73 research outputs found
NbSi nanowire quantum phase-slip circuits: dc supercurrent blockade, microwave measurements, and thermal analysis
We present a detailed report of microwave irradiation of ultranarrow superconducting nanowires. In our nanofabricated circuits containing a superconducting NbSi nanowire, a dc blockade of current flow was observed at low temperatures below a critical voltage Vc, a strong indicator of the existence of quantum phase-slip (QPS) in the nanowire. We describe the results of applying microwaves to these samples, using a range of frequencies and both continuous-wave and pulsed drive, in order to search for dual Shapiro steps which would constitute an unambiguous demonstration of quantum phase-slip. We observed no steps, and our subsequent thermal analysis suggests that the electron temperature in the series CrO resistors was significantly elevated above the substrate temperature, resulting in sufficient Johnson noise to wash out the steps. To understand the system and inform future work, we have constructed a numerical model of the dynamics of the circuit for dc and ac bias (both continuous-wave and pulsed drive signals) in the presence of Johnson noise. Using this model, we outline important design considerations for device and measurement parameters which should be used in any future experiment to enable the observation of dual Shapiro steps at experimentally accessible temperatures and, thus, lead to the development of a QPS-based quantum current standard
Thermal-Error Regime in High-Accuracy Gigahertz Single-Electron Pumping
Single-electron pumps based on semiconductor quantum dots are promising candidates for the emerging quantum standard of electrical current. They can transfer discrete charges with part-per-million (ppm) precision in nanosecond time scales. Here, we employ a metal-oxide-semiconductor silicon quantum dot to experimentally demonstrate high-accuracy gigahertz single-electron pumping in the regime where the number of electrons trapped in the dot is determined by the thermal distribution in the reservoir leads. In a measurement with traceability to primary voltage and resistance standards, the averaged pump current over the quantized plateau, driven by a 1-GHz sinusoidal wave in the absence of a magnetic field, is equal to the ideal value of ef within a measurement uncertainty as low as 0.27 ppm
A highly efficient method for the production and purification of recombinant human CXCL8
Chemokines play diverse and fundamental roles in the immune system and human disease, which has prompted their structural and functional characterisation. Production of recombinant chemokines that are folded and bioactive is vital to their study but is limited by the stringent requirements of a native N-terminus for receptor activation and correct disulphide bonding required to stabilise the chemokine fold. Even when expressed as fusion proteins, overexpression of chemokines in E. coli tends to result in the formation of inclusion bodies, generating the additional steps of solubilisation and refolding. Here we present a novel method for producing soluble chemokines in relatively large amounts via a simple two-step purification procedure with no requirements for refolding. CXCL8 produced by this method has the correct chemokine fold as determined by NMR spectroscopy and in chemotaxis assays was indistinguishable from commercially available chemokines. We believe that this protocol significantly streamlines the generation of recombinant chemokines
The protease associated (PA) domain in ScpA from Streptococcus pyogenes plays a role in substrate recruitment
Annually, over 18 million disease cases and half a million deaths worldwide are estimated to be caused by Group A Streptococcus. ScpA (or C5a peptidase) is a well characterised member of the cell enveleope protease family, which possess a S8 subtilisin-like catalytic domain and a shared multi-domain architecture. ScpA cleaves complement factors C5a and C3a, impairing the function of these critical anaphylatoxins and disrupts complement-mediated innate immunity. Although the high resolution structure of ScpA is known, the details of how it recognises its substrate are only just emerging. Previous studies have identified a distant exosite on the 2nd fibronectin domain that plays an important role in recruitment via an interaction with the substrate core. Here, using a combination of solution NMR spectroscopy, mutagenesis with functional assays and computational approaches we identify a second exosite within the protease-associated (PA) domain. We propose a model in which the PA domain assists optimal delivery of the substrate's C terminus to the active site for cleavage
Partitioning of on-demand electron pairs
We demonstrate the high fidelity splitting of electron pairs emitted on
demand from a dynamic quantum dot by an electronic beam splitter. The fidelity
of pair splitting is inferred from the coincidence of arrival in two detector
paths probed by a measurement of the partitioning noise. The emission
characteristic of the on-demand electron source is tunable from electrons being
partitioned equally and independently to electron pairs being split with a
fidelity of 90%. For low beam splitter transmittance we further find evidence
of pair bunching violating statistical expectations for independent fermions
Single-Hot-Electron Wave Packets for Quantum Electrical Metrology
Using a recently-developed time-of-flight measurement technique with 1 ps time resolution and
electron-energy spectroscopy, we developed a method to measure the longitudinal-optical-phonon
emission rate of hot electrons travelling along a depleted edge of a quantum Hall bar. A comparison
of the experimental results to a single-particle model implies that the main scattering mechanism
involves a two-step process via intra-Landau-level transition. We show this scattering can be suppressed by controlling the edge potential profile, and a scattering length > 1 mm can be achieved,
allowing the use of this system for scalable single-electron device applications
Towards a quantum representation of the ampere using single electron pumps
Electron pumps generate a macroscopic electric current by controlled
manipulation of single electrons. Despite intensive research towards a quantum
current standard over the last 25 years, making a fast and accurate quantised
electron pump has proved extremely difficult. Here we demonstrate that the
accuracy of a semiconductor quantum dot pump can be dramatically improved by
using specially designed gate drive waveforms. Our pump can generate a current
of up to 150 pA, corresponding to almost a billion electrons per second, with
an experimentally demonstrated current accuracy better than 1.2 parts per
million (ppm) and strong evidence, based on fitting data to a model, that the
true accuracy is approaching 0.01 ppm. This type of pump is a promising
candidate for further development as a realisation of the SI base unit ampere,
following a re-definition of the ampere in terms of a fixed value of the
elementary charge.Comment: 8 pages, 7 figure
Gigahertz quantized charge pumping in graphene quantum dots
Single electron pumps are set to revolutionize electrical metrology by
enabling the ampere to be re-defined in terms of the elementary charge of an
electron. Pumps based on lithographically-fixed tunnel barriers in mesoscopic
metallic systems and normal/superconducting hybrid turnstiles can reach very
small error rates, but only at MHz pumping speeds corresponding to small
currents of the order 1 pA. Tunable barrier pumps in semiconductor structures
have been operated at GHz frequencies, but the theoretical treatment of the
error rate is more complex and only approximate predictions are available.
