摘要:

AbstractThe surface of YbBa2Cu3O7‐δsingle crystals was investigated using a scanning tunneling microscope at room temperature in air. Growth steps with step heights equal to the lattice parametercas well as steps with step heights of multiples of 1.35 c were obtained. On smooth surfaces atoms arranged in a 3.8 Å square lattice could be resolved. We observed a tunneling‐induced etching effect leading to an etch pit on the surface. Furthermore, by applying voltage pulses with amplitude>5.0 V and duration>100 μs holes with diameters of ∼︁ 150 nm could be produced. Scanning tunneling spectroscopic measurements were performed yieldingI‐Vand lnI‐scharacteristics. The extremely small effective barrier height of ϕ = 0.3 eV inferred fromdlnI/dsmay be caused by dirt between the surf

ISSN:0003-3804    

DOI:10.1002/andp.19925040802

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractWe have measured the complex film impedance 1/σd(σ conductivity,dfilm thickness) of three YBaCuO thin films withd= 44, 115, and 168 nm on MgO substrates at 10.2 GHz in the temperature range between 300 and 4 K. BelowTc, the experimental results are discussed in terms of the two‐fluid model and the BCS theory. The residual resistance decreases with the film thickness. The thinnest film has a residual surface resistance of 3 · 10−4Ω. For this film, the complex microwave conductivity is calculated and compared with the models. Apart from the residual resistance, the measured conductivity is in agreement with the peak caused by the energy gap of the BCS theory. All measurements were performed with a cavity perturbation method which we have to our knowledge applied for the first time to superconducting thin films. The method allows to determine the complex impedance of films with arbitrary thickness. In particular, films with thicknesses small compared to the skin depth δ or the London penetration depth λ can be measured. Therefore, we are able to measure the impedance both in the normal and superconduct

ISSN:0003-3804    

DOI:10.1002/andp.19925040803

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractThe RPA is formulated in subspaces of coordinate‐like and momentum‐like 1phoperators. This allows to embed a large class of approximative schemes into a generalized RPA treatment. We give a detailed formulation in terms of wavefunctions in coordinate space which is ideally suited to practical programming. In particular, we work out the reduction to spherical tensors in the case of spherical symmetry which is most often the starting point in finite Fermion syst

ISSN:0003-3804    

DOI:10.1002/andp.19925040804

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractWe take up the flexible formulation of RPA in subspaces of coordinate‐like and momentum‐like 1phoperators which was developed in the preceeding paper. We draw the connection to many of the known collective approximations to RPA by proper choice of the subspace of operators. We finally apply the newly developed scheme to comparative studies of a hierarchy of approaches for the RPA spectra of doubly magic nuclei described with Skyrme for

ISSN:0003-3804    

DOI:10.1002/andp.19925040805

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractTunnelling in periodically driven bistable symmetric potential wells is investigated in an analytical approximation in a domain where the driving frequency is large compared to the tunnelling frequency and only the four lowest lying levels contribute significantly. The influence of finite level widths is taken into account, and a smooth variation of the amplitude of the driving field is allowed for.

ISSN:0003-3804    

DOI:10.1002/andp.19925040806

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

ISSN:0003-3804    

DOI:10.1002/andp.19925040801

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY