- title: 'Tkwant: a software package for time-dependent quantum transport'
  - Thomas Kloss
  - Joseph Weston
  - Benoit Gaury
  - Benoit Rossignol
  - Christoph Groth
  - Xavier Waintal
  abstract: "Tkwant is a Python package for the simulation of quantum nanoelectronics\n\
    devices to which external time-dependent perturbations are applied. Tkwant is\n\
    an extension of the Kwant package (https://kwant-project.org/) and can handle\n\
    the same types of systems: discrete tight-binding-like models that consist of\n\
    an arbitrary central region connected to semi-infinite electrodes. The problem\n\
    is genuinely many-body even in the absence of interactions and is treated\nwithin\
    \ the non-equilibrium Keldysh formalism. Examples of Tkwant applications\ninclude\
    \ the propagation of plasmons generated by voltage pulses, propagation of\nexcitations\
    \ in the quantum Hall regime, spectroscopy of Majorana fermions in\nsemiconducting\
    \ nanowires, current-induced skyrmion motion in spintronic\ndevices, multiple\
    \ Andreev reflection, Floquet topological insulators,\nthermoelectric effects,\
    \ and more. The code has been designed to be easy to use\nand modular. Tkwant\
    \ is free software distributed under a BSD license and can be\nfound at https://tkwant.kwant-project.org/."
  date: '2021-02-22T12:24:08Z'
  link: http://arxiv.org/abs/2009.03132v3
  ref: 2009.03132v3
  jref: New J. Phys. 23, 023025 (2021)
  jlink: http://dx.doi.org/10.1088/1367-2630/abddf7
- title: "The HANDE-QMC project: open-source stochastic quantum chemistry from the\n\
    \  ground state up"
  - James S. Spencer
  - Nick S. Blunt
  - Seonghoon Choi
  - Jiri Etrych
  - Maria-Andreea Filip
  - W. M. C. Foulkes
  - Ruth S. T. Franklin
  - Will J. Handley
  - Fionn D. Malone
  - Verena A. Neufeld
  - Roberto Di Remigio
  - Thomas W. Rogers
  - Charles J. C. Scott
  - James J. Shepherd
  - William A. Vigor
  - Joseph Weston
  - RuQing Xu
  - Alex J. W. Thom
  abstract: "Building on the success of Quantum Monte Carlo techniques such as diffusion\n\
    Monte Carlo, alternative stochastic approaches to solve electronic structure\n\
    problems have emerged over the last decade. The full configuration interaction\n\
    quantum Monte Carlo (FCIQMC) method allows one to systematically approach the\n\
    exact solution of such problems, for cases where very high accuracy is desired.\n\
    The introduction of FCIQMC has subsequently led to the development of coupled\n\
    cluster Monte Carlo (CCMC) and density matrix quantum Monte Carlo (DMQMC),\nallowing\
    \ stochastic sampling of the coupled cluster wave function and the exact\nthermal\
    \ density matrix, respectively. In this article we describe the HANDE-QMC\ncode,\
    \ an open-source implementation of FCIQMC, CCMC and DMQMC, including\ninitiator\
    \ and semi-stochastic adaptations. We describe our code and demonstrate\nits use\
    \ on three example systems; a molecule (nitric oxide), a model solid (the\nuniform\
    \ electron gas), and a real solid (diamond). An illustrative tutorial is\nalso\
    \ included."
  date: '2018-12-04T19:27:19Z'
  link: http://arxiv.org/abs/1811.11679v2
  ref: 1811.11679v2
- title: Transient and Sharvin resistances of Luttinger liquids
  - Thomas Kloss
  - Joseph Weston
  - Xavier Waintal
  abstract: "Although the intrinsic conductance of an interacting one-dimensional\
    \ system\nis renormalized by the electron-electron correlations, it has been known\
    \ for\nsome time that this renormalization is washed out by the presence of the\n\
    (non-interacting) electrodes to which the wire is connected. Here, we study the\n\
    transient conductance of such a wire: a finite voltage bias is suddenly applied\n\
    across the wire and we measure the current before it has enough time to reach\n\
    its stationary value. These calculations allow us to extract the Sharvin\n(contact)\
    \ resistance of Luttinger and Fermi liquids. In particular, we find\nthat a perfect\
    \ junction between a Fermi liquid electrode and a Luttinger liquid\nelectrode\
    \ is characterized by a contact resistance that consists of half the\nquantum\
    \ of conductance in series with half the intrinsic resistance of an\ninfinite\
    \ Luttinger liquid. These results were obtained using two different\nmethods:\
    \ a dynamical Hartree-Fock approach and a self-consistent Boltzmann\napproach.\
    \ Although these methods are formally approximate we find a perfect\nmatch with\
    \ the exact results of Luttinger/Fermi liquid theory."
