arXiv subjects

Di-Jing Huang

Publications and source records attributed to Di-Jing Huang.

At least 19 recordsLinked to original sources

arXiv2026

Selective coupling of high-order phonons in La2-xSrxCuO4

In crystals and molecules with strong coupling between the charge carriers and atomic vibrations, high order harmonics of the vibrational excitations can be observed with intensity following the Franck-Condon envelope. Here, we uncover a new regime of electron-phonon coupling in La2-xSrxCuO4 using resonant inelastic X-ray scattering. In the undoped compound, we find sharp peaks at approximately 85$\pm$5 meV, 180$\pm$7 meV, and 330$\pm$5 meV with no observable dispersion, and an absence of appreciable intensity from other modes observed in Raman spectroscopy in this energy range. The 180 meV and 330 meV excitations have energies approximately consistent with two-phonon and four-phonon in-plane Cu-O bond stretching mode, suggesting that the resonantly excited valence electrons strongly couple to this mode. These excitations exhibit significant doping dependence, eventually becoming unresolvable at 10% hole doping, where the low energy charge excitations are dominated by dispersive plasmons. The observation of selective coupling to even-ordered phonons and stark contrast with the Raman spectra indicate anomalous electron-phonon coupling beyond the Franck-Condon picture. One intriguing possibility is the existence of locally paired quasiparticles in insulating cuprates.

cond-mat.str-el↗

arXiv2026

Spectroscopic evidence for a molecular orbital Kondo insulator

A Kondo insulator (KI) is a prototypical example of a highly entangled phase of matter, where many-body interactions between local moments and delocalized electrons engender the non-magnetic insulating ground state. Conventionally, the local moments arise from atomic multiplet states with a narrow bandwidth, limiting Kondo coherence to low temperatures. Here, we realize a new paradigm for constructing the KI state with hybridized molecular orbitals in FeSb2. Resonant inelastic X-ray scattering (RIXS) at the Fe L-edge reveals distinct signatures of band-like continuum states and localized states. Comparisons with first-principles calculations establish a mixed-configuration ground state with hybridized Fe d-Sb p molecular orbitals as basis states. By systematically investigating the RIXS momentum, temperature, and doping dependences, we find propagating collective modes commensurate with many-body charge and spin excitations. Our results pave the way for understanding the emerging class of unconventional d electron insulators and engineering high temperature Kondo many-body states.

cond-mat.str-el↗

arXiv2026

Interlayer hybridization enables superconductivity in bilayer nickelates

Ruddlesden-Popper nickelates offer a new route to high-temperature superconductivity beyond the cuprates and iron-pnictides. However, the electronic reorganization that enables superconductivity in bilayer nickelates remain unresolved, largely due to the difficulty of directly probing the superconducting phase. Here, we overcome this limitation by stabilizing superconducting (La,Pr)$_3$Ni$_2$O$_7$ thin films with a protective capping layer, thereby enabling direct spectroscopic access via X-ray absorption and resonant inelastic X-ray scattering. We resolve the evolution of in-plane and out-of-plane electronic states, spin and orbital excitations, and spin-density-waves across insulating, superconducting, and metallic regimes. Combining experimental results with theoretical analysis, we show that the in-plane $d_{x^2-y^2}$ states form an itinerant backbone, while superconductivity emerges only when coherent $d_{z^2}$-$p_z$-$d_{z^2}$ interlayer hybridization develops, accompanied by suppressed static spin order and strongly damped spin excitations. Oxygen stoichiometry and epitaxial strain both act on this interlayer channel, placing superconductivity within a narrow window of interlayer coherence and correlation strength. These findings identify the microscopic ingredients required for superconductivity in bilayer nickelates and provide a multiorbital picture of its emergence.

cond-mat.supr-con↗

arXiv2026

Collective spin excitations in trilayer nickelate La$_4$Ni$_3$O$_{10}$

Ruddlesden-Popper (RP) nickelates have recently emerged as a new family of high-temperature superconductors. In bilayer RP nickelates, magnetic excitations with large exchange couplings have been observed, supporting a spin-mediated pairing mechanism. Whether comparable spin correlations persist in trilayer nickelates, however, remains unknown. Here, we present a Ni $L$-edge resonant inelastic X-ray scattering (RIXS) study of La$_4$Ni$_3$O$_{10}$ single crystals. While the orbital excitations remain similar to those of La$_3$Ni$_2$O$_{7}$, the collective spin excitations in La$_4$Ni$_3$O$_{10}$ exhibit a comparable bandwidth of about $60$ meV but substantially suppressed spectral weight, implying a weaker electronic correlation in the trilayer compounds. Our results underscore the three-dimensional and multi-orbital electronic character in La$_4$Ni$_3$O$_{10}$, highlighting important differences from the bilayer nickelates. These findings provide crucial insights into the evolution of magnetism across the RP nickelate family and its connection to superconductivity.

