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Probable observation of a supersolid helium phase
Physicists in the US have confirmed that solid helium can behave as a superfluid.
Earlier this year Eun-Seong Kim and Moses Chan of Pennsylvania State University observed
superfluid behaviour - fluid flow without viscosity - in solid helium that had been
confined in porous Vycor glass. However, the effect might have been due to the formation
of liquid-like layers in the pores. Now Kim and Chan have repeated the experiment with
bulk samples of solid helium and confirmed that it can enter a superfluid state
(E Kim and M H W Chan 2004 Science to appear).
Liquid helium-4 becomes superfluid when it is cooled to temperatures below 2.176 Kelvin.
However, theory predicts that it should also be possible to observe superfluid behaviour
in solid helium-4 under certain conditions.
One way to observe superfluidity in solid helium is to measure the resonant period of
a sample of the material in a piece of apparatus called a torsional oscillator.
This period depends on the moment of inertia of the sample, and this moment changes
when helium enters the superfluid state.
Kim and Chan measured a total of 17 samples
of solid helium-4 at pressures between 26 and 66 bars and found that they all became
superfluid at temperatures below 230 millikelvin. "Our experiment shows that the
superfluid-like behaviour is a general and intrinsic property of solid helium," they
write, "and not the result of confinement in any particular medium."
However, many of the details of the experiment are not yet understood.
In an accompanying article Tony Leggett of the University of Illinois in Urbana
writes that the experiment "will force theorists to revise dramatically the
generally accepted picture of crystalline solid helium-4."
Full article
Original article from Physicsweb
Negative refraction goes acoustic
Two physicists in China have shown that it should be possible to observe
negative refraction with sound waves. Xiangdong Zhang of Beijing Normal University
and Zhengyou Liu of Wuhan University have used computer simulations to design
two-dimensional phononic crystals that behave like the "left-handed" optical materials
that are able to negatively refract light (Appl. Phys. Lett. 85 341). They believe that
the phenomenon could lead to applications in acoustics, seismology and ultrasonics.
First proposed over thirty years ago, negative index materials bend light in the
opposite direction to ordinary materials. However, they were only demonstrated
experimentally in 2000. Now, Zhang and Liu have shown that negative refraction is
also possible with sound waves.
A phononic or sonic crystal is the acoustic equivalent of a photonic crystal -- a
material that contains a periodic arrangement of air-filled voids that have a lower
refractive index than the host material. It is the periodic variation of the refractive
index that creates an optical band gap in the photonic crystal, which means that only
certain wavelengths of light are able to pass through it. Similarly, phononic
crystals -- which consist of cylinders of one material embedded in a different
background medium -- contain acoustic gaps, which means that only certain wavelengths
of sound can pass through the material.
Zhang and Liu showed that negative acoustic refraction should occur in two systems:
steel cylinders in an air background, and water cylinders in a mercury background.
Moreover, they also have designed a two-dimensional acoustic "superlens" that
should -- like its optical equivalent -- be capable of sub-wavelength resolution
and reflection-free operation. The Chinese team believes that its acoustic system
will offer similar advantages.
"Extensive applications of such a phenomenon to acoustic devices are anticipated,"
Zhang told PhysicsWeb. "It is well known that acoustic devices that focus and image
sound waves are very important for medical, military and civilian applications".
Original article from Physicsweb
Hard disk-drive technology revolutionizes processing
This article reviews trends in magnetic hard disk drive (HDD) technology and
discusses underlying magnetoresistive phenomena, magnetic media materials issues,
and processes for fabricating leading-edge write and read heads for
state-of-the-art HDDs.
Original article from Pennet
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