arXiv subjects

H. Takagi

Publications and source records attributed to H. Takagi.

At least 19 recordsLinked to original sources

arXiv2026

High temperature transitions in Ruddlesden-Popper nickelates La$_{n+1}$Ni$_{n}$O$_{3n+1}$

The discovery of superconductivity at $15\,\mathrm{K}$ in the infinite-layer nickelate $(\mathrm{Nd},\mathrm{Sr})\mathrm{NiO}_2$, followed by superconductivity at $80\,\mathrm{K}$ in the Ruddlesden--Popper phase $\mathrm{La}_3\mathrm{Ni}_2\mathrm{O}_7$, has ushered in a new era of nickelate research. Despite this progress, large discrepancies between reports exist. Here, we investigate the complete series of bulk-stable $\mathrm{La}_{n+1}\mathrm{Ni}_n\mathrm{O}_{3n+1}$ compounds using a comprehensive set of experimental techniques, including PXRD, single-crystal XRD, electron microscopy, heat capacity, differential scanning calorimetry, magnetic susceptibility, and transport measurements, over a broad temperature range from $2$ to $1000\,\mathrm{K}$. By studying high-quality single crystals, we identify a previously underappreciated high-temperature phase transition in Ruddlesden--Popper nickelates $\mathrm{La}_{n+1}\mathrm{Ni}_n\mathrm{O}_{3n+1}$ distinct from the one going to a tetragonal phase.

cond-mat.supr-con↗

arXiv2026

Non-magnetic ground state in A$_2$WCl$_6$ (A = Cs, Rb, K): A face-centered cubic system of spin-orbit-entangled $J$ = 2 states

Heavy transition metal compounds with strong spin-orbit coupling appeared as a platform for $d$-electron multipolar physics. We report the electronic, magnetic, and structural properties of antifluorite-type tungsten chloride A$_2$WCl$_6$ (A = Cs, Rb, and K), comprising a face-centered cubic lattice of W$^{4+}$ ions. The 5$d^2$ configuration of W$^{4+}$ ions in a cubic environment yields a spin-orbit-entangled $J$ = 2 state, which has been discussed to give rise to multipolar ordering such as charge quadrupolar or magnetic octupolar ordering. We found that K$_2$WCl$_6$ undergoes a cubic-to-tetragonal structural transition which lifts the degeneracy of the $J$ = 2 state, leading to a non-magnetic singlet ground state. By contrast, Rb$_2$WCl$_6$ and Cs$_2$WCl$_6$ show no signs of phase transition and remain non-magnetic down to the lowest temperature measured. At low temperatures, signatures of weak structural anomalies were revealed, which may point to the presence of local distortions of the WCl$_6$ octahedra. We argue that the subtle structural distortion arises from the local quadrupolar component of the $J$ = 2 state but the frustrated quadrupolar interaction, together with chemical disorder, inhibits the formation of long-range quadrupolar ordering.

cond-mat.str-el↗

arXiv2025

Observation of collective charge excitations in a cuprate superconductor

Emergent symmetry breakings in condensed matter systems are often intimately linked to collective excitations. For example, the intertwined spin-charge stripe order in cuprate superconductors is associated with spin and charge excitations. While the collective behavior of spin excitations is well established, the nature of charge excitations remains to be understood. Here we present a high-resolution resonant inelastic x-ray scattering (RIXS) study of charge excitations in the stripe-ordered cuprate La$_{1.675}$Eu$_{0.2}$Sr$_{0.125}$CuO$_4$. The RIXS spectra consist of both charge and phonon excitations around the charge ordering wave vector. By modeling the momentum-dependent phonon intensity, the charge-excitation spectral weight is extracted for a wide range of energy. As such, we reveal the highly dispersive nature of the charge excitations, with an energy scale comparable to the spin excitations. Since charge order and superconductivity in cuprates are possibly driven by the same electronic correlations, determining the interaction strength underlying charge order is essential to establishing a comprehensive microscopic model of high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2024

Magnetic ground state of the dimer-based hexagonal perovskite Ba$_{3}$ZnRu$_{2}$O$_{9}$

