One-Lead Single-Electron Source with Charging Energy.

Single-electron sources, formed by a quantum dot (QD), are key elements for realizing electron analogue of quantum optics. We develop a new type of single-electron source with functionalities that are absent in existing sources. This source couples with only one lead. By an AC rf drive, it successively emits holes and electrons cotraveling in the lead, as in the mesoscopic capacitor. Thanks to the considerable charging energy of the QD, however, emitted electrons have energy levels a few tens of millielectronvolts above the Fermi level, so that emitted holes and electrons are split by a potential barrier on demand, resulting in a rectified quantized current. The resulting pump map exhibits quantized triangular islands, in good agreement with our theory. We also demonstrate that the source can be operated with another tunable-barrier single-electron source in a series double QD geometry, showing parallel electron pumping by a common gate driving.

Nanomachining-enabled strain manipulation of magnetic anisotropy in the free-standing GaMnAs nanostructures.

Strain perturbs atomic ordering in solids, with far-reaching consequences from an increased carrier mobility to localization in Si, stabilization of electric dipoles and nanomechanical transistor action in oxides, to the manipulation of spins without applying magnetic fields in n-GaAs. In GaMnAs, a carrier-mediated ferromagnetic semiconductor, relativistic spin-orbit interactions - highly strain-dependent magnetic interactions - play a crucial role in determining the magnetic anisotropy (MA) and anisotropic magnetoresistance (AMR). Strain modifies the MA and AMR in a nanomachined GaMnAs structure as measured by the anomalous Hall effect (AHE) and the planar Hall effect (PHE). Here, we report an MA modification by strain relaxation in an isolated GaMnAs Hall bar structure and by applying a range of local strains via fabricating asymmetrically mechanically buckled GaMnAs micro-Hall bar structures. In the AHE and PHE measurements, we observe a reduction in the in-plane MA and an enhancement in the out-of-plane MA as the compressive strain due to the lattice mismatch relaxes in the suspended structure. The functionality of such mechanical manipulation, as well as the two-level mechanical state and the corresponding AHE responses, is demonstrated by a fully scalable binary mechanical memory element in a GaMnAs single Hall cross structure.

Universality of periodicity as revealed from interlayer-mediated cracks.

A crack and its propagation is a challenging multiscale materials phenomenon of broad interest, from nanoscience to exogeology. Particularly in fracture mechanics, periodicities are of high scientific interest. However, a full understanding of this phenomenon across various physical scales is lacking. Here, we demonstrate periodic interlayer-mediated thin film crack propagation and discuss the governing conditions resulting in their periodicity as being universal. We show strong confinement of thin film cracks and arbitrary steering of their propagation by inserting a predefined thin interlayer, composed of either a polymer, metal, or even atomically thin graphene, between the substrate and the brittle thin film. The thin interlayer-mediated controllability arises from local modification of the effective mechanical properties of the crack medium. Numerical calculations incorporating basic fracture mechanics principles well model our experimental results. We believe that previous studies of periodic cracks in SiN films, self-de-bonding sol-gel films, and even drying colloidal films, along with this study, share the same physical origins but with differing physical boundary conditions. This finding provides a simple analogy for various periodic crack systems that exist in nature, not only for thin film cracks but also for cracks ranging in scale.

Free-Standing GaMnAs Nanomachined Sheets for van der Pauw Magnetotransport Measurements.

We report on the realization of free-standing GaMnAs epilayer sheets using nanomachining techniques. By optimizing the growth conditions of the sacrificial Al0.75Ga0.25As layer, free-standing metallic GaMnAs (with ~6% Mn) microsheets (with TC ~85 K) with integrated electrical probes are realized for magnetotransport measurements in the van der Pauw geometry. GaMnAs epilayer needs to be physically isolated to avoid buckling effects stemming from the release of lattice mismatch strain during the removal of the AlGaAs sacrificial layer. From finite element analysis, symmetrically placed and serpentine-shaped electrical leads induce minimal thermal stress at low temperatures. From magnetotransport measurements, changes in magnetic anisotropy are readily observed.

Tunable double and triple quantum dots in carbon nanotube with local side gates.

We demonstrate a simple but efficient design for forming tunable single, double and triple quantum dots (QDs) in a sub-µm-long carbon nanotube (CNT) with two major features that distinguish this design from that of traditional CNT QDs: the use of i) Al2Ox tunnelling barriers between the CNT and metal contacts and ii) local side gates for controlling both the height of the potential barrier and the electron-confining potential profile to define multiple QDs. In a serial triple QD, in particular, we find that a stable molecular coupling state exists between two distant outer QDs. This state manifests in anti-crossing charging lines that correspond to electron and hole triple points for the outer QDs. The observed results are also reproduced in calculations based on a capacitive interaction model with reasonable configurations of electrons in the QDs. Our design using artificial tunnel contacts and local side gates provides a simple means of creating multiple QDs in CNTs for future quantum-engineering applications.

Multimode circuit optomechanics near the quantum limit.

The coupling of distinct systems underlies nearly all physical phenomena. A basic instance is that of interacting harmonic oscillators, giving rise to, for example, the phonon eigenmodes in a lattice. Of particular importance are the interactions in hybrid quantum systems, which can combine the benefits of each part in quantum technologies. Here we investigate a hybrid optomechanical system having three degrees of freedom, consisting of a microwave cavity and two micromechanical beams with closely spaced frequencies around 32 MHz and no direct interaction. We record the first evidence of tripartite optomechanical mixing, implying that the eigenmodes are combinations of one photonic and two phononic modes. We identify an asymmetric dark mode having a long lifetime. Simultaneously, we operate the nearly macroscopic mechanical modes close to the motional quantum ground state, down to 1.8 thermal quanta, achieved by back-action cooling. These results constitute an important advance towards engineering of entangled motional states.

Scalable assembly method of vertically-suspended and stretched carbon nanotube network devices for nanoscale electro-mechanical sensing components.

For the first time, vertically suspended and stretched carbon nanotube network junctions were fabricated in large quantity via the directed assembly strategy using only conventional microfabrication facilities. In this process, surface molecular patterns on the side-wall of the Al structures were utilized to guide the assembly and alignment of carbon nanotubes in the solution. We also performed extensive experimental (electrical and mechanical) analysis and theoretical simulation about the vertically suspended single-walled carbon nanotube network junctions. The junctions exhibited semiconductor-like conductance behavior. Furthermore, we demonstrated gas sensing and electromechanical sensing using these devices.