Hole reduction and electron accumulation in YBa${}_2$Cu${}_3$O${}_y$ thin films using an electrochemical technique: Evidence for an $n$-type metallic state

We demonstrated extreme hole reduction and electron accumulation in YBa${}_2$Cu${}_3$O${}_y$ films by an electrochemical technique. Following the transition from superconductor to insulator, we succeeded in doping electrons as evidenced by the decrease in resistivity and sign change of the Hall coefficient. Further doping resulted in metallic $n$-type YBa${}_2$Cu${}_3$O${}_y$ with a carrier density of ∼2.5 × ${10}^{20}$ cm${}^{-3}$, but without any sign of superconductivity. The oxygen vacancies electrochemically induced in the CuO${}_2$ planes played a major role in the $p$-$n$ transition.

Figure 1

Evolution of ρ($T$) with the applied voltage $V_{\mathrm{EC}}$ for sample 1 (black circles) and with reaction time $t_{\mathrm{total}}$ for sample 5 (red circles) in KClO${}_4$/PEO. $V_{\mathrm{EC}}$ $=$ 0, 1.0, 2.0, and 3.0 V with fixed $t_{\mathrm{EC}}$ $=$ 2.5 h for sample 1 and $t_{\mathrm{total}}$ $=$ 0, 2.0, 4.0, 6.0, 8.0, 9.5, 10.5, 11.5, 12.5, 13.5, 14.5, and 15.5 h with fixed $V_{\mathrm{EC}}$ $=$ 1.0 V for sample 5, from bottom to top, respectively. Upper inset: Schematic drawing of the electrochemical cell. Lower inset: Plots of $T_c$ as a function of $n_{100\mathrm{K}}$ $=$ 1/${\mathit{eR}}_{\mathrm{H}}$(100 K) for sample 1 (black solid circle), sample 3 (open triangles, in CsClO${}_4$/PEO), sample 4 (solid triangles, in CsClO${}_4$/PEO), and sample 5 (red solid circles). Dotted line is a guide to the eyes.

Figure 2

Evolution of ρ($T$) with total reaction time $t_{\mathrm{total}}$ at fixed $V_{\mathrm{EC}}$ $=$ 1.0 V for sample 6 (heavily underdoped in as-grown condition).

Figure 3

Variation of the Hall number $n_{100\mathrm{K}}$ with reaction time $t_{\mathrm{total}}$ for sample 6 (heavily underdoped in the initial condition). Inset: Comparison of ρ($T$) curves with the same $\left|n_{100\mathrm{K}}\right|$ but opposite signs for sample 5 ($p$-type) and sample 6 ($n$-type).

Figure 4

Plots of $c$-axis lattice parameters before (open symbols) and after (solid symbols) electrochemical reactions for samples 5 (circles), 6 (triangles), 7 (squares), and 8 (diamonds) as a function of $n_{100\mathrm{K}}$. The shaded region corresponds to the relation of the $c$-axis parameter versus $n_{100\mathrm{K}}$ reported for YBa${}_2$Cu${}_3$O${}_y$ single crystals with various $y$ (Refs. 15 and16). Dotted line is a guide to the eyes. Inset: Comparison of (005) peaks in the measurements of x-ray diffraction patterns for sample 2 (as grown) and sample 6 (after electrochemical doping).