arXiv subjects

M. Horio

Publications and source records attributed to M. Horio.

At least 19 recordsLinked to original sources

arXiv2026

Differentiation of electron doping and oxygen reduction in electron-doped cuprates

Electron-doped cuprates require not only electron doping by chemical substitution but also post-growth reduction annealing for realizing superconductivity. However, electron concentration can also be varied by reduction annealing, making it challenging to disentangle the respective influences of electron concentration and oxygen non-stoichiometry. Here, by combining alkali-metal dosing and angle-resolved photoemission spectroscopy, we monitored changes in the electronic structure of an electron-doped cuprate while supplying additional electrons to its surface without modifying oxygen content. Whereas a Fermi surface reconstruction due to long-range antiferromagnetic order was suppressed by alkali-metal deposition, the pseudogap -- which is associated with short-range spin/charge correlations and can be suppressed by efficient reduction annealing -- was found to persist. The results highlight significant contribution of impurity oxygen atoms to pseudogap formation in electron-doped cuprates.

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arXiv2026

Ambipolar doping-induced surface in-gap state on Mott-insulating Ca$_2$RuO$_4$

We report an x-ray photoemission spectroscopy study of Ca$_2$RuO$_4$ surface-dosed with Cs alkali atoms and C$_{60}$ molecules. Due to its small ionization energy (large electron affinity), deposited Cs atoms (C$_{60}$ molecules) are expected to provide a solid surface with electrons (holes). Upon dosing the dopants to Mott-insulating Ca$_2$RuO$_4$, we found a new Ru $3d$ photoemission peak emerging on the lower binding-energy side, suggesting the creation of a core-hole screening channel associated with coherent Ru $4d$ states around the Fermi level. For both the Cs and C$_{60}$ dosing, this change occurred without an appreciable chemical potential jump. The coherent state, therefore, develops within the Mott gap through hybridization with the impurity level of the dopants. The present work highlights the flexibility of Mott-insulator surfaces as a playground for metal-insulator transitions.

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arXiv2025

Translational symmetry breaking in the electronic nematic phase of BaFe2As2

The microscopic origin of the nematicity, namely, four-fold rotational symmetry breaking in iron-based superconductors has been controversial since its discovery. In particular, its relationship with the stripe-type spin-density-wave order and the orthorhombic lattice distortion in the antiferromagnetic orthorhombic (AFO) phase, which exists at temperatures below the electronic nematic phase, has been highly debated. Here, we report on the temperature evolution of angle-resolved photoemission spectra of the parent compound BaFe2As2, ranging from the AFO to nematic to paramagnetic phases. The Dirac cone feature, which is formed in the AFO phase, is found to persist in the nematic phase, suggesting that an antiferroic order of the same periodicity as the AFO phase persists in the nematic phase. Considering the relatively shallow d_xy orbital in BaFe2As2, we propose that an antiferro-orbital order involving the d_xy and other orbitals takes place in the nematic phase.

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arXiv2025

Magnetic Excitations of a Half-Filled Tl-based Cuprate

Strong electron correlations drive Mott insulator transitions. Yet, there exists no framework to classify Mott insulators by their degree of correlation. Cuprate superconductors, with their tunable doping and rich phase diagrams, offer a unique platform to investigate the evolution of these interactions. However, spectroscopic access to a clean half-filled Mott-insulating state is lacking in compounds with the highest superconducting onset temperature. To fill this gap, we introduce a pristine, half-filled thallium-based cuprate system, Tl$_2$Ba$_5$Cu$_4$O$_{x}$. Using high-resolution resonant inelastic x-ray scattering, we probe long-lived magnon excitations and uncover a pronounced kink in the magnon dispersion, marked by a simultaneous change in group velocity and lifetime broadening. Modeling the dispersion within a Hubbard-Heisenberg approach, we extract the interaction strength and compare it with other cuprate systems. Our results establish a cuprate universal relation between electron-electron interaction and magnon zone-boundary dispersion. Superconductivity seems to be optimal at intermediate correlation strength, suggesting an optimal balance between localization and itinerancy.

