Isotropic Line Research Articles

Effect of dehydration on the ferrimagnetic resonance in [Cr(CN)6][Mn(S)-pnH(H2O)](H2O) single crystals

The effect of dehydration-induced structural transitions on the ferrimagnetic resonance spectra of the [Cr(CN)6][Mn(S)-pnH(H2O)](H2O) chiral molecular magnet has been studied for three crystalline modifications. The differences in the anisotropy field and exchange interaction constants due to the change in the dimensionality of the magnetic ordering upon the phase transitions have been established. In the dehydrated amorphous phase, apart from the ferrimagnetic resonance, additional isotropic magnetic resonance lines corresponding to the spin-glass state have been revealed.

Electron spin resonance in oriented nanowires Ge0.99Cr0.01

The electron spin resonance in Ge0.99Cr0.01 nanowires grown in nanopores of anodized aluminum oxide is investigated. At temperatures in the range 4–300 K, the electron spin resonance spectra of the nanowires exhibit an asymmetric isotropic resonance line corresponding to the paramagnetic resonance of charge carriers. At temperatures below 50–70 K, in the electron spin resonance spectra of the Ge0.99Cr0.01 nanowires, there appear a large number of additional resonance lines associated with the antiferromagnetic resonance in GeCr clusters. It is revealed that the effective g factor of the asymmetric Dyson line attributed to the charge carriers in germanium decreases regularly with an enhancement of the spin-orbit interaction in the series of Mn2+, Cr2+, Fe3+, and Co2+ transition metal ions used to dope germanium nanowires.

Measurement of Local Dissipation Scales in Turbulent Pipe Flow

Local dissipation scales are a manifestation of the intermittent small-scale nature of turbulence. We report the first experimental evaluation of the distribution of local dissipation scales in turbulent pipe flows for a range of Reynolds numbers: 2.4x10(4)<or=ReD<or=7.0x10(4). Our measurements at the nearly isotropic pipe center line and within the anisotropic logarithmic layer show excellent agreement with distributions that were previously calculated from numerical simulations of homogeneous isotropic box turbulence and with those predicted by theory. The reported results suggest a universality of the smallest-scale fluctuations around the classical Kolmogorov dissipation length.

Electron spin resonance of charge carriers and antiferromagnetic clusters in Ge0.99Cr0.01 nanowires

We report an electron spin resonance study of Ge nanowires doped with chromium. The Ge0.99Cr0.01 nanowires contain both diluted Сr3+ and Сr2+ ions and a large amount of Cr-based nanoclusters revealing spin-glass properties at T&amp;lt;60 K. The observed antiferromagnetic resonance at low temperatures is attributed to Ge–Cr nanoclusters. Additionally, we detect an asymmetric, isotropic resonant line clearly distinguished in the whole temperature range, T=4–300 K, which corresponds to a paramagnetic resonance of the charge carriers in the Ge0.99Cr0.01 nanowires. On the base of our previous studies, we compare properties of the charge carriers in germanium nanowires doped with different transition metals. We demonstrate that the effective g-factor related to charge carriers is reduced with an increase in the spin-orbital interaction in the row: Mn2+, Cr2+, Fe3+, and Co2+.

Cover Picture: Phys. Status Solidi A 206/5

AbstractThe Editor's Choice article “Analysis of polarization‐dependent photoreflectance studies for c ‐plane GaN films grown on a ‐plane sapphire” by M. Röppischer et al. (pp. 773–779) was selected among the contributions from the 3rd International Workshop on Modulation Spectroscopy of Semiconductor Structures (MS3) presented in this issue of physica status solidi (a).In the paper the optical properties of c ‐plane GaN layers grown on a ‐plane sapphire are investigated through the combination of experimental measurements and the calculation of transition energies and oscillator strengths. The front cover picture shows the calculated exciton transition energies at the center of the Brillouin zone for room temperature as well as the differences between them. The picture also illustrates the strain component parallel to the c‐axis as a function of the in‐plane strain. One can deduce from the upper three diagrams that the transition energies increase for net compressive strain, while they decrease for tensile strain. In addition, the diagrams are symmetric by the isotropic strain line, so one can exchange the values of strain for x‐ and y‐components, and the same transition energies are obtained. The measured photoreflectance signals of c‐plane GaN on a‐plane sapphire are well described by the calculated transition energies in connection with the oscillator strength (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Modified state parameter for characterizing static liquefaction of sand with fines

