Planar Superconductor-Normal-Superconductor Josephson Junctions in MgB2

Since the discovery of superconductivity in MgB2 considerable progress has been made in determining the physical properties of the material, which are promising for bulk conductors. Tunneling studies show that the material is reasonably isotropic and has a well-developed s-wave energy gap (∆), implying that electronic devices based on MgB2 could operate close to 30K. Although a number of groups have reported the formation of thin films by post-reaction of precursors, heterostructure growth is likely to require considerable technological development, making single-layer device structures of most immediate interest. MgB2 is unlike the cuprate superconductors in that grain boundaries do not form good Josephson junctions, and although a SQUID based on MgB2 nanobridges has been fabricated, the nanobridges themselves do not show junction-like properties. Here we report the successful creation of planar MgB2 junctions by localised ion damage in thin films. The critical current (IC) of these devices is strongly modulated by applied microwave radiation and magnetic field. The product of the critical current and normal state resistance (ICRN) is remarkably high, implying a potential for very high frequency applications.Comment: 7 pages including 4 figure

Spontaneous magnetization and Hall effect in superconductors with broken time-reversal symmetry

Broken time reversal symmetry (BTRS) in d wave superconductors is studied and is shown to yield current carrying surface states. The corresponding spontaneous magnetization is temperature independent near the critical temperature Tc for weak BTRS, in accord with recent data. For strong BTRS and thin films we expect a temperature dependent spontaneous magnetization with a paramagnetic anomaly near Tc. The Hall conductance is found to vanish at zero wavevector q and finite frequency w, however at finite q,w it has an unusual structure.Comment: 7 pages, 1 eps figure, Europhysics Letters (in press

Evaluation of an intervention program to promote neck care for computer user among staff and students of a Malaysian public university

Background: Computers are one of the most useful equipment developed in this electronic era. However incorrect posture and prolonged use of computers are risk factors for developing neck pain. A health promotion program was developed to educate computer users regarding neck care. The program was then evaluated to observe its effects in increasing knowledge, attitude and practice among the respondents. Materials and Methods: This was an intervention study conducted in a local public university. A set of materials consisting of a pamphlet, posters and questionnaire, were developed for the study. The pamphlet and posters were based on a study on exercises performed while sitting, while the questionnaire was based on a study assessing a neck care intervention program. The materials were assessed and validated prior to the program. Study variables were respondents’ characteristics, knowledge, attitude and practice/intention to practice neck care. The knowledge, attitude and practice/intention to practice were measured at baseline and post intervention and the changes were compared. Result: A total of 41 respondents participated. At baseline, only 61% of the respondents knew that daily computer use for 2 hours or more increased the risk of developing neck problem, however the percentage increased significantly after the program (P=0.001). The attitude toward the necessity to do exercise while working in front of the computer was 85.5% at the beginning but had also increased significantly after the program (P=0.008). All parameters forming the intention to practice good neck care (i.e. to sit with straight upper back, to do simple neck exercise while working in front of the computer and to adjust sitting position to maintain good posture) were significantly increased (P<0.05) after the intervention program. Conclusion: The knowledge and attitude regarding good neck care and the intention to practice good neck care significantly improved after the intervention program. More intervention programs to promote neck care for computer users should be carried out

Temperature and junction-type dependency of Andreev reflection in MgB2

We studied the voltage and temperature dependency of the dynamic conductance of normal metal-MgB2 junctions obtained either with the point-contact technique (with Au and Pt tips) or by making Ag-paint spots on the surface of high-quality single-crystal-like MgB2 samples. The fit of the conductance curves with the generalized BTK model gives evidence of pure s-wave gap symmetry. The temperature dependency of the gap, measured in Ag-paint junctions (dirty limit), follows the standard BCS curve with 2Delta/kTc = 3.3. In out-of-plane, high-pressure point contacts we obtained almost ideal Andreev reflection characteristics showing a single small s-wave gap Delta = 2.6 +/- 0.2 (clean limit). These results support the two-gap model of superconductivity, the presence of a modified layer at the surface of the crystals and an important and non-conventional role of the impurities in MgB2.Comment: 5 pages, 4 eps figures, SNS 2001 conferenc

