ISSN:0002-9505    

DOI:10.1119/1.16310

出版商:American Association of Physics Teachers    

年代:1990

数据来源: AIP

ISSN:0002-9505    

DOI:10.1119/1.16312

出版商:American Association of Physics Teachers    

年代:1990

数据来源: AIP

ISSN:0002-9505    

DOI:10.1119/1.16313

出版商:American Association of Physics Teachers    

年代:1990

数据来源: AIP

摘要:

This article surveys experiments with individual closely confined atomic particles since their beginning in 1973. Such experiments have improved information on the size of an elementary particle on the same level as a quark, the electron, by at least three orders of magnitude. Their extension to the optical region promises atomic clocks of a reproducibility improved up to 100 000 times. The most important of the techniques developed for continuously detecting, cooling, and spin‐state analyzing a permanently confined individual electron are described in some detail. The electron, trapped in ultrahigh vacuum at liquid helium temperature, is profitably viewed as a man‐made atom, geonium.

ISSN:0002-9505    

DOI:10.1119/1.16314

出版商:American Association of Physics Teachers    

年代:1990

数据来源: AIP

摘要:

The discovery of high‐temperature superconductivity in lamellar copper oxide materials is one of the most dramatic and unexpected events in physics in the last several decades. In this paper, which is the text of the 48th Richtmyer Lecture, the basic physics of this new phenomenon is discussed and it is explained why the problem is proving to be so difficult in spite of the unprecedented assault on it by many thousands of physicists, chemists, and materials scientists.

ISSN:0002-9505    

DOI:10.1119/1.16315

出版商:American Association of Physics Teachers    

年代:1990

数据来源: AIP

摘要:

It is a well‐known but frequently forgotten fact that the net external torque on a system of particles is equal to the rate of change of angular momentum only if the point about which moments are taken satisfies certain stringent conditions: The point must have zero acceleration, or it must be the center of mass, or its instantaneous acceleration must be directly toward or away from the center of mass. A simple problem in rigid‐body mechanics has been solved by Crawford, taking moments about the center of mass. It is shown here that the problem may also be solved using a point that doesnotsatisfy any of these conditions, in which case a more general equation is required. An external fixed point may also be used for calculating moments. When carefully executed, all correct approaches yield the same solution.

ISSN:0002-9505    

DOI:10.1119/1.16316

出版商:American Association of Physics Teachers    

年代:1990

数据来源: AIP

摘要:

A high‐Tcsuperconductor floating freely above a magnet of low symmetry remains rigidly suspended in the air in almost any position and orientation as if stuck in an invisible heap of sand. This striking effect is due to pinning of the magnetic flux lines inside the superconductor and is often overlooked, since usually magnets with rotational symmetry are used for levitation. Magnets with rotational symmetry allow for nearly undamped orbiting and rotation of the superconductor about the magnet’s symmetry axis. But even in this geometry, flux‐line pinning can be seen, since it forces the orbiting superconductor to turn the same face toward the axis. Superconductors with sufficiently strong pinning may even be suspended below a magnet.

ISSN:0002-9505    

DOI:10.1119/1.16317

出版商:American Association of Physics Teachers    

年代:1990

数据来源: AIP

摘要:

As an example of the application of functional integral techniques to condensed matter physics, the partition function for the BCS model for superconductivity is derived. The pairing term in the Hamiltonian is linearized by applying the Hubbard–Stratonovich transformation. It is shown that the standard result for the gap equation can be obtained by making a saddle‐point approximation to the functional integral.

ISSN:0002-9505    

DOI:10.1119/1.16318

出版商:American Association of Physics Teachers    

年代:1990

数据来源: AIP

摘要:

The one‐dimensional square quantum well of varying width is shown to be an instructive example for testing the appropriateness of approximate solutions in quantum mechanics. The range of wall speeds over which the adiabatic approximation is applicable is investigated and related to the initial energy of the quantum system for both expanding and contracting wells. However, of more interest is the applicability of the sudden approximation to these systems. It is shown that this approximation is appropriate for expanding wells at quite modest wall speeds, whereas the approximation cannot be applied for contracting wells no matter how rapidly the well width contracts. The physical reasons for the inapplicability of the sudden approximation for contracting wells is investigated.

ISSN:0002-9505    

DOI:10.1119/1.16319

出版商:American Association of Physics Teachers    

年代:1990

数据来源: AIP

摘要:

A simple and inexpensive mechanical model for the demonstration of chaos in a nonlinear deterministic system is described. Periodic, period‐doubled, and chaotic motions are exhibited by the model in agreement with the solutions of its governing differential equation. The model is sufficiently free from long‐term drifts to enable experimental Poincaré plots to be easily constructed. Such a simple mechanical model is shown to be ideal for use as a lecture room demonstration, as well as for a student laboratory.

ISSN:0002-9505    

DOI:10.1119/1.16320

出版商:American Association of Physics Teachers    

年代:1990

数据来源: AIP