Size-dependent threshold fields for Fröhlich conduction in niobium triselenide: Possible evidence for pinning by the crystal surface

The threshold field E_T for nonlinear “Fröhlich” conduction in niobium triselenide has been found to be correlated with the cross-sectional dimensions of the specimen. In 28 specimens from the same batch, E_T at 110 K was approximately proportional, over a range approaching 50 to 1, to the ratio C/A, where C is the circumference and A the area of the cross-section. No significant correlation was found between E_T and the resistance ratio R(293 K)/R(4.2 K), which was measured in 20 of the specimens and averaged 45. The diffraction evidence that the transverse phase coherence of the charge-density wave extends only over much smaller distances makes it unlikely that the cross-sectional dimensions influence E_T by limiting the Lee-Rice characteristic length for weak pinning by impurities. It is proposed that the pinning responsible for E_T is provided by the external surface of the crystal, supplemented in some cases by internal faults, rather than by impurities or other point defects within it.     

Search for kaonic nuclear state, K − pp, in the p + p → X + K + reaction with FOPI

We search for the most fundamental kaonic nuclear state, K ^??? pp by employing the reaction p?+?p→K ^?+??+?X at 3.0 GeV incident beam energy. The experiment makes use of the SIS-18 synchrotron and the ~4π acceptance FOPI apparatus in the cave-B at the GSI, Darmstadt, Germany and aims to collect full kinematics information of the reaction. We look for a signature of the X (?≡?K ^??? pp) in both missing mass spectrum by measuring K ^?+?, and invariant mass spectrum by measuring decay particles; X?→?Λ?+?p. The experiment is planned to take place in 2009.     

Limitations of the permeability-limited compartment model in estimating vascular permeability and interstitial volume fraction in DCE-MRI

Dynamic contrast-enhanced magnetic resonance imaging commonly uses compartment models to estimate tissue parameters in general and perfusion parameters in particular. Compartment models assume a homogeneous distribution of the injected tracer throughout the compartment volume. Since tracer distribution within a compartment cannot be assessed, the parameters obtained by means of a compartment model might differ from the actual physical values.This work systematically examines the widely used permeability-surface-limited one-compartment model to determine the reliability of the parameters obtained by comparing them with their actual values. A computer simulation was used to model spatial tracer distribution within the interstitial volume using diffusion of contrast agent in tissue. Vascular parameters were varied as well as tissue parameters. The vascular parameters used were capillary radius (4 and 12 μm), capillary permeability (from 0.03 to 3.3 μm/s) and intercapillary distances from 30 to 300 μm. The tissue parameters used were tortuosity (λ), porosity (α) and interstitial volume fraction (v_e).Our results suggest that the permeability-surface-limited compartment model generally underestimates capillary permeability for capillaries with a radius of 4 μm by factors from ≈0.03 for α=0.04, to ≈ 0.1 for α=0.2, to ≈ 0.5 for α=1.0. An overestimation of actual capillary permeability for capillaries with a radius of 12 μm by a factor of ≥1.3 was found for α=1.0, while α=0.2 yielded an underestimation by a factor of ≈0.3 and α=0.04 by a factor of ≈ 0.03. The interstitial volume fraction, v_e, obtained by the compartment model differed with increasing intercapillary distances and for low vessel permeability, whereas v_e was found to be estimated approximately accurately for P=0.3 μm/s and P=3.3 μm/s for vessel distances <100 μm.     

CP violation in t → W + b decays in two-Higgs doublet model

Due to the large mass of top quark, CP violation in the top-quark decay is sensitive to the interactions mediated by Higgs bosons. We consider CP violation in t → W ⁺ b decay by calculating consistently in unitary gauge the CP-violating up-down asymmetry of the leptons from W boson decays in t → W ⁺ b, defined by Grz?dkowski and Gunion, in the two-Higgs doublet model with CP-violating neutral sector. The asymmetry is shown to be at most of the order of (1–4) × 10^-4 for tan β = 1.0, where tan β is the ratio of vacuum expectation values for the two neutral Higgs bosons.     

