Carl-Olof Almbladh

Time-dependent quantum transport: A practical scheme using density functional theory

We present a computationally tractable scheme of time-dependent transport phenomena within openboundary time-dependent density functional theory. Within this approach all the response properties of a system are determined from the time propagation of the set of “occupied” Kohn-Sham orbitals under the influence of the external bias. This central idea is combined with an open-boundary description of the geometry of the system that is divided into three regions: left/right leads and the device region “real simulation region”. We have derived a general scheme to extract the set of initial states in the device region that will be propagated in time with proper transparent boundary-condition at the device/lead interface. This is possible due to a new modified Crank-Nicholson algorithm that allows an efficient time-propagation of open quantum systems. We illustrate the method in one-dimensional model systems as a first step towards a full first-principles implementation. In particular we show how a stationary current develops in the system independent of the transientcurrent history upon application of the bias. The present work is ideally suited to study ac transport and photon-induced charge-injection. Although the implementation has been done assuming clamped ions, we discuss how it can be extended to include dissipation due to electron-phonon coupling through the combined simulation of the electron-ion dynamics as well as electron-electron correlations.

Time-dependent quantum transport: An exact formulation based on TDDFT

An exact theoretical framework based on Time Dependent Density Functional Theory (TDDFT) is proposed in order to deal with the time-dependent quantum transport in fully interacting systems. We use a partition-free approach by Cini in which the whole system is in equilibrium before an external electric field is switched on. Our theory includes the interactions between the leads and between the leads and the device. It is well suited for calculating measurable transient phenomena as well as a.c. and other time-dependent responses. We show that the steady-state current results from a dephasing mechanism provided the leads are macroscopic and the device is finite. In the d.c. case, we obtain a Landauer-like formula when the effective potential of TDDFT is uniform deep inside the electrodes.

On the Theory of Photoemission

This paper is concerned with the a priori theory of the photoemission process in solids. Scattering-theoretical and response-theoretical approaches are discussed and a new demonstration of their equivalence is given. The elastic photoemission spectrum is analyzed in detail and an expression exact to all orders in interparticle interaction is obtained. This expression is similar to the renormalized three-current correlation formula used by Pendry and by others but includes also a screening and exchange-correlation correction to the optical field. The loss part of the spectrum is also discussed and a new quantum-mechanical approximation for the energy losses is obtained. For long mean free paths this correctly reduces to a three-step model in which the energy dissipation is given by a semi-classical transport equation.

Classical nuclear motion in quantum transport

An ab initio quantum-classical mixed scheme for the time evolution of electrode-device-electrode systems is introduced to study nuclear dynamics in quantum transport. Two model systems are discussed to illustrate the method. Our results provide the first example of current-induced molecular desorption as obtained from a full time-dependent approach and suggest the use of ac biases as a way to tailor electromigration. They also show the importance of nonadiabatic effects for ultrafast phenomena in nanodevices.

Self-consistent Hartree-Fock calculation of exchange effects in a weakly inhomogeneous electron gas☆

An exact calculation is given for the function Bx(n) which enters the Hohenberg-Kohn-Sham expansion, in powers of ▿n(r), of the echange energy of an inhomogeneous eletron gas. The calculation is made for arbitrary inter-particle interaction. It is shown that the gradient expansion does not exist, in the case of Coulomb interaction, unless correlations are taken into account.

Some open questions in TDDFT: Clues from lattice models and Kadanoff–Baym dynamics

Two aspects of TDDFT, the linear response approach and the adiabatic local density approximation, are examined from the perspective of lattice models. To this end, we review the DFT formulations on the lattice and give a concise presentation of the time-dependent Kadanoff-Baym equations, used to asses the limitations of the adiabatic approximation in TDDFT. We present results for the density response function of the 3D homogeneous Hubbard model, and point out a drawback of the linear response scheme based on the linearized Sham-Schluter equation. We then suggest a prescription on how to amend it. Finally, we analyze the time evolution of the density in a small cubic cluster, and compare exact, adiabatic-TDDFT and Kadanoff-Baym equations densities. Our results show that non-perturbative (in the interaction) adiabatic potentials can perform quite well for slow perturbations but that, for faster external fields, memory effects, as already present in simple many-body approximations, are clearly required

Can we always get the entanglement entropy from the Kadanoff-Baym equations? The case of the T-matrix approximation

We study the time-dependent transmission of entanglement entropy through an out-of-equilibrium model interacting device in a quantum transport set-up. The dynamics is performed via the Kadanoff-Baym equations within many-body perturbation theory. The double occupancy

Time-dependent transport through single molecules: nonequilibrium Green's functions

, needed to determine the entanglement entropy, is obtained from the equations of motion of the single-particle Greens function. A remarkable result of our calculations is that