Single Wall Carbon Nanotube Weak Links

We have reproducibly contacted gated single wall carbon nanotubes (SWCNT) to superconducting leads based on niobium. The devices are identified to belong to two transparency regimes: The Coulomb blockade and the Kondo regime. Clear signature of the superconducting leads is observed in both regimes and in the Kondo regime a narrow zero bias peak interpreted as a proximity induced supercurrent persist in Coulomb blockade diamonds with Kondo resonances.Comment: Proceeding for International Symposium on Mesoscopic Superconductivity and Spintronics 2006, NTT BRL, Atsugi, Japa

An improved 2.5 GHz electron pump: single-electron transport through shallow-etched point contacts driven by surface acoustic waves

We present an experimental study of a 2.5 GHz electron pump based on the quantized acoustoelectric current driven by surface acoustic waves (SAWs) through a shallow-etched point contact in a GaAs/AlGaAs heterostructure. At low temperatures and with an additional counter-propagating SAW beam, up to n = 20 current plateaus at I=nef could be resolved, where n is an integer, e the electron charge, and f the SAW frequency. In the best case the accuracy of the first plateau at 0.40 nA was estimated to be dI/I = +/- 25 ppm over 0.25 mV in gate voltage, which is better than previous results.Comment: 11 pages, 4 figure

Using Sedimentary and Geochemical Proxies for Little Ice Age Climate Climate Change Reconstructions, South Mainland Shetland

The Broo Site, located in South Mainland Shetland, is an archaeological site dated to the late 17th century that is believed to have been inhabited for a short period of time before massive sand blows completely buried the stone buildings. Previous investigations indicate deposition of thick sand layers in nearby lochs, potentially tied to Little Ice Age storminess; however the timing and mechanisms of deposition have yet to be determined. Analyses performed on eight cores from the nearby Lochs of Brow and Spiggie include bulk organic matter, stable isotope analysis, biogenic silica, grain size analysis, percent loss on ignition, magnetic susceptibility, and plutonium dating. The objectives of the study were to use sedimentary and geochemical proxies to examine changes in environmental conditions through the last few hundred years. Contiguous coarse-grained sediments were seen in the middle of most cores in the Loch of Brow. Confirmation of a minerogenic layer in the loch was identified through increases in magnetic susceptibility and sediment particle size within the unit, and visual identification. Bulk organic stable isotope analysis indicates a relatively stable carbon isotope signal with δ13C values ranging between -27‰ and -30‰. Stable nitrogen isotope values increased from 1-2‰ at the bottom of the core to 5‰ at the top of the core. Elevated C/N ratios indicate a terrestrial origin of organic matter in organic layers. The proposed models of sand genesis include marine inundation, aeolian deposition, and anthropogenic activity on the landscape, in particular increased agricultural activity, in conjunction with increased storminess of the Little Ice Age

Bedrock and Groundwater Interaction: Tracing the Potential for Cation Contamination in Groundwater, Small Point, Maine

Contamination of groundwater due to the leaching of cations from bedrock can be a serious threat to public health. It is therefore important for geologists to identify potential hazards to bedrock groundwater systems. Small Point, a peninsula located in mid-coast Maine, is an important study location as there is no municipal water supply; residents must rely either on groundwater or a rainwater cistern for drinking water. The objectives of my study are to determine the: (1) relationship between groundwater and leached cations from bedrock, (2) mineral phases where cations are present and the likelihood of mobilization, and (3) implications on the public, namely where are the best and worst places to extract groundwater. For this analysis 10 different sample lithologies, determined from extensive mapping of the Small Point peninsula were studied. These lithologies include Fe-S and Al bearing schists, amphibolites, calc-silicates, and a graphitic phyllite. Using leaching experiments, where each unit was crushed and exposed to slightly acidified rainwater (pH 5.5), it was determined which of the cations found in each rock unit may actually become mobilized within the groundwater system. Inductively coupled plasma atomic emission spectroscopy (ICP-AES), was used to complete this analysis. Leaching indicates that the most mobile cations are found in schists, in particular the Singing Sands Silver Schist(Ossss), as well as in the Icebox Amphibole-Rich Calc-Silicate(Oiacs) sample. Lithologies that did not mobilize cations easily include the Graphitic Phyllite (Ogp) and the West Marsh Schist (Owms). Using leaching experimetns in conjunction with the bedrock geology map developed by Sive (2012), it can be determined preliminarily which areas are the best and least suited to bedrock wells

Single wall carbon nanotube double quantum dot

We report on two top-gate defined, coupled quantum dots in a semiconducting single wall carbon nanotube, constituting a tunable double quantum dot system. The single wall carbon nanotubes are contacted by titanium electrodes, and gated by three narrow top-gate electrodes as well as a back-gate. We show that a bias spectroscopy plot on just one of the two quantum dots can be used to extract the addition energy of both quantum dots. Furthermore, honeycomb charge stability diagrams are analyzed by an electrostatic capacitor model that includes cross capacitances, and we extract the coupling energy of the double quantum dot.Comment: Published in Applied Physics Letters 4 December 2006. http://link.aip.org/link/?APL/89/23211

Different quantization mechanisms in single-electron pumps driven by surface acoustic waves

We have studied the acoustoelectric current in single-electron pumps driven by surface acoustic waves. We have found that in certain parameter ranges two different sets of quantized steps dominate the acoustoelectric current versus gate-voltage characteristics. In some cases, both types of quantized steps appear simultaneously though at different current values, as if they were superposed on each other. This could indicate two independent quantization mechanisms for the acoustoelectric current.Comment: 6 pages, 3 figure

Critical Current 0-π\pi Transition in Designed Josephson Quantum Dot Junctions

We report on quantum dot based Josephson junctions designed specifically for measuring the supercurrent. From high-accuracy fitting of the current-voltage characteristics we determine the full magnitude of the supercurrent (critical current). Strong gate modulation of the critical current is observed through several consecutive Coulomb blockade oscillations. The critical current crosses zero close to, but not at, resonance due to the so-called 0-$\pi$ transition in agreement with a simple theoretical model.Comment: 5 pages, 4 figures, (Supplementary information available at http://www.fys.ku.dk/~hij/public/nl_supp.pdf

Multiple Andreev reflections in diffusive SNS structures

We report new measurements on sup-gap energy structure originating from multiple Andreev reflections in mesoscopic SNS junctions. The junctions were fabricated in a planar geometry with high transparency superconducting contacts of Al deposited on highly diffusive and surface d-doped n++-GaAs. For samples with a normal GaAs region of active length 0.3um the Josephson effect with a maximal supercurrent Ic=3mA at T=237mK was observed. The sub-gap structure was observed as a series of local minima in the differential resistance at dc bias voltages V=2D/ne with n=1,2,4 i.e. only the even sub-gap positions. While at V=2D/e (n=1) only one dip is observed, the n=2, and the n=4 sub-gap structures each consists of two separate dips in the differential resistance. The mutual spacing of these two dips is independent of temperature, and the mutual spacing of the n=4 dips is half of the spacing of the n=2 dips. The voltage bias positions of the sub-gap differential resistance minima coincide with the maxima in the oscillation amplitude when a magnetic field is applied in an interferometer configuration, where one of the superconducting electrodes has been replaced by a flux sensitive open loop.Comment: 20 pages, 7 figure