Suppression of Superconducting Critical Current Density by Small Flux Jumps in MgB2MgB_2 Thin Films

By doing magnetization measurements during magnetic field sweeps on thin films of the new superconductor $MgB_2$, it is found that in a low temperature and low field region small flux jumps are taking place. This effect strongly suppresses the central magnetization peak leading to reduced nominal superconducting critical current density at low temperatures. A borderline for this effect to occur is determined on the field-temperature (H-T) phase diagram. It is suggested that the small size of the flux jumps in films is due to the higher density of small defects and the relatively easy thermal diffusion in thin films in comparison with bulk samples.Comment: 7 figures Phys. Rev. B accepted scheduled issue: 01 Feb 200

Local threshold field for dendritic instability in superconducting MgB2 films

Using magneto-optical imaging the phenomenon of dendritic flux penetration in superconducting films was studied. Flux dendrites were abruptly formed in a 300 nm thick film of MgB2 by applying a perpendicular magnetic field. Detailed measurements of flux density distributions show that there exists a local threshold field controlling the nucleation and termination of the dendritic growth. At 4 K the local threshold field is close to 12 mT in this sample, where the critical current density is 10^7 A/cm^2. The dendritic instability in thin films is believed to be of thermo-magnetic origin, but the existence of a local threshold field, and its small value are features that distinctly contrast the thermo-magnetic instability (flux jumps) in bulk superconductors.Comment: 6 pages, 6 figures, submitted to Phys. Rev.

Plasma-deposited fluorocarbon coatings for passive and active integrated optical devices

Amorphous fluorocarbon polymers are attractive materials for optical applications because of their high transparency at wavelengths up to 3 mu m due to the absence of C-H bonds. DuPont's amorphous fluoropolymer Teflon AF has been available since 1989, and Teflon AF films can be fabricated by spin, spray, or dip coating from solution or by use of compression or injection moulding from the melt. An alternative and promising route for processing fluorocarbon films is the use of a low-pressure plasma. This technique can be employed for plasma polymerization of suitable fluorocarbon monomers and for controlled etching of fluorocarbon materials. In the present work, we present the characteristics of plasma-polymerized tetrafluoroethylene layers with a refractive index of approximately 1.4. Reactive ion etching in an N2O plasma is used for patterning these layers, and also the spin-coated Teflon AF films with a refractive index of around 1.3. The possibility of fabricating passive and active polymer waveguide devices from fluorocarbon polymers is discussed, and estimates of expected waveguide performance are presented

In-situ pyroelectrical study of the polarization dynamics in nonlinear optical polymers and physico-chemical investigation of photo-induced degradation

The photo-isomerization of azo-chromophores in a nonlinear optical side-chain polymer permits the orientation of the dipoles under an applied electric field even well below the glass-transition temperature of the polymer. As the dipoles become orientable only within the penetration depth of the pump light, the technique is especially suitable for the preparation of poling patterns across the thickness of the polymer. Pyroelectrical investigations allow for an in-situ examination of the photo-induced poling process and for an analysis of the resulting nonuniform dipole-orientation patterns. For long-time exposures to the short-wavelength pump light, a photo-induced degradation of the chromophore molecules (photo-bleaching) is observed, which is analyzed by pyroelectrical (thermal) analysis, optical techniques and X-ray photoelectron spectroscopy