Effects of screening on quark-antiquark cross sections in quark-gluon plasma

Lowest-order cross sections for qq? production and annihilation can be approximately corrected for higher-order QCD effects by using a corrective K-factor. For energies where quark masses cannot be ignored, the K-factor is dominated by the wave function distortion arising from the initialor final-state interaction between the quark and the antiquark. We evaluate this K-factor for qq? production and annihilation in a quark-gluon plasma by taking into account the effects of Debye screening through a color-Yukawa potential. We present the corrective K-factor as a function of dimensionless parameters which may find applications in other systems involving attractive or repulsive Yukawa interactions. Prominent peaks of the K-factor occur for an attractive q-q? color-Yukawa interaction with Debye screening lengths of 0.835 and 3.23 times the Bohr radius, corresponding to two lowest s-wave qq? bound states moving into the continuum to become qq? resonances as the Debye screening length decreases. These resonances, especially the cc? and the bb? resonances, may be utilized to study the quark-gluon plasma by examining the systematics of the temperature dependence of heavy-quark pair production just above the threshold.     

Relation between contiguous Mie coefficients for perfectly conducting spheres

It is shown that when the size parameter x = ka in the scattering of electromagnetic waves by a perfectly conducting sphere is equal to an integer n, the Mie coefficients a_n and a_n−1 are equal. It is also shown that the other Mie coefficient b_n cannot be equal to b_n−1 for any finite x. The latter conclusion also holds in the case of scattering of a scalar particle by a hard sphere in quantum mechanics.     

Edge-based-attack induced cascading failures on scale-free networks

Most previous existing works on cascading failures only focused on attacks on nodes rather than on edges. In this paper, we discuss the response of scale-free networks subject to two different attacks on edges during cascading propagation, i.e., edge removal by either the descending or ascending order of the loads. Adopting a cascading model with a breakdown probability p of an overload edge and the initial load (k_ik_j)^α of an edge ij, where k_i and k_j are the degrees of the nodes connected by the edge ij and α is a tunable parameter, we investigate the effects of two attacks for the robustness of Barabási-Albert (BA) scale-free networks against cascading failures. In the case of α<1, our investigation by the numerical simulations leads to a counterintuitive finding that BA scale-free networks are more sensitive to attacks on the edges with the lowest loads than the ones with the highest loads, not relating to the breakdown probability. In addition, the same effect of two attacks in the case of α=1 may be useful in furthering studies on the control and defense of cascading failures in many real-life networks. We then confirm by the theoretical analysis these results observed in simulations.     

Low-field irreversibility line and its anisotropy of textured (Bi,Pb)2Sr2Ca2Cu3Oy silver-clad tapes

The irreversibility line H_irr(T) of textured (Bi,Pb)₂Sr₂Ca₂Cu₃O_y silver-clad tapes was obtained by ac susceptibility (ACS) and magnetoresistivity (MR) below 1500 G. It is found that, in the framework of flux motion, the H_irr(T) is determined by the thermally activated flux flow (TAFF) rather than the so-called giant flux creep, and the experimentally obtained H_irr(T) line is related to a certain resistivity criterion in both ACS and resistivity measurements. H_irr(T) is found to be significantly depressed by the amplitude of applied driving AC field and generally lower than that measured from the resistivity. The anisotropy in H_irr(T) is found to follow a scaling behaviour as described by the anisotropic effective mass model.     

Verification of radiotherapy doses by EPR dosimetry in patients' teeth

The aim of this work was to verify applicability of electron paramagnetic resonance (EPR) ex vivo dosimetry in teeth enamel for determination of doses absorbed by patients during radiotherapy with radiation fields covering head regions and to examine with what accuracy the doses predicted by radiotherapy treatment plan (RTP) can be confirmed by doses measured ex post by the EPR method. The doses were determined in 22 enamel samples obtained from 11 patients who, after their radiotherapy treatment underwent extraction of teeth due to medical reasons. The delivered doses were determined by measuring EPR signals in enamel samples from the extracted teeth; magnitude of these signals is proportional to concentration of stable free radicals induced by radiation in the hydroxyapatite content of enamel. The measured doses were compared with doses planned in the teeth locations by RTP systems. The relation between the measured (D_m) and the planned (D_p) doses can be described as a linear function: D_m = s·D_p + b, with the slope s = 0.93 ± 0.03 and the intercept b = 0.67 ± 1.26. The deviations between the measured and calculated doses were in the (−12.6%, +1.9%) range with the average deviation of – 4.6%. It is concluded, than more accurate measurements, achievable when using a higher calibration dose than in the present study, are necessary to confirm or to deny the observed bias between the measured and planned doses.     

Elastic stress fields caused by a dislocation in Ge x Si1−x /Si film-substrate system

The elastic stress fields caused by a dislocation in Ge_xSi_(1-x) epitaxial layer on Si substrate are investigated in this work. Based on the previous results in an anisotropic bimaterial system,the image method is further developed to determine the stress field of a dislocation in the film-substrate system under coupled condition. The film-substrate system is firstly transformed into a bimaterial system by distributing image dislocation densities on the position of the free surface. Then,the unknown image dislocation densities are solved by using boundary conditions,i.e.,traction free conditions on the free surface. Numerical simulation focuses on the Ge0.1Si0.9/Si film-substrate system. The effects of layer thickness,position of the dislocation and crystallographic orientation on the stress fields are discussed. Results reveal that both the stresses σxx,σxz at the free surface and the stress σxy,σyy,σyz on the interface are influenced by the layer thickness,but the former is stronger. In contrast to the weak dependence of stress field on the crystallographic orientation the stress field was strongly affected by dislocation position. The stress fields both in the film-substrate system and bimaterial system are plotted.     

Energy structure and absorption spectra of colloidal CdS nanocrystals in gelatin matrix

We investigate the energy structure of colloidal CdS nanocrystals by measuring the UV–vis absorption spectra. Nanocrystals were synthesized by sol–gel method in a gelatin matrix in the size range from 2.5 to 3.9 (±0.2) nm. In order to interpret the UV–vis absorption spectra we calculate the energy spectrum of electron quasi-stationary states using the model of open nanocrystal as well as the hole stationary spectrum in a two-band approach. It is shown that the main contribution to the absorption spectrum is made by interband transitions 1S_3/2→1S_e and 1P_3/2→1P_e, and its shape is determined by the size distribution of nanoparticles. For this system the estimated values of the effective masses of the heavy hole and light hole are 1.44m₀ and 0.28m₀, respectively.     

Thermal relaxation of charm in hadronic matter

The thermal relaxation rate of open-charm (D) mesons in hot and dense hadronic matter is calculated using empirical elastic scattering amplitudes. D-meson interactions with thermal pions are approximated by D^? resonances, while scattering off other hadrons (K, η, ρ, ω, K^?, N, Δ) is evaluated using vacuum scattering amplitudes as available in the literature based on effective Lagrangians and constrained by realistic spectroscopy. The thermal relaxation time of D-mesons in a hot π gas is found to be around 25-50 fm/c for temperatures T=150-180 MeV, which reduces to 10-25 fm/c in a hadron-resonance gas. The latter values, argued to be conservative estimates, imply significant modifications of D-meson spectra in heavy-ion collisions. Close to the critical temperature (Tc), the spatial diffusion coefficient (Ds) is surprisingly similar to recent calculations for charm quarks in the Quark-Gluon Plasma using non-perturbative T-matrix interactions. This suggests a possibly continuous minimum structure of Ds around Tc.     

Tuning fractional PID controllers for a Steward platform based on frequency domain and artificial intelligence methods

In this paper, two methods to tune a fractional-order PI ^λ D ^μ controller for a mechatronic system are presented. The first method is based on a genetic algorithm to obtain the parameter values for the fractionalorder PI ^λ D ^μ controller by global optimization. The second method used to design the fractional-order PI ^λ D ^μ controller relies on an auto-tuning approach by meeting some specifications in the frequency domain. The real-time experiments are conducted using a Steward platform which consists of a table tilted by six servo-motors with a ball on the top of the table. The considered system is a 6 degrees of freedom (d.o.f.) motion platform. The feedback on the position of the ball is obtained from images acquired by a visual sensor mounted above the platform. The fractional-order controllers were implemented and the performances of the steward platform are analyzed.     

Neubestimmung der absoluten Oszillatorenstärke der Eisen I-Resonanzlinieλ=3720 Å durch optische Doppelresonanz

The natural width of the strong Fe I spectral lineλ=3719,9 Å was determined in a double resonance experiment with an atomic beam as resonance cell and a hollow cathode as light source by the observation ofrf-induced magnetic dipole transitions between the Zeeman sublevels of the excited state in a weak static magnetic fieldH ₀. The optical line was isolated from disturbing neighbours in the same multiplet either roughly by an interference filter or strictly by a monochromator. From the widthΔν=5,35(14) MHz of the double resonance signal (monochromator value, including a small correction of 1.5% for the hyperfine structure of the 2.2% natural abundance of iron-57) the lifetimeτ of the (z ⁵ F ₅ ⁰ )-level is found to beτ=5,95 (16)×10^?8 sec. Thef-value of the transition to the ground state (a ⁵ D ₄) is calculated, by using the known relativef-values for the weak alternative transition possibilities to the (a ⁴ F ₅) and (a ⁵ F ₄) levels (λ=5012 and 5127 Å), to bef _λ3720=4,21 (11)×10^?2.     

An improved technique for the measurement of the complex susceptibility of magnetic colloids in the microwave region

Measurements by means of the short-circuit (S/C) and open circuit (O/C) transmission line techniques are well established methods for investigating the magnetic and dielectric properties of magnetic colloids, respectively. In particular, the S/C technique has been used in the investigation of the resonant properties of ferrofluids; resonance being indicated by the transition of the real component of the magnetic complex susceptibility, χ(ω)=χ′(ω)−iχ″(ω), from a positive to a negative value at a frequency, f_res. However, under certain circumstances, the accuracy of the S/C technique is affected by the dielectric properties of the sample, hence incurring errors in the measurement of χ(ω) and indeed of f_res. Here we present a model which, by combining short-circuit and open circuit measurements, is developed in a manner in which the permeability, μ, and the permittivity, ε, contribute simultaneously to the calculation of χ(ω), thereby providing superior experimental results in comparison to those obtained by the S/C technique alone. For the two ferrofluid samples measured it is demonstrated that the dielectric properties affect the high frequency content of the susceptibility spectrum.     

The rate matching effect: A hidden property of aperiodic stochastic resonance

We study a system S generating Poisson events, and a corresponding dichotomous signal as well, perturbed by a system P, also generating Poisson events and a corresponding dichotomous signal. The rates of events productions for system and perturbation are gS and gP, respectively. We call S events the events produced by the system S and P events those produced by the perturbation P. We show that this simple model reproduces the essence of recent experimental and theoretical results on aperiodic stochastic resonance. More remarkably, this simplified version of aperiodic stochastic resonance allows us to discover a property that has been overlooked by the earlier research work. The rate matching condition gS=gP is the border between two distinctly different conditions: For gS<gP, the P events are attractors of the S events and for gS>gP they become repellers of the S events. The transition from the former to the latter condition is very marked and takes place in a short region of either gS or gP, depending on which is the parameter changed, thereby resulting in a discontinuous transition.     

Electron density of states and superconducting Tc in A15-compounds

We compare the superconductivity parameters of V₃Ga and V₃Ge by the method of McMillan with the help of new neutron diffraction data obtained by P. Schweiss. We consider the strongly varying density of states in V₃Ga at the Fermi energy, a complication suggested by Labbé, Barisic, and Friedel. We find that increased electronic density of states contributes about equally with lattice softening to the enhancement of T_c in V₃Ga as compared to V₃Ge, although the values of N(O)J² = Mλ〈ω²〉 are roughly consistent with those obtained for some other metals by McMillan. The reason N(O) is not more effective in raising T_c is the partial compensation obtained from averaging over the narrow density-of-states peak which gives N ? 0.6N(O).     

Spectator and participator processes in the resonant photon-in and photon-out spectra at the Ce L3 edge of CeO2

We study both theoretically and experimentally the photon-in and photon-out spectra of CeO₂, which are caused by the Ce 2p to Ce 5d excitation followed by the three different de-excitation channels: (i) Ce 3d to Ce 2p (denoted by 3d-RXES), (ii) O 2p to Ce 2p (v-RXES), and (iii) Ce 5d to Ce 2p (RIXS). In 3d- and v-RXES, the 5d electron plays a role of a spectator, but in RIXS it is a participator. By extending our single impurity Anderson model (SIAM), which was used recently for our calculations of v- and 3d-RXES spectra of CeO₂, we study the polarization dependence in the spectator and participator spectra, and we perform more detailed calculations for 3d- and v-RXES spectral features, as well as new calculations for the RIXS spectrum with charge transfer excitations. The polarization dependence is different for the spectator and participator spectra; we have no polarization correlation between the incident and emitted photons for the spectator spectra but a strong polarization correlation for the participator spectrum. The theoretical calculations predict that the charge transfer excitations in RIXS occur in the transfer-energy range overlapped with v-RXES, but the RIXS and v-RXES spectra can be discriminated by taking advantage of the different polarization dependence. The overlapped RIXS and v-RXES spectra are observed successfully by our experiments and well reproduced by our SIAM calculations.