Electronic and transport properties in Ruddlesden-Popper neodymium nickelates ${\mathrm{Nd}}_{n+1}{\mathrm{Ni}}_n{\mathrm{O}}_{3n+1}$ ($n=1–5$)

A series of Ruddlesden-Popper nickelates ${\mathrm{Nd}}_{n+1}{\mathrm{Ni}}_n{\mathrm{O}}_{3n+1}$ ($n=1–5$) have been stabilized in thin film form using reactive molecular-beam epitaxy. X-ray diffraction and scanning transmission electron microscopy measurements suggest high crystalline quality of these films. The average Ni valence states in these compounds are in accordance with the nominal values, as verified by x-ray photoelectron spectroscopy. The metal-insulator transition temperature ($T_{\mathrm{MI}}$) shows a clear $n$ dependence for $n=3–5$ members. At low temperature, the resistivity for $n=3–5$ members exhibits a log $T$ dependence, which is like that reported in parent compounds of superconducting infinite-layer nickelates.

Figure 1

(a) Reflection high-energy electron diffraction (RHEED) intensity oscillations during the codeposition growth of $\mathrm{NdNi}{\mathrm{O}}_3$ films. Inset shows the RHEED pattern of the ${\mathrm{Nd}}_6{\mathrm{Ni}}_5{\mathrm{O}}_{16}$ film taken along the [100] pseudocubic direction. White solid line represents the intensity profile integrated along the vertical axis of the rectangular region marked with white dashed line. The half-order peaks caused by oxygen octahedral rotations were indicated by yellow arrows. (b) 2θ-ω x-ray diffraction (XRD) scans of 9-u.c.-thick ${\mathrm{Nd}}_{n+1}{\mathrm{Ni}}_n{\mathrm{O}}_{3n+1}$ Ruddlesden-Popper (RP) films with $n=1–5$ and 20-u.c.-thick $\mathrm{NdNi}{\mathrm{O}}_3$ film (the $n=\infty$ member of RP phases). Diffraction peaks of $\mathrm{LaAl}{\mathrm{O}}_3$ (LAO) substrate are denoted by asterisks. All plots are vertically offset for clarity. The peaks marked by # are background diffractions appearing occasionally depending on the exact configuration of the instrument. (c) Reciprocal space mapping (RSM) image of the 9-u.c.-thick ${\mathrm{Nd}}_6{\mathrm{Ni}}_5{\mathrm{O}}_{16}$ around LAO (103). (d) Atomic force microscopy (AFM) image of the 9-u.c.-thick ${\mathrm{Nd}}_6{\mathrm{Ni}}_5{\mathrm{O}}_{16}$ and corresponding line cut (inset) along the red dashed line. The root-mean-square roughness is 94 pm.

Figure 2

(a) Cross-sectional annular dark-field (ADF) scanning transmission electron microscopy (STEM) image of a 9-u.c.-thick ${\mathrm{Nd}}_6{\mathrm{Ni}}_5{\mathrm{O}}_{16}$ ($n=5$) film and the corresponding electron diffraction pattern shown in the inset. The subscript $p$ denotes the pseudocubic notation. Scale bar, 5 nm. (b) Enlarged view of the ADF-STEM image shown in (a) with the crystal structure model overlaid. Scale bar, 1 nm. (c)–(e) Atomic-resolution energy-dispersive x-ray spectroscopy (EDX) maps of Nd + Ni, Nd, and Ni acquired from the same area as (b).

Figure 3

(a) The Ni $2p_{3/2}$ spectra of $\mathrm{NdNi}{\mathrm{O}}_3$ and ${\mathrm{Nd}}_2\mathrm{Ni}{\mathrm{O}}_4$ fitted by multiplet splitting peaks using relative peak position, full width at half maximum, and area ratio suggested in Ref. [33]. (b) The Ni $2p_{3/2}$ spectra of $n=2–5$ Ruddlesden-Popper (RP) films fitted using nonlinear least-squares fitting (NLLSF) method as mentioned in the main text. Gradient blue area represents the presence of ${\mathrm{Ni}}^{2+}$ species after spectra deconvolution. (c) The proportion of ${\mathrm{Ni}}^{3+}$ and corresponding Ni valence states in $n=2–5$ RP films estimated from NLLSF analysis. Error bars represent uncertainty of the fitting process. Dashed line is the guide to the eyes.

Figure 4

The Nd $3d$ core-level spectra of Ruddlesden-Popper (RP) films, with the main peak position of Nd $3d_{5/2}$ spectra locating at ∼982 eV across the RP series.

Figure 5

The O $1s$ core-level spectra of Ruddlesden-Popper (RP) films. Following Ref. [42], four peaks (peaks A–D) with a Shirley-type background are included for the data fitting. The detailed fitting parameters are shown in Table 1.

Figure 6

(a) Temperature-dependent resistivities of $n=1–5$ Ruddlesden-Popper (RP) films and a 22-u.c.-thick $\mathrm{NdNi}{\mathrm{O}}_3$ film. Arrows indicate the temperature-sweeping direction. The temperature-sweeping rate is kept at 2 K/min. Schematic of the collinear four-point probe geometry used for the transport measurements is shown in the inset. (b) $T_{\mathrm{MI}}$ of RP films during the cooling process. Error bars represent sample-to-sample variation. (c) Semilogarithmic plot of resistivities of $n=3–5$ RP films during the cooling process. The black dashed lines are the corresponding log $T$ fits. Triangles indicate the upper temperature limit for log $T$ fits.

Figure 7

Resistivity data fitting to the semiconducting state of $n=3–5$ Ruddlesden-Popper (RP) compounds using the model combining thermally activated conduction and Mott variable range hopping, as proposed in Ref. [53]. Inset shows the fitting results for $\mathrm{NdNi}{\mathrm{O}}_3$.