Figure 1

(a) Sketch of the $\mathrm{NdBCO}/\mathrm{STO}$ interface with the labeling of the inequivalent copper and oxygen sites used throughout the text. The interface plane is shown as a dashed line separating the ${\mathrm{TiO}}_2$ plane of the STO (100) single crystal from the BaO plane of the NdBCO film. (b) In situ measured four point probe channel resistance of the three NdBCO devices: 4 u.c. (blue or dark gray diamonds) superconducting with critical temperature $T_c=20\mathrm{K}$ and carrier density $p_{\mathrm{pl}}=0.06(0)\mathrm{\text{holes}}/{\mathrm{CuO}}_2$ plane; 3 u.c. (red or gray circles) and 2 u.c. (black circles) insulating with $p_{\mathrm{pl}}=0.05(0)$ and $0.04(0)\mathrm{\text{holes}}/{\mathrm{CuO}}_2$ plane, respectively.Reuse & Permissions

Figure 2

The channel resistance of the 3 u.c. device, measured as a function of the number of holes per unit area induced by the electric field. In the inset $T_c$ vs doping (ppl) characteristics of the 4 u.c. (green or light gray circle), 3 u.c. (red or gray circle) and 2 u.c. (black circle) overlapped to the HTS dome (SC stands for superconducting and AF for antiferromagnetic phases). The horizontal bars around each experimental point are the estimated field effect, in the assumption that all the induced charge become carriers in the ${\mathrm{CuO}}_2$ planes of the first unit cell at the interface.Reuse & Permissions

Figure 3

Cu $L_3$ XAS spectra, with $\mathbf{E}\Vert ab$ ($I_{ab}$) and (b) $\mathbf{E}\Vert c$ ($I_c$), measured at low temperature (10 K). The ZR satellite peak intensity is related to the number of holes per ${\mathrm{CuO}}_2$ plane (see text). This feature decreases continuously when going from 4 u.c. (black squares) to 2 u.c. (blue or dark gray diamonds), in agreement with transport data.Reuse & Permissions

Figure 4

(a) Gate voltage dependence of the $I_{ab}$ XAS spectra in the 3 u.c. NdBCO device (the experimental configuration is shown in the inset): in the upper panel the $I_{ab}$ spectra are shown for negative (black lines) and positive (red or gray lines) gate voltages. The blue or dark gray line is acquired at zero field. In the lower panels the data are shown as color map pictures with different linear color scales around the main peak at 932 eV (left panel) and around the $\underline{c}3d^{10}\underline{L}$ satellite (right panel). (b) Effect of the field shown by subtracting to each $I_{ab}$ XAS spectra the reference scan acquired without the field (black closed and line joined circles). (c) Difference between $I_{ab}$ and $I_c$ XAS spectra at different gate voltages.Reuse & Permissions