C. S. Wu

Evidence for suppression of superconductivity by spin imbalance in Co-Al-Co single-electron transistors.

Spin imbalance can lead to suppression of superconductivity. We report the phenomena manifesting this effect under spin-polarized quasiparticle currents in ferromagnet-superconductor-ferromagnet single-electron transistors. The measured superconducting gap as a function of magnetic field reveals a dramatic decrease when the magnetizations of the two leads are misaligned. The effect of suppression increases with increasing source-drain voltage. A comparison with theoretical calculations is presented. This method may render it applicable to control superconductivity at low temperatures within low fields.

Effects of focused gallium ion-beam implantation on properties of nanochannels on silicon-on-insulator substrates

Gallium dopants have been introduced into micrometer and nanometer sized silicon-on-insulator devices by means of focused ion beam maskless implantation. Structures of implanted devices before and after annealing have been characterized by cross-sectional transmission electron microscopy and Raman spectroscopy. The implanted/annealed micrometer devices exhibit uniformly lower electric resistance due to the presence of dopants; and the nanometer scale devices also show lower resistance but with a large device-to-device fluctuation. The fluctuation is likely to be the result of statistical nonuniformity in the spatial distribution of the end-of-range damage on the nanometer scale.

Coupled single-electron transistors as a differential voltage amplifier

We have investigated a possible application of single-electron transistor (SET) devices for use as a differential voltage amplifier. The device consists of a box-SET and probe-SET coupled with each other through a tunnel junction, with the gate electrodes of the two SETs acting as differential signal inputs. The voltage across the probe-SET at a fixed bias current provides information about the charge states of both the probe-SET and the box-SET, which was confirmed by simulations based on the orthodox theory of single-electron tunnelling. When operated as a differential amplifier, the output probe-SET voltage signal was measured as a function of the two gate input signals. While the output signal was found to be proportional to the difference in the two input signals, it remained unchanged for input signals of the same amplitude (referred to as the common mode signal), and the common-mode rejection ratio was found to be 27.5 dB.

Shot noise of multichannel transport in mesoscopic junction systems

Shot noise, which accompanies the emergence of the quantization nature (discreteness) of electrons, indicates dynamic fluctuations in electrical current and further reveals underlying operations in mesoscopic applications. This work presents a multichannel junction device with which to study the shot noise of coupled-channel transports as in normal nanocircuits, and it describes the theoretical algorithms. The numerical results show that, in contrast to the indistinguishable results of average current on direct-current measurements, the shot noise related to a dynamic current definitely exhibits significant differences among devices with diverse multichannel set-ups and presents super-Poissonian behaviors associated with the splitting of tunneling states.

Solution-processed vanadium oxide interlayer for improving the performance of polymer/ZnO nanorod hybrid solar cells

A solution-processed vanadium oxide interlayer is introduced between the organic layer and the electrode for improving the performance of the polymer/ZnO nanorod hybrid solar cells. The results indicate that the vanadium oxide interlayer can serve as an electron-blocking layer to suppress the leakage current and an optical spacer to increase light absorption. As a result, the power conversion efficiency is improved from 2.52% to 3.56%, with a fill factor of 60% under 100 mW/cm2 irradiation.

Magnetic-field and temperature dependence of the energy gap in InN nanobelt

We present tunneling measurements on an InN nanobelt which shows signatures of superconductivity. Superconducting transition takes place at temperature of 1.3K and the critical magnetic field is measured to be about 5.5kGs. The energy gap extrapolated to absolute temperature is about 110μeV. As the magnetic field is decreased to cross the critical magnetic field, the device shows a huge zero-bias magnetoresistance ratio of about 400%. This is attributed to the suppression of quasiparticle subgap tunneling in the presence of superconductivity. The measured magnetic-field and temperature dependence of the superconducting gap agree well with the reported dependences for conventional metallic superconductors.

Improved performance of polymer/ZnO nanorod hybrid solar cells by slow drying of the photoactive layer

The slow drying of the photoactive layer in polymer/ZnO nanorod hybrid solar cells is studied as a way to improve device performance. The power conversion efficiency can be further improved to 3.05% with an enhanced fill factor of 55% by slowing the spin coating rate of photoactive layer. With the slower spin coating rate, the films of photoactive layer are thicker, and the polymer chains have longer time to self-organize and more effectively infiltrate into ZnO nanorod spacing, resulting in the higher crystallinity of polymer and light harvesting without sacrificing the carrier transportation.

Performance enhancement of organic/inorganic hybrid solar cells by improving the optical absorption of polymer

The performance of the polymer/ZnO nanorod hybrid solar cells based on poly(3-hexylthiophene) and methanofullerenes is improved with the enhanced optical absorption by increasing the thickness of the photoactive layer and introducing a solution-processed interlayer. The dependence of the optical absorption on the thickness of the photoactive layer is studied as a function of the spin-coating rate. With the slower spin-coating rate, the photoactive layer is thicker, and the polymer chains have longer time to self-organize and more effectively infiltrate into ZnO nanorod spacing. In addition, a solution-processed fullerene interlayer is introduced to modify the ZnO nanorod surface. With this interlayer, the optical absorption of the photoactive layer increases due to the better ordering of the photoactive layer. Our investigations show that the power conversion efficiency (PCE) is improved from 1.6% to 2.6% with the thickness of the photoactive layer from 240 nm to 350 nm by slowing the spin coating rate of the photoactive layer. Moreover, the PCE is also improved by the fullerene interlayer. The slow-drying method and the solution-processed fullerene interlayer both improve the crystallinity of the polymer and light harvesting.

One-dimensional arrays of superconducting quantum interference devices as magnetic-field-tuned superconducting detectors

This study experimentally demonstrates a superconducting detector that can be switched for charge signal and superconducting phase signal detection by tuning a magnetic field. In zero magnetic field, the one-dimensional array of superconducting quantum interference devices exhibits superconducting coherency and is phase sensitive. The array may become insulating and charge sensitive when a small perpendicular magnetic field is applied.

Cyclotron localization in a sub-10-nm silicon quantum dot single electron transistor

The authors have fabricated and measured a lateral Si single electron transistor consisting of a succession of a big island and small quantum dots. The big island gives rise to a small period Coulomb oscillation riding on the large irregular oscillation arising from the small quantum dots. The peaks of the latter shift in the presence of a magnetic field, which is analyzed in the context of field-induced Landau level shift with a soft-wall confinement potential. Furthermore, the current peak was suppressed for fields beyond a threshold value. An explanation based on cyclotron localization at noninteracting Landau levels is presented.