|Aktuelle Meldungen bei MM-Physik
14. Oktober 2000 © Schulphysik">email: Schulphysik
PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics AIP-PHYSICS NEWS:10Jahre alt News Number 504/505 10. Oktober, 2000 by Phillip F. Schewe and Ben Stein
|Update is now sent by email directly to
more than 17,000 readers. Of these half
are outside the US. Physics News Update items regularly
appear in Physics Today magazine, are posted on numerous
electronic bulletin boards, and are translated into a
dozen languages. All past Updates and a powerful engine
for searching the Update archive can be found at this
Affiliated websites include Physics News Graphics ( www.aip.org/physnews/graphics ), where figures connected to Update items are displayed; Physics News Select Articles, where certain physics journal articles are made available to science journalists in pdf format; and Physics News Links
( www.aip.org/physnews/links ), with connections to many other popular physics websites. Physics News Update is published by the American Institute of Physics, located in College Park, Maryland. In any given edition, Update can only sample a very few stories, but this week, by way of celebrating the diversity of physics research, we shall provide a panoramic snapshot of new results from around the world. (Abstracts for many of these articles can be found at the Online Journal Publishing Service at http://ojps.aip.org .
From the 2 October Physical Review Letters
NEGATIVE REFRACTION INDEX measured for a left-handed material (Smith and Knoll) INTENSE PROTON BEAMS, with up to 2 x 10^13 protons/burst above an energy of 10 MeV, produced with Petawatt laser irradiation of solids (Snavely et al.)
TEMPERATURE AND PRESSURE DEPENDENCE of sonoluminescence (Vasquez and Putterman). >From the 25 September Physical Review Letters:
ANTIDEUTERONS are made in high energy lead-lead collisions at CERN (Bearden et al.).
NEW ISOTOPES OF BOHRIUM, Bh-266 and Bh-267, are created and studied at LBL (Wilk et al.) METALLIC XENON is achieved by squeezing solid Xe up to pressures of 155 GPa (Eremets et al.). SOUND WAVES IN CARBON NANOTUBES can be excited with radio waves (Reulet et al.). A THERMODYNAMIC MODEL describes the proliferation of normal and malignant cells (Kuznetsov and Blokhin).
From Applied Physics Letters:
METAL WIRES LESS THAN 10 NM WIDE using epitaxy, cleaving, and etching techniques (Natelson et al., 25 Sept.)
LEFT HANDED METAMATERIAL: direct measurements of the permittivity and permeability for a material made of tiny rods and rings (Smith et al., 2 Oct.)
From Nature magazine
THE PHYSICS OF PANIC: modeling the behavior that can occur when, say, fire breaks out in a crowded theater (25 Sept., Helbing et al.).
SINGLE PHOTONS FROM SINGLE MOLECULES at user- specified times, with possible applications in quantum computation (25 Sept., Lounis and Moerner).
THE BLACK HOLE AT THE CENTER OF OUR GALAXY has been pinpointed with new higher accuracy (21 Sept., Ghez et al.). >From Science magazine:
OPTICAL ENTANGLEMENT OF EXCITONS (electron-hole pairs) in a quantum dot (Chen et al., Sept 15).
Other AIP journals: ICE NANOTUBES: simulations of hypothetical quasi-one- dimensional crystals built from water molecules (Koga et al., Journal of Chemical Physics, September).
OPTICAL TURBULENCE: the onset and recurrence of multiple filamentary structures in the propagation of high-power femtosecond laser pulses in air (Muloney et al., Chaos, September). SEARCH FOR FRACTIONAL-CHARGE PARTICLES in drops of suspensions of powdered meteorites and other special minerals (Loomba et al., Review of Scientific Instruments, Sept.). MODELING OF THE FOUR VORTICES produced in the wakes of jet aircraft (Fabre and Jacquin, Physics of Fluids, October).
From journals published by Elsevier (NL)
POSSIBLE DETECTION OF GRAVITATIONAL WAVES in future storage rings (Zer-Zion, Astroparticle Physics, November).
THE BERYLLIUM NEUTRINO FLUX inside the sun determined with helioseismology (Ricci and Villante, Physics Letters B, 31 August).