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arXiv2025

Enhanced superconducting gap in the outer CuO$_2$ plane of the trilayer cuprate (Hg,Re)Ba$_2$Ca$_2$Cu$_3$O$_{8+\delta}$

We report the first observation of a momentum-resolved superconducting gap in the Hg-based trilayer cuprate, which holds the highest record of superconducting transition temperature ($T_\mathrm{c}$) at ambient pressure. By angle-resolved photoemission spectroscopy utilizing a micro-focused beam, clear quasiparticle dispersions originating from the inner and outer CuO$_2$ planes (IP and OP, respectively) were separately identified. The magnitude of the superconducting gap for the IP was comparable to that of the Bi-based trilayer cuprate with a lower $T_\mathrm{c}$. In contrast, the superconducting gap for the OP was significantly larger than that of the Bi-based one. While strong pairing in the IP has been highlighted as the key element of trilayer cuprates, the present results suggest that the enhanced pairing energy in the OP is essential for the highest $T_\mathrm{c}$ at ambient pressure realized in the Hg-based trilayer cuprate.

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arXiv2024

Inhomogeneous reduction-annealing effects on the electron-doped cuprate superconductor revealed by micro-focused angle-resolved photoemission spectroscopy

The development of the protect-annealing method has extended the superconductivity of the electron-doped cuprate Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_{4}$ (PLCCO) into lower Ce concentrations, while the superconducting volume fraction decreases with underdoping. Employing angle-resolved photoemission spectroscopy with a micro-focused beam, we investigated the electronic structure of protect-annealed PLCCO ($x=0.08$) with small superconducting volume fraction. Significant spatial variation of Fermi surface area and shape was observed, suggesting inhomogeneity in electron concentrations and the pseudogap that competes with superconductivity. By performing measurements at dozens of different sample positions, negative and non-monotonic correlation was found between the electron concentration and pseudogap magnitude. The established correlation illustrates a systematic annealing dependence of the electronic structure where a pseudogap abruptly opens with insufficient oxygen reduction.

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arXiv2023

Magnon interactions in a moderately correlated Mott insulator

Quantum fluctuations in low-dimensional systems and near quantum phase transitions have significant influences on material properties. Yet, it is difficult to experimentally gauge the strength and importance of quantum fluctuations. Here we provide a resonant inelastic x-ray scattering study of magnon excitations in Mott insulating cuprates. From the thin film of SrCuO$_2$, single- and bi-magnon dispersions are derived. Using an effective Heisenberg Hamiltonian generated from the Hubbard model, we show that the single-magnon dispersion is only described satisfactorily when including significant quantum corrections stemming from magnon-magnon interactions. Comparative results on La$_2$CuO$_4$ indicate that quantum fluctuations are much stronger in SrCuO$_2$ suggesting closer proximity to a magnetic quantum critical point. Monte Carlo calculations reveal that other magnetic orders may compete with the antiferromagnetic N\'eel order as the ground state. Our results indicate that SrCuO$_2$ - due to strong quantum fluctuations - is a unique starting point for the exploration of novel magnetic ground states.

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arXiv2023

Orbital-selective metal skin induced by alkali-metal-dosing Mott-insulating Ca$_2$RuO$_4$

Doped Mott insulators are the starting point for interesting physics such as high temperature superconductivity and quantum spin liquids. For multi-band Mott insulators, orbital selective ground states have been envisioned. However, orbital selective metals and Mott insulators have been difficult to realize experimentally. Here we demonstrate by photoemission spectroscopy how Ca$_2$RuO$_4$, upon alkali-metal surface doping, develops a single-band metal skin. Our dynamical mean field theory calculations reveal that homogeneous electron doping of Ca$_2$RuO$_4$ results in a multi-band metal. All together, our results provide compelling evidence for an orbital-selective Mott insulator breakdown, which is unachievable via simple electron doping. Supported by a cluster model and cluster perturbation theory calculations, we demonstrate a novel type of skin metal-insulator transition induced by surface dopants that orbital-selectively hybridize with the bulk Mott state and in turn produce coherent in-gap states.

