Hydrogen hoops give superfluid clues

Cooled below 2.18 kelvins, helium becomes extremely odd stuff. It turns into a superfluid that can spontaneously crawl, frictionfree, over the lip of a cup (SN: 4/25/98, p. 271). For 6 decades, helium has been the only superfluid known. In the Sept. 1 Science, however, a group of German and Russian scientists offer experimental evidence that hydrogen might also become superfluid—under very specific circumstances.

Slava Grebenev of the Max Planck Institute in Göttingen, Germany and his colleagues captured rodlike, three-atom molecules called carbonyl sulfide within liquid-helium droplets. Then they painstakingly added rings of hydrogen molecules around the confined rods.

At a temperature of 0.38 K, a laser measurement of rotational energy suggested the rings were turning along with the carbonyl sulfide molecules. When the researchers then lowered droplet temperatures to 0.15 K, they no longer detected that characteristic rotational energy, so the sulfide molecules no longer seemed to be burdened by their hydrogen hoops. The researchers say these data point to the onset of frictionfree superfluidity in the encircling hydrogen.

Giacinto Scoles of Princeton University favors a different interpretation: The characteristic energy level isn’t observed because at 0.15 K, the complex may have too little energy to reach the rotational levels permitted by quantum mechanics and thus may not rotate at all.

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