arXiv subjects

Masahiko Isobe

Publications and source records attributed to Masahiko Isobe.

At least 19 recordsLinked to original sources

arXiv2026

Electronic tuning of the soft-phonon transport anomaly in Ta$_2$Ni(S$_x$Se$_{1-x}$)$_5$

Ta$_2$NiSe$_5$ continues to be investigated for its phase transition at $T_\textrm{c}$ = 326 K, where it develops both an electronic gap and a distortion of its Ta/Ni chains. One intriguing feature at $T_\textrm{c}$ seen in thermal transport is the giant anisotropic scattering of phonons moving perpendicular to the chains, which is apparently associated with the softening of a transverse acoustic phonon, but whose microscopic origin and significance demand clarification. By tuning the normal-state band overlap/gap with S substitution, we uncover a close connection between this soft-phonon transport anomaly and underlying electronic instabilities: When Ta$_2$Ni(S$_x$Se$_{1-x}$)$_5$ approaches a band insulator at high $x$, and signatures of the electronic transition are suppressed, the soft-phonon transport anomaly concomitantly vanishes. Our results establish the following picture for the Ta$_2$Ni(S$_x$Se$_{1-x}$)$_5$ family: Near the S end, a sole lattice instability gives rise to a weak structural transition with $T_\textrm{c}$ approaching 130 K. Near the Se end, additional electronic instabilities boost $T_\textrm{c}$ up to 326 K and amplify experimental signatures of the transition. The strong interaction between electrons, holes, and the lattice is manifested as a soft-phonon transport anomaly accompanied by electronic fluctuations, which include excitonic and hybridization-gap fluctuations.

cond-mat.str-el↗

arXiv2026

Imaging stripe dynamics in trilayer nickelate La$_4$Ni$_3$O$_{10}$

Since the discovery of high-temperature superconductivity in nickelate superconductors, it is an open question how closely the superconducting state resembles that of cuprate superconductors. One salient feature of the phase diagram of the high-temperature cuprate superconductors is stripe order. Despite their prevalence, real-space imaging has been limited to the charge sector. Here we use spin-polarised scanning tunnelling microscopy to visualize the local magnetic and charge distribution emerging due to a stripe order in the trilayer nickelate La$_4$Ni$_3$O$_{10}$. The stripe order exhibits a four unit cell periodicity, closely resembling that seen in cuprates, and opens a near-complete $\sim66\mathrm{meV}$ gap at the Fermi level. Crucially, discrete phase slips can be triggered by tunneling electrons above a $\sim 20\mathrm{meV}$ threshold, allowing imaging of stripe dynamics at the atomic scale. These results highlight the importance of correlation physics driving stripe-like orders in lanthanum nickelates with striking similarities to the cuprates.

cond-mat.str-el↗

arXiv2026

Topochemical Fluorination of La$_2$NiO$_{4+\delta}$ Single Crystals

Topochemical fluorination offers a low--temperature route for modifying the anion chemistry and electronic ground states of layered transition-metal oxides, providing access to metastable phases and functionalities that are not able to be achieved through conventional solid--state synthesis. Despite extensive work on polycrystalline samples and thin films, topochemical fluorination of bulk single crystals has not been studied, limiting insights into intrinsic structure property relationships. Here, we investigate the topochemical fluorination of optical float zone grown (OFZ) La$_2$NiO$_{4+\delta}$ single crystals using polymer-based PTFE, PVDF and inorganic CuF$_{2}$ fluorination agents and compare it to our topochemical pathways of reduction of LaNiO$_{3-x}$. By systematically investigating direct and indirect contact reaction pathways, we can understand fluorination mechanisms, quantify the degree of fluorine incorporation, and evaluate the resulting structural and magnetic modifications in a detail that was not possible in powder and thin films. Powder and single--crystal X-ray diffraction reveal that fluorination proceeds without destroying the Ruddlesden--Popper framework, while inducing lattice parameter changes consistent with anion intercalation in the bulk and ion exchange on the surface. This even induces a clear superstructure, which was not reported before and extends the understanding of anion insertion reactions beyond what is known on stage ordering in nickelates. Energy-dispersive X--ray spectroscopy confirms strong fluorine incorporation on the surface and reduced homogeneity in the bulk. Magnetic susceptibility measurements demonstrate a change in antiferromagnetic ordering upon fluorination.

