Cold nuclei, magnetic order, and hot spins.

Striving for a new low in solid-state physics, researchers have chilled the nuclei of rhodium atoms to a temperature of 280 picokelvins (pK), just barely above absolute zero. These experiments have also produced negative temperatures as "high" as -750 pK.

The results stem from a long-standing research effort by Pertti J. Hakonen, Olli V. Lounasmaa, and their coworkers at the Helsinki University of Technology in Finland. Their goal is to study interactions between the spinning nuclei of metal atoms to glean insights into the nature of magnetism. Because these spin interactions are extremely weak, the experiments must be done at very low temperatures.

"Nuclear spins in metals provide good models to investigate magnetism," Hakonen and Lounasmaa report in the Sept. 23 SCIENCE.

Ordinarily, temperature (measured in kelvins) describes the average energy associated with either the motion of free particles, such as electrons, in a material or the vibration of particles bound to certain sites in the material. In a metal at ultralow temperatures, however, the spinning nuclei and the free electrons may coexist at different temperatures. Hakonen and his colleagues focus on the spin behavior of atomic nuclei, particularly silver and rhodium.

When placed in a magnetic field at temperatures below I microkelvin, these nuclei organize themselves so that their spins tend to line up in the same direction as the magnetic field. As the temperature gets closer to absolute zero, the number of misaligned nuclear spins decreases. Spins that are initially oriented in the opposite direction of the magnetic field flip over to a parallel orientation, which has a lower energy.

By suddenly reversing the magnetic field, the researchers can create a situation in which nearly all of the nuclear spins are oriented in the opposite direction, or antiparallel to the magnetic field. Because more nuclei are in the high-energy state than in the low-energy state, the nuclei are said to have a negative spin temperature.

In this case, achieving even lower negative temperatures means adding energy to flip more spins into the higher-energy, antiparallel state. Thus, negative temperatures are actually "hotter" than positive ones, the researchers say. Similarly, a temperature of -750 pK is, technically speaking, warmer than one of -800 pK.