Local dielectric and strain measurements inYBa2Cu3O7−δ thin films by evanescent microscopy and Raman spectroscopy

Abstract

The near-field evanescent microwave microscope is based on a coaxial transmissionline resonator with a silver-plated tungsten tip protruding through an end-wallaperture. The sensor is used to measure local dielectric properties of thin-filmYBa2Cu3O7−δ deposited onthree different SrTiO3 bi-crystal substrates having mismatch grain boundary angles of3°,6°,and 12°. The measurements in the superconducting state are below critical temperature atT = 79.4 K. The dielectric property of the superconductor within the near field of thetip frustrates the electric field and measurably changes the transmission line’sresonant frequency. The shift of the resonator’s frequency is measured as afunction of tip–sample separation and associated changes in quality factor(ΔQ) image scans of the thin film are presented. A quantitative relationship between the real andimaginary parts of the local dielectric constant and the frequency shift using the method ofimages is established. The comparison between experimental data and theory based on thismethod is given and discussed. Raman measurements of the intergranular strain within theYBa2Cu3O7−δ thin filmdeposited on each SrTiO3 substrate in the region of the bi-crystal junction showed excellent correlation between grainboundary mismatch and induced grain boundary strain. Compressive strains normal to thea axis (i.e. tensilestrains normal to the b axis) were detected across the grain boundary. The magnitude of induced strain as well asits spread away from the grain boundary increased as the mismatch angle increased.