammation by inducing oxidative stress, resulting in up-regulation of pro-inflammatory transcription factors such as NF-κB (nuclear factor κB). These, in turn, regulate the generation of inflammatory mediators that lead to endothelial dysfunction and vascular injury. Inflammatory markers (e.g. C-reactive protein, chemokines and adhesion molecules) are increased in patients with hypertension and metabolic disorders, and predict the development of cardiovascular disease. Lifestyle modification and pharmacological approaches (such as drugs that target the renin–angiotensin system) may reduce blood pressure and inflammation in patients with hypertension and metabolic disorders, which will reduce cardiovascular risk, development of diabetes and cardiovascular morbidity and mortality.
Human arterial responses to isometric exercise: the role of the muscle metaboreflex
Authors: Davies Tom S. , Frenneaux Michael P. , Campbell Ross I. , White Michael J.
Published: March 2007

The effects of exercise on the distensibility of large and medium-sized arteries are poorly understood, but can be attributed to a combination of local vasodilator effects of exercise opposed by sympathetic vasoconstrictor tone. We sought to examine this relationship at the conduit artery level, with particular reference to the role of the sympatho-excitatory muscle metaboreflex. The effect of maintained muscle metaboreflex activation on a previously passive or exercised limb femoral artery was investigated. A total of ten healthy volunteers performed 2 min of isometric ankle plantar-flexion at 40% MVC (maximal voluntary force), in conjunction with 2 min of either non-ischaemic isometric HG (handgrip; control condition) or IHG (ischaemic HG) at 40% MVC. IHG was followed by 2 min of PECO (post-exercise circulatory occlusion) to maintain muscle metaboreflex activation. FTPWV [femoral–tibial PWV (pulse wave velocity)] was measured in the exercised or contralateral limb at baseline and immediately following calf exercise. BP (blood pressure) and HR (heart rate) were measured continuously throughout. In the HG condition, BP and HR returned promptly to baseline post-exercise, whereas exercised leg FTPWV was decreased (less stiff) by 0.6 m/s (P<0.05) and the non-exercised leg PWV was not changed from baseline. PECO caused a sustained increase in BP, but not HR, in the IHG condition. Contralateral leg PWV increased (stiffened) during PECO by 0.9 m/s (P<0.05), whereas exercised limb FTPWV was not changed from baseline. In conclusion, muscle metaboreflex activation causes a systemic stiffening of the arterial tree, which can overcome local exercise-induced decreases in arterial PWV.
Graded occlusion of perfused rat muscle vasculature decreases insulin action
Authors: Vollus Georgie C. , Bradley Eloise A. , Roberts Merren K. , Newman John M. B. , Richards Stephen M. , Rattigan Stephen , Barrett Eugene J. , Clark Michael G.
Published: March 2007

Insulin increases capillary recruitment in vivo and impairment of this may contribute to muscle insulin resistance by limiting either insulin or glucose delivery. In the present study, the effect of progressively decreased rat muscle perfusion on insulin action using graded occlusion with MS (microspheres; 15 μm in diameter) was examined. EC (energy charge), PCr/Cr (phosphocreatine/creatine ratio), AMPK (AMP-activated protein kinase) phosphorylation on Thr172 (P-AMPKα/total AMPK), oxygen uptake, nutritive capacity, 2-deoxyglucose uptake, Akt phosphorylation on Ser473 (P-Akt/total Akt) and muscle 2-deoxyglucose uptake were determined. Arterial injection of MS (0, 9, 15 and 30×106 MS/15 g of hindlimb muscle, as a bolus) into the pump-perfused (0.5 ml·min−1·g−1 of wet weight) rat hindlimb led to increased pressure (−0.5±0.8, 15.9±2.1, 28.7±4.6 and 60.3±9.4 mmHg respectively) with minimal changes in oxygen uptake. Nutritive capacity was decreased from 10.6±1.0 to 3.8±0.9 μmol·g−1 of muscle·h−1 (P<0.05) with 30×106 MS. EC was unchanged, but PCr/Cr was decreased dose-dependently to 61% of basal with 30×106 MS. Insulin-mediated increases in P-Akt/total Akt decreased from 2.15±0.35 to 1.41±0.23 (P<0.05) and muscle 2-deoxyglucose uptake decreased from 130±19 to 80±12 μg·min−1·g−1 of dry weight (P<0.05) with 15×106 MS; basal P-AMPKα in the absence of insulin was increased, but basal P-Akt/total Akt and muscle 2-deoxyglucose uptake were unaffected. In conclusion, partial occlusion of the hindlimb muscle has no effect on basal glucose uptake and marginally impacts on oxygen uptake, but markedly impairs insulin delivery to muscle and, thus, insulin-mediated Akt phosphorylation and glucose uptake.
Angiotensin II signal transduction through the AT1 receptor: novel insights into mechanisms and pathophysiology
Authors: Higuchi Sadaharu , Ohtsu Haruhiko , Suzuki Hiroyuki , Shirai Heigoro , Frank Gerald D. , Eguchi Satoru
Published: March 2007

