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Aktuelle Meldungen bei MM-Physik 02 März 2005 © email: Krahmer |
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Aktuelle Meldungen bei MM-Physik 02 März 2005 © email: Krahmer |
"The American Institute of Physics Bulletin of Physics News" AIP Auswahl 1/2004 by Phillip F. Schewe and Ben Stein, and James Riordon
| December 2, 2003 THE TOP PHYSICS STORIES OF 2003. The first three on our list concern the sharpening of our understanding of the big bang era, evidence for new quark groupings, and progress in manipulating quantum gases. At the largest size scale, new observations from the Wilkinson Microwave Anisotropy Probe (WMAP), the Sloan Digital Survey and other telescopes have reduced the uncertainties in the values of such cosmic parameters as the Hubble constant, the age of the universe, and the fractions of total energy vested in the form of dark and luminous matter www.aip.org/enews/physnews/2003/split/624-1.html ; www.aip.org/enews/physnews/2003/split/659-1.html Going to the opposite extreme, at the level of elementary matter, new data indicate that quarks needn't appear only in clumps of three (baryons) or two (mesons). Work at SLAC (US) and KEK (Japan) hint that quarks might also exist in "tetraquark" states advances in the use of attosecond laser pulses in studying chemical reactions the use of microfluidics---essentially the science of fluids on a chip---in processing bio-particles such as blood cells and DNA molecules evidence for the focusing of light in left-handed materials, materials with a negative index of refraction, and vindication of earlier research in this area |
| December 10, 2003 DO QUANTUM MEASUREMENTS CHANGE IF THE DETECTOR MOVES? For example, could a count of the number of photons in a burst of light depend on the location of the detector in an extreme gravitational field? These ideas, long pondered by physicists, might be verifiable in the lab, according to a new theory in which a Bose Einstein condensate (BEC) of cold atoms acts as a stand-in for the universal vacuum. The related notion that potential energy residing in the vacuum can influence the geometry of spacetime and thus the expansion of the cosmos could also be testable in a tabletop experiment here in Earth. The pertinent phenomenon that would facilitate this line of research is called the Unruh-Davies effect, which suggests that a detector accelerating (not just moving at a constant speed but actually moving ever faster) through a vacuum will effectively encounter photons coming out of the vacuum. (A related phenomenon is the Gibbons-Hawking effect, in which photons, "Hawking radiation," can be detected in the gravitationally intense region of a black hole). In the Unruh effect the energy needed to turn virtual photons into real photons would be supplied by the accelerating detector itself. The detector would see the vacuum not as an empty space but as a thermal bath of photons. The same effect can disrupt quantum teleportation (see the Update from a few weeks ago---http://www.aip.org/enews/physnews/2003/split/660-2.html ). The "temperature" of this bath would be proportional to the detector's acceleration. Actually observing such a thermal bath (equivalent to an effective temperature of something like 10^-15 K for a detector acceleration one hundred thousand times more than that felt by us on the surface of the Earth) with any foreseeable manmade detector is close to impossible, but two physicists at the Leopold-Franzens-Universitaet in Innsbruck, Petr Fedichev (peter.fedichev@uibk.ac.at) and Uwe Fischer (uwe.fischer@uni-tuebingen.de), believe the effect could be probed by studying how sound waves ripple through BECs in the lab. The superfluid condensate of atoms would correspond to the vacuum and phonons would be analogous to photons moving through a curved space-time. Before the experiment can be performed, larger BECs than used so far will be needed, as well as sharper optical manipulation of atoms in the BEC. (Physical Review Letters, 12 December 2003) |
| January 22, 2004 PULSARS WITH BOUNCE. Astronomers at the Max Planck Institute for Astrophysics in Munich and the University of Chicago have a new explanation for the curious high speeds of some pulsars moving through interstellar space. In gravitating themselves to death, some stars might suffer an asymmetric supernova. Since momentum must be conserved at all times, the imbalance in the explosion debris would be taken up by the remnant of the star, namely the spinning neutron star, or pulsar, that is left behind. Or rather, the pulsar won't be left behind, but will be kicked out into space away from the original stellar position with enough velocity (as much as 1000 km/sec) to be measurable by telescopes on Earth. For some time, one explanation for the pulsar velocities has been the idea that the emission of neutrinos from the newly formed neutron star causes the acceleration. Even a 1% asymmetry in the emission could result in pulsar speeds as great as 300 km/sec, but this line of thinking necessitates the presence of extreme conditions, such as magnetic fields of 10^16 gauss. Thomas Janka (thj@mpa-garching.mpg.de) and his colleagues believe the observed