16 research outputs found
A bound on the spectral radius of a weighted graph
Let G be simple, connected weighted graphs, where the edge weights are positive definite matrices. In this paper, we will give an upper bound on the spectral radius of the adjacency matrix for a graph G and characterize graphs for which the bound is attained
Ultrahigh-frequency surface acoustic wave generation for acoustic charge transport in silicon
We demonstrate piezo-electrical generation of ultrahigh-frequency surface acoustic waves on silicon substrates, using high-resolution UV-based nanoimprint lithography, hydrogen silsequioxane planarization, and metal lift-off. Interdigital transducers were fabricated on a ZnO layer sandwiched between two SiO2 layers on top of a Si substrate. Excited modes up to 23.5 GHz were observed. Depth profile calculations of the piezoelectric field show this multilayer structure to be suitable for acoustic charge transport in silicon at extremely high frequencies with moderate carrier mobility requirements
Ultrahigh-frequency surface acoustic wave transducers on ZnO/SiO2/Si using nanoimprint lithography
: Ultrahigh-frequency surface acoustic wave devices were fabricated on a ZnO/SiO2/Si substrate using step-and-flash nanoimprint lithography combined with hydrogen silsesquioxane (HSQ) planarization. Excellent critical dimension control was demonstrated for interdigital transducers with finger electrode widths from 125 down to 65 nm. Fundamental and higher-order Rayleigh modes up to 16.1 GHz were excited and detected, which is the highest frequency for ZnO-based transducers on silicon reported so far. Surface acoustic modes were confirmed with numerical simulations. Simulation results showed good agreement with the experimental data
High-frequency acoustic charge transport in GaAs nanowires
The oscillating piezoelectric fields accompanying surface acoustic waves are able to transport charge carriers in semiconductor heterostructures. Here, we demonstrate high-frequency (above 1 GHz) acoustic charge transport in GaAs-based nanowires deposited on a piezoelectric substrate. The short wavelength of the acoustic modulation, smaller than the length of the nanowire, allows the trapping of photo-generated electrons and holes at the spatially separated energy minima and maxima of conduction and valence bands, respectively, and their transport along the nanowire with a well defined acoustic velocity towards indium-doped recombination centers
Radio frequency occupancy state control of a single nanowire quantum dot
The excitonic occupancy state of a single, nanowire-based, heterostructure
quantum dot is dynamically programmed by a surface acoustic wave. The quantum
dot is formed by an interface or thickness fluctuation of a GaAs QW embedded in
a AlGaAs shell of a GaAs-AlGaAs core-shell nanowire. As we tune the time at
which carriers are photogenerated during the acoustic cycle, we find pronounced
intensity oscillations of neutral and negatively charged excitons. At high
acoustic power levels these oscillations become anticorrelated which enables
direct acoustic programming of the dot's charge configuration, emission
intensity and emission wavelength. Numerical simulations confirm that the
observed modulations arise from acoustically controlled modulations of the
electron and electron-hole-pair concentrations at the position of the quantum
dot.Comment: Revised manuscript, 12 pages, submitted to J. Phys. D as a
contribution in a Special Issue on Nanowire