Verification of d-wave pairing symmetry by microwave intermodulation distortion measurements in yttrium barium copper oxide

We report measurements of the temperature and power dependence of the microwave frequency intermodulation distortion (IMD) in high quality pulsed laser deposition (PLD) Yttrium Barium Copper Oxide (YBCO) on LaAlO 3 substrate. A low-temperature (T < 30 K) increase in IMD is the observation of an upturn of the nonlinear coefficient of the quadratic field dependence of the penetration depth. This IMD upturn is limited by the nonlinear Meissner effect that has been predicted for d-wave high-Tc superconductors. Various amounts of IMD increase are observed for different films with impurity (Ni, Zn and Ca) doping and other defects. The demonstration of the IMD upturn and the nonlinear Meissner effect were possible because the IMD measurement is an extremely sensitive method to detect the penetration depth change at even less than 0.01 nm. IMDs from various samples tend to merge at a single universal value at 0 K regardless of disorder, defects, and impurities due to the node singularity at 0 K. There is a similar converging trend in IMD towards the transition temperature T due to the quasiparticle thermal excitation and depletion of superelectrons. It is most likely that IMD has both intrinsic and extrinsic contributions. The 1/T 2 divergence at low T of the IMD was theoretically predicted and confirmed with high quality PLD YBCO on LaAlO 3 substrate from below 30 K. We found that Ni-doped YBCO gave an IMD increase comparable to high quality PLD YBCO. This finding agrees with the evidence that Ni (magnetic impurity) does not destroy superconductivity and preserves d-wave symmetry as shown in scanning tunnelling microscope experiments. The off-the-resonance excitation experiment indicates that the response time scale of nonlinearity in IMD generation is not affected by the magnitude of two-tone separation Af = f2 fi in YBCO on LaAlO 3 substrate. But, at low temperature (T < 20 K) and low output power at fundamental frequency (Pt < -30 dBm), the response time scale slows down IMD generation as the Af increases in YBCO on