Semiconductor devices and methods

A method of forming a semiconductor device includes the following steps: providing a plurality of semiconductor layers; providing means for coupling signals to and/or from layers of the device; providing a quantum well disposed between adjacent layers of the device; and providing a layer of quantum dots disposed in one of the adjacent layers, and spaced from the quantum well, whereby carriers can tunnel in either direction between the quantum well and the quantum dots.Board of Regents, University of Texas Syste

Dynamics of sliding drops on superhydrophobic surfaces

We use a free energy lattice Boltzmann approach to investigate numerically the dynamics of drops moving across superhydrophobic surfaces. The surfaces comprise a regular array of posts small compared to the drop size. For drops suspended on the posts the velocity increases as the number of posts decreases. We show that this is because the velocity is primarily determined by the contact angle which, in turn, depends on the area covered by posts. Collapsed drops, which fill the interstices between the posts, behave in a very different way. The posts now impede the drop behaviour and the velocity falls as their density increases.Comment: 7 pages, 4 figures, accepted for publication in Europhys. Let

Rheology of cholesteric blue phases

Blue phases of cholesteric liquid crystals offer a spectacular example of naturally occurring disclination line networks. Here we numerically solve the hydrodynamic equations of motion to investigate the response of three types of blue phases to an imposed Poiseuille flow. We show that shear forces bend and twist and can unzip the disclination lines. Under gentle forcing the network opposes the flow and the apparent viscosity is significantly higher than that of an isotropic liquid. With increased forcing we find strong shear thinning corresponding to the disruption of the defect network. As the viscosity starts to drop, the imposed flow sets the network into motion. Disclinations break-up and re-form with their neighbours in the flow direction. This gives rise to oscillations in the time-dependent measurement of the average stress.Comment: 4 pages, 4 figure

Quantum non-malleability and authentication

In encryption, non-malleability is a highly desirable property: it ensures that adversaries cannot manipulate the plaintext by acting on the ciphertext. Ambainis, Bouda and Winter gave a definition of non-malleability for the encryption of quantum data. In this work, we show that this definition is too weak, as it allows adversaries to "inject" plaintexts of their choice into the ciphertext. We give a new definition of quantum non-malleability which resolves this problem. Our definition is expressed in terms of entropic quantities, considers stronger adversaries, and does not assume secrecy. Rather, we prove that quantum non-malleability implies secrecy; this is in stark contrast to the classical setting, where the two properties are completely independent. For unitary schemes, our notion of non-malleability is equivalent to encryption with a two-design (and hence also to the definition of Ambainis et al.). Our techniques also yield new results regarding the closely-related task of quantum authentication. We show that "total authentication" (a notion recently proposed by Garg, Yuen and Zhandry) can be satisfied with two-designs, a significant improvement over the eight-design construction of Garg et al. We also show that, under a mild adaptation of the rejection procedure, both total authentication and our notion of non-malleability yield quantum authentication as defined by Dupuis, Nielsen and Salvail.Comment: 20+13 pages, one figure. v2: published version plus extra material. v3: references added and update

Jetting Micron-Scale Droplets onto Chemically Heterogeneous Surfaces

We report experiments investigating the behaviour of micron-scale fluid droplets jetted onto surfaces patterned with lyophobic and lyophilic stripes. The final droplet shape depends on the droplet size relative to that of the stripes. In particular when the droplet radius is of the same order as the stripe width, the final shape is determined by the dynamic evolution of the drop and shows a sensitive dependence on the initial droplet position and velocity. Numerical solutions of the dynamical equations of motion of the drop provide a close quantitative match to the experimental results. This proves helpful in interpreting the data and allows for accurate prediction of fluid droplet behaviour for a wide range of surfaces.Comment: 14 pages, accepted for publication in Langmui

Dimensional crossover and metal-insulator transition in quasi-two-dimensional disordered conductors

We study the metal-insulator transition (MIT) in weakly coupled disordered planes on the basis of a Non-Linear Sigma Model (NL$\sigma$M). Using two different methods, a renormalization group (RG) approach and an auxiliary field method, we calculate the crossover length between a 2D regime at small length scales and a 3D regime at larger length scales. The 3D regime is described by an anisotropic 3D NL$\sigma$M with renormalized coupling constants. We obtain the critical value of the single particle interplane hopping which separates the metallic and insulating phases. We also show that a strong parallel magnetic field favors the localized phase and derive the phase diagram.Comment: 16 pages (RevTex), 4 poscript figure

The accretion-diffusion scenario for metals in cool white dwarfs

We calculated diffusion timescales for Ca, Mg, Fe in hydrogen atmosphere white dwarfs with temperatures between 5000 and 25000 K. With these timescales we determined accretion rates for a sample of 38 DAZ white dwarfs from the recent studies of Zuckerman et al. (2003) and Koester et al. (2005). Assuming that the accretion rates can be calculated with the Bondi-Hoyle formula for hydrodynamic accretion, we obtained estimates for the interstellar matter density around the accreting objects. These densities are in good agreement with new data about the warm, partially ionized phase of the ISM in the solar neighborhood.Comment: To be published in A&

The Ultramassive White Dwarf EUVE J1746-706

We have obtained new optical and extreme ultraviolet (EUV) spectroscopy of the ultramassive white dwarf EUVE J1746-706. We revise Vennes et al.'s (1996a, ApJ, 467, 784) original estimates of the atmospheric parameters and we measure an effective temperature of 46,500 +/- 700 K and a surface gravity log g = 9.05 +/- 0.15 (~1.2 M_o), in agreement with Balmer line profiles and the EUV continuum. We derive an upper limit on the atmospheric abundance of helium of He/H = 1.3 x 10^{-4} and a neutral hydrogen column density in the local interstellar medium N_HI = 1.8 +/- 0.4 x 10^{19} cm^{-2} from the EUV spectrum. Our upper limit corresponds to half the helium abundance observed in the atmosphere of the ultramassive white dwarf GD 50. We discuss the possibility that EUVE J1746-706 represents an earlier phase of evolution relative to GD 50 and may, therefore, help us understand the origin and evolution of massive white dwarfs.Comment: 6 pages, 4 postscript figures, uses aastex, to be published in ApJ Letter

Control of drop positioning using chemical patterning

We explore how chemical patterning on surfaces can be used to control drop wetting. Both numerical and experimental results are presented to show how the dynamic pathway and equilibrium shape of the drops are altered by a hydrophobic grid. The grid proves a successful way of confining drops and we show that it can be used to alleviate {\it mottle}, a degradation in image quality which results from uneven drop coalescence due to randomness in the positions of the drops within the jetted array.Comment: 3 pages, 4 figure