1,991 research outputs found
Where Does the Density Localize? Convergent Behavior for Global Hybrids, Range Separation, and DFT+U
Approximate density functional theory (DFT) suffers from many-electron self-
interaction error, otherwise known as delocalization error, that may be
diagnosed and then corrected through elimination of the deviation from exact
piecewise linear behavior between integer electron numbers. Although paths to
correction of energetic delocalization error are well- established, the impact
of these corrections on the electron density is less well-studied. Here, we
compare the effect on density delocalization of DFT+U, global hybrid tuning,
and range- separated hybrid tuning on a diverse test set of 32 transition metal
complexes and observe the three methods to have qualitatively equivalent
effects on the ground state density. Regardless of valence orbital diffuseness
(i.e., from 2p to 5p), ligand electronegativity (i.e., from Al to O), basis set
(i.e., plane wave versus localized basis set), metal (i.e., Ti, Fe, Ni) and
spin state, or tuning method, we consistently observe substantial charge loss
at the metal and gain at ligand atoms (ca. 0.3-0.5 e or more). This charge loss
at the metal is preferentially from the minority spin, leading to increasing
magnetic moment as well. Using accurate wavefunction theory references, we
observe that a minimum error in partial charges and magnetic moments occur at
higher tuning parameters than typically employed to eliminate energetic
delocalization error. These observations motivate the need to develop
multi-faceted approximate-DFT error correction approaches that separately treat
density delocalization and energetic errors in order to recover both correct
density and magnetization properties.Comment: 34 pages, 11 figure
Non-adiabatic Josephson Dynamics in Junctions with in-Gap Quasiparticles
Conventional models of Josephson junction dynamics rely on the absence of low
energy quasiparticle states due to a large superconducting gap. With this
assumption the quasiparticle degrees of freedom become "frozen out" and the
phase difference becomes the only free variable, acting as a fictitious
particle in a local in time Josephson potential related to the adiabatic and
non-dissipative supercurrent across the junction. In this article we develop a
general framework to incorporate the effects of low energy quasiparticles
interacting non-adiabatically with the phase degree of freedom. Such
quasiparticle states exist generically in constriction type junctions with high
transparency channels or resonant states, as well as in junctions of
unconventional superconductors. Furthermore, recent experiments have revealed
the existence of spurious low energy in-gap states in tunnel junctions of
conventional superconductors - a system for which the adiabatic assumption
typically is assumed to hold. We show that the resonant interaction with such
low energy states rather than the Josephson potential defines nonlinear
Josephson dynamics at small amplitudes.Comment: 9 pages, 1 figur
Josephson effect in graphene SBS junctions
We study Josephson effect in graphene superconductor- barrier- superconductor
junctions with short and wide barriers of thickness and width , which
can be created by applying a gate voltage across the barrier region. We
show that Josephson current in such graphene junctions, in complete contrast to
their conventional counterparts, is an oscillatory function of both the barrier
width and the applied gate voltage . We also demonstrate that in the
thin barrier limit, where and keeping
finite, such an oscillatory behavior can be understood in terms of transmission
resonance of Dirac-Bogoliubov-de Gennes quasiparticles in superconducting
graphene. We discuss experimental relevance of our work.Comment: 7 Pg., 6 Figs, extended version submitted to PR
Transport Processes in Metal-Insulator Granular Layers
Tunnel transport processes are considered in a square lattice of metallic
nanogranules embedded into insulating host to model tunnel conduction in real
metal/insulator granular layers. Based on a simple model with three possible
charging states (, or 0) of a granule and three kinetic processes
(creation or recombination of a pair, and charge transfer) between
neighbor granules, the mean-field kinetic theory is developed. It describes the
interplay between charging energy and temperature and between the applied
electric field and the Coulomb fields by the non-compensated charge density.
The resulting charge and current distributions are found to be essentially
different in the free area (FA), between the metallic contacts, or in the
contact areas (CA), beneath those contacts. Thus, the steady state dc transport
is only compatible with zero charge density and ohmic resistivity in FA, but
charge accumulation and non-ohmic behavior are \emph{necessary} for conduction
over CA. The approximate analytic solutions are obtained for characteristic
regimes (low or high charge density) of such conduction. The comparison is done
with the measurement data on tunnel transport in related experimental systems.Comment: 10 pages, 11 figures, 1 reference corrected, acknowlegments adde
Josephson Junctions defined by a Nano-Plough
We define superconducting constrictions by ploughing a deposited Aluminum
film with a scanning probe microscope. The microscope tip is modified by
electron beam deposition to form a nano-plough of diamond-like hardness, what
allows the definition of highly transparent Josephson junctions. Additionally a
dc-SQUID is fabricated to verify appropriate functioning of the junctions. The
devices are easily integrated in mesoscopic devices as local radiation sources
and can be used as tunable on-chip millimeter wave sources
Turbulence in the mixing region between ducted coaxial streams
Turbulence in mixing region of ducted coaxial flow of air-air and air-Freon 12 combinatio
Proximity DC squids in the long junction limit
We report the design and measurement of
Superconducting/normal/superconducting (SNS) proximity DC squids in the long
junction limit, i.e. superconducting loops interrupted by two normal metal
wires roughly a micrometer long. Thanks to the clean interface between the
metals, at low temperature a large supercurrent flows through the device. The
dc squid-like geometry leads to an almost complete periodic modulation of the
critical current through the device by a magnetic flux, with a flux periodicity
of a flux quantum h/2e through the SNS loop. In addition, we examine the entire
field dependence, notably the low and high field dependence of the maximum
switching current. In contrast with the well-known Fraunhoffer-type
oscillations typical of short wide junctions, we find a monotonous gaussian
extinction of the critical current at high field. As shown in [15], this
monotonous dependence is typical of long and narrow diffusive junctions. We
also find in some cases a puzzling reentrance at low field. In contrast, the
temperature dependence of the critical current is well described by the
proximity effect theory, as found by Dubos {\it et al.} [16] on SNS wires in
the long junction limit. The switching current distributions and hysteretic IV
curves also suggest interesting dynamics of long SNS junctions with an
important role played by the diffusion time across the junction.Comment: 12 pages, 16 figure
Characterization of a Differential Radio-Frequency Single-Electron Transistor
We have fabricated and characterized a new type of electrometer that couples
two parallel single-electron transistors (SETs) to a radio-frequency tank
circuit for use as a differential RF-SET. We demonstrate operation of this
device in summing, differential, and single-SET operation modes, and use it to
measure a Coulomb staircase from a differential single Cooper-pair box. In
differential mode, the device is sensitive to uncorrelated input signals while
screening out correlated ones.Comment: 3 pages, 3 figures, submitted to Applied Physics Letter
Experimental study of the correlation length of critical-current fluctuations in the presence of surface disorder: Probing vortex long-range interactions
We report on critical currents and voltage noise measurements in Niobium
strips in the superconducting state, in the presence of a bulk vortex lattice
() and in the surface superconducting state ().
For homogeneous surfaces, the correlation length of the current fluctuations
can be associated with the electromagnetic skin depth of vortex superficial
instabilities. The modification of the surface state by means of low energy
irradiation induces a strong modification of the critical current and of the
noise. The appearance of a corner frequency in the spectral domain can be
linked with the low wave-vectors of the artificial corrugation. Since this
latter occurs only for , we propose that the long-range
interactions allow the correlation length to extend up to values imposed by the
surface topography.Comment: accepted for publication in PR
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