ISSN:0031-9228    

DOI:10.1063/1.2807454

出版商:AIP    

年代:1996

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2807455

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Earth's magnetic field has been a subject of curiosity for at least 3000 years and of quantitative study for more than 400 years. Geologic samples now extend our knowledge of geomagnetism back billions of years, and satellites and observatories log the tiniest changes in the strength and direction of the geomagnetic field. these investigations have revealed many intriguing characteristics of geomagnetism that any successful model of the phenomenon must explain—such as the stability of the geomagnetic field on time scales of105years, the field's predominantly dipole nature, the offset of its dipole axis from Earth's rotational axis, its temporal variability and so on. All these geomagnetic observations, however, have done little to suggest what a model of geomagnetism should look like.

ISSN:0031-9228    

DOI:10.1063/1.2807456

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

One issue that has driven many of the experiments on high‐temperature superconductivity is the nature of the wavefunction of the electron pairs that carry the supercurrent (seePHYSICS TODAY, May 1993, page 17). For conventional superconductors, this wavefunction is isotropic, having an angular momentuml = 0,that is, s‐wave. But for the anisotropic copper‐oxide materials that exhibit critical temperatures above 40 K, the wave‐function may not be so simple.

ISSN:0031-9228    

DOI:10.1063/1.2807457

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Agroup at the Ecole Normale Superieure in Paris has placed a helium atom in a state that is going in six directions at once—a very confusing state for an atom. In such a superposition state, also called a dark state, the atom can't absorb light because the absorption amplitudes cancel out by destructive interference. The temperature corresponding to the velocity spread of each of the six wavepackets is a factor of 20 below the so‐called recoil limit for laser‐cooled atoms. The three‐dimensional subrecoil experiment in helium was reported in the 4 December issue ofPhysical Review Lettersby John La wall (now at the National Institute of Standards and Technology in Gaithersburg, Maryland), Si‐mone Kulin, Bruno Saubamea, Nick Bigelow, Miche`le Leduc and Claude Cohen‐Tannoudji.

ISSN:0031-9228    

DOI:10.1063/1.2807458

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Oak Ridge National Laboratory has recently completed the construction of a radioactive ion beam facility to create nuclei at the limits of stability. The facility is now being commissioned. Created by reconfiguring two existing accelerators that have been used as a heavy‐ion facility, the new Holifield Radioactive Ion Beam Facility is expected to be ready for experiments next summer. Because of the decreasing ratio of protons to neutrons for heavy stable nuclei, says Jerry Garrett, scientific director of the facility, the most efficient technique for producing proton‐rich nuclei for nuclear structure studies involves heavy‐ion induced fusion‐evaporation reactions using proton‐rich projectiles and targets with nearly equal masses. “This new facility should be well suited for such studies of proton‐rich nuclei,” Garrett says.

ISSN:0031-9228    

DOI:10.1063/1.2807459

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

When a new crystalline material is discovered, one of the first fundamental properties to be determined is the atomic structure, defined by the minimum in the free energy with respect to the positions of the atoms. Another fundamental characteristic of interest is the curvature of the free energy in the vicinity of the minimum, and this would be manifest in the elastic constants for the material. As derivatives of the free energy, elastic constants are closely connected to thermodynamic properties of the material. They can be related to the specific heat, the Debye temperature and the Gruneisen parameter (which relates the thermal expansion coefficient to the specific heat at constant volume), and they can be used to check theoretical models. Extensive quantitative connections among thermodynamic properties can be made if the elastic constants are known as functions of temperature and pressure. The damping of elastic waves provides information on anharmonicity and on coupling with electrons and other relaxation mechanisms. The elastic properties are perhaps most valuable as probes of phase transitions, such as superconductivity transitions. Clearly precise and accurate measurements of elastic constants furnish significant information about materials.

ISSN:0031-9228    

DOI:10.1063/1.881483

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The rise of big science is often linked to World War II and the cold war. However, such an interpretation ignores the historical roots of big science in the six decades preceding the war. Here we discuss one aspect of the development of big science in America: the industrialization of astronomy and astronomy's emergence as the biggest of the sciences in an age of little science.

ISSN:0031-9228    

DOI:10.1063/1.881484

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Scientific societies were founded to foster communication among their members. As we move into the era of enhanced electronic communication, the research community is seeking to make the exchange of information among scientists faster and better, at reasonable cost. The Internet offers a wide variety of possibilities for accomplishing this goal.

ISSN:0031-9228    

DOI:10.1063/1.881598

出版商:AIP    

年代:1996

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.2807460

出版商:AIP    

年代:1996

数据来源: AIP