From Journals published by the Institute of Physics (UK)
MAGNETIC FIELDS AROUND BLACK HOLES and how they accelerate particles injected into the vicinity of black holes (Dovciak et al., European Journal of Physics, July).
PARTICLE DETECTORS USING DIAMOND are in demand owing to diamond's resistance to corrosion and radiation damage (Mainwood, Semiconductor Science and Technology, 9 Sept.). PHYSICS AT THE OLYMPICS: several feature articles (Physics World, Sept.)
From journals published in Russia:
"INFORMATIONAL" COOLING OF ATOMS is a kind of "Maxwell's Demon" sorting of cool and warm atoms using laser beams (Balykin and Letohov, 10 July JETP Letters).
ULTRAHIGH ENERGY COSMIC RAYS as possible evidence for extra dimensions (Konoplich and Rubin, 10 Aug. JETP Letters).
LOCALIZED LASER BULLETS: conditions under which laser solitons could be completely localized (Veretenov et al., Optics and Spectroscopy, Sept).
Other journals: WHITE DWARF MACHO BINARIES might make an important contribution to the gravitational wave background (Hiscock et al., Astrophysical Journal Letters, 1 September).
FLATLAND OPTICS: a study of the type of optics that one would find in Edwin Abbott's 19th century novel "Flatland" (Lohmann et al., Journal of the Optical Society of America A, Oct.).
TESTS AT THE HIGHEST ELECTRIC FIELDS. An
electron in orbit around a proton is not like a planet
circling a star. Not only does the electron (in a quantum
sense) not follow a trajectory of well-defined locations,
but space itself, in the case of a hydrogen atom, teems
with virtual particles such as photons and
electron-positron pairs popping into and out existence
owing to the energy vested in the electromagnetic field
surrounding the electron and nucleus. The presence of
these virtual particles can shift the allowed energies of
the electron, and measurements of this "Lamb
shift" (named for Willis Lamb) constitute the most
stringent test of quantum electrodynamics (QED) and
indeed the highest-precision test of any physical theory.
As strong as the electromagnetic field may be within the
hydrogen atom, however, it is small compared to the
electric field felt by the innermost electron in a
uranium atom. To get at this electron, and to test QED
amid the highest possible fields, physicists at the
Experimental Storage Ring (ESR) at the GSI lab in
Darmstadt, Germany send a beam of uranium atoms through
foils which successively strip all but one of the 92
electrons in the atom. The resultant ions, U91+, are a
sort of hydrogen atom with the E field turned way up: the
electric field felt by the lone electron is more than
10^16 V/cm, the strongest constant field in any lab. Even
the most intense laser electric field is about 10^12
V/cm. The measured value of the ground-state Lamb shift
is 468 eV with an uncertainty of 13 eV, and largely
agrees with QED predictions. The GSI scientists (Thomas
Stoehlker, email@example.com, 011-49- 615-971-2712) hope
soon to achieve 1 eV precision. (Stoehlker et al.,
Physical Review Letters, 9 Oct; Select articles.) SEMI-SUPERFLUIDITY.
Low-temperature superconductors resemble low-temperature
superfluid helium-4; in the first, pairs of electrons
condense into a macroscopically coherent quantum state,
which manifests itself as a resistanceless fluid, while
in the other helium-4 atoms condense into an analogous
state which manifests itself as a frictionless fluid.
Helium-3 can also form a coherent quantum state and exist
as a superfluid when helium-3 atoms pair up, but things
are more complicated because the He-3 pairs are magnetic,
while He-4 atoms and electron pairs are not. In recent
years one way of studying how He-3 atoms interact is to
loose a sample of the fluid into a sample of aerogel, the
nearly-as-light-as- air solid consisting of a diaphanous
forest of silicon pillars, some only 5 nm across. The
pillars serve as a sort of impurity and the superfluid
properties alter appropriately. The main change is that
the temperature at which this "dirty" He-3
becomes superfluid is depressed relative to the case for
unadulterated He-3. However, a new experiment at the
National Center for Scientific Research in Grenoble,
France (Yuriy Bunkov, firstname.lastname@example.org,
011-33-476-88-1252) reveals that He-3 shows some of the
properties of the quantum state at temperatures in
between the critical points for the pure and dirty cases.
