Dieter Schuh

Optically programmable electron spin memory using semiconductor quantum dots

The spin of a single electron subject to a static magnetic field provides a natural two-level system that is suitable for use as a quantum bit, the fundamental logical unit in a quantum computer. Semiconductor quantum dots fabricated by strain driven self-assembly are particularly attractive for the realization of spin quantum bits, as they can be controllably positioned, electronically coupled and embedded into active devices. It has been predicted that the atomic-like electronic structure of such quantum dots suppresses coupling of the spin to the solid-state quantum dot environment, thus protecting the ‘spin’ quantum information against decoherence. Here we demonstrate a single electron spin memory device in which the electron spin can be programmed by frequency selective optical excitation. We use the device to prepare single electron spins in semiconductor quantum dots with a well defined orientation, and directly measure the intrinsic spin flip time and its dependence on magnetic field. A very long spin lifetime is obtained, with a lower limit of about 20 milliseconds at a magnetic field of 4 tesla and at 1 kelvin.

Direct observation of controlled coupling in an individual quantum dot molecule.

We report the direct observation of quantum coupling in individual quantum dot molecules and its manipulation using static electric fields. A pronounced anticrossing of different excitonic transitions is observed as the electric field is tuned. A comparison of our experimental results with theory shows that the observed anticrossing occurs between excitons with predominant spatially direct and indirect character and reveals a field driven transition of the nature of the molecular ground state exciton wave function. Finally, the interdot quantum coupling strength is deduced optically and its dependence on the interdot separation is calculated.

Ultrastrong coupling of the cyclotron transition of a 2D electron gas to a THz metamaterial

Quantum Hall Meets Metamaterial Controlling and tuning light-matter interaction is crucial for fundamental studies of cavity quantum electrodynamics and for applications in classical and quantum devices. Scalari et al. (p. 1323) describe a system comprising an array of metamaterial split-ring resonators and a series of two-dimensional electronic gases (2DEG) formed in GaAs quantum wells. In a magnetic field, the electrons in the 2DEG performed cyclotron orbits and formed Landau levels. Strong coupling was observed between photon and magnetic cyclotron modes, producing a tunable semiconductor system for studying the light-matter interaction of two-level systems. A system of terahertz resonators coupled to two-dimensional electron gases presents a tunable test bed for the study of two-level physics. Artificial cavity photon resonators with ultrastrong light-matter interactions are attracting interest both in semiconductor and superconducting systems because of the possibility of manipulating the cavity quantum electrodynamic ground state with controllable physical properties. We report here experiments showing ultrastrong light-matter coupling in a terahertz (THz) metamaterial where the cyclotron transition of a high-mobility two-dimensional electron gas (2DEG) is coupled to the photonic modes of an array of electronic split-ring resonators. We observe a normalized coupling ratio, Ωωc=0.58, between the vacuum Rabi frequency, Ω, and the cyclotron frequency, ωc. Our system appears to be scalable in frequency and could be brought to the microwave spectral range with the potential of strongly controlling the magnetotransport properties of a high-mobility 2DEG.

Direct correlation of crystal structure and optical properties in wurtzite/zinc-blende GaAs nanowire heterostructures

A method for the direct correlation at the nanoscale of structural and optical properties of single GaAs nanowires is reported. Nanowires consisting of 100% wurtzite and nanowires presenting zinc-blende/wurtzite polytypism are investigated by photoluminescence spectroscopy and transmission electron microscopy. The photoluminescence of wurtzite GaAs is consistent with a band gap of 1.5 eV. In the polytypic nanowires, it is shown that the regions that are predominantly composed of either zinc-blende or wurtzite phase show photoluminescence emission close to the bulk GaAs band gap, while regions composed of a nonperiodic superlattice of wurtzite and zinc-blende phases exhibit a redshift of the photoluminescence spectra as low as 1.455 eV. The dimensions of the quantum heterostructures are correlated with the light emission, allowing us to determine the band alignment between these two crystalline phases. Our first-principles electronic structure calculations within density functional theory, employing a hybrid-exchange functional, predict band offsets and effective masses in good agreement with experimental results.

