Observation of a narrow superconducting transition at 6 GHz in crystals of Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

We report measurements of the temperature dependence of the surface resistivity of highquality Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ crystals at 5.95 GHz. A narrow transition with 89-K onset is observed. Resistivity in the normal state is found to be $R_{\mathrm{norm}}=126\mathrm{}\times {10}^{-3\mathrm{}}$ Ω. We establish an upper limit for the resistivity at 77 K of 1.0×${10}^{-3\mathrm{}}$ Ω and at 2 K of 1.5×${10}^{-5\mathrm{}}$ Ω. The dissipation in four crystals with a total area of about 20 ${\mathrm{mm}}^2$ is measured to be nearly independent of peak microwave magnetic field over the range of 0.5 to 93 G in sharp contrast to the strong field dependence observed in bulk sintered samples.