The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.

We report a significant enhancement of the upper critical field H c2 of different MgB 2 samples alloyed with nonmagnetic impurities. By studying films and bulk polycrystals with different resistivities ρ, we show a clear trend of H c2 increase as ρ increases. One particular high resistivity film had zero-temperature H c2 (0) well above the H c2 values of competing non-cuprate superconductors such as Nb 3 Sn and Nb-Ti. Our high-field transport measurements give record values H c2 ⊥ (0) ≈ 34T and H c2 || (0) ≈ 49 T for high resistivity films and H c2 (0) ≈ 29 T for untextured bulk polycrystals. The highest H c2 film also exhibits a significant upward curvature of H c2 (T), and temperature dependence of the anisotropy parameter γ(T) = H c2 || / H c2⊥ opposite to that of single crystals: γ(T) decreases as the temperature decreases, from γ(T c ) ≈ 2 to γ(0) ≈ 1.5. This remarkable H c2 enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivity in MgB 2 , which offers special opportunities for further H c2 increase by tuning of the impurity scattering by selective alloying on Mg and B sites. Our experimental results can be explained by a theory of two-gap superconductivity in the dirty limit. The very high values of H c2 (T) observed suggest that MgB 2 can be made into a versatile, competitive high-field superconductor.

Effects of induced biaxial strain on the electrical transport and magnetic properties of epitaxial thin films of SrRuO 3 and La 0.67 Sr 0.33 MnO 3 by structural transitions of ferroelectric BaTiO 3 substrates have been studied. Large jumps of electrical resistivity ͑ϳ5% in SrRuO 3 and ϳ12% in La 0.67 Sr 0.33 MnO 3 ͒ and low field magnetization ͑ϳ70% in La 0.67 Sr 0.33 MnO 3 ͒ have been observed in the films at the structural transition temperatures of BaTiO 3 substrate. The hysteretic jumps are reproducible through many thermal cycles, and they can be attributed to strain effects induced by the substrate. The use of phase transitions of ferroelectric substrates to manipulate lattice strain of epitaxial thin film heterostructures can be a useful way to modify the properties of perovskite oxides.

Two-dimensional (2D) piezoelectric hexagonal boron nitride nanoflakes (h-BN NFs) were synthesized by a mechanochemical exfoliation process and transferred onto an electrode line-patterned plastic substrate to characterize the energy harvesting ability of individual NFs by external stress. A single BN NF produced alternate piezoelectric output sources of ∼50 mV and ∼30 pA when deformed by mechanical bendings. The piezoelectric voltage coefficient (g 11 ) of a single BN NF was experimentally determined to be 2.35 × 10 −3 V•m•N −1 . The piezoelectric composite composed of BN NFs and an elastomer was spin-coated onto a bulk Si substrate and then transferred onto the electrode-coated plastic substrates to fabricate a BN NFs-based flexible piezoelectric energy harvester (f-PEH) which converted a piezoelectric voltage of ∼9 V, a current of ∼200 nA, and an effective output power of ∼0.3 μW. This result provides a new strategy for precisely characterizing the energy generation ability of piezoelectric nanostructures and for demonstrating f-PEH based on 2D piezomaterials.

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