Physics News Update, Number 493

Number 493, July 12, 2000 by Phillip F. Schewe and Ben Stein

A SUPERCONDUCTING TRANSISTOR-LIKE DEVICE Ihas been fabricated and tested by an Oxford-Naples group. The benefits of superconducting electronics are high switching speed, compatibility with sensors that operate at very low temperatures (where conventional transistors perform poorly), and high sensitivity to incoming electromagnetic radiation (useful in astronomical applications). The chief obstacle to superconducting electronics has been the lack of a 3-terminal device which can switch and amplify current.

Now, Norman Booth of Oxford (n.booth1@physics.ox.ac.uk, 011-44-1235-833486 or 01-204-348- 7219 in Canada for the summer) and Antonio Barone of the University of Naples (011-39-081-768-2416, barone@na.infn.it) and their colleagues have overcome this problem by placing together two tunnel junctions, to make a device consisting of three metallic layers separated by two insulating barriers (see figure at Physics News Graphics). A signal current or voltage applied to the first junction breaks some of the bound electron (Cooper) pairs in the superconducting injector layer, and one of the resulting two electronic excitations, called quasiparticles, tunnels into the superconducting part of the middle layer. If now the quasiparticle diffuses into the normal metal part of the middle layer it can give up its energy (which it got from the breakup of the Cooper pair) to many of the free electrons in the normal metal and these in turn can then tunnel through the second junction. This represents, from one side of the device to the other, a gain in current (as high as a factor of 70 in the Oxford-Naples experiments) and power (a factor as high as 1000).

Furthermore, the device operates (at a temperature of 4 K) on a power of about 1 microwatt and voltages of millivolts, in comparison to a power of 1-10 milliwatt and voltages of several volts for conventional transistors. This quasiparticle trapping transistor, which the researchers call a "quatratran," is the superconducting analog of both the pnp and npn transistors and can be batch produced with comparable fabrication techniques. (Pepe et al., Applied Physics Letters, 17 July 2000; Select Articles.)

INTENSITY MODULATED RADIOTHERAPY (IMRT) is an up-and-coming radiation therapy technique for cancer. Instead of employing radiation beams of uniform intensity, IMRT enables physicians to modify the intensity of each radiation beam in a sophisticated fashion. Firing non-uniform beams from several angles can allow physicians to deliver a higher dose of radiation to all parts of the three-dimensional volume of a tumor while sending less radiation to healthy surrounding tissue. IMRT will be the subject of many papers at the upcoming World Congress on Medical Physics and Biomedical Engineering, co-sponsored by the American Association of Physicists in Medicine, to take place from July 23-28 in Chicago. (Meeting information at www.wc2000.org/)

A TIME CAPSULE FROM ARCHIMEDES is how historian Reviel Netz describes a tenth century manuscript bearing seven different works composed by Archimedes in the third century BCE. The manuscript, referred to as the "Archimedes Palimpsest" (from the Greek word for rescraped, since the parchment was partially scraped clean and then overwritten in the twelfth century with a number of prayers), rested in obscurity for centuries until 1906 when a scholar recognized the underlying text and geometrical drawings as being by Archimedes.

The parchment quickly disappeared again, only to show up at auction in 1998. The purchaser, who remains anonymous, is now making the manuscript available for study. Of the seven different texts, several (such as Archimedes' treatises on buoyancy and on centers of gravity in planes) exist in other manuscripts, but in the case of two of the works, this manuscript represents the sole source. And one of these, a treatise called "Method," sets forth Archimedes' approach to using mathematical principles in solving physics problems and vice versa. For this reason, and because the Palimpsest "is, by a long stretch, the earliest evidence we have for Archimedes," Mr. Netz believes this is the most important manuscript associated with the man whom many regard at the most important scientist of antiquity. (Article by Netz in Physics Today, June 2000.)