The intracellular signal transduction of AngII (angiotensin II) has been implicated in cardiovascular diseases, such as hypertension, atherosclerosis and restenosis after injury. AT1 receptor (AngII type-1 receptor), a G-protein-coupled receptor, mediates most of the physiological and pathophysiological actions of AngII, and this receptor is predominantly expressed in cardiovascular cells, such as VSMCs (vascular smooth muscle cells). AngII activates various signalling molecules, including G-protein-derived second messengers, protein kinases and small G-proteins (Ras, Rho, Rac etc), through the AT1 receptor leading to vascular remodelling. Growth factor receptors, such as EGFR (epidermal growth factor receptor), have been demonstrated to be ‘trans’-activated by the AT1 receptor in VSMCs to mediate growth and migration. Rho and its effector Rho-kinase/ROCK are also implicated in the pathological cellular actions of AngII in VSMCs. Less is known about the endothelial AngII signalling; however, recent studies suggest the endothelial AngII signalling positively, as well as negatively, regulates the NO (nitric oxide) signalling pathway and, thereby, modulates endothelial dysfunction. Moreover, selective AT1-receptor-interacting proteins have recently been identified that potentially regulate AngII signal transduction and their pathogenic functions in the target organs. In this review, we focus our discussion on the recent findings and concepts that suggest the existence of the above-mentioned novel signalling mechanisms whereby AngII mediates the formation of cardiovascular diseases.
Spin noise fluctuations from paramagnetic molecular adsorbates on surfaces
Authors: Messina Paolo , Mannini Matteo , Caneschi Andrea , Gatteschi Dante , Sorace Lorenzo , Sigalotti Paolo , Sandrin Cristian , Prato Stefano , Pittana Paolo , Manassen Yishay
Published: March 2007

The measurement of spin noise in nuclei was demonstrated on bulk samples more than two decades ago. An ensemble of spins can produce a coherent signal at the Larmor frequency of a static magnetic field, known as spin noise, an effect due to the statistical polarization of small ensembles. The difficulty of these measurements is that the signal is extremely small—even if electron spins are detected. Although the statistical polarization of N spins dominates the Boltzmann statistics if N approaches unity, a more sensitive tool is requested to measure the polarization of the magnetic moment of a single spin. In this paper we report on the verification of recent results on the detection of spin noise from paramagnetic molecules of α,γ-bisdiphenylene-β-phenylallyl (BDPA) by Durkan and Welland [Appl. Phys. Lett. 80, 458 (2002)]. We also present results on a second paramagnetic specie 1,1-diphenyl-2-picrylhydrazyl (DPPH), deposited on Au(111) surfaces. Electron spin resonance spectra from ultrathin films of DPPH and BDPA grown on Au(111) are reported. We prove that the paramagnetic molecules preserve their magnetism on the surface. These data and a thorough analysis of the signal recovery apparatus help us to understand the low statistical recurrence of the spin noise in the data set. A detailed description of the experimental apparatus together with an analysis of the parameters that determine the sensitivity are also presented.
Linear theory of the electron beam-wave-plasma interactions in a magnetized plasma waveguide
Authors: Liu Shenggang , Wei Yanyu , Yuan XueSong , Yan Yang
Published: March 2007

