Jae-Hyuk Choi

The role of an amorphous carbon layer on a multi-wall carbon nanotube attached atomic force microscope tip in making good electrical contact to a gold electrode

Multi-wall carbon nanotube (MWNT) attached atomic force microscope (AFM) tips (MWNT tips) have good potential for use in AFM lithography. Good conducting MWNT tips are needed in such applications. However, characterizing the conductance of MWNT tips is nontrivial: making a good electrical contact between the MWNT and electrode is difficult. We observed that MWNT tips produced by hydrocarbon-deposition attachment usually do not make good electrical contacts to gold electrodes because of the thin and rough amorphous carbon layer on the MWNT that was unintentionally deposited during the attachment. We found that good contacts can be made if a more amorphous carbon layer is deposited to form a thick and smooth amorphous carbon layer on MWNTs. Good contact was made either by transformation of the amorphous carbon layer into a conducting or peel-off layer, exposing the bare MWNT surface. MWNT tips with an exposed MWNT surface showed the well-known high-current-flowing capacity and the stepped-cutting behavior of bare MWNTs. The peeling-off behavior of a thick amorphous carbon layer may be utilized in producing bare-surfaced MWNT tips that have good conductance and therefore are useful for applications.

Critical Properties of Submicrometer-Patterned Nb Thin Film

We have studied transport properties of submicrometer-patterned Nb thin films. Critical parameters, such as transition temperature and critical current density, were measured as functions of the film width, ranging from 50 nm to 5000 nm, and thickness, from 10 nm to 150 nm. Nb films were deposited by dc magnetron sputtering on Si substrates and patterned by lift-off with e-beam lithography. For a given film thickness, superconducting transition temperature, T c, decreased with decreasing film width below 200 nm. In the thickness (d ) dependence, T c dropped drastically for d les 20 nm due to proximity effect of surface layers, which are formed by strain or oxidation. The critical current density J c for a given film thickness increased gradually with decreasing width and decreased sharply below 200 nm. The gradual J c increase for wide strips is analyzed to be due to edge barrier effect for flux entry near the transition. The sharp drop below 200 nm is ascribed to the width variation of the size of about 20 nm along the strip and contamination of the film edge. These results are useful for designing and analyzing submicron-line-based superconducting electronic devices.

Specific Heat of Field-dependent Magnetic Orderings in HoNi2B2C

We have investigated the low-temperature specific heat of HoNi 2 B 2 C down to T = 1 K in various magnetic fields from 0 T to 5 T. At zero field, the specific heat exhibits a large peak at T N = 5.1 K, which is due to commensurate antiferromagnetic ordering, and other smaller peak at T 1 = 5.5 K, which is related to incommensurate ordering. As the field is increased up to 0.5 T, the first peak broadens and moves to a lower temperature, while the other peak at T 1 moves to a higher temperature, which indicates that the phase between T N and T 1 is more favored in magnetic fields. Among possible candidates for the phase, the a -axis modulation phase, which is an extension of a low temperature metamagnetic state, is the most probable. In the high-field region above 1 T, the two-peak feature is replaced by a broad-hill one that fades away with increasing fields, which is the typical smearing feature of the magnetic order at high fields.

Calibration of the spring constants of various AFM cantilevers with the small uncertainty level of 2

Calibration of the spring constants of atomic force microscopy (AFM) cantilevers is one of the issues in biomechanics and nanomechanics for quantified force metrology at pico- or nano Newton level. In this paper, we present an AFM cantilever calibration system: the Nano Force Calibrator (NFC), which consists of a precision balance and a one-dimensional stage. Three types of AFM cantilevers (contact and tapping mode) with different shapes (beam and V) and spring constants (42, 1, 0.06 Nm-1) are investigated using the NFC. The calibration results show that the NFC can calibrate the micro cantilevers ranging from 0.01~100 Nm-1 with uncertainties of about 2%

Dielectric characterization of relaxation and curing behavior of oriental lacquer

Dielectric analysis was used to investigate the effects of temperature and humidity on the curing behavior of oriental lacquer and to characterize the dielectric properties of the lacquer film. It was found that the oriental lacquer could not cure to its hardened state at relative humidity less than 50% in ambient temperature and that the cure time could be shortened tremendously by increasing the curing temperature. In order to study the dielectric properties of oriental lacquer film, two films were prepared at different curing temperatures. The glass transition and secondary relaxation temperatures of ordinary oriental lacquer film, room temperature cured purified lacquer, were observed at 45 and -40°C, respectively. The high temperature cured purified lacquer film showed a secondary relaxation at around -50°C. The relationship between thermodynamic properties and chemical structures was explored based on the analysis of the dielectric relaxation behavior using Cole-Cole plots and the dielectric relaxation intensity Ae.

Control Schemes of Quantum-Based Pico-Newton Force Measurement

We propose control-measurement schemes for flux-quantum-based pico-newton force metrology. A micron-sized superconducting ring placed in magnetic field gradients can serve as a pico-newton force gauge. Constant-force steps can be realized by controlling the flux quanta in the superconductor ring. Two different schemes are proposed to control and measure the number of flux quanta in the ring. They are the fixed bias-flux mode and the fixed bias-current mode. In the fixed bias-flux (bias-current) mode, external bias-flux (transport bias-current) is fixed just below the resistive transition and entry of the flux quanta is monitored by counting the voltage bumps with increasing the bias current (the external flux). At the targeted number of flux quanta, bias current and flux are reset to zero. The number of flux quanta trapped in the ring can be confirmed by counting the voltage bumps with increasing the bias current up to the maximum critical current of the ring. Simultaneous flux control and force measurement is made by measuring force steps in field gradient with counting the number of flux quanta exciting the ring with increasing the bias current.

Raman study of the Hg0.7Cr0.3Sr2CuO4+δ superconductors

Abstract The local environment of the apical oxygens (O A ) in the Sr-substituted mercury-based superconductor