23 research outputs found
Density of states and magnetoconductance of disordered Au point contacts
We report the first low temperature magnetotransport measurements on
electrochemically fabricated atomic scale gold nanojunctions. As , the
junctions exhibit nonperturbatively large zero bias anomalies (ZBAs) in their
differential conductance. We consider several explanations and find that the
ZBAs are consistent with a reduced local density of states (LDOS) in the
disordered metal. We suggest that this is a result of Coulomb interactions in a
granular metal with moderate intergrain coupling. Magnetoconductance of atomic
scale junctions also differs significantly from that of less geometrically
constrained devices, and supports this explanation.Comment: 5 pages, 5 figures. Accepted to PRB as Brief Repor
Shot-noise suppression by Fermi and Coulomb correlations in ballistic conductors
We investigate the injection of degenerate Fermi-Dirac electrons into a
multimode ballistic conductor under the space-charge limited regime. The
nonequilibrium current fluctuations were found to be suppressed by both Coulomb
and Fermi correlations. We show that the Fermi shot-noise suppression factor is
limited below by the value 2kT/epsilon_F, where T is the temperature and
epsilon_F the Fermi energy of the injected electrons. The Coulomb noise
suppression factor may attain much lower values epsilon_F/2qU, because of its
dependence on the applied bias U >> kT/q. The asymptotic behaviour of the
overall shot-noise suppression factor in a high degenerate limit was found to
be kT/qU, independently of the material parameters.Comment: 8 pages, 4 figures, minor changes, published versio
A Current Induced Transition in atomic-sized contacts of metallic Alloys
We have measured conductance histograms of atomic point contacts made from
the noble-transition metal alloys CuNi, AgPd, and AuPt for a concentration
ratio of 1:1. For all alloys these histograms at low bias voltage (below 300
mV) resemble those of the noble metals whereas at high bias (above 300 mV) they
resemble those of the transition metals. We interpret this effect as a change
in the composition of the point contact with bias voltage. We discuss possible
explanations in terms of electromigration and differential diffusion induced by
current heating.Comment: 5 pages, 6 figure
EMC testing of electricity meters using real-world and artificial current waveforms
In 2015, the energy measurement of some static electricity meters was found to be sensitive to specific conducted electromagnetic disturbances with very fast current changes caused by highly nonlinear loads, leading to meter errors up to several hundred percent. This article describes new results on the electromagnetic compatibility (EMC) of 16 different meters from all over Europe when exposed to real-world disturbance signals. Those test signals were obtained from household appliances and onsite measurements at metered supply points all over Europe. The results show that also the interference signals recorded onsite can cause measurement errors as large as several hundred percent, even for meters that pass the present EMC standards. This unambiguously demonstrates that the present immunity testing standards do not cover the most disturbing conducted interference occurring in present daily-life situations due to the increased use of nonlinear electronics. Furthermore, to enable the adoption of potential new test waveforms in future standards for electricity meter testing, artificial test waveforms were constructed based on real-world waveforms using a piece-wise linear model. These artificial test waveforms were demonstrated to cause meter errors similar to those caused by the original real-life waveforms they are representing, showing that they are suitable candidates for use in improved standardization of electricity meter testing.Postprint (published version
Quantum suppression of shot noise in field emitters
We have analyzed the shot noise of electron emission under strong applied
electric fields within the Landauer-Buttiker scheme. In contrast to the
previous studies of vacuum-tube emitters, we show that in new generation
electron emitters, scaled down to the nanometer dimensions, shot noise much
smaller than the Schottky noise is observable. Carbon nanotube field emitters
are among possible candidates to observe the effect of shot-noise suppression
caused by quantum partitioning.Comment: 5 pages, 1 fig, minor changes, published versio
Local densities, distribution functions, and wave function correlations for spatially resolved shot noise at nanocontacts
We consider a current-carrying, phase-coherent multi-probe conductor to which
a small tunneling contact is attached. We treat the conductor and the tunneling
contact as a phase-coherent entity and use a Green's function formulation of
the scattering approach. We show that the average current and the current
fluctuations at the tunneling contact are determined by an effective local
non-equilibrium distribution function. This function characterizes the
distribution of charge-carriers (or quasi-particles) inside the conductor. It
is an exact quantum-mechanical expression and contains the phase-coherence of
the particles via local partial densities of states, called injectivities. The
distribution function is analyzed for different systems in the zero-temperature
limit as well as at finite temperature. Furthermore, we investigate in detail
the correlations of the currents measured at two different contacts of a
four-probe sample, where two of the probes are only weakly coupled contacts. In
particular, we show that the correlations of the currents are at
zero-temperature given by spatially non-diagonal injectivities and
emissivities. These non-diagonal densities are sensitive to correlations of
wave functions and the phase of the wave functions. We consider ballistic
conductors and metallic diffusive conductors. We also analyze the Aharonov-Bohm
oscillations in the shot noise correlations of a conductor which in the absence
of the nano-contacts exhibits no flux-sensitivity in the conductance.Comment: 17 pages, 8 figure
Density functional method for nonequilibrium electron transport
We describe an ab initio method for calculating the electronic structure,
electronic transport, and forces acting on the atoms, for atomic scale systems
connected to semi-infinite electrodes and with an applied voltage bias. Our
method is based on the density functional theory (DFT) as implemented in the
well tested Siesta approach (which uses non-local norm-conserving
pseudopotentials to describe the effect of the core electrons, and linear
combination of finite-range numerical atomic orbitals to describe the valence
states). We fully deal with the atomistic structure of the whole system,
treating both the contact and the electrodes on the same footing. The effect of
the finite bias (including selfconsistency and the solution of the
electrostatic problem) is taken into account using nonequilibrium Green's
functions. We relate the nonequilibrium Green's function expressions to the
more transparent scheme involving the scattering states. As an illustration,
the method is applied to three systems where we are able to compare our results
to earlier ab initio DFT calculations or experiments, and we point out
differences between this method and existing schemes. The systems considered
are: (1) single atom carbon wires connected to aluminum electrodes with
extended or finite cross section, (2) single atom gold wires, and finally (3)
large carbon nanotube systems with point defects.Comment: 18 pages, 23 figure
Three-phase state estimation in the medium-voltage network with aggregated smart meter data
In distribution networks, the lack of measurement data is usually thought to be an inevitable bottleneck of conventional grid operation and planning. Recently, the availability of smart meters in the distribution network has provided an opportunity to improve the network observability. In medium-voltage (MV) distribution networks, there is an increasing demand to use aggregated smart meter data for the state estimation, instead of adopting pseudo-measurements with a low level of accuracy. However, the performance of an estimator requires good knowledge of the available measurements, in terms of both expected values and associated uncertainties. Therefore, this paper intends to firstly pave a new way of utilizing smart meter data gathered from the low-voltage (LV) feeders in a concrete and reliable manner. For the purpose of state estimation in MV distribution networks, smart meter data is to be processed through three steps: phase identification, data aggregation and uncertainty evaluation. The feasibility of the proposed method is verified on the IEEE European LV Test Feeder with a set of real-world smart meter data. Afterwards, the influence of the aggregated smart meter data on the three-phase state estimation are investigated on the modified IEEE 13-node test system and IEEE 34-node test system. Simulation results show that the effect of aggregated smart meter data on the accuracy of state estimators is dependent on both the accuracy level of the aggregated data and the measurement configuration in the network. Furthermore, the use of aggregated smart meter data is shown to be able to provide improved state estimation