cond-mat.supr-con↗

arXiv2026

Possibility of ferro-octupolar order in Ba$_2$CaOsO$_6$ assessed by X-ray magnetic dichroism measurements

Localized $5d^2$ electrons in a cubic crystal field possess multipoles such as electric quadrupoles and magnetic octupoles. We studied the cubic double perovskite Ba$_2$CaOsO$_6$ containing the Os$^{6+}$ ($5d^2$) ions, which exhibits a phase transition to a `hidden order' below $T^* \sim$ 50 K, by X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at the Os $L_{2,3}$ edge. The cubic ligand-field splitting between the $t_{2g}$ and $e_g$ levels of Os $5d$ was deduced by XAS to be $\sim$4 eV. Ligand-field (LF) multiplet calculation under fictitious strong magnetic fields indicated that the exchange interaction between nearest-neighbor octupoles should be as strong as $\sim$1.5 meV if a ferro-octupolar order is stabilized in the `hidden-ordered' state, consistent with the exchange interaction of $\sim$1 meV previously predicted theoretically using model and density functional theory calculations. The temperature dependence of the XMCD spectra was consistent with a $\sim$18 meV residual cubic splitting of the lowest $J_{\rm eff} =$ 2 multiplet state into the non-Kramers $E_g$ doublet ground state and the $T_{2g}$ triplet excited state.

cond-mat.str-el↗

arXiv2025

Distinct orbital contributions to electronic and magnetic structures in La$_{4}$Ni$_{3}$O$_{10}$

High-T$_c$ superconductivity has recently been discovered in Ruddlesden-Popper phase nickelates under pressure, where the low-energy electronic structure is dominated by Ni $d_{x^2 - y^2}$ and $d_{z^2}$ orbitals. However, the respective roles of these orbitals in superconductivity remain unclear. Here, by combining X-ray absorption, electron energy loss spectroscopy, and density functional theory calculations on La$_{4}$Ni$_{3}$O$_{10}$ single crystals, we identify ligand holes in the $p_{x,y}$ orbitals of planar oxygen and the $p_z$ orbitals of apical oxygen, which hybridize with the Ni $d_{x^2-y^2}$ and $d_{z^2}$ orbitals, respectively. These ligand holes enable orbital-selective O K-edge resonant inelastic X-ray scattering (RIXS) study, which reveals that $d_{x^2-y^2}$ states dominate the low-energy charge excitations and are more itinerant. We also observe a $\sim$0.1 eV bimagnon through RIXS and Raman spectroscopy, which leads to an interlayer superexchange interaction J$_z$ of $\sim$50 meV. Our results reveal distinct contributions of Ni $d_{x^2-y^2}$ and $d_{z^2}$ orbitals to the electronic and magnetic structure and provide direct experimental insights to understand the RP-phase nickelate superconductors.

cond-mat.supr-con↗

arXiv2025

Electronic Energy Scales of Cr$X_3$ ($X$ = Cl, Br, and I) using High-resolution X-ray Scattering

Chromium tri-halides Cr$X_3$ ($X$ = Cl, Br, and I) have recently become a focal point of research due to their intriguing low-temperature,layer-dependent magnetism that can be manipulated by an electric field. This makes them essential candidates for spintronics applications. These magnetic orders are often related to the electronic structure parameters, such as spin-orbit coupling (SOC), Hund's coupling ($J_H$), $p-d$ covalency, and inter-orbital Coulomb interactions. Accurately determining such parameters is paramount for understanding Cr$X_3$ physics. We have used ultra high-resolution resonant inelastic x-ray scattering (RIXS) spectroscopy to study Cr$X_3$ across phase transition temperatures. Ligand field multiplet calculations were used to determine the electronic structure parameters by incorporating the crystal field interactions in a distorted octahedral with $C_3$ symmetry. These methods provide the most detailed description of Cr$X_3$ magneto-optical and electronic energetic (terms) to date. For the first time, the crystal field distortion parameters $Dσ$ and $Dτ$ were calculated, and the energies of $d$ orbitals have been reported. Our RIXS spectroscopic measurements reveal a clear energy separation between spin-allowed quartet states and spin-forbidden doublet states in Cr$X_3$. The role of SOC in Cr $2p$ orbitals for the spin-flip excitations has been demonstrated. The determined 10$Dq$ values are in good agreement with the spectrochemical series, and Racah B follows the Nephelauxetic effect. Such precise measurements offer insights into the energy design of spintronic devices that utilize quantum state tuning within 2D magnetic materials.

