At Japan’s National Institute for Materials Science, Takayoshi Sasaki and colleagues created the superconductor from layers of cobalt oxide separated by Na⁺ ions and H₂O. The new superconductor is unusual in containing H₂O, which has the effect of doubling the interlayer distance to 2 nm. This appears to be essential to triggering the superconducting behavior. The crystal structure of the oxyhydrate has some similarities to the hexagonal structure displayed by superconducting MgB₂.

The citation rate of #5 is being boosted by a technical development announced at Brookhaven National Laboratory early in 2004. Sangmoon Park, a chemist at Brookhaven, has devised a safe method of making the superconductor. His research has eliminated processes requiring volatile and inflammable liquids that produce much hazardous waste. His procedure uses plain water! (S. Park, et al., Phys. Rev. B, 68[18]: art. no. 180505, 2003.) The new synthesis process will be helpful in developing real-world applications because water-based superconductors are expected to be more versatile. Metal superconductors are brittle and cannot be made into pliable objects.

The Top Ten this period has a second paper #10 on low-temperature physics, which describes a truly remarkable way to make a molecular Bose-Einstein condensate (BEC). The starting point for the experiment, conducted at NIST (Boulder, Colorado), is the production of an ultracold degenerate gas of ⁴⁰K through laser trapping and evaporative cooling. This atomic gas obeys Fermi statistics. Markus Greiner and colleagues then used a magnetic field to tune the atom-atom interactions in this gas in such a way that atom pairs are converted to weakly bound ⁴⁰K₂ molecules. Critically, the survival time of the molecules can be much longer than the collision time in the gas. The NIST researchers observed a bimodal momentum distribution among these molecules, following which the phase transition to BEC occurred. Currently there are experiments running in several labs to unlock the physics of mixtures of BEC and Fermi gases at nanoKelvin temperatures, with a view to tuning the system continuously between the two limits of BEC and Fermi gas.

An enduring mystery in observational astronomy is the origin of gamma-ray bursts (GRBs), first noticed in the 1960s. Since 1997, the BeppoSAX satellite has provided rapid and accurate positions for several GRBs a year. Optical astronomers have seized on these prized coordinates to observe the faint afterglows, and thereby measure the redshifts, which establish that GRBs are at cosmological distances. The afterglow phenomenon suggests that at least some GRBs are associated with the core collapse of massive stars. However, direct evidence of a link between a supernova explosion and a GRB has been hard to secure. That’s because GRBs are typically at a redshift z ~ 1, which is too distant for obtaining a good spectrum of the accompanying star.

On March 29, 2003, the situation changed dramatically when the High Energy Transient Explorer II satellite registered an extremely intense GRB lasting 25s. This produced a bright afterglow which a host of ground-based telescopes observed for several weeks. By the end of the first week, the spectroscopic campaign led by Tom Matheson (Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts) began to see light from an underlying supernova emerge, and with the passing days this became more prominent. The subtraction of the continuum from the afterglow left a spectrum typical of a hypernova, a powerful supernova with high ejection speed. This category of exploding star arises as a result of sudden and catastrophic collapse of the core of a massive star. Hot Paper #7 provides the first solid evidence of a direct link between a gamma-ray burst and a hypernova collapse.