---------- Forwarded message ---------- Date: Thu, 5 May 2005 11:46:33 -0400 From: [EMAIL PROTECTED] To: [EMAIL PROTECTED] Subject: Physics News Update 730
PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 730 May 5, 2005 by Phillip F. Schewe, Ben Stein
ROOM TEMPERATURE LIQUID SODIUM can occur but only under pressures of a million atmospheres. Melting is a mystery. It happens when the thermal agitation among atoms in a solid overcomes the inter-atom bonds. Applying pressure to a solid sample usually helps to negate the effect of thermal agitation and so the melting temperature usually goes up with pressure. In a few materials, such as water, above a certain pressure the melting point begins to drop. Now, the most dramatic case yet seen of such a "negative melting curve" has been studied by scientists at the Carnegie Institution of Washington looking at one of the simplest metals known, sodium. What happens is this: With zero pressure applied, sodium melts at a temperature of 371 K. As pressure is added, the melting temperature goes up too, up to 1000 K at a pressure of 30 giga-pascals (30 GPa), or about 300,000 atm. Then strange things happen. As the pressure is taken up further, the melting point starts to drop, reaching a low of 300 K (below its ambient melting point) at pressures of 118 GPa (see graph at www.aip.org/png). All previous materials exhibiting negative melting curves have gone negative very reluctantly, over pressure ranges of a few GPa or temperature ranges of a few K. Sodium, by contrast, goes negative over a range of 700 K and 80 GPa. According to Carnegie researcher Eugene Gregoryanz ([EMAIL PROTECTED]), at a pressure of a million atmospheres his sodium sample melts at room temperature. The liquid is denser than the solid (water shares this trait), and might have strange plastic or mechanical properties. It might even be superconducting under some circumstances, he says. (Gregoryanz et al., Physical Review Letters, upcoming article)
AN OPTICAL CONVEYOR BELT for moving sub-micron objects has been achieved by collaborating physicists at the Institute of Scientific Instruments in Brno, Czech Republic and at the University of St. Andrews in Scotland. Their set-up used a special type of non-diffracting laser light that forms a very narrow beam existing over long distance without changing its width. Two such counter-propagating laser beams establish up a lace-like standing wave pattern which can suspend and hold tiny polystyrene spheres of just the right size. The balls, which range in size from 400 nm to one micron, have a density comparable to water. Previously, scientists have used such non-diffracting "optical lace" beams to move particles with the force of radiation pressure, but without the ability to stop them using only a single beam. The Czech and Scottish researchers, by contrast, set up a light lace pattern with numerous knots, corresponding to intensity maxima (antinodes) of the standing wave. Furthermore a particle can be confined near a knot and all the knots can then be moved simultaneously over large distances by changing the relative phases of the counter-propagating laser beams. Moreover thanks to the self-healing property of the non-diffracting beams, many particles can be confined simultaneously in the standing wave structure (near the knots) without significantly spoiling the beam properties. The positioning accuracy, related to the precision of the phase shift and the optical trap depth (the size of the knots), is at the micron level and will get better. Pavel Zemanek ([EMAIL PROTECTED]) says that possible applications for his device include the delivery of biological or colloidal microparticles or even ultracold atoms. (Cizmar et al., Applied Physics Letters, 25 April 2005; lab site at http://www.isibrno.cz/omitec/index.php?swt.html ) (A few years we wrote about a different kind of photon conveyor belt: http://www.aip.org/pnu/1997/split/pnu321-1.htm )
CORRECTION: In the item on pyrofusion (Update 729, Item 1), the tungsten tip is actually positively charged, so that it and the pyroelectric crystal both repel the positive deuterium ions towards a solid deuterium-containing target.
*********** PHYSICS NEWS UPDATE is a digest of physics news items arising from physics meetings, physics journals, newspapers and magazines, and other news sources. It is provided free of charge as a way of broadly disseminating information about physics and physicists. For that reason, you are free to post it, if you like, where others can read it, providing only that you credit AIP. Physics News Update appears approximately once a week.
AUTO-SUBSCRIPTION OR DELETION: By using the expression "subscribe physnews" in your e-mail message, you will have automatically added the address from which your message was sent to the distribution list for Physics News Update. If you use the "signoff physnews" expression in your e-mail message, the address in your message header will be deleted from the distribution list. Please send your message to: [EMAIL PROTECTED] (Leave the "Subject:" line blank.)