An experimental study was carried out to investigate the static liquefaction behaviour of sand with a small amount of plastic and nonplastic fines. Five series of tests were conducted in drained and undrained conditions. The drained test results indicate not only that the failure line coincides with the critical state, but also that the development of volumetric strain during shearing was not sensitive to the initial confining pressure. In both isotropically and anisotropically consolidated undrained tests, a so-called “reverse behaviour” was consistently observed. The results were also interpreted in the critical state framework. The critical and steady state (CS/SS) data were found to trace along the same curve in e–log( p′) space, irrespective of the stress history and effective stress paths. A comparison between the isotropic consolidation line (ICL) and critical state (CS) curve showed that a small amount of fines can significantly change the shape and position of the ICL relative to the CS curve. Furthermore, the soil behaviour manifested in both drained and undrained shearing led to the development of a modified state parameter.

EPR study of local symmetry sites ofCe3+inPb1−xCexA(A=S, Se, and Te)

The local site symmetry of ${\text{Ce}}^{3+}$ ions in the diluted magnetic semiconductors ${\text{Pb}}_{1\ensuremath{-}x}{\text{Ce}}_{x}A$ ($A=\text{S}$, Se, and Te) has been investigated by electron-paramagnetic resonance (EPR). The experiments were carried out on single crystals with cerium concentration $x$ ranging from 0.001 to 0.035. The isotropic line due to ${\text{Ce}}^{3+}$ ions located at the substitutional Pb cation site with octahedral symmetry was observed for all the studied samples. We determined the effective Land\'e factors to be $g=1.333$, 1.364, and 1.402 for $A=\text{S}$, Se, and Te, respectively. The small difference with the predicted Land\'e factor $g$ of 10/7 for the ${\ensuremath{\Gamma}}_{7}$ $(J=5/2)$ ground state was attributed to crystal-field admixture. In addition, EPR lines from ${\text{Ce}}^{3+}$ ions located at sites with small distortion from the original octahedral symmetry were also observed. Two distinct sites with axial distortion along the $⟨001⟩$ crystallographic direction were identified and a third signal in the spectrum was attributed to sites with the cubic symmetry distorted along the $⟨110⟩$ direction. The distortion at these distinct Ce sites is attributed to Pb lattice vacancies near the cerium ions that compensate for its donor activity.

Demonstration That CobG, the Monooxygenase Associated with the Ring Contraction Process of the Aerobic Cobalamin (Vitamin B12) Biosynthetic Pathway, Contains an Fe-S Center and a Mononuclear Non-heme Iron Center

The ring contraction process that occurs during cobalamin (vitamin B(12)) biosynthesis is mediated via the action of two enzymes, CobG and CobJ. The first of these generates a tertiary alcohol at the C-20 position of precorrin-3A by functioning as a monooxygenase, a reaction that also forms a gamma lactone with the acetic acid side chain on ring A. The product, precorrin-3B, is then acted upon by CobJ, which methylates at the C-17 position and promotes ring contraction of the macrocycle by catalyzing a masked pinacol rearrangement. Here, we report the characterization of CobG enzymes from Pseudomonas denitrificans and Brucella melitensis. We show that both contain a [4Fe-4S] center as well as a mononuclear non-heme iron. Although both enzymes are active in vivo, the P. denitrificans enzyme was found to be inactive in vitro. Further analysis of this enzyme revealed that the mononuclear non-heme iron was not reducible, and it was concluded that it is rapidly inactivated once it is released from the bacterial cell. In contrast, the B. melitensis enzyme was found to be fully active in vitro and the mononuclear non-heme iron was reducible by dithionite. The reduced mononuclear non-heme was able to react with the oxygen analogue NO, but only in the presence of the substrate precorrin-3A. The cysteine residues responsible for binding the Fe-S center were identified by site-directed mutagenesis. A mechanism for CobG is presented.

STUDY OF MODE PROPAGATION IN PSEUDOCHIRAL TRANSMISSION LINES

In this paper, a generic planar transmission line fllled, homogeneously, with a pseudochiral omega medium is considered. It is shown that only a uniaxial omega medium can support TE and TM modes separately. Thus, for such a medium, the flelds and modal equations for TE, TM and TEM mode propagation are obtained. The special case of parallel plate waveguide is solved, and the efiect of pseudochirality parameter › on the propagation constant and cutofi frequency is considered. For TEM propagation, an equivalent circuit is given which is difierent from the common isotropic transmission line model. Finally, a pseudochiral stripline is analyzed, and the elements of the equivalent circuit are calculated. The results show that the properties of the line vary as the pseudochirality parameter changes.

Stereological Estimates of Alveolar Number and Size and Capillary Length and Surface Area in Mice Lungs

The major function of the lung is gas exchange and depends on alveolar and capillary parameters such as surface area and volume. The number of alveoli may report on the nature of structural changes in lung parenchyma during development, illness or changing environmental factors. We therefore developed an efficient and easily applicable stereological design for estimating and monitoring these structural parameters in the mouse lung. The estimation of volume fractions of different lung compartments has been carried out by point counting. A combination of cycloid grids superimposed on vertical sections was used to estimate the capillary surface area with isotropic test lines. Capillary length could be measured using the harmonic mean of the surface weighted diameter. The Euler characteristic applied in the physical fractionator with varying but known sampling fractions (Horovitz-Thompson estimator) enabled us to estimate alveolar number. In adult mice lungs, we obtained total values for alveolar number of 2.31 x 10(6) alveoli in a pair of lungs, alveolar surface area of 82.2 cm(2), capillary surface area of 124 cm(2), and capillary length of 1.13 km. All values are corrected for tissue shrinkage. With this study we present a highly efficient combination of several design-based stereological tools for the unbiased estimation of alveolar number and volume as well as length, surface area, and diameter of capillaries in the mice lung. Anat Rec, 292:113-122, 2009. (c) 2008 Wiley-Liss, Inc.

Heteronuclear dipolar decoupling effects on multiple‐quantum and satellite‐transition magic‐angle spinning NMR spectra

We here report on the influence of heteronuclear dipolar decoupling on the (27)Al 3QMAS, 5QMAS, and the double-quantum filter-satellite-transition magic-angle spinning (DQF-STMAS) spectra of a strongly dipolar-coupled system, gibbsite. The requirements for heteronuclear dipolar decoupling increase with the order of coherence evolving in the indirect dimension of a two-dimensional (2D) experiment. The isotropic line width of the high-resolution 2D spectra, in samples like gibbsite, is composed of four parts: the distribution of isotropic shifts (delta(ISO), delta(QIS)), the homogeneous broadening related to the proton-proton flip-flop terms, the (27)Al-(27)Al homonulcear dipolar couplings, and the (1)H-(27)Al heteronuclear dipolar couplings. It is shown that, even in the case of gibbsite, where a strong proton-proton bath exists, the main resolution limiting factor in these experiments resides in the (1)H-(27)Al dipolar interaction.

