arXiv subjects

Kenji Ishii

Publications and source records attributed to Kenji Ishii.

17 recordsLinked to original sources

arXiv2026

Evolution of crystal field and intraionic interactions in the ilmenite $A$IrO$_3$ ($A$ = Mg, Zn, Cd) and hyperhoneycomb $\beta$-ZnIrO$_3$

Spin-orbit Mott insulators with the $t_{2g}^5$ electron configuration are promising platforms for the Kitaev spin liquid, yet fine-tuning of their crystal structures is essential to suppress non-Kitaev interactions. Here, we investigate the local electronic structures of the ilmenite iridates $A\mathrm{IrO}_3$ ($A = \mathrm{Mg}, \mathrm{Zn}, \mathrm{Cd}$) and the hyperhoneycomb $\beta\text{-}\mathrm{ZnIrO}_3$ using Ir $L_3$-edge resonant inelastic x-ray scattering (RIXS). Multiplet analysis of the RIXS spectra reveals a systematic evolution of the crystal field and intraionic interaction parameters upon chemical substitution at the $A$-site. We observe an enhancement of the trigonal distortion with increasing $A$-site ionic radius. This provides a microscopic explanation for the deviation from the ideal $J=1/2$ state and the antiferromagnetic interactions identified in $\mathrm{CdIrO}_3$. Furthermore, the local multiplet parameters of ilmenite $\mathrm{ZnIrO}_3$ and hyperhoneycomb $\beta\text{-}\mathrm{ZnIrO}_3$ are found to be nearly identical, demonstrating that their different magnetic ground states are primarily governed by their distinct lattice structures rather than the single-ion properties. These findings establish a solid foundation for understanding how local crystal-field distortions control the magnetic Hamiltonian in Kitaev candidate materials.

cond-mat.str-el↗

arXiv2025

Observation of spin-conserving two-spinon continuum in the $S$=1/2 antiferromagnetic chain system Sr$_2$CuO$_3$ using Cu $K$-edge resonant inelastic x-ray scattering

We report a Cu $K$-edge resonant inelastic x-ray scattering (RIXS) study of spin excitations in the $S$=1/2 antiferromagnetic chain system Sr$_2$CuO$_3$. The spectral weight observed below the charge-transfer gap appears in two-spinon continuum, indicating the fractionalization of a spin-conserving ($\Delta S = 0$) magnetic excitation into two-spinon states. The intensity of these excitations reaches a maximum near the midpoint between the center and the boundary of the Brillouin zone, and decreases toward the zone boundary; this behavior contrasts with that of the spin-flip ($\Delta S = 1$) excitations typically observed via inelastic neutron scattering or Cu $L_3$-edge RIXS. The momentum dependence of the intensity is described by the spin-exchange dynamical structure factor. A phenomenological analysis of the symmetry between the polarization and the $d$ orbital explains the resonance condition for the two-spinon excitations.

cond-mat.str-el↗

arXiv2024

Persistent quantum vibronic dynamics in a $5d^1$ double perovskite oxide

Quantum entanglement between the spin, orbital, and lattice degrees of freedom in condensed matter systems can emerge due to an interplay between spin-orbit and vibronic interactions. Heavy transition metal ions decorated on a face-centered cubic lattice, for example, in $5d^1$ double perovskites, are particularly suited to support these quantum entangled states, but direct evidence has not yet been presented. In this work, we report additional peaks in the low-energy spectra of a $5d^1$ double perovskite, Ba$_2$CaReO$_6$, which cannot be explained by adopting a purely classical description of lattice vibrations. Instead, our theoretical analysis demonstrates that these spectroscopic signatures are characteristic of orbital-lattice entangled states in Ba$_2$CaReO$_6$. Crucially, both theory and experiment demonstrate that these quantum-entangled states persist to low temperatures, despite the onset of multipolar order.

