Coexistence of Midgap Antiferromagnetic and Mott States in Undoped, Hole- and Electron-Doped Ambipolar Cuprates

We report the first observation of the coexistence of a distinct midgap state and a Mott state in undoped and their evolution in electron and hole-doped ambipolar ${\mathrm{Y}}_{0.38}{\mathrm{La}}_{0.62}({\mathrm{Ba}}_{0.82}{\mathrm{La}}_{0.18}{)}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ films using spectroscopic ellipsometry and x-ray absorption spectroscopies at the O $K$ and Cu $L_{3,2}$ edges. Supported by theoretical calculations, the midgap state is shown to originate from antiferromagnetic correlation. Surprisingly, while the magnetic state collapses and its correlation strength weakens with dopings, the Mott state in contrast moves toward a higher energy and its correlation strength increases. Our result provides important clues to the mechanism of electronic correlation strengths and superconductivity in cuprates.

Figure 1

(a) Crystal structure and (b) phase diagram of $p$-type and $n$-type YLBLCO. Note that while $T_c$ is from our transport measurements, $T_N$ is estimated according to the relationship between the Nèel temperature and carrier concentration from Ref. [16]. Optical conductivity (${\sigma }_1$) of (c) $p$-type and (d) $n$-type YLBLCO for various carrier concentrations and insulator $P0$ from spectroscopic ellipsometry measurements. Theoretical calculations of ${\sigma }_1$ of (e) $p$-type and (f) $n$-type YLBLCO for various carrier concentrations and insulator $P0$.

Figure 2

Low-energy region (preedge peak) of the O $K$-edge absorption spectra of (a) $p$-type and (c) $n$-type YLBLCO with various carrier concentrations including an insulator $P0$ for $\mathbf{E}\perp c$. Cu $L_3$ edge absorption spectra of (b) $p$-type and (d) $n$-type YLBLCO with various carrier concentration including an insulator $P0$ for $\mathbf{E}\perp c$.

Figure 3

(a) The computed self-energy dressed spectral weight spectrum for three representative dopings, insulating $P0$, $P4$, and $N4$. (b) Real (${\mathrm{\Sigma }}^{\prime }$) and (c) imaginary part (${\mathrm{\Sigma }}^{\prime \prime }$) of self-energy of $p$-type (left) and $n$-type (right) YLBLCO for various carrier concentrations and $P0$. (d) The computed $k$-dependent mass renormalization factor for three representative dopings, $P0$, $P4$, and $N4$. (e) DOS of $p$-type (left) and $n$-type (right) YLBLCO for various carrier concentrations and $P0$.

Figure 4

(a) Contrasting carrier-concentration dependences of the magnetic and charge-transfer gaps in $p$-type and $n$-type YLBLCO. Present theoretical and experimental results (measured by spectroscopic ellipsometry). (b) Carrier-concentration dependences of the energy position of peaks $A$ ($A^{\prime }$), and $B$ for $p$-type and $n$-type YLBLCO extracted XAS at the O $K$ edge (cf. Fig. 2). The shadow and the dashed lines are to guide the eye. (c) Proposed electronic structure model of samples $N2$, $P0$, $P1$, and $P4$, as derived from the analyses of XAS and SE data. The black (empty) bands are occupied (unoccupied) by electrons. There are lower (LHB) and upper Hubbard bands (UHB) separated by the Mott-Hubbard gap, lower (LMB) and upper magnetic bands (UMB) separated by the antiferromagnetic gap, and the Zhang-Rice (ZR) feature. The “0 eV” represent the Fermi level. We note that for $P0$ and $n$-type cases the ZR feature occurs below the Fermi level and is accessible in SE but not in XAS. While for $p$-type cases, the ZR feature occurs above the Fermi level and is accessible in both SE and XAS.