effects are better explained if the asymmetries come not from neutrino emission but from the way matter reacts with neutrinos shooting into (and heating) the infalling stellar layers that are about to be flung back out into space during the supernova explosion. In other words, the irregularities arise not from particle physics but from the purely hydrodynamic effects of a gust of neutrinos plowing into a layer of material, a process in which small instabilities in a shock front can quickly grow much larger. Scheck et al., Physical Review Letters, 9 January 2004; see colorful illustrations at www.mpa-garching.mpg.de/mpa/research/current_research/hl2003-10/hl2003-10-en.html LARGE SCALE STRUCTURES IN THE EARLY UNIVERSE are also larger than expected. Like the presence of surprisingly early mature galaxies at a redshift of about 2 (see the item above) another result at the AAS meeting suggests that the standard cosmological model---or at least that part of it devoted to galaxy formation---is in need of revision. A group of astronomers using the Blanco Telescope of the Inter-American Observatory in Chile and the Anglo-Australian Telescope in Australia reported seeing a grouping of 37 galaxies, all at a redshift close to 2.38, spread 300 million light years across the sky. Povilas Palunas (University of Texas) said that this constitutes the largest observed structure in the distant universe. According to models that simulate how the hot diffuse matter of the infant cosmos distilled into a web of knots and filaments, such an immense agglomeration should not have arisen so quickly. The statistical case for saying that this sampling of bright galaxies (fainter galaxies could not be seen) is truly a coherent structure and not just a chance juxtaposition can be expressed as a probability with 1000-to-1 odds, a likelihood obtained by looking not at the specific arrangement of galaxies themselves but at the daunting amount of void between the galaxies. Gerard Williger (Johns Hopkins) said that he and his colleagues would naturally like next to sample adjoining volumes of deep space in order to test the proposition that the hasty filimentation of matter seen in this initial data set (the observed galaxies lie in the southern constellation "Grus") is not an isolated incident www.gsfc.nasa.gov/topstory/2004/filament.html |
| February 2, 2004 ELEMENT 115 HAS BEEN DISCOVERED at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia. JINR physicists and their longtime collaborators from Lawrence Livermore National Lab in the US produced 4 atoms of the new superheavy element by striking a target of americium-243 atoms with a beam of calcium-48 ions. The beam energy used, 248 MeV, was chosen to produce just the right energy conditions for making the amalgamated nucleus but not causing it to break up, at least not right away. The long lifetime observed for element 115 suggests that physicists might be getting closer to the "island of stability," the presumed region on the chart of possible nuclear isotopes for which certain combinations of protons and neutrons (collectively known as nucleons, the regular constituents of all nuclei) are much more stable than some of the other heavy nuclei made artificially at accelerators. In general, nature doesn't produce elements heavier than uranium (element 92) and scientists must resort to colliding smaller nuclei to build up heavyweight elements. In previous experiments conducted by the same team at Dubna, evidence has been recorded for elements 114 and 116. One sequential decay event corresponding to element 118 was also seen. (Claims for a separate discovery of element 118 by a group at the Lawrence Berkeley National Lab in the US were later withdrawn.) In the new experiment, using the same approach, a beam of calcium-48 atoms (atomic number, or Z, equal to 20) was plowed into a target of americium-243 atoms (Z=95). By bringing together element 95 with element 20, four atoms of element 115 were created The nuclei of these precious atoms apparently lived for 90 msec. They expired in the following way: by decaying first to element 113 by the emission of an alpha particle (a nuclear morsel consisting of two protons and two neutrons); thence to element 111 by alpha emission again; and then by three more alpha decay steps to element 105 ("Dubnium") which, after the delay of a whole day (almost an eternity in nuclear physics) from the time of the original interaction, finally fissioned. Besides being a very difficult physics experiment to carry out, this work represents a great feat of nuclear chemistry, since it entailed sifting 4 atoms out of trillions of candidates. In other words, the gas-filled separator, employing chemistry, proved to be just as important as the accelerator. In the past decade or so even-Z superheavy nuclei---112, 114, 116, 118---were sought at Dubna chiefly because of the facility's intense beams of Ca-48 and the ready availability of even-Z actinide targets. By the way, this experiment also marks the discovery of a second element, 113, which had not been seen before either. (Oganessian et al., Physical Review C, upcoming article; contact Yuri Oganessian at JINR, oganessian@flnr.jinr.ru, 011-7-09621-62151; Ken Moody at Livermore, 925-423-4585, moody3@llnl.gov, or Mark Stoyer, mastoyer@llnl.gov, cell phone 301-661-1169; background article by Oganessian in Scientific American, Jan 2000.) |
