S. Debald

Bio: S. Debald is an academic researcher from University of Hamburg. The author has contributed to research in topics: Quantum dot & Phonon. The author has an hindex of 4, co-authored 7 publications receiving 203 citations.

Rashba effect and magnetic field in semiconductor quantum wires

Abstract: We investigate the influence of a perpendicular magnetic field on the spectral and spin properties of a ballistic quasi-one-dimensional electron system with Rashba effect. The magnetic field strongly alters the spin-orbit induced modification to the subband structure when the magnetic length becomes comparable to the lateral confinement. A subband-dependent energy splitting at $k=0$ is found which can be much larger than the Zeeman splitting. This is due to the breaking of a combined spin orbital-parity symmetry.

Spin-orbit-driven coherent oscillations in a few-electron quantum dot.

Abstract: We propose an experiment to observe coherent oscillations in a single quantum dot with the oscillations driven by spin-orbit interaction. This is achieved without spin-polarized leads, and relies on changing the strength of the spin-orbit coupling via an applied gate pulse. We derive an effective model of this system which is formally equivalent to the Jaynes-Cummings model of quantum optics. For parameters relevant to an InGaAs dot, we calculate a Rabi frequency of 2 GHz.

Control of dephasing and phonon emission in coupled quantum dots

Abstract: We predict that phonon subband quantization can be detected in the nonlinear electron current through double quantum dot qubits embedded into nanosize semiconductor slabs, acting as phonon cavities. For particular values of the dot level splitting A, piezoelectric or deformation potential scattering is either drastically reduced as compared to the bulk case, or strongly enhanced due to phonon van Hove singularities. By tuning A via gate voltages, one can either control dephasing, or strongly increase emission into phonon modes with characteristic angular distributions.

Interaction and confinement in nanostructures : Spin-orbit coupling and electron-phonon scattering

