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发信人: dijiao (PSUer), 信区: Physics 标题: New probable signature of supersolid 发信站: BBS 珞珈山水站 (Wed Oct 31 05:36:26 2007) 原始链接： http://physicsworld.com/cws/article/news/31583;jsessionid=8EC9328EAF00CA9CF168 73510FBD63CB#comments Oct 25, 2007 New supersolid signature sighted Evidence is mounting for the existence of a strange new state of matter called a "supersolid", in which a small fraction of ultracold helium decouples from the rest of the solid and flows effortlessly through the material as if it wer e not there. Although the first clear signs of supersolidity were obtained thr ee years ago by Moses Chan and colleagues at Pennsylvania State University in the US, subsequent research cast doubt on those findings. Now, however, Chan h as measured the specific heat of several helium-4 samples and has found a peak in the data that he says is a "probable" signature of the supersolid phase (N ature 449 1025). SupersolidSupersolidity was first predicted in 1969 by Russian theorists Alexa nder Andreev and Ilya Liftshitz. They said that lattice vacancies in solid hel ium could all collapse into the same quantum state if the helium were cooled t o an extremely cold temperature. This Bose-Einstein condensate (BEC) of vacanc ies would behave as a coherent entity, moving throughout the rest of the solid like a superfluid. In 2004 Chan and his graduate student Eun-Seong Kim found the first evidence f or superfluidity in a torsional oscillator, which consisted of a cylindrical c ell filled with high-pressure helium-4. The cell, which was suspended from a r od, was rotated back and forth while being cooled. When the temperature reache d about 200 mK, the researchers saw a sudden change in the oscillation period of the cell, which they interpreted as evidence that about 1% of the helium ha d “decoupled” from the solid helium-4 and was not oscillating. Although this was taken as a sign of supersolidity, subsequent experiments by Chan and others revealed that the decoupling temperature and the percentage de coupled varied significantly from sample to sample. This led some physicists t o suggest that the observed effect was not caused by condensing vacancies, but rather by either the flow of superfluid helium along grain boundaries in poly crystalline samples, or by a transition to a “superglass” phase. Earlier thi s year Chan and colleagues repeated their torsion experiments on a single-crys tal of helium-4 – which had no grain boundaries. This time the decoupling occ urred at the lower temperature of 75 mK with only 0.3% of the sample decouplin g – the strongest evidence yet that the vacancies became a superfluid. Now Chan and colleagues have measured the specific heat – the energy required to change the temperature of a material – of several helium-4 samples. They found peaks in the specific heat at about 75 mK, which they concluded to be a “probable” signature of the supersolid phase. “If there is a real phase tra nsition from the normal solid to the supersolid phase, there should be a therm odynamic signature, like a peak in the specific heat of solid helium”, explai ned Chan. However, the experiment was not designed to measure the simultaneous decoupling of mass, and therefore the team cannot be certain that the peak is associated with supersolidity. The team also found that the temperature dependence of the specific heat was a t odds with that expected if a glassy state was forming in the solid – ruling out the superglass explanation for decoupling. The peak in the heat capacity suggests that the supersolid state emerges in a second-order or continuous phase transition – just like superfluidity. “Ther e is no theoretical consensus on the nature of the supersolid phase”, said Ch an, “[but] based on our knowledge of superfluids, the transition is more like ly to be second order, than say first order”. Chan told physicsworld.com that the team have incorporated new and more sensit ive thermometers into their apparatus, which should allow them to get a better idea of the exact shape of the peak. By analyzing the shape, the team could g et a better understanding of the nature of the phase transition. Chan is also keen to look for other signatures of supersolidity such as “second sound”, w hich is a dramatic increase in the thermal conductivity of a material when it becomes a superfluid. -- 每天，开心由己 ※ 来源:·珞珈山水BBS站 http://bbs.whu.edu.cn·[FROM: 128.118.49.*]
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发信人: dijiao (PSUer), 信区: Physics
标题: New probable signature of supersolid
发信站: BBS 珞珈山水站 (Wed Oct 31 05:36:26 2007)

