ISSN:0031-9228    

DOI:10.1063/1.2802857

出版商:AIP    

年代:1998

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.882086

出版商:AIP    

年代:1998

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.882087

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Aquantum dot, as its name implies, is a minuscule region of metallic or semiconductor material whose dimensions can be as small as a few tens of nanometers on a side. It has been likened to an artificial atom because it carries a discrete charge and quantized electronic energy levels (see the article by Marc Kastner in PHYSICSTODAY, January 1993, page 24). That analogy has now been taken a step further with the demonstration that a quantum dot interacts with nearby metallic leads in much the same way that a single magnetic impurity interacts with a surrounding metallic substance—in the phenomenon known as the Kondo effect.

ISSN:0031-9228    

DOI:10.1063/1.882089

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Since the mid‐1970s, an “ozone hole” has developed in the stratosphere over Antarctica each austral summer. The Arctic, by contrast, has been immune to such severe depletion because it is much less susceptible to developing a strong vortex, a pattern of winds that encircles the pole, isolating a continent‐sized body of air in which all the conditions for ozone depletion can be established. In March 1997, however, the Arctic stratosphere behaved more like its southern counterpart than had ever been observed before, and Arctic ozone levels hit record lows for March. (See the figures on page 19.) The observations were reported in eight papers in the 15 November 1997 issue ofGeophysical Research Letters.

ISSN:0031-9228    

DOI:10.1063/1.882090

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Over the past ten years, laser intensities have increased by more than four orders of magnitude to reach enormous intensities of1020 W/cm2.The field strength at these intensities is on the order of a teravolt per centimeter, or a hundred times the Coulombic field binding the ground state electron in the hydrogen atom. The electrons driven by such a field are relativistic, with an oscillatory energy of 10 MeV. At these intensities, the light pressure,P = I/c,is extreme, on the order of giga‐ to terabars. The laser interacting with matter—solid, gas, plasma—generates high‐order harmonics of the incident beam up to the 3 nm wavelength range, energetic ions or electrons with mega‐electron‐volt energies (figure 1), gigagauss magnetic fields and violent accelerations of1021g(gis Earth's gravity). Finally, the interaction of an ultraintense beam with superrelativistic particles can produce fields approaching the critical field in which an electron gains in one Compton wavelength an energy equal to twice its rest mass. Under these conditions, one observes nonlinear quantum electrody‐namical effects. In many ways, this physical environment of extreme electric fields, magnetic fields, pressure, temperature and acceleration can be found only in stellar interiors or close to the horizon of a black hole. It is fascinating to think that an astrophysical environment governed by hydrodynamics, radiation transport and gravitational interaction can be re‐created in university laboratories for extremely short times, switching the role of the scientist from voyeur to actor.

ISSN:0031-9228    

DOI:10.1063/1.882131

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The unexpected discovery of the cosmic microwave background by Arno Penzias and Robert Wilson in 1965 is now the centerpiece of our understanding of the Big Bang and the subsequent evolution of the universe. (SeePHYSICS TODAY, November 1997, page 32.) The discovery also set off something of a race to verify one of its implications for cosmic rays. In 1966, Kenneth Greisen (Cornell University) pointed out that the most energetic cosmic‐ray particles would be affected by interaction with the ubiquitous photons of this microwave background. Greisen predicted that, if cosmic‐ray sources were far enough away from us and if their energy spectrum extended beyond1020 eV,then the ultra‐high‐energy protons and nuclei would interact inelastically with the backgound radiation.

ISSN:0031-9228    

DOI:10.1063/1.882132

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

If you are in the habit of watching your drip coffee maker prepare your morning beverage, you'll occasionally witness something interesting: As the coffee drips into the pot, you'll often see one of the drops land on the surface and remain intact for a second or so before coalescing with the bulk liquid. This temporary noncoalescence of two bodies of the same liquid is neither an isolated phenomenon nor one that has been observed only in the era of drip coffee makers. As early as 1879, Lord Rayleigh examined how water jets bounce over each other, and in 1881 Osborne Reynolds observed what he called floating drops. (See the box on page 40.) A century later, “a scientific curiosity” was how Jearl Walker referred to the phenomenon.

ISSN:0031-9228    

DOI:10.1063/1.882133

出版商:AIP    

年代:1998

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.882091

出版商:AIP    

年代:1998

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.882092

出版商:AIP    

年代:1998

数据来源: AIP