The theoretical study on the electron beam-wave interactions in a plasma waveguide immersed in a finite magnetic field is given in the article, in which both the plasma and the electron beam are considered as special media. Making use of the constitutive transformation and the Lorentz transformation in the four-dimensional space, the permittivity tensor of the stationary magnetized plasma, the permittivity tensor, the permeability tensor, and the chiral tensor of the electron beam in the rest (laboratory) frame are acquired. Therefore, two coupled wave equations for the magnetized plasma and electron beam have been obtained and the dispersion relations are then achieved by solving these coupled equations together with the boundary conditions including the surface current density due to the ripple of the plasma/beam. As an example of the applications of this approach, the beam-wave interactions in a practical plasma Cherenkov maser have been studied and the numerical calculations have been carried out in detail. It has been found that the present approach is more accurate and can provide clearer mode information for the electron beam-wave interactions in a magnetized plasma waveguide. This approach can be exploited in a number of electron beam-wave interaction systems including some kinds of free electron devices, plasma filled Cherenkov radiated free electron lasers and masers.
Interpretation of distributed-feedback-laser spectrum using reflection properties of Bloch waves
Authors: Maslov A. V. , Ning C. Z.
Published: March 2007

We demonstrate that the phase and amplitude of the reflection coefficient for Bloch waves can be useful tools in understanding and predicting the spectral characteristics of distributed-feedback lasers. We further apply the Bloch wave formalism for engineering the asymmetry of the spectral response with respect to the gap through a judicious termination of the laser facets.
Structural phase transitions in pentacosane C25H52
Authors: Mukherjee Prabir K.
Published: March 2007

The pentacosane undergoes a series of structural phase transitions in addition to the melting transition according to an x-ray scattering study. A phenomenological theory is developed to describe the structural phase transitions between different phases observed in pentacosane. We present a detailed analysis of the different phases and analyze the temperature anomalies of the elastic constants and heat capacity. The thermodynamic anomalies on the different phase transitions are described by the coupling between the order parameters and the elastic strains. The theoretical predictions are found to be in good qualitative agreement with experimental results.
Electric-field-dependent charge transport in organic thin-film transistors
Authors: Wang Liang , Fine Daniel , Basu Debarshi , Dodabalapur Ananth
Published: March 2007

This article reports the experimental study of the electric-field-dependent charge transport mechanisms in polycrystalline organic thin-film field-effect transistors. This work represents the quantitative measurement of the temperature and electric-field dependences of the mobility in organic thin-film transistors with scaled device geometry when carrier densities are at levels of practical importance. The true behavior of field-dependent mobility was extracted by minimizing contact effects consistently over a range of channel lengths. In these partially ordered systems, experimental data suggest that thermally activated and field-assisted hopping transport between disorder-induced localized states dominates over intrinsic polaronic transport seen in organic single crystals. The experimental results were found to exhibit a Frenkel-Poole-type dependence consistently over a wide range of channel lengths, fields, and temperatures.
Reduced dynamics of coupled harmonic and anharmonic oscillators using higher-order perturbation theory
Authors: Schröder Markus , Schreiber Michael , Kleinekathöfer Ulrich
Published: March 2007

Several techniques to solve a hierarchical set of equations of motion for propagating a reduced density matrix coupled to a thermal bath have been developed in recent years. This is either done using the path integral technique as in the original proposal by Tanimura and Kubo [J. Phys. Soc. Jpn. 58, 101 (1998)] or by the use of stochastic fields as done by Yan et al. [Chem. Phys. Lett. 395, 216 (2004)]. Based on the latter ansatz a compact derivation of the hierarchy using a decomposition of the spectral density function is given in the present contribution. The method is applied to calculate the time evolution of the reduced density matrix describing the motion in a harmonic, an anharmonic, and two coupled oscillators where each system is coupled to a thermal bath. Calculations to several orders in the system-bath coupling with two different truncations of the hierarchy are performed. The respective density matrices are used to calculate the time evolution of various system properties and the results are compared and discussed with a special focus on the convergence with respect to the truncation scheme applied.
Roles of noise in single and coupled multiple genetic oscillators
Authors: Yoda Mitsumasa , Ushikubo Tomohiro , Inoue Wataru , Sasai Masaki
Published: March 2007

