Braca Grim Bajke Pdf
Braca Grim Bajke Pdf
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Najlepse Bajke Sveta Braca Grim
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Grim StatisticsGunre may surpass autoaccidents as a cause of death
The European tourists whowere shot by highway hun-ters in Florida were driv-ing cars that were legally requiredto have seat belts and may evenhave been equipped with airbags.
Whereas nationwide concernwith automobile safety has led toimproved crash-worthiness andtougher laws for drunken driving,the number of deaths caused bygunfire continues to increase. Willthe declining curve of auto-relat-ed mortality intersect with the risingcurve of deaths from firearm use?
The most authoritative statistics in-dicate that the question is not will?
but when? According to Garen Winte-mute of the University of California atDavis, guns may move into first placeduring the next decade.
Wintemutes comparison of gunand automobile mortality statis-tics (left) was published in theJournal of the American MedicalAssociation. The date on which thenation achieves the crossoversome reports reveal that Louisi-ana and Texas have already donesodepends on the stability ofcurrent trends. Deaths from gun-shot wounds have increased rap-idly during the past ve years (af-ter a decade of decline), whereasautomobile fatalities are fallingfaster than usual, as they tend todo in bad economic times. If thisnew pattern persists, more peo-ple will die from gunfire than inauto accidents during 1994. But
if long-term historical trends reassertthemselves, the crossover will wait un-til a few years after the turn of the century. Paul Wallich
SCIENCE AND THE CITIZEN
14 SCIENTIFIC AMERICAN November 1993
Dots IncredibleControlling single electronsin a quantum dot
Manipulating small numbers ofatomic particles seems to havebecome a standard part of therepertoire of physics. So devotees of theart are being dazzled by a supreme featof nanoscale sleight of hand, which hasbeen achieved by researchers at AT&TBell Laboratories.
The Bell Labs workers, Raymond C.Ashoori, now at the Massachusetts In-stitute of Technology, andHorst L. Stormer and their col-leagues, report in Physical Re-view Letters that they can con-trol the behavior of as few as one or two electrons in apatch of semiconducting ma-terial that is only a few tens of nanometers square. This lev-el of resolution was previous-ly thought to be unattainable.The success should enable in-vestigators to explore quan-tum phenomena that have nev-er been observed in an experi-mental setting and might serveas a basis for signicant tech-nological advances.
The semiconductor specksare known as quantum dots,
or articial atoms. Although many realatoms actually constitute a quantumdot, the electronic properties of a dotmake it the equivalent of an individualatom. Like a real atom, a quantum dotharbors distinct numbers of electrons.But rather than being held in place bythe charge of a nucleus, the electronsin an articial atom are conned byboundaries of a material. Trapped insuch a box, the electrons occupy dis-crete energy levels, just as they dowhen bound by a real nucleus. A quan-tum dot is constructed from a lm ofsemiconducting material, such as galli-um arsenide, sandwiched between two
insulating layers. The lithographic pro-cesses used to etch circuit patterns canform the articial atoms [see Dimin-ishing Dimensions, by Elizabeth Cor-coran; SCIENTIFIC AMERICAN, November1990; and Quantum Dots, by Mark A.Reed, January].
Detailed studies of the properties ofquantum dots have been dicult. Thestandard method of examining theirelectronic characteristicsmeasuringthe charge owing through themwaslimited in resolution. The current issmall, and you have to put 30 to 40 elec-trons into the articial atom before cur-rent ows, according to Marc A. Kast-
ner, an M.I.T. investigator whoalso explores articial atoms.
But Ashoori had a dream oflooking at electrons one byone as they accumulate toform an articial atom. Whileworking at Bell Labs, he andhis colleagues decided to trymeasuring changes in theamount of charge (that is, thecapacitance) caused by the dotrather than the amount of cur-rent owing though it. Thetechnique, single-electron ca-pacitance spectroscopy, callsfor placing an articial atombetween two electrically con-ducting plates. We then applya tickling voltage to induce anelectron from one of the plates
LONG-TERM MORTALITY TRENDS for motor vehi-cles and firearms (colors) converge in 2003, short-term ones (black) in 1994.
QUANTUM DOTS are fabricated inside metal disks aboutone micron in diameter. A contact loop collars the middledisk and transmits the signals from the tunneling elec-trons inside to measurement devices.
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Copyright 1993 Scientific American, Inc.