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arXiv2023

Influence of oxygen-coordination number on the electronic structure of single-layer La-based cuprates

We present an angle-resolved photoemission spectroscopy study of the single-layer T*-type structured cuprate SmLa$_{1-x}$Sr$_x$CuO$_4$ with unique five-fold pyramidal oxygen coordination. Upon varying oxygen content, T*-SmLa$_{1-x}$Sr$_x$CuO$_4$ evolved from a Mott-insulating to a metallic state where the Luttinger sum rule breaks down under the assumption of a large hole-like Fermi surface. This is in contrast with the known doping evolution of the structural isomer La$_{2-x}$Sr$_x$CuO$_4$ with six-fold octahedral coordination. In addition, quantitatively characterized Fermi surface suggests that the empirical $T_\mathrm{c}$ rule for octahedral oxygen-coordination systems does not apply to T*-SmLa$_{1-x}$Sr$_x$CuO$_4$. The present results highlight unique properties of the T*-type cuprates possibly rooted in its oxygen coordination, and necessitate thorough investigation with careful evaluation of disorder effects.

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arXiv2023

Oxygen on-site Coulomb energy in Pr$_{1.3-x}$La$_{0.7}$Ce$_x$CuO$_{4}$ and Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ and its relation with Heisenberg exchange

We study the electronic structure of electron-doped Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_{4}$ (PLCCO ; $T_{c}$ = 27 K, x = 0.1) and hole-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212 ; $T_{c}$ = 90 K) cuprate superconductors using x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (Res-PES). From Res-PES across the O K-edge and Cu L-edge, we identify the O 2p and Cu 3d partial density of states (PDOS) and their correlation satellites which originate in two-hole Auger final states. Using the Cini-Sawatzky method, analysis of the experimental O 2p PDOS shows an oxygen on-site Coulomb energy for PLCCO to be $U_{p}$ = 3.3$\pm$0.5 eV and for Bi2212, $U_{p}$ = 5.6$\pm$0.5 eV, while the copper on-site Coulomb correlation energy, $U_{d}$ = 6.5$\pm$0.5 eV for Bi2212. The expression for the Heisenberg exchange interaction $J$ in terms of the electronic parameters $U_{d}$, $U_{p}$, charge-transfer energy $\Delta$ and Cu-O hopping $t_{pd}$ obtained from a simple Cu$_2$O cluster model is used to carry out an optimization analysis consistent with $J$ known from scattering experiments. The analysis also provides the effective one band on-site Coulomb correlation energy $\tilde{U}$ and the effective hopping $\tilde{t}$. PLCCO and Bi2212 are shown to exhibit very similar values of $\tilde{U}$/$\tilde{t}$ $\sim$9-10, confirming the strongly correlated nature of the singlet ground state in the effective one-band model for both the materials.

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arXiv2022

Pseudogap Suppression by Competition with Superconductivity in La-Based Cuprates

We have carried out a comprehensive high-resolution angle-resolved photoemission spectroscopy (ARPES) study of the pseudogap interplay with superconductivity in La-based cuprates. The three systems La$_{2-x}$Sr$_x$CuO$_4$, La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$, and La$_{1.8-x}$Eu$_{0.2}$Sr$_x$CuO$_4$ display slightly different pseudogap critical points in the temperature versus doping phase diagram. We have studied the pseudogap evolution into the superconducting state for doping concentrations just below the critical point. In this setting, near optimal doping for superconductivity and in the presence of the weakest possible pseudogap, we uncover how the pseudogap is partially suppressed inside the superconducting state. This conclusion is based on the direct observation of a reduced pseudogap energy scale and re-emergence of spectral weight suppressed by the pseudogap. Altogether these observations suggest that the pseudogap phenomenon in La-based cuprates is in competition with superconductivity for anti-nodal spectral weight.

