Scattering Properties of Spin-Polarized Liquid 3He

In this paper, the scattering properties of spin-polarized liquid ³He (³He^??) are studied using the Galitskii-Migdal-Feynman (GMF) formalism. The effective cross sections??including the total, diffusion and viscosity cross sections??are calculated. It is found that these cross sections tend to decrease with increasing spin polarization f. The S-wave scattering cross section is the most significant partial wave contributing to the total cross section at low energy. This contribution decreases with increasing f; whereas the contribution of the higher angular-momentum waves, especially the P-wave, increases with increasing f. The most prominent features of our calculations are a resonance and a Ramsauer-Townsend minimum in the cross sections at low temperatures. For comparison purposes, the effective cross sections in the Brueckner-Bethe-Goldstone (BBG) formalism are calculated. These remain zero up to the Fermi momentum, beyond which they are equal to those given by the GMF formalism. We deduce that hole-hole scattering plays an essential role in the scattering properties.     

DC and AC electrical characteristics of porous GaAs/p-GaAs heterostructure

This paper reports on studying Au/porous GaAs/p⁺-GaAs heterostructure through current-voltage I (V), conductance- and capacitance-frequency dependencies (G (f) and C (f)). Diode parameters such as ideality factor and zero bias barrier energy have been calculated by using Schottky model corrected from the series resistance. The obtained ideality factor is generally high and it was explained by space charge limited current (SCLC) regime, characterised by the presence of single trapping level. A model based upon TFE tunnelling of carriers at reverse current was used to explain the non-saturation of reverse current after series resistance correction. The G (f) characteristics exhibit the presence of three frequency regions. In the first region the conductance is independent of frequency. Moreover, in the second region the AC conductance increases with increasing frequency and approximately follows ω^s dependence. These results were interpreted as the result of the transport of injected carrier via hopping mechanism involving defect. In the third region a peak conductance appeared after which the conductance decreased with increasing frequency. C (f) measurements exhibit that the capacitance behaviour is typical of material with traps. From G (f) and C (f) measurement trap density and relaxation time were calculated.     

Microscopic structure and energy transfer of vacancy-related defect pairs with Erbium in wide-gap semiconductors

Electron Paramagnetic Resonance (EPR) measurements of Erbium-doped 6H-SiC and wurtzite GaN samples are compared to total energy calculations based on density functional theory (DFT) in order to investigate the well-known luminescence of the intra 4f-shell transition at 1540 nm, useful in light-emitting diodes or lasers. The highly correlated f-electrons of Erbium (Er) have been treated within an LDA+U approach. We discus how pairs of an Er-ion with intrinsic defects can be responsible in GaN and SiC for relaxing the selection rules for intra 4f-shell transitions: In GaN our EPR investigation indicates the presence of a nitrogen vacancy next to the Er-ion. Through controlled generation of intrinsic defects in 6H-SiC single crystals and EPR measurements we support the corresponding model in SiC, that predicts defect pairs of an Er-ion and a neighboring carbon vacancy. In other words, low-energy irradiation seems to be a promising way to enhance the Er-luminescence desired for device applications.     

The effect of transpiration on self-similar boundary layer flow over moving surfaces

The simultaneous effects of normal transpiration through and tangential movement of a semi-infinite plate on self-similar boundary layer flow beneath a uniform free stream is considered. The flow is therefore governed by a plate velocity parameter λ and a transpiration parameter μ and the computed wall shear stress parameter is f″(0). Dual solutions are found for each value of μ in λ–f″(0) parameter space. It is shown that the range of known dual solutions for zero transpiration increases with suction and decreases with blowing. A stability analysis for this self-similar flow reveals that, for each value of μ, lower solution branches are unstable while upper solution branches are stable.     

Comparison of fatigue failure criterion in flexural fatigue test

This study compares traditional stiffness and energy based fatigue failure criteria with the fatigue failure criterion based on the viscoelastic continuum damage (VECD) approach. In traditional approach, fatigue failure is defined as the number of cycles at which the stiffness of a material reduces by 50% (N_f50). In energy based approach, fatigue failure is defined by the number of cycles at the maximum energy ratio or Rowe’s maximum stiffness defined by stiffness multiplied by the corresponding number of the cycle (E * N). In VECD approach, fatigue failure is defined by the number of loading cycles at the inflection point of the normalized pseudostiffness (C) versus damage variable (S) curve. It is shown that a correlation exits between traditional criteria and VECD criteria. It is shown that maximum energy ratio or Rowe’s maximum stiffness based fatigue life is higher than the traditional fatigue life (N_f50). This indicates the traditional approach is conservative. A strong correlation of fatigue was observed between the VECD fatigue criterion and energy ratio based fatigue criteria. However, the fatigue life by VECD approach is always less than the fatigue life by energy ratio or Rowe’s maximum stiffness.     

On Saint-Venant’s principle in the theory of Cosserat elastic shells

In this paper, we consider the linear theory of Cosserat elastic shells made from an homogeneous and anisotropic material. We study the equilibrium of cylindrical shells loaded by contact forces and couples acting on the end edges. We prove the principle of Saint-Venant in the following formulation: if a system of loads has zero resultant force and moment at each end edge, then the strain energy U_z contained in that portion of the cylindrical shell situated beyond a distance z from the load region has an exponential decay as a function of z.     

Impurity-Related Local Density-of-States Modulation in?Bi2Sr2Ca(Cu1?x Zn x )2O8+�� Probed by Scanning Tunneling Spectroscopy

We have performed scanning tunneling spectroscopy on a single crystal of Bi₂Sr₂Ca(Cu_1?x Zn _x )₂O_8+?? (x=0.005) and found several regions where at least two Zinc (Zn) impurities aggregate within a few nm. In these regions, there are two-dimensional local density-of-states (LDOS) modulations with the period of about 1.41a ₀ (a ₀ is Cu?CO?CCu distance) along the direction tilted 45° from Cu?CO?CCu direction within the energy of below 10 mV. Since these modulations are suppressed in a map of the ratio between LDOS at a positive energy and at its negative, it is presumable that these modulations do not stem from the simple quasiparticle interference whose pattern should be enhanced in the ratio map. We suggest the possible explanations with respect to the origin of the observed modulations.     

Strong ergodicity of systems with the average shadowing property

Let X be a compact metric space and f?: X → X be a continuous map. It is proved that if f has the average shadowing property and an invariant Borel probability measure with full support, then for all n ≥ 1, the map f × f ×?????× f (n times) and hence f ⁿ are totally strongly ergodic.     

Electronic structure and the structure of the order parameter in high-T c superconductors based on copper oxides and iron pnictides

Experimental data on the symmetry of the superconducting state in the high-T _c superconductors based on copper oxides and iron pnictides have been analyzed using a space group approach to the wavefunction of a Cooper pair. We have found the irreducible representations of the crystal symmetry groups of these materials for singlet states with zero total momentum corresponding to experimental data on the structure of their order parameter.     

The irregular set for maps with the specification property has full topological pressure

Let (X, d) be a compact metric space, f : X ? X be a continuous map with the specification property and ? : X ? ? a continuous function. We consider the set of points for which the Birkhoff average of ? does not exist (which we call the irregular set for ?) and show that this set is either empty or carries full topological pressure (in the sense of Pesin and Pitskel see, for example [Y.B. Pesin, Dimension Theory in Dimensional Systems: Contemporary Views and Applications, University of Chicago Press, Chicago, 1997]). We formulate various equivalent natural conditions on ? that completely describe when the latter situation holds and give examples of interesting systems to which our results apply but were not previously known. As an application, we show that for a suspension flow over a continuous map with specification, the irregular set carries full topological entropy.     

High Frequency Sound in Superfluid 3He-B

We present measurements of the absolute phase velocity of transverse and longitudinal sound in superfluid ³He-B at low temperature, extending from the imaginary squashing mode to near pair-breaking. Changes in the transverse phase velocity near pair-breaking have been explained in terms of an order parameter collective mode that arises from f-wave pairing interactions, the so-called J=4^? mode. Using these measurements, we establish lower bounds on the energy gap in the B-phase. Measurement of attenuation of longitudinal sound at low temperature and energies far above the pair-breaking threshold, are in agreement with the lower bounds set on pair-breaking. Finally, we discuss our estimations for the strength of the f-wave pairing interactions and the Fermi liquid parameter, F ₄ ^s .     

Pseudorandom numbers for conformal measures

We propose a new algorithm for generating pseudorandom (pseudo-generic) numbers of conformal measures of a continuous map T acting on a compact space X and for a Hölder continuous potential φ : X → ?. In particular, we show that this algorithm provides good approximations to generic points for hyperbolic rational functions of degree two and the potential ?h log |T′|, where h denotes the Hausdorff dimension of the Julia set of T.     

