The discovery of superconductivity at 39 K in MgB 2 1 raises many issues. One of the central questions is whether this new superconductor resembles a hightemperature-cuprate superconductor or a lowtemperature metallic superconductor in terms of its current carrying characteristics in applied magnetic fields. In spite of the very high transition temperatures of the cuprate superconductors, their performance in magnetic fields has several drawbacks 2 . Their large anisotropy restricts high bulk current densities to much less than the full magnetic field-temperature (H-T) space over which superconductivity is found. Further, weak coupling across grain boundaries makes transport current densities in untextured polycrystalline forms low and strongly magnetic field sensitive 3,4 . These studies of MgB 2 address both issues. In spite of the multi-phase, untextured, nano-scale sub-divided nature of our samples, supercurrents flow throughout without the strong sensitivity to weak magnetic fields characteristic of Josephson-coupled grains 3 . Magnetization measurements over nearly all of the superconducting H-T plane show good temperature scaling of the flux pinning force, suggestive of a current density determine d by flux pinning. At least two length scales are suggested by the magnetization and magneto optical (MO) analysis but the cause of this seems to be phase inhomogeneity, porosity, and minority insulating phase such as MgO rather than by weakly coupled grain boundaries. Our results suggest that polycrystalline ceramics of this new class of superconductor will not be compromised by the weak link problems of the high temperature superconductors, a conclusion with enormous significance for applications if higher temperature analogs of this compound can be discovered.The principal samples were synthesized by direct reaction of bright Mg flakes (Aldrich Chemical) and sub-micron amorphous B powder (Callery Chemical). Starting materials were lightly mixed in half-gram batches, and pressed into pellets. These pellets were placed on Ta foil, which was in turn placed on Al 2 O 3 boats, and fired in a tube furnace under a mixed gas of 95% Ar 5% H 2 for 1 hour at 600 C, 1 hour at 800 C, and 1 hour at 900 C, and then lightly ground. The resulting powders were pressed into pellets and then hot pressed at 10 kbar at temperatures between 650 and 800 °C for periods between 1 and 5.5 hours. Disks ~4 mm in diameter and ~1 mm thick were cut from these pellets for property characterization. As noted later, this process cannot yet be considered optimum.Magnetization properties were examined in SQUID and vibrating sample magnetometers (VSM) in applied fields up to 14 T from 4.2 to above T c . Figure 1 shows onset T c values of 37-38 K for the above samples and for commercial MgB 2 powder (99.5%, ~2 µm diameter by examination, CERAC). Sample 1 and the commercial powder show smooth transitions with some temperature dependence of the zerofield cooled (ZFC) shielded moment, while sample 3 exhibits a step, indicative of non-uniformity in su...

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.

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

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Nature © Macmillan Publishers Ltd 1998 8 letters to nature 906 NATURE | VOL 392 | 30 APRIL 1998 peak I can be used for the fabrication of efficient polymer photovoltaic cells 4 .In excited states that have their origin in a transition from a delocalized orbital to a delocalized orbital, both carriers, electron and hole, are similarly extended, and therefore spatially overlapped. The same is true for excited states that involve transitions from a localized orbital to another localized orbital. However, in an excited state formed by a transition from a localized orbital to a delocalized orbital, the hole is confined to the phenylene ring, while the electron is delocalized over the chain (or vice versa). We consider that this is the reason for the higher photocurrent yields of these states.These results, which have antecedents in early work on molecular crystals (refs 25-27 and refs therein), have more general implications for the understanding of electronic excitations in low-dimensional systems 10,11 . The presence of charge-transfer states that separate more readily into free electron and holes can provide a natural description for the progression from molecular to semiconductor models of electronic excitations. Ⅺ

The commonly observed linear sections in the voltage-current characteristics of Ag-sheathed BSCCO-2212 and BSCCO-2223 composites were studied. One source of such characteristics is due to ohmic losses in the silver at the current feed points. Characteristic current transfer lengths are of the order of millimetres at 4.2 K and tenths of millimetres at 77 K. However, linear components can also be observed well away from current feed points. These were found to be associated with regions of locally reduced critical current, which provoke local transfer of the excess current into the Ag, thus producing local, ohmic V -I characteristics. Because of the finite current transfer length, some ohmic voltage can be detected even outside regions of reduced critical current. When the silver was removed from lower critical current regions, ohmic voltages were no longer observed, even when the critical current was severely reduced by local damage with a laser. Within the range of electric field covered by this experiment (0.05-100 µV cm −1 ), we conclude that all ohmic voltages are produced by current flow in Ag.

Due to its complex pathogenesis, the prevention and therapization of Alzheimer’s disease (AD) remains a serious challenge. Crocin, the main compound isolated from Crocus sativus L., demonstrates various pharmacological activities including anti-apoptotic properties. The present study investigated the neuroprotective effect of crocin and the underlying mechanisms. In l-glutamate-damaged HT22 cells, 3-h crocin pretreatment strongly enhanced the HT22 cell viability, reduced the apoptotic rate, mitigated mitochondrial dysfunction, suppressed intracellular reactive oxygen species (ROS) accumulation and Ca2+ overload compared with untreated cells. Additionally, crocin significantly decreased the expression levels of Bax, Bad and cleaved caspase-3 and increased the expression levels of B-cell lymphoma-extra large, phosphorylated (P-) protein kinase B and P-mammalian target of rapamycin compared with untreated cells. In mice with AD induced by d-galactose and aluminum trichloride, crocin substantially improved the cognition and memory abilities of the mice as measured by their coordination of movement in an open field test, and reduced their escape time in the Morris water maze test compared with untreated mice. Biochemical analysis confirmed that crocin was able to reduce the Aβ1-42 content in the mouse brains, increase the levels of glutathione peroxidase, superoxide dismutase, acetylcholine and choline acetyltransferase, and reduce the levels of ROS and acetylcholinesterase in the serum, cerebral cortex and hypothalamus compared with untreated mice. Immunohistochemical analysis demonstrated that crocin reduced Aβ1-42 deposition in the hippocampus of the brains of treated mice compared with untreated mice. In conclusion, crocin demonstrates good prospects in the treatment of AD through the oxidative stress-associated apoptosis signaling pathway.