| March 3, 2004 THE ACCELERATING EXPANSION of the universe, the notion that the big bang enlargement of spacetime is not slowing down but actually gathering speed, has received new experimental support in the form of supernova observations made by the Hubble Space Telescope (HST). Previous evidence for such a cosmic acceleration consisted of studies of the dimness of remote supernovas ( http://www.aip.org/enews/physnews/1998/split/pnu355-1.htm ), and represented a major revision for some scientists who had long thought that the mutual gravity among galaxies would slow or even reverse the cosmological expansion. The new HST observations consist of reexaminations of 170 previously studied supernovas and the announcement of 16 new objects, including 6 of the 7 most distant type Ia supernovas yet recorded. The new data are in line with the accelerating-expansion hypothesis employing the mysterious mechanism usually referred to as "dark energy." The energy of the universe would be divided up as follows: 29% in the form of matter (dark plus luminous) and 71% as dark energy. (NASA press conference, 20 Feb; Riess et al., preprint astro-ph/0402512 ) BUBBLE FUSION, the apparent generation of fusion energy through the violent collapse of bubbles in a liquid tank, has been reported in a paper about to be published in Physical Review E (Taleyarkhan et al., upcoming, probably March 2004). The paper, a followup to a controversial report published two years ago ( http://www.aip.org/enews/physnews/2002/split/579-2.html ), reports "statistically significant neutron and gamma ray emissions" after sound waves and pulsed neutrons hit a chilled liquid acetone tank spiked with deuterium fuel. The researchers (Rusi Taleyarkhan, formerly at Oak Ridge but now at Purdue, 765-494-0198, rusi@purdue.edu ) report the observation of flashes of light (sonoluminescence) as well as the emission of neutrons with energies of less than or equal to 2.5 MeV---what you would expect if pairs of deuterium atoms were fusing together to produce energy in their setup. While the researchers describe various improvements to their experimental setup, in response to comments received in their original paper 2 years ago, critics (including Aaron Galonsky, Michigan State, galonsky@nscl.msu.edu, 850-267-8976 by phone until April 1) still have a number of concerns. According to Galonsky, the data for neutron emissions is lumped together with data of gamma-ray emissions. While separating neutron and gamma-ray signals is challenging, it is necessary to have a clean neutron-only spectrum to have an unambiguous demonstration of nuclear fusion. Willy Moss of Livermore (925-422-7302, wmoss@llnl.gov) says "Although I believe that thermonuclear sonofusion [not to be confused with cold fusion] may not be impossible...I am still not convinced... I believe that additional tests need to be done and many should have been performed and discussed in the paper, for example...if neutrons are being generated, then how about moving the scintillator further away from the sample to see if the signal decreases, due to the decreasing solid angle of the detector?" (Other experts, Richard Lahey, RPI, laheyr@rpi.edu , 518-276-6614, a co-author on the paper; Mike Saltmarsh, Oak Ridge, 865-576-6915, saltmars@mail.phy.ornl.gov, co-author of a paper that attempted to duplicate the initial results but reported a null result---see Shapira and Saltmarsh, Phys Rev Lett, 19 August 2002) |
| March 10, 2004 MULTILEVEL MOLECULAR MEMORY. In conventional memory cells a bit of information is either a zero or one. (In hypothetical quantum computers, a bit could be both a zero and a one at the same time, but that kind of nimble balancing is years away from exploitation and so bits continue to be bi-level.) In the meantime one way of cramming more data into a fixed lateral region on a data storage device, other than shrinking the cell's size, is to store more than one bit in each memory cell. This is one goal of molecular electronics (or "moletronics") where, for instance, one would like to store information in the form of parcels of charge placed at several active sites around a single molecule. A USC/NASA-Ames collaboration has taken a step in the direction of such a chemical memory by producing a memory cell with three different controllable bit states, with a total of 8 (2 raised to the 3rd power) distinct levels. This multilevel molecular memory unit works by charging or discharging "molecular wires" consisting of molecules (attached to an underlying nanowire) into different chemically reduced or oxidized (redox) states. (See the figure at http://www.aip.org/mgr/png/2004/213.htm ) The information stored in the unit can be read back out by sampling the resistance of the nanowire; the attached redox molecules act, in effect, as chemical gates for controlling the number of electrons in the nanowire. In tests so far the data written this way has survived for as long as 600 hours, compared to retention times of a few hours for one-bit-per-cell molecular memories. The researchers (contact Chongwu Zhou, USC, chongwuz@usc.edu. 213 740 4708) are attempting to make more extended memory chips using the new principle. Data density rates as high as 40 Gbits/cm^2 are expected. (Li et al., Applied Physics Letters, cover story in the 15 March 2004 issue;) |
