Distinction between the normal-state gap and superconducting gap of electron-doped cuprates

We report the study of temperature- and magnetic-field dependent in-plane tunneling conductance on the electron-doped ${\text{Nd}}_{1.85}{\text{Ce}}_{0.15}{\text{CuO}}_{4-y}$ and ${\text{Pr}}_{1-x}{\text{LaCe}}_x{\text{CuO}}_{4-y}$ single crystals. The previously reported normal state gap (NSG) ${\Delta }_{\text{ps}}$ and superconducting gap ${\Delta }_{\text{sc}}$ were observed simultaneously in superconducting state. Combined with an investigation on the evolution of low-energy spectral weight with temperature, our data present evidence for electron-doped cuprates that ${\Delta }_{\text{ps}}$ and ${\Delta }_{\text{sc}}$ are most possibly two different energy scales with very different field dependencies, and that superconductivity is condensed from the residual quasiparticle density of states (DOS) of the NSG state.

Figure 1

(Color online) (a) Conductance vs voltage for ncco-op25 measured at various temperatures. (b) Spectra measured in superconducting state $\left(T=2\text{K}\right)$ and normal state $\left(T=28\text{K}\right)$, respectively. The thick solid line indicates the spectrum of 28 K by subtracting the normal state resistance of the sample $\left(R_s=1.85\Omega \right)$. (c) Comparison of two spectra measured at the same location whereas with different junction resistances, the solid line indicates the background according to the spectra of above $T_c$ (see text).

Figure 2

(Color online) (a) Conductance vs voltage for ncco-op25 in both superconducting state $\left(H=0\text{T}\right)$ and normal state $\left(H=9.5\text{T}\right)$. The hatched area indicates the lost low-energy spectral weight in normal state. (b) Field dependence of normalized spectra for ncco-op25. The normal state data with $H=9–12\text{T}$ overlap each other. The thin gray lines correspond to the fields from 0 T to 5.5 T, and the thick black line indicates the spectra of $H=6\text{T}$. (c) Field dependence of normalized spectra for plcco-un19. All curves except the lowest one are offset upwards for clarity. Both the superconducting coherence peaks and the NSG edges are indicated by short black bars.

Figure 3

(Color online) Temperature dependence of normalized spectra for (a) $H=0\text{T}$ and (b) $H>H_{c2}$. (c) Temperature dependence of the integrated spectral weight for the spectra shown in (a) and (b), respectively. (d) Difference in the integrated spectral weight between superconducting state and NSG state. The solid line is the theoretical calculation according to the BTK theory.

Figure 4

(Color online) Evolution of spectra at $T=2\text{K}$ with increasing field for various doping levels: (a) plcco-un19, (b) plcco-un23, (c) ncco-op25, and (d) plcco-ov17. For each dope, all the data of $H<H_{c2}$ are normalized by the one with the highest field and offset relatively in order to demonstrate the evolution of the coherence peaks as indicated by those two symmetric thin lines.

Figure 5

(Color online) Field dependence of the magnitude of ${\Delta }_{\text{sc}}$ which was estimated from the peak positions of the spectra shown in Fig. 4. The solid line is a guide to the eye.