554 research outputs found
Interfacial Studies in Semiconductor Heterostructures by X-Ray Diffraction Techniques
X-ray radiation is a non-destructive probe well suited to assess structural perfection of semiconductor material. Three techniques are used to study the interfacial roughness, period fluctuations and annealing-induced interdiffusion in various superlattice structures. Reflectivity of long period Si/Si1-xGex multiple quantum wells reveals an asymmetry oriented along the direction of miscut in the interface roughness with the Si1-xGex to Si interfaces being about twice as rough (0.5 versus 0.3 nm) as the Si to Si1-xGex interfaces. For Si-Si0.65Ge0.35 multiple quantum wells, diffuse scattering is minimal for a growth temperature of 550°C and increases substantially at very low (250°C) or high (750°C) growth temperatures. In (SimGen)p short period superlattices, the X-ray reflectivity data are consistent with interfacial mixing over about two monolayers and thickness fluctuations of about 5% vertically in the structures. For superlattices grown on vicinal surfaces, the roughness spectrum is correlated with the surface miscut orientation. Double-crystal X-ray diffraction using symmetrical and asymmetrical reflections has been used to study epitaxial lattice distortion and strain relaxation in InGaAs/GaAs heterostructures grown on (100) on-orientation and 2° off (100) GaAs surfaces. It is shown that thick InGaAs films retain an appreciable fraction of their initial strain and that their crystal lattice is triclinically distorted. The magnitude of the deformation is larger when growth is carried out on a vicinal surface
Strong subadditivity and the covariant holographic entanglement entropy formula
Headrick and Takayanagi showed that the Ryu-Takayanagi holographic
entanglement entropy formula generally obeys the strong subadditivity (SSA)
inequality, a fundamental property of entropy. However, the Ryu-Takayanagi
formula only applies when the bulk spacetime is static. It is not known whether
the covariant generalization proposed by Hubeny, Rangamani, and Takayanagi
(HRT) also obeys SSA. We investigate this question in three-dimensional
AdS-Vaidya spacetimes, finding that SSA is obeyed as long as the bulk spacetime
satisfies the null energy condition. This provides strong support for the
validity of the HRT formula.Comment: 38 page
Pressure-dependent transition from atoms to nanoparticles in magnetron sputtering: Effect on WSi2 film roughness and stress
We report on the transition between two regimes from several-atom clusters to
much larger nanoparticles in Ar magnetron sputter deposition of WSi2, and the
effect of nanoparticles on the properties of amorphous thin films and
multilayers. Sputter deposition of thin films is monitored by in situ x-ray
scattering, including x-ray reflectivity and grazing incidence small angle
x-ray scattering. The results show an abrupt transition at an Ar background
pressure Pc; the transition is associated with the threshold for energetic
particle thermalization, which is known to scale as the product of the Ar
pressure and the working distance between the magnetron source and the
substrate surface. Below Pc smooth films are produced, while above Pc roughness
increases abruptly, consistent with a model in which particles aggregate in the
deposition flux before reaching the growth surface. The results from WSi2 films
are correlated with in situ measurement of stress in WSi2/Si multilayers, which
exhibits a corresponding transition from compressive to tensile stress at Pc.
The tensile stress is attributed to coalescence of nanoparticles and the
elimination of nano-voids.Comment: 16 pages, 10 figures; v3: published versio
The Simplicial Ricci Tensor
The Ricci tensor (Ric) is fundamental to Einstein's geometric theory of
gravitation. The 3-dimensional Ric of a spacelike surface vanishes at the
moment of time symmetry for vacuum spacetimes. The 4-dimensional Ric is the
Einstein tensor for such spacetimes. More recently the Ric was used by Hamilton
to define a non-linear, diffusive Ricci flow (RF) that was fundamental to
Perelman's proof of the Poincare conjecture. Analytic applications of RF can be
found in many fields including general relativity and mathematics. Numerically
it has been applied broadly to communication networks, medical physics,
computer design and more. In this paper, we use Regge calculus (RC) to provide
the first geometric discretization of the Ric. This result is fundamental for
higher-dimensional generalizations of discrete RF. We construct this tensor on
both the simplicial lattice and its dual and prove their equivalence. We show
that the Ric is an edge-based weighted average of deficit divided by an
edge-based weighted average of dual area -- an expression similar to the
vertex-based weighted average of the scalar curvature reported recently. We use
this Ric in a third and independent geometric derivation of the RC Einstein
tensor in arbitrary dimension.Comment: 19 pages, 2 figure
Ricci flow and black holes
Gradient flow in a potential energy (or Euclidean action) landscape provides
a natural set of paths connecting different saddle points. We apply this method
to General Relativity, where gradient flow is Ricci flow, and focus on the
example of 4-dimensional Euclidean gravity with boundary S^1 x S^2,
representing the canonical ensemble for gravity in a box. At high temperature
the action has three saddle points: hot flat space and a large and small black
hole. Adding a time direction, these also give static 5-dimensional
Kaluza-Klein solutions, whose potential energy equals the 4-dimensional action.
