摘要:

AbstractWe use superconducting Al‐tunnel junctions as tunable phonon generators in the meV‐range to determine ground state splitting at zero stress of positively charged states associated with single acceptors in Si and double acceptors in Ge. From the stress and energy dependence of the conductivity induced by high frequency phonon irradiation of the corresponding two‐ and three‐hole states we find that the splitting is below 0.1 meV for the ground states of Si:B+and Ge:Be+, and 1 meV and 1.2 meV for Si:Ga+and Si:Al+, respectively. These level separations are comparable to those found previously for the ground states of the corresponding acceptor bound excitons A0X. For the deeper acceptors Si:In+, Si:Tl+, and Ge:Zn+the results are qualitatively di

ISSN:0003-3804    

DOI:10.1002/andp.19955070602

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe dynamics and evolution of molecular clouds, which are the main sites of active star formation in our Galaxy, are governed by the interaction of the self‐gravity, magnetic fields, and ambipolar diffusion, in the form of waves and instability. In our earlier paper (Part 1), we carried out a detailed planar modal analysis. The present paper (Part 2) is an extension of Part 1 in order to include the three‐dimensionality and the finite size of the cloud as well as the nonlinear effects.A cylindrical modal approach is developed to take into account the three‐dimensionality and the finite size of the cloud as well as the special direction of the mean fieldB0. Dispersion relation and solutions of such cylindrical modes are obtained. It is shown that, in the most unstable direction (∥B0), the growth rate is considerably reduced by the finite lateral size compared with the planar mode of the same wavelength.Nonlinear effects of the magnetic field and magnetic waves are discussed, with particular attention paid to their dependence on the coupling factor σ which is the ratio between the mean collision frequency of a neutral with ions and the gravitational response frequency. It is shown that fast magnetosonic waves are as important as Alfvén waves in the global support of the cloud. In order that the lower limit of the wavelengths in themoderately dissipative rangeof such waves is small compared with the cloud size, σ should be larger than 5. It is also shown that σ should be larger than 7.3 in order for the density growth of the neutral fluid in a free‐fall time to be smaller than 30%. A typical value of σ ≈ 11 in molecular clouds is estimated. This corresponds to an ionization rate ζ = 10−17and a metal depletion δ = 0.1. For the clouds with such value of σ, both the density growth and the flux loss are smaller than 20% in a free‐fall time.It is shown that a self‐adjusting mechanism is able to slow down the global collapse at the early stage of cloud evolution in terms of the interaction between the global collapse and the then existing Alfvén and fast magnetosonic waves, which originate from the inhomogeneous velocity and density distributions in the cloud. Such interaction will not only strengthen these waves, but also create outwards decaying amplitudes of the field perturbation and therefore generate outward net magnetic forces to support the cloud against global collapse. The same mechanism also works for refreshing the outwards propagating Alfvén and fast magnetosonic waves caused by star‐forming or core‐forming activities, if the total energy supply rate due to these activities is lower than the total dissipation rate of these waves. In this way, a significant portion of the released gravitational energy during the global collapse is tapped and turned into the magnetic waves to slow down the global collapse itself.In terms of such a mechanism, the property that the dissipation rate of Alfvén and fast magnetosonic waves increases with the wave number leads to a simple explanation of the coexistence of the global quasi‐stability and the local instability (formation of dense cores) in molecular clouds with cloud ma

ISSN:0003-3804    

DOI:10.1002/andp.19955070603

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractA 5D Machian generalization of Einstein's theory of gravitation is described in which the cosmological constant appears as the contribution of the rest of the universe to the gravitational field of an “isolated” source. This work amounts to a Machian interpretation of the Kaluza‐Klein theory. The standard cosmological models are derived in this framework, and it is shown that ‐ in conformity with the interpretation presented here ‐ the cosmological constant does not directly contribute to the geometric description of the universe a

ISSN:0003-3804    

DOI:10.1002/andp.19955070604

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe standard formalism of quantum theory is enhanced and definite meaning is given to the concepts of experiment, measurement and event. Within this approach one obtains a uniquely defined piecewise deterministic algorithm generating quantum jumps, classical events and histories of single quantum objects. The wave‐function Monte Carlo method of Quantum Optics is generalized and promoted to the level of a fundamental process generating all the real events in Nature. The already worked out applications include SQUID‐tank model and generalized cloud chamber model with GRW spontaneous localization as a particular case. Differences between the present approach and quantum measurement theories based on environment‐induced master equations are stressed. Questions: what is classical, what is time, and what observers are addressed. Possible applications of the new approach are suggested, among them connection between the stochastic commutative geometry and Connes' noncommutative formulation of the Standard Model, as well as potential applications to the theory and practice of quantum comp

ISSN:0003-3804    

DOI:10.1002/andp.19955070605

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractBy systematically assigning weights to kinetically‐controlled folding pathways we introduce a novel scheme of statistical inference. We provide supporting experimental evidence to show that this approach is suitable to explain the expediency and robustness of RNA folding. The statistical scheme is constructed in four stages, the last of which leads to a suitable representation that allows for direct comparison with experiment: a) An appropriate space of folding histories is defined; b) The space is endowed with a measure and in this way an ensemble is defined; c) The ensemble is systematically simplified by coarse‐graining each copy or replica of conformation space. This procedure entails lumping together rapidly‐interconverting conformations; d) A base‐pair probability matrix (BPPM) is introduced by representing all structures contributing to the ensemble at a given instant. Thus, we take a convenient cross‐section of the ensemble by taking a fixed instant in time. The BPPM is contrasted vis‐a‐vis experimental information on biologically‐competent conformations. This last procedure is paramount to verify the theory. Moreover, the essential properties of folding are captured by showing that the statistical weight is concentrated on a very limited domain of closely‐related folding pathways whose biological competence has been establish

ISSN:0003-3804    

DOI:10.1002/andp.19955070606

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

ISSN:0003-3804    

DOI:10.1002/andp.19955070601

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY