Source capture
Authors Leonard M. Verhoff, Michael Seidl, Liang Si, Karsten Held
arXiv / Publisher https://arxiv.org/abs/2609.21665
PDF https://arxiv.org/pdf/2609.21665
Relevance score 4.399
Primary category cond-mat.str-el
Published 2026-09-21
Research paradigm Theoretical
Sample form Unknown

Summary

The orbital character of the so-called A-pocket in infinite-layer nickelates remains under debate. We investigate bulk NdNiO2 using density-functional theory combined with dynamical mean-field theory and identify the A-pocket as a strongly hybridized state with substantial interstitial-s, Nd-dxy and Ni-dxz/yz contributions. Our angle-resolved photoemission spectroscopy simulations reproduce the main polarization-dependent features measured experimentally. Removing the interstitial-s matrix element leads to a clear disagreement with the measured Fermi surface maps, even though the interstitial-s contribution is not dominant. Suppressing instead the Nd-dxy or Ni-dxz/yz matrix elements leaves the maps almost unchanged, because these are small at the zone corner. More generally, our results demonstrate that assigning the microscopic origin of spectral features in infinite-layer nickelates requires treating orbital hybridizations and photoemission matrix elements together.

Materials

  • bulk NdNiO2

Methods

Keywords

  • a pocket
  • orbital character
  • orbital hybridization
  • photoemission matrix elements
  • fermi surface maps
  • polarization dependence
  • interstitial s state
  • nd dxy state
  • ni dxz/yz state
  • matrix element effect
  • bonding antibonding state
  • spectral weight
  • electronic correlations

Highlights

  • Assigning the microscopic origin of spectral features in infinite-layer nickelates requires treating orbital hybridizations and photoemission matrix elements together.
  • Removing the interstitial-s matrix element leads to a clear disagreement with the measured Fermi surface maps, even though the interstitial-s contribution is not dominant.
  • Suppressing the Nd-dxy or Ni-dxz/yz matrix elements leaves the maps almost unchanged, because these matrix elements are small at the zone corner.
  • The interstitial-s orbital dominates the photoemission signal of the A-pocket, not the wave function.
  • A simplified weighted-trace description of ARPES is generally not valid in multi-orbital systems because off-diagonal elements of the spectral function combine coherently with orbital-dependent matrix elements and can produce interferences.
  • The relevant Wannier orbitals forming the low-energy description of NdNiO2 strongly deviate from perfect atomic-like orbitals, mostly due to inter-orbital hybridizations with O-p, which can completely alter matrix elements and the interpretation of the photocurrent.

Conclusions

  • Our DFT+DMFT calculations identify the electron pocket state of infinite-layer nickelates around the A-momentum as a strongly hybridized state with substantial interstitial-s, Nd-dxy, and Ni-dxz/yz character near the Fermi energy.
  • The A-pocket band crosses the Fermi energy because the bonding combination of the interstitial-s and Nd-dxy orbitals leads to a strong dispersion, and this bonding combination further hybridizes and reconstructs with the Ni-dxz/yz band.
  • The A-pocket is predominantly Ni-dxz/yz at its bottom at the A-momentum, has a strong admixture of Ni-dxz/yz, interstitial-s, and Nd-dxy around the Fermi energy, and is predominantly interstitial-s towards the Gamma point.
  • Local DMFT correlations strongly renormalize the Ni-dxz/yz band but shift the A-pocket only slightly upward in energy, otherwise hardly changing its dispersion, so correlations only weakly modify the orbital composition of the A-pocket at the Fermi level.
  • Our DFT+DMFT simulations of ARPES spectra well reproduce the principal polarization-dependent symmetries and qualitative intensity variations of the A-pocket in the measured Fermi surface maps.
  • The apparent pure interstitial-s character of the A-pocket in ARPES is due to a matrix-element effect; under the experimental conditions, the Ni-dxz/yz and Nd-dxy contributions are suppressed compared to the interstitial-s state, so the interstitial-s orbital contributes much more to the ARPES signal than its admixture to the wavefunction would suggest.
  • Neither the experimental nor the simulated data require the A-pocket to have purely or predominantly interstitial-s character; instead the measured maps are fully consistent with the hybridized A-pocket found in DFT and DFT+DMFT.

