Remotely-doped graded potential well structures

We present a new method to obtain high-mobility three-dimensional electron gas systems. We have achieved control of carrier density and of carrier profile by growth of the first remotely-doped parabolic potential well structures. Computer-controlled molecular beam epitaxy is used to grow a layer of ultra-fine superlattices with a programmable composition gradient. This produces conduction-band potentials which, in the absence of doping, are equivalent to the potential profiles of fixed charge distributions. When conduction electrons are introduced into these graded wells through remote doping of the barrier regions, they distribute themselves in such a way as to produce a uniform chemical potential at thermal equilibrium. We illustrate through computer simulations employing Fermi statistics that electrons introduced into a wide parabolic potential well distribute themselves uniformly. More significantly, the carrier distribution in the well is remarkably insensitive to the dopant sheet charge in the barrier, the more so at lower temperatures. We have fabricated remotely-doped graded potential well structures of the proposed type by molecular beam epitaxy. These structures exhibit the above effects. Measured mobilities of such three-dimensional electron gases grown using the GaAs/Al_xGa_1−xAs system are higher than those of bulk-doped GaAs doped to give the same uniform electron concentration.     

Flow and heat transfer for a power-law electrically conducting fluid flowing between parallel plates under transverse magnetic field with viscous dissipation

The flow and heat transfer problem with viscous dissipation for electrically conducting non-Newtonian fluids with power-law model in the thermal entrance region of two parallel plates with magnetic field under constant heat flux and constant wall temperature conditions has been studied. The governing equations have been solved numerically using quasilinearization technique and implicit finite-difference scheme. It has been found that the effect of viscous dissipation on heat transfer is quite significant for heating and cooling conditions at the wall.     

Spectroscopically determined electronegativity values for heavy elements

A new scale of electronegativity based on X-ray spectroscopic data was earlier presented by Mande and others for the elements₃Li to₅₄Xe. The present paper reports an extension of this scale carried out for most of the heavier elements lying between₅₅Cs and₉₂U. The present approach is based on the physical interpretation of electronegativity of an atom as the attractive electrostatic force it exerts at a distance equal to its covalent radius. To estimate this force, experimental spectroscopic data have been employed. The merits of such an approach have been pointed out recently by Mullay and may be of interest in the context of the intimate relation found between electronegativity and superconducting transition temperature.     

Microexplosions in tellurite glasses

Femtosecond laser pulses were used to produce localized damage in the bulk and near the surface of baseline, Al₂O₃-doped and La₂O₃-doped sodium tellurite glasses. Single or multiple laser pulses were non-linearly absorbed in the focal volume by the glass, leading to permanent changes in the material in the focal volume. These changes were caused by an explosive expansion of the ionized material in the focal volume into the surrounding material, i.e. a microexplosion. The writing of simple structures (periodic array of voxels, as well as lines) was demonstrated. The regions of microexplosion and writing were subsequently characterized using scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS) and atomic force microscopy (AFM). Fingerprints of microexplosions (concentric lines within the region and a concentric ring outside the region), due to the shock wave generated during microexplosions, were evident. In the case of the baseline glass, no chemistry change was observed within the region of the microexplosion. However, Al₂O₃-doped and La₂O₃-doped glasses showed depletion of the dopant from the edge to the center of the region of the microexplosions, indicating a chemistry gradient within the regions. Interrogation of the bulk- and laser-treated regions using micro-Raman spectroscopy revealed no structural change due to the microexplosions and writing within these glasses. Received: 27 December 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. +1-509/376-3108, E-mail: sk.sundaram@pnl.gov     

Surface segregation and surface electronic interactions in PtCu

Photoemission and Auger electron spectroscopy on Pt_0.98Cu_0.02 show that the (110) face has over twice as much Cu surface segregation as the (111) face. The Cu 3d-derived surface “density of states” differ strikingly in peak shape and in width (by 0.5 eV). The centroids, compared with bulk Cu d states, are shifted by more than 0.3 eV towards the Fermi level. This is the first experimental correlation between surface segregation and surface bonding.     

Characterization of high intensity focused ultrasound transducers using acoustic streaming

A new approach for characterizing high intensity focused ultrasound (HIFU) transducers is presented. The technique is based upon the acoustic streaming field generated by absorption of the HIFU beam in a liquid medium. The streaming field is quantified using digital particle image velocimetry, and a numerical algorithm is employed to compute the acoustic intensity field giving rise to the observed streaming field. The method as presented here is applicable to moderate intensity regimes, above the intensities which may be damaging to conventional hydrophones, but below the levels where nonlinear propagation effects are appreciable. Intensity fields and acoustic powers predicted using the streaming method were found to agree within 10% with measurements obtained using hydrophones and radiation force balances. Besides acoustic intensity fields, the streaming technique may be used to determine other important HIFU parameters, such as beam tilt angle or absorption of the propagation medium.