Optical spectroscopies

The most promising applications of high-T_c materials are devices patterned from thin films. Good superconducting properties of the samples, i.e. high critical current densities and high transition temperatures, are a prerequisite for applications. These properties, however, are strongly influenced by the amount of grain boundaries, which is closely related to the epitaxial quality. We have shown that Raman spectroscopy is a powerful tool to investigate non-destructively the epitaxy and the stoichiometry of YBa₂Cu₃O₇ films with high spatial resolution. Scanning micro-Raman spectroscopy (SMRS) yields these informations in both lateral dimensions. We also use low-temperature scanning laser microscopy (LTSLM) to investigate local electrical properties of devices consisting of high-T_c superconductors. In these experiments a dc-current biased sample at a temperature close to the superconducting transition is scanned with a focused laser beam causing a local heating at the point (x,y) of the beam focus. The detected voltage signal V(x,y) yields informations about critical temperatures T_c,0(x,y), transition widths DT_c(x,y), and critical current densities j_c(x,y) with a spatial resolution of about 1-2 mm limited by the diameter of the laser spot. The figure displays results of an investigation of a crossover, where an upper YBa₂Cu₃O₇ bridge crosses an underlying one which is seperated by an insulating layer or SrTiO₃. The device is patterned in multi-layer technique using photo lithography and argon ion milling.

Fig. 1: Comparison of the voltage response of a crossover (lower bridge from top to bottom (right) and upper bridge from left to right) at 87 K (a) with the local epitaxial quality (b). Red regions correspond to high voltages and bad qualities, respectively.

Spectroscopic ellipsometry is a highly effective technique to gain information about the complex dielectric function e(w) = e₁(w) + ie₂(w) in thin films and devices. Layer thickness, surface roughness, and the manipulation of the dielectric function in a structured film itself can be determined.

The example below in Fig. 2 shows the metal-to-insulator transition of a La_0.7Sr_0.3MnO₃ thin film, which was structured by grain boundaries running in a chess-board like fashion across the film. This film was grown by Z. Ivanov (Chalmers University) and shows a "low-field"-component in its colossal magnetoresistance, which is important in order to achieve a potential device application of those materials.

Fig. 2: The figure on the right shows the real part of the pseudo-dielectric function in a temperature range between 500 K and 30 K. At low energies < 1 eV a metal-to-insulator transition yields a temperature dependent plasma frequency. At temperatures below 210 K it indicates a persistent release of charge carriers (inset). At higher energies we observe changes in the spectral range typical for interband transitions. These indicate a structural change at the transition. This corresponds well to the inelastic light scattering data shown before. This work was performed at the University of Illinois at Urbana-Champaign.