W. C. Lee

Energy gap of quaternary compound superconductor YBi2B2C and LuNi2B2C by point-contact spectroscopy

Abstract Tunneling spectra of a single crystalline YNi 2 B 2 C ( T c = 15.4 K) and LuNi 2 B 2 C ( T c = 15.7 K) have been measured by a point-contact spectroscopy using a low-temperature scanning tunneling microscope. The conductance curve of these materials shows a smeared gap structure with an almost linear density of states inside of the gap region as well as a sizable zero-bias conductance. The linear conductance inside the gap region has been explained by a proximity-induced energy gap in the degraded surface normal layer. The resulting energy gap at 4.2 K is 3.2 meV and 3.3 meV for YNi 2 B 2 C and LuNi 2 B 2 C, respectively. A simple analysis using a smeared BCS density of states, on the other hand, yields energy gap of 2.55 meV and 2.65 meV. Temperature dependence of the energy gap in LuNi 2 B 2 C shows a reasonable agreement with the BCS type temperature dependence except a slight deviation near T c .

Surface barrier effect of isothermal magnetization of grain-aligned HgBa2Ca2Cu3O8

Abstract Isothermal magnetization of grain-aligned HgBa 2 Ca 2 Cu 3 O 8 as a function of the applied magnetic field parallel to the c -axis was measured. The curves were analyzed by using the extended critical state model which includes the combined effects of the surface barrier and the magnetization due to pinning. It is shown that the effect of the surface barrier in this mercury-based superconductor is significant especially at the high-temperature region.

Vortex fluctuation of grain-aligned HgBa2Ca2Cu3O8+δ

Abstract We have measured the reversible magnetization M(H, T) for grain-aligned HgBa 2 Ca 2 Cu 3 O 8+δ . Our data exhibit a clear vortex fluctuation about 6 K below the superconducting onset temperature. The derived in-plane magnetic penetration depth λ ab (0) is 1700 A which is similar to those of other high- T c superconductors. In addition, the zero-temperature GL coherence length ξ ab (0) is 22±5 A, which implies GL parameter ϰ =77±13. This ϰ is much greater than that of HgBa 2 CuO 6 , but similar to that of YBa 2 Cu 3 O 7 .

Determination of the precise upper critical field of YNi2B2C superconductor

Abstract The paramagnetic susceptibility of YNi 2 B 2 C single crystal in a high magnetic field is found to be about 2×10 −5 cm 3 /mol, which is one order of magnitude smaller than the previous reports. The paramagnetic behavior is caused by the sample impurity, not by the intrinsic magnetism of the YNi 2 B 2 C superconductor and shows simple Curie–Weiss behavior. In this case, an upper critical field of the YNi 2 B 2 C can be easily obtained by extracting the paramagnetic contribution. The upper critical field shows an anomalous behavior, the steep rising at the low temperature region.

Methods to Minimize or Adjust for Healthy Worker Effect in Occupational Epidemiology

Healthy worker effect (HWE) refers to the consistent tendency for actively employed individuals to have a more favorable mortality experience than the population at large. Although HWE has been well known since the 1970s, only a few studies in occupational epidemiology have attempted to fully define and evaluate HWE. HWE can be separated into effects on the initial hiring into the workforce (healthy worker hire effect) and those on continuing employment (healthy worker survival effect). In this review, we summarize the methods for minimizingor adjusting for the healthy worker effect available in occupational epidemiology. It is noteworthy that healthy worker survival effect appears complicated, considering that employment status plays simultaneous roles as a counfounding variable and intermediate variable, whereas healthy worker hire effect may be adjusted by incorporating health status at baseline into the statistical model. In addition, two retrospective cohort studies for workers in the semiconductor industry and Vietnam veterans in Korea, respectively, were introduced, and their results were explained in terms of healthy worker effect.

