Dynamics and Fluctuations in Single-Electron Tunneling Devices
Doktorsavhandling, 2018

In recent years, it has been routinely achieved to build nanoscale electronic devices, which generate current pulses carrying only a single elementary charge. Realizations of these single-electron emitters are based on time-dependently driven quantum dots, on single-electron turnstiles built from superconductor/normal-metal hybrid structures, and also on nanosystems employing Lorentzian voltage pulses or surface acoustic waves. In this thesis, we present theoretical studies of single-electron transport in nanoscale devices of this kind. A central focus, besides extending the understanding of the physics in these devices, is the development and application of complementary theoretical methods. This multi-method approach allows us to highlight the assets and limitations of different theories, to compare the accuracy of results and the necessary analytical/computational efforts, and, most importantly, to find novel and fruitful method combinations.

In this thesis, we first propose a novel clocked spin-current source, which consists of a superconducting island tunnel coupled to two superconducting contacts via a ferromagnetic insulator layer. We demonstrate that this nanostructure can be operated as an emitter of a precise quantized spin current and we point out its working principle as well as its experimental feasibility.

The second device we analyze is a single-electron source, which is built from an interacting quantum dot with tunnel coupling to a single contact. The single-electron emission is triggered by a slow time-dependent gate-voltage driving, and we present a comprehensive study of the noise spectrum of the emitted current signal. The noise contains information on the system's excitation spectrum and its dynamics, and it also reveals signatures of Coulomb interaction. To derive the noise spectra over a large frequency range, we extend a real-time diagrammatic perturbative method in the tunnel coupling to finite noise frequencies in the presence of the slow time-dependent drive. We then perform a harmonic decomposition of the noise spectra, present an interpretation of the noise in terms of individual fluctuation processes, and point out characteristic signatures for the interplay between Coulomb interaction and the time-dependent driving.

Third, we turn to time-dependent density-functional theory, which is a numerical method, and we transfer insights from the diagrammatic calculations to this theory. This novel combination of methods allows us to develop a nonadiabatic (i.e. time-nonlocal) approximation of this theory's exchange-correlation potential. We relate properties of the exchange-correlation potential to physical time scales of the electron dynamics and we apply it to obtain numerical time evolutions of single and multiple quantum dots coupled to a shared electron reservoir. In addition, we extend this combination of methods to another nanosystem, namely an interacting quantum dot coupled to two contacts and exposed to time-dependent gate and bias voltages. The results presented in this part of the thesis constitute a significant step towards the application of time-dependent density-functional theory for the description of charge dynamics in complex single-electron tunneling devices.

quantum dot

time-dependent density-functional theory

perturbation theory

single-electron source


Niklas Dittmann

Chalmers, Mikroteknologi och nanovetenskap (MC2), Tillämpad kvantfysik

Clocked single-spin source based on a spin-split superconductor

New Journal of Physics,; Vol. 18(2016)p. Article Number: 083019 -

Artikel i vetenskaplig tidskrift

Nonadiabatic Dynamics in Single-Electron Tunneling Devices with Time-Dependent Density-Functional Theory

Physical Review Letters,; Vol. 120(2018)

Artikel i vetenskaplig tidskrift

Niklas Dittmann, Nicole Helbig, Dante Kennes, Dynamics of the Anderson impurity model: benchmarking a non-adiabatic exchange-correlation potential in TDDFT

Niklas Dittmann, Nicole Helbig, Nonadiabatic dynamics of a biased quantum dot with time-dependent density-functional theory

In electronic circuits, charge is typically transported by propagating electrons, where each electron carries a single elementary charge through the circuit. This granular nature of charge can play a significant role for its dynamics, in particular, in nanostructures that are cooled down to a few Kelvin. For example, nanodevices which emit single electrons into a conductor, triggered by carefully arranged time-dependent voltages, are today routinely realized in low-temperature laboratories around the world. These experimental achievements ask for appropriate developments also in theoretical physics. However, a key difficulty for theory is posed by the complex interplay of different time and energy scales, which influence the dynamics of these devices. Also, the Coulomb interaction, which leads to a repulsive force between electrons, is in most cases rather challenging to include. In this thesis, in order to advance our theoretical toolbox, I present a novel method combination that allows for an accurate and at the same time numerically efficient theoretical description of charge dynamics in so-called single-electron tunneling devices. In addition, in order to gain a deeper understanding of the dynamics of two specific nanodevices, I present studies of their time-dependent currents and current fluctuations.


Nanovetenskap och nanoteknik (2010-2017)


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Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4509



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