Symmetry and Temperature dependence of the Order parameter in MgB2 from point contact measurements

We have performed differential conductance versus voltage measurements of Au/MgB2 point contacts. We find that the dominant component in the conductance is due to Andreev reflection. The results are fitted to the theoretical model of BTK for an s-wave symmetry from which we extract the value of the order parameter (Delta) and its temperature dependence. From our results we also obtain a lower experimental bound on the Fermi velocity in MgB2.Comment: 7 pages (Including figure captions) and 4 figure

Optical conductivity and penetration depth in MgB2

The complex conductivity of a MgB2 film has been investigated in the frequency range 4 cm^{-1}< nu < 30 cm^{-1} and for temperatures 2.7 K < T <300 K. The overall temperature dependence of both components of the complex conductivity is reminiscent of BCS-type behavior, although a detailed analysis reveals a number of discrepancies. No characteristic feature of the isotropic BCS gap temperature evolution is observed in the conductivity spectra in the superconducting state. A peak in the temperature dependence of the real part of the conductivity is detected for frequencies below 9 cm^{-1}. The superconducting penetration depth follows a T^2 behavior at low temperatures.Comment: 4 pages, 4 figure

Local and macroscopic tunneling spectroscopy of Y(1-x)CaxBa2Cu3O(7-d) films: evidence for a doping dependent is or idxy component in the order parameter

Tunneling spectroscopy of epitaxial (110) Y1-xCaxBa2Cu3O7-d films reveals a doping dependent transition from pure d(x2-y2) to d(x2-y2)+is or d(x2-y2)+idxy order parameter. The subdominant (is or idxy) component manifests itself in a splitting of the zero bias conductance peak and the appearance of subgap structures. The splitting is seen in the overdoped samples, increases systematically with doping, and is found to be an inherent property of the overdoped films. It was observed in both local tunnel junctions, using scanning tunneling microscopy (STM), and in macroscopic planar junctions, for films prepared by either RF sputtering or laser ablation. The STM measurements exhibit fairly uniform splitting size in [110] oriented areas on the order of 10 nm2 but vary from area to area, indicating some doping inhomogeneity. U and V-shaped gaps were also observed, with good correspondence to the local faceting, a manifestation of the dominant d-wave order parameter

A New Method of Probing the Phonon Mechanism in Superconductors including MgB2_{2}

Weak localization has a strong influence on both the normal and superconducting properties of metals. In particular, since weak localization leads to the decoupling of electrons and phonons, the temperature dependence of resistance (i.e., $\lambda_{tr}$) is decreasing with increasing disorder, as manifested by Mooij's empirical rule. In addition, Testardi's universal correlation of $T_{c}$ (i.e., $\lambda$) and the resistance ratio (i.e., $\lambda_{tr}$) follows. This understanding provides a new means to probe the phonon mechanism in superconductors including MgB$_{2}$. The merits of this method are its applicability to any superconductors and its reliability because the McMillan's electron-phonon coupling constant $\lambda$ and $\lambda_{tr}$ change in a broad range, from finite values to zero, due to weak localization. Karkin et al's preliminary data of irradiated MgB$_{2}$ show the Testardi correlation, indicating that the dominant pairing mechanism in MgB$_{2}$ is the phonon-mediated interaction.Comment: 9 pages, latex, 3 figure

Scanning Tunneling Spectroscopy in MgB 2

We present extensive Scanning Tunneling Spectroscopy (STM/S) measurements at low temperatures in the multiband superconductor MgB$_2$. We find a similar behavior in single crystalline samples and in single grains, which clearly shows the partial superconducting density of states of both the $\pi$ and $\sigma$ bands of this material. The superconducting gaps corresponding to both bands are not single valued. Instead, we find a distribution of superconducting gaps centered around 1.9mV and 7.5mV, corresponding respectively to each set of bands. Interband scattering effects, leading to a single gap structure at 4mV and a smaller critical temperature can be observed in some locations on the surface. S-S junctions formed by pieces of MgB$_2$ attached to the tip clearly show the subharmonic gap structure associated with this type of junctions. We discuss future developments and possible new effects associated with the multiband nature of superconductivity in this compound.Comment: 11 pages, 6 figures, submitted to Physica