11B NMR measurement of vortex dynamics in YNi2B2C

Abstract 11 B pulsed nuclear magnetic resonance (NMR) measurements have been performed on a single crystal of YNi 2 B 2 C superconductor to investigate vortex structures and dynamics. With lowering temperature below T c , we have observed distinct features of NMR data; namely, thermal hysteresis of 11 B NMR shift, motional narrowing of line width and a double-peak structure of T 2 −1 . These features indicate significant thermal motion of vortices in spite of low T c and nearly isotropic 3D superconductor.

Various vortex phase transitions in a weakly disordered superconductor YNi2B2C single crystal

Abstract The mixed state of the weakly disordered superconductor, YNi 2 B 2 C, is found to be similar to that of high- T c materials due to its large fluctuations. The very sharp resistive transition with a kink and the S-shaped I–V curve appear below H = 1T indicating a lattice melting transition. The scaling behavior of the vortex glass transition in R-T and I – V curves for H ≥ 2 T is observed with critical exponents, ν = 1.23 ± 0.02 and z = 5.46 ± 0.32, which are field-independent. The vortex solid state changes from the vortex lattice at low fields to the vortex glass at high fields due to field-enhanced pinning effect. The type of multicriticality is not yet conclusive.

Magnetic phase transitions of the DyNi2B2C single crystal

Abstract The magnetization isotherms M(H) of DyNi 2 B 2 C single crystal were measured with magnetic fields perpendicular and parallel to the c axis. Three magnetically ordered states were observed only for fields perpendicular to the c axis. From the M(T,H) data, the magnetic phase diagram in the H - T plane was constructed for the H⊥ c-axis.

Ramans study of Y1−xPrxBa2Cu3O7−δ and YB2Cu3−xZnxO7−δ single crystals

Abstract Single crystals with known T c values of Y 1− x Pr x Ba 2 Cu 3 O 7−δ (YPr1:2:3) and YBa 2 Cu 3− x Zn 3− x Zn x O 7−δ (YZn1:2:3) systems are studied by Raman measurements. The Raman spectra for (YPr1:2:3) single crystals show that the frequencies of Ba and O z modes increase as the Pr content increases. The results are consistent with the hole-localization scheme proposed for the suppression of superconductivity in the polycrystalline YPr1:2:3 systems. On the other hand, in the YZn1:2:3 system, all the Raman modes do not change in frequencies. However, the FWHM of the Cu(2) mode increases with the decrease of T c , indicating strong scattering of charge carriers by the substituted Zn ions in the CuO 2 planes. The induced disorder in the CuO 2 planes may be related with suppression of T c in the YZn1:2:3 system. Thus, the suppression mechanism in the YZn1:2:3 systems seems to be different from that in the YPr1:2:3 systems.

Food Safety Perceptions on Pesticide Contamination among Koreans -Based on the 2008 Social Survey-

Objective: This study aimed to examine the level of risk perception related to pesticide contamination of food products and associated sociodemographic factors, as well as investigate the association with environmental pollution preventive behaviors. Methods: Based on data from Social Survey conducted in 2008 among a nationally representative sample of the population 15 years or older, we estimated proportions and 95% confidence intervals of the risk perception of pesticide contamination of food products. Logistic regression analyses were used to analyze the effects of sociodemographic status on risk perception. Results: Among respondents, those who feel anxious about the risks of pesticide contamination of domestic food products accounted for 40.4%, whereas the proportion rose to 87.0% with imported food products. These numbers showed that levels of anxiety about the pesticide contamination of imported food products were ranked at the top, and those for domestic products were at the bottom of the list of all environmental pollutants examined in the survey. Significantly related factors to anxiety about pesticide contamination of food products included women, the 40-50 age group, university graduates, married, non-manual employment, having school-aged children, and owning one`s own home. The population attempting to prevent environmental pollution showed higher levels of anxiety about pesticide contamination of food products than did those who were not taking such steps. Conclusion: Compared to other forms of environmental pollution, Koreans show the highest levels of anxiety about pesticide contamination of imported food products. Therefore, it is necessary to establish appropriate regulations and transparent risk communication with the public about the safety of imported food.