cond-mat.mtrl-sci↗

arXiv2025

Enhanced Superconductivity and Mixed-dimensional Behaviour in Infinite-layer Samarium Nickelate Thin Films

Rare-earth infinite-layer nickelates represent an emerging class of unconventional superconductors, with materials synthesis largely limited to early lanthanide compounds. Here, we report the synthesis and characterization of phase-pure superconducting samarium-based infinite-layer nickelate thin films, including the first demonstration of Sm$_{1-x}$Sr$_x$NiO$_2$, along with co-doped variants incorporating europium and calcium. These films, grown on LSAT (001) substrates, exhibit coherent lattice structures up to $\sim$ 9 nm thickness with minimal stacking faults. The co-doped compounds achieve a record-small $c$-axis parameter of 3.26 Å and display remarkable superconducting transition temperatures up to 32.5 K. These results establish a clear correlation between decreasing $c$-axis parameter and increasing critical temperature across different rare-earth systems. In addition, angle-dependent magnetoresistance investigations reveal the existence of a hybrid mixture of 2D and 3D superconductivity in this novel system with enhanced coupling between the rare-earth 5d and Ni 3d orbitals, confirmed by resonant inelastic X-ray scattering experiments. As the concentration of Eu increases, the system exhibits a clear tendency towards 3D superconductivity. Furthermore, we observe distinctive negative magnetoresistance in the europium-containing samples. These findings advocate clear materials design principles for higher transition temperatures and exotic physics in infinite-layer nickelate superconductors through structural engineering of the rare-earth site.

cond-mat.supr-con↗

arXiv2025

Persistent paramagnons in high-temperature infinite-layer nickelate superconductors

The recent discovery of high-temperature superconductivity in hole-doped SmNiO$_2$, exhibiting the record-high transition temperature $T_c$ among infinite-layer (IL) nickelates, has opened a new avenue for exploring design principles of superconductivity. Experimentally determining the electronic structure and magnetic interactions in this new system is crucial to elucidating the mechanism behind the enhanced superconductivity. Here, we report a Ni $L$-edge resonant inelastic x-ray scattering (RIXS) study of superconducting Sm-based IL nickelate thin films Sm$_{1-x-y-z}$Eu$_x$Ca$_y$Sr$_z$NiO$_2$ (SECS). Dispersive paramagnonic excitations are observed in both optimally and overdoped SECS samples, supporting a spin-fluctuation-mediated pairing scenario. However, despite the two-fold enhancement of $T_c$ in the Sm-based nickelates compared to their Pr-based counterparts, the effective exchange coupling strength is reduced by approximately $20\%$. This behavior contrasts with hole-doped cuprates, where magnetic interactions correlate positively with $T_c$, highlighting essential differences in their superconducting mechanisms.

cond-mat.supr-con↗

arXiv2024

Low temperature dynamic polaron liquid in a manganite exhibiting colossal magnetoresistance

Polarons - fermionic charge carriers bearing a strong companion lattice deformation - exhibit a natural tendency for self-localization due to the recursive interaction between electrons and the lattice. While polarons are ubiquitous in insulators, how they evolve in transitions to metallic and superconducting states in quantum materials remains an open question. Here, we use resonant inelastic x-ray scattering (RIXS) to track the electron-lattice coupling in the colossal magneto-resistive bi-layer manganite La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ across its metal-to-insulator transition. The response in the insulating high-temperature state features harmonic emissions of a dispersionless oxygen phonon at small energy transfer. Upon cooling into the metallic state, we observe a drastic redistribution of spectral weight from the region of these harmonic emissions to a broad high energy continuum. In concert with theoretical calculations, we show that this evolution implies a shift in electron-lattice coupling from static to dynamic lattice distortions that leads to a distinct polaronic ground state in the low temperature metallic phase - a dynamic polaron liquid.

cond-mat.str-el↗

arXiv2024

Giant X-ray circular dichroism in a time-reversal invariant altermagnet

X-ray circular dichroism, arising from the contrast in X-ray absorption between opposite photon helicities, serves as a spectroscopic tool to measure the magnetization of ferromagnetic materials and identify the handedness of chiral crystals. Antiferromagnets with crystallographic chirality typically lack X-ray magnetic circular dichroism because of time-reversal symmetry, yet exhibit weak X-ray natural circular dichroism. Here, we report the observation of giant natural circular dichroism in the Ni $L_3$-edge X-ray absorption of Ni$_3$TeO$_6$, a polar and chiral antiferromagnet with effective time-reversal symmetry. To unravel this intriguing phenomenon, we propose a phenomenological model that classifies the movement of photons in a chiral crystal within the same symmetry class as that of a magnetic field. The coupling of X-ray polarization with the induced magnetization yields giant X-ray natural circular dichroism, revealing the altermagnetism of Ni$_3$TeO$_6$. Our findings provide evidence for the interplay between magnetism and crystal chirality in natural optical activity. Additionally, we establish the first example of a new class of magnetic materials exhibiting circular dichroism with time-reversal symmetry.