Here, we present a monolithic, fixed barrier single electron pump made entirely
from graphene. We demonstrate pump operation at frequencies up to 1.4 GHz, and
predict the error rate to be as low as 0.01 parts per million at 90 MHz.
Combined with the record-high accuracy of the quantum Hall effect and proximity
induced Josephson junctions, accurate quantized current generation brings an
all-graphene closure of the quantum metrological triangle within reach.
Envisaged applications for graphene charge pumps outside quantum metrology
include single photon generation via electron-hole recombination in
electrostatically doped bilayer graphene reservoirs, and for readout of
spin-based graphene qubits in quantum information processing.Comment: 13 pages, 11 figures, includes supplementary informatio
The regional and global significance of nitrogen removal in lakes and reservoirs
Author Posting. © The Author(s), 2008. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Biogeochemistry 93 (2009): 143-157, doi:10.1007/s10533-008-9272-x.Human activities have greatly increased the transport of biologically available N through
watersheds to potentially sensitive coastal ecosystems. Lentic water bodies (lakes and
reservoirs) have the potential to act as important sinks for this reactive N as it is
transported across the landscape because they offer ideal conditions for N burial in
sediments or permanent loss via denitrification. However, the patterns and controls on
lentic N removal have not been explored in great detail at large regional to global scales.
In this paper we describe, evaluate, and apply a new, spatially explicit, annual-scale,
global model of lentic N removal called NiRReLa (Nitrogen Retention in Reservoirs and
Lakes). The NiRReLa model incorporates small lakes and reservoirs than have been
included in previous global analyses, and also allows for separate treatment and analysis
of reservoirs and natural lakes. Model runs for the mid-1990s indicate that lentic systems
are indeed important sinks for N and are conservatively estimated to remove 19.7 Tg N
yr-1 from watersheds globally. Small lakes (< 50 km2) were critical in the analysis,
retaining almost half (9.3 Tg N yr-1) of the global total. In model runs, capacity of lakes
and reservoirs to remove watershed N varied substantially (0-100%) both as a function of
climate and the density of lentic systems. Although reservoirs occupy just 6% of the
global lentic surface area, we estimate they retain approximately 33% of the total N
removed by lentic systems, due to a combination of higher drainage ratios (catchment
surface area : lake or reservoir surface area), higher apparent settling velocities for N, and
greater N loading rates in reservoirs than in lakes. Finally, a sensitivity analysis of
NiRReLa suggests that, on-average, N removal within lentic systems will respond more
strongly to changes in land use and N loading than to changes in climate at the global
scale.The NSF26 Research Coordination Network on denitrification for support for collaboration
(award number DEB0443439 to S.P. Seitzinger and E.A. Davidson). This project was
also supported by grants to J.A. Harrison from California Sea Grant (award number
RSF8) and from the U.S. Geological Survey 104b program and R. Maranger (FQRNT
Strategic Professor)
Wave Vector Difference of Magnetic Bragg Reflections and Low Energy Magnetic Excitations in Charge-stripe Ordered La2NiO4.11
We report on the magnetism of charge-stripe ordered La2NiO4.11±0.01 by neutron scattering and μSR. On going towards zero energy transfer there is an observed wave vector offset in the centring of the magnetic excitations and magnetic Bragg reflections, meaning the excitations cannot be described as Goldstone modes of the magnetic order. Weak transverse field μSR measurements determine the magnetically order volume fraction is 87% from the two stripe twins, and the temperature evolution of the magnetic excitations is consistent with the low energy excitations coming from the magnetically ordered volume of the material. We will discuss how these results contrast with the proposed origin of a similar wave vector offset recently observed in a La-based cuprate, and possible origins of this effect in La2NiO4.11
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