  date: '2018-04-26T08:00:21Z'
  link: http://arxiv.org/abs/1710.00895v2
  ref: 1710.00895v2
  jref: Phys. Rev. B 97, 165134 (2018)
  jlink: http://dx.doi.org/10.1103/PhysRevB.97.165134
- title: Cooperative Charge Pumping and Enhanced Skyrmion Mobility
  - Adel Abbout
  - Joseph Weston
  - Xavier Waintal
  - Aurelien Manchon
  abstract: "The electronic pumping arising from the steady motion of ferromagnetic\n\
    skyrmions is investigated by solving the time evolution of the Schrodinger\nequation\
    \ implemented on a tight-binding model with the statistical physics of\nthe many-body\
    \ problem. It is shown that the ability of steadily moving\nskyrmions to pump\
    \ large charge currents arises from their non-trivial magnetic\ntopology, i.e.\
    \ the coexistence between spin-motive force and topological Hall\neffect. Based\
    \ on an adiabatic scattering theory, we compute the pumped current\nand demonstrate\
    \ that it scales with the reflection coefficient of the\nconduction electrons\
    \ against the skyrmion. Finally, we propose that such a\nphenomenon can be exploited\
    \ in the context of racetrack devices, where the\nelectronic pumping enhances\
    \ the collective motion of the train of skyrmions."
  date: '2018-04-06T21:14:34Z'
  link: http://arxiv.org/abs/1804.02460v1
  ref: 1804.02460v1
  jref: Phys. Rev. Lett. 121, 257203 (2018)
  jlink: http://dx.doi.org/10.1103/PhysRevLett.121.257203
- title: Towards Realistic Time-Resolved Simulations of Quantum Devices
  - Joseph Weston
  - Xavier Waintal
  abstract: "We report on our recent efforts to perform realistic simulations of large\n\
    quantum devices in the time domain. In contrast to d.c. transport where the\n\
    calculations are explicitly performed at the Fermi level, the presence of\ntime-dependent\
    \ terms in the Hamiltonian makes the system inelastic so that it\nis necessary\
    \ to explicitly enforce the Pauli principle in the simulations. We\nillustrate\
    \ our approach with calculations for a flying qubit interferometer, a\nnanoelectronic\
    \ device that is currently under experimental investigation. Our\ncalculations\
    \ illustrate the fact that many degrees of freedom (16,700\ntight-binding sites\
    \ in the scattering region) and long simulation times (80,000\ntimes the inverse\
    \ Bandwidth of the tight-binding model) can be easily achieved\non a local computer."
  date: '2016-04-05T09:39:35Z'
  link: http://arxiv.org/abs/1604.01198v1
  ref: 1604.01198v1
  jref: J Comput Electron 15, 1148 (2016)
  jlink: http://dx.doi.org/10.1007/s10825-016-0855-9
- title: "A linear-scaling source-sink algorithm for simulating time-resolved\n  quantum\
    \ transport and superconductivity"
  - Joseph Weston
  - Xavier Waintal
  abstract: "We report on a \"source-sink\" algorithm which allows one to calculate\n\
    time-resolved physical quantities from a general nanoelectronic quantum system\n\
    (described by an arbitrary time-dependent quadratic Hamiltonian) connected to\n\
    infinite electrodes. Although mathematically equivalent to the non equilibrium\n\
    Green's function formalism, the approach is based on the scattering wave\nfunctions\
    \ of the system. It amounts to solving a set of generalized\nSchr\\\"odinger equations\
    \ which include an additional \"source\" term (coming from\nthe time dependent\
    \ perturbation) and an absorbing \"sink\" term (the electrodes).\nThe algorithm\
    \ execution time scales linearly with both system size and\nsimulation time allowing\
    \ one to simulate large systems (currently around $10^6$\ndegrees of freedom)\
    \ and/or large times (currently around $10^5$ times the\nsmallest time scale of\
    \ the system). As an application we calculate the\ncurrent-voltage characteristics\
    \ of a Josephson junction for both short and long\njunctions, and recover the\
    \ multiple Andreev reflexion (MAR) physics. We also\ndiscuss two intrinsically\
    \ time-dependent situations: the relaxation time of a\nJosephson junction after\
    \ a quench of the voltage bias, and the propagation of\nvoltage pulses through\
    \ a Josephson junction. In the case of a ballistic, long\nJosephson junction,\
    \ we predict that a fast voltage pulse creates an oscillatory\ncurrent whose frequency\
    \ is controlled by the Thouless energy of the normal\npart. A similar effect is\
    \ found for short junctions, a voltage pulse produces\nan oscillating current\
    \ which, in the absence of electromagnetic environment,\ndoes not relax."
  date: '2015-10-20T17:05:29Z'
  link: http://arxiv.org/abs/1510.05967v1
  ref: 1510.05967v1
  jref: Phys. Rev. B 93, 134506 (2016)
  jlink: http://dx.doi.org/10.1103/PhysRevB.93.134506
- title: Probing (topological) Floquet states through DC transport
  - Michel Fruchart
  - Pierre Delplace
  - Joseph Weston
  - Xavier Waintal
  - David Carpentier
  abstract: "We consider the differential conductance of a periodically driven system\n\
    connected to infinite electrodes. We focus on the situation where the\ndissipation\
    \ occurs predominantly in these electrodes. Using analytical\narguments and a\
    \ detailed numerical study we relate the differential\nconductances of such a\
    \ system in two and three terminal geometries to the\nspectrum of quasi-energies\
    \ of the Floquet operator. Moreover these differential\nconductances are found\
    \ to provide an accurate probe of the existence of gaps in\nthis quasi-energy\
    \ spectrum, being quantized when topological edge states occur\nwithin these gaps.\
    \ Our analysis opens the perspective to describe the\nintermediate time dynamics\
    \ of driven mesoscopic conductors as topological\nFloquet filters."
  date: '2015-10-06T13:09:09Z'
  link: http://arxiv.org/abs/1507.00152v2
  ref: 1507.00152v2
  jref: Physica E 75 (2016) 287-294
  jlink: http://dx.doi.org/10.1016/j.physe.2015.09.035
- title: Manipulating Andreev and Majorana Bound States with microwaves
  - Joseph Weston
  - Benoit Gaury
  - Xavier Waintal
  abstract: "We study the interplay between Andreev (Majorana) bound states that form\
    \ at\nthe boundary of a (topological) superconductor and a train of microwave\
    \ pulses.\nWe find that the extra dynamical phase coming from the pulses can shift\
    \ the\nphase of the Andreev reflection, resulting in the appear- ance of dynamical\n\
    Andreev states. As an application we study the presence of the zero bias peak\n\
    in the differential conductance of a normal-topological superconductor junction\n\
    - the simplest, yet somehow ambiguous, experimental signature for Majorana\nstates.\
    \ Adding microwave radiation to the measuring electrodes provides an\nunambiguous\
    \ probe of the Andreev nature of the zero bias peak."
  date: '2015-07-30T13:19:58Z'
  link: http://arxiv.org/abs/1411.6885v2
  ref: 1411.6885v2
  jref: Phys. Rev. B 92, 020513 (2015)
  jlink: http://dx.doi.org/10.1103/PhysRevB.92.020513
- title: AC Josephson effect without superconductivity
  - Benoit Gaury
  - Joseph Weston
  - Xavier Waintal
  abstract: "Superconductivity derives its most salient features from the coherence\
    \ of its\nmacroscopic wave function. The associated physical phenomena have now\
    \ moved\nfrom exotic subjects to fundamental building blocks for quantum circuits\
    \ such\nas qubits or single photonic modes. Here, we theoretically find that the\
    \ AC\nJosephson effect---which transforms a DC voltage $V_b$ into an oscillating\n\
    signal $cos(2eV_b t/ \\hbar)$---has a mesoscopic counterpart in normal\nconductors.\
    \ We show that on applying a DC voltage $V_b$ to an electronic\ninterferometer,\
    \ there exists a universal transient regime where the current\noscillates at frequency\
    \ $eV_b/h$. This effect is not limited by a\nsuperconducting gap and could, in\
    \ principle, be used to produce tunable AC\nsignals in the elusive $0.1-10$ THz\
    \ \"terahertz gap\"."
  date: '2014-07-15T08:46:27Z'
  link: http://arxiv.org/abs/1407.3911v1
  ref: 1407.3911v1
  jref: Nature Communications 6, 6524 (2015)
  jlink: http://dx.doi.org/10.1038/ncomms7524
- title: Classical and quantum spreading of a charge pulse
  - Benoit Gaury
  - Joseph Weston
  - Christoph Groth
  - Xavier Waintal
  abstract: "With the technical progress of radio-frequency setups, high frequency\
    \ quantum\ntransport experiments have moved from theory to the lab. So far the\
    \ standard\ntheoretical approach used to treat such problems numerically--known\
    \ as Keldysh\nor NEGF (Non Equilibrium Green's Functions) formalism--has not been\
    \ very\nsuccessful mainly because of a prohibitive computational cost. We propose\
    \ a\nreformulation of the non-equilibrium Green's function technique in terms\
    \ of the\nelectronic wave functions of the system in an energy-time representation.\
    \ The\nnumerical algorithm we obtain scales now linearly with the simulated time\
    \ and\nthe volume of the system, and makes simulation of systems with 10^5 - 10^6\n\
    atoms/sites feasible. We illustrate our method with the propagation and\nspreading\
    \ of a charge pulse in the quantum Hall regime. We identify a classical\nand a\
    \ quantum regime for the spreading, depending on the number of particles\ncontained\
    \ in the pulse. This numerical experiment is the condensed matter\nanalogue to\
    \ the spreading of a Gaussian wavepacket discussed in quantum\nmechanics textbooks."
