The strange rules of quantum mechanics apply mainly to the atomic and subatomic domains. Only rarely do those rules manifest themselves on a larger scale, as in the case of superconductors, which let electricity pass without resistance. Physicists prize phenomena such as these because they offer a sometimes bizarre, big-screen picture of quantum mechanics in action.
Now, European researchers report that they may have discovered a thoroughly unexpected example of large-scale quantum behavior. It takes place in ultracold samples of certain types of glass. The experimenters stumbled upon it while trying to improve on low-temperature thermometers by using glasses whose capacitance, or ability to store charge, varies with temperature.
The investigators expected magnetic fields to have negligible influence on capacitance readings for the glass. To their surprise, magnetic field variations affected the measurements 10,000 times more strongly than anticipated.
“These are very strange experimental results, indicating very new physics,” says Christian Enss of the University of Heidelberg. The outsize response to magnetic fields starts to kick in below about 100 millikelvins (mK) and is stronger at much lower temperatures, report Enss, Peter Strehlow of the Physikalisch-Technische Bundesanstalt in Berlin, and their colleagues in the Feb. 28 Physical Review Letters.
The researchers discovered that below 5.84 mK, a critical temperature at which the response intensifies, even magnetic fields a hundredth as strong as Earth’s weak field alter glass capacitance.
“I find this work extremely exciting,” says Douglas D. Osheroff of Stanford University. “I don’t know if I would call this a new quantum phenomenon, but it is certainly very interesting.”
Working with the experimenters, some theorists have come up with an explanation for the magnetic response. They make use of an idea developed in 1972 that attributes certain properties of the glasses to the motion of particles—probably ions or ion clusters—via the quantum effect called tunneling. Chilling the glass quells heat-related jitters of its ions enough for tunneling motions to become coordinated, the new theory proposes. The ions’ synchronized movement generates a magnetic field, which interacts with external fields.
Above 5.84 mK, synchronization occurs only in patches a few micrometers across. Below that temperature, the theorists say, all the sample’s ions abruptly fall into the same quantum mechanical state. Similarly, atoms in a Bose-Einstein condensate (SN: 7/15/95, p. 36) and electrons in a superconductor share a single state.
Stefan Kettemann and Peter Fulde, both of the Max Planck Institute for Complex Physical Systems in Dresden, and Strehlow described that theory in the Nov. 22, 1999 Physical Review Letters.
Although the newly reported data seem to indicate a “real effect,” Anthony J. Leggett of the University of Illinois at Urbana-Champaign argues that impurities that have intrinsic magnetism might account for it.
Enss says the researchers ruled that out by determining that the glass contains too few magnetic impurities to have an appreciable effect.
A skeptical Philip W. Anderson of Princeton University, who helped devise the 1972 tunneling theory, says that researchers have yet to identify what entities do the tunneling—ions, groups of ions, or something else. “What kinds of conclusions can you draw about ‘a riddle, wrapped in a mystery inside an enigma’?” he asks, quoting Winston Churchill.
Enss concedes that the presence of a large-scale quantum state in glass below 5.84 mK remains unproven. He and his coworkers have begun experiments to test for definitive evidence, such as interference effects between samples. “There might be other explanations [than large-scale quantum effects],” he says, “but so far there isn’t one.”