Werner Wegscheider

Energy spectra of quantum rings.

Quantum mechanical experiments in ring geometries have long fascinated physicists. Open rings connected to leads, for example, allow the observation of the Aharonov–Bohm effect, one of the best examples of quantum mechanical phase coherence. The phase coherence of electrons travelling through a quantum dot embedded in one arm of an open ring has also been demonstrated. The energy spectra of closed rings have only recently been studied by optical spectroscopy. The prediction that they allow persistent current has been explored in various experiments. Here we report magnetotransport experiments on closed rings in the Coulomb blockade regime. Our experiments show that a microscopic understanding of energy levels, so far limited to few-electron quantum dots, can be extended to a many-electron system. A semiclassical interpretation of our results indicates that electron motion in the rings is governed by regular rather than chaotic motion, an unexplored regime in many-electron quantum dots. This opens a way to experiments where even more complex structures can be investigated at a quantum mechanical level.

Experimental Separation of Rashba and Dresselhaus Spin Splittings in Semiconductor Quantum Wells

The relative strengths of Rashba and Dresselhaus terms describing the spin-orbit coupling in semiconductor quantum well (QW) structures are extracted from photocurrent measurements on n-type InAs QWs containing a two-dimensional electron gas (2DEG). This novel technique makes use of the angular distribution of the spin-galvanic effect at certain directions of spin orientation in the plane of a QW. The ratio of the relevant Rashba and Dresselhaus coefficients can be deduced directly from experiment and does not relay on theoretically obtained quantities. Thus our experiments open a new way to determine the different contributions to spin-orbit coupling.

Spin-galvanic effect

There is much recent interest in exploiting the spin of conduction electrons in semiconductor heterostructures together with their charge to realize new device concepts. Electrical currents are usually generated by electric or magnetic fields, or by gradients of, for example, carrier concentration or temperature. The electron spin in a spin-polarized electron gas can, in principle, also drive an electrical current, even at room temperature, if some general symmetry requirements are met. Here we demonstrate such a ‘spin-galvanic’ effect in semiconductor heterostructures, induced by a non-equilibrium, but uniform population of electron spins. The microscopic origin for this effect is that the two electronic sub-bands for spin-up and spin-down electrons are shifted in momentum space and, although the electron distribution in each sub-band is symmetric, there is an inherent asymmetry in the spin-flip scattering events between the two sub-bands. The resulting current flow has been detected by applying a magnetic field to rotate an optically oriented non-equilibrium spin polarization in the direction of the sample plane. In contrast to previous experiments, where spin-polarized currents were driven by electric fields in semiconductor, we have here the complementary situation where electron spins drive a current without the need of an external electric field.

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.

In-plane gates and nanostructures fabricated by direct oxidation of semiconductor heterostructures with an atomic force microscope

The surface of shallow Ga[Al]As heterostructures is locally oxidized with an atomic force microscope. The electron gas underneath the oxide is depleted. We demonstrate experimentally that these depleted regions of the two-dimensional electron gas can be made highly resistive at liquid nitrogen temperatures. Thus, local anodic oxidation of high electron mobility transistors with an atomic force microscope provides a novel method to define nanostructures and in-plane gates. Two examples, namely antidots and quantum point contacts as in-plane gate transistors have been fabricated and their performance at low temperatures is discussed.

Optical spectroscopy of a GaAs/AlGaAs quantum wire structure using near‐field scanning optical microscopy

We report the first spectroscopic study using a low temperature near‐field scanning optical microscope. We have studied an array of GaAs/AlGaAs cleaved edge overgrowth quantum wires. The three luminescence peaks originate from different structures in the sample: The (001)‐oriented multiple quantum wells, the (110)‐oriented single quantum well, and the quantum wires. The linewidth of the quantum wire emission is related to roughness in the (110)‐oriented single quantum well. Quenching of the multiple quantum wells and single quantum well emission near the quantum wires is attributed to diffusion of photoexcited carriers into the wires.

Silicon/germanium strained layer superlattices

High quality Si/Ge strained layer superlattices are achieved by low temperature molecular beam epitaxy on Si, SixGe1−x and Ge substrates. Various characterization techniques are used to obtain information on critical thickness, strain distribution, misfit dislocations, interface sharpness and superlattice periodicity. The band structure is strongly influenced by strain and zone folding effects. Two-dimensional electron systems can be realized in the wider gap Si layers due to the strain-induced lowering of the conduction band. New optical transitions in the infrared regime are observed with short period Si/Ge superlattices.

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.

Morphology and flexibility of graphene and few-layer graphene on various substrates

We report on detailed microscopy studies of graphene and few-layer graphene produced by mechanical exfoliation on various semiconducting substrates. We demonstrate the possibility to prepare and analyze graphene on (001)-GaAs, manganese p-doped (001)-GaAs, and InGaAs substrates. The morphology of graphene on these substrates was investigated by scanning electron and atomic force microscopies and compared to layers on SiO2. It was found that graphene sheets strongly follow the texture of the sustaining substrates independent on doping, polarity, or roughness. Furthermore resist residues exist on top of graphene after a lithographic step. The obtained results provide the opportunity to research the graphene-substrate interactions.

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.