Watson Kuo

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.

Controlled placement and electrical contact properties of individual multiwalled carbon nanotubes on patterned silicon chips

A scheme that allows on-chip growth of multiwalled carbon nanotubes at designed locations is demonstrated. The nanotubes were grown by thermal chemical vapor deposition and were contacted to nanoscaled Cr electrodes fabricated by standard e-beam lithography techniques. The contacts were found to be Ohmic with resistance values on the order of 103 Ω at room temperature. Remarkably, the contacts showed weak temperature dependence down to 40 mK and were insensitive to the magnetic field up to 5 T.

Anderson localization in strongly coupled gold-nanoparticle assemblies near the metal–insulator transition

An experimental study was made to explore the charge transport properties of a two-dimensional network of self-assembled gold nanoparticles, which were prepared near the metal-insulator transition. The insulating devices demonstrated single-charge tunneling and resonant tunneling at mK temperatures. A magnetic field perpendicular to the substrate could suppress the Coulomb oscillations, suggesting that the charge interactions were due to dynamical charge inhomogeneities, rather than single-nanoparticle charging effects.

Gate-controlled spin polarized current in ferromagnetic single electron transistors

Gate voltage can be used to tune polarization of current passing through a ferromagnetic single electron transistor when spin accumulate in the central electrode. The shift in spin chemical potential acts as charge offsetin the island and alternates the gate dependence of spin current. We demonstrate this phenomenon by applying master equation calculations to ferromagnetic/normal metal/ferromagnetic single electron transistors. Taking advantage of this effect, one can use ferromagnetic single electron transistors as a tunable current polarizer.

Phase diffusions due to radio-frequency excitations in one-dimensional arrays of superconductor/ insulator/superconductor junctions

The dc response of one-dimensional (1D) arrays of super- conductor/insulator/superconductor (SIS) junctions under radio-frequency (RF) irradiation has been experimentally studied. The zero-bias resistance as a function of RF amplitude in the low-frequency regime shows an oscillatory behaviour, and can be explained using the phase fluctuation model for Cooper- pair tunnelling. It follows that an effective classical detector theory is applicable and the oscillation period can be compared with the superconducting gap. The results suggest a primary and direct RF detection scheme using 1D SIS junction arrays.

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.

Large microwave response of one-dimensional Josephson junction arrays in charge dominant regime

By using photoresistance measurement, one-dimensional (1D) Josephson junction arrays can be used as primary radio-frequency and microwave detectors. The response can be explained by the microwave-enhanced phase diffusion both in the superconducting phase and charge dominant limits. Free from the screening effect due to mobile charges when the junctions were strongly coupled via the Josephson effect, the 1D array exhibited large microwave response in the charge dominant limit. Used as an in-line detector of guided microwaves, the array produces a negligible change of about in the microwave transmission.

Effect of the electromagnetic environment on the dynamics of charge and phase particles in one-dimensional arrays of small Josephson junctions

The effect of the electromagnetic environment on the dynamics of quasi-particles, Cooper pairs and phase particles in one-dimensional arrays of small Josephson junctions is investigated experimentally and theoretically. It is found that the environment enhances the phase ordering and thus suppresses quasi-particle tunneling at high temperature and localization of Cooper pairs at low temperature. The dynamics is studied in the context of phase-charge duality, and the experimental results are quantitatively analyzed in both charge-ordered and phase-ordered regimes. Based on these analyses, a low-temperature phase diagram as well as a finite-temperature crossover phase diagram are constructed and compared to the experimental diagrams.

Magnetoresistance fluctuations in a weak disorder indium nitride nanowire

We report measurements of magnetoresistance (MR) fluctuations in a weak disorder indium nitride nanowire. The MR fluctuations are reproducible, aperiodic and symmetric in magnetic field but are asymmetric upon reversal of bias direction. The fluctuations are analysed for both perpendicular and parallel external magnetic field configurations in the light of tunnel magnetoresistance at low field and impurity scattering at higher field. The asymmetry in bias reversal is caused by breakdown of time reversal symmetry.

Reduction of modal length using Josephson junction array confined cavity

We propose an architecture in which the coupling between a cavity and superconducting qubits is enhanced by reducing the cavity modal length. The cavity is connected to outside transmission lines at the both ends by two short Josephson junction arrays. The latter plays a role of distributed reflector, which can provide an arbitrary negative phase to the reflected wave below its photonic bandgap. With this phase contribution from the Josephson junction arrays, we show, in the context of Fabry-Perot resonance, that the modal length can be 2 orders shorter than the wavelength.