cond-mat.str-el↗

arXiv2023

Charge redistribution, charge order and plasmon in La$_{2-x}$Sr$_{x}$CuO$_{4}$/La$_{2}$CuO$_{4}$ superlattices

Interfacial superconductors have the potential to revolutionize electronics, quantum computing, and fundamental physics due to their enhanced superconducting properties and ability to create new types of superconductors. The emergence of superconductivity at the interface of La$_{2-x}$Sr$_{x}$CuO$_{4}$/La$_{2}$CuO$_{4}$ (LSCO/LCO), with a T$_c$ enhancement of $\sim$ 10 K compared to the La$_{2-x}$Sr$_{x}$CuO$_{4}$ bulk single crystals, provides an exciting opportunity to study quantum phenomena in reduced dimensions. To investigate the carrier distribution and excitations in interfacial superconductors, we combine O K-edge resonant inelastic X-ray scattering and atomic-resolved scanning transmission electron microscopy measurements to study La$_{2-x}$Sr$_{x}$CuO$_{4}$/La$_{2}$CuO$_{4}$ superlattices (x=0.15, 0.45) and bulk La$_{1.55}$Sr$_{0.45}$CuO$_{4}$ films. We find direct evidence of charge redistribution, charge order and plasmon in LSCO/LCO superlattices. Notably, the observed behaviors of charge order and plasmon deviate from the anticipated properties of individual constituents or the average doping level of the superlattice. Instead, they conform harmoniously to the effective doping, a critical parameter governed by the T$_c$ of interfacial superconductors.

cond-mat.supr-con↗

arXiv2022

Prevailing charge order in overdoped cuprates beyond the superconducting dome

The extremely overdoped cuprates are generally considered to be Fermi liquid metals without exotic orders, whereas the underdoped cuprates harbor intertwined states. Contrary to this conventional wisdom, using Cu $L_3$ edge and O $K$ edge resonant x-ray scattering, we reveal a charge order (CO) in overdoped La$_{2-x}$Sr$_x$CuO$_4$ (0.35 $\leq$ x $\leq$ 0.6) beyond the superconducting dome. This CO has a periodicity of $\sim$ 6 lattice units with correlation lengths of $\sim 3 - 20$ lattice units. It shows similar in-plane momentum and polarization dependence and dispersive excitations as the CO of underdoped cuprates, but its maximum intensity differs along the c-direction and persists up to 300 K. This CO cannot be explained by either the Fermi surface instability or the doped Hubbard model and its origin remains to be understood. Our results suggest that CO is prevailing in the overdoped metallic regime and superconductivity emerges out of the CO phase upon decreasing hole carriers.

cond-mat.supr-con↗

arXiv2020

Post-growth annealing effects on charge and spin excitations in Nd$_{2-x}$Ce$_x$CuO$_4$

We report a Cu K- and L$_3$-edge resonant inelastic x-ray scattering study of charge and spin excitations of bulk Nd$_{2-x}$Ce$_x$CuO$_4$, with focus on post-growth annealing effects. For the parent compound Nd$_2$CuO$_4$ ($x = 0$), a clear charge-transfer gap is observed in the as-grown state, whereas the charge excitation spectra indicate that electrons are doped in the annealed state. This is consistent with the observation that annealed thin-film and polycrystalline samples of RE$_2$CuO$_4$ (RE = rare earth) can become metallic and superconducting at sufficiently high electron concentrations without Ce doping. For $x = 0.16$, a Ce concentration for which it is known that oxygen reduction destroys long-range antiferromagnetic order and induces superconductivity, we find that the high-energy spin excitations of non-superconducting as-grown and superconducting annealed crystals are nearly identical. This finding is in stark contrast to the significant changes in the low-energy spin excitations previously observed via neutron scattering.

cond-mat.supr-con↗

arXiv2020

Revealing the site selective local electronic structure of Co$_{3}$O$_{4}$ using $2p3d$ resonant inelastic X-ray scattering