原始链接：

http://physicsworld.com/cws/article/news/31583;jsessionid=8EC9328EAF00CA9CF168
73510FBD63CB#comments

Oct 25, 2007

New supersolid signature sighted

Evidence is mounting for the existence of a strange new state of matter called
a "supersolid", in which a small fraction of ultracold helium decouples from
the rest of the solid and flows effortlessly through the material as if it wer
e not there. Although the first clear signs of supersolidity were obtained thr
ee years ago by Moses Chan and colleagues at Pennsylvania State University in
the US, subsequent research cast doubt on those findings. Now, however, Chan h
as measured the specific heat of several helium-4 samples and has found a peak
in the data that he says is a "probable" signature of the supersolid phase (N
ature 449 1025).

SupersolidSupersolidity was first predicted in 1969 by Russian theorists Alexa
nder Andreev and Ilya Liftshitz. They said that lattice vacancies in solid hel
ium could all collapse into the same quantum state if the helium were cooled t
o an extremely cold temperature. This Bose-Einstein condensate (BEC) of vacanc
ies would behave as a coherent entity, moving throughout the rest of the solid
like a superfluid.

In 2004 Chan and his graduate student Eun-Seong Kim found the first evidence f
or superfluidity in a torsional oscillator, which consisted of a cylindrical c
ell filled with high-pressure helium-4. The cell, which was suspended from a r
od, was rotated back and forth while being cooled. When the temperature reache
d about 200 mK, the researchers saw a sudden change in the oscillation period
of the cell, which they interpreted as evidence that about 1% of the helium ha
d “decoupled” from the solid helium-4 and was not oscillating.

Although this was taken as a sign of supersolidity, subsequent experiments by
Chan and others revealed that the decoupling temperature and the percentage de
coupled varied significantly from sample to sample. This led some physicists t
o suggest that the observed effect was not caused by condensing vacancies, but
rather by either the flow of superfluid helium along grain boundaries in poly
crystalline samples, or by a transition to a “superglass” phase. Earlier thi
s year Chan and colleagues repeated their torsion experiments on a single-crys
tal of helium-4 – which had no grain boundaries. This time the decoupling occ
urred at the lower temperature of 75 mK with only 0.3% of the sample decouplin
g – the strongest evidence yet that the vacancies became a superfluid.

Now Chan and colleagues have measured the specific heat – the energy required
to change the temperature of a material – of several helium-4 samples. They
found peaks in the specific heat at about 75 mK, which they concluded to be a
“probable” signature of the supersolid phase. “If there is a real phase tra
nsition from the normal solid to the supersolid phase, there should be a therm
odynamic signature, like a peak in the specific heat of solid helium”, explai
ned Chan. However, the experiment was not designed to measure the simultaneous
decoupling of mass, and therefore the team cannot be certain that the peak is
associated with supersolidity.

The team also found that the temperature dependence of the specific heat was a
t odds with that expected if a glassy state was forming in the solid – ruling
out the superglass explanation for decoupling.

The peak in the heat capacity suggests that the supersolid state emerges in a
second-order or continuous phase transition – just like superfluidity. “Ther
e is no theoretical consensus on the nature of the supersolid phase”, said Ch
an, “[but] based on our knowledge of superfluids, the transition is more like
ly to be second order, than say first order”.

Chan told physicsworld.com that the team have incorporated new and more sensit
ive thermometers into their apparatus, which should allow them to get a better
idea of the exact shape of the peak. By analyzing the shape, the team could g
et a better understanding of the nature of the phase transition. Chan is also
keen to look for other signatures of supersolidity such as “second sound”, w
hich is a dramatic increase in the thermal conductivity of a material when it
becomes a superfluid.

--
每天，开心由己

※ 来源:·珞珈山水BBS站

http://bbs.whu.edu.cn·[FROM: 128.118.49.*]

[返回单文区目录]