cond-mat.str-el↗

arXiv2026

Anisotropic magnetoelastic coupling in the honeycomb magnet Na$_3$Co$_2$SbO$_6$

We present magnetization and dilatometry measurements on the honeycomb cobaltate Na$_3$Co$_2$SbO$_6$ and map out its detailed field-temperature phase diagram down to sub-Kelvin temperatures. Our data for in-plane magnetic fields show a strongly anisotropic $c^{*}$-axis lattice response, which is dominated by the variation of Co--O--Co bond angles according to \textit{ab initio} calculations. At $T = 0.4$~K, the magnetization $M(B)$ exhibits step-like features that are also highly anisotropic. In the case of $B \parallel b$, a small hysteresis observed around the second field-induced magnetic transition ($B_{c2}$) indicates its first-order character, whereas divergence of the magnetic Gr\"uneisen parameter at $B_{c2}$ is suppressed upon cooling and signals the absence of quantum critical behavior upon entering the field-polarized state. None of our thermodynamic measurements provide evidence for a field-induced quantum spin liquid state near or above $B_{c2}$.

cond-mat.str-el↗

arXiv2025

Magnetism of the alternating monolayer-trilayer phase of La$_3$Ni$_2$O$_7$

Understanding the magnetic ground state of Ruddlesden-Popper nickelates is crucial, as these materials exhibit superconductivity under high pressure and host competing electronic orders that may play a key role in the pairing mechanism. In this work, we investigate the magnetic properties of the alternating monolayer-trilayer phase of La$_3$Ni$_2$O$_7$ (1313-La$_3$Ni$_2$O$_7$) using muon-spin rotation/relaxation ($\mu$SR) under both ambient and hydrostatic pressure conditions. The monolayer-trilayer phase develops incommensurate magnetic order below approximately 150 K, with a mean ordering temperature of $T_{SDW} \simeq 123$ K and a transition width of $\Delta T_{SDW} \simeq 15$ K. The abrupt onset of the internal magnetic field indicates a first-order-like transition. Hydrostatic pressure ($p$) suppresses the magnetic ordering temperature at a rate of $dT_{SDW}/d p \simeq -3.9$ K/GPa, demonstrating a progressive destabilization of the ordered state. By comparison with the bilayer 2222-La$_3$Ni$_2$O$_7$ and the trilayer 3333-La$_4$Ni$_3$O$_{10}$ systems, and within a unified phenomenological framework, systematic trends are identified linking the pressure dependence of $T_{SDW}$, the (in)commensurability of the magnetic order, and the character of the magnetic transition. These trends consistently indicate a gradual reduction of electronic correlation strength from the bilayer to the monolayer-trilayer and trilayer nickelates. This hierarchy suggests that the higher superconducting transition temperature observed in the 2222 phase may be closely connected to its more strongly correlated electronic nature. These results position the alternating monolayer-trilayer 1313-La$_3$Ni$_2$O$_7$ as an intermediate member linking the magnetic behavior of the bilayer 2222-La$_3$Ni$_2$O$_7$ and the trilayer 3333-La$_4$Ni$_3$O$_{10}$ Ruddlesden-Popper compounds.

cond-mat.supr-con↗

arXiv2025

Comparative Raman study of Ruddlesden-Popper nickelates and the monolayer-trilayer polymorph

Ruddlesden-Popper (RP) nickelates have attracted intense interest following the discovery of superconductivity in several members of the series, including bilayer (BL) La$_3$Ni$_2$O$_7$, trilayer (TL) La$_4$Ni$_3$O$_{10}$, and structural polymorphs composed of monolayer-bilayer or monolayer-trilayer (ML-TL) units. However, an inherent propensity of the RP series to form intergrown phases during single-crystal synthesis, together with spatial variations in oxygen stoichiometry, has complicated the determination of their intrinsic material properties. As a consequence, conflicting reports have emerged on both their electronic phase transitions and lattice dynamics. In this work, we perform a comparative study of the phononic and electronic Raman responses of high-quality ML-TL single crystals and contrast them with those of other RP nickelates, using samples with optimized oxygen content. We establish several Raman spectral features that enable unambiguous phase identification across the series. Moreover, we uncover characteristics in the phononic and electronic Raman response of ML-TL that are not reflected in the pure ML and TL compounds. We attribute these differences to a distinctive electronic structure arising from self-doping and confinement effects induced by the ML unit within the ML-TL lattice architecture.

cond-mat.str-el↗

arXiv2025

Substrate tuning of the structural and electronic transition in thin flakes of the excitonic insulator candidate Ta$_2$NiSe$_5$

Ta$_2$NiSe$_5$ continues to draw interest for its 326 K phase transition, whose dual electronic and structural nature reflects a complex interplay of electron-hole (excitonic) and electron-lattice interactions. Most studies that have attempted to decipher the relative importance of these interactions, particularly through charge transfer, have been limited to bulk samples. We utilized a thin-flake approach to modify the excitonic interactions in Ta$_2$NiSe$_5$ via an underlying film of Au. Using polarized Raman spectroscopy, we found that four layers of Ta$_2$NiSe$_5$ supported on conducting Au show a transition temperature that is both reduced by over 100 K and broadened due to an interfacial charge gradient effect, manifesting the presence of excitonic interactions. In contrast, four layers of Ta$_2$NiSe$_5$ supported on insulating Al$_2$O$_3$ show nearly bulk-like properties. We also report the development of an all-dry exfoliation and transfer protocol that generalizes substrate engineering for strongly correlated van der Waals materials.

cond-mat.str-el↗

arXiv2025

Terahertz light driven coherent excitation of a zone-folded Raman-active phonon mode in the Spin-Ladder System $\alpha'$-NaV$_2$O$_5$

We investigate the out-of-equilibrium lattice dynamics in the spin-ladder system $\alpha'$-NaV$_2$O$_5$ using intense terahertz (THz) pump and near-infrared (NIR) probe spectroscopy. When quasi-single-cycle THz pulses interact with $\alpha'$-NaV$_2$O$_5$ in its low-temperature, dimerized charge-ordered phase, they induce coherent oscillations in the time domain at the zone-folded Raman-active phonon frequency of 1.85 THz. By combining pump-probe measurements with lattice dynamics modeling based on equation-of-motion approach, we propose that these oscillations arise from a nonlinear coupling between Raman-active and infrared (IR)-active phonon modes, with the latter being resonantly excited by the THz pulses. In contrast, excitation with NIR femtosecond laser pulses does not produce measurable vibrational dynamics, highlighting the unique potential of THz-driven, nonlinear light-matter interactions for the coherent and selective control of structural dynamics in quantum materials.

cond-mat.mtrl-sci↗

arXiv2025

Topological Metal-Insulator Transition within the Ferromagnetic state

A major challenge in condensed matter physics is integrating topological phenomena with correlated electron physics to leverage both types of states for next-generation quantum devices. Metal-insulator transitions (MITs) are central to bridging these two domains while simultaneously serving as 'on-off' switches for electronic states. Here, we demonstrate how the prototypical material of K2Cr8O16 undergoes a ferromagnetic MIT accompanied by a change in band topology. Through inelastic x-ray and neutron scattering experiments combined with first-principles theoretical calculations, we demonstrate that this transition is not driven by a Peierls mechanism, given the lack of phonon softening. Instead, we establish the transition as a topological MIT within the ferromagnetic phase (topological-FM-MIT) with potential axionic properties, where electron correlations play a key role in stabilizing the insulating state. This work pioneers the discovery of a topological-FM-MIT and represents a fundamentally new class of topological phase transitions, revealing a unique pathway through which magnetism, topology, and electronic correlations interact.

