Robert H. Blick

Anomalous Kondo Effect in a Quantum Dot at Nonzero Bias

We present measurements on the Kondo effect in a small quantum dot connected strongly to one lead and weakly to the other. The conductance of the dot reveals an offset of the Kondo resonance at zero magnetic field. While the resonance persists in the negative bias regime, it is suppressed in the opposite direction. This demonstrates the pinning of the Kondo resonance to the Fermi levels of the leads.

Magnetotransport measurements on freely suspended two-dimensional electron gases

We present magnetotransport measurements on freely suspended two-dimensional electron gases from AlxGa1-xAs/GaAs heterostructures. The technique to realize such devices relies on a specially molecular beam epitaxy grown GaAs/AlxGa1-xAs-heterostructure, including a sacrificial layer. We fabricated simple mini-Hall-bars as well as quantum cavities and quantum dot systems. We find well-pronounced Shubnikov–de Haas oscillations and observe commensurability resonances, allowing characterization of the electron gas in these 100-nm thin membranes.

Determination of the complex microwave photoconductance of a single quantum dot

A small quantum dot containing approximately 20 electrons is realized in a two-dimensional electron system of an AlxGa1-xAs/GaAs heterostructure. Conventional transport and microwave spectroscopy reveal the dot’s electronic structure. By applying a coherently coupled two-source technique, we are able to determine the complex microwave-induced tunnel current. The amplitude of this photoconductance resolves photon-assisted tunneling (PAT) in the nonlinear regime through the ground state and an excited state as well. The out-of-phase component (susceptance) allows us to study charge relaxation within the quantum dot on a time scale comparable to the microwave beat period.

Freely suspended two-dimensional electron gases

We present a new technique allowing us to build freely suspended two-dimensional electron gases from AlGaAs/GaAs/AlAs heterostructures. This technique relies on an MBE-grown structures that includes a sacrificial layer.

Upscaling high-quality CVD graphene devices to 100 micron-scale and beyond

We describe a method for transferring ultra large-scale chemical vapor deposition-grown graphene sheets. These samples can be fabricated as large as several cm2 and are characterized by magneto-transport measurements on SiO2 substrates. The process we have developed is highly effective and limits damage to the graphene all the way through metal liftoff, as shown in carrier mobility measurements and the observation of the quantum Hall effect. The charge-neutral point is shown to move drastically to near-zero gate voltage after a 2-step post-fabrication annealing process, which also allows for greatly diminished hysteresis.

Strain-induced Dirac state shift in topological insulator Bi2Se3 nanowires

In this study, we demonstrate the possibility to tune Dirac surface states of a three-dimensional topological insulator (TI) by applying external strain to single-crystalline Bi2Se3 nanowires (NWs). The NWs were placed over 200u2009nm deep trenches, which leads to a significant bending, resulting in tensile strain at the bottom surface of the wire and compressive strain at its top surface. By performing low-temperature magnetotransport measurements, we were able to show that TI surfaces under compressive or tensile strain ( ϵ=±0.1%) experience a significant Dirac shift of ΔE=∓30u2009meV as compared to relaxed surfaces. For surface states under tensile strain, an increased carrier mobility is indicated. The opportunity to externally tune the Dirac states therefore could lead to further improvement in future TI devices.

Guided Growth and Electrical Probing of Neurons on Arrays of Biofunctionalized GaAs/InGaAs Semiconductor Microtubes

We demonstrate embedded growth of cortical mouse neurons in dense arrays of semiconductor microtubes (see Figure (a,b)). The microtubes, fabricated from a strained GaAs/InGaAs heterostructure, guide axon growth through them and thus, enable the outgrowth of complex, artificial neuronal networks (see Figure (c)). At the same time, in situ electrical sensing is made possible. We present methods of stimulating and sensing action potentials, where electrodes are embedded inside the microtubes (see Figure d)). The wrapping of these electrodes around the axon greatly increases the contact area, and, with the fabrication of multiple electrodes along the tube length allow for the measurement of action potential propagation along single axons. The coaxial nature of the microtubes - similar to myelin - is expected to enhance the signal transduction along the axon.Our choice of GaAs, an optical III-V semiconductor, offers a variety of advantages over Si despite its toxicity: Its electron velocity and mobility is generally higher than, resulting in lower noise levels of possible electronic devices. We present a technique of suppressing arsenic toxicity and prove its efficiency by the results of neuronal cell culture.View Large Image | View Hi-Res Image | Download PowerPoint Slide

Effects of electron confinement on the acoustoelectric current in suspended quantum point contacts

An acoustoelectric current driven through a quantum point contact (QPC) on a suspended nanobridge by surface acoustic waves displays a non-trivial behavior in the presence of a perpendicular magnetic field. Our study reveals that the dependencies of this current on the QPC gate voltage and magnetic field can be explained by a variable material parameter σm. We develop a theoretical model for this phenomenon based on the modification of the Coulomb interaction and, correspondingly, the electron-SAWs coupling in the presence of the electron confinement.

Dynamic Rabi oscillations in a quantum dot embedded in a nanobridge in the presence of surface acoustic waves

A quantum dot is created within a suspended nanobridge containing a two-dimensional electron gas. The electron current through this dot exhibits well-pronounced Coulomb blockade oscillations. When surface acoustic waves (SAW) are driven through the nanobridge, Coulomb blockade peaks are shifted. To explain this feature, we derive the expressions for the quantum dot level populations and electron currents through these levels and show that SAW-induced Rabi oscillations lead to the observed phenomenology.

Dynamic control for nanostructures through slowly ramping parameters.

We propose a nanostructure control method which uses slowly ramping parameters. We demonstrate the dynamics of this method in both a nonlinear classical system and a quantum system. When a quantum mechanical two-level atom (quantum dot) is irradiated by an electric field with a slowly increasing frequency, there exists a sudden transition from ground (excited) to excited (ground) state. This occurs when the ramping rate is smaller than the square of the Rabi frequency. The transition arises when its instant frequency-the time derivative of the driving field phase-matches the resonance frequency, satisfying the Fermi golden rule. We also find that the parameter ramping is an efficient control manner for classical nanomechanical shuttles. For ramping of driving amplitudes, the shuttles mechanical oscillation is amplified and even survives when the ramping is stopped outside the original oscillation region. This strange oscillation is due to the entrance into a multistable dynamic region in phase space. For ramping of driving frequencies, an onset of oscillation arises when the instant frequency enters the oscillation region. Thus, regardless of being classical or quantum, the instant frequency is physically relevant. We discuss in which conditions the dynamic control is efficient.