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arXiv2022

Competition between spin ordering and superconductivity near the pseudogap boundary in La2-xSrxCuO4: insights from NMR

When superconductivity is suppressed by high magnetic fields in La2-xSrxCuO4, striped antiferromagnetic (AFM) order becomes the magnetic ground state of the entire pseudogap regime, up to its end at the doping p* [M. Frachet, I. Vinograd et al., Nat. Phys. 16, 1064 (2020)]. Glass-like freezing of this state is detected in 139La NMR measurements of the spin-lattice relaxation rate 1/T1. Here, we present a quantitative analysis of 1/T1 data in the hole-doping range p=x=0.12-0.171, based on the Bloembergen-Purcell-Pound (BPP) theory, modified to include statistical distribution of parameters arising from strong spatial inhomogeneity. We observe spin fluctuations to slow down at temperatures T near the onset of static charge order and, overall, the effect of the field B may be seen as equivalent to strengthening stripe order by approaching p=0.12 doping. In details however, our analysis reveals significant departure from usual field-induced magnetic transitions. The continuous growth of the amplitude of the fluctuating moment with increasing B suggests a nearly-critical state in the B->0 limit, with very weak quasi-static moments possibly confined in small areas like vortex cores. Further, the nucleation of spin order in the vortex cores is shown to account quantitatively for both the value and the p dependence of a field scale characterizing bulk spin freezing. The correlation time of the fluctuating moment appears to depend exponentially on B/T (over the investigated range). This explains the timescale dependence of various experimental manifestations, including why, for transport measurements, the AFM moments may be considered static over a considerable range of B and T. These results make the high-field magnetic ground state up to p* an integral part of the discussion on putative quantum criticality.

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arXiv2022

Fate of charge order in overdoped La-based cuprates

In high-temperature cuprate superconductors, stripe order refers broadly to a coupled spin and charge modulation with a commensuration of eight and four lattice units, respectively. How this stripe order evolves across optimal doping remains a controversial question. Here we present a systematic resonant inelastic x-ray scattering (RIXS) study of weak charge correlations in La2-xSrxCuO4 (LSCO) and La1.8-xEu0.2SrxCuO4 (LESCO). Ultra high energy resolution experiments demonstrate the importance of the separation of inelastic and elastic scattering processes. Upon increasing doping x, the long-range temperature dependent stripe order is found to be replaced by short-range temperature independent correlations at a critical point xc = 0.15 distinct from the pseudogap critical doping. We argue that the doping and temperature independent short-range correlations originate from unresolved electron-phonon coupling that broadly peaks at the stripe ordering vector. In LSCO, long-range static stripe order vanishes in a quantum critical point around optimal doping.

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arXiv2022

Uniaxial Pressure Induced Stripe Order Rotation in La$_{1.88}$Sr$_{0.12}$CuO$_4$

Static stripe order is detrimental to superconductivity. Yet, it has been proposed that transverse stripe fluctuations may enhance the inter-stripe Josephson coupling and thus promote superconductivity. Direct experimental studies of stripe dynamics, however, remain difficult. From a strong-coupling perspective, transverse stripe fluctuations are realized in the form of dynamic "kinks" -- sideways shifting stripe sections. Here, we show how modest uniaxial pressure tuning reorganizes directional kink alignment. Our starting point is La$_{1.88}$Sr$_{0.12}$CuO$_4$, where transverse kink ordering results in a rotation of stripe order away from the crystal axis. Application of mild uniaxial pressure changes the ordering pattern and pins the stripe order to the crystal axis. This reordering occurs at a much weaker pressure than that to detwin the stripe domains and suggests a rather weak transverse stripe stiffness. Weak spatial stiffness and transverse quantum fluctuations are likely key prerequisites for stripes to coexist with superconductivity.