The noisy fluctuation of chemical reactions should profoundly affect the oscillatory dynamics of gene circuit. In this paper a prototypical genetic oscillator, repressilator, is numerically simulated to analyze effects of noise on oscillatory dynamics. The oscillation is coherent when the protein number and the rate of the DNA state alteration are within appropriate ranges, showing the phenomenon of coherence resonance. Stochastic fluctuation not only disturbs the coherent oscillation in a chaotic way but also destabilizes the stationary state to make the oscillation relatively stable. Bursting in translation, which is a source of intense stochastic fluctuation in protein numbers, suppresses the destructive effects of the finite leakage rate of protein production and thus plays a constructive role for the persistent oscillation. When multiple repressilators are coupled to each other, the cooperative interactions among repressilators enhance the coherence in oscillation but the dephasing fluctuation among multiple repressilators induces the amplitude fluctuation in the collective oscillation.
Effect of concentrated energy deposition on the aerodynamic drag of a blunt body in hypersonic flow
Authors: Satheesh K. , Jagadeesh G.
Published: March 2007

Experimental results on the effect of energy deposition using an electric arc discharge, upstream of a 60° half angle blunt cone configuration in a hypersonic flow stream is reported. Investigations involving drag measurements and high speed Schlieren flow visualization have been carried out in a hypersonic shock tunnel using air and argon as the test gases; and an unsteady drag reduction of about 50% (maximum reduction) has been observed in the energy deposition experiments done in argon environment. These studies also show that the effect of discharge on the flow field is more pronounced in argon environment as compared to air, which confirms that thermal effects are mainly responsible for flow alteration with discharge. It has also been observed that the interaction of the hypersonic flow with the discharge filament results in the development of an unsteady flow field.
Geometry optimization using improved virtual orbitals: A complete active space numerical gradient approach
Authors: Chaudhuri Rajat K. , Freed Karl F.
Published: March 2007

The improved virtual orbital-complete active space configuration interaction (IVO-CASCI) method is extended to enable geometry optimization and the calculation of vibrational frequencies for ground and excited states using numerical energy gradients. Applications consider the ground state geometries and vibrational frequencies of the Be2, LiF, H2S, and HCN molecules, as well as excited state properties for HCN, systems that are sufficiently complex to access the efficacy of the method. Comparisons with other standard approaches (self-consistent field, second order Möller-Plesset perturbation theory, complete active space self-consistent field, and coupled cluster singles and doubles methods) demonstrate that the numerical gradient version of the IVO-CASCI approach generally fares comparable to or better for all systems studied. The accurate estimates for the Be2 bond length and vibrational frequency are notable since many other computationally facile methods produce poor results.
Thermionic power generation at high temperatures using SiGe∕Si superlattices
Authors: Vashaee Daryoosh , Shakouri Ali
Published: March 2007

Recent studies have predicted that heterostructure superlattices can enhance the effective thermoelectric power factor significantly through selective emission of hot carriers via thermionic emission. Here, we study the potential of SiGe∕Si superlattices for power generation at high temperatures. A detailed theory based on Boltzmann transport equation is developed which takes into account multiple valleys. We show that thermionic emission provides only a modest improvement in the power factor. This is due to the fact that SiGe is a multivalley semiconductor and it has a large density of states. With reasonable dopings, Fermi energy in SiGe alloy is very close to the band minimum so that the symmetry of the differential conductivity does not change very much with small barrier superlattices. Particularly at high temperatures when the thermal spread of the carriers is much larger than the Fermi energy in the band, superlattice energy filtering is not effective.
Nature of the cubic to rhombohedral structural transformation in (AgSbTe2)15(GeTe)85 thermoelectric material
Authors: Cook B. A. , Kramer M. J. , Wei X. , Harringa J. L. , Levin E. M.
Published: March 2007

