Figure 1

Schematic of electronic structure evolution with hole doping in the single-layer nickelate $R_{2-x}$Sr${}_x$NiO${}_4$. (a) Checkerboard (CB)-type charge ordering on the NiO${}_2$ plane at $x=0.5$. (b) Underlying two-dimensional hole Fermi surface with k-dependent pseudogap at $x=1.1$, reproduced from the previous ARPES study.[17] Solid and dotted curves represent the Fermi momentum positions where one can see a quasiparticle peak and a high-energy pseudogap structure, respectively. A three-dimensional electron pocket centered only in $k_z=0$ plane is also shown.Reuse & Permissions

Figure 2

(a) Polarization-dependent XAS spectra at O $K$-edge for E $\parallel a$ and E $\parallel c$. Some pre-edge peaks as labeled with letters (A-D) are clearly discerned in the lower energy region (528-533 eV) next to the main peak centered at about 536 eV. (b) Schematic of all the transition processes corresponding to the pre-edge peaks. Filled arrows added in the left and right sides represent the electrons which are transferred from the O 1$s$ state to the initial Ni 3$d$ $e_g$ orbital state shown in the middle. Detailed doping variation of the O $K$-pre-edge spectra for (c) E $\parallel a$ and (d) E $\parallel c$. Gaussian fittings for the pre-edge peaks corresponding to the transition processes (B, C, and D) as depicted in (b) are indicated by dashed, solid, and chain curves, respectively. The peaks corresponding to the bracketed final states in the panel C of (b) cannot be clearly discerned probably because such higher-energy peaks are easily hidden by the main or other pre-edge peaks. We confirm that variation of the fitting procedures even considering the extra peaks causes negligible changes in the estimated site occupancy shown in Fig. 3a within the error bar and no difference in our conclusion.Reuse & Permissions

Figure 3

(a) Doping variation of the site occupancy with the respective valence and orbital states for the hole doping in Nd${}_{2-x}$Sr${}_x$NiO${}_4$, as estimated by fitting and quantifying the peaks B-D in the O $K$-pre-edge spectra for E $\parallel c$. Room-temperature lattice constants along (b) $a$- and (c) $c$-axis directions for Nd${}_{2-x}$Sr${}_x$NiO${}_4$ single crystals. The crystal structure is low-temperature orthorhombic for $x\le 0.45$ and high-temperature tetragonal for $x\ge 0.50$, respectively.Reuse & Permissions

Figure 4

Temperature and doping variations of the in-plane (a) resistivity ${\rho }_{ab}$ and (b) Hall coefficient $R_{\mathrm{H}}$ in $R_{2-x}$Sr${}_x$NiO${}_4$. Dotted, solid, and broken lines show the results obtained for $R=\mathrm{La}$, Nd, and Eu, respectively. Little $R$ dependence is observed as also confirmed in the previous reports.[15, 17] (c) Doping dependence of the low-temperature specific heat in a form of $C/T$ vs. $T^2$ plot for $R=\mathrm{Eu}$. Straight lines represent the extrapolation toward zero temperature to estimate the electronic specific heat coefficient ($\gamma$, Fig. 6c). The deviation from the lines below $T\sim 1$ K arises from the nuclear Schottky component. In addition, the curved shape with the steeper slope at $x=0.7$ is due to an additional low-temperature entropy component appearing around the low-doping region.[34]Reuse & Permissions

Figure 5

(a)-(c) Temperature variation of the in-plane optical conductivity spectra ${\sigma }_{ab}\left(\omega \right)$ in Nd${}_{2-x}$Sr${}_x$NiO${}_4$ with $x=1.0$, 1.1, and 1.3. (d)-(f) Temperature dependence of the inverse of the Hall coefficient $1/R_{\mathrm{H}}$ and the low-energy spectral weight (effective number of electrons $\Delta N_{\mathrm{eff}}$ at ${\omega }_{\mathrm{c}}=0.2$ eV, see text for details) for the corresponding doping levels. Remarkable decrease and increase of $1/R_{\mathrm{H}}$ and $\Delta N_{\mathrm{eff}}$ are observed below the characteristic temperature $T^{*}$ (indicated by vertical shadows), respectively.Reuse & Permissions

Figure 6

Evolution of (a) the in-plane conductivity ${\sigma }_{ab}$ and (b) the Hall coefficient $R_{\mathrm{H}}$ in the $T$-$x$ diagram. Doping variation of (c) the electronic specific-heat coefficient $\gamma$ and (d) the pseudogap (PG) magnitude ${\Delta }_{\mathrm{PG}}$ (see text for details). They indicate the insulator (I)-metal (M) ground state boundary and the proximate critical doping region (CR), roughly denoted by the vertical broken lines. (e) Electronic phase diagram of $R_{2-x}$Sr${}_x$NiO${}_4$ around the metal-insulator transition. The pseudogap formation temperature $T^{*}$ (see text for details) are presented together with the checkerboard (CB)-type charge order and antiferromagnetic (AF) spin order transition temperatures (upward triangles,[13] downward triangles,[15] and squares[29]). The open and filled marks indicate the data for $R=\mathrm{La}$ and Nd, respectively; in this system the critical temperatures of the spin and charge orderings have been confirmed to be least influenced by the variation of the $R$ species.[10, 13, 15] The solid and broken curves are merely the guide to the eyes.Reuse & Permissions