Byung-Chill Woo

Electric property evolution of structurally defected multilayer graphene.

We report on the influence of structural disorder on the electrical properties of multilayer graphene (MLG). Exponential decreases in the conductance and transconductance with increase of defects in the MLG were observed, which could be explained by the percolation and the variable range hopping conduction. An enhancement of p-type nature with increasing disorders was considered to be the result of oxygen doping in the graphene sheets introduced by oxygen plasma. The rapid increase of low-frequency noise was attributed to the formation of conductive network through the continuum percolation, as the low-frequency noise could be increased by the enhanced carrier scattering at the defect sites. We hope that our result should suggest a simple method of tuning the electrical properties of graphene.

Design of the KRISS watt balance

We report the design of the KRISS watt balance, which includes a magnet, a guiding stage and a coil position measurement system. The KRISS watt balance incorporates a closed-type cylindrical permanent magnet and a motion guiding stage. For the magnet, a flux shunt is used to reduce flux changes due to temperature variations. A piston gauge is used to achieve linearity in the motion guiding stage. In the weighing mode, the residual force between the weight of the test mass and the Lorentz force generated in a coil is measured by a commercial weighing cell. In the dynamic mode, a linear motor in the motion guiding stage vertically translates the coil and the weighing cell. The in-plane motion of the coil is measured by position sensors, and the out-of-plane motion is measured by single-pass homodyne interferometers.

Biosensor utilizing resist-derived carbon nanostructures

The authors present a biosensor using pyrolyzed electron beam resist nanostructures as an active conducting channel. Versatile, arbitrarily shaped nanostructures such as nanowires, nanodots, and suspended nanobridges are fabricated by a facile electron beam resist thermal decomposition method. The nanostructures typically show 15–21nm thickness, 100–200nm width, 0.6nm roughness, and p-type majority conduction with tailored resistivity of 5.2–0.75Ωcm. Streptavidin-biotin binding and pH dependent conductance modulation are demonstrated using pyrolyzed resist based devices.

Low-field method for measuring proton gyromagnetic ratio

A measurement of the proton gyromagnetic ratio in water by the low-field method is in progress. The Earths magnetic field is compensated down to 1 nT at the center of Helmholtz coils in the constructed nonmagnetic facilities. The solenoid former of fused silica has been fabricated by the screw grinding and lapping within the dimensional variations of a few micrometers. >

Collective dynamics of Josephson fluxons in Bi2Sr2CaCu2O8+δ intrinsic junctions

Abstract The dynamic characteristics of Josephson fluxons in naturally stacked intrinsic Josephson junctions in Bi2Sr2CaCu2O8+δ single crystals were studied experimentally. The Josephson fluxons were generated either by irradiating microwaves or by applying dc magnetic fields in parallel with the junction planes. In the presence of a finite tunneling bias current the fluxons are driven along the junctions in resonance with the excitation modes of Josephson plasma and thus exhibit a variety of dynamic characteristics. Thus in the flux-flow state of fluxons in both cases above, for a relatively low vortex density, sub-branch splitting of the ‘supercurrent’ branch in the current–voltage (I–V) characteristics was observed, which corresponded to different plasma excitation modes. For a higher vortex density, on the other hand, all the quasiparticle branches merged into a single non-hysteretic I–V curve which exhibited step-like kink features, very similar to the recently predicted ones corresponding to different moving fluxon patterns. The detailed agreement with the theoretical prediction, however, is yet to be confirmed.

Piston guide and magnet assembly for the KRISS watt balance

The piston guide and magnet assembly for the KRISS watt balance were tested. The velocity of the piston guide was controlled within 4×10-3 at the velocity of 2 mm s-1 using a linear motor and linear encoder feedback. The magnet assembly which consists of two permanent magnet rings and closed yokes was fabricated. B-field at the air gap of 25 mm was measured using a Hall probe. The B-field uniformity was within 5×10-4 over the vertical distance of 24 mm.

Gravity measurements for the KRISS watt balance

The gravity was investigated to obtain the local acceleration due to gravity at the position of the test mass in the KRISS watt balance system. Absolute gravity, vertical gravity gradient, and horizontal variations of gravity were measured. The vertical attraction force on the test mass due to the watt balance system was calculated using an analogue of gravity to the electrostatic force. The gravity correction due to major components of the KRISS watt balance was -33.4 μGal. The gravity study will contribute to the KRISS watt balance experiment to prepare forthcoming mass redefinition.

Anomalous negative magnetoresistance of multi-walled carbon nanotube with Ni78Fe22 electrodes

Abstract Electrical transport properties of individual multi-walled carbon nanotubes in contact with the ferromagnetic electrodes, Ni78Fe22, were studied at low temperatures. The magnetoresistance curve was non-hysteretic and exhibited pronounced dip structure at the external field of 160 mT . The magnetoresistance ratio depends strongly both on bias current and temperature.

Micromachined thin-film multijunction thermal converters developed at KRISS

Micromachined thin-film multijunction thermal converters (MJTCs) suitable as high performance ac-dc transfer standards have been fabricated and studied at KRISS. This paper describes their thermal design and the materials chosen to improve performance. And finally performance data are given over a wide range of frequencies and conditions.

Well-defined plateaus of the conductance in two-terminal device of nonsuspended graphene

Quantized conductance, reflecting the half-integer quantum Hall effect in a non-suspended single-layer graphene, was clearly observed in a two-terminal device at 2 K and 8 T, particularly at ±2e2/h and ±6e2/h. Dependence of the conductance quantization upon temperature and the magnetic field strength was systematically investigated. In the case of v = +2, the quantized behavior was discernable even at 180 K. Resistance of Dirac point decreased linearly with decrease of temperature and increases below 20 K, implying the behavior of quantum Hall insulator at high magnetic field.