| April 1, 2004
THE CORE-MANTLE BOUNDARY, halfway down to the center of the Earth, has become a bit more understandable because of new laboratory studies of the behavior of rock under pressure and because of new computer simulations predicting the existence of another polymorph of the mineral MgSiO3 that is more stable than the other phase previously known. Previous seismic assessment of the so called D" layer just above the core-mantle boundary has been puzzling geoscientists. The most prevalent mineral at great depths is MgSiO3, a mineral generally configured as a perovskite, a class of ceramic crystal in which three chemical elements in the ratio 1:1:3 form a distorted cubic structural unit. But some scientists believe that the perovskite cannot avoid dissociation amid the hard conditions at the core-mantle boundary. One lab study of perovskites subjected to the conditions of high pressures and temperatures that approximate the D" layer, indicated that the mineral had survived in a new form. In other words, the great pressures and temperatures bring about a phase transition in the mineral. The scientists, at the Tokyo Institute of Technology, scattered x rays from their sample in its squeezed form. The x-ray data has now been analyzed by collaborators at the University of Minnesota and the results, along with first principles calculations, were reported at last week's APS March Meeting in Montreal. Minnesota scientists Jun Tsuchiya, Taku Tsuchiya, Koichiro Umemoto, and Renata Wentzcovitch (papers L28.9 and L28.11) said that the new form of MgSiO3, called "post perovskite," should be stable at the D" layer. Its anisotropic structure, apparently unknown so far, could account for some of the seismic irregularities (changes in the speed of seismic waves) at those depths. DATING WATER AND TRACING BONES to high precision will be more widely available for geological and biomedical applications thanks to state-of-the-art atom counting techniques. In a pair of new papers, Zheng-Tian Lu of Argonne National Laboratory (lu@anl.gov) and his colleagues have demonstrated two new applications of Atom Trap Trace Analysis (ATTA; see Update 416), in which researchers trap desired isotopes with lasers and magnetic fields and then count them with laser techniques. ATTA has now been used to count krypton-81 atoms in groundwater samples in the ancient waters of the Sahara. Kr-81 (half life=229,000 years) is a rare isotope produced by the cosmic rays in the atmosphere, and accompanies more common forms of atmospheric krypton. Trapped in water underneath the Sahara, the abundance of Kr-81 relative to other Kr isotopes provides information on how long the water has been there. Extracting krypton from the Nubian aquifer in the western Sahara, and using the ATTA technique, the researchers found that the water's age ranges from 200,000 to a million years old, depending upon the sample location. In another application, researchers used ATTA to count individual calcium-41 atoms released from the bones of a human subject. This isotope is injected into osteoporosis patients and subsequent measurements of its abundance can be used to monitor bone loss and retention rates. Until now, medical researchers had to rely upon particle accelerators to perform this task. But the smaller and potentially cheaper ATTA is now precise enough to do the job, with the ability to detect one Ca-41 atom per 10^8-10^10 calcium atoms. With further increases in precision (in which one Ca-41 atom can be detected amidst 10^15 other calcium atoms) the technique could be ideal for archaeological dating (half-life of Ca-41 = 103,000 year) of ancient bones ranging from 50,000 to 500,000 years old. (Sturchio et al., Geophysical Research. Letters, 12 March 2004; and Moore et al., Physical Review. Letters., upcoming article) |
| April 8, 2004 MRI WITH 80-NM RESOLUTION, far better than for the best medical scans, has been achieved with a device that combines atomic force microscope (AFM) and nuclear magnetic resonance (NMR; also known as magnetic resonance imaging, or MRI) technology. In the hybrid methodology called magnetic resonance force microscopy (MRFM), a tiny magnetized particle is attached to a cantilever which is then brought near a sample which surrounded by a coil that emits radio waves. When a tiny magnetic domain in the sample feels just the right amount of magnetic field from the nearby coil and magnetic particle it will vigorously interact with them resonantly. (The tiny volume being probed is referred to as a voxel, and the sample-coil-particle combination is equivalent to the setup in a standard MRI machine for imaging, say, a tumor.) The sample-particle resonant interaction causes the cantilever to oscillate (the particle on the cantilever is like a man bouncing resonantly, higher and higher, on a diving board). The oscillating cantilever, monitored with a laser beam, is then scanned from place to place, filling out a two-dimensional and then