We investigate the magnetic ground state of single crystals of the ruthenium-dimer-based hexagonal perovskite Ba$_{3}$ZnRu$_{2}$O$_{9}$ using magnetic susceptibility and resonant inelastic x-ray scattering (RIXS) measurements. While a previous study on powder samples exhibited intriguing magnetic behavior, questions about whether the spin state within a Ru$_{2}$O$_{9}$ dimer is a conventional $S = 3/2$ dimer or an orbital-selective $S = 1$ dimer were raised. The RIXS spectra reveal magnetic excitations from Hund's intraionic multiplet and intradimer spin-triplet transitions. The observed transition energies of the Hund's intraionic multiplets align with the $S=3/2$ ground state, contrasting with the theoretically proposed orbital-selective $S=1$ dimer state. High-temperature magnetic susceptibility analysis confirms the realization of the spin $S=3/2$ dimer state, and the extracted intradimer coupling is consistent with the spin-triplet transition energy observed in the RIXS spectra. These results highlights the ability of "spectroscopic fingerprinting" by RIXS to determine the magnetic ground states of complex materials.

cond-mat.str-el↗

arXiv2024

Spin-stripe order tied to the pseudogap phase in La1.8-xEu0.2SrxCuO4

Although spin and charge stripes in high-Tc cuprates have been extensively studied, the exact range of carrier concentration over which they form a static order remains uncertain, complicating efforts to understand their significance. In La2-xSrxCuO4 (LSCO) and in zero external magnetic field, static spin stripes are confined to a doping range well below p*, the pseudogap boundary at zero temperature. However, when high fields suppress the competing effect of superconductivity, spin stripe order is found to extend up to p*. Here, we investigated La1.8-xEu0.2SrxCuO4 (Eu-LSCO) using 139La nuclear magnetic resonance and observe field-dependent spin fluctuations suggesting a similar competition between superconductivity and spin order as in LSCO. Nevertheless, we find that static spin stripes are present practically up to p* irrespective of field strength: the stronger stripe order in Eu-LSCO prevents superconductivity from enforcing a non-magnetic ground state, except very close to p*. Thus, spin-stripe order is consistently bounded by p* in both LSCO and Eu-LSCO, despite their differing balances between stripe order and superconductivity. This indicates that the canonical stripe order, where spins and charges are intertwined in a static pattern, is fundamentally tied to the pseudogap phase, though the exact nature of this connection has yet to be elucidated. Any stripe order beyond the pseudogap endpoint must then be of a different nature: either spin and charge orders remain intertwined, but both fluctuating, or only spin order fluctuates while charge order remains static. The presence of spin-stripe order up to p*, the pervasive, slow, and field-dependent spin-stripe fluctuations, as well as the electronic inhomogeneity documented in this work, must all be carefully considered in discussions of Fermi surface transformations, quantum criticality, and strange metal behavior.

cond-mat.supr-con↗

arXiv2024

Electronic structure and resonant inelastic x-ray scattering in Ta2NiSe5

We study the electronic structure of Ta2NiSe5 in its low-temperature semiconducting phase, using resonant inelastic x-ray scattering (RIXS) at the Ta L3 edge. We also investigate the electronic properties of Ta2NiSe5 within the density-functional theory using the generalized gradient approximation in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. While ARPES, dc transport, and optical measurements indicate that Ta2NiSe5 is a small band-gap semiconductor, DFT gives a metallic nonmagnetic solution in Ta2NiSe5 . To obtain the semiconducting ground state in Ta2 NiSe5 we use a self-interaction correction (SIC) procedure by introducing an orbital-dependent potential Vl into the Hamiltonian. We investigate theoretically the x-ray absorption spectroscopy (XAS) and RIXS spectra at the Ni and Ta L3 edges and analyze the spectra in terms of interband transitions. We investigate the RIXS spectra as a function of momentum transfer vector Q and incident photon energy. Because Ta2 NiSe5 possesses only fully occupied (Ni 3d and Se 4p) and completely empty (Ta 5d) shells with the formal valencies Ta5+ (5d0), Ni0 (3d10 ), and Se2- (4p6 ), both the Ni and Ta L3 RIXS spectra belong to a charge transfer type with ligand-to-metal excitations.