Yielding and shear behaviour of cement-treated dredged materials

The mechanical behaviour of three cement-treated dredged materials (DM) was studied in the laboratory by isotropic compression and undrained triaxial tests. The intrinsic properties of the untreated and reconstituted soil were used as a reference framework to interpret the effect of cementation on the mechanical behaviour of the cement-treated DM. The isotropic compression line of the reconstituted soil is termed the intrinsic normal compression line (INCL). It is found that the isotropic compression lines of the cement-treated DM lie above the INCL, i.e. the void ratio of the cement-treated soil is higher than that of the reconstituted soil for a given confining pressure. The yield stress measured from the isotropic compression tests increases with increasing cement content and curing time. The pre-yield and post-yield shear responses of the cement-treated DM are different. Pre-yield soil specimens show a tendency to dilate, but post-yield soil specimens show a tendency to contract. Failure envelopes of the cement treated DM lie above that of the reconstituted DM. Linear Mohr–Coulomb failure envelopes were observed for the range of applied stress used in the study. The cohesion intercept increases with increasing cement content and curing time. The strength enhancement of the cement-treated DM is the result of the structures developed during cementation process. Based on a recently proposed soil water transfer model, the change in normalised bound water content, (Δ m bw) N, is used to evaluate the combined effects of cement content and curing period on the mechanical behaviour of the cement-treated DM. It is found that there exists a threshold value of (Δ m bw) N, beyond which the yield stress and cohesion intercept increases nonlinearly with increasing (Δ m bw) N. The threshold value is equal to 0.37 for the three DM used in the study.

Exceptional collections for Grassmannians of isotropic lines

We construct a full exceptional collection of vector bundles in the derived categories of coherent sheaves on the Grassmannian of isotropic two-dimensional subspaces in a symplectic vector space of dimension 2n and in an orthogonal vector space of dimension 2n + 1 for all n.

Computational modelling of nematic phase ordering by film and droplet growth over heterogeneous substrates

This paper presents a computational study of defect nucleation associated with the kinetics of the isotropic‐to‐nematic phase ordering transition over heterogeneous substrates, as it occurs in new liquid crystal biosensor devices, based on the Landau–de Gennes model for rod‐like thermotropic nematic liquid crystals. Two regimes are identified due to interfacial tension inequalities: (i) nematic surface film nucleation and growth normal to the heterogeneous substrate, and (ii) nematic surface droplet nucleation and growth. The former, known as wetting regime, leads to interfacial defect shedding at the moving nematic‐isotropic interface. The latter droplet regime, involves a moving contact line, and exhibits two texturing mechanisms that also lead to interfacial defect shedding: (a) small and large contact angles of drops spreading over a heterogeneous substrate, and (b) small drops with large curvature growing over homogeneous patches of the substrate. The numerical results are consistent with qualitative defect nucleation models based on the kinematics of the isotropic–nematic interface and the substrate–nematic–isotropic contact line. The results extend current understanding of phase ordering over heterogeneous substrates by elucidating generic defect nucleation processes at moving interfaces and moving contact lines.

Solid-State NMR Studies on Ionic closo-Dodecaborates

Alkali metal dodecahydro-closo-dodecaborates M2[B12H12] (M = K, Rb, Cs, NH4, N(CH3)4) and the perhalogenated cesium salts Cs2[B12X12] (X = Cl, Br, I) are studied by solid-state 11B nuclear magnetic resonance (NMR) spectroscopy as well as X-ray diffraction (XRD) and differential scanning calorimetry. The present work addresses the molecular dynamics of the anionic [B12X12]2− icosahedra which is examined by variable-temperature 11B NMR line shape studies between 120 and 370 K. Characteristic line shape effects are observed which strongly depend on the actual substituent X and the counterion M+. All alkali metal dodecahydro-closo-dodecaborates M2 [B12H12] exhibit at elevated temperatures 11B NMR spectra with a single isotropic line which proves the presence of an efficient molecular process, resulting in dynamic (rotational) disorder along with vanishing dipolar and quadrupolar interactions. The positional order of the boron clusters, however, remains unaffected, as shown by the XRD data. At lower temperatures, the underlying motions are frozen on the NMR timescale resulting in characteristic 11B NMR spectra with a dominant homonuclear 11B–11B dipolar splitting. The per-halogenated cesium salts Cs2[B12X12] behave differently. Hence, from the experimental 11B NMR spectra at room temperature a substantial mobility is only seen for the [B12Cl12]2− anion. Obviously, the degree of anion mobility depends on the size of the substituent X in the [B12X12]2− clusters (X = H, Cl, Br, I). A quantitative analysis of the experimental 11B NMR spectra of the alkali metal dodecahydro-closo-dodecaborates M2 [B12H12] is achieved by line shape simulations, considering [B12H12]2− ions undergoing reorientational jumps between icosahedral sites. From the motional correlation times the activation energies are derived. It is found that a correlation exists between the activation energies, the motional correlation times and the lattice constant. Hence, the activation energies and correlation times strongly increase with decreasing size of the cation M+, which reflects an increasing sterical hindrance due to a decreasing crystallo-graphic lattice constant in the same direction.

The undrained cyclic behaviour of a carbonate sand

A series of undrained cyclic triaxial tests was carried out on a carbonate sand that had previously been the subject of extensive research for monotonic loading. It was found that the generation of pore pressures under cyclic loading depended on the same factors as they did for monotonic loading, so that the state of the soil in the plane of specific volume against logarithm of mean effective pressure was of particular significance. For states that were not initially on the normal compression line, the pore pressure changes were also found to be highly dependent on how the initial state was achieved: whether it was by compaction and first loading, or by overconsolidation. Failure was found to occur for a much lower number of cycles from states on the one-dimensional normal compression line than for states on the isotropic normal compression line, and cyclic loading to failure was found to be associated with much less particle breakage than monotonic loading. When the stiffnesses measured during cycling had been normalised for those factors that influence the monotonic stiffness of the soil tested, it was still found that for two-way cyclic loading there was a component of the reduction in stiffness that might be attributed to changes of fabric due to the alternation in the direction of the major principal stress.

Side Chain and Backbone Dynamics of Phospholamban in Phospholipid Bilayers Utilizing 2H and 15N Solid-State NMR Spectroscopy

2H and 15N solid-state NMR spectroscopic techniques were used to investigate both the side chain and backbone dynamics of wild-type phospholamban (WT-PLB) and its phosphorylated form (P-PLB) incorporated into 1-palmitoyl-2-oleoyl-sn-glycerophosphocholine (POPC) phospholipid bilayers. 2H NMR spectra of site-specific CD3-labeled WT-PLB (at Leu51, Ala24, and Ala15) in POPC bilayers were similar under frozen conditions (-25 degrees C). However, significant differences in the line shapes of the 2H NMR spectra were observed in the liquid crystalline phase at and above 0 degrees C. The 2H NMR spectra indicate that Leu51, located toward the lower end of the transmembrane (TM) helix, shows restricted side chain motion, implying that it is embedded inside the POPC lipid bilayer. Additionally, the line shape of the 2H NMR spectrum of CD3-Ala24 reveals more side chain dynamics, indicating that this residue (located in the upper end of the TM helix) has additional backbone and internal side chain motions. 2H NMR spectra of both WT-PLB and P-PLB with CD3-Ala15 exhibit strong isotropic spectral line shapes. The dynamic isotropic nature of the 2H peak can be attributed to side chain and backbone motions to residues located in an aqueous environment outside the membrane. Also, the spectra of 15N-labeled amide WT-PLB at Leu51 and Leu42 residues showed only a single powder pattern component indicating that these two 15N-labeled residues located in the TM helix are motionally restricted at 25 degrees C. Conversely, 15N-labeled amide WT-PLB at Ala11 located in the cytoplasmic domain showed both powder and isotropic components at 25 degrees C. Upon phosphorylation, the mobile component contribution increases at Ala11. The 2H and 15N NMR data indicate significant backbone motion for the cytoplasmic domain of WT-PLB when compared to the transmembrane section.

Using 31P MAS NMR to monitor a gel phase thermal disorder transition in sphingomyelin/cholesterol bilayers

The impact of low cholesterol concentrations on an egg sphingomyelin bilayer is investigated using 31P magic angle spinning (MAS) NMR spectroscopy. The magnitude of the isotropic 31P MAS NMR line width is used to monitor the main gel to liquid crystalline phase transition, along with a unique gel phase pretransition. In addition, the 31P chemical shift anisotropy (CSA) and spin–spin relaxation times ( T 2), along with the effects of spinning speed, proton decoupling and magnetic field strength, are reported. The variation of this unique gel phase thermal pretransition with the inclusion of 5 through 21 mol% cholesterol is presented and discussed.

EPR study ofEu2+inPb1−xEuxSelayers grown on a Si substrate

Electron paramagnetic resonance (EPR) of ${\mathrm{Eu}}^{2+}$ ions in ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}\mathrm{Se}$ layers grown by molecular beam epitaxy on Si substrate has been investigated. The measurements were performed on layers with thickness around $1.5\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ and europium concentration $x=0.01$, 0.06, 0.08, and 0.10. The spin Hamiltonian (SH) parameters of the single ion anisotropy were obtained from the sample with $x=0.01$. Room temperature $(300\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ EPR measurements showed that the site symmetry of the ${\mathrm{Eu}}^{2+}$ ions is cubic and the SH parameters were found to be $g=1.970\ifmmode\pm\else\textpm\fi{}0.001$, ${b}_{4}=270.5\ifmmode\pm\else\textpm\fi{}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, and ${b}_{6}=\ensuremath{-}2.0\ifmmode\pm\else\textpm\fi{}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. However, at $T=4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, it was found that the cubic symmetry is axially distorted along the [111] direction, perpendicular to the layer surface. The change in the EPR spectra at $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ was interpreted by incorporating an axial component in the SH with ${b}_{2}=\ensuremath{-}41\ifmmode\pm\else\textpm\fi{}3\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. The observed distortion was attributed to strain induced by thermal expansion caused by a mismatch between the layer and the substrate. For $x=0.01$, we also observed a partially resolved hyperfine structure due to the $^{151}\mathrm{Eu}$ and $^{153}\mathrm{Eu}$ isotopes and a structure of lines due to Eu-Eu pair spectra. For samples with higher Eu concentration $x=0.06$ and $x=0.08$, the spectrum consisted of the isolated ${\mathrm{Eu}}^{2+}$ ions EPR lines superimposed on a broad single isotropic line at $g\ensuremath{\approx}2$, which was attributed to signal from Eu clusters. For the sample with $x=0.10$, we only observed a single broad line centered on $g\ensuremath{\approx}2$. The number of single ${\mathrm{Eu}}^{2+}$ ions was determined from the EPR lines intensities at $T=300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The results show that the distribution of the ${\mathrm{Eu}}^{2+}$ over the cation sites is not random.

Color centers in topaz: comparison between neutron and gamma irradiation

Neutron- and gamma irradiation-induced paramagnetic centers in natural colorless topaz from four different Brazilian localities were studied by electron paramagnetic resonance (EPR) and optical absorption as a function of irradiation dose and thermal treatment. Gamma irradiation doses were applied up to 1,000 kGy with a dose rate of 15 kGy h−1. For the neutron irradiation experiments, a neutron flux of 4 × 1012 cm−2 s−1 was used with an integrated flux of up to 1 × 1018 cm−2. From the experiments, it is concluded that brownish colors are induced by gamma-rays and may be associated with a single broad isotropic EPR line with g = 2.015(2). Both the EPR line and the related optical absorption band at 460 nm (2.7 eV) are lost during thermal treatments between 150 and 200°C. Fast neutrons create the paramagnetic peroxyl radicals O2 − and the paramagnetic O− hole centers. The O− centers have the same thermal stability as the optical absorption band at 620 nm (2.0 eV). It is confirmed that the absorption due to the O− center is responsible for the blue color in topaz. Both color centers and their absorption bands are discussed in the context of O− bound small polarons.