The existence of a large thermoelectric figure of merit in (AgSbTe2)15(GeTe)85 has been known for many years. However, the nature of the crystallographic transformation in these materials from a high-temperature cubic to a low-temperature rhombohedral polymorph and its effect on electrical transport has not been clearly established. Transmission electron microscopy studies were performed that show extensive twinning in the low-temperature structure, resulting from lattice strain during the dilation along the (111) crystallographic direction. Analysis of differential scanning calorimetric studies indicates that the transformation is of second order, so that the high-temperature cubic phase is nonquenchable. High-temperature x-ray diffraction was performed to establish the transformation temperature, which was found to be complete upon heating at a temperature of 510K. Results of electrical conductivity measurements as a function of temperature on as-cast samples are discussed in terms of the observed twinning.
Thick film polymer-ceramic composites for pyroelectric applications
Authors: Dietze M. , Krause J. , Solterbeck C.-H. , Es-Souni M.
Published: March 2007

Thick films of 0-3 composites of lead-zirconate-titanate ceramic and polyvinylidene-trifluorethylene copolymer have been produced by spin coating on gold-coated silicon wafers. The dielectric properties were investigated as a function of ceramic volume fraction and temperature. Pyroelectric measurements were undertaken by temperature modulation with a Peltier element. Additionally, the pyroelectric response has been investigated up to 3000Hz using a modulated laser. The piezoelectric response of the composites obtained by using a laser vibrometer are also reported. It is shown that the dielectric constant increases with increasing volume fraction of ceramic and that it reaches a maximum at a temperature in the range of 65–70°C due to the ferroelectric-paraelectric phase transition of the polymer matrix. The pyroelectric coefficient increases to 92μCm−2K−1 at a ceramic volume fraction of 20%. Furthermore the effective piezoelectric charge coefficient d33 of the composite almost vanishes at this composition. This composites show relatively high pyroelectric figures of merit and may be a potential candidate for pyroelectric sensor applications.
Judd-Ofelt analysis of spectroscopic property of Er3+ in congruent and near-stoichiometric Zn∕Er-codoped LiNbO3 crystals
Authors: Zhang De-Long , Hua Ping-Rang , Xu Yu-Heng , Pun E. Y. B.
Published: March 2007

Congruent LiNbO3 crystals doped with 5.5∕1, 6∕0.15, and 7∕0.8mol%∕mol% of Zn∕Er were thermally treated at 1120°C over 101h using vapor transport equilibration (VTE) technique. Unpolarized visible and infrared absorption spectra of the VTE-treated and corresponding as-grown crystals were recorded at room temperature. It is shown that VTE treatment has brought these crystals closer to stoichiometric composition. X-ray powder diffraction and optical absorption results show that the VTE treatment induces the crystalline phase transformation with respect to Er3+ from the original Er3+:LiNbO3 to the ErNbO4 in all crystals studied. The formation of the ErNbO4 phase results in substantial absorption changes that include the drop of the transparency in the visible region, the reduction of Er3+ absorption, and the Zn-doping-level-dependent changes in spectral shape and peaking position at 0.98 and 1.5μm regions. Based upon the measured absorption spectra, the influences of Zn-doping level and VTE treatment on the Er3+ spectroscopic properties were studied by using the Judd-Ofelt (JO) theory. The results show that the VTE treatment has a little effect on the fluorescence branch ratio, while a large effect on the JO parameters, oscillator strength, and electron transition probability. The Zn doping level affects the JO parameters, oscillator strength, and transition probabilities of some individual manifolds. In addition, the VTE treatment leads to the increases of the theoretical radiative lifetimes of almost all manifolds considered. For the emission at 1.5μm, the lifetime increase is as much as 30% and the theoretical results are in well agreement with the experimental data. The VTE-induced lifetime increase is qualitatively explained.
Impact of the etching gas on vertically oriented single wall and few walled carbon nanotubes by plasma enhanced chemical vapor deposition
Authors: Gohier A. , Minea T. M. , Djouadi M. A. , Granier A.
Published: March 2007