cond-mat.str-el↗

arXiv2024

Dynamic Jahn-Teller effect in the strong spin-orbit coupling regime

Exotic quantum phases, arising from a complex interplay of charge, spin, lattice and orbital degrees of freedom, are of immense interest to a wide research community. A well-known example of such an entangled behavior is the Jahn-Teller effect, where the lifting of orbital degeneracy proceeds through lattice distortions, often accompanied by ordering of spins and metal-insulator transitions. Static distortions, including cooperative behavior, have been associated with colossal magneto-resistance, multiferroicity, high-$T_\mathrm{C}$ superconductivity and other correlated phenomena. Realizations of the dynamic Jahn-Teller effect, on the other hand, are scarce since the preservation of vibronic symmetries requires subtle tuning of the local environment. Here we demonstrate that a highly-symmetrical 5d$^1$ double perovskite Ba$_2$MgReO$_6$, comprising of a 3D array of isolated ReO$_6$ octahedra, fulfils these requirements, resulting in a unique case of a dynamic Jahn-Teller system with strong spin-orbit coupling. Thermodynamic and resonant inelastic x-ray scattering experiments undoubtedly show that the Jahn-Teller instability leads to a ground-state doublet, invoking a paradigm shift for this family of compounds. The restoration of vibronic degrees of freedom arises from a quantum-mechanical zero-point motion, as revealed by detailed quantum chemistry calculations. The dynamic state of ReO$_6$ octahedra persists down to the lowest temperatures, where a multipolar order sets in, allowing for investigations of the interplay between a dynamic JT effect and strongly correlated electron behavior.

cond-mat.str-el↗

arXiv2020

Localized character of charge excitations for La$_{2-x}$Sr$_{x}$NiO$_{4+δ}$ revealed by oxygen $K$-edge resonant inelastic X-ray scattering

We performed a resonant inelastic X-ray scattering (RIXS) study of La$_{2-x}$Sr$_{x}$NiO$_{4+δ}$ (LSNO) at the oxygen $K$ edge to investigate the nature of the doped holes with regard to charge excitations. Charge excitations of the hole-doped nickelates are found to be almost independent of momentum transfer, indicating that the doped holes are strongly localized in character. Additionally, conspicuous changes in energy position are in temperature dependence. These characters are observed in stark contrast to those of the high-$T_{c}$ cuprate La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO), where delocalized doped holes form charge excitations with sizable momentum dependence in the CuO$_2$ plane. This distinct nature of charge excitations of doped holes is consistent with the metallicity of the materials and could be caused by strong electron-phonon coupling and weak quantum spin fluctuation in the nickelates.

cond-mat.str-el↗

arXiv2020

Revisiting the phase diagram of T*-type La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$CuO$_4$ using Oxygen $K$-edge X-ray absorption spectroscopy

Oxygen $K$-edge X-ray absorption spectroscopy measurements were conducted on T*-type La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$CuO$_4$ (LESCO) to estimate the hole density ($n_{\rm h}$) and investigate the oxidation annealing effect on $n_{\rm h}$. A drastic increase in $n_{\rm h}$ due to annealing was found. The increase in $n_{\rm h}$ cannot be explained solely by the oxygen gain due to annealing, suggesting that delocalized holes are introduced into the CuO$_2$ plane. A phase diagram of LESCO was redrawn against $n_{\rm h}$.

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

Distinct variation of electronic states due to annealing in La$_{1.8}$Eu$_{0.2}$CuO$_4$ and Nd$_{2}$CuO$_4$

We performed Cu {\it K}-edge X-ray absorption fine structure measurements on T'-type La$_{1.8}$Eu$_{0.2}$CuO$_4$ (LECO) and Nd$_2$CuO$_4$ (NCO) to investigate the variation in the electronic state associated with the emergence of superconductivity due to annealing. The X-ray absorption near-edge structure spectra of as-sintered (AS) LECO are quite similar to those of AS NCO, indicating that the ground state of AS T'-type LECO is a Mott insulator. We found a significant variation of the electronic state at the Cu sites in LECO due to annealing. The electron density after annealing ($n_{\rm AN}$) was evaluated for both superconducting LECO and non-superconducting NCO and found to be 0.40 and 0.05 electrons per Cu, respectively. In LECO but not in NCO, extended X-ray absorption fine structure analysis revealed a softening in the strength of the Cu-O bond in the CuO$_2$ plane due to annealing, which is consistent with the screening effect on phonons in the metallic state. Since the amounts of oxygen loss due to annealing ($δ$) for LECO and NCO are comparable with each other, these results suggest distinct electron-doping processes in the two compounds. That electron-doping in NCO approximately follows the relation $n_{\rm AN}=2δ$ can be understood if electrons are doped through oxygen deficiency, but the anneal-induced metallic nature and large $n_{\rm AN}$ of LECO suggest a variation of the electronic band structure causes self-doping of carriers. The origin of the difference in doping processes due to annealing is discussed in connection with the size of the charge transfer gap.

cond-mat.supr-con↗

arXiv2019

Reciprocity between local moments and collective magnetic excitations in the phase diagram of BaFe$_2$(As$_{1-x}$P$_x$)$_2$

