32 research outputs found
Toward bio-inspired information processing with networks of nano-scale switching elements
Unconventional computing explores multi-scale platforms connecting
molecular-scale devices into networks for the development of scalable
neuromorphic architectures, often based on new materials and components with
new functionalities. We review some work investigating the functionalities of
locally connected networks of different types of switching elements as
computational substrates. In particular, we discuss reservoir computing with
networks of nonlinear nanoscale components. In usual neuromorphic paradigms,
the network synaptic weights are adjusted as a result of a training/learning
process. In reservoir computing, the non-linear network acts as a dynamical
system mixing and spreading the input signals over a large state space, and
only a readout layer is trained. We illustrate the most important concepts with
a few examples, featuring memristor networks with time-dependent and history
dependent resistances
Interference effects in phtalocyanine controlled by H-H tautomerization: a potential two-terminal unimolecular electronic switch
We investigate the electrical transport properties of two hydrogen tautomer
configurations of phthalocyanine (H2Pc) connected to cumulene and gold leads.
Hydrogen tautomerization affects the electronic state of H2Pc by switching the
character of molecular orbitals with the same symmetry close to the Fermi
level. The near degeneracy between the HOMO and HOMO-1 leads to pronounced
interference effects, causing a large change in current for the two tautomer
configuratons, especially in the low-bias regime. Two types of planar junctions
are considered: cumulene-H2Pc-cumulene and gold-H2Pc-gold. Both demonstrate
prominent difference in molecular conductance between ON and OFF states. In
addition, junctions with gold leads show pronounced negative differential
resistance (NDR) at high bias voltage, as well as weak NDR at intermediate
bias.Comment: 10 pages, 7 figures, accepted for publication in Physical Review
Controlling Josephson transport by manipulation of Andreev levels in ballistic mesoscopic junctions
We discuss how to control dc Josephson current by influencing the structure
and nonequilibrium population of Andreev levels via external electrostatic
gates, external current injection and electromagnetic radiation. In particular
we will consider the "giant" Josephson current in "long" SIS tunnel junctions
and the regular and anomalous nonequilibrium Josephson currents in three
terminal SNS junctions. We will briefly discuss applications to the Josephson
field effect transistor (JOFET) and to the newly invented Josephson
interference transistor (JOINT).Comment: 10 pages, 3 figures; contribution to a special volume of
Superlattices and Microstructures journal (ed. P.F. Bagwell
Multiple Andreev reflections as a transport problem in energy space
We present an approach for analyzing the dc current in voltage biased quantum
superconducting junctions. By separating terms from different -particle
processes, we find that the -particle current can be mapped on the problem
of wave transport through a potential structure with barriers. We discuss
the relation between resonances in such structures and the subgap structures in
the current-voltage characteristics. At zero temperature we find, exactly, that
only processes creating real excitations contribute to the current. Our results
are valid for a general SXS-junction, where the X-region is an arbitrary
non-superconducting region described by an energy-dependent transfer matrix.Comment: 11 pages, 4 figures, submitted to Superlattices and Microstructure
Resonant transport through midgap states in voltage-biased Josephson junctions of d-wave superconductors
We study theoretically the ac Josephson effect in voltage biased planar
junctions of d-wave superconductors. For some orientations of the
superconductors a current peak is found at finite voltage in the
current-voltage characteristics. We pick out the relevant physical processes
and write down an analytical formula for the current which clearly shows how
the midgap state acts as a resonance and produces the peak. We present a
possible explanation for the zero-bias conductance peak, recently found in
experiments on grain boundary junctions of high-temperature superconductors, in
terms of resonant transmission through midgap state of quasiparticles
undergoing multiple Andreev reflections. We note that within our framework the
zero-bias conductance peak appears in rather transparent Josephson junctions of
d-wave superconductors.Comment: 10 pages, 5 figures, Submitted to a special volume of "Superlattices
and Microstructures
Full Frequency Back-Action Spectrum of a Single Electron Transistor during Qubit read-out
We calculate the spectral density of voltage fluctuations in a Single
Electron Transistor (SET), biased to operate in a transport mode where
tunneling events are correlated due to Coulomb interaction. The whole spectrum
from low frequency shot noise to quantum noise at frequencies comparable to the
SET charging energy is considered. We discuss the back-action
during read-out of a charge qubit and conclude that single-shot read-out is
possible using the Radio-Frequency SET.Comment: 4 pages, 5 figures, submitted to PR
Attosecond electron-spin dynamics in Xe 4d photoionization
The photoionization of xenon atoms in the 70-100 eV range reveals several
fascinating physical phenomena such as a giant resonance induced by the dynamic
rearrangement of the electron cloud after photon absorption, an anomalous
branching ratio between intermediate Xe states separated by the spin-orbit
interaction and multiple Auger decay processes. These phenomena have been
studied in the past, using in particular synchrotron radiation, but without
access to real-time dynamics. Here, we study the dynamics of Xe 4d
photoionization on its natural time scale combining attosecond interferometry
and coincidence spectroscopy. A time-frequency analysis of the involved
transitions allows us to identify two interfering ionization mechanisms: the
broad giant dipole resonance with a fast decay time less than 50 as and a
narrow resonance at threshold induced by spin-flip transitions, with much
longer decay times of several hundred as. Our results provide new insight into
the complex electron-spin dynamics of photo-induced phenomena
Superconducting d-wave junctions: The disappearance of the odd ac components
We study voltage-biased superconducting planar d-wave junctions for arbitrary
transmission and arbitrary orientation of the order parameters of the
superconductors. For a certain orientation of the superconductors the odd ac
components disappear, resulting in a doubling of the Josephson frequency. We
study the sensitivity of this disappearance to orientation and compare with
experiments on grain boundary junctions. We also discuss the possibility of a
current flow parallel to the junction.Comment: 5 pages, 3 figure
Full frequency voltage noise spectral density of a single electron transistor
We calculate the full frequency spectral density of voltage fluctuations in a
Single Electron Transistor (SET), used as an electrometer biased above the
Coulomb threshold so that the current through the SET is carried by sequential
tunnel events. We consider both a normal state SET and a superconducting SET.
The whole spectrum from low frequency telegraph noise to quantum noise at
frequencies comparable to the SET charging energy , and high
frequency Nyquist noise is described. We take the energy exchange between the
SET and the measured system into account using a real-time diagrammatic Keldysh
technique. The voltage fluctuations determine the back-action of the SET onto
the measured system and we specifically discuss the case of superconducting
charge qubit read-out and measuring the so-called Coulomb staircase of a single
Cooper pair box.Comment: 14 pages, 18 figures, submitted to PR