Hot Paper #7 reports for the first time the successful growth of p-type ZnO films by molecular beam epitaxy, a discovery which opens the door to junctions and quantum wells based on ZnO. The new process first requires the fabrication of ZnO film, which is achieved by heating pure Zn to 450 – 700° C in a flow of research-grade O₂. The team led by David Look (Wright State University, Dayton, Ohio) found that if a small amount of N₂ is added to the gas flux, p-type ZnO films were deposited with a thickness of 1.9 mm. Laboratory study of these films showed that they have the appropriate electrical properties for junction fabrication.

The ZnO excitement continues with newcomer #10, which announces a spectacular breakthrough in the growth of ZnO nanorods. One-dimensional nanowires and nanorods are attracting huge interest because of their physical properties and their potential for application to a great diversity of electronic and photonic devices. The growth of nanowires requires impurities as catalysts to precipitate the formation of droplets for deposition on a preferred site. Unfortunately the catalyst can be unintentionally incorporated as a contaminant detrimental to device fabrication.

A completely new approach to nanorod growth is heralded in #10, in which a group of physicists based at Pohang University, South Korea, demonstrate a unique catalyst-free method of making ZnO nanorods. Their method of chemical vapor deposition can produce high-quality ZnO nanorods on a large scale at low cost. For Science Watch, Gyu-Chui Yi made the following comment: "Since our method does not need any metal catalyst it enables us to grow high-purity nanorods. The nanorods grown by our method are vertically aligned on the substrate and exhibit uniform thickness and length distributions." More recent publications from this group report the realization of a metal/semiconductor junction, nanodiode arrays, nanorod quantum structures, and light-emitting nanorod devices. Paper #10 opens a window on a new world of amazing quantum devices.

The quantum physics theme continues with #9, reporting the properties of a degenerate Fermi gas. Whatever is that? To answer this question we need to remember that the elementary constituents of matter are divided into fermions and bosons, both of which behave according to their angular momentum, or spin. Fermions have an intrinsic spin that is an odd multiple of the Planck constant whereas the bosons acquire an even multiple. The behavioral properties come from quantum statistics, which dictate that at ultracold temperatures, the properties of fermions and bosons are dramatically different.

Paper #9 showcases the bizarre behavior of fermionic ⁶Li atoms, which the CO₂ laser cradles as a cigar-shaped cloud of atoms, 1 mm long and 0.1 mm wide. When the trap is suddenly switched off, the gas balloons along the transverse direction while staying motionless along the cigar axis. Ordinary gas would quickly assume a spherical shape. The anisotropy seen by the Duke researchers may be indicative of superfluidity. This laboratory realization of a strongly interacting Fermi gas provides a tabletop forum for checking the theory of more exotic fermionic matter, such as quark matter or the interiors of neutron stars. 

Dr. Simon Mitton is the Senior Fellow of
St Edmund’s College, University of Cambridge, U.K.