Vertically oriented single wall nanotubes (SWNTs) and few walled nanotubes (FWNTs) have been grown by electronic cyclotron resonance plasma enhanced chemical vapor deposition (PECVD) on silica flat substrates. The impact of the plasma parameters on SWNT and FWNT growth has been investigated using two different etching gas mixtures, namely, C2H2∕NH3 and C2H2∕H2 with various ratios and applied bias voltages. Kinetic studies are also proposed in order to describe the FWNT growth mechanism by plasma techniques. A key role played by the reactive gas (NH3 and H2) is observed in the PECVD process, contrary to multiwalled nanotube growth. It is demonstrated that the balance between FWNT growth versus FWNT etching can be widely modulated by varying the gas mixture and bias voltage. It is shown that the use of hydrogen for hydrocarbon gas dilution restricts the destruction of SWNT and FWNT by the plasma species (ions and radicals).
GaAs whiskers grown by metal-organic vapor-phase epitaxy using Fe nanoparticles
Authors: Regolin Ingo , Khorenko Victor , Prost Werner , Tegude Franz J. , Sudfeld Daniela , Kästner Jochen , Dumpich Günter , Hitzbleck Klemens , Wiggers Hartmut
Published: March 2007

GaAs nanowhiskers were grown by metal-organic vapor-phase epitaxy on (111)B GaAs substrates. The diameter of the nanowhiskers was defined by monodisperse Fe nanoparticles deposited on the GaAs substrate from the vapor phase. The growth temperature of the whiskers was investigated from 480to520°C. The whiskers are preferentially directed along the crystal orientations of ⟨001⟩, ⟨111⟩, and their equivalents. High-resolution transmission electron microscopy characterization including energy disperse x-ray spectroscopy measurements revealed not only iron oxide but also arsenic inside the seed particle at the top of the GaAs whiskers. This indicates that the particle stays at the top during the whisker growth.
Scattering loss and effective index step of asymmetric cladding surface-etched distributed Bragg reflector lasers at 850 nm
Authors: Price R. K. , Elarde V. C. , Coleman J. J.
Published: March 2007

The reflectivity of surface-etched distributed Bragg reflector (DBR) laser gratings, suitable for incorporation in photonic integrated circuits, is determined from a series of samples having varied grating etch depths. The effective index step and DBR scattering loss are used to characterize the relationship between the physical structure and associated optical characteristics of asymmetric cladding surface-etched lasers at 850 nm. These results enable the fabrication of frequency-selective DBR gratings with a predetermined grating index step and reflectivity.
Microcrystalline, nanocrystalline, and ultrananocrystalline diamond chemical vapor deposition: Experiment and modeling of the factors controlling growth rate, nucleation, and crystal size
Authors: May P. W. , Ashfold M. N. R. , Mankelevich Yu. A.
Published: March 2007

Ar ∕ CH 4 ∕ H 2 gas mixtures have been used to deposit microcrystalline diamond, nanocrystalline diamond, and ultrananocrystalline diamond films using hot filament chemical vapor deposition. A three-dimensional computer model was used to calculate the gas phase composition for the experimental conditions at all positions within the reactor. Using the experimental and calculated data, we show that the observed film morphology, growth rate, and across-sample uniformity can be rationalized using a model based on competition between H atoms, CH3 radicals, and other C1 radical species reacting with dangling bonds on the surface. Proposed formulas for growth rate and average crystal size are tested on both our own and published experimental data for Ar∕CH4∕H2 and conventional 1% CH4∕H2 mixtures, respectively.
A high accuracy technique to measure the electrical conductivity of liquids using small test samples
Authors: Wu Jianping , Stark J. P. W.
Published: March 2007

