Resistivity at low temperatures in electron-doped cuprate superconductors

We measured the magnetoresistance as a function of temperature down to 20 mK and magnetic field for a set of underdoped ${\text{Pr}}_{1.88}{\text{Ce}}_{0.12}{\text{CuO}}_{4-\delta }$ thin films with controlled oxygen content. This allows us to access the edge of the superconducting dome on the underdoped side. The sheet resistance increases with increasing oxygen content whereas the superconducting transition temperature is steadily decreasing down to zero. Upon applying various magnetic fields to suppress superconductivity we found that the sheet resistance increases when the temperature is lowered. It saturates at very low temperatures. These results, along with the magnetoresistance, cannot be described in the context of zero-temperature two-dimensional superconductor-to-insulator transition nor as a simple Kondo effect due to scattering off spins in the copper-oxide planes. We conjecture that due to the proximity to an antiferromagnetic phase magnetic droplets are induced. This results in negative magnetoresistance and in an upturn in the resistivity.

Figure 1

(Color online) Sheet resistance as a function of temperature for the various samples. Inset: zoom on the low-temperature region for the superconducting samples (S2-S4).

Figure 2

(Color online) Inset: black circles are the resistivity data as a function of temperature for sample S2. The solid red line is a second degree polynomial fit to the high-temperature regime (above the upturn). Main plot: the transition temperature $T_c$ for the various samples is plotted against $R_{◻}\left(0\right)$ obtained from an extrapolation to zero temperature of the fit, as demonstrated in the inset.

Figure 3

(Color online) The sheet resistance (samples S2-S4) as a function of magnetic field for various temperatures (from right to left): 2, 1.5, 1, 0.7, 0.5, 0.35, 0.22, 0.12, 0.05, and 0.03 K (0.03 K is not available for S3). Lower right panel: a broader field and resistance view.

Figure 4

(Color online) The sheet resistance as a function of temperature for various magnetic fields and for the different samples. In the lower right panel we zoom on the low-temperature and low-field behavior of sample 2.

Figure 5

(Color online) Field dependence of the spin magnetoresistance at $T=1.5\text{K}$ (using the data of Ref. 12). The spin magnetoresistance exhibits linear field dependence even at relatively high fields. The spin magnetoresistance drops to zero for $x\ge 0.16$ at 1.5 K.