192 research outputs found
Classical and quantum spreading of a charge pulse
With the technical progress of radio-frequency setups, high frequency quantum
transport experiments have moved from theory to the lab. So far the standard
theoretical approach used to treat such problems numerically--known as Keldysh
or NEGF (Non Equilibrium Green's Functions) formalism--has not been very
successful mainly because of a prohibitive computational cost. We propose a
reformulation of the non-equilibrium Green's function technique in terms of the
electronic wave functions of the system in an energy-time representation. The
numerical algorithm we obtain scales now linearly with the simulated time and
the volume of the system, and makes simulation of systems with 10^5 - 10^6
atoms/sites feasible. We illustrate our method with the propagation and
spreading of a charge pulse in the quantum Hall regime. We identify a classical
and a quantum regime for the spreading, depending on the number of particles
contained in the pulse. This numerical experiment is the condensed matter
analogue to the spreading of a Gaussian wavepacket discussed in quantum
mechanics textbooks.Comment: 4 pages, 5 figures; to be published in IEEE Xplore, in Proceedings to
IEEE 17th International Workshop on Computational Electronics 2014, June 3 -
6, 2014, Paris, France. Correction of typographic mistakes and update of ref.
1
Tunable thermopower in a graphene-based topological insulator
Following the recent proposal by Weeks et al., which suggested that indium
(or thallium) adatoms deposited on the surface of graphene should turn the
latter into a quantum spin Hall (QSH) insulator characterized by a sizeable
gap, we perform a systematic study of the transport properties of this system
as a function of the density of randomly distributed adatoms. While the samples
are, by construction, very disordered, we find that they exhibit an extremely
stable QSH phase with no signature of the spatial inhomogeneities of the adatom
configuration. We find that a simple rescaling of the spin-orbit coupling
parameter allows us to account for the behaviour of the inhomogeneous system
using a homogeneous model. This robustness opens the route to a much easier
experimental realization of this topological insulator. We additionally find
this material to be a very promising candidate for thermopower generation with
a target temperature tunable from 1 to 80K and an efficiency ZT close to 1.Comment: 7 pages, 5 figure
Graphene-based heterojunction between two topological insulators
Quantum Hall (QH) and quantum spin Hall (QSH) phases have very different edge
states and, when going from one phase to the other, the direction of one edge
state must be reversed. We study this phenomena in graphene in presence of a
strong perpendicular magnetic field on top of a spin-orbit (SO) induced QSH
phase. We show that, below the SO gap, the QSH phase is virtually unaffected by
the presence of the magnetic field. Above the SO gap, the QH phase is restored.
An electrostatic gate placed on top of the system allows to create a QSH-QH
junction which is characterized by the existence of a spin-polarized chiral
state, propagating along the topological interface. We find that such a setup
naturally provides an extremely sensitive spin-polarized current switch.Comment: 10 pages, 5 figure
Numerical simulations of time resolved quantum electronics
This paper discusses the technical aspects - mathematical and numerical -
associated with the numerical simulations of a mesoscopic system in the time
domain (i.e. beyond the single frequency AC limit). After a short review of the
state of the art, we develop a theoretical framework for the calculation of
time resolved observables in a general multiterminal system subject to an
arbitrary time dependent perturbation (oscillating electrostatic gates, voltage
pulses, time-vaying magnetic fields) The approach is mathematically equivalent
to (i) the time dependent scattering formalism, (ii) the time resolved Non
Equilibrium Green Function (NEGF) formalism and (iii) the partition-free
approach. The central object of our theory is a wave function that obeys a
simple Schrodinger equation with an additional source term that accounts for
the electrons injected from the electrodes. The time resolved observables
(current, density. . .) and the (inelastic) scattering matrix are simply
expressed in term of this wave function. We use our approach to develop a
numerical technique for simulating time resolved quantum transport. We find
that the use of this wave function is advantageous for numerical simulations
resulting in a speed up of many orders of magnitude with respect to the direct
integration of NEGF equations. Our technique allows one to simulate realistic
situations beyond simple models, a subject that was until now beyond the
simulation capabilities of available approaches.Comment: Typographic mistakes in appendix C were correcte
Pushing the limit of quantum transport simulations
Simulations of quantum transport in coherent conductors have evolved into