cond-mat.str-el↗

arXiv2023

Planar thermal Hall effect from phonons in cuprates

A surprising "planar" thermal Hall effect, whereby the field is parallel to the current, has recently been observed in a few magnetic insulators, and this has been attributed to exotic excitations such as Majorana fermions or chiral magnons. Here we investigate the possibility of a planar thermal Hall effect in three different cuprate materials, in which the conventional thermal Hall conductivity $\kappa_{\rm {xy}}$ (with an out-of-plane field perpendicular to the current) is dominated by either electrons or phonons. Our measurements show that the planar $\kappa_{\rm {xy}}$ from electrons in cuprates is zero, as expected from the absence of a Lorentz force in the planar configuration. By contrast, we observe a sizable planar $\kappa_{\rm {xy}}$ in those samples where the thermal Hall response is due to phonons, even though it should in principle be forbidden by the high crystal symmetry. Our findings call for a careful re-examination of the mechanisms responsible for the phonon thermal Hall effect in insulators.

cond-mat.str-el↗

arXiv2023

Spontaneous topological Hall effect induced by non-coplanar antiferromagnetic order in intercalated van der Waals materials

In ferromagnets, electric current generally induces transverse Hall voltage in proportion to magnetization (anomalous Hall effect), and it is frequently used for electrical readout of the up and down spin states. While these properties are usually not expected in antiferromagnets, recent theoretical studies predicted that non-coplanar antiferromagnetic order with finite scalar spin chirality (i.e. solid angle spanned by neighboring spins) can often induce large spontaneous Hall effect even without net magnetization or external magnetic field. This phenomenon, i.e. spontaneous topological Hall effect, can potentially be used for the efficient electrical readout of the antiferromagnetic states, but its experimental verification has long been elusive due to the lack of appropriate materials hosting such exotic magnetism. Here, we report the discovery of all-in-all-out type non-coplanar antiferromagnetic order in triangular lattice compounds CoTa3S6 and CoNb3S6, by performing the detailed magnetic structure analysis based on polarized neutron scattering experiments as well as systematic first-principles calculations. These compounds are reported to host unconventionally large spontaneous Hall effect despite their vanishingly small net magnetization, and our analysis revealed that it can be well explained in terms of topological Hall effect, which originates from the fictitious magnetic field associated with scalar spin chirality in non-coplanar antiferromagnetic orders. The present results indicate that the scalar spin chirality mechanism can offer a promising route to realize giant spontaneous Hall response even in compensated antiferromagnets, and highlight intercalated van der Waals magnets as an unique quasi-two-dimensional material platform to enable various nontrivial manner of electrical reading and possible writing of non-coplanar antiferromagnetic domains.

cond-mat.mtrl-sci↗

arXiv2023

Epitaxial growth and scanning tunneling microscopy of LiV$_2$O$_4$ thin films on SrTiO$_3$(111)

LiV$_2$O$_4$ is a mixed-valent spinel oxide and one of a few transition-metal compounds to host a heavy fermion phase at low temperatures. While numerous experimental studies have attempted to elucidate how its 3$d$ electrons undergo giant mass renormalization, spectroscopic probes that may provide crucial hints, such as scanning tunneling microscopy (STM), remain to be applied. A prerequisite is atomically flat and pristine surfaces, which, in the case of LiV$_2$O$_4$, are difficult to obtain by cleavage of small, three-dimensional crystals. We report the epitaxial growth of LiV$_2$O$_4$ thin films with bulklike properties on SrTiO$_3$(111) via pulsed laser deposition and stable STM imaging of the LiV$_2$O$_4$(111) surface. The as-grown films were transferred $ex$ $situ$ to a room-temperature STM, where subsequent annealing with optional sputtering in ultrahigh vacuum enabled compact islands with smooth surfaces and a hexagonal 1$\times$1 atomic lattice to be resolved. Our STM measurements provide insights into growth mechanisms of LiV$_2$O$_4$ on SrTiO$_3$(111), as well as demonstrate the feasibility of performing surface-sensitive measurements of this heavy fermion compound.

cond-mat.mtrl-sci↗

arXiv2022

Pressure suppression of the excitonic insulator state in Ta2NiSe5 observed by optical conductivity