New simplified approach for obtaining a reliable plateau value in fatigue analysis of bituminous materials

The Plateau Value (PV) based on the Ratio of Dissipated Energy Change (RDEC) approach has been proven to provide a unique relationship with the fatigue life, N_f, independent of loading mode, temperature and frequency. In this paper, a new simplified approach is proposed to compute the unique energy parameter PV. The results of this study show that applying the proposed method allows a unique fatigue power law to be produced and eliminates the ambiguity and variability in calculating PV. However, the study also shows that the PV-N_f fatigue curves for bituminous materials are dependent on material type which is contradictory to other studies dealing with PV. The study also highlights the importance of appropriately identifying the fatigue failure point prior to commencing any fatigue analysis.     

Enhancement of Superconducting Temperatures in Cs-based Superconductors

A material, when superconductive, has an electrical resistance of exactly zero below a critical temperature (T _c). This is universal in superconductors. We firstly verify our approaches by comparing our preliminary calculations for resistance with previous available measurements. After the intensive computations, we illustrate the effect of activation volume and energy on the enhancement of T _c (up to 50 K) for the Cs _x Fe_2?y Se_2?z (0≤x,y,z≤1) superconductor by using the verified Eyring’s transition-state approach which is based on the quantum chemistry to treat the activated complex: Cs _x Fe_2?y Se_2?z . Our results suggest that tuning activation volume is the efficient way to enhance the superconducting temperature.     

Go with the FLOW: visualizing spatiotemporal dynamics in optical widefield calcium imaging

Widefield calcium imaging has recently emerged as a powerful experimental technique to record coordinated large-scale brain activity. These measurements present a unique opportunity to characterize spatiotemporally coherent structures that underlie neural activity across many regions of the brain. In this work, we leverage analytic techniques from fluid dynamics to develop a visualization framework that highlights features of flow across the cortex, mapping wavefronts that may be correlated with behavioural events. First, we transform the time series of widefield calcium images into time-varying vector fields using optic flow. Next, we extract concise diagrams summarizing the dynamics, which we refer to as FLOW (flow lines in optical widefield imaging) portraits. These FLOW portraits provide an intuitive map of dynamic calcium activity, including regions of initiation and termination, as well as the direction and extent of activity spread. To extract these structures, we use the finite-time Lyapunov exponent technique developed to analyse time-varying manifolds in unsteady fluids. Importantly, our approach captures coherent structures that are poorly represented by traditional modal decomposition techniques. We demonstrate the application of FLOW portraits on three simple synthetic datasets and two widefield calcium imaging datasets, including cortical waves in the developing mouse and spontaneous cortical activity in an adult mouse.     

On the Euler equation appearing in the synthesis of radiating systems

A problem appearing when the Euler equation is used in the theory of antenna synthesis is considered. Behavior of a solution to this equation at the ends of the synthesis interval contradicts the Meixner conditions, according to which this solution must tend to zero as a square root of the distance to the ends. It is shown that this contradiction arises if we seek solutions in the L ₂ space and disappears if an L ₂ subspace with limited energy norm is selected as the space of solutions. In this case, the Euler equation arises from a variational minimization problem for a functional involving the norm of current determined in the energy space.     

Invariant scrambled sets and distributional chaos

In their famous paper ‘Period three implies chaos’, Li and Yorke started a study of a very important phenomena in dynamical systems (known presently under the name Li–Yorke chaos). Recently, it was proved by Du that an interval map f is turbulent if and only if there is an invariant scrambled set for f. We extend this approach and prove that exactly the same characterization is valid for distributional chaos.     

Low Temperature Scanning Tunneling Spectroscopy Studies of High J c NdBa2Cu3O7 − δ Single Crystals

We report low temperature scanning tunneling spectroscopy (STS) studies on NdBa₂Cu₃O_{7 ? δ} (Nd123) single crystals. Two characteristic spectra with a nearly ohmic background spectrum and a V-shaped background are observed on the cleaved surfaces. The former spectum shows an inhomogeneous distribution of a superconducting energy gap (2Δ) for STS differential conductance map. The latter shows a homogeneous distribution. It is probable that those differences are attributed to a tunneling current from different surface layers. The temperature dependence of tunneling conductance spectrum with a V-shaped background reveals that the superconducting gap disappears around T _c, and no pseudogap behavior exists above T _c.     

The Pole Term in Linear Response Theory: An Example From the Transverse Response of the Electron Gas

In linear response theory, the dielectric response at zero frequency sometimes appears to violate the f-sum rule, which has apparent implications for causality. Here, we study the origin of this apparent discrepancy, focusing on Lindhard’s formula for the transverse response of the electron gas. At non-zero frequency, first-order poles contribute to the imaginary part of the dielectric function in the usual way. At zero frequency, second-order poles contribute in a way which forces a careful consideration of the notation of summation and integration to avoid an error. A compact formula for the contribution of the second-order poles is presented. The sense in which the f-sum rule is satisfied is discussed.