142 research outputs found
Wigner transport equation with finite coherence length
The use of the Wigner function for the study of quantum transport in open
systems present severe criticisms. Some of the problems arise from the
assumption of infinite coherence length of the electron dynamics outside the
system of interest. In the present work the theory of the Wigner function is
revised assuming a finite coherence length. A new dynamical equation is found,
corresponding to move the Wigner momentum off the real axis, and a numerical
analysis is performed for the case of study of the onedimensional potential
barrier.Comment: 14 pages, 1 figure. Revised text. Added new reference
Effect of symmetry in the many-particle Wigner funcion
An analysis of the Wigner function for identical particles is presented. Four situations have been considered. (i) The first is scattering process between two indistinguishable particles described by a minimum uncertainty wave packets showing the exchange and correlation effects in Wigner phase space. (ii) An equilibrium ensemble of N particles in a one-dimensional box and in a one-dimensional harmonic potential is considered second, showing that the reduced one-particle Wigner function, as a function of the energy defined in the Wigner phase space, tends to the Fermi-Dirac or to the Bose-Einstein distribution function, depending on the considered statistics. (iii) The third situation is reduced one-particle transport equation for the Wigner function, in the case of interacting particles, showing the need for the two-particle reduced Wigner function within the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy scheme. (iv) Finally, the electron-phonon interaction in the two-particle case is considered, showing coparticipation of two electrons in the interaction with the phonon bath
Transport scaling limits of ovonic Devices: a simulative approach
The transport scaling limits of Ovonic devices are studied by means of a numerical solution of a time- and space-dependent transport models based on a set of equations that provide a good physical grasp of the microscopic process at hand. The predictivity of the approach has been confirmed through the comparison with recent experimental results where the parasitic effects have been reduced by the use of top-technology measuring equipments. The present analysis is performed for the AgInSbTe chalcogenide, since this material exibits a steep threshold-switching dynamics which makes it promising for high-speed non-volatile memory applications
Intrinsic Electric Oscillations of Ovonic Devices towards the TeraHerz limit
The time-dependent response of Ovonic devices to an electric potential ramp signal
is analysed by means of an enhanced version of a previously published time-dependent charge-
transport model proposed by the authors. Depending on the inevitable parasitics of the system,
either stable or oscillating solutions are found according to the position of the load line. The
model also allows for speculations on the potential of Ovonic materials in the design of high-
frequency oscillating circuits close to the terahertz range
Quantum logic gates based on coherent electron transport in quantum wires
It is shown that the universal set of quantum logic gates can be realized using solid-state quantum bits based on coherent electron transport in quantum wires. The elementary quantum bits are realized with a proper design of two quantum wires coupled through a potential barrier Numerical simulations show that (a) a proper design of the coupling barrier allows one to realize any one-qbit rotation and (b) Coulomb interaction between two qbits of this kind allows the implementation of the CNOT gate. These systems are based on a mature technology and seem to be integrable with conventional electronics
Microscopic theory of quantum-transport phenomena in mesoscopic systems: A Monte Carlo approach
A theoretical investigation of quantum-transport phenomena in mesoscopic
systems is presented. In particular, a generalization to ``open systems'' of
the well-known semiconductor Bloch equations is proposed. The presence of
spatial boundary conditions manifest itself through self-energy corrections and
additional source terms in the kinetic equations, whose form is suitable for a
solution via a generalized Monte Carlo simulation. The proposed approach is
applied to the study of quantum-transport phenomena in double-barrier
structures as well as in superlattices, showing a strong interplay between
phase coherence and relaxation.Comment: to appear in Phys. Rev. Let
Nanoscale Phase Change Memory with Graphene Ribbon Electrodes
Phase change memory (PCM) devices are known to reduce in power consumption as
the bit volume and contact area of their electrodes are scaled down. Here, we
demonstrate two types of low-power PCM devices with lateral graphene ribbon
electrodes: one in which the graphene is patterned into narrow nanoribbons and
the other where the phase change material is patterned into nanoribbons. The
sharp graphene "edge" contacts enable switching with threshold voltages as low
as ~3 V, low programming currents (<1 {\mu}A SET, <10 {\mu}A RESET) and ON/OFF
ratios >100. Large-scale fabrication with graphene grown by chemical vapor
deposition also enables the study of heterogeneous integration and that of
variability for such nanomaterials and devices.Comment: submitted to Applied Physics Letter
Multi-dimensional modeling and simulation of semiconductor nanophotonic devices
Self-consistent modeling and multi-dimensional simulation of semiconductor nanophotonic devices is an important tool in the development of future integrated light sources and quantum devices. Simulations can guide important technological decisions by revealing performance bottlenecks in new device concepts, contribute to their understanding and help to theoretically explore their optimization potential. The efficient implementation of multi-dimensional numerical simulations for computer-aided design tasks requires sophisticated numerical methods and modeling techniques. We review recent advances in device-scale modeling of quantum dot based single-photon sources and laser diodes by self-consistently coupling the optical Maxwell equations with semiclassical carrier transport models using semi-classical and fully quantum mechanical descriptions of the optically active region, respectively. For the simulation of realistic devices with complex, multi-dimensional geometries, we have developed a novel hp-adaptive finite element approach for the optical Maxwell equations, using mixed meshes adapted to the multi-scale properties of the photonic structures. For electrically driven devices, we introduced novel discretization and parameter-embedding techniques to solve the drift-diffusion system for strongly degenerate semiconductors at cryogenic temperature. Our methodical advances are demonstrated on various applications, including vertical-cavity surface-emitting lasers, grating couplers and single-photon sources
Semiconductor Superlattices: A model system for nonlinear transport
Electric transport in semiconductor superlattices is dominated by pronounced
negative differential conductivity. In this report the standard transport
theories for superlattices, i.e. miniband conduction, Wannier-Stark-hopping,
and sequential tunneling, are reviewed in detail. Their relation to each other
is clarified by a comparison with a quantum transport model based on
nonequilibrium Green functions. It is demonstrated how the occurrence of
negative differential conductivity causes inhomogeneous electric field
distributions, yielding either a characteristic sawtooth shape of the
current-voltage characteristic or self-sustained current oscillations. An
additional ac-voltage in the THz range is included in the theory as well. The
results display absolute negative conductance, photon-assisted tunneling, the
possibility of gain, and a negative tunneling capacitance.Comment: 121 pages, figures included, to appear in Physics Reports (2001
- …