a three-dimensional map of the resonant interaction. (The scanned, oscillating cantilever plus laser readout is the AFM part of the setup.) The goal is not to help surgeons (the best medical MRI has a spatial resolution of about a tenth of a millimeter) but to be able to scan and image small objects---especially particles of biological importance, such as viruses and proteins---with atomic-scale resolution. In other words, you would like to increase the sensitivity so as to map the presence of single spins. The voxel in this case would be shrunk to less that than 1 nm. A new experiment at the University of Washington is far from reaching this goal, but researchers have improved sensitivity by a factor of almost 10,000 from the time of the earliest MRFM imaging papers in 1996. (For a report from 1997, see http://www.aip.org/enews/physnews/1997/split/pnu313-1.htm). The higher sensitivity in general comes by shrink the apparatus and cooling things (currently, to 80 K) as much as possible, the better to read out the oscillations and position the sample with greater accuracy. The Washington voxel of 80 nm---how big is it? One of the team members, John Sidles (206-543-3690, s idles@u.washington.edu) says that about a million of these voxels could fit inside a typical blood cell. (Chao, Dougherty, Garbini, Sidles, Review of Scientific Instruments, May 2004; website, courses.washington.edu/goodall/MRFM ) Other groups are working in this area and are attempting to marshal the requisite equipment needed for single-spin imaging. According to Joseph Shih-hui Chao, one of the authors, this would include millikelvin temperatures, 30-nm-sized magnetic particles, sub-nm positioning accuracy, and yet softer cantilevers. ENTANGLEMENT BETWEEN A PHOTON AND A TRAPPED ATOM has been directly observed for the first time, offering a method for establishing links between quantum memories over appreciable distances. Entanglement--a sort of arranged marriage between two or more particles--has usually been directly measured between species of the same kind, such as all photons or all atoms. In recent experiments, however, University of Michigan researchers achieve inter-species entanglement by trapping a cadmium ion with electric fields. They put the trapped cadmium's outer electron into an excited (high-energy) state. The atom immediately decays to one of two ground (low-energy) states--let's call them A and B--while emitting a photon. State A represents the case in which the spin of the atom's outer electron is lined up with the spin of the atom's nucleus; B represents the case in which the electron's spin is opposite to that of the nucleus. The photon's polarization--the direction of its electric field--correlates with the resulting ground state of the atom. In other words, if the atom decays to state A, the photon's electric field rotates clockwise, and if it decays to state B, counterclockwise. Because each path is equally likely, quantum mechanics forces us to consider both decay routes as occurring at the same time. So once the atom decays, both it and the photon essentially carry out both possibilities--each enters a "superposition" of two states. Meanwhile, their properties remain interdependent--or correlated--with each another. As a result, the atom and photon are in an entangled superposition. While the individual participants are in fuzzy, unresolved states, the terms of their marriage are perfectly defined. However, measuring the photon--the act of observing it--forces the photon to make a commitment. Upon measurement it must assume one polarization state or another--clockwise or counterclockwise. And this in turn forces the atom to collapse into state A (if the polarization is clockwise) or state B (if polarization is counterclockwise). One could conduct powerful logic operations based on these interdependencies. This cross-species entanglement technique has shortcomings--researchers cannot actively create an entangled state but must wait for it to occur by detecting the photon, so the entanglement is immediately destroyed and efficiency is not high. However, if two remotely located trapped atoms simultaneously decay in the same way as reported in this experiment, and the two emitted photons are jointly detected after interfering on a beamsplitter, then the two atoms become entangled and available for subsequent use for long-distance quantum computing and quantum communication. (Blinov et al., Nature, 11 March 2004; contact Chris Monroe, crmonroe@umich.edu) |
| April 13, 2004
