9,421 research outputs found
Coulomb blockade in two island systems with highly conductive junctions
We report measurements on single-electron pumps, consisting of two metallic
islands formed by three tunnel junctions in series. We focus on the
linear-response conductance as a function of gate voltage and temperature of
three samples with varying system parameters. In all cases, strong quantum
fluctuation phenomena are observed by a log(k_B T/(2 E_co)) reduction of the
maximal conductance, where E_co measures the coupling strength between the
islands. The samples display a rich phenomenology, culminating in a
non-monotonic behavior of the maximal conductance as a function of temperature
Tunable reflection minima of nanostructured antireflective surfaces
Broadband antireflection schemes for silicon surfaces based on the moth-eye principle and comprising arrays of subwavelength-scale pillars are applicable to solar cells, photodetectors, and stealth technologies and can exhibit very low reflectances. We show that rigorous coupled wave analysis can be used to accurately model the intricate reflectance behavior of these surfaces and so can be used to explore the effects of variations in pillar height, period, and shape. Low reflectance regions are identified, the extent of which are determined by the shape of the pillars. The wavelengths over which these low reflectance regions operate can be shifted by altering the period of the array. Thus the subtle features of the reflectance spectrum of a moth-eye array can be tailored for optimum performance for the input spectrum of a specific application
Effect of combined addition of graphene oxide and citric acid on superconducting properties of MgB₂Effect of combined addition of graphene oxide and citric acid on superconducting properties of MgB₂
In the present work, polycrystalline samples with compositions MgB₂ + 3wt% GO + x wt% C₆H₈O₇ (x = 0, 5 and 10) have been synthesized to study the effect of combined addition of graphene oxide (GO) and citric acid (C₆H₈O₇) on superconducting properties of MgB₂. X-ray diffraction studies show the formation of hexagonal crystal structure of MgB₂ with space group P6/mmm in all synthesized samples. We observe that the addition of GO in the sample improves the grain connectivity and consequently enhances the critical current density significantly with no substantial change in Tc. However for this sample, there is no significant improvement in Hc2 and Hirr. With the combined addition of GO and citric acid, the JC(H), Hc2 and Hirr are observed to improve substantially as compared to the pristine MgB₂ and GO added MgB₂ samples. For example JC(10 K, 5 T) of sample x = 10 has improved by a factor of ∼15 as compared to pure MgB₂ sample and by a factor of ∼5.5 as compared to the x = 0 sample. Furthermore, Hc2(0) for x = 10 sample has enhanced by 13 T as compared to pure MgB₂ while it has increased by ∼10 T in comparison to x = 0 sample. Enhanced flux pinning has been observed with the combined addition of GO and citric acid
Assessing runoff generation in riparian wetlands : monitoring groundwater-surface water dynamics at the micro-catchment scale
Acknowledgements: We would like to thank the European Research Council (ERC, project GA 335910 VeWa) for funding.Peer reviewedPostprin
Coagulation by Random Velocity Fields as a Kramers Problem
We analyse the motion of a system of particles suspended in a fluid which has
a random velocity field. There are coagulating and non-coagulating phases. We
show that the phase transition is related to a Kramers problem, and use this to
determine the phase diagram, as a function of the dimensionless inertia of the
particles, epsilon, and a measure of the relative intensities of potential and
solenoidal components of the velocity field, Gamma. We find that the phase line
is described by a function which is non-analytic at epsilon=0, and which is
related to escape over a barrier in the Kramers problem. We discuss the
physical realisations of this phase transition.Comment: 4 pages, 3 figure
Charge ordering and magneto-polarons in NaCoO
Using spectral ellipsometry, we have measured the dielectric function of a
NaCoO crystal that exhibits bulk antiferromagnetism with
T=19.8 K. We identify two prominent transitions as a function of
temperature. The first one at 280 K involves marked changes of the electronic
and the lattice response that are indicative of charge ordering in the
CoO layers. The second transition coincides with T=19.8 K and
reveals a sizeable spin-charge coupling. The data are discussed in terms of
charge ordering and formation of magneto-polarons due to a charge-induced
spin-state transition of adjacent Co ions
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