[en] The equations of motion for convection in dilute ³He-superfluid-⁴He mixtures are the same as those for convection in a conventional pure fluid with the addition of several correction terms. Fetter has considered, for a horizontally infinite layer with realistic boundary conditions, the effect of these corrections on the critical Rayleigh number, R_c. The results are a perturbation expansion for R_c to lowest order in three perturbation terms, var-epsilon ₁, var-epsilon ₂, var-epsilon ₃. In order to make a comparison with recent precise experiments which have yielded R_c as a function of the layer height d, the authors have carried out several calculations. First they show that the analysis can be recast as an expansion in inverse powers of d². They then carry out a complete expansion to O(d^-6). Up to O(d^-4), the expansion involves only the ratio (λ₀/d) where λ₀ is a length scale which is intrinsic to superfluid mixtures. They consider the effect of the superfluid perturbations on both the critical Rayleigh numbers and wavevectors. These are shifted very little as long as λ₀/d is small; the crossover from large to small occurs for λ₀/d∼0.1. They also solve a simplified version of the stability problem which contains the dominant superfluid effect. The simplified problem is Hermitian, and is therefore amenable to an exact solution. A comparison with experimental data for R_c and the simplified model shows excellent agreement with the calculations. 12 refs., 7 figs

[en] We report observations of chaotic bursting in a fluid, a convecting layer of ³He--superfluid-⁴He. At an aspect ratio Γ=6.0 and Prandtl number Pr=0.3 we find intermittent bursts out of a fully developed chaotic state. Above the onset of bursting the average length of a burst-free region varies as (R-R_b)^-γ with γ=4.6. The average length of a burst varies as (R-R_b)^δ with δ=2.0. A particularly novel state was seen for Γ=4.25 and Pr=0.12, where there occurs a reversible switching transition involving two chaotic attractors

[en] We report in situ doping of GaN with the rare earth element Nd by plasma-assisted molecular beam epitaxy. For the highest Nd effusion cell temperatures, Rutherford backscattering and secondary ion mass spectroscopy data indicate ∼5 at. % Nd in epilayers grown on c-plane sapphire. X-ray diffraction found no evidence of phase segregation under nitrogen-rich conditions with up to ∼1 at. % Nd, with the highest luminescence intensities corresponding to doping of ∼0.5 at. %. Spectral correlation of the Nd emission multiplets for above (325 nm) and below (836 nm) GaN bandgap excitations implies enhanced substitutional doping at the Ga site

[en] Strain-compensated InGaN/AlGaN structures can enable the growth of thick layers of InGaN epitaxial films far beyond the critical thickness for InGaN grown pseudomorphically to GaN. In this paper, we demonstrate the epitaxial growth of high-quality strain-compensated a-plane In_0.12Ga_0.88N/Al_0.19Ga_0.81N superlattices up to 5 times thicker than the critical thickness on free-standing a-plane GaN substrates by plasma-assisted molecular beam epitaxy (PA-MBE). The superlattices consist of 50 to 200 periods of 10 nm thick In_0.12Ga_0.88N and 6 nm thick Al_0.19Ga_0.81N layers. The structures are characterized using a double crystal X-ray diffractometer, asymmetric reciprocal space mapping, and atomic force microscopy. We use X-ray diffraction to determine the strain, composition, degree of relaxation, and superlattice period of our samples. The structural characteristics of periodic structures containing from 50 to 200 periods are compared to single layer, uncompensated In_0.12Ga_0.88N films. A 100 period structure exhibited only 15% relaxation compared to 69% relaxation for the bulk In_0.12Ga_0.88N film grown with the same total InGaN thickness but without strain-compensating layers. (copyright 2015 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

[en] The role of the In adlayer on the morphological and structural properties of nonpolar a-plane InN films was elucidated during the plasma-assisted molecular beam epitaxy on freestanding GaN. Reflection high energy electron diffraction during In adsorption experiments on a-plane InN surfaces revealed a stable In adlayer coverage of ∼2 ML. This In adlayer-mediated growth was responsible for achieving atomically smooth surfaces (rms roughness of <1 nm), phase-pure material with lower x-ray rocking curve widths (Δω<0.5 deg.), lower crystal mosaic tilt/twist, and decreased stacking fault densities, compared to N-rich conditions. The photoluminescence peak emission and band gap energy of the a-plane InN films were ∼0.63 and ∼0.7 eV, respectively

[en] This study reports on the growth of high-quality nonpolar m-plane [1100] InN films on free-standing m-plane GaN substrates by plasma-assisted molecular beam epitaxy. Optimized growth conditions (In/N ratio ∼1 and T=390-430 deg. C) yielded very smooth InN films with undulated features elongated along the [1120] orientation. This directionality is associated with the underlying defect structure shown by the anisotropy of x-ray rocking curve widths parallel to the [1120] (i.e., 0.24 deg. - 0.34 deg.) and [0001] (i.e., 1.2 deg. - 2.7 deg.) orientations. Williamson-Hall analysis and transmission electron microscopy identified the mosaic tilt and lateral coherence length and their associations with different densities of dislocations and basal-plane stacking faults. Ultimately, very low band gap energies of ∼0.67 eV were measured by optical absorption similar to the best c-plane InN

