Visualizing landscapes of the superconducting gap in heterogeneous superconductor thin films: geometric influences on proximity effects

The proximity effect is a central feature of superconducting junctions as it underlies many important applications in devices and can be exploited in the design of new systems with novel quantum functionality. Recently, exotic proximity effects have been observed in various systems, such as superconductor-metallic nanowires and graphene-superconductor structures. However, it is still not clear how superconducting order propagates spatially in a heterogeneous superconductor system. Here we report intriguing influences of junction geometry on the proximity effect for a 2D heterogeneous superconductor system comprised of 2D superconducting islands on top of a surface metal. Depending on the local geometry, the superconducting gap induced in the surface metal region can either be confined to the boundary of the superconductor, in which the gap decays within a short distance (~ 15 nm), or can be observed nearly uniformly over a distance of many coherence lengths due to non-local proximity effects.Comment: 17 pages, 4 figure

Controlling spin-orbit coupling to tailor type-II Dirac bands

NiTe2, a type-II Dirac semimetal with strongly tilted Dirac band, has been explored extensively to understand its intriguing topological properties. Here, using density-functional theory (DFT) calculations, we report that the strength of spin-orbit coupling (SOC) in NiTe2 can be tuned by Se substitution. This results in negative shifts of the bulk Dirac point (BDP) while preserving the type-II Dirac band. Indeed, combined studies using scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) confirm that the BDP in the NiTe2-xSex alloy moves from +0.1 eV (NiTe2) to -0.3 eV (NiTeSe) depending on the Se concentrations, indicating the effective tunability of type-II Dirac fermions. Our results demonstrate an approach to tailor the type-II Dirac band in NiTe2 by controlling the SOC strength via chalcogen substitution. This approach can be applicable to different types of topological materials.Comment: 25 pages, 4 figure

Low temperature scanning tunneling microscope study of low-dimensional superconductivity on metallic nanostructures

textSuperconductivity is a remarkable quantum phenomenon in which a macroscopic number of electrons form a condensate of Cooper pairs that can be described by a single quantum wave function. According to the celebrated Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, there is a minimum length scale (the coherence length) below which the condensate has a rigid quantum phase. The fate of superconductivity in a system with spatial dimensions smaller than [the coherence length] has been the subject of intense interest for decades and recent studies of superconductivity in ultra-thin epitaxial metal films have revealed some surprising behaviors in light of BCS theory. Notably, it was found that superconductivity remains robust in thin lead films with thicknesses orders of magnitude smaller than the coherence length (i.e. in the extreme two dimensional limit). Such studies raise the critical question: what happens to superconductivity as all dimensions are reduced toward the zero dimensional limit? By controlling the lateral size of ultra thin 2D islands, we systematically address this fundamental question with a detailed scanning tunneling microscopy/spectroscopy study. We show that as the lateral dimension is reduced, the strength of the superconducting order parameter is also reduced, at first slowly for dimensions larger than the bulk coherence length, and then dramatically at a critical length scale of ~ 40nm. We find this length scale corresponds to the lateral decay length of the order parameter in an island containing regions of different heights and different superconducting strength. Overall, our results suggest that fluctuation corrections to the BCS theory are important in our samples and may need to be systematically addressed by theory.Physic

Magneto-Acupuncture Stimuli Effects on Ultraweak Photon Emission from Hands of Healthy Persons

AbstractWe investigated ultraweak photon emissions from the hands of 45 healthy persons before and after magneto-acupuncture stimuli. Photon emissions were measured by using two photomultiplier tubes in the spectral range of UV and visible. Several statistical quantities such as the average intensity, the standard deviation, the δ-value, and the degree of asymmetry were calculated from the measurements of photon emissions before and after the magneto-acupuncture stimuli. The distributions of the quantities from the measurements with the magneto-acupuncture stimuli were more differentiable than those of the groups without any stimuli and with the sham magnets. We also analyzed the magneto-acupuncture stimuli effects on the photon emissions through a year-long measurement for two subjects. The individualities of the subjects increased the differences of photon emissions compared to the above group study before and after magnetic stimuli. The changes on the ultraweak photon emission rates of hand for the magnet group were detected conclusively in the quantities of the averages and standard deviations

Estimation of mass transport parameters of gases for quantifying CH4 oxidation in landfill soil covers

Methane (CH4), which is one of the most abundant anthropogenic greenhouse gases, is produced from landfills. CH4 is biologically oxidized to carbon dioxide, which has a lower global warming potential than methane, when it passes through a cover soil. In order to quantify the amount of CH4 oxidized in a landfill cover soil, a soil column test, a diffusion cell test, and a mathematical model analysis were carried out. In the column test, maximum oxidation rates of CH4 (Vmax) showed higher values in the upper part of the column than those in the lower part caused by the penetration of O2 from the top. The organic matter content in the upper area was also higher due to the active microbial growth. The dispersion analysis results for O2 and CH4 in the column are counter-intuitive. As the upward flow rate of the landfill gas increased, the dispersion coefficient of CH4 slightly increased, possibly due to the effect of mechanical dispersion. On the other hand, as the upward flow rate of the landfill gas increased, the dispersion coefficient of O2 decreased. It is possible that the diffusion of gases in porous media is influenced by the counter-directional flow rate. Further analysis of other gases in the column, N2 and CO2, may be required to support this hypothesis, but in this paper we propose the possibility that the simulations using the diffusion coefficient of O2 under the natural condition may overestimate the penetration of O2 into the soil cover layer and consequently overestimate the oxidation of CH4.The financial support for this research was provided by the Brain Korea 21 Project. The authors also thank the Research Insti- Upward flow rate (ml min-1) 0 10 15 Ds O2 (cm2 sec-1) 0.010 0.015 0.020 0.025 0.030 R2 = 0.98 5 Fig. 10. Estimated diffusion coefficients of O2 under various upward flow rates. Table 4 Comparison of diffusion coefficients determined by diffusion cell tests and parameter estimation Derived Ds CH4 (cm2 s 1) Ds O2 (cm2 s 1) Derived from diffusion cell tests 0.0299 0.0231 Derived from parameter estimation Column A 0.0360 0.0200 Column B 0.0350 0.0160 Column C 0.0357 0.0190 Ds CH4 (cm2 sec-1) 0.025 0.030 0.035 0.040 0.045 0.050 Upward flow rate (ml min-1) 0 5 10 15 Fig. 9. Estimated dispersion coefficients of CH4 under various upward flow rates. 874 J. Im et al. / Waste Management 29 (2009) 869–875 tute of Engineering Science at Seoul National University for their technical assistance