Abstract: It is the purpose of this work to study the interplay of interaction and confinement in nanostructures using two examples. In part I, we investigate the effects of spin-orbit interaction in parabolically confined ballistic quantum wires and few-electron quantum dots. In general, spin-orbit interaction couples the spin of a particle to its orbital motion. In nanostructures, the latter can easily be manipulated by means of confining potentials. In the first part for this work, we answer the question how the spatial confinement influences spectral and spin properties of electrons in nanostructures with substantial spin-orbit coupling. The latter is assumed to originate from the structure inversion asymmetry at an interface. Thus, the spin-orbit interaction is given by the Rashba model. For a quantum wire, we show that one-electron spectral and spin properties are governed by a combined spin orbital-parity symmetry of wire. The breaking of this spin parity by a perpendicular magnetic field leads to the emergence of a significant energy splitting at k=0 and hybridisation effects in the spin density. Both effects are expected to be experimentally accessible by means of optical or transport measurements. In general, the spin-orbit induced modifications of the subband structure are very sensitive to weak magnetic fields. Because of magnetic stray fields, this implies several consequences for future spintronic devices, which depend on ferromagnetic leads. For the spin-orbit interaction in a quantum dot, we derive a model, inspired by an analogy with quantum optics. This model illuminates most clearly the dominant features of spin-orbit coupling in quantum dots. The model is used to discuss an experiment for observing coherent oscillations in a single quantum dot with the oscillations driven by spin-orbit coupling. The oscillating degree of freedom represents a novel, composite spin-angular momentum qubit. In part II, the interplay of mechanical confinement and electron-phonon interaction is investigated in the transport through two coupled quantum dots. Phonons are quantised modes of lattice vibration. Geometrical confinement in nanomechanical resonators strongly alters the properties of the phonon system. We study a free-standing quantum well as a model for a nano-size planar phonon cavity. We show that coupled quantum dots are a promising tool to detect phonon quantum size effects in the electron transport. For particular values of the dot level splitting Delta, piezo-electric or deformation potential scattering is either drastically reduced as compared to the bulk case, or strongly enhanced due to van-Hove singularities in the phonon density of states. By tuning Delta via gate voltages, one can either control dephasing in double quantum dot qubit systems, or strongly increase emission of phonon modes with characteristic angular distributions. In dieser Arbeit betrachten wir das Zusammenspiel von Wechselwirkung und raumlicher Beschrankung anhand von zwei Beispielen. In Teil I untersuchen wir Effekte der Spin-Bahn-Wechselwirkung in ballistischen Quantendrahten und Quantenpunkten. Die Spin-Bahn-Wechselwirkung koppelt den Spinfreiheitgrad eines Teilchens an seine orbitale Bewegung, die sich in Nanostrukturen leicht durch beschrankende Potentiale beeinflussen lasst. Im ersten Teil dieser Arbeit betrachten wir, wie die spektralen und Spineigenschaften in Systemen mit substantieller Spin-Bahn-Wechselwirkung von der raumlichen Beschrankung beeinflusst werden. Wir nehmen an, dass die Spin-Bahn-Wechselwirkung durch die Raumspiegelungsasymmetrie in einer Inversionsschicht bestimmt wird und beschreiben sie daher durch das Rashba Modell. Wir zeigen, dass in einem Quantendraht die spektralen und Spineigenschaften eines Elektrons durch eine kombinierte Spin-Raumparitatssymmetrie bestimmt werden. Das Aufheben dieser Symmetrie durch ein senkrechtes Magnetfeld fuhrt zu einer ausgepragten Energieaufspaltung bei k=0 und Hybridisierungseffekten in der Spindichte. Es ist zu erwarten, dass beide Effekte fur optische oder Transportexperimente zuganglich sind. Die von der Spin-Bahn-Wechselwirkung stammenden Modifikationen der Subbandstruktur sind sehr empfindlich gegenuber schwachen Magnetfeldern. Dies hat Konsequenzen fur zukunftigen Spintronikbauteile, die von ferromagnetischen Zuleitungen abhangen (Streufelder). Inspiriert von einer Analogie zur Quantenoptik, leiten wir am Beipiel des Quantenpunkts ein effektives Modell her, das die Hauptmerkmale der Spin-Bahn-Wechselwirkung in Quantenpunkten verdeutlicht. In diesem Modell diskutieren wir ein Experiment zur Beobachtung von spinbahngetriebenen koharenten Oszillationen in einem einzelnen Quantenpunkt. Der oszillierende Freiheitsgrad stellt ein neues Qubit dar, das sich aus Spin und Drehimpuls zusammensetzt. In Teil II untersuchen wir das Zusammenspiel von mechanischer Beschrankung und Elektron-Phonon-Wechselwirkung im Transport durch zwei gekoppelte Quantenpunkte. Phononen sind quantisierte Gitterschwingungen deren Eigenschaften stark von der Beschrankung in nanomechanischen Resonatoren beeinflusst werden. Am Beispiel einer ebenen Phononenkavitat zeigen wir, dass gekoppelte Quantenpunkte einen vielversprechenden Detektor zum Nachweis von Phonon-"quantum-size"-Effekten im elektronischen Transport darstellen. Fur gewisse Werte des Energieabstands Delta der Quantenpunkte wird die Streuung durch das piezoelektrische oder Deformationspotential entweder drastisch unterdruckt oder durch van-Hove Singularitaten in der Zustandsdichte der Phononen enorm verstarkt. Die Anderung von Delta ermoglicht es daher, Kontrolle uber die Dephasierung in Doppelquantenpunkt-basierten Qubit-Systemen zu erlangen, oder die Emission in Phononmoden mit charakteristischer Winkelverteilung zu verstarken.

Spin transport through nanostructures

Abstract: The influences of non-Fermi liquid correlations and spin-orbit scattering on the transport properties of quantum nanostructures connected to interacting leads are studied. Signatures of the spin are investigated in the transport. One-dimensional quantum dot is studied in the sequential tunnelling regime using the master equation approach. Quantum coherent conductance is calculated using the transfer matrix method for a quasi-one dimensional system with the Rashba coupling Hamiltonian, and for a two -dimensional quantum dot in a multi-terminal geometry modelled by the Ando Hamiltonian. In the sequential tunnelling regime, states with a higher total spin can be stabilized by suitably adjusting bias and gate voltages. Spin polarized current can be achieved by locally applying a magnetic field. For coherent linear transport through a multi-terminal device at zero ma gnetic field, we find a spin polarized current at certain energies, induced by spin-orbit scattering. Transport properties of nanostructures related to the charge of the electron have been extensively studied during the past tw o decades, since the discovery of the conductance quantisation of quantum point contacts and two-dimensional electron systems in a quantising magnetic field, and of the Coulomb blockade in quantum dots connected to conducting leads by high tunnel barriers ** . The latter can be used to construct a single electron tr ansistor (SET), in which switching between non-conducting and conducting states is done essentially with a single charge. Transport phenomena in nanostructures related to the spin of the electron have attracted considerable interest only during the past few years *** , after the theoretical

Spins in few-electron quantum dots

Abstract: The canonical example of a quantum-mechanical two-level system is spin. The simplest picture of spin is a magnetic moment pointing up or down. The full quantum properties of spin become apparent in phenomena such as superpositions of spin states, entanglement among spins, and quantum measurements. Many of these phenomena have been observed in experiments performed on ensembles of particles with spin. Only in recent years have systems been realized in which individual electrons can be trapped and their quantum properties can be studied, thus avoiding unnecessary ensemble averaging. This review describes experiments performed with quantum dots, which are nanometer-scale boxes defined in a semiconductor host material. Quantum dots can hold a precise but tunable number of electron spins starting with 0, 1, 2, etc. Electrical contacts can be made for charge transport measurements and electrostatic gates can be used for controlling the dot potential. This system provides virtually full control over individual electrons. This new, enabling technology is stimulating research on individual spins. This review describes the physics of spins in quantum dots containing one or two electrons, from an experimentalist’s viewpoint. Various methods for extracting spin properties from experiment are presented, restricted exclusively to electrical measurements. Furthermore, experimental techniques are discussed that allow for 1 the rotation of an electron spin into a superposition of up and down, 2 the measurement of the quantum state of an individual spin, and 3 the control of the interaction between two neighboring spins by the Heisenberg exchange interaction. Finally, the physics of the relevant relaxation and dephasing mechanisms is reviewed and experimental results are compared with theories for spin-orbit and hyperfine interactions. All these subjects are directly relevant for the fields of quantum information processing and spintronics with single spins i.e., single spintronics.

Coherent Control of a Single Electron Spin with Electric Fields

Abstract: Manipulation of single spins is essential for spin-based quantum information processing Electrical control instead of magnetic control is particularly appealing for this purpose, because electric fields are easy to generate locally on-chip We experimentally realized coherent control of a single-electron spin in a quantum dot using an oscillating electric field generated by a local gate The electric field induced coherent transitions (Rabi oscillations) between spin-up and spin-down with 90 degrees rotations as fast as approximately 55 nanoseconds Our analysis indicated that the electrically induced spin transitions were mediated by the spin-orbit interaction Taken together with the recently demonstrated coherent exchange of two neighboring spins, our results establish the feasibility of fully electrical manipulation of spin qubits

Spin dynamics in semiconductors

Abstract: This article reviews the current status of spin dynamics in semiconductors which has achieved a lot of progress in the past years due to the fast growing field of semiconductor spintronics. The primary focus is the theoretical and experimental developments of spin relaxation and dephasing in both spin precession in time domain and spin diffusion and transport in spacial domain. A fully microscopic many-body investigation on spin dynamics based on the kinetic spin Bloch equation approach is reviewed comprehensively.

Direct measurement of the spin-orbit interaction in a two-electron InAs nanowire quantum dot.

Abstract: We demonstrate control of the electron number down to the last electron in tunable few-electron quantum dots defined in catalytically grown InAs nanowires. Using low temperature transport spectroscopy in the Coulomb blockade regime, we propose a method to directly determine the magnitude of the spin-orbit interaction in a two-electron artificial atom with strong spin-orbit coupling. Because of a large effective g factor |g*|=8±1, the transition from a singlet S to a triplet T+ ground state with increasing magnetic field is dominated by the Zeeman energy rather than by orbital effects. We find that the spin-orbit coupling mixes the T+ and S states and thus induces an avoided crossing with magnitude ΔSO=0.25±0.05 meV. This allows us to calculate the spin-orbit length λSO≈127 nm in such systems using a simple model. (Less)