A large quantity of a sample is usually required in electrical conductivity measurements of fluids. If precise electrical conductivity measurements are to be performed while using small test samples, it is necessary to study the effects of both fringe electric fields and interfacial properties. These effects were investigated here by using a novel technique termed the triangular waveform voltage method. The results using this technique revealed: (1) the fringe electric fields may be identified as an increase in apparent electrode area; and (2) the interfacial effects may be simply treated as an interfacial resistance. It was found that the increase of the electrode area due to the fringe electric fields may be reduced to below 1% at small electrode separations; the interfacial resistance is largely dependent of measurement frequency, electrode separation, and the electrical conductivity of the liquid. In general we show that the impact of interfacial resistance can be significantly reduced and the error in the electrical conductivity measurements for small liquid samples can be reduced from 40% to become less than ±1% in the tested NaCl(aq) solutions, spanning three decades of concentration, by adopting this new method.
Deformation and molecular chain evolution of poly(vinylidene fluoride-trifluoroethylene) copolymer films under combined electrical and mechanical loads
Authors: Fang F. , Yang W. , Zhang M. Z. , Zhang F. C.
Published: March 2007

Deformation and molecular chain evolution under combined electrical and mechanical loads are characterized for poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer films with 55 and 68 mol % VDF content [termed P(VDF-TrFE) 55∕45 and P(VDF-TrFE) 68∕32, respectively]. In both copolymer films, the direction of the applied electric field with respect to the stress field direction has significant influence on the stress-strain behavior and the evolution of molecular chain structure. When the electric field is applied parallel to the tension direction, the molecular chains have the tendency to convolute, which prevents the films from acquiring a more oriented fibrillar structure compared with film structures under pure mechanical tensile stress. Consequently, the yield strength is enhanced while the fracture strain is reduced. When the electric field is applied along the film thickness direction, it causes randomly oriented dipoles in the films to orient preferentially along the direction of the external electric field. The combination of the mechanical tensile load and the electric field in the film thickness direction contributes to a well aligned and closely packed molecular chain structure, and consequently both UTS and fracture strain are increased. The VDF content dependence on the stress-strain behavior is discussed.
Effects of the MnO additives on the properties of Pb(Fe2∕3W1∕3)–PbTiO3 relaxors: Comparison of empirical law and experimental results
Authors: Hong Cheng-Shong , Chu Sheng-Yuan , Su Wen-Chang , Chang Ren-Chuan , Nien Hsiau-Hsian , Juang Yung-Der
Published: March 2007

In this paper, pure and 0.15wt%MnO additives of (1−x)Pb(Fe2∕3W1∕3)O3–xPbTiO3 [(1−x)PFW−xPT] with x=0.1, 0.2, 0.3, and 0.4 were synthesized by the conventional solid state reaction method. The dielectric diffusion properties were investigated with the empirical law. The lattice structure was found to transfer from tetragonal to pseudocubic and the diffusion phase characteristic is more notable as decreasing PbTiO3 compositions or adding MnO additives. Moreover, the dielectric loss is improved and the space charge polarization effect is vanished as adding MnO additives in 0.7PFW-0.3PT compounds. We suggested it was probably due to the lead vacancy which is substituted by the manganese ions or the interstitial manganese ions intervene into the interstice between the positive and negative ions.
Rayleigh-Taylor instabilities in reaction-diffusion systems inside Hele-Shaw cell modified by the action of temperature
Authors: García Casado Gustavo , Tofaletti Lorena , Müller Darío , D’Onofrio Alejandro
Published: March 2007

The influence of temperature in the buoyancy driven Rayleigh-Taylor instability of reaction-diffusion fronts is investigated experimentally in Hele-Shaw cells. The acid autocatalysis of chlorite-tetrathionate reaction coupled to molecular diffusion yields exothermic planar reaction-diffusion fronts separating two miscible reactant and product solutions. The resulting chemical front moves downwards invading the fresh reactants, leaving the products of the reaction behind it. The density of the product solution is higher than the reactant solution; hence, the traveling front is buoyantly unstable and develops density fingers in time (Rayleigh-Taylor instability) when the products are above the reactants. The kinetic constant of a chemical reaction varies due to thermal effects. This may stabilize the exothermic descending front when temperature is increased, so that the mixing zone decreases, modifying the fingering patterns, until it almost disappears. The authors study the influence of the temperature variation on the instability pattern figure observed in the chlorite-tetrathionate reaction for long times, corresponding to the nonlinear regime.
Polarization transfer from photon to electron spin in g factor engineered quantum wells
Authors: Kosaka H. , Mitsumori Y. , Rikitake Y. , Imamura H.
Published: March 2007

