An important predicted, but so far uncharacterized, property of the new superconductor MgB 2 is electronic anisotropy arising from its layered crystal structure. Here we report on three caxis oriented thin films, showing that the upper critical field anisotropy ratio H c2 || /H c2 ⊥ is 1.8 to 2.0, the ratio increasing with higher resistivity. Measurements of the magnetic field-temperature phase diagram show that flux pinning disappears at H* ≈ 0.8H c2is strongly enhanced by alloying to 39 T for the highest resistivity film, more than twice that seen in bulk samples.The discovery of superconductivity at almost 40 K in MgB 2 has reawakened the search for high critical temperature T c in compounds with light elements [1]. In spite of the high T c of bulk MgB 2 samples, the upper critical field H c2 (T) at which bulk superconductivity is destroyed and the irreversibility field H*(T) at which bulk supercurrent densities disappear are both comparatively low. The maximum extrapolations of µ 0 H c2 (0) give 16-18 T, while H*(0) is about 0.5H c2 (0) [2-8]. µ 0 H*(4.2 K) is thus 7 T, well below the 10.5 T irreversibility field of Nb47wt.%Ti, for which T c is 9 K and µ 0 H c2 (4.2 K) is ~12 T [9]. At present it is not known whether the low irreversibility field of MgB 2 is related to its electronic anisotropy, a problem that is well known in the strongly anisotropic, high-temperature copperoxide superconductors [10]. Since MgB 2 consists of alternating

Motivated by emerging needs for accurate force measurements in the nanotechnology and biophysics areas, we present an atomic force microscope (AFM) cantilever calibration system, the ‘nano force calibrator’ (NFC), consisting of a microbalance and a precision translation stage. Calibration using the NFC has proved to be a reliable and accurate method through a series of experiments with a commercial piezoresistive AFM cantilever. In these experiments, linearity, repeatability and reproducibility of measurements were investigated along with the effects of calibration conditions, such as orientation of the cantilever and temperature. Uncertainty analysis shows that the stiffness and force sensitivity are determined to be 3.385 N m−1 and 0.6490 µN Ω−1, which are traceable to the Système International d'Unités (SI units). The relative standard uncertainties of both the stiffness and sensitivity are approximately 0.4% or conservatively 0.5%.

We have demonstrated the feasibility of using the nano force calibrator (NFC), consisting of a microbalance and a nano-stage, as a calibration device, which can accurately determine normal spring constants (k) of various atomic force microscope (AFM) cantilevers with traceability to the Système International d'Unités (SI units). From very compliant (k < 0.1 N m−1) to stiff (k > 10 N m−1) cantilevers, three types of commercial levers with different shapes (beam and V) and operating modes (contact and tapping) were chosen to test NFC calibration performances. We have found that all types of levers could be well characterized by the NFC even when a small force (approximately 500 nN) was used to calibrate a soft cantilever (k < 0.1 N m−1). We declared the relative standard uncertainty of the spring constant calibration of our method to be better than 1%, based on calibration results and uncertainty analysis. Because of its small calibration uncertainty, the NFC is recommendable for accurate calibration of AFM cantilevers and as a reference method for assessing other popularly used calibration methods.

We report measurements of the transport and the magnetic properties of high-quality, sub-millimeter-sized MgB 2 single crystals with clear hexagonal-plate shapes. The low-field magnetization and the magnetic hysteresis curves show the bulk pinning of these crystals to be very weak. The Debye temperature of ⌰ D ϳ1100 K, obtained from the zero-field resistance curve, suggests that the normal-state transport properties are dominated by electron-phonon interactions. The resistivity ratio between 40 K and 300 K was about 5, and the upper critical field anisotropy ratio was 3.0Ϯ0.2 at temperatures around 32 K.

We present a highly elastic and wearable piezoresistive strain sensor based on three-dimensional, micro-porous graphene-coated polydimethylsiloxane (PDMS) sponge suitable for being attached on human skin. The proposed strain sensors are simply fabricated by a sugar templating process and dip coating method based graphene ink in a facile and cost effective manner. The fabricated graphene-coated PDMS sponge shows highly stable mechanical properties in various tensile stress-strain test. A graphene thin film coated onto the backbone of PDMS sponges is used as the sensing materials of piezoresisitve strain sensors. The changes in resistance of the devices are highly stable, repeatable, and reversible when various strain is applied. Furthermore, the strain sensors show excellent sensing performance under different strain rate and mechanically robustness enough to be worked stably under repeated loads without any degradation.