cond-mat.str-el↗

arXiv2025

Spinon mediation of witness spin dynamics in herbertsmithite

The kagome lattice of spin-1/2 copper atoms in herbertsmithite is conjectured to sustain a quantum spin liquid state with spinon quasiparticles. Ideally, the kagome crystal planes are each separated by a plane of spinless zinc atoms. However, in real crystals some spin-1/2 copper atoms substitute randomly onto these inter-kagome zinc sites. Here we reconceptualize such 'impurity' atoms as quantum witness spins whose dynamics is designed to probe the spin liquid state. We then introduce spin noise spectroscopy to measure the frequency and temperature dependence of witness spin dynamics, demonstrating that their phenomenology is consistent with extensive interactions between witness spins mediated by propagation of spinons through a quantum spin liquid. Ultimately, a sharp transition occurs at around 260 mK, below which the properties of both spin noise and magnetic susceptibility suggest that the witness spins form a spin glass phase. Among theoretical models considered, we demonstrate that our observations are only consistent with spinon-mediated interactions between witness spins by either a Z2 or U(1) quantum spin liquid, with the former model more closely matching the data. Our work demonstrates that quantum mechanical witness spins may now conceivably be used as a widely applicable probe of quantum spin liquid physics.

cond-mat.str-el↗

arXiv2025

Magnetic Frustration in CuYbSe$_2$: an Yb-Based Triangular Lattice Selenide

The Yb based triangular lattice delafossites $A$Yb$X_2$ ($A$ = alkali metal, $X$ = O, S, Se) have recently been studied as quantum spin liquid candidates. We report the synthesis of powders and single crystals of CuYbSe$_2$ with a perfect triangular lattice of Yb$^{3+}$ moments. Magnetic susceptibility and heat capacity measurements reveal no evidence of long-range magnetic ordering down to $1.8$~K in spite of a significant antiferromagnetic exchange between Yb$^{3+}$ moments, suggesting a frustrated magnetic system. Electrical resistivity measurements indicate insulating behavior, consistent with the localized nature of magnetic moments. Heat capacity reveals that CuYbSe$_2$ can be treated as an effective spin $S = 1/2$ triangular lattice antiferromagnet below $\sim 50$~K. Magnetic susceptibility measurements on single crystals reveals weak magnetic anisotropy. These properties position CuYbSe$_2$ as a promising candidate for a quantum spin liquid state and as a new platform for exploring exotic magnetic ground states in triangular lattice systems.

cond-mat.str-el↗

arXiv2025

Lattice dynamics of the infinite-layer nickelate LaNiO$_2$

Infinite-layer (IL) nickelates have rapidly emerged as a new class of superconductors. However, due to the technical challenges of their topotactic synthesis, they have so far been realized primarily as thin films or polycrystalline powder samples, limiting comprehensive investigations of fundamental physical properties such as the lattice dynamics. Here, we present a time-of-flight inelastic neutron scattering study on a sample composed of a large number of co-aligned bulk crystals of the IL nickelate LaNiO$_2$. We observe several dispersive phonon branches, which are in good agreement with lattice dynamical calculations based on density-functional perturbation theory. In addition, we compare the characteristics of selected LaNiO$_2$ phonon modes to those of isostructural cuprate superconductors. Our findings provide a reference point for future experimental and theoretical efforts aimed at understanding the interplay between lattice dynamics and electronic properties in IL nickelates.

cond-mat.str-el↗

arXiv2025

Exploring Low-Dimensional Magnetism in Cobalt Vanadates, ${A}$CoV$_{2}$O$_{7}$~(${A}$~=~Ca, Sr) : Crystal Growth and Magnetic Properties of Effective Spin-1/2 Zigzag Chains