The authors demonstrate polarization transfer from a photon to an electron spin intermediated by a light-hole exciton in a GaAs∕AlGaAs quantum well, which has an engineered electron g factor of less than 0.01 for an in-plane magnetic field. Negative spin polarization was clearly observed at the selective excitation of the light-hole exciton from two-color time-resolved Kerr rotation. This demonstration is a necessary step towards demonstrating coherence transfer from a photon to an electron spin, which is necessary for building a quantum repeater used for long distance quantum communications.
Growth, microstructure, and ferroelectric properties of Pb(Zr0.4Ti0.6)O3∕PbZrO3 superlattices prepared on SrTiO3 (100) substrates by pulsed laser deposition
Authors: Bao Dinghua , Scholz Roland , Alexe Marin , Hesse Dietrich
Published: March 2007

Artificially designed Pb(Zr,Ti)O3 (PZT)∕PbZrO3 (PZ) superlattices were grown on (100)-oriented SrTiO3 (STO) substrates with and without SrRuO3 bottom electrodes by pulsed laser deposition. Their microstructure was characterized using x-ray diffraction and transmission electron microscopy. Different from single PZT films having a c-axis orientation on STO (100) substrates, during growth the PZT and PZ layers in the superlattices turn to an a-axis orientation from the c-axis orientation of the first PZT layer. This growth behavior is explained by minimization of the respective interfacial lattice mismatch. The superlattices have a rather smooth morphology and sharp PZT/PZ interfaces. At the latter, misfit dislocations have been found occasionally. There are indications for the absence of the orthorhombic, antiferroelectric phase in part of the PZ layers, possibly due to strain hindering the phase transition. An elongated, “linearized” shape of the ferroelectric hysteresis loop has been found, with a remanent polarization (2Pr) of 17μC∕cm2 and a coercive field of about 110kV∕cm, which seems to be the result of a superposition of ferroelectric and antiferroelectric switching.
Temporally evolved recoil pressure driven melt infiltration during laser surface modifications of porous alumina ceramic
Authors: Harimkar Sandip P. , Samant Anoop N. , Dahotre Narendra B.
Published: March 2007

Laser surface modification of porous alumina ceramic with a high power laser is associated with a series of physical processes such as heating, melting, and evaporation of material. Above certain threshold laser intensity (∼1010W∕m2), rapid evaporation at melt surface generates strong recoil pressures. These laser-induced evaporation recoil pressures tend to drive the flow of molten material into the porous substrate thus contributing to the overall observed depth of melting. This paper presents a three-dimensional thermal model to calculate the temporal evolution of temperature during laser surface modifications of alumina ceramic. This is followed by the determination of recoil pressures at the evaporating surface based on experimentally verified physical model of melt hydrodynamics and laser-induced evaporation proposed by Anisimov [Sov. Phys. JETP 27, 182 (1968)]. Finally, Carman-Kozeny equations were employed to analyze the effect of recoil pressure on the depth of infiltration which is subsequently integrated with the calculated depth of melting from thermal model. Such an integrative approach results in better agreement of the predicted values of depths of melting with the experimental values.
Influence of electron beam exposure on crystallization of phase-change materials
Authors: Pandian Ramanathaswamy , Kooi Bart J. , De Hosson Jeff Th. M. , Pauza Andrew
Published: March 2007

Isothermal crystallization of amorphous SbxTe films capped with ZnS-SiO2 or GeCrN layers was performed using in situ heating within a transmission electron microscope. The effect of the electron beam of the microscope on the crystallization process was investigated. It was found that electron irradiation during the crystallization process leads to a continuous increase in the crystal growth velocity. For SbxTe sandwiched between ZnS-SiO2 the effect of the electron beam was equivalent to a temperature rise of about 10 K, without affecting the activation energy for growth. However, for SbxTe sandwiched between GeCrN the activation energy for growth was also decreased due to electron beam exposure. The observed variations in the crystal growth rates are attributed to relaxations within the initial amorphous phase initiated by thermal energy and/or electron irradiation.