| 1. |
- Dittmann, Niklas, 1987, et al.
(författare)
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Clocked single-spin source based on a spin-split superconductor
- 2016
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 18:8, s. Article Number: 083019 -
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Tidskriftsartikel (refereegranskat)abstract
- We propose an accurate clocked single-spin source for ac-spintronic applications. Our device consists of a superconducting island covered by a ferromagnetic insulator (FI) layer through which it is coupled to superconducting leads. Single-particle transfer relies on the energy gaps and the island's charging energy, and is enabled by a bias and a time-periodic gate voltage. Accurate spin transfer is achieved by the FI layer which polarizes the island, provides spin-selective tunneling barriers and improves the precision by suppressing Andreev reflection. We analyze realistic material combinations and experimental requirements which allow for a clocked spin current in the MHz regime. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
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| 2. |
- Dittmann, Niklas, 1987
(författare)
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Dynamics and Fluctuations in Single-Electron Tunneling Devices
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 3. |
- Dittmann, Niklas, 1987, et al.
(författare)
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Dynamics of the Anderson impurity model: Benchmarking a nonadiabatic exchange-correlation potential in time-dependent density-functional theory
- 2019
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 99:7
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Tidskriftsartikel (refereegranskat)abstract
- In this comparative study we benchmark a recently developed nonadiabatic exchange-correlation potential within time-dependent density-functional theory (TDDFT) [Phys. Rev. Lett. 120, 157701 (2018)PRLTAO0031-900710.1103/PhysRevLett.120.157701] by (a) validating the transient dynamics using a numerically exact density-matrix renormalization-group approach as well as by (b) comparing the RC time, a typical linear-response quantity, to up to second-order perturbation theory results. As a test bed we use the dynamics of the single-impurity Anderson model. These benchmarks show that the nonadiabatic potential yields quantitatively accurate results for the transient dynamics for temperatures of the order of the hybridization strength, while the TDDFT RC times quantitatively agree with those from second-order perturbation theory for temperatures which are large compared to the hybridization strength. Both results are particularly intriguing given the relatively low numerical cost of a TDDFT calculation.
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| 4. |
- Dittmann, Niklas, 1987, et al.
(författare)
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Equilibrium finite-frequency noise of an interacting mesoscopic capacitor studied in time-dependent density functional theory
- 2018
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 969:1
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Konferensbidrag (refereegranskat)abstract
- We calculate the frequency-dependent equilibrium noise of a mesoscopic capacitor in time-dependent density functional theory (TDDFT). The capacitor is modeled as a single-level quantum dot with on-site Coulomb interaction and tunnel coupling to a nearby reservoir. The noise spectra are derived from linear-response conductances via the fluctuation-dissipation theorem. Thereby, we analyze the performance of a recently derived exchange-correlation potential with time-nonlocal density dependence in the finite-frequency linear-response regime. We compare our TDDFT noise spectra with real-time perturbation theory and find excellent agreement for noise frequencies below the reservoir temperature.
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| 5. |
- Dittmann, Niklas, 1987, et al.
(författare)
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Finite-frequency noise of interacting single-electron emitters: Spectroscopy with higher noise harmonics
- 2018
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 98:11
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Tidskriftsartikel (refereegranskat)abstract
- We derive the symmetrized current-noise spectrum of a quantum dot, which is weakly tunnel-coupled to an electron reservoir and driven by a slow time-dependent gate voltage. This setup can be operated as an on-demand emitter of single electrons into a mesoscopic conductor. By extending a real-time diagrammatic technique which is perturbative in the tunnel coupling, we obtain the time-resolved finite-frequency noise as well as its decomposition into noise harmonics in the presence of both strong Coulomb interaction and slow time-dependent driving. We investigate the noise over a large range of frequencies and point out where the interplay of Coulomb interaction and driving leads to unique signatures in finite-frequency noise spectra, in particular in the first harmonic. Besides that, we employ the first noise harmonic as a spectroscopic tool to access individual fluctuation processes. We discuss how the inverse noise frequency sets a time scale for fluctuations, which competes with time scales of the quantum-dot relaxation dynamics as well as the driving.
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| 6. |
- Dittmann, Niklas, 1987, et al.
(författare)
-
Nonadiabatic Dynamics in Single-Electron Tunneling Devices with Time-Dependent Density-Functional Theory
- 2018
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 120:15
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Tidskriftsartikel (refereegranskat)abstract
- We simulate the dynamics of a single-electron source, modeled as a quantum dot with on-site Coulomb interaction and tunnel coupling to an adjacent lead in time-dependent density-functional theory. Based on this system, we develop a time-nonlocal exchange-correlation potential by exploiting analogies with quantum-transport theory. The time nonlocality manifests itself in a dynamical potential step. We explicitly link the time evolution of the dynamical step to physical relaxation timescales of the electron dynamics. Finally, we discuss prospects for simulations of larger mesoscopic systems.
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