Nonlinear magnetic field dependence of the conductance in d-wave NIS tunnel junctions

The ab-plane NIS-tunnelling conductance in d-wave superconductors shows a zero-bias conductance peak which is predicted to split in a magnetic field. In a pure d-wave superconductor the splitting is linear for fields small on the scale of the thermodynamic critical field. The field dependence is shown to be nonlinear, even at low fields, in the vicinity of a surface phase transition into a local time-reversal symmetry breaking state. The field evolution of the conductance is sensitive to temperature, doping, and the symmetry of the sub-dominant pairing channel.Comment: 4 pages, 4 figure

Contacting single bundles of carbon nanotubes with alternating electric fields

Single bundles of carbon nanotubes have been selectively deposited from suspensions onto sub-micron electrodes with alternating electric fields. We explore the resulting contacts using several solvents and delineate the differences between Au and Ag as electrode materials. Alignment of the bundles between electrodes occurs at frequencies above 1 kHz. Control over the number of trapped bundles is achieved by choosing an electrode material which interacts strongly with the chemical functional groups of the carbon nanotubes, with superior contacts being formed with Ag electrodes.Comment: 4 pages, RevTe

Reversible Metal-Semiconductor Transition of ssDNA-Decorated Single-Walled Carbon Nanotubes

A field effect transistor (FET) measurement of a SWNT shows a transition from a metallic one to a p-type semiconductor after helical wrapping of DNA. Water is found to be critical to activate this metal-semiconductor transition in the SWNT-ssDNA hybrid. Raman spectroscopy confirms the same change in electrical behavior. According to our ab initio calculations, a band gap can open up in a metallic SWNT with wrapped ssDNA in the presence of water molecules due to charge transfer.Comment: 13 pages, 6 figure

The neonicotinoid insecticide Imidacloprid repels pollinating flies and beetles at field-realistic concentrations

Neonicotinoids are widely used systemic insecticides which, when applied to flowering crops, are translocated to the nectar and pollen where they may impact upon pollinators. Given global concerns over pollinator declines, this potential impact has recently received much attention. Field exposure of pollinators to neonicotinoids depends on the concentrations present in flowering crops and the degree to which pollinators choose to feed upon them. Here we describe a simple experiment using paired yellow pan traps with or without insecticide to assess whether the commonly used neonicotinoid imidacloprid repels or attracts flying insects. Both Diptera and Coleoptera exhibited marked avoidance of traps containing imidacloprid at a field-realistic dose of 1 μg L-1, with Diptera avoiding concentrations as low as 0.01 μg L-1. This is to our knowledge the first evidence for any biological activity at such low concentrations, which are below the limits of laboratory detection using most commonly available techniques. Catch of spiders in pan traps was also slightly reduced by the highest concentrations of imidacloprid used (1 μg L-1), but catch was increased by lower concentrations. It remains to be seen if the repellent effect on insects occurs when neonicotinoids are present in real flowers, but if so then this could have implications for exposure of pollinators to neonicotinoids and for crop pollination. © 2013 Easton, Goulson

Dissociation of ssDNA - Single-Walled Carbon Nanotube Hybrids by Watson-Crick Base Pairing

The unwrapping event of ssDNA from the SWNT during the Watson-Crick base paring is investigated through electrical and optical methods, and binding energy calculations. While the ssDNA-metallic SWNT hybrid shows the p-type semiconducting property, the hybridization product recovered metallic properties. The gel electrophoresis directly verifies the result of wrapping and unwrapping events which was also reflected to the Raman shifts. Our molecular dynamics simulations and binding energy calculations provide atomistic description for the pathway to this phenomenon. This nano-physical phenomenon will open up a new approach for nano-bio sensing of specific sequences with the advantages of efficient particle-based recognition, no labeling, and direct electrical detection which can be easily realized into a microfluidic chip format.Comment: 4 pages, 4 figure

Subharmonic gap structure in d-wave superconductors

We present a self-consistent theory of current-voltage characteristics of d-wave/d-wave contacts at arbitrary transparency. In particular, we address the open problem of the observation of subharmonic gap structure (SGS) in cuprate junctions. Our analysis shows that: (i) the SGS is possible in d-wave superconductors, (ii) the existence of bound states within the gap results in an even-odd effect in the SGS, (iii) elastic scattering mechanisms, like impurities or surface roughness, may suppress the SGS, and (iv) in the presence of a magnetic field the Doppler shift of the Andreev bound states leads to a very peculiar splitting of the SGS, which is an unambiguous fingerprint of d-wave superconductivity.Comment: Revtex4, 4 pages, 5 figure

Electronic Devices Based on Purified Carbon Nanotubes Grown By High Pressure Decomposition of Carbon Monoxide

The excellent properties of transistors, wires, and sensors made from single-walled carbon nanotubes (SWNTs) make them promising candidates for use in advanced nanoelectronic systems. Gas-phase growth procedures such as the high pressure decomposition of carbon monoxide (HiPCO) method yield large quantities of small diameter semiconducting SWNTs, which are ideal for use in nanoelectronic circuits. As-grown HiPCO material, however, commonly contains a large fraction of carbonaceous impurities that degrade properties of SWNT devices. Here we demonstrate a purification, deposition, and fabrication process that yields devices consisting of metallic and semiconducting nanotubes with electronic characteristics vastly superior to those of circuits made from raw HiPCO. Source-drain current measurements on the circuits as a function of temperature and backgate voltage are used to quantify the energy gap of semiconducting nanotubes in a field effect transistor geometry. This work demonstrates significant progress towards the goal of producing complex integrated circuits from bulk-grown SWNT material.Comment: 6 pages, 4 figures, to appear in Nature Material

Tomography of pairing symmetry from magnetotunneling spectroscopy -- a case study for quasi-1D organic superconductors

We propose that anisotropic $p$-, $d$-, or $f$-wave pairing symmetries can be distinguished from a tunneling spectroscopy in the presence of magnetic fields, which is exemplified here for a model organic superconductor ${(TMTSF)}_{2}X$. The shape of the Fermi surface (quasi-one-dimensional in this example) affects sensitively the pairing symmetry, which in turn affects the shape (U or V) of the gap along with the presence/absence of the zero-bias peak in the tunneling in a subtle manner. Yet, an application of a magnetic field enables us to identify the symmetry, which is interpreted as an effect of the Doppler shift in Andreev bound states.Comment: 4 papegs, 4 figure

Evidence of a d to s-wave pairing symmetry transition in the electron-doped cuprate superconductor Pr2−x_{2-x}Cex_xCuO4_4

We present point contact spectroscopy (PCS) data for junctions between a normal metal and the electron doped cuprate superconductor Pr$_{2-x}$Ce$_x$CuO$_4$ (PCCO). For the underdoped compositions of this cuprate ($x \sim 0.13$) we observe a peak in the conductance-voltage characteristics of the point contact junctions. The shape and magnitude of this peak suggests the presence of Andreev bound states at the surface of underdoped PCCO which is evidence for a d-wave pairing symmetry. For overdoped PCCO ($x \sim 0.17$) the PCS data does not show any evidence of Andreev bound states at the surface suggesting an s-wave pairing symmetry.Comment: 4 pages Latex, 4 eps figures included. Submitted to Phys. Rev. Let