摘要:

At a metal-insulator transition of Anderson type caused by a change in volume or composition in an alloy, the conductivity at the limit of low temperatures goes continuously to zero; there is no minimum metallic conductivity. Long and Pepper in 1984, however, gave theoretical reasons for expecting that a transition induced by astrongmagnetic field could prove an exception and supported their conjecture by experiments on doped InP. Recently however, Biskupski, El Kaaouachi and Briggs in 1992 came to contrary conclusions also for InP. The theoretical position reviewed also by Kaveh and Mott in 1985 is reconsidered in this letter, and it is concluded that, under certain very restricted conditions, σmin(≈003e2/ħ) can be observed, and a hypothesis for the differing results of these two groups of workers is proposed.

ISSN:0950-0839    

DOI:10.1080/09500839208214686

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Local-density-functional theory is used to determine the structure of molecular water along the c channel in α-quartz. We use a 105-atom cluster terminated by H atoms and find that water has a large effect on the silicate lattice. In particular it flips a Si─O─Si unit, which normally points inwards towards the centre of the c channel, outwards with the 0 atom of water exposed to the two Si atoms. The water molecule is bound with polarization forces between it and the deformed network.

ISSN:0950-0839    

DOI:10.1080/09500839208214687

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Local-density-functional theory is used to evaluate the structure, vibrational bands and electronic properties of two point defects in diamond a pair of adjacent substitutional N atoms and a vacancy surrounded by four N atoms. These defects are important as they are possible models of the A and B centres seen in type Ia diamond. For the A centre we find a N─N bond length of 2·14 Å, an occupied electronic energy level near midgap and four infrared absorption bands with frequencies close to the experimental values. For the model of the B centre we find a N─C length of 1·49 Å an occupied triplet electronic level within the gap consistent with the recent suggestion that the N9 absorption band is due to this defect and again vibratory modes in close agreement with experiment. These results provide strong evidence as to the validity of the models for the A and B centres.

ISSN:0950-0839    

DOI:10.1080/09500839208214688

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Submolecular features of the C60buckyballs have been resolved at the ambient temperature by scanning tunnelling microscopy (STM). The possible causes of freezing of the rotational motion of the C60molecules are discussed.

ISSN:0950-0839    

DOI:10.1080/09500839208214689

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

It need not be considered surprising that many metals attain and retain the b.c.c. structure at very high pressures, since there is a sense in which this structure is close packed (emphatically closer than f.c.c.), namely in the sense of putting nuclei close to every point of the space in which the array of nuclei resides. Closeness in this sense, it is suggested, is particularly relevant to the structure of metals, which are not assemblies of hard spheres. The case is presented by way of a consideration of Voronoi cells, which provide a straightforward way of testing for closeness in this sense for any array.

ISSN:0950-0839    

DOI:10.1080/09500839208214690

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

A specially designed photoconductivity experiment directly shows that, above room temperature, the steady-state recombination lifetime of the free electrons in undoped amorphous silicon can be described by the free-electron quasi-Fermi energy alone, without the need to invoke the carrier generation rate or the temperature.

ISSN:0950-0839    

DOI:10.1080/09500839208214691

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

It is shown that Auger processes at negative U centres may be one of the efficient possible channels for recombination in chalcogenide amorphous semiconductors (CASs). Estimations of the hopping Auger recombination (AR) probability of the ex- charge carriers at the negative U centres are performed. At moderately low temperatures, hopping AR is probably the cause of the photoconductivity observed in CASs.

ISSN:0950-0839    

DOI:10.1080/09500839208214692

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor