Evidence of a nodeless superconducting gap in PrCeCuO from magnetic penetration depth measurements

We have measured the inverse-squared magnetic penetration depth, $\lambda ^{-2}$(T), at 50 kHz of films of the electron-doped cuprate superconductor Pr$_{2-x}$Ce$_{x}$CuO$_{4-\delta }$ over a range of Ce dopings, 0.124 $\le $ x $\le $ 0.144, that extends from underdoped to overdoped. The maximum T$_{C}$ was 24 K at x = 0.131. The films were grown by mbe on SrTiO$_{3}$ substrates that had been buffered with a thin layer of the insulating parent compound, Pr$_{2}$CuO$_{4-\delta }$, to obtain the cleanest possible films. Resistivity decreased smoothly and monotonically with doping. We used a two-coil mutual inductance technique to determine the film conductivity $\sigma $ down to about 0.5 K, and we obtained $\lambda ^{-2}$ from $\sigma _{2}$ in the usual fashion. We found that $\lambda ^{-2}$(T) was flat at low temperatures. That is, $\lambda ^{-2}$(T)/$\lambda ^{-2}$(0) changed by less than the experimental noise of 0.15{\%} over a factor of 3 or more change in T. Fits to the low-T data yield minimum a gap value, $\Delta _{min}$(0)/k$_{B}$T$_{C}$, that is unity near optimal doping and decreases with over- and underdoping. This talk will compare our results with other penetration depth measurements that find quadratic behavior at low T, consistent with a d-wave gap and with phase sensitive measurements.