1,721,056 research outputs found
Single Atom Electronics
No full textThis thesis describes a series of experiments on the electronic properties of individual shallow dopant atoms in silicon. Shallow dopants are impurity atoms that bind either a single electron or hole and can therefore be con- sidered as the solid state analogue to the hydrogen atom. As the transistor density increases, critical device dimensions are fast approaching the effective Bohr radius of shallow dopant atoms. This offers the compelling possibility to utilize the quantum nature of dopant atoms to enhance the functionality if semiconductor nano-devices. The emphasis of the experimental work described in this thesis is on the interaction between single dopant atoms and their environment. When dopant atoms are embedded within a nano-structure quantum confinement at the interface will strongly perturb the wavefunction of the dopant-bound electron (or hole), as a consequence the wavefunctions and energies will no longer be that of hydrogenic states. Moreover, dielectric mismatch between the semicon- ductor and its surroundings will influence the energy spectra of dopant atoms near the interface. Generally speaking, the presence of interfaces will break the tetrahedral symmetry of the silicon crystal and will cause degeneracies to be lifted, drastically shifting the electronic states of dopant atoms with respect to dopant-bound states in bulk silicon. Scanning tunneling spectroscopy (STS) is a unique method that allows for the spatially resolved investigation of the electronic structure of sub-surface dopant atoms. Unlike in other transport measurements, both lateral position and depth of single dopant atoms can unambiguously be determined. Chapters 3 and 4 of this thesis describe experiments aimed at studying the energy spectra as a function of depth of individual sub-surface dopant atoms by means of electron transport through the localized dopant states. Interface enhancement of the ionization energy, and as a consequence deactivation of dopants near the interface, is a major concern for doping nano-structures. Chapter 3 describes the experimental investigation of the effect of the interface on the ionization energy of single sub-surface acceptors. It is worthwhile mentioning here that the vacuum-silicon interface yields the largest possible dielectric mismatch attainable for silicon. The depth of individual ac- ceptors is measured by the influence of the ionized acceptor nucleus on the local density of valence band states. The ionization energy is determined from the voltage at which resonant tunneling through the localized acceptor state occurs. An absolute energy scale is provided by the thermal broadening of the conductance peaks. It is explicitly demonstrated that acceptors in silicon less than a Bohr radius away from the interface maintain a bulk-like ionization energy. Building on the methods described in Chapter 3, measurements of the excited state spectra of single sub-surface acceptors are presented in Chapter 4. Interface induced spin-orbit splitting of the four-fold degenerate ground state of boron in silicon results in the formation of two Kramers doublets. The observed enhancement of this splitting for acceptors close to the interface, and moreover the ability to controllably tune this splitting will have strong implications for quantum computation schemes based on the spin of acceptor-bound holes. One of the key challenges in single atom electronics is the strict require- ments for dopant placement. Recent developments in scanning tunneling microscopy (STM) based bottom-up fabrication have paved the way for atomically precise dopant based electronic devices. Chapter 5 illustrates, for the first time, how low temperature scanning tunneling spectroscopy can be used in conjunction with bottom-up dopant engineering. Transport measurements on single phosphorus donors deliberately placed five monolayers beneath the surface of a p-type silicon substrate serve as a proof-of-principle for STS studies on atomically precise dopant structures. Chapter 6 describes an experiment where, for the first time, the quantum states of a single arsenic donor embedded in a nano-scale field-effect transistor are utilized to increase the device functionality of the transistor. By integrating two single-atom transistors in a circuit a classical logic operation, namely a full addition, is performed using only a fraction of the transistors required in a conventional complementary-metal-oxide-semiconductor circuit.Quantum NanoscienceApplied Science
Electron transport through single donors in silicon
No full text-Kavli Institute of Nanoscience DelftApplied Science
Quantum transport in strongly interacting one-dimensional nanostructures
