Detailed measurements of the critical current density j c of YBa 2 Cu 3 O 7Ϫ␦ films grown by pulsed laser deposition reveal the increase of j c as a function of film thickness. Both this thickness dependence and the field dependence of the critical current are consistently described using a generalization of the theory of strong pinning of Ovchinnikov and Ivlev ͓Phys. Rev. B 43, 8024 ͑1991͔͒. From the model, we deduce values of the defect density (10 21 m Ϫ3 ) and the elementary pinning force, which are in good agreement with the generally accepted values for Y 2 O 3 inclusions. In the absence of clear evidence that the critical current is determined by linear defects or modulations of the film thickness, our model provides an alternative explanation for the rather universal field dependence of the critical current density found in YBa 2 Cu 3 O 7Ϫ␦ films deposited by different methods.
Self-assembled thin films of gold nanoparticles of 4–5nm, prepared on glass using aliphatic dithiols of different hydrocarbon chain lengths as interparticle linker molecules, have been studied by x-ray methods and dc conductivity. X-ray data revealed small spacer-dependent cluster size variations in the films. Conductivity, in the temperature range of 4.2–300K, showed a strong relation with spacer length, indicating that different mechanisms of conduction from metal-like through hopping to tunneling dominate, depending on temperature range and particle spacing. The results demonstrate that the electronic and optical properties of such films can be precisely controlled by interparticle distance.
Vortex thermal fluctuations in heavily underdoped Bi(2)Sr(2)CaCu(2)O(8+delta) (T(c)=69.4 K) are studied using Josephson plasma resonance. From the zero-field data, we obtain the c-axis penetration depth lambda(L,c)(0)=230+/-10 micrometer and the anisotropy ratio gamma(T). The low plasma frequency allows us to study phase correlations over the whole vortex solid state and to extract a wandering length r(w) of vortex pancakes. The temperature dependence of r(w) as well as its increase with dc magnetic field is explained by the renormalization of the vortex line tension by the fluctuations, suggesting that this softening is responsible for the dissociation of the vortices at the first order transition.
Pulsed laser deposition of high Tc compounds onto unheated substrates, resulting in amorphous thin films, preserves to a great extent the composition of matter ejected from the target. This composition is of primary interest, both for understanding the dynamics of laser–target interaction and for practical (optimization) reasons. We have investigated the structure of amorphous and crystalline YBaCuO films obtained both in on-axis and off-axis deposition geometries, and correlated the results with optical and transport properties of these films. X-ray scattering reveals in amorphous films the existence of: (i) amorphous continuum of spatially disordered atoms, (ii) small (10–40 Å) amorphous clusters which can be considered as mesoscopic order fluctuations in the amorphous continuum, and (iii) slightly larger (50–250 Å) crystalline clusters exhibiting quasi-two dimensional (00l) or (11l) long range order. Crystalline films are predominantly (00l) oriented. Optical spectra of both crystalline and amorphous films show regions of enhanced attenuation caused by free charge carriers. Spectra of amorphous samples containing small crystalline clusters exhibit features which we relate to the electron localization caused by quantum size effects. Transport measurements are in good agreement with the structural and optical results. Conductivity of the on-axis films is 3–4 orders of magnitude higher than that of the off-axis films. The (nonlinear) conductivity of amorphous films increases with temperature and remains constant below 200 K. We suggest that besides the usual variable range hopping conduction mechanism, classical tunneling of charge carriers at constant energy between metallic (crystalline) clusters is present. Interestingly, the amorphous off-axis films exhibit a periodic repetition of the elements of atomic order or disorder along the direction of the plasma plume and undergo a structural transition of (11l)→(00l) type.
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