Suppression of low-energy Andreev states by a supercurrent in YBa_2Cu_3O_7-delta

We report a coherence-length scale phenomenon related to how the high-Tc order parameter (OP) evolves under a directly-applied supercurrent. Scanning tunneling spectroscopy was performed on current-carrying YBa_2Cu_3O_7-delta thin-film strips at 4.2K. At current levels well below the theoretical depairing limit, the low-energy Andreev states are suppressed by the supercurrent, while the gap-like structures remain unchanged. We rule out the likelihood of various extrinsic effects, and propose instead a model based on phase fluctuations in the d-wave BTK formalism to explain the suppression. Our results suggest that a supercurrent could weaken the local phase coherence while preserving the pairing amplitude. Other possible scenarios which may cause the observed phenomenon are also discussed.Comment: 6 pages, 4 figures, to appear in Physical Review

Scanning SQUID Susceptometry of a paramagnetic superconductor

Scanning SQUID susceptometry images the local magnetization and susceptibility of a sample. By accurately modeling the SQUID signal we can determine the physical properties such as the penetration depth and permeability of superconducting samples. We calculate the scanning SQUID susceptometry signal for a superconducting slab of arbitrary thickness with isotropic London penetration depth, on a non-superconducting substrate, where both slab and substrate can have a paramagnetic response that is linear in the applied field. We derive analytical approximations to our general expression in a number of limits. Using our results, we fit experimental susceptibility data as a function of the sample-sensor spacing for three samples: 1) delta-doped SrTiO3, which has a predominantly diamagnetic response, 2) a thin film of LaNiO3, which has a predominantly paramagnetic response, and 3) a two-dimensional electron layer (2-DEL) at a SrTiO3/AlAlO3 interface, which exhibits both types of response. These formulas will allow the determination of the concentrations of paramagnetic spins and superconducting carriers from fits to scanning SQUID susceptibility measurements.Comment: 11 pages, 13 figure

Measurement of Cosmic-ray Muons and Muon-induced Neutrons in the Aberdeen Tunnel Underground Laboratory

We have measured the muon flux and production rate of muon-induced neutrons at a depth of 611 m water equivalent. Our apparatus comprises three layers of crossed plastic scintillator hodoscopes for tracking the incident cosmic-ray muons and 760 L of gadolinium-doped liquid scintillator for producing and detecting neutrons. The vertical muon intensity was measured to be $I_{\mu} = (5.7 \pm 0.6) \times 10^{-6}$ cm$^{-2}$s$^{-1}$sr$^{-1}$. The yield of muon-induced neutrons in the liquid scintillator was determined to be $Y_{n} = (1.19 \pm 0.08 (stat) \pm 0.21 (syst)) \times 10^{-4}$ neutrons/($\mu\cdot$g$\cdot$cm$^{-2}$). A fit to the recently measured neutron yields at different depths gave a mean muon energy dependence of $\left\langle E_{\mu} \right\rangle^{0.76 \pm 0.03}$ for liquid-scintillator targets.Comment: 14 pages, 17 figures, 3 table

Local tunneling probe of (110) Y_0.95Ca_0.05Ba_2Cu_3O_7-delta thin films in a magnetic field

Scanning tunneling spectroscopy was performed on (110)-oriented thin films of Ca-overdoped Y$_{0.95}$Ca$_{0.05}$Ba$_2$Cu$_3$O$_{7-\delta}$ at 4.2K, to probe the local evolution of Andreev$-$Saint-James surface states in a c-axis magnetic field. In zero field, we observed conductance spectra with spontaneously-split peaks and spectra with unsplit zero-bias peaks. The former showed enhanced splitting with field, and the latter showed threshold splitting above finite fields. Although both field evolutions can be described in terms of screening and orbital supercurrents, within the framework of $d\pm i\alpha$ pairing ($d$=$d_{x^2-y^2}$; $\alpha$=$d_{xy}$,$s$), the enhanced splitting is consistent with only the $d$ + $i\alpha$ state. Our results have direct implications on the stability of broken time-reversal symmetry in cuprate superconductors.Comment: 4 pages, 4 figure

Development and Evaluation of a Positive Youth Development Course for University Students in Hong Kong

With higher education, university graduates are important elements of the labor force in knowledge-based economies. With reference to the mental health and developmental problems in university students, there is a need to review university's role in nurturing holistic development of students. Based on the positive youth development approach, it is argued that promoting intrapersonal competencies is an important strategy to facilitate holistic development of young people in Hong Kong. In The Hong Kong Polytechnic University, a course entitled Tomorrow's Leader focusing on positive youth development constructs to promote student well-being will be offered on a compulsory basis starting from 2012/13 academic year under the new undergraduate curriculum structure. The proposed course was piloted in 2010/11 school year. Different evaluation strategies, including objective outcome evaluation, subjective outcome evaluation, process evaluation, and qualitative evaluation, are being carried out to evaluate the developed course. Preliminary evaluation findings based on the piloting experience in 2010/11 academic year are presented in this paper

Crystallization and preliminary crystallographic analysis of the central domain of Drosophila Dribble, a protein that is essential for ribosome biogenesis. Addendum

An addition to the paper by Cheng et al. [(2010), Acta Cryst. F66, 546–548]