We investigate mixed-valence oxide Co$_3$O$_4$ using Co $2p3d$ resonant inelastic X-ray scattering (RIXS). By setting resonant edges at Co$^{2+}$ and Co$^{3+}$ ions, the $dd$ excitations on the two Co sites are probed selectively, providing detailed information on the local electronic structure of Co$_3$O$_4$. The $2p3d$ RIXS result reveals the $^4$T$_{2}$ excited state of tetrahedral Co$^{2+}$ site at 0.5 eV beyond the discriminative power of optical absorption spectroscopies. Additionally, the $^3$T$_{2g}$ excited stated at 1.3 eV is uniquely identified for the octahedral Co$^{3+}$ site. Guided by cluster multiplet simulations, the ground-state character of the Co$^{2+}$ and Co$^{3+}$ site is determined to be high-spin $^4$A$_{2}$(T$_d$) and low-spin $^1$A$_{1g}$(O$_h$), respectively. This indicates that only the Co$^{2+}$ site is magnetically active site at low-temperatures in Co$_3$O$_4$. The ligand-to-metal charge transfer analysis suggests a formation of a strong covalent bonding between Co and O ions at the Co$^{3+}$ site, while Co$^{2+}$ is rather ionic.

cond-mat.mtrl-sci↗

arXiv2020

Temperature evolution of magnetic phases near the thickness-dependent metal-insulator transition in La$_{1-x}$Sr$_x$MnO$_3$ thin films observed by XMCD

Perovskite-type manganites, which are well-known for their intriguing physical properties such as colossal magnetoresistance (CMR) and half metalicity, have been considered as candidate materials for spintronics. However, their ferromagnetic (FM) properties are often suppressed in thin films when the thickness is reduced down to several monolayers (MLs). In order to investigate how the magnetic phases evolve near the paramagnetic (PM)-to-FM phase transition boundary, we have performed temperature-dependent x-ray magnetic circular dichroism (XMCD) experiments on a La$_{1-x}$Sr$_{x}$MnO$_3$ (LSMO, $x=0.4$) thin film, whose thickness (8 ML) is close to the boundary between the FM-metallic and the PM-insulating phases. By utilizing the element-selectiveness of XMCD, we have quantitatively estimated the fractions of the PM and superparamagnetic (SPM) phases as well as the FM one as a function of temperature. The results can be reasonably described based on a microscopic phase-separation model.

cond-mat.mtrl-sci↗

arXiv2018

Excitonic dispersion of the intermediate-spin state in LaCoO$_3$ revealed by resonant inelastic X-ray scattering

More than 50 years ago the electron-hole attraction was proposed to drive narrow gap semiconductors or semimetals to a new phase, the excitonic insulator. The experimental proof of its existence in bulk materials remains elusive. In strongly correlated insulators, the proximity of the excitonic insulator phase is reflected by the presence of dispersive electron-hole excitations with a small gap above a singlet ground state. Recently, such an excitation spectrum was proposed to be realized in perovskite oxide LaCoO$_3$. In this Letter we use Co $L_3$-edge resonant inelastic X-ray scattering to put this proposal to experimental test.

cond-mat.str-el↗

arXiv2017

Strong localization of oxidized Co3+ state in cobalt-hexacyanoferrate

Secondary batteries are important energy storage devices for a mobile equipment, an electric car, and a large-scale energy storage. Nevertheless, variation of the local electronic state of the battery materials in the charge (or oxidization) process are still unclear. Here, we investigated the local electronic state of cobalt-hexacyanoferrate (Na$_x$Co[Fe(CN)$_6$]$_{0.9}$), by means of resonant inelastic X-ray scattering (RIXS) with high energy resolution (~100 meV). The L-edge RIXS is one of the most powerful spectroscopic technique with element- and valence-selectivity. We found that the local electronic state around Co$^{2+}$ in the partially-charged Na$_{1.1}$Co$^{2+}$$_{0.5}$Co$^{3+}$$_{0.5}$[Fe$^{2+}$(CN)$_6$]$_{0.9}$ film (x = 1.1) is the same as that of the discharged Na$_{1.6}$Co$^{2+}$[Fe$^{2+}$(CN)$_6$]$_{0.9}$ film (x = 1.6) within the energy resolution, indicating that the local electronic state around Co$^{2+}$ is invariant against the partial oxidization. In addition, the local electronic state around the oxidized Co$^{3+}$ is essentially the same as that of the fully-charged film Co$^{3+}$[Fe$^{2+}$(CN)$_6$]$_{0.3}$[Fe$^{3+}$(CN)$_6$]$_{0.6}$ (x = 0.0) film. Such a strong localization of the oxidized Co$^{3+}$ state is advantageous for the reversibility of the redox process, since the localization reduces extra reaction within the materials and resultant deterioration.

cond-mat.mtrl-sci↗