Number 863, May 1, 2008 by Phillip F. Schewe and Jason S. Bardi
Giant Piezoresistance.
A new experiment, conducted by scientists from France, Switzerland, and the UK, has recorded the largest ever change in a bulk material’s electrical resistance brought about by stretching the material at room temperature. Piezoresistance
is one of several phenomena in which a resistance change, prompted by a change in another physical parameter, can be used in making sensitive sensors. In magnetoresistance, for example, the force from a tiny magnetic domain can alter the resistance of a circuit in a scanner directly overhead. A pronounced form of this effect, giant magnetoresistance, is at the heart of the billion-dollar hard-drive industry, earning two pioneer scientists the Nobel prize in physics in 2007.
In piezoresistance, by contrast, it’s not a tiny magnetic field but the tiny mechanical stretching of a material that alters the resistance, which in turn registers as an electrical signal. Piezoresistive devices have been in use for some time. In simple metal-foil versions, the kind used in monitoring the integrity of concrete walls or in monitoring prosthetic limbs, the change in resistance per unit of strain (a ratio referred to as the gage factor) typically has a value of about 2. For the more expensive silicon-based piezoresistors, the kind used in cell phones and airbag accelerometers, the gage factor is usually about 100.
In the new experiment a metal/silicon hybrid piezoresistance sample yielded a gage factor of 900, the largest ever seen at room temperature in a bulk material. (Larger gage factors have been observed at impractically low temperatures where quantum effects accentuate the piezoresistance.) Giant piezoresistive structures should be good news for the designers of MEMS (microelectromechanical systems) devices where it is important to measure ultra-small accelerations, or atomic-scale deflections. Alternatively, higher sensitivity to movement can be translated into lower power requirements when (as in cell phones) battery energy is at a
premium.
One of the researchers, Alistair Rowe of the Ecole Polytechnique in Palaiseau, France (alistair.rowe@polytechnique.edu, 33-169-3347-87) says that the giant-piezoresistance materials probably wouldn’t be used directly as a storage medium but more likely as a method for reading mechanically-stored information in devices like IBM’s “Millipede.” (Rowe et al., Physical Review Letters, 11 April 2008)
Multicolor Atom Laser.
A new theoretical study suggests that intense atom lasers would not be mono-chromatic. In ordinary optical lasers the coordinated build-up of a coherent light pulse ensures that all light waves possess a single energy; in other words, all the laser light would have a single color. By analogy, the action of an atom laser, consisting of the release of gaseous atoms (in the form of waves) from the magnetic-field constraints used to create a Bose Einstein condensate (BEC), should be monochromatic. After all, the atoms in a BEC have been chilled to such an extreme extent that all the atoms possess the same energy.
Stephen Choi and his colleagues at the University of Massachusetts Boston maintain that in dilute BECs this is indeed the case. But in dense BECs, frequent collisions among atoms will promote classes of atoms to higher energies as well. In effect, the inter-atomic interactions "generate" atom waves at alternative, harmonic energies.
The UMB researchers next show how this harmonic generation can be demonstrated. They do this by simulating what happens when a BEC is sent through an interferometer, a device in which a coherent train of waves is divided into two components, steered along different paths, and then recombined to create a characteristic interference pattern.
In this simulated interferometer the BEC atoms don't move through space. Instead the dividing, steering, and recombining are carried out by subtly modulating the magnetic fields used to confine the BEC in the first place.
The presence of extra "harmonic" atom waves will make the resultant interferometer pattern more complicated. But this, Choi (stephen.choi@umb.edu) says, is not all bad. It can, for example, lead to greater sensitivity when the atom interferometer is put to use in certain devices, such as gyroscopes. (Choi et al., Physical Review A, April 2008)
Iron Superconductivity.
The highest superconductivity transition temperature for a non-copper material has been achieved: 44 K by Japanese scientists at the Nihon University, the Frontier Research Center, and the Tokyo Institute of Technology and 55 K at the Institute of Physics in Beijing. To attain a comparatively high transition temperature, the Japanese researchers had to squeeze their La-O-F-Fe-As sample with a pressure of 4 giga-pascals. (Takahashi et al., Nature, 23 April 2008) Even higher transition temperatures (55 K) in arsenic-iron compounds are reported to have been made in China (see Science, 25 April 2008).
Graphine Quantum Dots.
Physicists at the University of Manchester have created single-electron transistors as small as 30 nm on two-dimensional carbon. (Ponomarenko et al., Science, 18 April 2008)
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