Time-resolved ultrafast photocurrents and terahertz generation in freely suspended graphene

Graphene, a two-dimensional layer of carbon atoms, is a promising building block for a wide range of optoelectronic devices owing to its extraordinary electrical and optical properties, including the ability to absorb ~2% of incident light over a broad wavelength range. While the RC-limited bandwidth of graphene-based photodetectors can be estimated to be as large as 640 GHz, conventional electronic measurement techniques lack for analysing photocurrents at such frequencies. Here we report on time-resolved picosecond photocurrents in freely suspended graphene contacted by metal electrodes. At the graphene-metal interface, we demonstrate that built-in electric fields give rise to a photocurrent with a full-width-half-maximum of ~4 ps and that a photothermoelectric effect generates a current with a decay time of ~130 ps. Furthermore, we show that, in optically pumped graphene, electromagnetic radiation up to 1 THz is generated. Our results may prove essential to build graphene-based ultrafast photodetectors, photovoltaic cells and terahertz sources.

Tunneling Anisotropic Magnetoresistance and Spin-Orbit Coupling in Fe/GaAs/Au Tunnel Junctions

We report the observation of tunneling anisotropic magnetoresistance effect in the epitaxial metal-semiconductor system Fe/GaAs/Au. The observed twofold anisotropy of the resistance can be switched by reversing the bias voltage, suggesting that the effect originates from the interference of the spin-orbit coupling at the interfaces. Corresponding model calculations reproduce the experimental findings very well.

Rashba and Dresselhaus spin splittings in semiconductor quantum wells measured by spin photocurrents

The spin-galvanic effect and the circular photogalvanic effect induced by terahertz radiation are applied to determine the relative strengths of Rashba and Dresselhaus band spin splitting in 001-grown GaAs and InAs based two dimensional electron systems. We observed that shifting the -doping plane from one side of the quantum well to the other results in a change of sign of the photocurrent caused by Rashba spin splitting while the sign of the Dresselhaus term induced photocurrent remains. The measurements give the necessary feedback for technologists looking for structures with equal Rashba and Dresselhaus spin splittings or perfectly symmetric structures with zero Rashba constant.

Electrical spin injection and detection in lateral all-semiconductor devices

Both electrical injection and detection of spin-polarized electrons are demonstrated in a single wafer all-semiconductor GaAs-based lateral spintronic device, employing p+-(Ga,Mn)As/n+-GaAs ferromagnetic Esaki diodes as spin aligning contacts. Spin-dependent phenomena, such as spin precession and spin-valve effect, are observed in nonlocal signal and the measurements reveal the unusual origin of the latter in the investigated devices. The conversion of spin-polarized holes into spin-polarized electrons via Esaki tunneling leaves its mark in a bias dependence of the spin-injection efficiency, which at maximum reaches the value of 50%.

Ferromagnetic GaAs/GaMnAs Core−Shell Nanowires Grown by Molecular Beam Epitaxy

GaAs/GaMnAs core-shell nanowires were grown by molecular beam epitaxy. The core GaAs nanowires were synthesized under typical nanowire growth conditions using gold as catalyst. For the GaMnAs shell the temperature was drastically reduced to achieve low-temperature growth conditions known to be crucial for high-quality GaMnAs. The GaMnAs shell grows epitaxially on the side facets of the core GaAs nanowires. A ferromagnetic transition temperature of 20 K is obtained. Magnetic anisotropy studies indicate a magnetic easy axis parallel to the nanowire axis.

Position controlled self-catalyzed growth of GaAs nanowires by molecular beam epitaxy

GaAs nanowires are grown by molecular beam epitaxy using a self-catalyzed, Ga-assisted growth technique. Position control is achieved by nano-patterning a SiO(2) layer with arrays of holes with a hole diameter of 85 nm and a hole pitch varying between 200 nm and 2 µm. Gallium droplets form preferentially at the etched holes acting as catalyst for the nanowire growth. The nanowires have hexagonal cross-sections with {110} side facets and crystallize predominantly in zincblende. The interdistance dependence of the nanowire growth rate indicates a change of the III/V ratio towards As-rich conditions for large hole distances inhibiting NW growth.