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arXiv2021

Evidence for a square-square vortex lattice transition in a high-$T_\textrm{c}$ cuprate superconductor

Using sound velocity and attenuation measurements in high magnetic fields, we identify a new transition in the vortex lattice state of La$_{2-x}$Sr$_{x}$CuO$_4$ (LSCO). The transition, observed in magnetic fields exceeding 35 T and temperatures far below zero field $T_c$, is detected in the compression modulus of the vortex lattice, at a doping level $x=p=0.17$. Our theoretical analysis based on Eilenberger theory of vortex lattice shows that the transition corresponds to the long-sought 45 degrees rotation of the square vortex lattice, predicted to occur in $d$-wave superconductors near a van Hove singularity.

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arXiv2021

Superconducting gap and pseudogap in the surface states of the iron-based superconductor PrFeAsO$_{1-y}$ studied by angle-resolved photoemission spectroscopy

In order to study the possible superconductivity at the polar surfaces of 1111-type iron-based superconductors, which is doped with a large amount of holes in spite of the electron doping in bulk materials, we have performed angle-resolved photoemission spectroscopy (ARPES) studies on superconducting PrFeAsO$_{1-y}$~crystals. We have indeed observed the opening of a superconducting gap on surface-derived hole pockets as well as on a bulk-derived hole pocket. The superconducting gap is found to open on the surface-derived hole pockets below the bulk $T_c$, which suggests that the surface superconductivity is possibly induced by proximity effect from the bulk. We have also observed the opening of a large pseudogap on the surface-derived hole pockets, which is similar to the pseudogap in 122-type bulk superconductors doped with a smaller amount of holes. This suggests that the opening of a large pseudogap is a characteristic property of hole-doped iron-based superconductors.

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arXiv2021

Electronic reconstruction forming a $C_2$-symmetric Dirac semimetal in Ca$_3$Ru$_2$O$_7$

Electronic band structures in solids stem from a periodic potential reflecting the structure of either the crystal lattice or an electronic order. In the stoichiometric ruthenate Ca$_3$Ru$_2$O$_7$, numerous Fermi surface sensitive probes indicate a low-temperature electronic reconstruction. Yet, the causality and the reconstructed band structure remain unsolved. Here, we show by angle-resolved photoemission spectroscopy, how in Ca$_3$Ru$_2$O$_7$ a $C_2$-symmetric massive Dirac semimetal is realized through a Brillouin-zone preserving electronic reconstruction. This Dirac semimetal emerges in a two-stage transition upon cooling. The Dirac point and band velocities are consistent with constraints set by quantum oscillation, thermodynamic, and transport experiments, suggesting that the complete Fermi surface is resolved. The reconstructed structure -- incompatible with translational-symmetry-breaking density waves -- serves as an important test for band structure calculations of correlated electron systems.

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arXiv2021

Nematicity in the pseudogap state of cuprate superconductors revealed by angle-resolved photoemission spectroscopy

The nature of the pseudogap and its relationship with superconductivity are one of the central issues of cuprate superconductors. Recently, a possible scenario has been proposed that the pseudogap state is a distinct phase characterized by spontaneous rotational symmetry breaking called "nematicity" based on transport and magnetic susceptibility measurements, where the symmetry breaking was observed below the pseudogap temperature $T^*$. Here, we report a temperature-dependent ARPES study of nematicity in slightly overdoped Bi${}_{1.7}$Pb${}_{0.5}$Sr${}_{1.9}$CaCu${}_2$O${}_{8+δ}$ triggered by a uniaxial strain applied along one of the Cu-O bond directions. While the nematicity was enhanced in the pseudogap state as in the previous studies, it was suppressed in the superconducting state. These results indicate that the pseudogap state is characterized by spontaneous rotational symmetry breaking and that the nematicity may compete with superconductivity. These new experimental insights may provide clues for the nature of the pseudogap and its relation to the superconductivity.

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