We report the successful growth of high-quality single crystals of \ACVO, a quasi-one-dimensional zigzag chain compound containing Co$^{2+}$ ions, using the optical floating zone method. The crystal growth was stabilized under high-pressure argon-oxygen gas with slow growth rates, overcoming challenges associated with the incongruent melting behavior of this material. X-ray diffraction confirms the zigzag arrangement of Co$^{2+}$ ions, forming a quasi-one-dimensional chain structure. Magnetic susceptibility and heat capacity measurements reveal an antiferromagnetic phase transition at the Néel temperature ($T_{\text{N}} \sim 3.5$ K) and negative Curie-Weiss temperatures, indicative of dominant antiferromagnetic interactions. The distorted CoO$_6$ octahedral geometry and strong spin-orbit coupling suggest that Co$^{2+}$ ions likely exhibit an effective $J = 1/2 $ Kramers doublet state. The results presented here demonstrate the potential of \ACVO\ as a platform for investigating low-dimensional magnetism and quantum magnetic phenomena. These insights shed light on the role of the ${A}$-site ion in tuning the magnetic interactions, which will foster future research into the field-induced behavior in these cobalt vanadates.

cond-mat.str-el↗

arXiv2025

Cluster Spin Glass State in Ba$_3$Sb$_{1+x}$Co$_{2-x}$O$_{9-δ}$: Cation Disorder and Mixed-Valence Co Dimers

We investigate the structural, magnetic, and thermodynamic properties of \BSCO\ ($x$ = 0.04, $δ$ = 0.54), a hexagonal perovskite featuring face-sharing CoO$_6$ octahedra that forms Co dimers. DC and AC magnetization measurements reveal a frequency-dependent spin-freezing transition consistent with glassy dynamics. AC susceptibility fits best to the Vogel-Fulcher model, indicating collective freezing of interacting spin clusters. Isothermal magnetization follows the Langevin function, suggesting finite-sized magnetic clusters rather than isolated paramagnetic moments. Non-equilibrium dynamics, evidenced by thermoremanent magnetization and memory effects, further support a spin-glass-like state. Heat capacity shows no sharp anomalies, and neutron powder diffraction confirms the absence of magnetic Bragg peaks down to 1.5~K, ruling out long-range magnetic order. Rietveld refinement reveals significant Co/Sb intersite disorder ($\sim$~30\pct) and oxygen non-stoichiometry, introducing exchange randomness and frustration that drive the spin-glass-like behavior. Electrical resistivity exhibits Arrhenius-type temperature dependence with an activation energy of 0.173~eV, consistent with semiconducting behavior. Temperature-dependent X-ray diffraction shows no structural phase transitions, confirming that the spin-glass-like state is not lattice-driven. Our results establish \BSCO\ as a cluster spin-glass candidate, where Co dimers, disorder, and geometric frustration prevent long-range order, leading to slow spin dynamics. These findings highlight the role of cation disorder and oxygen vacancies in stabilizing unconventional magnetic states in cobalt-based hexagonal perovskites.

cond-mat.str-el↗

arXiv2025

Direct Evidence of Metal-Ligand Redox in Li-ion Battery Positive Electrodes

Describing Li-ion battery positive electrodes in terms of distinct transition metal or oxygen redox regimes can lead to confusion in understanding metal-ligand hybridisation, oxygen dimerisation, and degradation. There is a pressing need to study the electronic structure of these materials and determine the role each cation and anion plays in charge compensation. Here, we employ transition metal L-edge X-ray Resonance Photoemission Spectroscopy in conjunction with Single Impurity Anderson models, Self-consistent Real Space Multiple Scattering spectral simulations, and Dynamical Mean-Field theory calculations to directly evaluate the redox mechanisms in (de-)lithiated battery electrodes. This approach reconciles the redox description of two canonical cathodes -- LiMn$_{0.6}$Fe$_{0.4}$PO$_{4}$ and LiNiO$_{2}$ -- in terms of varying degrees of charge transfer using the established Zaanen-Sawatzky-Allen framework, common to condensed matter physics. In LiMn$_{0.6}$Fe$_{0.4}$PO$_{4}$, the absence of charge transfer means capacity arises due to the depopulation of metal $\textit{3d}$ states, i.e. conventional metal redox. Whereas, in LiNiO$_{2}$, charge transfer dominates and redox occurs through the formation and elimination of ligand hole states. This work clarifies the role of oxygen in Ni-rich system and provides a framework to explain how capacity can be extracted from oxygen-dominated states in highly covalent systems without needing to invoke dimerisation.