The layered chalcogenide Ta2NiSe5 has recently attracted much interest as a strong candidate for the long sought excitonic insulator (EI). Since the physical properties of an EI are expected to depend sensitively on the external pressure, it is important to clarify the pressure evolution of microscopic electronic state in Ta2NiSe5. Here we report the optical conductivity [s(w)] of Ta2NiSe5 measured at high pressures to 10 GPa and at low temperatures to 8 K. With cooling at ambient pressure, s(w) develops an energy gap of about 0.17 eV and a pronounced excitonic peak at 0.38 eV, as already reported in the literature. Upon increasing pressure, the energy gap becomes narrower and the excitonic peak is broadened. Above a structural transition at Ps~3 GPa, the energy gap becomes partially filled, indicating that Ta2NiSe5 is a semimetal after the EI state is suppressed by pressure. At higher pressures, s(w) exhibits metallic characteristics with no energy gap. The detailed pressure evolution of s(w) is presented, and discussed mainly in terms of a weakening of excitonic correlation with pressure.

cond-mat.str-el↗

arXiv2022

Measurements of protons and charged pions emitted from $ν_μ$ charged-current interactions on iron at a mean neutrino energy of 1.49$\,$GeV using a nuclear emulsion detector

This study conducted an analysis of muons, protons, and charged pions emitted from $ν_μ$ charged-current interactions on iron using a nuclear emulsion detector. The emulsion detector with a 65$\,$kg iron target was exposed to a neutrino beam corresponding to 4.0$\times$10$^{19}$ protons on target with a mean neutrino energy of 1.49$\,$GeV. The measurements were performed at a momentum threshold of 200 (50)$\,$MeV/$c$ for protons (pions), which are the lowest momentum thresholds attempted up to now. The measured quantities are the multiplicities, emission angles, and momenta of the muons, protons, and charged pions. In addition to these inclusive measurements, exclusive measurements such as the muon-proton emission-angle correlations of specific channels and the opening angle between the protons of CC0$π$2$p$ events were performed. The data were compared to Monte Carlo (MC) predictions and some significant differences were observed. The results of the study demonstrate the capability of detailed measurements of neutrino-nucleus interactions using a nuclear emulsion detector to improve neutrino interaction models.

hep-ex↗

arXiv2022

Creating and controlling Dirac fermions, Weyl fermions, and nodal lines in the magnetic antiperovskite Eu$_3$PbO

The band topology of magnetic semimetals is of interest both from the fundamental science point of view and with respect to potential spintronics and memory applications. Unfortunately, only a handful of suitable topological semimetals with magnetic order have been discovered so far. One such family that hosts these characteristics is the antiperovskites, A$_3$BO, a family of 3D Dirac semimetals. The A=Eu$^{2+}$ compounds magnetically order with multiple phases as a function of applied magnetic field. Here, by combining band structure calculations with neutron diffraction and magnetic measurements, we establish the antiperovskite Eu$_3$PbO as a new topological magnetic semimetal. This topological material exhibits a multitude of different topological phases with ordered Eu moments which can be easily controlled by an external magnetic field. The topological phase diagram of Eu$_3$PbO includes an antiferromagnetic Dirac phase, as well as ferro- and ferrimagnetic phases with both Weyl points and nodal lines. For each of these phases, we determine the bulk band dispersions, the surface states, and the topological invariants by means of $\textit{ab-initio}$ and tight-binding calculations. Our discovery of these topological phases introduces Eu$_3$PbO as a new platform to study and manipulate the interplay of band topology, magnetism, and transport.

cond-mat.mtrl-sci↗

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.

cond-mat.supr-con↗

arXiv2022

Probing the interlayer coupling in 2$H$-NbS$_2$ via soft x-ray angle-resolved photoemission spectroscopy

In the large family of two-dimensional (2D) layered materials including graphene, its honeycomb analogs, and transition-metal dichalcogenides, the interlayer coupling plays a rather intriguing role. On the one hand, the weak van der Waals interaction that holds the layers together endows these compounds with quasi-2D properties, which might imply small interlayer effects on the electronically active bands. On the other hand, the oft-witnessed differences in electronic, optical, and magnetic behaviors of monolayers, bilayers, and multilayers of the same compound must have as their microscopic origin the detailed interlayer hopping parameters. Given the few experimental reports that have attempted to explicitly extract these parameters, we employ soft-x-ray angle-resolved photoemission spectroscopy (SX-ARPES) to probe the interlayer coupling in superconducting 2$H$-NbS$_2$. We visualize the S 3$p_z$ bands that disperse with respect to the out-of-plane momentum and introduce a simple tight-binding model to extract the interlayer hopping parameters. From first-principles calculations, we clarify how atomic distances and the proper accounting for screening via hybrid functionals influence these bands. The knowledge of interlayer hopping parameters is particularly pertinent in NbS$_2$, where recent experiments have uncovered fingerprints of finite-momentum superconductivity in the bulk material and heterostructures.

