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TY  - THES
AU  - Riwar, Roman-Pascal
TI  - Current and noise in interacting quantum pumps
CY  - Aachen
PB  - Publikationsserver der RWTH Aachen University
M1  - RWTH-CONV-144301
SP  - 180 S. : graph. Darst.
PY  - 2013
N1  - Zsfassung in dt. und engl. Sprache. - Prüfungsjahr: 2013. - Publikationsjahr: 2014
N1  - Aachen, Techn. Hochsch., Diss., 2013
AB  - This thesis is a theoretical study of the current and zero-frequency noise through interacting quantum systems coupled to reservoirs. We investigate the electron transport between system and reservoirs, when both the system and (possibly) the reservoirs are subject to an external time-dependent driving. We focus on systems so small that the confinement gives rise to quantised energy levels, and quantum coherences become important in order to describe the transport. If only very few levels participate in the transport, we refer to the system as a quantum dot, which is the main building block for our theoretical investigation. In this thesis we consider transport that arises due to a time-dependent driving of the system. The transport behaviour depends strongly on how fast the driving is with respect to the time scales that govern the system dynamics. A major focus in this thesis is on adiabatic driving, i.e., a driving so slow that the system can almost immediately follow the time-dependent modulation. Adiabatic time-dependent driving can be used in metrology, to realise very precise current sources in order to establish a quantum standard for the ampere. In fact, an adiabatic single electron pump driven in a periodic fashion, can transport one electron per pumping cycle. The reliability of quantised pumping depends on the driving speed, as too fast driving may result in the system failing to follow the time-dependent driving. We study this aspect in collaboration with an experimental group, who realised an electron pump based on two atomic dopants in a silicon nanowire. The experimentalists aim at measuring an adiabatic quantised electron transport, and moreover investigate a cross-over to a nonadiabatic regime where charging errors occur due to fast driving. We derive a formalism that can provide a complete picture of the pumping mechanism, and accurately describes the experiment, including different nonadiabatic effects. There occurs a purely nonadiabatic current signal where no adiabatic transport is possible. We show that this additional signal is of high value for spectroscopy, as it gives information about the system parameters governing the nonadiabatic processes. Furthermore, the electron spin can encode logical information. In the field of spintronics, the aim is to not only use the electron spin as the logical entity, but to coherently manipulate and transport it. Spin-dependent transport may emerge in the presence of ferromagnets: when two ferromagnets are connected via a thin metallic layer, there arises the giant magnetoresistance, where the conductance decreases significantly for antiparallel alignment of the magnetisations. This effect is of importance for instance in the readout of magnetic hard drives. Based on this, there has been a lot of interest to study spin valves in the context of quantum transport, i.e., where the metallic layer is replaced by a quantum dot. In this thesis we study in particular the charge and spin transport in a double quantum dot contacted to normal metal as well as ferromagnetic contacts, due to a time-dependent modulation of the energy levels of the quantum dots. In the adiabatic driving regime, we examine the possibility of pure spin pumping in the absence of a pumped charge. Also we study how the relative orientation of the ferromagnets' spin polarisation affects the charge transport. In general, the current through a quantum system is accompanied by fluctuations. These fluctuations can be quantified by the zero-frequency current noise, given as the variance of the number of electrons that arrive at a contact averaged over a large measuring time. In quantum transport there are two intrinsic sources of noise: thermal fluctuations and shot noise due to the electronic charge being quantised. While in an equilibrium situation the thermal contribution is dominant, for strong nonequilibrium the shot noise starts to abound. The noise signal contains crucial information about the system and its statistics. In particular the Coulomb interaction may give rise to correlations in the electron transport, visible in the noise. When considering adiabatic quantum pumps, an additional pumping noise contribution arises due to the driving. This pumping noise has been studied extensively for the case of weakly interacting systems. So far, there has been no theoretical treatment of the pumping noise in strongly interacting quantum dot pumps. Here, we study the pumping noise for the specific model of a single-level quantum dot pump, and focus on the interplay of quantum and interaction effects in the pumping noise. As a substantial part of this thesis we derive a formalism based on real-time diagrammatics. By means of this formalism, we shed light onto the characteristics and statistics of the quantum pump. Moreover, we will be able to show that the zero-frequency noise gives information about the time-resolved pumping current.
KW  - Quantenmechanik (SWD)
KW  - Elektronischer Transport (SWD)
KW  - Funkelrauschen (SWD)
KW  - Periodische Störung (SWD)
KW  - Coulomb-Wechselwirkung (SWD)
KW  - Vielteilchensystem (SWD)
LB  - PUB:(DE-HGF)11
UR  - https://publications.rwth-aachen.de/record/229331
ER  -