FIRST EVIDENCE FOR SUPERFLUIDITY IN AN ATOM-BASED FERMI GAS has been observed by researchers at Duke University (including John Thomas, 919-660-2508, jet@phy.duke.edu, and Michael Gehm, mgehm@ee.duke.edu, 919-403-5003). In a Physical Review Letters paper published online today http://link.aps.org/abstract/PRL/v92/e150402 , the researchers have observed an ultracold gas of lithium-6 atoms acting as one big vibrating "jelly." While the jelly-like (or "hydrodynamic") behavior could arise in ordinary versions of ultracold lithium gases, the researchers found evidence that their gas was a superfluid, a "perfect" jelly which vibrates for a long time after being shaken. The properties of the atomic jelly can provide information on much smaller superfluid systems (such as a quark-gluon plasma) and much larger ones (neutron stars). The behavior of the jelly could even help determine whether it's physically possible to create superconductors which operate well above room temperature, which could lead to breakthroughs from widely available energy-saving power lines to magnetically levitated trains. What's shared by all these systems, from a quark-gluon plasma to neutrons in neutron stars, is that they are made of strongly interacting pairs of "spin-up" and "spin-down" particles (spin up/down is analogous to the atoms having bar magnets pointing in opposite directions). To produce the observed behavior, the researchers believe that the interaction mechanism among their lithium-6 atoms is in a weird "cross-over regime" (see Update 671), a condition in which the atom pairs are neither molecules (in which case they would form a molecular Bose Einstein condensate, see Update 663) nor they type of weakly bound Cooper pairs found in conventional superconductors. In their experiment, the researchers cooled and trapped lithium-6 atoms with a focused laser beam, whose electric field confined the atoms. The researchers made sure the atoms were in a 50-50 mixture of spin-up and spin-down states. They then used their optical system to lower the temperature of atoms via "evaporative cooling" (i.e., allowing hotter atoms to escape to lower the overall temperature of the gas). Next, they tested the gas's ability to act like a vibrating "jelly." To start vibrations in the gas, they turned off the trapping laser for a short time, allowing the gas to expand, and then turned the laser back on again. At this point the gas cloud was quivering, and the researchers took a series of pictures to show these vibrations (see movie at www.aip.org/mgr/png). They measured the cloud's frequency of vibration, as well as how long the vibrations persist. In one case, they adjusted the magnetic field so that the atoms were strongly interacting. In this case, they found a frequency of vibration of 2837 Hz, in very close agreement with a theoretical prediction of 2830 Hz for a hydrodynamic Fermi gas. Lowering the temperature of the gas caused the vibrations or "oscillations" to last for a longer time, in contrast to an ordinary hydrodynamic gas, in which a lower temperature would cause the oscillations to "damp" or die out more quickly. The Duke physicists ruled out two non-superfluid scenarios for the behavior, namely that the oscillations were caused by (1) a high rate of atomic collisions (however, in this scenario, the oscillations would die out more quickly as the temperature is lowered) and (2) a collisionless gas that oscillates via mean-field interactions, the net effect of many atom-to-atom interactions (however, the predicted vibration frequency for this scenario differs by 500 Hz from the observations). Still, the researchers do not have an iron-clad case for superfluidity yet, in large part because the theory for strongly interacting superfluid Fermi gases is incomplete. Namely, there is no prediction of how the damping times of the vibrations should increase with decreasing temperature, which would help to identify a "transition temperature" below which superfluidity would occur. (In their setup, the Duke team started seeing evidence for superfluidity at temperatures below 0.4 to 0.7 Microkelvin.) In summary, the experiments constitute first evidence for what could plausibly be superfluid behavior based on pairs of fermion atoms in a gas. The photos provide macroscopic information (i.e., viewing the overall gas that's visible to the naked eye) that complement the "microscopic" information provided by other groups (Update 671), which probe the pairing of spin-up and spin-down atoms. (Kinast et al., Physical Review Letters, 16 April 2004) GREATLY IMPROVED SOLAR CELLS might result from the use of a photophysical process in which for each incident solar photon not one but two excitons (electron-hole pairs) are created. As with photosynthesis what happens in a solar cell is the conversion of light energy into a small current of electrons; in plants the freed electrons helps to build glucose; in solar cells the currents are collected in the form of electricity. Victor Klimov and Richard Schaller at Los Alamos have enhanced the phenomenon called "impact ionization," which can significantly improve the efficiency of the conversion of solar energy to electrical current. Normally, an incident photon striking a semiconductor produces an electron-hole pair plus a bit of heat. By using sub-10-nm sized nanoparticles made of lead and selenium atoms, the Los Alamos scientists encourage the interaction to spawn a second exciton instead of the heat. Although they haven't yet built a working solar cell, they are the first to demonstrate the efficacy of getting the PbSe nanocrystals to render more photo-current. Implementing the new process might result in efficiency gains of more then 35% in the conversion of light to current. (Physical Review Letters, upcoming article; contact Victor Klimov, 505-699-7541, klimov@lanl.gov; http://quantumdot.lanl.gov) |
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