cond-mat.supr-con↗

arXiv2022

Crystallization of heavy fermions via epitaxial strain in spinel LiV$_{2}$O$_{4}$ thin film

The mixed-valent spinel LiV$_{2}$O$_{4}$ is known as the first oxide heavy-fermion system. There is a general consensus that a subtle interplay of charge, spin, and orbital degrees of freedom of correlated electrons plays a crucial role in the enhancement of quasi-particle mass, but the specific mechanism has remained yet elusive. A charge-ordering (CO) instability of V$^{3+}$ and V$^{4+}$ ions that is geometrically frustrated by the V pyrochlore sublattice from forming a long-range CO down to $T$ = 0 K has been proposed as a prime candidate for the mechanism. To uncover the hidden CO instability, we applied epitaxial strain from a substrate on single-crystalline thin films of LiV$_{2}$O$_{4}$. Here we show a strain-induced crystallization of heavy fermions in a LiV$_{2}$O$_{4}$ film on MgO, where a charge-ordered insulator comprising of a stack of V$^{3+}$ and V$^{4+}$ layers along [001], the historical Verwey-type ordering, is stabilized by the in-plane tensile and out-of-plane compressive strains from the substrate. Our discovery of the [001] Verwey-type CO, together with previous realizations of a distinct [111] CO, evidence the close proximity of the heavy-fermion state to degenerate CO states mirroring the geometrical frustration of the pyrochlore lattice, which supports the CO instability scenario for the mechanism behind the heavy-fermion formation.

cond-mat.str-el↗

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.

cond-mat.supr-con↗

arXiv2022

Perspective: Molecular beam epitaxy of antiperovskite oxides

Antiperovskites, or inverse perovskites, have recently emerged as a material class with a plethora of promising electronic properties. This perspective describes the molecular beam epitaxy (MBE) growth of oxide antiperovskites Sr$_3$PbO and Sr$_3$SnO. We show that MBE offers great potential not only in growing antiperovskites with high structural quality, but also in providing a means to seamlessly connect with advanced characterization tools, including x-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED), and scanning tunneling microscopy (STM), to facilitate the analyses of their intrinsic properties. The initial results point toward the feasibility of atomically controlled antiperovskite growth, which could open doors to study topological and correlated electronic states in an electronic environment quite distinct from what is available in conventional complex oxides.

cond-mat.mtrl-sci↗

arXiv2022

Origin of oscillatory structures in the magnetothermal conductivity of the putative Kitaev magnet $\alpha$-RuCl$_3$

The layered honeycomb magnet $\alpha$-RuCl$_3$ has been suggested to exhibit a field-induced quantum spin liquid state, in which the reported large thermal Hall effect close to the half-quantized value still remains a subject of debate. Recently, oscillatory structures of the magnetothermal conductivity were reported and interpreted as quantum oscillations of charge-neutral particles. To investigate the origin of these oscillatory structures, we performed a comprehensive measurement of the in-plane magnetothermal conductivity $\kappa(H)$ down to low temperature (100 mK), as well as magnetization $M$, for single crystals grown by two different techniques: Bridgman and chemical vapor transport. The results show a series of dips in $\kappa(H)$ and peaks in the field derivative of $M$ located at the same fields independent of the growth method. We argue that these structures originate from field-induced phase transitions rather than quantum oscillations. The positions of several of these features are temperature-dependent and connected to the magnetic phase transitions in zero field: the main transition at 7 K and weaker additional transitions which likely arise from secondary phases at 10 K and 13 K. In contrast to what is expected for quantum oscillations, the magnitude of the structure in $\kappa(H)$ is smaller for the higher conductivity crystal and decreases rapidly upon cooling below 1 K.

cond-mat.str-el↗