Orbital and spin character of doped carriers in infinite-layer nickelates

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Orbital and spin character of doped carriers in infinite-layer nickelates

M. Rossi, H. Lu, A. Nag, D. Li, M. Osada, K. Lee, B. Y. Wang, S. Agrestini, M. Garcia-Fernandez, J. J. Kas, Y.-D. Chuang, Z. X. Shen, H. Y. Hwang, B. Moritz, Ke-Jin Zhou, T. P. Devereaux, and W. S. Lee

Phys. Rev. B 104, L220505 – Published 8 December 2021

Abstract

The recent discovery of superconductivity in ${Nd}_{1 - x} {Sr}_{x} {NiO}_{2}$ has drawn significant attention in the field. A key open question regards the evolution of the electronic structure with respect to hole doping. Here we exploit x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) to probe the doping-dependent electronic structure of ${Nd}_{1 - x} {Sr}_{x} {NiO}_{2}$ . Upon doping, a high-energy feature in Ni $L_{3}$ -edge XAS develops in addition to the main absorption peak, while XAS at the O $K$ -, Nd $M_{3}$ - and Nd $M_{5}$ -edge exhibits a much weaker response. This implies that doped holes are mainly introduced into Ni $3 d$ states. By comparing our data to atomic multiplet calculations including $D_{4 h}$ crystal field, the doping-induced feature in Ni $L_{3}$ -edge XAS is consistent with a $d^{8}$ spin-singlet state in which doped holes reside in the $3 d_{x^{2} - y^{2}}$ orbitals. This is further supported by the softening of RIXS orbital excitations due to doping, corroborating with the Fermi level shift associated with increasing holes in the Ni $3 d_{x^{2} - y^{2}}$ orbital.

Received 7 October 2020
Revised 2 August 2021
Accepted 1 November 2021

DOI:https://doi.org/10.1103/PhysRevB.104.L220505

Physics Subject Headings (PhySH)

Electronic structure Impurities in superconductors Superconducting gap Superconducting phase transition

Cuprates Thin films

Methods in superconductivity Resonant inelastic x-ray scattering X-ray absorption spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

M. Rossi ¹, H. Lu^1,2, A. Nag ³, D. Li ^1,4, M. Osada ^1,5, K. Lee^1,2, B. Y. Wang^1,2, S. Agrestini³, M. Garcia-Fernandez³, J. J. Kas^6,1, Y.-D. Chuang ⁷, Z. X. Shen^1,2,4,8, H. Y. Hwang^1,4,8, B. Moritz¹, Ke-Jin Zhou³, T. P. Devereaux^1,5,8,*, and W. S. Lee ^1,†

¹Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
²Department of Physics, Stanford University, Stanford, California 94305, USA
³Diamond Light Source, Harwell Campus, Didcot OX11 0DE, United Kingdom
⁴Department of Applied Physics, Stanford University, Stanford, California 94305, USA
⁵Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA
⁶Department of Physics, University of Washington, Seattle, Washington 98195, USA
⁷Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 6-2100, Berkeley, California 94720, USA
⁸Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305, USA

^*Corresponding author: tpd@stanford.edu
^†Corresponding author: leews@stanford.edu

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Vol. 104, Iss. 22 — 1 December 2021

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Images

Figure 1
(a) Total electron yield Ni $L_{3}$ -edge XAS of ${Nd}_{1 - x} {Sr}_{x} {NiO}_{2}$ . A constant background has been removed and the postedge intensity has been set to 1. The drawing shows the direction of the photon polarization $ε$ in the sample reference frame $a b c$ . (b) Total electron yield Nd $M_{3}$ -edge XAS. A constant background has been removed and the peak intensity has been set to 1 for better comparison of the peak region. The inset zooms into the energy range corresponding to the leading edge.
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Figure 2
RIXS intensity maps of $Nd {NiO}_{2}$ (a) and ${Nd}_{0.775} {Sr}_{0.225} {NiO}_{2}$ (b) as a function of incident photon energy across the Ni $L_{3}$ edge. RIXS spectra were collected at an incidence angle of $35^{\circ}$ and scattering angle of $154^{\circ}$ . The dashed line marks the incident energy selected for the measurement of the angular dependence of RIXS spectra. Data reported in (a) are taken from Ref. [38].
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Figure 3
(a) Schematics of the RIXS process at the Ni $L_{3}$ edge. (b) Ni cation (orange sphere) surrounded by O anions (black spheres) and sketch of the experimental geometry. The incoming photon wave-vector $k_{in}$ forms an angle $θ$ with the sample $a b$ plane. The scattered photon wave-vector $k_{out}$ forms an angle $Θ$ with $k_{in}$ . The incident photon polarization is either parallel ( $π$ ) or perpendicular ( $σ$ ) to the scattering plane, which coincides with the $a c$ plane. (c, d) Stack plot of RIXS spectra of $Nd {NiO}_{2}$ (c) and ${Nd}_{0.875} {Sr}_{0.125} {NiO}_{2}$ (d) collected at various incidence angles $θ$ and fixed scattering angle $Θ$ . The incident energy was fixed to 852.5 eV for $Nd {NiO}_{2}$ [(dashed line in Fig. 2] and 852.44 eV for ${Nd}_{0.875} {Sr}_{0.125} {NiO}_{2}$ . (e) Angular dependence of the intensity of $d d$ transitions calculated for the angles of (c).
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Figure 4
(a) Doping dependence of the RIXS spectra of ${Nd}_{1 - x} {Sr}_{x} {NiO}_{2}$ measured at various incidence angles with $Θ = 154^{\circ}$ and $π$ incident photon polarization. (b) Two-level diagrams showing the competition between Hund's coupling and crystal-field splitting $D$ . (c) XAS spectra of $Nd {NiO}_{2}$ (black dots) and ${Nd}_{0.775} {Sr}_{0.225} {NiO}_{2}$ (red diamonds) measured at $θ = 90^{\circ} (ε ∥ a$ , filled markers) and $θ = 10^{\circ}$ ( $ε$ almost parallel to $c$ , empty marker). The spectra of the undoped compound are scaled and shifted to match the main peak of the doped sample. The difference between the spectra of ${Nd}_{0.775} {Sr}_{0.225} {NiO}_{2}$ and $Nd {NiO}_{2}$ is plotted as blue lines. (d) Multiplet calculations of a $d^{8}$ site in spin-singlet ( $S = 0$ , dark gray) and spin-triplet ( $S = 1$ , light gray) configurations, respectively. Solid and dotted lines correspond to light polarization parallel to the $a$ and $c$ axes, respectively.
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