cond-mat.mtrl-sci↗

arXiv2025

Hund flat band in a frustrated spinel oxide

Electronic flat bands associated with quenched kinetic energy and heavy electron mass have attracted great interest for promoting strong electronic correlations and emergent phenomena such as high-temperature charge fractionalization and superconductivity. Intense experimental and theoretical research has been devoted to establishing the rich non-trivial metallic and heavy fermion phases intertwined with such localized electronic states. Here, we investigate the transition metal oxide spinel LiV2O4, an enigmatic heavy fermion compound lacking localized f orbital states. We use angle-resolved photoemission spectroscopy and dynamical mean field theory to reveal a new kind of correlation-induced flat band with suppressed inter-atomic electron hopping arising from intra-atomic Hund coupling. The appearance of heavy quasiparticles is ascribed to a proximate orbital-selective Mott state characterized by fluctuating local moments as evidenced by complementary magnetotransport measurements. The spectroscopic fingerprints of long-lived quasiparticles and their disappearance with increasing temperature further support the emergence of a high-temperature bad metal state observed in transport data. This work resolves a long-standing puzzle on the origin of heavy fermion behavior and unconventional transport in LiV2O4. Simultaneously, it opens a new path to achieving flat bands through electronic interactions in d-orbital systems with geometrical frustration, potentially enabling the realization of exotic phases of matter such as the fractionalized Fermi liquids.

cond-mat.str-el↗

arXiv2024

Unraveling nano-scale effects of topotactic reduction in LaNiO$_2$ crystals

Infinite-layer nickelates stand as a promising frontier in the exploration of unconventional superconductivity. Their synthesis through topotactic oxygen reduction from the parent perovskite phase remains a complex and elusive process. This study delves into the nano-scale effects of the topotactic lattice transformation within LaNiO$_2$ crystals. Leveraging high-resolution scanning transmission electron microscopy and spectroscopy, our investigations uncover a panorama of structural alterations, including grain boundaries and coherent twin boundaries, triggered by reduction-induced transformations. In addition, our analyses unveil the formation of an oxygen-rich disordered transition phase encircling impurities and pervading crystalline domains, and the internal strain is accommodated by grain boundary formation. By unraveling these nano-scale effects, our findings provide insights into the microscopic intricacies of the topotactic reduction process elucidating the transition from the perovskite to the infinite-layer phase within nickelate bulk crystals.

cond-mat.mtrl-sci↗

arXiv2024

Electronic structure of the alternating monolayer-trilayer phase of La3Ni2O7

Recent studies of La$_3$Ni$_2$O$_7$ have identified a bilayer (2222) structure and an unexpected alternating monolayer-trilayer (1313) structure, both of which feature signatures of superconductivity near 80 K under high pressures. Using angle-resolved photoemission spectroscopy, we measure the electronic structure of 1313 samples. In contrast to the previously studied 2222 structure, we find that the 1313 structure hosts a flat band with a markedly different binding energy, as well as an additional electron pocket and band splittings. By comparison to local-density approximation calculations, we find renormalizations of the Ni-$d_{z^2}$ and Ni-$d_{x^2-y^2}$ derived bands to be about 5 to 7 and about 4 respectively, suggesting strong correlation effects. These results reveal important differences in the electronic structure brought about by the distinct structural motifs with the same stoichiometry. Such differences may be relevant to the putative high temperature superconductivity.

cond-mat.supr-con↗