arXiv subjects

Hans-Andreas Engel

Publications and source records attributed to Hans-Andreas Engel.

15 recordsLinked to original sources

arXiv2007

Hanle Effect near Boundaries

The Hanle effect describes suppression of spin polarization due to precession in a magnetic field. This is a standard spintronics tool and it gives access to the spin lifetime of samples in which spins are generated homogeneously. We examine the Hanle effect when spins are generated at a boundary of a diffusive sample by the extrinsic spin Hall effect. We show that the Hanle curve is spatially dependent and that the "apparent" spin lifetime, given by its inverse half-width, is shorter near the boundary even if the spin relaxation rate is homogenous.

cond-mat.mes-hall↗

arXiv2007

Theory of Spin Hall Effects in Semiconductors

Spin Hall effects are a collection of phenomena, resulting from spin-orbit coupling, in which an electrical current flowing through a sample can lead to spin transport in a perpendicular direction and spin accumulation at lateral boundaries. These effects, which do not require an applied magnetic field, can originate in a variety of intrinsic and extrinsic spin-orbit coupling mechanisms and depend on geometry, dimension, impurity scattering, and carrier density of the system--making the analysis of these effects a diverse field of research. In this article, we give an overview of the theoretical background of the spin Hall effects and summarize some of the most important results. First, we explain effective spin-orbit Hamiltonians, how they arise from band structure, and how they can be understood from symmetry considerations; including intrinsic coupling due to bulk inversion or structure asymmetry or due to strain, and extrinsic coupling due to impurities. This leads to different mechanisms of spin transport: spin precession, skew scattering, and side jump. Then we present the kinetic (Boltzmann) equations, which describe the spin-dependent distribution function of charge carriers, and the diffusion equation for spin polarization density. Next, we define the notion of spin currents and discuss their relation to spin polarization. Finally, we explain the electrically induced spin effects; namely, spin polarization and currents in bulk and near boundaries (the focus of most current theoretical research efforts), and spin injection, as well as effects in mesoscopic systems and in edge states.

cond-mat.mes-hall↗

arXiv2007

Spin generation away from boundaries by nonlinear transport

In several situations of interest, spin polarization may be generated far from the boundaries of a sample by nonlinear effects of an electric current, even when such a generation is forbidden by symmetry in the linear regime. We present an analytically solvable model where spin accumulation results from a combination of current gradients, nonlinearity, and cubic anisotropy. Further, we show that even with isotropic conductivity, nonlinear effects in a low symmetry geometry can generate spin polarization far away from boundaries. Finally, we find that drift from the boundaries results in spin polarization patterns that dominate in recent experiments on GaAs by Sih et al. [Phys. Rev. Lett. 97, 096605 (2006)].

cond-mat.mes-hall↗

arXiv2006

Quantum optical interface for gate-controlled spintronic devices

We describe an opto-electronic structure in which charge and spin degrees of freedom in electrical gate-defined quantum dots can be coherently coupled to light. This is achieved via electron-electron interaction or via electron tunneling into a proximal self-assembled quantum dot. We illustrate potential applications of this approach by considering several quantum control techniques, including optical read-out of gate-controlled semiconductor quantum bits and controlled generation of entangled photon-spin pairs.

cond-mat.mes-hall↗

arXiv2006

Out-of-plane spin polarization from in-plane electric and magnetic fields

We show that the joint effect of spin-orbit and magnetic fields leads to a spin polarization perpendicular to the plane of a two-dimensional electron system with Rashba spin-orbit coupling and in-plane parallel dc magnetic and electric fields, for angle-dependent impurity scattering or nonparabolic energy spectrum, while only in-plane polarization persists for simplified models. We derive Bloch equations, describing the main features of recent experiments, including the magnetic field dependence of static and dynamic responses.

cond-mat.mes-hall↗

arXiv2005

Theory of Spin Hall conductivity in n-doped GaAs

We develop a theory of extrinsic spin currents in semiconductors, resulting from spin-orbit coupling at charged scatterers, which leads to skew scattering and side jump contributions to the spin Hall conductance. Applying the theory to bulk n-GaAs, without any free parameters, we find spin currents that are in reasonable agreement with recent experiments by Kato et al. [Science 306, 1910 (2004)].

cond-mat.mes-hall↗

arXiv2004

Controlling Spin Qubits in Quantum Dots

We review progress on the spintronics proposal for quantum computing where the quantum bits (qubits) are implemented with electron spins. We calculate the exchange interaction of coupled quantum dots and present experiments, where the exchange coupling is measured via transport. Then, experiments on single spins on dots are described, where long spin relaxation times, on the order of a millisecond, are observed. We consider spin-orbit interaction as sources of spin decoherence and find theoretically that also long decoherence times are expected. Further, we describe the concept of spin filtering using quantum dots and show data of successful experiments. We also show an implementation of a read out scheme for spin qubits and define how qubits can be measured with high precision. Then, we propose new experiments, where the spin decoherence time and the Rabi oscillations of single electrons can be measured via charge transport through quantum dots. Finally, all these achievements have promising applications both in conventional and quantum information processing.

cond-mat.mes-hall↗

arXiv2004

Probing Single-Electron Spin Decoherence in Quantum Dots using Charged Excitons

We propose to use optical detection of magnetic resonance (ODMR) to measure the decoherence time T_{2} of a single electron spin in a semiconductor quantum dot. The electron is in one of the spin 1/2 states and a circularly polarized laser can only create an optical excitation for one of the electron spin states due to Pauli blocking. An applied electron spin resonance (ESR) field leads to Rabi spin flips and thus to a modulation of the photoluminescence or, alternatively, of the photocurrent. This allows one to measure the ESR linewidth and the coherent Rabi oscillations, from which the electron spin decoherence can be determined. We study different possible schemes for such an ODMR setup, including cw or pulsed laser excitation.

cond-mat.mes-hall↗

arXiv2004

Optical Detection of Single-Electron Spin Decoherence in a Quantum Dot

We propose a method based on optically detected magnetic resonance (ODMR) to measure the decoherence time $T_{2}$ of a single electron spin in a semiconductor quantum dot. The electron spin resonance (ESR) of a single excess electron on a quantum dot is probed by circularly polarized laser excitation. Due to Pauli blocking, optical excitation is only possible for one of the electron-spin states. The photoluminescence is modulated due to the ESR which enables the measurement of electron spin decoherence. We study different possible schemes for such an ODMR setup.

cond-mat.mes-hall↗

arXiv2003

Asymmetric Quantum Shot Noise in Quantum Dots

We analyze the frequency-dependent noise of a current through a quantum dot which is coupled to Fermi leads and which is in the Coulomb blockade regime. We show that the asymmetric shot noise as function of frequency shows steps and becomes super-Poissonian. This provides experimental access to the quantum fluctuations of the current. We present an exact calculation for a single dot level and a perturbative evaluation of the noise in Born approximation (sequential tunneling regime but without Markov approximation) for the general case of many levels with charging interaction.

cond-mat.mes-hall↗

arXiv2003

Measurement efficiency and n-shot read out of spin qubits

We consider electron spin qubits in quantum dots and define a measurement efficiency e to characterize reliable measurements via n-shot read outs. We propose various implementations based on a double dot and quantum point contact (QPC) and show that the associated efficiencies e vary between 50% and 100%, allowing single-shot read out in the latter case. We model the read out microscopically and derive its time dynamics in terms of a generalized master equation, calculate the QPC current and show that it allows spin read out under realistic conditions.

cond-mat.mes-hall↗

arXiv2001

Single Spin Dynamics and Decoherence in a Quantum Dot via Charge Transport

We investigate the spin dynamics of a quantum dot with a spin-1/2 ground state in the Coulomb blockade regime and in the presence of a magnetic rf field leading to electron spin resonances (ESR). We show that by coupling the dot to leads, spin properties on the dot can be accessed via the charge current in the stationary and non-stationary limit. We present a microscopic derivation of the current and the master equation of the dot using superoperators, including contributions to decoherence and energy shifts due to the tunnel coupling. We give a detailed analysis of sequential and co-tunneling currents, for linearly and circularly oscillating ESR fields, applied in cw and pulsed mode. We show that the sequential tunneling current exhibits a spin satellite peak whose linewidth gives a lower bound on the decoherence time T_2 of the dot-spin. Similarly, the spin decoherence can be accessed also in the cotunneling regime via ESR induced spin flips. We show that the conductance ratio of the spin satellite peak and the conventional peak due to sequential tunneling saturates at the universal conductance ratio of 0.71 for strong ESR fields. We describe a double-dot setup which generates spin dependent tunneling and acts as a current pump (at zero bias), and as a spin inverter which inverts the spin-polarization of the current. We show that Rabi oscillations of the dot-spin induce coherent oscillations in the time-dependent current. These oscillations are observable in the time-averaged current as function of ESR pulse-duration, and they allow one to access the spin coherence directly in the time domain. We analyze the measurement and read-out process of the dot-spin via currents in spin-polarized leads and identify measurement time and efficiency by calculating the counting statistics, noise, and the Fano factor.

cond-mat.mes-hall↗

arXiv2000

Detection of Single Spin Decoherence in a Quantum Dot via Charge Currents

We consider a quantum dot attached to leads in the Coulomb blockade regime which has a spin 1/2 ground state. We show that by applying an ESR field to the dot-spin the stationary current in the sequential tunneling regime exhibits a resonance whose line width is determined by the single-spin decoherence time T_2. The Rabi oscillations of the dot-spin are shown to induce coherent current oscillations from which T_2 can be deduced in the time domain. We describe a spin-inverter which can be used to pump current through a double-dot via spin flips generated by ESR.

cond-mat.mes-hall↗

arXiv2000

Conductance fluctuations in diffusive rings: Berry phase effects and criteria for adiabaticity

We study Berry phase effects on conductance properties of diffusive mesoscopic conductors, which are caused by an electron spin moving through an orientationally inhomogeneous magnetic field. Extending previous work, we start with an exact, i.e. not assuming adiabaticity, calculation of the universal conductance fluctuations in a diffusive ring within the weak localization regime, based on a differential equation which we derive for the diffuson in the presence of Zeeman coupling to a magnetic field texture. We calculate the field strength required for adiabaticity and show that this strength is reduced by the diffusive motion. We demonstrate that not only the phases but also the amplitudes of the h/2e Aharonov-Bohm oscillations are strongly affected by the Berry phase. In particular, we show that these amplitudes are completely suppressed at certain magic tilt angles of the external fields, and thereby provide a useful criterion for experimental searches. We also discuss Berry phase-like effects resulting from spin-orbit interaction in diffusive conductors and derive exact formulas for both magnetoconductance and conductance fluctuations. We discuss the power spectra of the magnetoconductance and the conductance fluctuations for inhomogeneous magnetic fields and for spin-orbit interaction.

cond-mat.mes-hall↗

arXiv2000

Spintronics and Quantum Dots for Quantum Computing and Quantum Communication

Control over electron-spin states, such as coherent manipulation, filtering and measurement promises access to new technologies in conventional as well as in quantum computation and quantum communication. We review our proposal of using electron spins in quantum confined structures as qubits and discuss the requirements for implementing a quantum computer. We describe several realizations of one- and two-qubit gates and of the read-in and read-out tasks. We discuss recently proposed schemes for using a single quantum dot as spin-filter and spin-memory device. Considering electronic EPR pairs needed for quantum communication we show that their spin entanglement can be detected in mesoscopic transport measurements using metallic as well as superconducting leads attached to the dots.

cond-mat.mes-hall↗