Charge order is a central feature of the physics of cuprate superconductors and is known to arise from a modulation of holes with primarily oxygen character. Low-valence nickelate superconductors ...also host charge order, but the electronic character of this symmetry breaking is unsettled. Here, using resonant inelastic x-ray scattering at the Ni \(L_2\)-edge, we identify intertwined involvements of Ni \(3d_{x^2-y^2}\), \(3d_{3z^2-r^2}\), and O \(2p_{\sigma}\) orbitals in the formation of diagonal charge order in an overdoped low-valence nickelate La\(_{4}\)Ni\(_{3}\)O\(_{8}\). The Ni \(3d_{x^2-y^2}\) orbitals, strongly hybridized with planar O \(2p_{\sigma}\), largely shape the spatial charge distribution and lead to Ni site-centered charge order. The \(3d_{3z^2-r^2}\) orbitals play a small, but non-negligible role in the charge order as they hybridize with the rare-earth \(5d\) orbitals. Our results reveal that the low-energy physics and ground-state character of these nickelates are more complex than those in cuprates.
Science Advances 9, eadg3710 (2023) In the study of dynamic charge order correlations in the cuprates, most high
energy-resolution resonant inelastic x-ray scattering (RIXS) measurements have
focused ...on momenta along the high-symmetry directions of the copper oxide
plane. However, electron scattering along other in-plane directions should not
be neglected as they may contain information relevant, for example, to the
origin of charge order correlations or to our understanding of the isotropic
scattering responsible for strange metal behavior in cuprates. We report
high-resolution resonant inelastic x-ray scattering (RIXS) experiments that
reveal the presence of dynamic electron correlations over the $q_x$-$q_y$
scattering plane in underdoped
$\textrm{Bi}_2\textrm{Sr}_2\textrm{Ca}\textrm{Cu}_2\textrm{O}_{8+\delta}$ with
$T_c=54$ K. We use the softening of the RIXS-measured bond stretching phonon
line as a marker for the presence of charge-order-related dynamic electron
correlations. The experiments show that these dynamic correlations exist at
energies below approximately $70$ meV and are centered around a quasi-circular
manifold in the $q_x$-$q_y$ scattering plane with radius equal to the magnitude
of the charge order wave vector, $q_{CO}$. We also demonstrate how this
phonon-tracking procedure provides the necessary experimental precision to rule
out fluctuations of short-range directional charge order (i.e. centered around
$q_x=\pm q_{CO}, q_y=0$ and $q_x=0, q_y=\pm q_{CO}$) as the origin of the
observed correlations.
High-resolution resonant inelastic x-ray scattering (RIXS) at the oxygen K edge has been used to study the orbital excitations of Ca sub(2) RuO sub(4) and Sr sub(2) RuO sub(4). In combination with ...linear dichroism x-ray absorption spectroscopy, the ruthenium 4d-orbital occupation and excitations were probed through their hybridization with the oxygen p orbitals. These results are described within a minimal model, taking into account crystal field splitting and a spin-orbit coupling lambda sub(so) = 200 meV. The effects of spin-orbit interaction on the electronic structure and implications for the Mott and superconducting ground states of (Ca,Sr) sub(2) RuO sub(4) are discussed.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
Phys. Rev. X 12, 011055 (2022) The discovery of superconductivity in square-planar low valence nickelates
has ignited a vigorous debate regarding their essential electronic properties:
Do these ...materials have appreciable oxygen charge-transfer character akin to
the cuprates, or are they in a distinct Mott-Hubbard regime where oxygen plays
a minimal role? Here, we resolve this question using O $K$-edge resonant
inelastic x-ray scattering (RIXS) measurements of the low valence nickelate
La$_{4}$Ni$_{3}$O$_{8}$ and a prototypical cuprate La$_{2-x}$Sr$_{x}$CuO$_{4}$
($x=0.35$). As expected, the cuprate lies deep in the charge-transfer regime of
the Zaanen-Sawatzky-Allen scheme. The nickelate, however, is not well described
by either limit of the ZSA scheme and is found to be of mixed
charge-transfer/Mott-Hubbard character with the Coulomb repulsion $U$ of
similar size to the charge-transfer energy $\Delta$. Nevertheless, the
transition-metal-oxygen hopping is larger in La$_{4}$Ni$_{3}$O$_{8}$ than in
La$_{2-x}$Sr$_{x}$CuO$_{4}$, leading to a significant superexchange interaction
and an appreciable hole occupation of the ligand O orbitals in
La$_{4}$Ni$_{3}$O$_{8}$ despite its larger $\Delta$. Our results clarify the
essential characteristics of low valence nickelates and put strong constraints
on theoretical interpretations of superconductivity in these materials.
The transition metal chalcogenide Ta$_2$NiSe$_5$ undergoes a second-order
phase transition at $T_c=328$ K involving a small lattice distortion. Below
$T_c$, a band gap at the center of its Brillouin ...zone increases up to about
0.35 eV. In this work, we study the electronic structure of Ta$_2$NiSe$_5$ in
its low-temperature semiconducting phase, using resonant inelastic x-ray
scattering (RIXS) at the Ni $L_3$-edge. In addition to a weak fluorescence
response, we observe a collection of intense Raman-like peaks that we attribute
to electron-hole excitations. Using density functional theory calculations of
its electronic band structure, we identify the main Raman-like peaks as
interband transitions between valence and conduction bands. By performing
angle-dependent RIXS measurements, we uncover the dispersion of these
electron-hole excitations that allows us to extract the low-energy boundary of
the electron-hole continuum. From the dispersion of the valence band measured
by angle-resolved photoemission spectroscopy, we derive the effective mass of
the lowest unoccupied conduction band.
The rich phenomenology engendered by the coupling between the spin and
orbital degrees of freedom has become appreciated as a key feature of many
strongly-correlated electron systems. The resulting ...emergent physics is
particularly prominent in a number of materials, from Fe-based unconventional
superconductors to transition metal oxides, including manganites and vanadates.
Here, we investigate the electronic ground states of $\alpha$-Sr$_2$CrO$_4$, a
compound that is a rare embodiment of the spin-1 Kugel-Khomskii model on the
square lattice -- a paradigmatic platform to capture the physics of coupled
magnetic and orbital electronic orders. We have used resonant X-ray diffraction
at the Cr-$K$ edge to reveal N\'{e}el magnetic order at the in-plane wavevector
$\mathbf{Q}_N = (1/2, 1/2)$ below $T_N = 112$ K, as well as an additional
electronic order at the 'stripe' wavevector $\mathbf{Q}_s = (1/2, 0)$ below
T$_s$ $ \sim 50$ K. These findings are examined within the framework of the
Kugel-Khomskii model by a combination of mean-field and Monte-Carlo approaches,
which supports the stability of the spin N\'{e}el phase with subsequent
lower-temperature stripe orbital ordering, revealing a candidate mechanism for
the experimentally observed peak at $\mathbf{Q}_s$. On the basis of these
findings, we propose that $\alpha$-Sr$_2$CrO$_4$ serves as a new platform in
which to investigate multi-orbital physics and its role in the low-temperature
phases of Mott insulators.
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