Christian Heyn

Three-terminal energy harvester with coupled quantum dots

Rectification of thermal fluctuations in mesoscopic conductors is the key idea behind recent attempts to build nanoscale thermoelectric energy harvesters to convert heat into useful electric power. So far, most concepts have made use of the Seebeck effect in a two-terminal geometry, where heat and charge are both carried by the same particles. Here, we experimentally demonstrate the working principle of a new kind of energy harvester, proposed recently, using two capacitively coupled quantum dots. We show that, due to the novel three-terminal design of our device, which spatially separates the heat reservoir from the conductor circuit, the directions of charge and heat flow become decoupled. This enables us to manipulate the direction of the generated charge current by means of external gate voltages while leaving the direction of heat flow unaffected. Our results pave the way for a new generation of multi-terminal nanoscale heat engines.

Optical Modes Excited by Evanescent-Wave-Coupled PbS Nanocrystals in Semiconductor Microtube Bottle Resonators

We report on optical modes in rolled-up microtube resonators that are excited by PbS nanocrystals filled into the microtube core. Long ranging evanescent fields into the very thin walled microtubes cause strong emission of the nanocrystals into the resonator modes and a mode shift after a self-removal of the solvent. We present a method to precisely control the number, the energy and the localization of the modes along the microtube axis.

HIGH-SENSITIVE SUPERCONDUCTING MAGNETOMETRY ON A TWO-DIMENSIONAL ELECTRON GAS UP TO 10 TESLA

We report on new magnetization studies on a two-dimensional electron system (2DES) revealing spin splitting of the Landau levels. For this, we have built a high-sensitive susceptometer consisting of a low-noise thin-film dc superconducting quantum interference device (SQUID) with a multiturn input coil and a wire-wound first-order gradiometer. The system noise level is only 40×10 −6 Φ 0 /√ (Hz) down to a frequency of a few Hz in unshielded environment. In background fields up to 10 T, the system exhibits significant low-frequency noise. At frequencies above 1 kHz, however, the SQUID sensitivity is barely affected and we have reached a value of about 10 −14 J/T at 1 T and better than 10 −13 J/T at 10 T. With this, we have studied the de Haas–van Alphen effect for a tunable 2DES starting from zero carrier density.

Rolled-up nanotechnology for the fabrication of three-dimensional fishnet-type GaAs-metal metamaterials with negative refractive index at near-infrared frequencies

We propose and demonstrate the fabrication of a three-dimensional fishnet metamaterial by utilizing rolled-up nanotechnology. It consists of 6 alternating layers of silver and (In)GaAs with an array of subwavelength holes “drilled” by focused ion beams. By means of finite-integration technique simulations, we show that the fabricated structure is a single-negative material possessing a negative real part of the refractive index in the near-infrared regime. We show that the fabricatedmaterial can be made double negative by slightly changing the size of the holes.

Dynamics of mass transport during nanohole drilling by local droplet etching

Local droplet etching (LDE) utilizes metal droplets during molecular beam epitaxy for the self-assembled drilling of nanoholes into III/V semiconductor surfaces. An essential process during LDE is the removal of the deposited droplet material from its initial position during post-growth annealing. This paper studies the droplet material removal experimentally and discusses the results in terms of a simple model. The first set of experiments demonstrates that the droplet material is removed by detachment of atoms and spreading over the substrate surface. Further experiments establish that droplet etching requires a small arsenic background pressure to inhibit re-attachment of the detached atoms. Surfaces processed under completely minimized As pressure show no hole formation but instead a conservation of the initial droplets. Under consideration of these results, a simple kinetic scaling model of the etching process is proposed that quantitatively reproduces experimental data on the hole depth as a function of the process temperature and deposited amount of droplet material. Furthermore, the depth dependence of the hole side-facet angle is analyzed.

Correlation effects in wave function mapping of molecular beam epitaxy grown quantum dots.

We investigate correlation effects in the regime of a few electrons in uncapped InAs quantum dots by tunneling spectroscopy and wave function (WF) mapping at high tunneling currents where electron-electron interactions become relevant. Four clearly resolved states are found, whose approximate symmetries are roughly s and p, in order of increasing energy. Because the major axes of the p-like states coincide, the WF sequence is inconsistent with the imaging of independent-electron orbitals. The results are explained in terms of many-body tunneling theory, by comparing measured maps with those calculated by taking correlation effects into account.

Geometry-enhanced magnetoresistance of narrow Au∕InAs hybrid structures incorporating a two-dimensional electron system

We have investigated the magnetoresistance of metal-semiconductor hybrid structures at 4.2 K. The devices consisted of polycrystallineAu and an InAs-based heterostructure that hosted a high-mobility two-dimensional electron system. We have varied the electrical parameters and, in particular, the width-to-length ratio W ∕ L of the linear hybrid structures. The recently discovered extraordinary magnetoresistanceeffect is most pronounced for narrow devices with W ∕ L ⩽ 0.05 , consistent with our theoretical prediction. Relative resistance changes with a factor of 1000 are observed at 1 T. To achieve this an interfaceresistance of only 10 − 8 Ω cm 2 is a prerequisite. This can be prepared routinely by in situ cleaved edge overgrowth.

Far-infrared excitations below the Kohn mode: Internal motion in a quantum dot

We have investigated the far-infrared response of quantum dots in modulation doped GaAs heterostructures. We observe novel modes at frequencies below the center-of-mass Kohn mode. Comparison with Hartree-RPA calculations show that these modes arise from the flattened potential in our field-effect confined quantum dots. They reflect pronounced relative motion of the charge density with respect to the center-of-mass.

Imaging correlated wave functions of few-electron quantum dots: Theory and scanning tunneling spectroscopy experimentsa)

We show both theoretically and experimentally that scanning tunneling spectroscopy (STS) images of semiconductor quantum dots may display clear signatures of electron-electron correlation. We apply many-body tunneling theory to a realistic model, which fully takes into account correlation effects and dot anisotropy. Comparing measured STS images of freestanding InAs quantum dots with those calculated by the full configuration interaction method, we explain the wave-function sequence in terms of images of one- and two-electron states. The STS map corresponding to double charging is significantly distorted by electron correlation with respect to the noninteracting case.

Guided neuronal growth on arrays of biofunctionalized GaAs/InGaAs semiconductor microtubes

We demonstrate embedded growth of cortical mouse neurons in dense arrays of semiconductor microtubes. The microtubes, fabricated from a strained GaAs/InGaAs heterostructure, guide axon growth through them and potentially enable electrical and optical probing of propagating action potentials. The coaxial nature of the microtubes—similar to myelin—is expected to enhance the signal transduction along the axon. We present a technique of suppressing arsenic toxicity and prove the success of this technique by overgrowing neuronal mouse cells.