Main claims

  • The A-pocket in infinite-layer nickelates is a strongly hybridized state with substantial interstitial-s, Nd-dxy, and Ni-dxz/yz character.
    • Evidence: DFT+DMFT projections show that at the Fermi level the A-pocket has about 30% interstitial-s, 22% Nd-dxy, and 48% Ni-dxz/yz character.,The A-pocket band arises from an interstitial-s and Nd-dxy bonding state that further hybridizes and reconstructs with the Ni-dxz/yz band.
  • The interstitial-s matrix element is essential to reproduce measured polarization-dependent ARPES maps even though interstitial-s is not dominant in the spectral weight.
    • Evidence: Suppressing the interstitial-s matrix element leads to clear disagreement with measured Fermi surface maps, especially for linear horizontal polarization.,Interstitial-s provides only about 30% of the spectral weight but dominates the A-pocket photoemission signal.
  • The apparent pure interstitial-s character of the A-pocket in ARPES is a matrix-element effect.
    • Evidence: Ni-dxy and Ni-dxz/yz matrix elements are small at the Brillouin zone corner, so suppressing them leaves the ARPES maps almost unchanged.,The measured maps are consistent with the hybridized pocket found in DFT and DFT+DMFT.
  • DMFT correlations only weakly modify the orbital composition of the A-pocket at the Fermi level.
    • Evidence: The A-pocket is weakly correlated and shifts only slightly upward in DMFT.,The orbital contributions change only slightly from about 30% interstitial-s, 25% Nd-dxy, and 44% Ni-dxz/yz in DFT to about 30%, 22%, and 48% in DMFT.
  • Assigning the microscopic origin of spectral features in infinite-layer nickelates requires treating orbital hybridizations and photoemission matrix elements together.
    • Evidence: Removing the interstitial-s matrix element causes disagreement with experiment, while suppressing Nd-dxy or Ni-dxz/yz channels has only a minor effect.,Matrix elements must be evaluated with the actual Wannier functions rather than with atomic-like orbitals.
  • The ARPES data of Li et al. do not contradict the hybridized A-pocket and can neither confirm nor exclude the Ni-dxy and Nd-dxy admixture under the measured geometry.
    • Evidence: ARPES in the experimental geometry is essentially blind to the Ni-dxy and Nd-dxy admixture because their matrix elements are suppressed compared with the interstitial-s channel.,Simulated Fermi surface maps with the full hybridized pocket are consistent with the measured polarization-dependent features.

Workflow

  • model_construction — The low-energy electronic structure of bulk NdNiO2 is modeled from first principles.
    • Materials: bulk NdNiO2; Nd-4f states; Ni-3d orbitals; Nd-5d orbitals; interstitial-s orbital
    • Methods: crystal structure setup with P4/mmm symmetry and lattice constants a=3.854 Angstrom, c=3.265 Angstrom; GGA-PBE exchange-correlation; open-core approximation for Nd-4f states
    • Observations: Non-magnetic DFT incorrectly places Nd-4f states at the Fermi level, motivating the open-core treatment.
  • DFT_calculation — DFT identifies the A-pocket band as involving interstitial-s, Nd-dxy, and Ni-dxz/yz character.
    • Materials: bulk NdNiO2; Ni-3d states; Nd-5d states; interstitial-s state
    • Methods: WIEN2K all-electron full-potential DFT; GGA-PBE
    • Observations: DFT shows an A-pocket derived from a hybridized bonding state about 2 eV above the Fermi level, with an antibonding partner near 4 eV.; The A-pocket band is flat from the zone center toward M, then hybridizes with Ni and Nd states toward the zone corner.
  • Wannier_downfolding — An 11-band Wannier model captures the relevant low-energy orbitals and hybridizations.
    • Materials: Bloch states; Ni-3d-derived states; Nd-5d-derived states; interstitial-s state
    • Methods: maximally localized Wannier functions via WANNIER90 interfaced by WIEN2WANNIER; downfolding to an 11-band tight-binding model
    • Observations: The interstitial-s state lies between two Ni sites where apical oxygen would sit in the perovskite parent phase.; The Ni- and Nd-like Wannier orbitals contain some O-2p weight and are not pure atomic orbitals.
  • DMFT_calculation — DMFT correlations preserve the hybridized character of the A-pocket near the Fermi level.
    • Materials: Ni impurity; Nd impurity; interstitial-s state
    • Methods: Hubbard-Kanamori interaction with Anisimov double counting; DMFT impurity solved by continuous-time quantum Monte Carlo in hybridization expansion on the Matsubara axis using W2DYNAMICS; analytic continuation to real frequencies using maximum entropy in anacont; Dyson equation for the lattice Green's function
    • Observations: DMFT shifts the A-pocket slightly upward and strongly renormalizes the Ni-dxz/yz band.; At the DMFT Fermi crossing, the A-pocket contributions are about 30% interstitial-s, 22% Nd-dxy, and 48% Ni-dxz/yz.
  • ARPES_simulation — The A-pocket ARPES intensity is dominated by the interstitial-s channel through matrix-element effects, not by its spectral weight alone.
    • Materials: Wannier orbitals; DMFT spectral-function matrix; dipole matrix elements
    • Methods: Wannier-ARPES; Fermi golden rule and sudden approximation; plane-wave final-state approximation; coherent sum over orbital matrix elements including interorbital interference; controlled suppression of selected matrix-element channels
    • Observations: Simulated polarization-dependent Fermi surface maps reproduce the main experimental features.; Removing the interstitial-s matrix element causes clear disagreement with measured maps, especially for linear horizontal polarization.; Suppressing Nd-dxy or Ni-dxz/yz channels leaves the maps almost unchanged because these matrix elements are small at the Brillouin zone corner.
  • comparison_and_interpretation — The measured ARPES maps do not contradict a hybridized A-pocket and require simultaneous treatment of orbital hybridization and photoemission matrix elements.
    • Materials: simulated ARPES maps; experimental ARPES maps from Li et al. [21]
    • Methods: qualitative comparison of polarization-dependent Fermi surface maps; orbital-channel decomposition; comparison of atomic-like and Wannier matrix elements
    • Observations: Full DFT+DMFT plus plane-wave final-state simulations match the principal polarization-dependent features.; The atomic-orbital weighted-trace analysis of Li et al. neglects the interstitial-s channel and coherent off-diagonal spectral matrix elements.; Wannier orbitals deviate from atomic-like orbitals and can completely alter matrix elements and photocurrent interpretation.