53 research outputs found
Transport Properties of Holographic Defects
We study the charge transport properties of fields confined to a
(2+1)-dimensional defect coupled to (3+1)-dimensional super-Yang-Mills at
large-\nc and strong coupling, using AdS/CFT techniques applied to linear
response theory. The dual system is described by \nf probe D5- or D7-branes
in the gravitational background of \nc black D3-branes. Surprisingly, the
transport properties of both defect CFT's are essentially identical -- even
though the D7-brane construction breaks all supersymmetries. We find that the
system possesses a conduction threshold given by the wave-number of the
perturbation and that the charge transport arises from a quasiparticle spectrum
which is consistent with an intuitive picture where the defect acquires a
finite width. We also examine finite- modifications arising from
higher derivative interactions in the probe brane action.Comment: 54 pages, 22 figures, references added, minor changes to figures and
comments, final version published in JHE
The transformation of diamond to graphite: Experiments reveal the presence of an intermediate linear carbon phase
Natural diamonds that have been partially replaced by graphite have been observed to occur in natural rocks. While the graphite-to-diamond phase transition has been extensively studied the opposite of this (diamond to graphite) remains poorly understood. We performed high-pressure and temperature hydrous and anhydrous experiments up to 1.0 GPa and 1300 °C using Amplex premium virgin synthetic diamonds (20–40 μm size) as the starting material mixed with Mg(OH) as a source of HO for the hydrous experiments. The experiments revealed that the diamond-to-graphite transformation at P = 1.0 GPa and T = 1300 °C was triggered by the presence of HO and was accomplished through a three-stage process. Stage 1: diamond reacts with a supercritical HO producing an intermediate 200–500 nm size “globular carbon” phase. This phase is a linear carbon chain; i.e. a polyyne or carbyne. Stage 2: the linear carbon chains are unstable and highly reactive, and they decompose by zigzagging and cross-linking to form sp-hybridized structures. Stage 3: normal, disordered, and onion-like graphite is produced by the decomposition of the sp-hybridized carbon chains which are re-organized into sp bonds. Our experiments show that there is no direct transformation from sp C-bonds into sp C-bonds. Our hydrous high-pressure and high-temperature experiments show that the diamond-to-graphite transformation requires an intermediate metastable phase of a linear hydrocarbon. This process also provides a simple mechanism for the substitution of other elements into the graphite structure (e.g. H, S, O)
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Subnanosecond, high voltage photoconductive switching in GaAs
We are conducting research on the switching properties of photoconductive materials to explore their potential for generating high-power microwaves (HPM) and for high rep-rate switching. We have investigated the performance of Gallium Arsenide (GaAs) in linear mode (the conductivity of the device follows the optical pulse) as well as an avalanche-like mode (the optical pulse only controls switch closing). Operating in the linear mode, we have observed switch closing times of less than 200 ps with a 100 ps duration laser pulse and opening times of less than 400 ps at several kV/cm fields using neutron irradiated GaAs. In avalanche and lock-on modes, high fields are switched with lower laser pulse energies, resulting in higher efficiencies; but with measurable switching delay and jitter. We are currently investigating both large area (1 cm{sup 2}) and small area (<1 mm{sup 2}) switches illuminated by AlGaAs laser diodes at 900 nm and Nd:YAG lasers at 1.06 {mu}m
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