Studying the helium using nuclear magnetic resonance
(NMR) techniques, the Grenoble venture to suggest that
one reason for the anomalous behavior might be the
existence of a new type of superfluidity, one involving
the coupling of not two but four helium-3 atoms. (Bunkov
et al., Physical Review Letters, 16 Oct; Select
NON-QUANTIZED MAGNETIC BUNDLES INSIDE SUPERCONDUCTORS. What happens when a superconductor is placed in a magnetic field? Currents will be induced inside the sample which generate a magnetic field of their own, neutralizing the external field. This exclusion of the external field is called the Meissner effect. If the field is strong enough, however, some of the external field lines will be able to penetrate the superconductor, although only by organizing themselves into flux bundles (also called vortices) of discrete sizes. That is, the bundles are commonly thought to possess a flux in multiples of a basic unit equal to Planck's constant divided by 2 times the charge of the electron. Decades ago theorists pointed out that this is indeed the case for flux bundles deep inside superconductors but not for bundles near the boundary of the sample. Now researchers at the University of Nijmegen in Holland (Andre Geim, email@example.com) and the University of Antwerp in Belgium have demonstrated this experimentally, verifying that some flux vortices do not encompass quantum values of the basic unit of magnetism; indeed some vortices have but a tiny fraction (as small as 1%) of the unit value. (Geim et al., Nature, 7 September 2000; for experimental background, also see Geim et al., Physical Review Letters, 14 August 2000.)
SINGLE-MOLECULE STM CHEMISTRY. The versatility and exactitude of the scanning tunneling microscope (STM) is demonstrated in a new experiment at the Free University of Berlin, where scientists have for the first time manipulated single molecules to perform a complete chemical reaction. Saw-Wai Hla (011-49-30-838-52-813) and his colleagues start with several iodobenzene (C6H5I) molecules resting on a terraced copper substrate. Then they dissociate some of the molecules into iodine and phenyl (C6H5) by injecting electrons from the STM tip. Next the iodine atoms are herded up and moved away with the STM tip. Now the tip pulls one phenyl close to another; they are not yet chemically bonded, though: pulling on one phenyl does not bring the other one along. Finally, another splash of electrons from the tip effectively welds the two phenyls together; proof that binding occurs is that when one phenyl is pulled with the tip, the other comes along for the ride (see a drawing of this sequence at www.aip.org/physnews/graphics). In summary, the making of C12H10 molecules from C6H5I molecules, normally carried out on a copper catalyst and using thermal activation (a process chemists call the Ullmann reaction), has here been forced to proceed by employing one molecule at a time at a cryogenic temperature of 20 K. The researchers believe that new manmade molecules, never before seen in nature, can be engineered in this way, including the selective detachment or replacement of parts of larger molecules for individual assembling of molecular based nano-devices. (Hla et al., Physical Review Letters, 25 Sept; Select Articles.)
ENTANGLED PHOTONS CAN DEFEAT THE DIFFRACTION LIMIT, a new paper suggests. This might lead to a much sharper form of microchip lithography than is possible with "classical" photons. The factor that ordinarily determines how small a standard lithography technique can write features on a chip is known as the diffraction limit, or Rayleigh criterion, which says that you can't inscribe a feature, or see a detail, smaller than half the wavelength of the light or other radiation used to perform the task. But new research (Jonathan Dowling, JPL/Caltech, 818-393-5343, Jonathan.P.Dowling@jpl.nasa.gov) shows that the Rayleigh criterion applies to classical physics but not quantum physics. In their proposal for "quantum interferometric lithography," two entangled photons enter a setup containing mirrors and beamsplitters. The two photons--acting as a single unit--constitute a light wave which is split up and then recombined on a surface, creating patterns on the surface equivalent to those that would be made by a single photon with half the wavelength. On a 2-D surface, this would allow researchers to write features four times smaller than prescribed by the Rayleigh limit. Preparing three entangled photons (still more difficult) and sending them through the device would create even better results: effectively a single photon with a third of the wavelength, enabling nine-fold smaller features on a 2-D surface. Although more work is needed to realize this proposal, the technique potentially allows the creation of features smaller than 25 nm, the size limit below which classical computer designs would begin to fail because of phenomena such as electron tunneling. (Boto et al., Physical Review Letters, 25 Sept 2000; Select Articles.)