mature techniques that are used in fields of physics ranging from electrical
engineering to quantum nanoelectronics and material science. The most efficient
general-purpose algorithms have a computational cost that scales as in 3D, which on the one hand is a substantial improvement over older
algorithms, but on the other hand still severely restricts the size of the
simulation domain, limiting the usefulness of simulations through strong
finite-size effects. Here, we present a novel class of algorithms that, for
certain systems, allows to directly access the thermodynamic limit. Our
approach, based on the Green's function formalism for discrete models, targets
systems which are mostly invariant by translation, i.e. invariant by
translation up to a finite number of orbitals and/or quasi-1D electrodes and/or
the presence of edges or surfaces. Our approach is based on an automatic
calculation of the poles and residues of series expansions of the Green's
function in momentum space. This expansion allows to integrate analytically in
one momentum variable. We illustrate our algorithms with several applications:
devices with graphene electrodes that consist of half an infinite sheet;
Friedel oscillation calculations of infinite 2D systems in presence of an
impurity; quantum spin Hall physics in presence of an edge; the surface of a
Weyl semi-metal in presence of impurities and electrodes connected to the
surface. In this last example, we study the conduction through the Fermi arcs
of the topological material and its resilience to the presence of disorder. Our
approach provides a practical route for simulating 3D bulk systems or surfaces
as well as other setups that have so far remained elusive.Comment: 20 pages, 13 figure
Designing potentials by sculpturing wires
Magnetic trapping potentials for atoms on atom chips are determined by the
current flow in the chip wires. By modifying the shape of the conductor we can
realize specialized current flow patterns and therefore micro-design the
trapping potentials. We have demonstrated this by nano-machining an atom chip
using the focused ion beam technique. We built a trap, a barrier and using a
BEC as a probe we showed that by polishing the conductor edge the potential
roughness on the selected wire can be reduced. Furthermore we give different
other designs and discuss the creation of a 1D magnetic lattice on an atom
chip.Comment: 6 pages, 8 figure
Quantum Monte Carlo for correlated out-of-equilibrium nanoelectronic devices
International audienceWe present a simple, general purpose, quantum Monte-Carlo algorithm for out-of-equilibrium interacting nanoelectronics systems. It allows one to systematically compute the expansion of any physical observable (such as current or density) in powers of the electron-electron interaction coupling constant U. It is based on the out-of-equilibrium Keldysh Green's function formalism in real-time and corresponds to evaluating all the Feynman diagrams to a given order U n (up to n = 15 in the present work). A key idea is to explicitly sum over the Keldysh indices in order to enforce the unitarity of the time evolution. The coefficients of the expansion can easily be obtained for long time, stationary regimes, even at zero temperature. We then illustrate our approach with an application to the Anderson model, an archetype interacting mesoscopic system. We recover various results of the literature such as the spin susceptibility or the " Kondo ridge " in the current-voltage characteristics. In this case, we found the Monte-Carlo free of the sign problem even at zero temperature , in the stationary regime and in absence of particle-hole symmetry. The main limitation of the method is the lack of convergence of the expansion in U for large U , i.e. a mathematical property of the model rather than a limitation of the Monte-Carlo algorithm. Standard extrapolation methods of divergent series can be used to evaluate the series in the strong correlation regime
Theoretical, numerical, and experimental study of a flying qubit electronic interferometer
We discuss an electronic interferometer recently measured by Yamamoto et al.
This "flying quantum bit" experiment showed quantum oscillations between
electronic trajectories of two tunnel-coupled wires connected via an
Aharanov-Bohm ring. We present a simple scattering model as well as a numerical
microscopic model to describe this experiment. In addition, we present new
experimental data to which we confront our numerical results. While our
analytical model provides basic concepts for designing the flying qubit device,
we find that our numerical simulations allow to reproduce detailed features of
the transport measurements such as in-phase and anti-phase oscillations of the
two output currents as well as a smooth phase shift when sweeping a side gate.
Furthermore, we find remarkable resemblance for the magneto conductance
oscillations in both conductance and visibility between simulations and
experiments within a specific parameter range.Comment: 10 pages, 13 figure
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