Abstract: When liquid 4He is cooled below 2.176 K, it undergoes a phase transition—Bose–Einstein condensation—and becomes a super- fluid with zero viscosity. Once in such a state, it can flow without dissipation even through pores of atomic dimensions. Although it is intuitive to associate superflow only with the liquid phase, it has been proposed theoretically that superflow can also occur in the solid phase of 4He. Owing to quantum mechanical fluctuations, delocalized vacancies and defects are expected to be present in crystalline solid 4He, even in the limit of zero temperature. These zero-point vacancies can in principle allow the appearance of superfluidity in the solid. However, in spite of many attempts, such a 'supersolid' phase has yet to be observed in bulk solid 4He. Here we report torsional oscillator measurements on solid helium confined in a porous medium, a configuration that is likely to be more heavily populated with vacancies than bulk helium. We find an abrupt drop in the rotational inertia5 of the confined solid below a certain critical temperature. The most likely interpretation of the inertia drop is entry into the supersolid phase. If confirmed, our results show that all three states of matter—gas, liquid and solid—can undergo Bose–Einstein condensation.Probable observation of a supersolid helium phase, , Nature, 1/2004, Volume 427, Issue 6971, p.225 - 227, (2004)
|Element name||Element symbol||Atomic number|
Here is a list of the elements sorted by alphabetically by element name.
Sponsoring an elementThe visible form of the sponsorship can be along the following lines (but if this is not what you had in mind, we may be able to work together towards your needs):
- banner ad (468 x 60 pixels) at top of all pages for that element
- additional small graphic button (120 x 60 pixels) on the left menu panel
- small "advertorial" on an appropriate page for that element, 30-60 words and a small image, set in a panel on that page.
WebElements user profileWebElements users are intelligent and well-educated. According to an online survey responded to by 25000 users:
- about 65% of all users are from USA
- about 60% of all users are under 21 (average age 24, median age 16-17)
- about 50% of all users over 21 are degree holders
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The BBC is airing some "periodic tales" on Radio 4. Familiar Radio 4 voices introduce elements from the Periodic Table and the unique roles they play in human existence - with a little help from the irreverent Tom Lehrer. Listen to these ten elements:
- Krypton: Heidli Nicklaus on the Superman element, krypton
- Helium: Brian Perkins dramatises the effects of Helium
- Silver: Trevor Harrison (Eddie Grundy in the Archers) finds some unusual properties of Silver
- Cobalt: Hedli Nicklaus (Cathy Perks) takes on the goblin element of cobalt
- Selenium: Carole Boyd (The Archers' Linda Snell) unearths selenium
- Oxygen: Brian Perkins bravely dramatises the effects of oxygen
- Arsenic: Charlotte Green takes on the deadly history of arsenic
- Mercury: Carole Boyd (Linda Snell) reflects on mercury, the poisonous liquid metal
- Iodine: Charlotte Green on the discovery of iodine's essential place in brain development
- Nickel: Trevor Harrison reveals that the space station Mir is largely made of nickel
[[Note added Dec 2009: sadly these recordings no longer exist on the BBC site. I did offer to host them here but no luck]]
In a letter to Nature E. Kim and M. H. W. Chan (Pennsylvania State University, USA) note that when liquid 4He is cooled below 2.176 K, it undergoes a phase transition and becomes a superfluid with zero viscosity. They claim that in addition to superflow in the liquid phase, superflow can also occur under some conditions in the solid phase of one of the helium isotopes (4He), and present results to back this up. In other words - evidence for a "supersolid". A supersolid behaves like a superfluid (flows without resistance) although it has crystalline solid characteristics.1
Earth's most severe mass extinction - an event 250 million years ago that wiped out 90 percent of all marine species and 70 percent of land vertebrates - was triggered by a collision with a comet or asteroid, according to a team led by The University of Washington, Seattle, USA. Evidence is based upon elegant findings involving carbon molecules called buckminsterfullerenes (C60, Buckyballs) with the gases helium and argon trapped inside their cage structures.
The scientists do not know the site of the impact 250 million years ago, when all Earth's land formed a supercontinent called Pangea. However, the space body left a calling card - a much higher level of complex carbon molecules called buckminsterfullerenes, or Buckyballs, with the noble (or chemically nonreactive) gases helium and argon trapped inside their cage structures. Fullerenes, which contain 60 or more carbon atoms and have a structure resembling a soccer ball or a geodesic dome, are named for Buckminster Fuller, who invented the geodesic dome.
The researchers know these particular Buckyballs are extraterrestrial because the noble gases trapped inside have an unusual ratio of isotopes. For instance, terrestrial helium is mostly helium-4 and contains only a small amount of helium-3, while extraterrestrial helium - the kind found in these fullerenes - is mostly helium-3.
"These things form in carbon stars. That's what's exciting about finding fullerenes as a tracer," according to Luann Becker, one of scientific team involved. The extreme temperatures and gas pressures in carbon stars are perhaps the only way extraterrestrial noble gases could be forced inside a fullerene, she said. These gas-laden fullerenes were formed outside the Solar System, and their concentration at the Permian-Triassic boundary means they were delivered by a comet or asteroid.