  date: '2014-07-15T07:48:11Z'
  link: http://arxiv.org/abs/1406.7232v2
  ref: 1406.7232v2
  jref: "Proceedings of the 17th International Workshop on Computational\n  Electronics\
    \ (Paris, France, June 3-6, 2014), p1-p4. Published by IEEE"
  jlink: http://dx.doi.org/10.1109/IWCE.2014.6865808
- title: "Stopping electrons with radio-frequency pulses in the quantum Hall\n  regime"
  - Benoit Gaury
  - Joseph Weston
  - Xavier Waintal
  abstract: "Most functionalities of modern electronic circuits rely on the possibility\
    \ to\nmodify the path fol- lowed by the electrons using, e.g. field effect\ntransistors.\
    \ Here we discuss the interplay between the modification of this\npath and the\
    \ quantum dynamics of the electronic flow. Specifically, we study\nthe propagation\
    \ of charge pulses through the edge states of a two-dimensional\nelectron gas\
    \ in the quantum Hall regime. By sending radio-frequency (RF)\nexcitations on\
    \ a top gate capacitively coupled to the electron gas, we\nmanipulate these edge\
    \ state dynamically. We find that a fast RF change of the\ngate voltage can stop\
    \ the propagation of the charge pulse inside the sample.\nThis effect is intimately\
    \ linked to the vanishing velocity of bulk states in\nthe quantum Hall regime\
    \ and the peculiar connection between momentum and\ntransverse confinement of\
    \ Landau levels. Our findings suggest new possibilities\nfor stopping, releasing\
    \ and switching the trajectory of charge pulses in\nquantum Hall systems."
  date: '2014-05-14T14:53:05Z'
  link: http://arxiv.org/abs/1405.3520v1
  ref: 1405.3520v1
  jref: Phys. Rev. B 90, 161305(R) (2014)
  jlink: http://dx.doi.org/10.1103/PhysRevB.90.161305
- title: Numerical simulations of time resolved quantum electronics
  - Benoit Gaury
  - Joseph Weston
  - Matthieu Santin
  - Manuel Houzet
  - Christoph Groth
  - Xavier Waintal
  abstract: "This paper discusses the technical aspects - mathematical and numerical\
    \ -\nassociated with the numerical simulations of a mesoscopic system in the time\n\
    domain (i.e. beyond the single frequency AC limit). After a short review of the\n\
    state of the art, we develop a theoretical framework for the calculation of\n\
    time resolved observables in a general multiterminal system subject to an\narbitrary\
    \ time dependent perturbation (oscillating electrostatic gates, voltage\npulses,\
    \ time-vaying magnetic fields) The approach is mathematically equivalent\nto (i)\
    \ the time dependent scattering formalism, (ii) the time resolved Non\nEquilibrium\
    \ Green Function (NEGF) formalism and (iii) the partition-free\napproach. The\
    \ central object of our theory is a wave function that obeys a\nsimple Schrodinger\
    \ equation with an additional source term that accounts for\nthe electrons injected\
    \ from the electrodes. The time resolved observables\n(current, density. . .)\
    \ and the (inelastic) scattering matrix are simply\nexpressed in term of this\
    \ wave function. We use our approach to develop a\nnumerical technique for simulating\
    \ time resolved quantum transport. We find\nthat the use of this wave function\
    \ is advantageous for numerical simulations\nresulting in a speed up of many orders\
    \ of magnitude with respect to the direct\nintegration of NEGF equations. Our\
    \ technique allows one to simulate realistic\nsituations beyond simple models,\
    \ a subject that was until now beyond the\nsimulation capabilities of available\
    \ approaches."
  date: '2014-02-18T16:43:03Z'
  link: http://arxiv.org/abs/1307.6419v4
  ref: 1307.6419v4
  jref: Physics Reports 534, 1-37 (2014)
  jlink: http://dx.doi.org/10.1016/j.physrep.2013.09.001