ISSN:0003-3804    

DOI:10.1002/andp.19925040102

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractOn freshly cleaved highly oriented pyrolytic graphite we observed large‐scale superperiodicities by a scanning tunneling microscope at room temperature in air. Several hexagonal superstructures with periods of 30 nm, 4.2 nm, 2.4 nm, and 2.0 nm, respectively, and a strip‐like superstructure with a period of 1 nm were obtained. With exception of the largest hexagonal superperiodicity (30 nm spacing), all other superstructures are superimposed on the atomic corrugation of graphite. The origin of these superstructures is not clear yet. We assume that they arise from crystal defects in graphite. The hexagonal superstructure may be caused by the Moiré effect due to the rotational misorientation of the two top layers or of two successive layers near the sur

ISSN:0003-3804    

DOI:10.1002/andp.19925040103

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractDoping amorphous GaxAr1−xmixtures with the strong spin‐orbit scatterer Bi has a dramatic effect on the metal‐insulator transition (MIT) occurring in this system at a critical metal atomic concentrationxc: (i) the MIT is shifted fromx′c= 0.36 ± 0.01 (corresponding to a critical metal volume fractionv′c= 0.19 ± 0.01) of the undoped system to a lower value ofxc= 0.25 ± 0.01 (vc= 0.14 ± 0.01) for (Ga0.9Bi0.1)xAr1−xand (ii) the critical exponentvandgof the dc conductivity and Hall coefficient, respectively, change fromv′ = 0.5 ± 0.1 andg′ = 0 for the undoped samples tov= 1.3 ± 0.3 andg= 0.5 ± 0.1

ISSN:0003-3804    

DOI:10.1002/andp.19925040104

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractStarting from a model of free Fermions in two dimensions with an arbitrary strong effective interaction, we derive a Ginzburg‐Landau theory describing the crossover from BCS‐superconductivity to Bose‐condensation. We find a smooth crossover from the standard BCS‐limit to a Gross‐Pitaevski type equation for the order parameter in a Bose superfluid. The mean field transition temperature exhibits a maximum at a coupling strength, where the behaviour crosses over from BCS to Bose like with corresponding values of 2 Δ0/Tc≈ 5 which are characteristic for highTcsup

ISSN:0003-3804    

DOI:10.1002/andp.19925040105

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractWe have calculated electron‐phonon matrix elements relevant for indirect optical absorption, intervalley matrix elements and intravalley deformation potentials as well as splitting parameters for thek=0states in silicon and germanium within the rigid quasi‐ion model. In contrast to the “atomic” potentials in conventional rigid‐ion models the quasi‐ion potentials are well‐defined. The agreement with experiment and with other theoretical results is satisfactory in most cases. We have also studied the role of distortion corrections to the rigid quasi‐ion approximation. We find that the influence of distortions is small, indicating that the rigid quasi‐ion model is an adequate description ‐ at least in case of

ISSN:0003-3804    

DOI:10.1002/andp.19925040106

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractWe have calculated the temperature dependence of the direct and indirect band gaps in silicon and germanium. The electron‐phonon potential as well as the phonon frequencies and eigenvectors are calculated consistently within the rigid quasi‐ion model. Comparison is made with experiment and with the theoretical results of Allen and Cardona and Lautenschlager et

ISSN:0003-3804    

DOI:10.1002/andp.19925040107

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractWe discuss a variational ground state wave function for the symmetric periodic Anderson model with commensurate spin density order. The energy of this ansatz is evaluated in closed form. Our approach generalizes a variational treatment proposed recently by Strack and Vollhardt and results in significantly lower energy. Contrary to the Gutzwiller ansatz the wave function recovers the Schrieffer‐Wolff limit for large Coulomb repulsion. We clarify the relation of our approach to unrestricted Hartree‐Fock and present a comparison with existing quantum Monte Carlo calculations for one dimens

ISSN:0003-3804    

DOI:10.1002/andp.19925040108

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractRecently general conditions have been given which imply the existence of the Feynman integral as a Hida distribution. Here we perform an explicit construction of these distributions for a large class of Schrödinger problems

ISSN:0003-3804    

DOI:10.1002/andp.19925040109

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractFor short times and small clusters we check the validity of the 1935 Becker‐Döring equation by Monte Carlo simulations in the square Ising model at and near its critical point; some discrepancies are fou

ISSN:0003-3804    

DOI:10.1002/andp.19925040110

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractWe study the kinetics of interconversion of conformational substates in RNA folding by means of a Monte‐Carlo simulation. We establish a scaling law relating the size of the barriers with the length of a random uncorrelated RNA chain. The way the relaxation timescales diverge in the thermodynamic limit coincides with the behavior of nonergodic relaxation timescales for spin glasse

ISSN:0003-3804    

DOI:10.1002/andp.19925040111

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY