摘要:

AbstractThe acoustic and dielectric properties of different glasses at audio frequencies and temperatures below 1 K have been investigated with the vibrating reed and a capacitance bridge technique. We found the temperature dependence of the absorption of vitreous silica (Suprasil W) to agree with the predictions of the tunneling model which is commonly used to explain the low temperature behaviour of amorphous materials. The variation of the sound velocity and of the dielectric constant, however, shows significant deviations from the expected behaviour which cannot be accounted for by a simple modification of the model. Instead, it seems to be necessary to introduce a temperature dependence of some relevant model parameters. Moreover, at very low temperatures (T<0.1 K) the sound velocity strongly depends on the excitation levels. The absence of this effect at higher temperatures proves that it can be ascribed to a nonlinear response of tunneling systems. Similar results were found in sound velocity measurements on a cover glass and on a superconducting metallic glass (Pd30Zr70,Tc= 2.6 K), which indicates that these features are a general aspect of the dynamics of tunneling states in glasses. In contrast to the insulating glasses we found that in Pd30Zr70also the internal friction is strain dependent.

ISSN:0003-3804    

DOI:10.1002/andp.19945060502

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractWe present the first systematic study of potential energy curves and prolate‐oblate shape transitions of sodium clusters with 8

ISSN:0003-3804    

DOI:10.1002/andp.19945060503

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractWe study the conductance of a single particle on a ring subject to an arbitrary dc electric field, which is generated by a linearly in time increasing magnetic flux. The full quantum mechanical time development is calculated numerically by splitting the dynamics into independent consecutive Zener tunneling transitions and free motion on the ring. The Zener transitions occur near the avoided crossings of the bandstructure which arises from the adiabatic eigenstates as a function of flux in the presence of a static scattering potential. To account for the necessary dissipation the particle is coupled to an appropriate oscillator bath which is adjusted to give a strictly linear current‐voltage characteristic for arbitrary voltage and temperature in theabsenceof scattering. Taking a single δ‐function scatterer we find that the dissipative coupling eliminates the localization in energy space found previously and leads to a well defined resistive steady state. The scattering introduces reproducible fluctuations around the average Ohmic behavior which are caused by coherent backscattering. Their magnitude depends on the strength of the scattering potential and decays slowly for large voltages. The associated correlation energy is determined by the uncertainty of the eigenstates due to the dissipative bath coupling. Thermal averaging leads to a decrease of the conductance fluctuations proportional t

ISSN:0003-3804    

DOI:10.1002/andp.19945060504

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThe 1 D one‐band Hubbard model with different repulsive on‐site interactions on even (U+V>0) and odd (U‐V>0) sites, supplemented by the correlated‐hopping term (t*>0), describing the modification of the electron hopping by the presence of other particles on the sites, is considered as a 1 D model forCuOsystems. The ground state phase diagram is studied within the framework of the bosonization technique and renormalization group analysis valid for weak coupling. Depending on the choice of model parameters, the following sequences of phase transitions with increasing bandfilling occur: 1) metal‐insulator‐metal (fort* ⩽U/4); 2) metal‐insulator‐metal‐superconductor\documentclass{article}\pagestyle{empty}\begin{document}$ ({\rm for}U/4

ISSN:0003-3804    

DOI:10.1002/andp.19945060505

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThe dynamics of a coupled model (harmonic oscillator‐relativistic scalar field) in Conformal Robertson‐Walker (k= +1) spacetimes is investigated. The exact radiation‐reaction equation of the source‐including the retarded radiation terms due to the closed space geometry – is obtained and analyzed. A suitable family of Lyapunov functions is constructed to show that, if the spacetime expands monotonely, then the source's energy damps. A numerical simulation of this equation for expanding Universes, with and without Future Event Horizon, is

ISSN:0003-3804    

DOI:10.1002/andp.19945060506

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

ISSN:0003-3804    

DOI:10.1002/andp.19945060501

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY