Supersolid14961639_64500983
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New observations of an apparent superfluid component close loopholes in an earlier experiment, but present their own set of mysteries.
S
Magnesium
Figure 1. Torsion oscillator. Solid helium occupies a 1-cm diameter annular cbannel in a cylindrical cell suspended by a torsion rod. With the help of the electrodes attached to the side of the cell, a lock-in amplifier can keep the oscillator in resonance. The superfluid behavior observed in the cell goes away when a magnesium barrier blocks the channel. (Courtesy of Eun-Seong Kim and Moses Chan.) November 2004 Physics Today 23
Search and Dis#39;t Go With the Flow
a factor of about 60. The observation of an apparent decoupling in tbe bulk solid answers a major concern tbat had been raised uppose you could freeze the water "Kim and Chan are clearly seeing about the earlier experiments witb in a washing machine while it was something really fascinating," says porous media. Particularly in the operating. Once the water had solidi- Robert Hallock of tbe University of tiny-pored Vycor, it was not clear what fied, the machine's agitators would Massacbusetts Amherst. "I'm not sure formed in the pores. One possibility cause the entire icy mass to oscil- what it means yet, but I trust their was that a fluid layer lined the pore surfaces and was masquerading as a late—until the motor burned out. In observations." supersolid. Such a fluid layer may January of this year, Eun-Seong Kim have been present in the bulk experiand Moses Chan, both at Pennsylva- Getting in the groove nia State University, reported an ex- In their bulk-helium experiment, the ment too, but with a thickness of just periment' that has some of the flavor Penn State researchers introduced an atom or two, it couldn't have been of the washing-machine fantasy, but liquid 'He into a narrow annular responsible for the observed drop in which yielded a surprising result (see channel near the outside of tbe tor- period. PHYSICS TODAY, April 2004, page 21). sional oscillator illustrated in figure Conversely, the results in Vycor They introduced liquid helium-4 into 1, and solidified the *He at high pres- strengthen tbe interpretation that the a torsional oscillator, solidified the liq- sure. They then electrically drove the bulk-helium observations were uid at high pressure, and lowered the oscillator and measured tbe resonant caused by the decoupling of a solid temperature. When the temperature period as they lowered the tempera- component from the bulk. The Unifell below about 175 mK, tbe period of ture of the solid; by changing tbe driv- versity of Alberta's John Beamish the resonant oscillation also began to ing voltage, Kim and Chan adjusted points out that, absent tbe Vycor refall. About l'/( of the ^He in the oscil- the oscillation amplitude and thus the sults, the decoupling might have bad lator appeared not to move with the maximum speed of the channel. a more conventional explanation hehulk oscillation; it behaved as a suAs figure 2 shows, the resonant pe- cause solids are not entirely rigid and perfluid would. riod began to decrease as the temper- sometimes can deform. If one rotates The 'He, though, was embedded in ature fell below about 250 mK. The a cup of coffee back and forth. Vycor glass, a highly porous material period drop was most pronounced for Beamish points out, the coffee—-a conwith a pore size just several nanome- small maximum speeds and seemed ventional viscous liquid, not a superters across. That porous structure, to saturate below 5 /j,m/s or so. The fluid—does not rotate as quickly in with its large surface area, compli- drop in period indicates a correspon- the center of the cup as it does near cated the interpretation of the de- ding drop in the moment of inertia of tbe edge. If a similar effect were to crease in oscillation period. A concep- the material in tbe cell's annular occur with the solid 'He in the oscillatually cleaner experiment would use a channel. Presumably, some of the tor's annular channel, it could mimic torsion oscillator tbat contains an an- solid material had decoupled from the decoupling. But that explanation is nular channel filled with solid ''He. bulk. That so-called supersolid was not compatible with the observed deKim and Chan have now done that not participating in the overall oscil- coupling in the small pores of Vycor; hulk-helium experiment,- and the re- latory motion. When a magnesium all tbe coffee confmed within a thin sults are much like those of the ear- barrier was introduced into tbe oscil- capillary will rotate together. lier Vycor work and a subsequent sim- lator to block tbe annular cbannel, The drop in the bulk helium's moilar experiment with porous gold. the change in period was reduced by ment of inertia can be explained without invoking macroscopic supersolid flow. Anthony Leggett of the University of Illinois at Urbana-Champaign notes that one can't rule out the possibility Magnesium Filling line barrier that Kim and Chan are observing a nonequilibrium phenomenon with a
S
Magnesium
Figure 1. Torsion oscillator. Solid helium occupies a 1-cm diameter annular cbannel in a cylindrical cell suspended by a torsion rod. With the help of the electrodes attached to the side of the cell, a lock-in amplifier can keep the oscillator in resonance. The superfluid behavior observed in the cell goes away when a magnesium barrier blocks the channel. (Courtesy of Eun-Seong Kim and Moses Chan.) November 2004 Physics Today 23
Search and Dis#39;t Go With the Flow
a factor of about 60. The observation of an apparent decoupling in tbe bulk solid answers a major concern tbat had been raised uppose you could freeze the water "Kim and Chan are clearly seeing about the earlier experiments witb in a washing machine while it was something really fascinating," says porous media. Particularly in the operating. Once the water had solidi- Robert Hallock of tbe University of tiny-pored Vycor, it was not clear what fied, the machine's agitators would Massacbusetts Amherst. "I'm not sure formed in the pores. One possibility cause the entire icy mass to oscil- what it means yet, but I trust their was that a fluid layer lined the pore surfaces and was masquerading as a late—until the motor burned out. In observations." supersolid. Such a fluid layer may January of this year, Eun-Seong Kim have been present in the bulk experiand Moses Chan, both at Pennsylva- Getting in the groove nia State University, reported an ex- In their bulk-helium experiment, the ment too, but with a thickness of just periment' that has some of the flavor Penn State researchers introduced an atom or two, it couldn't have been of the washing-machine fantasy, but liquid 'He into a narrow annular responsible for the observed drop in which yielded a surprising result (see channel near the outside of tbe tor- period. PHYSICS TODAY, April 2004, page 21). sional oscillator illustrated in figure Conversely, the results in Vycor They introduced liquid helium-4 into 1, and solidified the *He at high pres- strengthen tbe interpretation that the a torsional oscillator, solidified the liq- sure. They then electrically drove the bulk-helium observations were uid at high pressure, and lowered the oscillator and measured tbe resonant caused by the decoupling of a solid temperature. When the temperature period as they lowered the tempera- component from the bulk. The Unifell below about 175 mK, tbe period of ture of the solid; by changing tbe driv- versity of Alberta's John Beamish the resonant oscillation also began to ing voltage, Kim and Chan adjusted points out that, absent tbe Vycor refall. About l'/( of the ^He in the oscil- the oscillation amplitude and thus the sults, the decoupling might have bad lator appeared not to move with the maximum speed of the channel. a more conventional explanation hehulk oscillation; it behaved as a suAs figure 2 shows, the resonant pe- cause solids are not entirely rigid and perfluid would. riod began to decrease as the temper- sometimes can deform. If one rotates The 'He, though, was embedded in ature fell below about 250 mK. The a cup of coffee back and forth. Vycor glass, a highly porous material period drop was most pronounced for Beamish points out, the coffee—-a conwith a pore size just several nanome- small maximum speeds and seemed ventional viscous liquid, not a superters across. That porous structure, to saturate below 5 /j,m/s or so. The fluid—does not rotate as quickly in with its large surface area, compli- drop in period indicates a correspon- the center of the cup as it does near cated the interpretation of the de- ding drop in the moment of inertia of tbe edge. If a similar effect were to crease in oscillation period. A concep- the material in tbe cell's annular occur with the solid 'He in the oscillatually cleaner experiment would use a channel. Presumably, some of the tor's annular channel, it could mimic torsion oscillator tbat contains an an- solid material had decoupled from the decoupling. But that explanation is nular channel filled with solid ''He. bulk. That so-called supersolid was not compatible with the observed deKim and Chan have now done that not participating in the overall oscil- coupling in the small pores of Vycor; hulk-helium experiment,- and the re- latory motion. When a magnesium all tbe coffee confmed within a thin sults are much like those of the ear- barrier was introduced into tbe oscil- capillary will rotate together. lier Vycor work and a subsequent sim- lator to block tbe annular cbannel, The drop in the bulk helium's moilar experiment with porous gold. the change in period was reduced by ment of inertia can be explained without invoking macroscopic supersolid flow. Anthony Leggett of the University of Illinois at Urbana-Champaign notes that one can't rule out the possibility Magnesium Filling line barrier that Kim and Chan are observing a nonequilibrium phenomenon with a