[en] Highlights: • A new mechanism for active metamaterial in THz is proposed and implemented. • A tunable complex oxide thin film is in the core of the proposed mechanism. • This active metamaterial will have faster response and better wear and tear. • The active metamaterial can be used to determine dielectric properties of the film. - Abstract: Design, fabrication and characterization of an active single-negative metamaterial device in the terahertz (THz) spectrum is presented. The device is constructed by vertical split-ring unit cells which incorporate a Ba_0.6Sr_0.4TiO₃ (BST) thin film whose dielectric constant can be altered by applying DC bias to change the capacitance of the split-ring resonators. A numerical model of the device that predicts a resonant frequency as a function of BST dielectric constant is presented. The resonant frequency shift due to the applied DC bias is experimentally verified and correlated with the model. The resonant frequency shift occurs as a direct result of the bias controlled index of refraction change in BST which significantly enhances the device response speed and durability compared with previously demonstrated micro-electro-mechanical systems based tunable metamaterial devices. In addition, this device can be used to characterize dielectric properties of the tunable oxide thin films in low THz spectrum (such as tunability) for many applications.

[en] Passive scalar transport involves complex interactions between advection and diffusion, where the global transport rate depends upon scalar diffusivity and the values of the (possibly large) set of parameters controlling the advective flow. Although computation of a single solution of the advection-diffusion equation (ADE) is simple, in general it is prohibitively expensive to compute the parametric variation of solutions over the full parameter space Q, even though this is crucial for, e.g. optimization, parameter estimation, and elucidating the global structure of transport. By decomposing the flows within Q so as to exploit symmetries, we derive a spectral method that solves the ADE over Q three orders of magnitude faster than other methods of similar accuracy. Solutions are expressed in terms of the exponentially decaying natural periodic patterns of the ADE, sometimes called 'strange eigenmodes'. We apply the method to the experimentally realisable rotated arc mixer chaotic flow, both to establish numerical properties and to calculate the fine-scale structure of the global solution space for transport in this chaotic flow. Over 10⁵ solutions within Q are resolved, and spatial pattern locking, a symmetry breaking transition to disordered spatial patterns, and fractally distributed optima in transport rate are observed. The method exhibits exponential convergence, and efficiency increases with resolution of Q

[en] Laminar mixing by the inline-mixing principle is a key to many industrial fluids-engineering systems of size extending from micrometers to meters. However, insight into fundamental transport phenomena particularly under the realistic conditions of three-dimensionality (3D) and fluid inertia remains limited. This study addresses these issues for inline mixers with cylindrical geometries and adopts the Rotated Arc Mixer (RAM) as a representative system. Transport is investigated from a Lagrangian perspective by identifying and examining coherent structures that form in the 3D streamline portrait. 3D effects and fluid inertia introduce three key features that are not found in simplified configurations: transition zones between consecutive mixing cells of the inline-mixing flow; local upstream flow (in certain parameter regimes); transition/inertia-induced breaking of symmetries in the Lagrangian equations of motion (causing topological changes in coherent structures). Topological considerations strongly suggest that there nonetheless always exists a net throughflow region between inlet and outlet of the inline-mixing flow that is strictly separated from possible internal regions. The Lagrangian dynamics in this region admits representation by a 2D time-periodic Hamiltonian system. This establishes one fundamental kinematic structure for the present class of inline-mixing flows and implies universal behavior in that all states follow from the Hamiltonian breakdown of one common integrable state. A so-called period-doubling bifurcation is the only way to eliminate transport barriers originating from this state and thus is a necessary (yet not sufficient) condition for global chaos. Important in a practical context is that a common simplification in literature, i.e., cell-wise fully-developed Stokes flow (“2.5D approach”), retains these fundamental kinematic properties and deviates from the generic 3D inertial case only in a quantitative sense. This substantiates its suitability for (at least first exploratory) studies on (qualitative) mixing properties

[en] Countless theoretical/numerical studies on transport and mixing in two-dimensional (2D) unsteady flows lean on the assumption that Hamiltonian mechanisms govern the Lagrangian dynamics of passive tracers. However, experimental studies specifically investigating said mechanisms are rare. Moreover, they typically concern local behavior in specific states (usually far away from the integrable state) and generally expose this indirectly by dye visualization. Laboratory experiments explicitly addressing the global Hamiltonian progression of the Lagrangian flow topology entirely from integrable to chaotic state, i.e., the fundamental route to efficient transport by chaotic advection, appear non-existent. This motivates our study on experimental visualization of this progression by direct measurement of Poincaré sections of passive tracer particles in a representative 2D time-periodic flow. This admits (i) accurate replication of the experimental initial conditions, facilitating true one-to-one comparison of simulated and measured behavior, and (ii) direct experimental investigation of the ensuing Lagrangian dynamics. The analysis reveals a close agreement between computations and observations and thus experimentally validates the full global Hamiltonian progression at a great level of detail