he Hot Papers in Physics for this period feature the rebirth of some former stars: Bose-Einstein condensate (#6) and the properties of superconducting copper oxides (#7, #9, #10). At the top of the table Lisa Randall and Raman Sundrum remain in a strong position at #1 on extra spatial dimensions.

A spectacular property of liquid ⁴He is its superfluidity at temperatures below 2.17 K, caused because many of the atoms dive into the same energy state, forming a Bose condensate. In this circumstance the atoms are forced to act in lockstep, and will assume identical quantum energy states. The superfluid behavior is revealed by a famous "rotating bucket" experiment in which liquid helium comes to rest inside a spinning container for a sufficiently small rotation rate. Above this critical rotation frequency the helium’s angular momentum is quantized, with a singularity called a vortex associated with each unit of angular momentum. This vortex concept is central to understanding superfluid behavior because all atoms in a vortex behave identically. One consequence of this marshalling of the atoms is entirely frictionless flow, since any energy-losing collisions with the wall of the container are forbidden by the requirement to maintain the same energy state for every atom.

In 1995 physicists succeeded in making a Bose-Einstein condensate (BEC) by chilling Rb atoms to nK temperatures, where they act in unison as a quantum blob which was expected to have properties in common with ⁴He. This experimental breakthrough stimulated theoretical work on the formation of vortices in atomic BEC. The He rotating bucket cannot be used for gaseous BEC because the vortices are larger than the magnetic trap, so a different method has to be used to transfer the quantized angular momentum.

Paper #6 reports the creation of vortices in ⁸⁷Rb, an isotope with two internal spin states, both of which can exist simultaneously to form a two-component BEC. The team lead by BEC pioneers Eric Cornell and Carl Weiman used a combination of microwave and laser techniques to induce conversion between the two components and stir in angular momentum. By optical interference techniques they produced vortices and observed their evolution on a time scale of ~1s. Just below the Top Ten a further paper on BEC vortices hovers in position #13 (K.W. Madison, et al.Phys. Rev. Lett.84: 806-09, 2000). These French physicists have used a focused laser beam as an optical spoon to stir up to four vortices at a time with lifetimes of 0.4 - 1 s. The multiple vortices are observed to mutually repel one another.

Superfluidity and superconductivity are strongly related, the latter being a special case of the former for a flowing medium carrying electrical charge. The high-temperature superconductors are ceramic copper oxides which carry electricity without resistance at temperatures below about -233 K. Pushing this critical temperature (T_c) higher, to room temperature even, is a major goal, which can only be realized with better understanding of chemical bonding and electrical charge distribution in the rare-earth copper oxides (or cuprates). Their Cu and O atoms are bonded in planes and chains, revealed by new imaging techniques.

Above T_c the electrical charge in several cuprates is arranged in patterns of stripes. The presence of these stripes in cuprate LSCO is the topic of #7 and #10. Both papers strive to understand whether the charge stripe order exists in the superconducting regime (temperature <T_c), and what importance, if any, it has for the superconducting phenomenon. Paper #10 describes a nuclear quadrupole resonance technique for measuring charge-stripe order in cuprates. The application of the technique to underdoped LSCO clearly shows the persistence of stripe order right through the entire superconducting regime. The study of stripe order has proved elusive, since it manages to elude experimental detection, but #7 looks at the same oxide through neutron powder-diffraction data. This technique shows how the topology of the CuO6 octahedra is related to the electronic system and the charge stripes.

Finally, on high-T_c materials is #9 on the pseudogap in the electron excitation spectrum of cuprate superconductors. Understanding the physics of these superconductors is a highest-priority intellectual puzzle. Guy Deutscher of Tel Aviv University suggests that BEC behavior or the formation of striped phases may provide a framework for getting to the bottom of what makes T_c ceramics tick. 

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