Doping dependence of the chemical potential and surface electronic structure in ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$ and ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ using hard x-ray photoemission spectroscopy

The electronic structure of ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$ and ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ for various values of $x$ has been investigated using hard x-ray photoemission spectroscopy. The experimental results establish that the cleaving of ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$ compounds occurs predominantly in the ${\text{BaCuO}}_3$ complex, leading to charged surfaces at higher $x$ and to uncharged surfaces at lower $x$ values. The bulk component of the core-level spectra exhibits a shift in binding energy as a function of $x$, from which a shift of the chemical potential as a function of hole concentration in the ${\text{CuO}}_2$ layers could be derived. The doping dependence of the chemical potential across the transition from a Mott-Hubbard insulator to a Fermi-liquid-like metal is very different in these two series of compounds. In agreement with previous studies in the literature the chemical-potential shift in ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ is close to zero for small hole concentrations. In ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$, similar to all other doped cuprates studied so far, a strong shift of the chemical potential at low hole doping is detected. However, the results for the inverse charge susceptibility at small $x$ shows a large variation between different doped cuprates. The results are discussed in view of various theoretical models. None of these models turns out to be satisfactory.

Figure 1

(Color online) Crystal structure of ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_7$ (orthorhombic; space group $Pmmm$). O(1) sites are empty in the crystal structure of ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_6$ leading to a tetragonal structure.

Figure 2

(Color online) $\text{Y}3d$ core-level spectra of ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$ for various O concentrations. (a) Normal-emission spectra collected with $h\nu =2010\text{eV}$. (b) Analogous spectra collected at the same photon energy but at $35°$ emission with respect to surface normal. (c) Normal-emission spectra collected with 2010 eV photons (open circles) and 6030 eV photons (closed circles [green in online color version]) for $x=0.45$. The red solid lines superimposed on the experimental spectra are the total fit results and the thin blue lines represent the spin-orbit split components of the fit.

Figure 3

(Color online) (a) $\text{Ba}4d$ core-level spectra ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$ for various $x$ values collected at normal emission with $h\nu =2010\text{eV}$. (b) $\text{Ba}4d$ spectra from ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6.45}$, collected at $35°$ emission with 2010 eV photons, and at normal emission with both 2010 and 6030 eV excitation energies. (c) A zoom in on the $\text{Ba}3d_{5/2}$ spin-orbit split component for various values of $x$ collected at normal emission with $h\nu =2010\text{eV}$.

Figure 4

(Color online) (a) Fits to the $\text{Ba}4d$ core-level spectra of ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$ for various $x$ values collected at normal emission with 2010 eV photon energy. (b) Fits to the $\text{Ba}4d$ spectra collected at normal emission with 6030 eV photons. (c) The surface contribution extracted from the fits, as a function of $x$, whereby the shorthand notation 2K and 6K has been used to designate the use of 2010 and 6030 eV photons, respectively. S.C. represents surface contribution.

Figure 5

(Color online) $\text{O}1s$ core-level spectra of ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$ for various $x$ values, collected using the photon energies (and emission angles) indicated, whereby NE stands for normal emission.

Figure 6

(Color online) $\text{O}1s$ core-level spectra of (a) ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6.45}$ and (b) ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{7.0}$ collected under conditions giving differing surface sensitivity. For the panels containing spectra: from top to bottom the bulk sensitivity is increasing. The red (blue) lines represent the simulated total (partial) spectral functions. (c) The areas (i.e., intensities) of the features A, B, and C as a function of $x$.

Figure 7

(Color online) $\text{Cu}2p$ spectra of ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$ for various $x$ values collected at normal emission using 2010 eV photon energy.

Figure 8

(Color online) (a) $\text{Cu}2p_{3/2}$ spectra from ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$ for various $x$ values recorded with 2010 eV photons. (b) Analogous data for the doping levels $x=0.15$, 0.45, and 1.0 collected with $h\nu =2010\text{eV}$ and 6030 eV.

Figure 9

(Color online) (a) $\text{La}3d_{5/2}$ feature and its satellite of ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ for various $x$ values are shown for 2010 eV (open circles) and 6030 eV (solid circles) photon energies. The solid line represents the 6030 eV spectra shifted by about 0.2 eV toward higher binding energies. (b) $\text{La}3d_{5/2}$ spectra using 2010 eV photons for different values of $x$.

Figure 10

(Color online) $\text{La}4d$ spectra of ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ for various values of $x$. The open and closed circles represent spectra collected using 2010 and 6030 eV photon energies. The solid line superimposed on the experimental data represent the fit comprising a pair of spin-orbit split doublets, each of which is shown by a green/gray shaded surface.

Figure 11

(Color online) (a) $\text{O}1s$ core-level spectra of ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ for various $x$ values, corresponding to 2010 and 6030 eV photon energies are shown by open and solid circles, respectively. (b) 2010 eV spectra for different values of $x$ reveal an energy shift with increasing $x$.

Figure 12

(Color online) Fit to the $\text{O}1s$ features of ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ for various $x$ values obtained at (a) 2010 eV and (b) 6030 eV photon energies exhibiting two distinct features in all the cases. (c) The peak position and profiles of the $\text{O}1s$ spectra in ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ are compared to those of the two ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_7$ end members.

Figure 13

(Color online) $\text{Cu}2p_{3/2}$ spectra of ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ for various values of $x$ measured with a photon energy of 2010 eV. While the satellite position shows only a small shift, the main peak exhibits significant change. The data for $x=0.0$ and 0.2 are superimposed with data for $x=0.04$ (red triangles).

Figure 14

(Color online) $\text{Cu}2p_{3/2}$ spectra of ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ for various $x$ values collected using 2010 eV (solid circles) and 6030 eV (open circles) photons. A distinct signature of three features A, B, and C is visible in the spectra of the main line region.

Figure 15

(Color online) binding-energy shift of the various bulk-related core-levels lines as a function of (a) $x$ and of (b) hole concentration in the ${\text{CuO}}_2$ layers in ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$ samples. The analogous energy shifts for ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ samples are also shown in (b). The solid lines represent a guide to the eye.