No full textIn this thesis we study quantum transport in several one-dimensional systems with strong electronic interactions. The first chapter contains an introduction to the concepts treated throughout this thesis, such as the Aharonov-Bohm effect, the Kondo effect, the Fano effect and quantum state transfer. It also includes a brief historical introduction to these phenomena. The next three chapters discuss electronic transport in strongly interacting systems with a focus on the transport produced by the Kondo effect. The final chapter deals with spin quantum state transfer, where we analytically address the idea of having a one-dimensional spin chain as a quantum data bus in a quantum computer. In chapter 2 we model a system of coupled donors to explain experimental data of conductance, and use this model to investigate coherence and correlation effects. The two donors are strongly coupled to two leads in a parallel configuration embedded in a nano-wire field effect transistor. We then model the system as an Aharonov-Bohm ring with a strongly interacting quantum dot in each arm, and calculate the conductance in the middle of the Coulomb diamond when the system is in the Kondo regime. In the experimental data, interference was observed when a magnetic field was applied [Fig. 2.3(a)]. This interference shows a dependence on the Aharonov-Bohm phase picked up by electrons traversing the structure. This means that donors can be coupled coherently through a many-body state (the Kondo state). Calculations show the non-monotonic behavior of the conductance that was seen in the experimental data [Fig. 2.4(a)]. The conductance decreases since the Kondo effect is destroyed by the magnetic field, and at the same time an oscillatory behavior appears due to the magnetic phase picked up by the electrons going through the parallel structure. Our results improve the general understanding of possible interference effects in an atomic system, especially in the regime where strong interactions take over. Non-symmetric conductance resonances were observed in the data used in chapter 2 when the transport regime of one of the quantum dots changes from Coulomb blockade to sequential tunneling. We model this situation in chapter 3 and arrive at an analytical expression of conductance which we rewrite as a Fano equation. We demonstrate that the strongly interacting quantum dot creates a Kondo scattering channel which serves as a continuum and interferes with the resonant quantum dot, hence producing a non-symmetric Fano like shape in the conductance. Simulations were done using experimental parameters and good agreement with the data is found [Fig. 3.3]. Furthermore, we predict that even if the interacting channel is fully in the Kondo regime, we can use the magnetic flux to diminish its contribution by lowering the characteristic Kondo temperature (Kondo state broadening), producing an alteration in the electron’s path preference. The next challenge consisted of modeling a strongly-interacting chain of atoms, and study the impact of disorder on the Kondo conductance. In chapter 4 we model the energy levels of the quantum dots to be in the middle of the Coulomb blockade region without disorder. Transport calculations of the atomic chains show that in the weak disorder regime conductance drops with increasing disorder, which is surprising and not expected as the disorder is screened by the pinning of the Kondo state at the Fermi level. We demonstrate that the cause of this decrease is an induced non-screened disorder due to the local distribution of Kondo temperatures along the chain. We also show that weak disorder increases the Kondo temperature of a chain without disorder. We propose two experimental scanning tunneling microscopy setups where the impact of local Kondo temperatures can be observed [Fig. 4.5]. It has been reported that quantum state transfer (QST) can be achieved in a Heisenberg spin chain consisting of three spins. Then it might also be possible to achieve QST in longer spin chains if they can be modeled by an effective three-spin system during the complete quantum state transfer. This idea is formally discussed in chapter 5. We propose simple protocols to achieve quantum state transfer across a spin bus with high accuracy. We propose an effective toy model and apply our findings to a spin chain with a sender and a receiver qubit. We find that within the scope of the effective model the control of only the couplings of the spin bus to the sender and receiver qubits yield high fidelity. We also find an interesting high fidelity region that cannot be described by the effective toy model, and predict the high fidelities to be a consequence of a time-independent first excited state energy. We apply the socalled superadiabatic formalism, which makes the evolution 100% adiabatic, and find fidelities that are equal to one. We derive an approximate Hamiltonian containing parameters that correspond to physical (experimental) knobs, and demonstrate that this Hamiltonian improves the fidelity in both of the treated protocols [Fig. 5.9].Quantum NanoscienceApplied Science
Ethnicity and changes in weaving technology in Cyprus and the Eastern Mediterranean during the 12th. cent. B.C.
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Two dimensional photonic crystal devices
No full textKavli Institute of Nanoscience DelftApplied Science
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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