| 查看: 238 | 回复: 0 | |||
| 当前主题已经存档。 | |||
[交流]
PHYSICS NEWS UPDATE 769
|
|||
|
PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 769 March 17, 2006 by Phillip F. Schewe, Ben Stein, and Davide Castelvecchi THE INFLATIONARY BIG BANG MODEL has passed a crucial test as scientists working on the Wilkinson Microwave Anisotropy Probe released a long-awaited second set of data at a press conference held March 17. WMAP was launched in 2001 to map the anisotropies in the cosmic microwave background (CMB) with far greater precision than the Cosmic Background Explorer, the predecessor that first discovered the anisotropies in 1990s. The earlier release of WMAP data 3 years ago nailed down several grand features of the universe that had previously been known only very roughly, including: the time of recombination (380,000 years after the big bang, when the first atoms were formed); the age of the universe (13.7 billion years, plus or minus 200 million years); and the makeup of the universe (with dark energy accounting for 73% of all energy---see update 624, http://aip.org/pnu/2003/split/624-1.html). Since that 2003 announcement WMAP researchers have painstakingly worked to reduce the uncertainties in their results. The big new thing in yesterday's announcement, based on three years of data, was the release of a map of the sky containing information about the microwaves' polarization. The microwaves are partly polarized (oriented) from the time of their origin (emerging from the so called sphere of last scattering---see http://www.aip.org/pnu/2002/split/591-1.html ) and partly polarized by scattering (later on their journey to Earth) from the pervasive plasma (mostly ionized hydrogen) created when ultraviolet radiation from the first generation of stars struck surrounding interstellar gas. WMAP now estimates that this reionization, effectively denoting the era of the first stars, occurred 400 million years after the big bang, instead of 200 million years as had been previously thought. The main step forward is that smaller error bars, courtesy of the polarization map and the much better temperature map across the sky (with an uncertainty of only 200 nK), provide a new estimate for the inhomogeneities in the CMB's temperature. The simplest model, called Harrison-Zeldovic, posits that the spectrum of inhomogeneities should be flat; that is, the inhomogeneities should have the same variation at all scales. Inflation, on the other hand, predicts a slight deviation from this flatness. The new WMAP data for the first time measures the spectrum with enough precision to show a preference for inflation rather than the Harrison-Zeldovic spectrum---a test that was long-awaited as inflation's smoking gun. (Papers available at map.gsfc.nasa.gov/m_mm/pub_papers/threeyear.html ) TWO-DIMENSIONAL CARBON, OR GRAPHENE, has many of the interesting properties possessed by one-dimensional carbon (in the form of nanotubes): electrons can move at high speed and suffer little energy loss. According to Walt deHeer (Georgia Tech), who spoke at this week's meeting of the American Physical Society (APS) in Baltimore, graphene will provide a more controllable platform for integrated electronics than is possible with nanotubes since graphene structures can be fabricated lithographically as large wafers. Single sheets of graphene were only isolated in 2004 by Andre Geim (Univ. Manchester). In graphene, electron velocity is independent of energy. That is, electrons move as if they were light waves; they act as if they were massless particles. This extraordinary property was elucidated in November 2005 through experiments (see background article in the Jan. 2006 Physics Today) using the quantum Hall effect (QHE), in which electrons, confined to a plane and subjected to high magnetic fields, execute only prescribed quantum trajectories. These tests were conducted by groups represented at the APS meeting by Geim and Philip Kim (Columbia Univ.). The QHE studies also revealed that when an electron completes a full circular trajectory in the imposed magnetic field, its wavefunction (encapsulating the electron's quantum wave nature) is shifted by 180 degrees. This modification, called "Berry's phase," acts to reduce the propensity for electrons to scatter in the backwards direction; this in turn helps reduce electron energy loss. Geim reported a new twist to this story. Studying QHE in graphene bilayers he observed a new version of QHE, featuring a doubled Berry's phase of 360 degrees. Also, Geim drew a comparison to certain cosmologies in which multiple universes can co-exist, each with its own set of physical constants; in graphene, he said, where electrons move in a light-like way, with a fast speed (but one somewhat less than the speed of light in vacuum), the parameter which sets the scale of the electromagnetic force, namely the fine structure constant (defined as e^2/hc), has a value of roughly 2.0 rather than the customary 1/137. The goal now is to learn more graphene physics and then worry about applications. For example, Walt deHeer reported that a plot of resistance versus applied magnetic field had a fractal shape. DeHeer said that has so far has no explanation for this. As for applications, he said that on an all-graphene chip, linking components with the usual metallic interconnects (which tends to disrupt quantum relations) would not be necessary. Thus the wave nature of electrons could be more fully exploited for quantum-information purposes. De Heer's group so far has been attempting to build circuitry in this way; they have made graphene structures (including a graphene transistor) as small as 80 nm and expect to get down to the 10-nm size. |
回复此楼» 猜你喜欢
体制内长辈说体制内绝大部分一辈子在底层，如同你们一样大部分普通教师忙且收入低
已经有18人回复
-
面上可以超过30页吧？
已经有7人回复
-
网上报道青年教师午睡中猝死、熬夜猝死的越来越多，主要哪些原因引起的？
已经有5人回复
-
“人文社科而论，许多学术研究还没有达到民国时期的水平”
已经有6人回复
-
版面费该交吗
已经有13人回复
-
为什么中国大学工科教授们水了那么多所谓的顶会顶刊，但还是做不出宇树机器人？
已经有10人回复
-
什么是人一生最重要的？
已经有4人回复