The small black hole has a Gross-Perry-Yaffe-type negative mode, and is
therefore unstable under Ricci flow. We numerically simulate the two flows
seeded by this mode, finding that they lead to the large black hole and to hot
flat space respectively, in the latter case via a topology-changing
singularity. In the context of string theory these flows are world-sheet
renormalization group trajectories. We also use them to construct a novel free
energy diagram for the canonical ensemble.Comment: 31 pages, 14 color figures. v2: Discussion of the metric on the space
of metrics corrected and expanded, references adde
Numerical solution to the hermitian Yang-Mills equation on the Fermat quintic
We develop an iterative method for finding solutions to the hermitian
Yang-Mills equation on stable holomorphic vector bundles, following ideas
recently developed by Donaldson. As illustrations, we construct numerically the
hermitian Einstein metrics on the tangent bundle and a rank three vector bundle
on P^2. In addition, we find a hermitian Yang-Mills connection on a stable rank
three vector bundle on the Fermat quintic.Comment: 25 pages, 2 figure
Influence of Annealing on the Interface Structure and Strain Relief in Si/Ge Heterostructures on (100) Si
Research work on the general problem of the nature and thermal stability of the Si/Ge semiconductor interface is reviewed. We report on our recent studies of the interface structure in [(Si)m(Ge)n]p superlattices and (Ge)n layers buried in Si as revealed by Raman scattering, extended X-ray absorption fine structure, and X-ray techniques. Strain relaxation and interdiffusion in the superlattices caused by annealing have been investigated, and it is found that considerable strain-enhanced intermixing together with partial relaxation of Ge-Ge bonds occurs even for very short anneal times at 700°C. Further annealing leads to diffusion at a much slower rate and to the eventual formation of an alloy layer. The Ge-Ge bond lengths in as-grown samples are that expected for a fully strained Ge layer. Similar studies of the (Ge)n layers reveal that two-dimensional pseudomorphic growth proceeds up to n = 5, probably mediated by a Si-Ge interface interdiffusion over one or two monolayers of approximately 20%. A n = 12 layer gave evidence of strain relaxation by the introduction of dislocations and clustering. Interdiffusion proceeds rapidly on annealing at 750°C
Investigating Off-shell Stability of Anti-de Sitter Space in String Theory
We propose an investigation of stability of vacua in string theory by
studying their stability with respect to a (suitable) world-sheet
renormalization group (RG) flow. We prove geometric stability of (Euclidean)
anti-de Sitter (AdS) space (i.e., ) with respect to the simplest
RG flow in closed string theory, the Ricci flow. AdS space is not a fixed point
of Ricci flow. We therefore choose an appropriate flow for which it is a fixed
point, prove a linear stability result for AdS space with respect to this flow,
and then show this implies its geometric stability with respect to Ricci flow.
The techniques used can be generalized to RG flows involving other fields. We
also discuss tools from the mathematics of geometric flows that can be used to
study stability of string vacua.Comment: 29 pages, references added in this version to appear in Classical and
Quantum Gravit
Transport and Photo-Conduction in Carbon Nanotube Fibers
We have characterized the conductivity of carbon nanotubes (CNT) fibers
enriched in semiconducting species as a function of temperature and pulsed
laser irradiation of 266 nm wavelength. While at high temperatures the response
approaches an Arrhenius law behavior, from room temperature down to 4.2 K the
response can be framed, quantitatively, within the predictions of the
fluctuation induced tunneling which occurs between the inner fibrils (bundles)
of the samples and/or the elementary CNTs constituting the fibers. Laser
irradiation induces an enhancement of the conductivity, and analysis of the
resulting data confirms the (exponential) dependence of the potential barrier
upon temperature as expected from the fluctuation induced tunneling model. A
thermal map of the experimental configuration consisting of laser-irradiated
fibers is also obtained via COMSOL simulations in order to rule out bare
heating phenomena as the background of our experiments. (*) AuthorComment: 13 pages and 7 figure
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