Unconventional superconductivity arises at the border between the strong coupling regime with local magnetic moments and the weak coupling regime with itinerant electrons, and stems from the physics of criticality that dissects the two. Unveiling the nature of the quasiparticles close to quantum criticality is fundamental to understand the phase diagram of quantum materials. Here, using resonant inelastic x-ray scattering (RIXS) and Fe-K$_β$ emission spectroscopy (XES), we visualize the coexistence and evolution of local magnetic moments and collective spin excitations across the superconducting dome in isovalently-doped BaFe$_2$(As$_{1-x}$P$_x$)$_2$ (0.00$\leq$x$\leq0.$52). Collective magnetic excitations resolved by RIXS are gradually hardened, whereas XES reveals a strong suppression of the local magnetic moment upon doping. This relationship is captured by an intermediate coupling theory, explicitly accounting for the partially localized and itinerant nature of the electrons in Fe pnictides. Finally, our work identifies a local-itinerant spin fluctuations channel through which the local moments transfer spin excitations to the particle-hole (paramagnons) continuum across the superconducting dome.

cond-mat.supr-con↗

arXiv2019

Ordering of hidden multipoles in spin-orbital entangled 5$d^{1}$ Ta chlorides

Spin-orbit coupling of as large as a half eV for electrons in 5$d$ orbitals often gives rise to the formation of spin-orbital entangled objects, characterized by the effective total angular momentum $J_{eff}$. Of particular interest are the $J_{eff}$ = 3/2 states realized in 5$d^{1}$ transition metal ions surrounded by an anion octahedron. The pure $J_{eff}$ = 3/2 quartet does not have any magnetic dipolar moment (<$M$> = 0) but hosts hidden pseudo-dipolar moments accompanied by charge quadrupoles and magnetic octupoles. Cs$_2$TaCl$_6$ and Rb$_2$TaCl$_6$ are correlated insulators with 5$d^{1}$ Ta$^{4+}$ ions in a regular Cl octahedron. Here we demonstrate that these Ta chlorides have a substantially suppressed effective magnetic dipolar moment of ~ 0.2 $μ_B$. Two phase transitions are observed at low temperatures that are not pronounced in the magnetization but accompanied with large electronic entropy of $R$ln4. We ascribe the two transitions to the ordering of hidden multipoles.

cond-mat.str-el↗

arXiv2019

Charge Excitations in Nd$_{2-x}$Ce$_x$CuO$_4$ Observed with Resonant Inelastic X-ray Scattering: Comparison of Cu K-edge with Cu L$_3$-edge

We report a Cu K-edge resonant inelastic x-ray scattering (RIXS) study of momentum-dependent charge excitations in Nd$_{2-x}$Ce$_x$CuO$_4$ ($x$ = 0.075 and 0.18). The peak position and width of the excitations coincide excellently with those observed in Cu L$_3$-edge RIXS. It demonstrates that the same charge excitations are observed at the two edges.

cond-mat.str-el↗

arXiv2018

Ce-doping and reduction annealing effects on electronic states in Pr$_{2-x}$Ce$_{x}$CuO$_4$ studied by Cu ${K}$-edge X-ray absorption spectroscopy

We investigated Ce-substitution and reduction annealing effects on the electronic states at copper sites by Cu ${K}$-edge x-ray absorption near-edge structure measurements in Pr$_{2-x}$Ce$_x$CuO$_{4+α-δ}$ (PCCO) with varying $x$ and $δ$ (the amount of oxygen loss during annealing) values. Absorption near-edge spectra were modified by Ce-substitution and reduction annealing in a similar manner with increasing $x$ and $δ$. Considering electron doping by Ce-substitution, this similarity indicates an increase of electron number at the copper sites due to annealing $n_{\rm AN}$. Thus, the total number of electrons is determined by the amount of Ce and oxygen ions. Furthermore, quantitative analyses of the spectra clarified that the number of Cu$^+$ sites, corresponding to the induced electron number by Ce-substitution $n_{\rm Ce}$ increases linearly with $x$ in the as-sintered PCCO ($δ=0$), whereas $n_{\rm AN}$ is not exactly equal to twice of $δ$, which is expected from charge neutrality. For each $x$-fixed sample, $n_{\rm AN}$ tends to exceed 2$δ$ with increasing $δ$, suggesting the emergence of two types of carrier due to annealing.

cond-mat.supr-con↗

arXiv2016

Magnetic moment evolution and spin freezing in doped BaFe$_{2}$As$_{2}$

Fe-K$_β$ X-ray emission spectroscopy measurements reveal an asymmetric doping dependence of the magnetic moments $μ_\text{bare}$ in electron- and hole-doped BaFe$_{2}$As$_{2}$. At low temperature, $μ_\text{bare}$ is nearly constant in hole-doped samples, whereas it decreases upon electron doping. Increasing temperature substantially enhances $μ_\text{bare}$ in the hole-doped region, which is naturally explained by the theoretically predicted crossover into a spin-frozen state. Our measurements demonstrate the importance of Hund's coupling and electronic correlations, especially for hole-doped BaFe$_{2}$As$_{2}$, and the inadequacy of a fully localized or fully itinerant description of the 122 family of Fe pnictides.

cond-mat.supr-con↗

arXiv2014

Bipartite magnetic parent phases in the iron-oxypnictide superconductor

High-temperature (high-$T_{\rm c}$) superconductivity appears as a consequence of the carrier-doping of an undoped parent compound exhibiting antiferromagnetic order; thereby, ground-state properties of the parent compound are closely relevant to the superconducting state. On the basis of the concept, a spin-fluctuation has been addressed as an origin of pairing of the superconducting electrons in cuprates. Whereas, there is growing interest in the pairing mechanism such as an unconventional spin-fluctuation or an advanced orbital-fluctuation due to the characteristic multi-orbital system in iron-pnictides. Here, we report the discovery of an antiferromagnetic order as well as a unique structural transition in electron-overdoped LaFeAsO$_{1-x}$H$_x$ ($x$ ~ 0.5), whereby another parent phase was uncovered, albeit heavily doped. The unprecedented two-dome superconducting phases observed in this material can be interpreted as a consequence of the carrier-doping starting from the original at $x\sim0$ and advanced at $x\sim0.5$ parent phases toward the intermediate region. The bipartite parent phases with distinct physical properties in the second magnetic phase provide us with an interesting example to illustrate the intimate interplay among the magnetic interaction, structural change and orbital degree of freedom in iron-pnictides.

cond-mat.str-el↗

arXiv2014

Engineering spin-orbital magnetic insulator by tailoring superlattices

Novel interplay of spin-orbit coupling and electron correlations in complex Ir oxides recently emerged as a new paradigm for correlated electron physics. Because of a large spin-orbit coupling of ~0.5 eV, which is comparable to the transfer energy t and the crystal field splitting $Δ$ and Coulomb U, a variety of ground states including magnetic insulator, band insulator, semimetal and metal, shows up in a narrow materials phase space. Utilizing such subtle competition of the ground states, we successfully tailor a spin-orbital magnetic insulator out of a semimetal SrIrO$_3$ by controlling dimensionality using superlattice of [(SrIrO$_3$)$_m$, SrTiO$_3$] and show that a magnetic ordering triggers the transition to magnetic insulator. Those results can be described well by a first-principles calculation. This study is an important step towards the design and the realization of topological phases in complex Ir oxides with very strong spin-orbit coupling.

cond-mat.str-el↗

arXiv2012

Disentangling a quantum antiferromagnet with resonant inelastic X-ray scattering

Low-dimensional copper oxide lattices naturally manifest electronic states with strong short range quantum entanglement, which are known to lead to remarkable emergent material properties. However the nanometer scale many-body wavefunction is challenging to measure or manipulate in a simple way. In this study, X-ray induced $dd$ electronic transitions are used to suppress spin entanglement across a single lattice site in the spin-1/2 antiferromagnetic chain compound SrCuO$_2$, revealing a class of cuprate magnetic excitations that result from breaking the spin chain. These measurements are the first to employ two closely spaced X-ray resonances of copper (M$_2$ and M$_3$) as a form of natural 2-slit interferometer to distinguish between different types of electronic transition and resolve how they influence the dynamics of nearby spin-entangled electrons.

cond-mat.str-el↗

arXiv2002

Ferromagnetism and giant magnetoresistance in the rare earth fullerides Eu6-xSrxC60

We have studied crystal structure, magnetism and electric transport properties of a europium fulleride Eu6C60 and its Sr-substituted compounds, Eu6-xSrxC60. They have a bcc structure, which is an isostructure of other M6C60 (M represents an alkali atom or an alkaline earth atom). Magnetic measurements revealed that magnetic moment is ascribed to the divalent europium atom with S = 7/2 spin, and a ferromagnetic transition was observed at TC = 10 - 14 K. In Eu6C60, we also confirm the ferromagnetic transition by heat capacity measurement. The striking feature in Eu6-xSrxC60} is very large negative magnetoresistance at low temperature; the resistivity ratio ρ(H = 9 T)/ρ(H = 0 T) reaches almost 10^{-3} at 1 K in Eu6C60. Such large magnetoresistance is the manifestation of a strong pi-f interaction between conduction carriers on C60 and 4f electrons of Eu.

cond-mat.mtrl-sci↗